The Silicon-Controlled Rectifier (SCR)

Shockley diodes are curious devices, but rather limited in application. Their usefulness may be expanded, however, by equipping them with another means of latching. In doing so, each becomes true amplifying devices (if only in an on/off mode), and we refer to these as silicon-controlled rectifiers, or SCRs.

The progression from Shockley diode to SCR is achieved with one small addition, actually nothing more than a third wire connection to the existing PNPN structure: (Figure below)

The Silicon-Controlled Rectifier (SCR)

If an SCR's gate is left floating (disconnected), it behaves exactly as a Shockley diode. It may be latched by breakover voltage or by exceeding the critical rate of voltage rise between anode and cathode, just as with the Shockley diode. Dropout is accomplished by reducing current until one or both internal transistors fall into cutoff mode, also like the Shockley diode. However, because the gate terminal connects directly to the base of the lower transistor, it may be used as an alternative means to latch the SCR. By applying a small voltage between gate and cathode, the lower transistor will be forced on by the resulting base current, which will cause the upper transistor to conduct, which then supplies the lower transistor's base with current so that it no longer needs to be activated by a gate voltage. The necessary gate current to initiate latch-up, of course, will be much lower than the current through the SCR from cathode to anode, so the SCR does achieve a measure of amplification.

This method of securing SCR conduction is called triggering, and it is by far the most common way that SCRs are latched in actual practice. In fact, SCRs are usually chosen so that their breakover voltage is far beyond the greatest voltage expected to be experienced from the power source, so that it can be turned on only by an intentional voltage pulse applied to the gate.

It should be mentioned that SCRs may sometimes be turned off by directly shorting their gate and cathode terminals together, or by "reverse-triggering" the gate with a negative voltage (in reference to the cathode), so that the lower transistor is forced into cutoff. I say this is "sometimes" possible because it involves shunting all of the upper transistor's collector current past the lower transistor's base. This current may be substantial, making triggered shut-off of an SCR difficult at best. A variation of the SCR, called a Gate-Turn-Off thyristor, or GTO, makes this task easier. But even with a GTO, the gate current required to turn it off may be as much as 20% of the anode (load) current! The schematic symbol for a GTO is shown in the following illustration: (Figure below)

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The Gate Turn-Off thyristor (GTO)

SCRs and GTOs share the same equivalent schematics (two transistors connected in a positive-feedback fashion), the only differences being details of construction designed to grant the NPN transistor a greater β than the PNP. This allows a smaller gate current (forward or reverse) to exert a greater degree of control over conduction from cathode to anode, with the PNP transistor's latched state being more dependent upon the NPN's than vice versa. The Gate-Turn-Off thyristor is also known by the name of Gate-Controlled Switch, or GCS.

A rudimentary test of SCR function, or at least terminal identification, may be performed with an ohmmeter. Because the internal connection between gate and cathode is a single PN junction, a meter should indicate continuity between these terminals with the red test lead on the gate and the black test lead on the cathode like this: (Figure below)

Rudimentary test of SCR

All other continuity measurements performed on an SCR will show "open" ("OL" on some digital multimeter displays). It must be understood that this test is very crude and does not constitute a comprehensive assessment of the SCR. It is possible for an SCR to give good ohmmeter indications and still be defective. Ultimately, the only way to test an SCR is to subject it to a load current.

If you are using a multimeter with a "diode check" function, the gate-to-cathode junction voltage indication you get may or may not correspond to what's expected of a silicon PN junction (approximately 0.7 volts). In some cases, you will read a much lower junction voltage: mere hundredths of a volt. This is due to an internal resistor connected between the gate and cathode incorporated within some SCRs. This resistor

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is added to make the SCR less susceptible to false triggering by spurious voltage spikes, from circuit "noise" or from static electric discharge. In other words, having a resistor connected across the gate-cathode junction requires that a strong triggering signal (substantial current) be applied to latch the SCR. This feature is often found in larger SCRs, not on small SCRs. Bear in mind that an SCR with an internal resistor connected between gate and cathode will indicate continuity in both directions between those two terminals: (Figure below)

Larger SCRs have gate to cathode resistor.

"Normal" SCRs, lacking this internal resistor, are sometimes referred to as sensitive gate SCRs due to their ability to be triggered by the slightest positive gate signal.

The test circuit for an SCR is both practical as a diagnostic tool for checking suspected SCRs and also an excellent aid to understanding basic SCR operation. A DC voltage source is used for powering the circuit, and two pushbutton switches are used to latch and unlatch the SCR, respectively: (Figure below)

SCR testing circuit

Actuating the normally-open "on" pushbutton switch connects the gate to the anode, allowing current from the negative terminal of the battery, through the cathode-gate PN junction, through the switch, through the load resistor, and back to the battery. This gate current should force the SCR to latch on, allowing current to go directly from cathode to anode without further triggering through the gate. When the "on" pushbutton is released, the load should remain energized.

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Pushing the normally-closed "off" pushbutton switch breaks the circuit, forcing current through the SCR to halt, thus forcing it to turn off (low-current dropout).

If the SCR fails to latch, the problem may be with the load and not the SCR. A certain minimum amount of load current is required to hold the SCR latched in the "on" state. This minimum current level is called the holding current. A load with too great a resistance value may not draw enough current to keep an SCR latched when gate current ceases, thus giving the false impression of a bad (unlatchable) SCR in the test circuit. Holding current values for different SCRs should be available from the manufacturers. Typical holding current values range from 1 milliamp to 50 milliamps or more for larger units.

For the test to be fully comprehensive, more than the triggering action needs to be tested. The forward breakover voltage limit of the SCR could be tested by increasing the DC voltage supply (with no pushbuttons actuated) until the SCR latches all on its own. Beware that a breakover test may require very high voltage: many power SCRs have breakover voltage ratings of 600 volts or more! Also, if a pulse voltage generator is available, the critical rate of voltage rise for the SCR could be tested in the same way: subject it to pulsing supply voltages of different V/time rates with no pushbutton switches actuated and see when it latches.

In this simple form, the SCR test circuit could suffice as a start/stop control circuit for a DC motor, lamp, or other practical load: (Figure below)

DC motor start/stop control circuit

Another practical use for the SCR in a DC circuit is as a crowbar device for overvoltage protection. A "crowbar" circuit consists of an SCR placed in parallel with the output of a DC power supply, for placing a direct short-circuit on the output of that supply to prevent excessive voltage from reaching the load. Damage to the SCR and power supply is prevented by the judicious placement of a fuse or substantial series resistance ahead of the SCR to limit short-circuit current: (Figure below)

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Crowbar circuit used in DC power supply

Some device or circuit sensing the output voltage will be connected to the gate of the SCR, so that when an overvoltage condition occurs, voltage will be applied between the gate and cathode, triggering the SCR and forcing the fuse to blow. The effect will be approximately the same as dropping a solid steel crowbar directly across the output terminals of the power supply, hence the name of the circuit.

Most applications of the SCR are for AC power control, despite the fact that SCRs are inherently DC (unidirectional) devices. If bidirectional circuit current is required, multiple SCRs may be used, with one or more facing each direction to handle current through both half-cycles of the AC wave. The primary reason SCRs are used at all for AC power control applications is the unique response of a thyristor to an alternating current. As we saw, the thyratron tube (the electron tube version of the SCR) and the DIAC, a hysteretic device triggered on during a portion of an AC half-cycle will latch and remain on throughout the remainder of the half-cycle until the AC current decreases to zero, as it must to begin the next half-cycle. Just prior to the zero-crossover point of the current waveform, the thyristor will turn off due to insufficient current (this behavior is also known as natural commutation) and must be fired again during the next cycle. The result is a circuit current equivalent to a "chopped up" sine wave. For review, here is the graph of a DIAC's response to an AC voltage whose peak exceeds the breakover voltage of the DIAC: (Figure below)

DIAC bidirectional response

With the DIAC, that breakover voltage limit was a fixed quantity. With the SCR, we have control over exactly when the device becomes latched by triggering the gate at any point in time along the waveform. By connecting a suitable control circuit to the

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gate of an SCR, we can "chop" the sine wave at any point to allow for time-proportioned power control to a load.

Take the circuit in Figure below as an example. Here, an SCR is positioned in a circuit to control power to a load from an AC source.

SCR control of AC power

Being a unidirectional (one-way) device, at most we can only deliver half-wave power to the load, in the half-cycle of AC where the supply voltage polarity is positive on the top and negative on the bottom. However, for demonstrating the basic concept of time-proportional control, this simple circuit is better than one controlling full-wave power (which would require two SCRs).

With no triggering to the gate, and the AC source voltage well below the SCR's breakover voltage rating, the SCR will never turn on. Connecting the SCR gate to the anode through a standard rectifying diode (to prevent reverse current through the gate in the event of the SCR containing a built-in gate-cathode resistor), will allow the SCR to be triggered almost immediately at the beginning of every positive half-cycle: (Figure below)

Gate connected directly to anode through a diode; nearly complete half-wave current through load.

We can delay the triggering of the SCR, however, by inserting some resistance into the gate circuit, thus increasing the amount of voltage drop required before enough gate current triggers the SCR. In other words, if we make it harder for electrons to flow through the gate by adding a resistance, the AC voltage will have to reach a higher

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point in its cycle before there will be enough gate current to turn the SCR on. The result is in Figure below.

Resistance inserted in gate circuit; less than half-wave current through load.

With the half-sine wave chopped up to a greater degree by delayed triggering of the SCR, the load receives less average power (power is delivered for less time throughout a cycle). By making the series gate resistor variable, we can make adjustments to the time-proportioned power: (Figure below)

Increasing the resistance raises the threshold level, causing less power to be delivered to the load. Decreasing the resistance lowers the threshold level, causing more power to be delivered to the load.

Unfortunately, this control scheme has a significant limitation. In using the AC source waveform for our SCR triggering signal, we limit control to the first half of the waveform's half-cycle. In other words, it is not possible for us to wait until after the wave's peak to trigger the SCR. This means we can turn down the power only to the point where the SCR turns on at the very peak of the wave: (Figure below)

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Circuit at minimum power setting

Raising the trigger threshold any more will cause the circuit to not trigger at all, since not even the peak of the AC power voltage will be enough to trigger the SCR. The result will be no power to the load.

An ingenious solution to this control dilemma is found in the addition of a phase-shifting capacitor to the circuit: (Figure below)

Addition of a phase-shifting capacitor to the circuit

The smaller waveform shown on the graph is voltage across the capacitor. For the sake of illustrating the phase shift, I'm assuming a condition of maximum control resistance where the SCR is not triggering at all with no load current, save for what little current goes through the control resistor and capacitor. This capacitor voltage will be phase-shifted anywhere from 0o to 90o lagging behind the power source AC waveform. When this phase-shifted voltage reaches a high enough level, the SCR will trigger.

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With enough voltage across the capacitor to periodically trigger the SCR, the resulting load current waveform will look something like Figure below)

Phase-shifted signal triggers SCR into conduction.

Because the capacitor waveform is still rising after the main AC power waveform has reached its peak, it becomes possible to trigger the SCR at a threshold level beyond that peak, thus chopping the load current wave further than it was possible with the simpler circuit. In reality, the capacitor voltage waveform is a bit more complex that what is shown here, its sinusoidal shape distorted every time the SCR latches on. However, what I'm trying to illustrate here is the delayed triggering action gained with the phase-shifting RC network; thus, a simplified, undistorted waveform serves the purpose well.

SCRs may also be triggered, or "fired," by more complex circuits. While the circuit previously shown is sufficient for a simple application like a lamp control, large industrial motor controls often rely on more sophisticated triggering methods. Sometimes, pulse transformers are used to couple a triggering circuit to the gate and cathode of an SCR to provide electrical isolation between the triggering and power circuits: (Figure below)

Transformer coupling of trigger signal provides isolation.

When multiple SCRs are used to control power, their cathodes are often not electrically common, making it difficult to connect a single triggering circuit to all SCRs equally. An example of this is the controlled bridge rectifier shown in Figure below.

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Controlled bridge rectifier

In any bridge rectifier circuit, the rectifying diodes (in this example, the rectifying SCRs) must conduct in opposite pairs. SCR1 and SCR3 must be fired simultaneously, and SCR2 and SCR4 must be fired together as a pair. As you will notice, though, these pairs of SCRs do not share the same cathode connections, meaning that it would not work to simply parallel their respective gate connections and connect a single voltage source to trigger both: (Figure below)

This strategy will not work for triggering SCR2 and SCR4 as a pair.

Although the triggering voltage source shown will trigger SCR4, it will not trigger SCR2 properly because the two thyristors do not share a common cathode connection to reference that triggering voltage. Pulse transformers connecting the two thyristor gates to a common triggering voltage source will work, however: (Figure below)

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Transformer coupling of the gates allows triggering of SCR2 and SCR4 .

Bear in mind that this circuit only shows the gate connections for two out of the four SCRs. Pulse transformers and triggering sources for SCR1 and SCR3, as well as the details of the pulse sources themselves, have been omitted for the sake of simplicity.

Controlled bridge rectifiers are not limited to single-phase designs. In most industrial control systems, AC power is available in three-phase form for maximum efficiency, and solid-state control circuits are built to take advantage of that. A three-phase controlled rectifier circuit built with SCRs, without pulse transformers or triggering circuitry shown, would look like Figure below.

Three-phase bridge SCR control of load

REVIEW: A Silicon-Controlled Rectifier, or SCR, is essentially a Shockley diode with

an extra terminal added. This extra terminal is called the gate, and it is used to trigger the device into conduction (latch it) by the application of a small voltage.

To trigger, or fire, an SCR, voltage must be applied between the gate and cathode, positive to the gate and negative to the cathode. When testing an SCR, a momentary connection between the gate and anode is sufficient in polarity, intensity, and duration to trigger it.

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SCRs may be fired by intentional triggering of the gate terminal, excessive voltage (breakdown) between anode and cathode, or excessive rate of voltage rise between anode and cathode. SCRs may be turned off by anode current falling below the holding current value (low-current dropout), or by "reverse-firing" the gate (applying a negative voltage to the gate). Reverse-firing is only sometimes effective, and always involves high gate current.

A variant of the SCR, called a Gate-Turn-Off thyristor (GTO), is specifically designed to be turned off by means of reverse triggering. Even then, reverse triggering requires fairly high current: typically 20% of the anode current.

SCR terminals may be identified by a continuity meter: the only two terminals showing any continuity between them at all should be the gate and cathode. Gate and cathode terminals connect to a PN junction inside the SCR, so a continuity meter should obtain a diode-like reading between these two terminals with the red (+) lead on the gate and the black (-) lead on the cathode. Beware, though, that some large SCRs have an internal resistor connected between gate and cathode, which will affect any continuity readings taken by a meter.

SCRs are true rectifiers: they only allow current through them in one direction. This means they cannot be used alone for full-wave AC power control.

If the diodes in a rectifier circuit are replaced by SCRs, you have the makings of a controlled rectifier circuit, whereby DC power to a load may be time-proportioned by triggering the SCRs at different points along the AC power waveform.

TRIACS

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SCRs are unidirectional (one-way) current devices, making them useful for controlling DC only. If two SCRs are joined in back-to-back parallel fashion just like two Shockley diodes were joined together to form a DIAC, we have a new device known as the TRIAC: (Figure below)

The TRIAC SCR equivalent and, TRIAC schematic symbol

Because individual SCRs are more flexible to use in advanced control systems, these are more commonly seen in circuits like motor drives; TRIACs are usually seen in simple, low-power applications like household dimmer switches. A simple lamp dimmer circuit is shown in Figure below, complete with the phase-shifting resistor-capacitor network necessary for after-peak firing.

TRIAC phase-control of power

TRIACs are notorious for not firing symmetrically. This means these usually won't trigger at the exact same gate voltage level for one polarity as for the other. Generally speaking, this is undesirable, because unsymmetrical firing results in a current waveform with a greater variety of harmonic frequencies. Waveforms that are symmetrical above and below their average centerlines are comprised of only odd-numbered harmonics. Unsymmetrical waveforms, on the other hand, contain even-numbered harmonics (which may or may not be accompanied by odd-numbered harmonics as well).

In the interest of reducing total harmonic content in power systems, the fewer and less diverse the harmonics, the better -- one more reason individual SCRs are favored over TRIACs for complex, high-power control circuits. One way to make the TRIAC's

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current waveform more symmetrical is to use a device external to the TRIAC to time the triggering pulse. A DIAC placed in series with the gate does a fair job of this: (Figure below)

DIAC improves symmetry of control

DIAC breakover voltages tend to be much more symmetrical (the same in one polarity as the other) than TRIAC triggering voltage thresholds. Since the DIAC prevents any gate current until the triggering voltage has reached a certain, repeatable level in either direction, the firing point of the TRIAC from one half-cycle to the next tends to be more consistent, and the waveform more symmetrical above and below its centerline.

Practically all the characteristics and ratings of SCRs apply equally to TRIACs, except that TRIACs of course are bidirectional (can handle current in both directions). Not much more needs to be said about this device except for an important caveat concerning its terminal designations.

From the equivalent circuit diagram shown earlier, one might think that main terminals 1 and 2 were interchangeable. These are not! Although it is helpful to imagine the TRIAC as being composed of two SCRs joined together, it in fact is constructed from a single piece of semiconducting material, appropriately doped and layered. The actual operating characteristics may differ slightly from that of the equivalent model.

This is made most evident by contrasting two simple circuit designs, one that works and one that doesn't. The following two circuits are a variation of the lamp dimmer circuit shown earlier, the phase-shifting capacitor and DIAC removed for simplicity's sake. Although the resulting circuit lacks the fine control ability of the more complex version (with capacitor and DIAC), it does function: (Figure below)

This circuit with the gate to MT2 does function.

Suppose we were to swap the two main terminals of the TRIAC around. According to the equivalent circuit diagram shown earlier in this section, the swap should make no difference. The circuit ought to work: (Figure below)

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With the gate swapped to MT1, this circuit does not function.

However, if this circuit is built, it will be found that it does not work! The load will receive no power, the TRIAC refusing to fire at all, no matter how low or high a resistance value the control resistor is set to. The key to successfully triggering a TRIAC is to make sure the gate receives its triggering current from the main terminal 2 side of the circuit (the main terminal on the opposite side of the TRIAC symbol from the gate terminal). Identification of the MT1 and MT2 terminals must be done via the TRIAC's part number with reference to a data sheet or book.

REVIEW: A TRIAC acts much like two SCRs connected back-to-back for bidirectional

(AC) operation. TRIAC controls are more often seen in simple, low-power circuits than

complex, high-power circuits. In large power control circuits, multiple SCRs tend to be favored.

When used to control AC power to a load, TRIACs are often accompanied by DIACs connected in series with their gate terminals. The DIAC helps the TRIAC fire more symmetrically (more consistently from one polarity to another).

Main terminals 1 and 2 on a TRIAC are not interchangeable. To successfully trigger a TRIAC, gate current must come from the main

terminal 2 (MT2) side of the circuit!

LIGHT ACTIVATED SCR (LASCR)

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Like bipolar transistors, SCRs and TRIACs are also manufactured as light-sensitive devices, the action of impinging light replacing the function of triggering voltage.

Optically-controlled SCRs are often known by the acronym LASCR, or Light Activated SCR. Its symbol, not surprisingly, looks like Figure below.

Light activated SCR

Optically-controlled TRIACs don't receive the honor of having their own acronym, but instead are humbly known as opto-TRIACs. Their schematic symbol is shown in Figure below.

Opto-TRIAC

Optothyristors (a general term for either the LASCR or the opto-TRIAC) are commonly found inside sealed "optoisolator" modules.

SILICON CONTROLLED SWITCH

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If we take the equivalent circuit for an SCR and add another external terminal, connected to the base of the top transistor and the collector of the bottom transistor, we have a device known as a silicon-controlled-switch, or SCS: (Figure below)

The Silicon-Controlled Switch(SCS)

This extra terminal allows more control to be exerted over the device, particularly in the mode of forced commutation, where an external signal forces it to turn off while the main current through the device has not yet fallen below the holding current value. Note that the motor is in the anode gate circuit in Figure below. This is correct, although it doesn't look right. The anode lead is required to switch the SCS off. Therefore the motor cannot be in series with the anode.

SCS: Motor start/stop circuit, equivalent circuit with two transistors.

When the "on" pushbutton switch is actuated, the voltage applied between the cathode gate and the cathode, forward-biases the lower transistor's base-emitter junction, and turning it on. The top transistor of the SCS is ready to conduct, having been supplied with a current path from its emitter terminal (the SCS's anode terminal) through resistor R2 to the positive side of the power supply. As in the case of the SCR, both transistors turn on and maintain each other in the "on" mode. When the lower transistor turns on, it conducts the motor's load current, and the motor starts and runs.

The motor may be stopped by interrupting the power supply, as with an SCR, and this is called natural commutation. However, the SCS provides us with another means of

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turning off: forced commutation by shorting the anode terminal to the cathode. [GE1] If this is done (by actuating the "off" pushbutton switch), the upper transistor within the SCS will lose its emitter current, thus halting current through the base of the lower transistor. When the lower transistor turns off, it breaks the circuit for base current through the top transistor (securing its "off" state), and the motor (making it stop). The SCS will remain in the off condition until such time that the "on" pushbutton switch is re-actuated.

REVIEW: A silicon-controlled switch, or SCS, is essentially an SCR with an extra gate

terminal. Typically, the load current through an SCS is carried by the anode gate

and cathode terminals, with the cathode gate and anode terminals sufficing as control leads.

An SCS is turned on by applying a positive voltage between the cathode gate and cathode terminals. It may be turned off (forced commutation) by applying a negative voltage between the anode and cathode terminals, or simply by shorting those two terminals together. The anode terminal must be kept positive with respect to the cathode in order for the SCS to latch.

Thyristor

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An SCR rated about 100 amperes, 1200 volts mounted on a heat sink - the two small wires are the gate trigger leads

The thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased (that is, as long as the voltage across the device has not reversed).

Some sources define silicon controlled rectifiers and thyristors as synonymous.

Function

The thyristor is a four-layer semiconducting device, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to p-type material near to the cathode. (A variant called an SCS—Silicon Controlled Switch—brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled Bipolar Junction Transistors, arranged to cause the self-latching action.

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Thyristors have three states:

1. Reverse blocking mode — Voltage is applied in the direction that would be blocked by a diode

2. Forward blocking mode — Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction

3. Forward conducting mode — The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"

Function of the gate terminal

The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).

Layer diagram of thyristor.

When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting (On state).

If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on state suddenly.

It should be noted that once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until both: (a) the potential VG is removed and (b) the current through the device (anode−cathode) is less than the

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holding current specified by the manufacturer. Hence VG can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator.

These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger current (IGT). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor.

Switching characteristics

In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH).

V - I characteristics.

A thyristor can be switched off if the external circuit causes the anode to become negatively biased. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.

After a thyristor has been switched off by forced commutation, a finite time delay must have elapsed before the anode can be positively biased in the off-state. This minimum delay is called the circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.

For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of tQ are required. Such fast thyristors are made by diffusing into the silicon heavy metals ions such as gold or platinum which act as charge combination centres. Alternatively, fast thyristors may be made by neutron irradiation of the silicon.

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Applications

Load voltage regulated by thyristor phase control.Upper trace: load voltageLower trace: trigger signal.

Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to automatically switch off; referred to as Zero Cross operation. The device can be said to operate synchronously as, once the device is open, it conducts current in phase with the voltage applied over its cathode to anode junction with no further gate modulation being required to replicate; the device is biased fully on. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature.

Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers.

Phase control (PFC), also called phase cutting, is a method of pulse width modulation (PWM) for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform.

Overview

Phase fired control is often used to control the amount of voltage, current or power that a power supply feeds to its load. It does this in much the same way that a pulse width modulated (PWM) supply would pulse on and off to create an average value at its output. If the supply has a DC output, its time base is of no importance in deciding when to pulse the supply on or off, as the value that will be pulsed on and off is continuous.

PFC differs from PWM in that it addresses supplies that output a modulated waveform, such as the sinusoidal AC waveform that the national grid outputs. Here, it becomes important for the supply to pulse on and off at the correct position in the modulation cycle for a known value to be achieved; for example, the controller could

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turn on at the peak of a waveform or at its base if the cycle's time base were not taken into consideration.

Phase fired controllers take their name from that fact that they trigger a pulse of output at a certain phase of the input's modulation cycle. In essence, a PFC is a PWM controller that can synchronise itself with the modulation present at the input.

Most phase fired controllers use thyristors or other solid state switching devices as their control elements. Thyristor based controllers may utilise Gate Turn Off (GTO) thyristors, allowing the controller to not only decide when to pulse the output on but also when to turn it off, rather than having to wait for the waveform to pass within the element's Zero Cross Point.

Output reduction by bucking

A phase fired controller, like a buck topology switched-mode power supply, is only able to deliver an output maximum equal to that which is present at its input, minus any losses occurring in the control elements themselves. Provided the modulation during each cycle is predictable or repetitive, as it is on the national grid's AC mains, to obtain an output lower than its input, a phase fired control simply switches off for a given phase angle of the input's modulation cycle. By triggering the device into conduction at a phase angle greater than 0 degrees, a point after the modulation cycle starts, a fraction of the total energy within each cycle is present at the output.

'Boosting' by derating

To achieve a 'boost' like effect, the PFC designs must be derated such that maximum present at the input is higher than the nominal output requirements. When the supply is first turned on or operating under nominal conditions, the controller will continually be delivering less than 100% of its input. When a boost is required, the controller delivers a percentage closer to 100% of the maximum input available.

Derating of mains powered, phase fired controllers is important as they are often used to control resistive loads, such as heating elements. Over time, the resistance of heating elements can increase. To account for this, a phase fired control must be able to provide some degree of extra voltage to draw the same heating current through the element. The only way of achieving this is to purposely design the supply to require less than 100% of the input's modulation cycle when the elements are first put in place, progressively opening the supply up towards delivering 100% of the input modulation cycle as the elements age.

Applications

Previously, extremely expensive and heavy multi-tapped transformers were used as the supplies for such elements, with the corresponding winding tap being connected to the element to produce the desired temperature. This limited the temperature resolution to the number of tap combinations available. They often find their way into controllers designed for equipment such as electric ovens and furnaces.

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In modern, usually high power, equipment, the transformer is replaced with phase fired controllers connecting the load directly to the mains, resulting in a substantially cheaper and lighter system. However, the method is usually limited to use in equipment that would be unrealistic without it. This is because removal of the mains transformer means that the load is in direct galvanic contact with the input. For industrial ovens and furnaces the input is often the national grid AC, which is itself galvanically referenced to the Earth. With the controller's output referenced to the Earth, a user need only be in contact with the Earth and one of the output terminals to risk receiving an electrical shock. With many high power pieces of equipment running from three phase 415 V, high current capable inputs and having the entirety of any metallic housing or framework present Earthed (grounded), this is a serious risk that must be assessed with care.

History

The first patent for phase fired controllers derives from 1912.[citation needed] However realization was first possible in 1920s, when mercury arc valve rectifiers with control grids became available. However, this method of voltage regulation not very common at the time, because of the limitations mercury arc valves. It became widespread with the invention of solid-state thyristors at the end of 1950s.

Thyristors can also be found in power supplies for digital circuits, where they can be used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components. The thyristor is used in conjunction with a zener diode attached to its gate, and when the output voltage of the supply rises above the zener voltage, the thyristor conducts, shorting the power supply output to ground (and in general blowing an upstream fuse).

The first large scale application of thyristors, with associated triggering diac, in consumer products related to stabilized power supplies within color television receivers in the early 1970s. The stabilized high voltage DC supply for the receiver was obtained by moving the switching point of the thyristor device up and down the falling slope of the positive going half of the AC supply input (if the rising slope was used the output voltage would always rise towards the peak input voltage when the device was triggered and thus defeat the aim of regulation). The precise switching point was determined by the load on the output DC supply as well fluctuations on the input AC supply. They proved to be unpopular with the AC grid power supplier companies because the simultaneous switching of many television receivers, all at approximately the same time, introduced asymmetry into the supply waveform and, as a consequence injected DC back into the grid with a tendency towards saturation of transformer cores and overheating. Thyristors were largely phased out in this kind of application by the end of the decade. Thyristors have been used for decades as lighting dimmers in television, motion pictures, and theater, where they replaced inferior technologies such as autotransformers and rheostats. They have also been used in photography as a critical part of flashes (strobes).

Snubber circuits

Because thyristors can be triggered on by a high rate of rise of off-state voltage, in many applications this is prevented by connecting a resistor-capacitor (RC) snubber

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circuit between the anode and cathode terminals in order to limit the dV/dt (i.e., rate of change of voltage versus time).

Motor Speed Control using Thytistor drives

A thyristor drive is a motor drive circuit where AC supply current is regulated by a thyristor phase control to provide variable voltage to a DC motor.

Thyristor drives are very simple and were first introduced in the 1960s. They remained the predominant type of industrial motor controller until the end of the 1980s when the availability of low cost electronics led to their replacement by chopper drives for high performance systems and inverters for high reliability with AC motors.

They are still employed in very high power applications, such as locomotives, where the high power capability of the thyristors and the simplicity of the design can make them a more attractive proposition than transistor based controllers.

A derivative of the thyristor drive is the simple AC phase controller. This uses a single phase controlled triac to provide a variable voltage AC output for regulating a universal motor. This is the type of motor speed control most commonly used in domestic appliances, such as food mixers, and small AC powered tools, such as electric drills.

HVDC electricity transmission

Since modern thyristors can switch power on the scale of megawatts, thyristor valves have become the heart of high-voltage direct current (HVDC) conversion either to or from alternating current. In the realm of this and other very high power applications, both electronically switched (ETT) and light switched (LTT) thyristors[4] are still the primary choice. The valves are arranged in stacks usually suspended from the ceiling of a transmission building called a valve hall. Thyristors are arranged into a Graetz bridge circuit and to avoid harmonics are connected in series to form a 12 pulse converter. Each thyristor is cooled with deionized water, and the entire arrangement becomes one of multiple identical modules forming a layer in a multilayer valve stack called a quadruple valve. Three such stacks are typically hung from the ceiling of the valve building of a long distance transmission facility.[5][6]

Comparisons to other devices

The functional drawback of a thyristor is that, like a diode, it only conducts in one direction. A similar self-latching 5-layer device, called a TRIAC, is able to work in both directions. This added capability, though, also can become a shortfall. Because the TRIAC can conduct in both directions, reactive loads can cause it to fail to turn

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off during the zero-voltage instants of the ac power cycle. Because of this, use of TRIACs with (for example) heavily-inductive motor loads usually requires the use of a "snubber" circuit around the TRIAC to assure that it will turn off with each half-cycle of mains power. Inverse parallel SCRs can also be used in place of the triac; because each SCR in the pair has an entire half-cycle of reverse polarity applied to it, the SCRs, unlike TRIACs, are sure to turn off. The "price" to be paid for this arrangement, however, is the added complexity of two separate but essentially identical gating circuits.

An earlier gas filled tube device called a Thyratron provided a similar electronic switching capability, where a small control voltage could switch a large current. It is from a combination of "thyratron" and "transistor" that the term "thyristor" is derived.

Although thyristors are heavily used in megawatt scale rectification of AC to DC, in low and medium power (from few tens of watts to few tens of kilowatts) they have almost been replaced by other devices with superior switching characteristics like MOSFETs or IGBTs. One major problem associated with SCRs is that they are not fully controllable switches. The GTO (Gate Turn-off Thyristor) and IGCT are two related devices which address this problem. In high-frequency applications, thyristors are poor candidates due to large switching times arising from bipolar conduction. MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only majority carriers carry the current).

Failure modes

As well as the usual failure modes due to exceeding voltage, current or power ratings, thyristors have their own particular modes of failure, including:

Turn on di/dt — in which the rate of rise of on-state current after triggering is higher than can be supported by the spreading speed of the active conduction area (SCRs & triacs).

Forced commutation — in which the transient peak reverse recovery current causes such a high voltage drop in the sub-cathode region that it exceeds the reverse breakdown voltage of the gate cathode diode junction (SCRs only).

Silicon carbide thyristors

In recent years, some manufacturers have developed thyristors using Silicon carbide (SiC) as the semiconductor material. These have applications in high temperature environments, being capable of operating at temperatures up to 350 °C.

Types of thyristors SCR — Silicon Controlled Rectifier

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ASCR — Asymmetrical SCR RCT — Reverse Conducting Thyristor LASCR — Light Activated SCR, or LTT — Light triggered thyristor DIAC & SIDAC — Both forms of trigger devices BOD — Breakover Diode — A gateless thyristor triggered by avalanche

current, used in protection applications TRIAC — Triode for Alternating Current — A bidirectional switching device

containing two thyristor structures GTO — Gate Turn-Off thyristor IGCT — Integrated Gate Commutated Thyristor

o MA-GTO — Modified Anode Gate Turn-Off thyristor o DB-GTO — Distributed Buffer Gate Turn-Off thyristor

MCT — MOSFET Controlled Thyristor — It contains two additional FET structures for on/off control.

o BRT — Base Resistance Controlled Thyristor SITh — Static Induction Thyristor, or FCTh — Field Controlled Thyristor

containing a gate structure that can shut down anode current flow.

LASS — Light Activated Semiconducting Switch

The GTO is a tri state device. with an 8-function setup.

DIACS

The DIAC, or diode for alternating current, is a trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. When this occurs,

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the resistance of the diode abruptly decreases, leading to a sharp decrease in the voltage drop across the diode and, usually, a sharp increase in current flow through the diode. The diode remains "in conduction" until the current flow through it drops below a value characteristic for the device, called the holding current. Below this value, the diode switches back to its high-resistance (non-conducting) state. This behavior is bidirectional, meaning typically the same for both directions of current flow.

Typical Diac voltage and current relationships. Once the voltage exceeds the turn-on threshold, the device turns on and the voltage rapidly falls while the current increases.

Most DIACs have a breakdown voltage around 30 V. In this way, their behavior is somewhat similar to (but much more precisely controlled and taking place at lower voltages than) a neon lamp.

DIACs have no gate electrode, unlike some other thyristors they are commonly used to trigger, such as TRIACs. Some TRIACs contain a built-in DIAC in series with the TRIAC's "gate" terminal for this purpose.

DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2.

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A diac is a  two-electrode, three-layer bidirectional avalanche diode that can be switched from the off state to the on state for either polarity of applied voltage.

Fig. 18 shows the junction diagram and schematic symbol for a diac; Fig. 19 shows the voltage-current characteristic.

This three-layer trigger diode is similar in construction to a bipolar transistor, but differs from it

in that the doping concentrations at the two junctions are approximately the same and there is no contact made to the base layer. The equal doping levels result in a symmetrical  bidirectional  switching characteristic, as shown in Fig. 19. When. an 

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increasing positive or negative voltage is applied across the terminals of the diac, a minimum (leakage) current I (BO)flows through the device until the voltage reaches the breakover point V(BO). The reverse-biased junction then undergoes avalanche breakdown and, beyond this point, the device exhibits a negative-resistance characteristic, i.e., current through the device increases substantially with decreasing  voltage.

Diacs are primarily used as triggering devices in thyristor phase-control circuits used for light dimming, universal motor-speed control, heat control, and similar applications. Fig. 20 shows the  general circuit diagram for a diac/triac phase-control circuit.

Diacs are bidirectional diodes that switch AC voltages and trigger silicon controlled rectifiers (SCRs) and triacs. SCRs are four-layer (PNPN) thyristors with an input terminal (gate), an output terminal (anode), and a common terminal (cathode) for both the input and output. Triacs are three-terminal silicon devices that function as two SCRs configured in an inverse, parallel arrangement, so as to provide load current during both halves of the AC supply voltage. Diacs, which are similar to open base NPN transistors, exhibit a high-impedance blocking state up to a voltage breakover point above which negative resistance is achieved. Except for a small leakage amount, diacs do not conduct current until a breakover voltage is attained. Because they are bidirectional, diacs are used as firing devices in phase control such as light dimmers and motion speed controls. 

Performance specifications for diacs include breakover voltage, breakover voltage symmetry, breakover current, output voltage, repetitive peak on-state current, and power dissipation. Breakover voltage (VBO), the voltage at which diacs begin to conduct, is measured between the input and output terminals when diacs switch on. Breakover voltage symmetry ( VBO) is the maximum breakover voltage range with a specified capacitance when diacs are connected in parallel. Measured during the “on” state, output voltage (VO) is the voltage across a 20-ohm resistor in series with a diac during the discharge of a specified capacitor. Repetitive peak on-state current (ITRM) is the maximum limiting peak on-state current, including all repetitive transient currents, for which diacs are rated. Power dissipation (Pd) is the power dissipated by diacs during the “on” state.

Diac

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A diac is a form of solid-state switch used to switch AC voltage; it belongs to the class of switches known as thyristers. It is like a junction transistor without a base lead (it is a two-lead device) and accomplishes its switching action by breakdown at a certain voltage. There are also four layer devices with a similar mode of operation known as four-layer diodes.

Triac

The triode AC switch (TRIAC) is a power-switching device as is the SCR. The TRIAC conducts currents in both directions while the SCR allows current in only one direction. A common application is for lighting controllers. In response to a trigger, the triac conducts until the AC voltage applied reaches zero, then blocks flow until the next trigger occurs. Since a trigger can cause it to trigger current in either direction, it is an efficient power controller from essentially zero to full power.

ThyristersThyrister is the name given to semiconductor switches in which a large current can be switched by a small gate current. They are usually three-lead devices where the gate signal on one lead controls the current between the other leads. Examples are the silicon-controlled rectifier (SCR) which conducts current in one direction and the triac which is a double SCR which conducts in both directions. There are some two lead varieties like the diac in which a zener type breakdown provides the trigger to start conduction.

Silicon Controlled RectifierThe SCR is a power-switching device commonly used for lighting control, motor speed control and other variable power applications

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The silicon-controlled rectifier is like a junction transistor with a fourth layer and therefore three p-n junctions. The two outer junctions are forward biased by the voltage as shown, but the inner junction is reverse biased. A small current in the gate electrode can turn on the current, and it will stay on until the driving voltage is removed. It is called a rectifier because it conducts current in only direction. If AC voltage is applied, then it can be turned on by a pulse and remain on until the end of that half cycle. Timed 60 Hz triggers can by used to control power by changing the trigger point within the half cycle.

UJT’s and PUT’s

Unijunction transistor

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Circuit symbol

A unijunction transistor (UJT) is an electronic semiconductor device that has only one junction. The UJT has three terminals: an emitter (E) and two bases (B1 and B2). The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type and it is heavily doped. The resistance between B1 and B2, when the emitter is open-circuit is called interbase resistance.

There are two types of unijunction transistor:

The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length, defining the device parameter η. The 2N2646 is the most commonly used version of the UJT.

The programmable unijunction transistor, or PUT, is a close cousin to the thyristor. Like the thyristor it consists of four P-N layers and has an anode and a cathode connected to the first and the last layer, and a gate connected to one of the inner layers. They are not directly interchangeable with conventional

UJTs but perform a similar function. In a proper circuit configuration with two "programming" resistors for setting the parameter η, they behave like a conventional UJT. The 2N6027 is an example of such a device.

The UJT is biased with a positive voltage between the two bases. This causes a potential drop along the length of the device. When the emitter voltage is driven approximately one diode voltage above the voltage at the point where the P diffusion (emitter) is, current will begin to flow from the emitter into the base region. Because the base region is very lightly doped, the additional current (actually charges in the base region) causes conductivity modulation which reduces the resistance of the portion of the base between the emitter junction and the B2 terminal. This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected. Overall, the effect is a negative resistance at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuits.

Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1970's and early 1980's because they allowed simple oscillators to be built using just

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one active device. Later, as Integrated Circuits became more popular, oscillators such as the 555 timer IC became more commonly used.

In addition to its use as the active device in relaxation oscillators, one of the most important applications of UJTs or PUTs are to trigger thyristors (SCR, TRIAC, etc.). In fact, a DC voltage can be used to control a UJT or PUT circuit such that the "on-period" increases with an increase in the DC control voltage. This application is important for large AC current control.

Unijunction Transistor

      The unijunction transistor(UJT) is a three terminal device with characteristics very different from the conventional 2 junction, bipolar transistor. It is a pulse generator with the trigger or control signal applied at the emitter . This trigger voltage is a fraction (n) of interbase voltage, Vbb.The UJT circuit symbol, junction schematic, and characteristic curve are shown below.

     The emitter terminal does not inject current into the base region until its voltage reaches Vp. Once Vp is reached the base circuit conducts and a postive pulse appears at the B1 terminal and a negative pulse at B2. The UJT incorporates a negative resistance region, a low emitter current, and a high output pulse current at terminals B1 and B2, making it an ideal pulse trigger. A simple RC timer circuit using a UJT is shown below.

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The very basic specifications of a UJT are:

(a) Vbb(max) - The maximum interbase voltage that can be applied to the UJT(b) Rbb-the interbase resistance of the UJT(c) n - The intrinsic standoff ratio which defines Vp.(d) Ip - The peakpoint emitter current

UJT CHARACTERISTICS …. some more information:

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As observed during plotting of the characteristics, in practice, as the emitter voltage is increased gradually upwards from 0 volts, at about 7.2 volts the emitter-base1 junction of the diode gets forward biased and at the peak point voltage, there is a sudden and quick changeover, in the sense that the emitter current rises sharply and the emitter voltage drops down to the valley point voltage, from the peak point voltage. This observed change is as follows:

From Peak Point Voltage of 7.2 volts, the voltage falls down to the Valley Point Voltage which is about 3.2 volts. The rise in the emitter current level is from 0 mA. to about 3.4 mA. Under this condition onwards, the UJT emitter-base 1 junction is in saturation region. The characteristics from the Peak Point Voltage up to the Valley Point Voltage represents the negative resistance region. Beyond this if the emitter voltage is increased more than 3.2 volts, say, 3.4 or 3.6 or 3.8 volts, correspondingly the emitter current also increases (following ohms law). Increasing the emitter voltage further will further increase the emitter current.

The only way to therefore bring down this saturation current flowing through the device when Valley Point Voltage is reached, is to reduce the emitter voltage and make it less than 3.2 volts. Reducing this voltage and making it 3 volts, 2.8 volts, 2.6 volts, etc and further, still keeps the UJT in full conduction or saturation, and somewhere around 2.2 volts or so, the current drops down to 0mA (or for that matter the emitter current takes on the reverse direction of the order of microamps). This lowering of the emitter voltage below the Valley Point Voltage is necessary, in order to bring the UJT out of the breakdown state. And after this the only way to make the UJT start conducting is increase the emitter voltage until the “diode” at the emitter junction becomes forward biased, at which conduction starts and breakdown takes place again.

PUT PROGRAMMABLE UJTThe Programmable Unijunction Transistor behaves much like a unijunction transistor (UJT), but is "programmable" via external resistors (that is, you can use two resistors to set a PUT's peak voltage). Note that the name is a bit of a misnomer -- as a thyristor, it is a four layer device, unlike a true unijunction transistor which has but two layers.

Like other thyristors, a PUT looks much like a junction transistor with a fourth layer and therefore a total of three P-N junctions. Meanwhile, a third terminal, the gate (G), makes a PUT function like a hybrid of transistor and diode:

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PUTs are essentially special-purpose devices in electronics, used for lighting control, motor speed control and other variable power applications. In combination with an SCR they can, though, make a mean solar engine.

In a pinch, you can build up something much like a PUT from discrete transistors wired as a complementary feedback pair:

Here, as soon as any current flows in either transistor, this current becomes base current for the other transistor, and both transistors turn on hard. This means you can only build up this circuit using low-leakage transistors ('though this should be the case with any decent-quality modern transistor ).

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Programmable UJT

Programmable UJT

The programmable unijunction transistor (PUT) is not a unijunction transistor at all. The fact that the V-I characteristics and applications of both are similar prompted the choice of labels.

It is also a four-layer P-N-P-N solid-state device with a gate connected directly to the sandwiched N-type layer. The basic structure, schematic symbol and the basic biasing arrangement of PUT are shown in figures respectively. As the symbol indicates, it is essentially an SCR with a control mechanism that permits a duplication of the characteristics of the typical SCR. The term “programmable” is applied because the inter base resistance RBB, the intrinsic stand-off ratio Ƞ and peak-point voltage VP, as defined in UJT can be programmed to any desired values through external resistors RB

and RB2 and the supply voltage VBB. From figure we see that by voltage divider rule when IG = 0,

VG =   (RB1 / RB1 + RB2 )  VBB =  Ƞ  VBB

Consider figure The P-N-P-N device shown in figure has its gate connected to the junction of external resistors RB and RB . The four-layer construction shown in figure indicates that the anode-gate junction is forward biased when the anode becomes positive with respect to gate. When this occurs, the device is turned on. The anode-to-cathode voltage VAK then drops to a low level, and the device conducts heavily until the input voltage become too low to sustain conduction. It is seen that this action stimulates the performance of a UJT. The anode of the device acts as the emitter of UJT.

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Characteristics of Programmable UJT

The typical characteristics of the device are shown in figure. The firing or peak-point potential is given as

VP = Ƞ VBB + VB  as defined for the UJT.

However VP represents the voltage drop VAK in figure [ the forward voltage drop across the conducting diode]. For silicon VB is typically 0.7 V.

In PUT RB1 and RB2 are the external resistors to the device permitting the adjustment of Ƞ and hence VG while in the UJT both RB1 and RB2 represent the bulk resistance and ohmic base contacts of the device (both inaccessible).  Although the characteristics of the PUT and UJT are similar, the peak and valley currents of the PUT are typically lower than those of a UJT of a similar rating. In addition, the minimum operating voltage of PUT is also lower than that of UJT.

Application of PUT

PUT Relaxation Oscillator

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PUT, because of its superiority over UJT, replaces UJT. One popular application of PUT is in the relaxation oscillator shown in figure. The instant the supply VBB is switched on, the capacitor starts charging toward VBB volts, since there is no anode current at this point. The instant the voltage across the capacitor equals VP, the device fires and anode current IA = IP is established through the PUT. As soon as the device fires, the capacitor starts discharging rapidly through the low on-resistance of the PUT and RK. Consequently, a voltage spike is produced across RK during the discharge. As soon as the capacitor C gets discharged, the PUT turns off and the charging cycle starts all over again as narrated above.

The time period required to attain the firing potential VP is given approximately by the expression

T = RC loge = VBB / VBB – VP = Ƞ VBB

At the point of firing of PUTIP R = VBB – VP

If R is too large, the current IP cannot be established, and the device will not fire

So RMAX = VBB – VP / IP

Similarly  RMIN = VBB – VV / IV

VBB – VP / IP > R > VBB – VV / IV

Programmable uni-junction transistors (PUT) are three-terminal thyristors that are triggered into conduction when the voltage at the anode exceeds the voltage at the gate. The PUT is similar to the UJT, but its intrinsic standoff ratio can be set by two external resistors. Hence, the name "programmable" is used. A PUT is a more advanced version of a unijunction transistor (UJT). In a programmable unijunction transistor, operating characteristics such as base-to-base resistance, intrinsic standoff voltage, valley current, and peak current can be programmed by setting the values of two external resistors. Applications for programmable unijunction transistors (PUT) include thyristor triggers, oscillators, pulse, and timing circuits, with frequencies up to 10 kHz. An integrated circuit can include not only an integrated circuit chip, but also a circuit transistor such as a programmable unijunction transistor.

Performance specifications for programmable unijunction transistors (PUT) include peak current (with RG of 10K ohms and 1M ohms), valley current (with RG of 10K ohms and 1M ohms), gate-to-cathode forward voltage, gate-to-cathode reverse voltage, gate-to-anode reverse voltage, anode-to-cathode voltage, peak non-repetitive forward current, peak repetitive forward current, peak repetitive forward current, DC forward anode current, DC gate current, power dissipation, storage temperature, operating junction temperature. Programmable unijunction transistors (PUT) can be packaged individually or in standard packaging for high-volume requirements, such as automatic insertion equipment.

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THYRISTOR / SCR THEORY

As stated before, Bell Laboratories were the first to fabricate a silicon-based semiconductor device called thyristor. Its first prototype was introduced by GEC (USA) in 1957. This company did a great deal of pioneering work about the utility of thyristors in industrial applications. Later on, many other devices having characteristics similar to that of a thyristor were developed. These semiconductor devices, with their characteristics identical with that of a thyristor, are triac, diac, silicon-controlled switch, programmable unijunction transistor (PUT), GTO, RCT etc. This whole family of semiconductor devices is given the name thyristor. Thus the term thyristor denotes a family of semiconductor devices used for power control in dc and ac systems. One oldest member of this thyristor family, called silicon-controlled rectifier (SCR), is the most widely used device. At present, the use of SCR is so vast that over the years, the word thyristor has become synonymous with SCR. It appears that the term thyristor is now becoming more common than the actual term SCR. In this book, the term SCR and thyristor will be used at random for the same device SCR. Other members of thyristor family are also discussed in this category.

A thyristor has characteristics similar to a thyratron tube. But from the construction view point, a thyristor (a pnpn device) belongs to transistor (pnp or npn device) family. The name ‘thyristor’, is derived by a combination of the capital letters from THYRatron and transISTOR. This means that thyristor is a solid state device like a transistor and has characteristics similar to that of a thyratron tube. The present-day reader may not be familiar with thyratron tube as this is not being taught these days.

MOS CONTROLLED THYRISTOR

An MCT is a new device in the field of semiconductor-controlled devices. It is basically a thyristor with two MOSFETs built into the gate structure. One MOSFET is used for turning on the MCT and the other for turning off the device. An MCT is a high-frequency, high:power, low-conduction drop switching device.

An MCT combines into it the features of both conventional four-layer thyristor having regenerative action and MOS-gate structure. However, in MCT, anode is the reference with  respect to which, all gate signals are applied. In a conventional SCR, cathode is the reference terminal for gate signals.

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The equivalent circuit of MCT is shown in Fig. 2.21 (a). It consists of one on-FET, one off-FET and two transistors. The on-FET is ap-channel MOSFET and off-FET is an rc-channel MOSFET. An arrow towards the gate terminal indicates n-channel MOSFET and the arrow away from the gate terminal as the p-channel MOSFET. The two transistors in the equivalent circuit indicate that there is regenerative feedback in the MCT just as it is in an ordinary thyristor. Fig. 2.21 (6) gives the circuit symbol of an MCT

An MCT is turned-on by a negative voltage pulse at the gate with respect to the anode and is turned-off by a positive voltage pulse. Working of MCT can be understood better by referring to Fig. 2.21 (a).

Turn-on process. As stated above, MCT is turned on by applying a negative voltage pulse at the gate with respect to anode. In other words, for turning on MCT, gate is made negative with respect to anode by the voltage pulse between gate and anode. With the application of this negative voltage pulse, on-FET gets >turned-on and off-FET is off. With on-FET on, current begins to flow from anode A, through on-FET and then as the base current and emitter current of npn transistor and then to cathode C. This turns on npn transistor. As a result, collector current begins to flow in npn transistor. As off-FET is off, this collector current of npn transistor acts as the base current of pnp transistor. Subsequently, pnp transistor is also turned on. Once both the transistors are on, regenerative action of the connection scheme takes place and the thyristor or MCT is turned on.

Note that on-FET and pnp transistor are in parallel when thyristor is in conduction state. During the time MCT is on, base current of npn transistor flows mainly through pnp transistor because of its better conducting property.

Turn-off process. For turning-off the MCT, off-FET (or n -channel MOSFET) is energized by positive voltage pulse at the gate. With the application of positive voltage pulse, off-FET is turned on and on-FET is turned off. After off-FET is turned on, emitter-base terminals of pnp transistor are short circuited by off-FET So now anode current begins to flow through off’-FET and therefore base current of pnp transistor begins to decrease. Further, collector current of pnp transistor that forms the base current of npn transistor also begins to decrease.

As a consequence, base currents of both pnp and npn transistors, now devoid of stored charge in their n and p bases respectively, begin to decay. This regenerative action eventually turns off the MCT.

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An MCT has the following merits :

(i)        Low forward conduction drop,

(ii)       fast turn-on and turn-off times,(iii)   low switching losses and

(iv)    high gate input impedance, which allows simpler design of drive circuits.

An MCT is a brand-new device which is likely to be available commercially very soon. As it possesses highly adaptable features for its use as a switching device, it seems to have tremendous scope for its widespread applications. Its potential applications include dc and ac motor drives, UPS systems, induction heating, dc-dc converters, power line conditioners etc. It may, in the near future, challenge the existence of most of the available devices like -   resistors, GTOs, BJTs, IGBTs (7).

NEW SEMICONDUCTING MATERIALS

At present, silicon enjoys monopoly as a semiconductor material for the commercial production of power-control devices. This is because silicon is cheaply available and semiconductor devices of any size can be easily fabricated on a single silicon chip. There are, however, new types of materials like gallium arsenic (GaAs), silicon carbide and diamond which possess the desirable properties required for switching devices. At present, state-of-the-art technology for these materials is primitive compared with silicon, and many more years of research investment are required before these materials become commercially viable for the production of power-controlled devices. Superconductive materials may also be used in the manufacture of such devices, but work in this direction has not yet been reported.

Germanium is not used in the fabrication of thyristors because of the following reasons:

1. Germanium has much lower thermal conductivity; its thermal resistance is, therefore, more. As a consequence, germanium thyristors suffer from more losses, more temperature rise and therefore lower operating life.

2. Its breakdown voltage is much less than that of silicon. It means that germanium thyristor can be built for small voltage ratings only.

3. Germanium is much costlier than silicon.

TERMINAL CHARACTERISTICS OF A THYRISTOR

Thyristor is a four layer, three-junction, p-n-p-n semiconductor switching device. It has three terminals ; anode, cathode and gate. Fig. 4.1 (a) gives constructional details of a typical thyristor. Basically, a thyristor consists of four layers of alternate p-type and n-type silicon semiconductors forming three junctions J1, J2 and J3 as shown in Fig. 4.1 (a). The threaded portion is for the purpose of tightening the thyristor to the frame or heat sink with the help of a nut. Gate terminal is usually kept near the cathode terminal Fig. 4.1 (a). Schematic diagram and circuit symbol for a thyristor are shown respectively in Figs. 4.1 (b) and (c). The terminal connected to outer p region is called anode (A), the terminal connected to outer n region is called cathode and that

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connected to inner p region is called the gate (G). For large current applications, thyristors need better cooling ; this is achieved to a great extent by mounting them onto heat sinks. SCR rating has improved considerably since its introduction in 1957. Now SCRs of voltage rating 10 kV and an rms current rating of 3000 A with corresponding power-handling capacity of 30 MW are available. Such a high power thyristor can be switched on by a low voltage supply of about 1 A and 10 W and this gives us an idea of the immense power amplification capability (= 3 x 106) of this device. As SCRs are solid state devices, they are compact, possess high reliability and have low loss. Because of these useful features, SCR is almost universally employed these days for all high power-controlled devices.

An SCR is so called because silicon is used for its construction and its operation as a rectifier (very low resistance in the forward conduction and very high resistance in the reverse direction) can be controlled. Like the diode, an SCR is an unidirectional device that blocks the current flow from cathode to anode. Unlike the diode, a thyristor also blocks the current flow from anode to cathode until it is triggered into conduction by a proper gate signal between gate and cathode terminals.

For engineering applications of thyristors, their terminal characteristics must be known. In this article, their static V-I characteristics, dynamic characteristics during turn-on and turn-off processes and their gate characteristics are discussed.

STATIC CHARACTERISTICS OF A THYRISTOR

An elementary circuit diagram for obtaining static V-I characteristics of a thyristor is shown in Fig. 4.2 (a). The anode and cathode are connected to main source through the load. The gate and cathode are fed from a source Es which provides positive gate current from gate to cathode.

Fig. 4.2 (b) shows static V-I characteristics of a thyristor. Here Va is the anode voltage across thyristor terminals A, K and Ia is the anode current. Typical SCR V-I characteristic shown in Fig. 4.2 (b) reveals that a thyristor has three basic modes of operation ; namely, reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. These three modes of operation are now discussed below :

Reverse Blocking Mode: When cathode is made positive with respect to anode with switch S open, Fig. 4.2 (a), thyristor is reverse biased as shown in Fig. 4.3 (a). Junctions J1 J3 are seen to be reverse biased whereas junction J2 is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them. A small leakage current of the order of a few milliamperes (or a few microamperes depending upon the SCR rating) flows. This is reverse blocking

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mode, called the off-state, of the thyristor. If the reverse voltage is increased, then at a critical breakdown level, called reverse breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. A large current associated with VBR gives rise to more losses in the SCR. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltage applied across a thyristor is less than VBR, the device offers a high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as an open switch.

Note that V-I characteristic after avalanche breakdown during reverse blocking mode is applicable only when load resistance is zero, Fig. 4.2 (b). In case load resistance is present, a large anode current associated with avalanche breakdown at VBR would cause substantial voltage drop across load and as a result, V-I characteristic in third quadrant would bend to the right of vertical line drawn at VBR.

Forward Blocking Mode : When anode is positive with respect to the cathode, with gate circuit open, thyristor is said to be forward biased as shown in Fig. 4.3 (b). It is seen from this figure that junctions J1, J3 are forward biased but junction J2 is reverse biased. In this mode, a small current, called forward leakage current, flows as shown in Figs. 4.2 (b) and 4.3 (b). In case the forward voltage is increased, then the reverse biased junction J2 will have an avalanche breakdown at a voltage called forward

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breakover voltage VB0. When forward voltage is less than VBO, SCR offers a high impedance. Therefore, a thyristor can be treated as an open switch even in the forward blocking mode.

Forward Conduction Mode : In this mode, thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward breakover voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending on the rating of SCR. It may be seen from Fig. 4.2 (b) that this voltage drop increases slightly with an increase in anode current. In conduction mode, anode current is limited by load impedance alone as voltage drop across SCR is quite small. This small voltage drop vT across the device is due to ohmic drop in the four layers.

TURNING ON THE THYRISTOR:

With anode positive with respect to cathode, a thyristor can be turned on by any one of the following techniques :

(a) Forward voltage triggering          (b) gate triggering

(c) dv/dt triggering              (d)Temperature triggering

(e)Light triggering.

These methods of turning-on a thyristor are now discussed one after the other.

(a) Forward Voltage Triggering: When anode to cathode forward voltage is increased with gate circuit open, the reverse biased junction J2 will break. This is known as avalanche breakdown and the voltage at which avalanche occurs is called forward breakover voltage VB0. At this voltage, thyristor changes from off-state (high voltage with low leakage current) to on-state characterised by low voltage across thyristor with large forward current. As other junctions J1, J3 are already forward biased, breakdown of junction J2 allows free movement of carriers across three junctions and as a result, large forward anode-current flows. As stated before, this forward current is limited by the load impedance. In practice, the transition from off-state to on-state obtained by exceeding VB0 is never employed as it may destroy the device.

The magnitudes of forward and reverse breakover voltages are nearly the same and both are temperature dependent. In practice, it is found that VBR is slightly more than VB0. Therefore, forward breakover voltage is taken as the final voltage rating of the device during the design of SCR applications.

After the avalanche breakdown, junction J2 looses its reverse blocking capability. Therefore, if the anode voltage is reduced below VB0 SCR will continue conduction of the current. The SCR can now be turned off only by reducing the anode current below a certain value called holding current (defined later).

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(b) Gate Triggering : Turning on of thyristors by gate triggering is simple, reliable and efficient, it is therefore the most usual method of firing the forward biased SCRs. A thyristor with forward breakover voltage (say 800 V) higher than the normal working voltage (say 400 V) is chosen. This means that thyristor will remain in forward blocking state with normal working voltage across anode and cathode and with gate open. However, when turn-on of a thyristor is required, a positive gate voltage between gate and cathode is applied. With gate current thus established, charges are injected into the inner p layer and voltage at which forward breakover occurs is reduced. The forward voltage at which the device switches to on-state depends upon the magnitude of gate current. Higher the gate current, lower is the forward breakover voltage

When positive gate current is applied, gate P layer is flooded with electrons from the cathode. This is because cathode N layer is heavily doped as compared to gate P layer. As the thyristor is forward biased, some of these electrons reach junction J2. As a result, width of depletion layer around junction J2 is reduced. This causes the junction J2 to breakdown at an applied voltage lower than forward breakover voltage VB0. If magnitude of gate current is increased, more electrons will reach junction J2 ,as a consequence thyristor will get turned on at a much lower forward applied voltage.

Fig. 4.2 (b) shows that for gate current Ig = 0, forward breakover voltage is VB0. For Igl

, forward breakover voltage, or turn-on voltage is less than VB0 For Ig2 > Ig1 , forward breakover voltage is still further reduced. The effect of gate current on the forward breakover voltage of a thyristor can also be illustrated by means of a curve as shown in Fig. 4.4. For Ig < oa, forward breakover voltage remains almost constant at VB0. For gate currents Ig1 , Ig2 and Ig3 the values of forward breakover voltages are ox, oy and oz, respectively as shown. In Fig. 4.2 (b), the curve marked Ig = 0 is actually for gate current less than oa. In practice, the magnitude of gate current is more than the minimum gate current required to turn on the SCR. Typical gate current magnitudes are of the order of 20 to 200 mA.

Once the SCR is conducting a forward current, reverse biased junction J2 no longer exists. As such, no gate current is required for the device to remain in on-state. Therefore, if the gate current is removed, the conduction of current from anode to

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cathode remains unaffected. However, if gate current is reduced to zero before the rising anode current attains a value, called the latching current, the thyristor will turn-off again. The gate pulse width should therefore be judiciously chosen to ensure that anode current rises above the latching current. Thus latching current may be defined as the minimum value of anode current which it must attain during turn-on process to maintain conduction when gate signal is removed.

Once the thyristor is conducting, gate loses control. The thyristor can be turned-off (or the thyristor can be returned to forward blocking state) only if the forward current falls below a low-level current called the holding current. Thus holding current may be defined as the minimum value of anode current below which it must fall for turning-off the thyristor. The latching current is higher than the holding current. Note that latching current is associated with turn-on process and holding current with turn-off process. It is usual to take latching current as two to three times the holding current . In industrial applications, holding current (typically 10 mA) is almost taken as zero.

(c)   dv/dt  Triggering :

dv/dt triggering : The reversed biased junction J2 behaves like a capacitor because of the space-charge present there. Let the capacitance of this junction be Cj. For any capacitor, i = C dv/dt.In case it is assumed that entire forward voltage va appears across reverse biased junction J2 then charging current across the junction is given by

i = Cj dva /dt

This charging or displacement current across junction J2 is collector currents of Q2 and Q1 Currents IC2, IC1 will induce emitter current in Q2, Q1 In case rate of rise of anode voltage is large, the emitter currents will be large and as a result, α1+ α2 will approach unity leading to eventual switching action of the thyristor.

(d) Temperature Triggering : During forward blocking, most of the applied voltage appears across reverse biased junction J2. This voltage across junction J2 associated with leakage current may raise the temperature of this junction. With increase in temperature, leakage current through junction J2 further increases. This cumulative process may turn on the SCR at some high temperature.

(e) Light Triggering: For light-triggered SCRs, a recess (or niche) is made in the inner p-layer as shown in Fig. 4.5 (a). When this recess is irradiated, free charge carriers (holes and electrons) are generated just like when gate signal is applied between gate and cathode. The pulse of light of appropriate wavelength is guided by optical fibres for irradiation. If the intensity of this light thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a thyristor is known as light-activated SCR (LASCR).

LASCR may be triggered with a light source or with a gate signal. Sometimes a combination of both light source and gate signal is used to trigger an SCR. For this, the gate is biased with voltage or current slightly less than that required to turn it on, now a beam of light directed at the inner p-layer junction turns on the SCR. The light

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intensity required to turn-on the SCR depends upon the voltage bias given to the gate. Higher the voltage (or current) bias, lower the light intensity required.

Light-triggered thyristors have now been used in high-voltage direct current (HVDC) transmission systems. In these several SCRs are connected in series-parallel combination and their light-triggering has the advantage of electrical isolation between power and control circuits.

TURNING OFF THE THYRISTOR (COMMUTATION):

Static and switching characteristics of thyristors are always taken into consideration for economical and reliable design of converter equipment. Static characteristics of a thyristor have already been examined. In this part of the section; switching, dynamic or transient, characteristics of thyristors are discussed.

During turn-on and turn-off processes, a thyristor is subjected to different voltages across it and different currents through it. The time variations of the voltage across a thyristor and the current through it during turn-on and turn-off processes give the dynamic or switching characteristics of a thyristor. Here, first switching characteristics during turn-on are described and then the switching characteristics during turn-off

Switching Characteristics during Turn-off

Thyristor turn-off means that it has changed from on to off state and is capable of blocking the forward voltage. This dynamic process of the SCR from conduction state to forward blocking state is called commutation process or turn-off process.

Once the thyristor is on, gate loses control. The SCR can be turned off by reducing the anode current below holding current . If forward voltage is applied to the SCR at the moment its anode current falls to zero, the device will not be able to block this forward voltage as the carriers (holes and electrons) in the four layers are still favourable for conduction. The device will therefore go into conduction immediately even though gate signal is not applied. In order to obviate such an occurrence, it is

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essential that the thyristor is reverse biased for a finite period after the anode current has reached zero.

The turn-off time decreases with an increase in the magnitude of reverse voltage, particularly in the range of 0 to – 50 V. This is because high reverse voltage sucks out the carriers out of the junctions Jl , J3 and the adjacent transition regions at a faster rate.

The thyristor turn-off time tq is applicable to an individual SCR. In actual practice, thyristor (or thyristors) form a part of the power circuit. The turn-off time provided to the thyristor by the practical circuit is called circuit turn-off time tc.

Thyristors with slow turn-off time (50 – 100 (usee) are called converter grade SCRs and those with fast turn-off time (3 – 50 µsec) are called inverter-grade SCRs. Converter-grade SCRs are cheaper and are used where slow turn-off is possible as in phase-controlled rectifiers, ac voltage controllers, cycloconverters etc. Inverter-grade SCRs are costlier and are used in inverters, choppers and force-commutated converters.

SCR TURN-ON TIME

As stated before, thyristor is considered to be a charge controlled device. Thus, higher the magnitude of gate current pulse, lesser is the time to inject the required charge for turning-on the thyristor. Therefore, SCR turn-on time can be reduced by using gate current of higher magnitude. It should be ensured that pulse width is sufficient to allow the anode current to exceed the latching current. In practice, gate pulse width is usually taken as equal to, or greater than, SCR turn-on time. With pulse triggering, greater amount of gate power dissipation can be allowed

A duty cycle is defined as the ratio of pulse-on period to periodic time of pulse. In Fig. 4.12 (a), pulse-on period is T and periodic time is T1. Therefore, duty cycle δ is given by

δ = T/ T1

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Sometimes the pulses of Fig. 4.12 (a) are modulated to generate a train of pulses as shown in Fig. 4.12 (b). This technique of firing the thyristor is called high-frequency carrier gating. The advantages offered by this method of firing the SCRs are lower rating, reduced dimensions and therefore an overall economical design of the pulse transformer needed for isolating the low power circuit from the main power circuit.

There is also prescribed a peak reverse voltage (gate negative with respect to cathode) that can be applied across gate-cathode terminals. Any voltage signal, given by the trigger circuit (or by any interference), exceeding this prescribed limit of about 5 to 20 V may damage the gate circuit. For preventing the occurrence of such hazards, a diode is connected either in series with the gate circuit or across the gate-cathode terminals as shown in Fig. 4.12 (c). Diode across the gate-cathode terminals, called clamping diode, prevents the gate-cathode voltage from becoming more than about 1 V. Diode in series with gate circuit prevents the flow of negative gate source current from becoming more than small reverse leakage current.

The magnitude of gate voltage and gate current for triggering an SCR is inversely proportional to junction temperature. Thus, at very low temperatures, gate voltage and gate current must have high values in order to ensure turn-on. But Pgm should not be exceeded in any case.

The resistor Rl , connected across gate-cathode terminals, Fig. 4.10 (b), also serves to bypass a part of the thermally-generated leakage current across junction J2 when SCR is in the forward blocking mode ; this improves the thermal stability of SCR.

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FINGER VOLTAGE:

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SNUBBER CIRCUITS:

A snubber circuit consists of a series combination of resistance R s and capacitance Cs

in parallel with the thyristor as shown in Fig. 4.25. Strictly speaking, a capacitor C s in parallel with the device is sufficient to prevent unwanted dv/dt triggering of the SCR. When switch S is closed, a sudden voltage appears across the circuit. Capacitor C s

behaves like a short circuit, therefore voltage across SCR is zero. With the passage of time, voltage across Cs builds up at a slow rate such that dv/dt across Cs and therefore across SCR is less than the specified maximum dv/dt rating of the device. Here the question arises that if Cs is enough to prevent accidental turn-on of the device by dv/dt, what is the need of putting Rs in series with Cs ? The answer to this is as under.

Before SCR is fired by gate pulse, Cs charges to full voltage Vs. When the SCR is turned on, capacitor discharges through the SCR and sends a current equal to V s / (resistance of local path formed by Cs and SCR). As this resistance is quite low, the turn-on di/dt will tend to be excessive and as a result, SCR may be destroyed. In order to limit the magnitude of discharge current, a resistance Rs is inserted in series with Cs

as shown in Fig. 4.25. Now when SCR is turned on, initial discharge current V s/Rs is relatively small and turn-on di/dt is reduced.

In actual practice ; Rs, Cs and the load circuit parameters should be such that dv/dt across Cs during its charging is less than the specified dv/dt rating of the SCR and discharge current at the turn-on of SCR is within reasonable limits. Normally, Rs Cs

and load circuit parameters form an underdamped circuit so that dv/dt is limited to acceptable values.

The design of snubber circuit parameters is quite complex.. In practice, designed snubber parameters are adjusted up or down in the final assembled power circuit so as to obtain a satisfactory performance of the power electronics system.

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OVERVOLTAGE PROTECTION

Thyristors are very sensitive to overvoltages just as other semi-conductor devices are. Overvoltage transients are perhaps the main cause of thyristor failure. Transient overvoltages cause either maloperation of the circuit by unwanted turn-on of a thyristor or permanent damage to the device due to reverse breakdown. A thyristor may be subjected to internal or external overvoltages ; the former is caused by the thyristor operation whereas the latter comes from the supply lines or the load circuit.

(i) Internal overvoltages. Large voltages may be generated internally during the commutation of a thyristor. After thyristor anode current reduces to zero, anode current reverses due to stored charges. This reverse recovery current rises to a peak value at which time the SCR begins to block. After.this peak, reverse recovery current decays abruptly with large di/dt. Because of the series inductance L of the SCR circuit, large transient voltage L di/dt is produced. As this internal overvoltage may be several times the breakover voltage of the device, the thyristor may be destroyed permanently.

{ii) External overvoltages. External overvoltages are caused due to the interruption of current flow in an inductive circuit and also due to lightning strokes on the lines feeding the thyristor systems. When a thyristor converter is fed through a transformer, voltage transients are likely to occur when the transformer primary is energised or de-energised. Such overvoltages may cause random turn on of a thyristor. As a result, the overvoltages may appear across the load causing the flow of large fault currents. Overvoltages may also damage the thyristor by an inverse breakdown. For reliable operation, the overvoltages must be suppressed by adopting suitable techniques.

Suppression of overvoltages. In order to keep the protective components to a minimum, thyristors are chosen with their peak voltage ratings of 2.5 to 3 times their normal peak working voltage. The effect of overvoltages is usually minimised, by using RC circuits and non-linear resistors called voltage clamping devices.

The RC circuit, called snubber circuit, is connected across the device to be protected, see Fig. 4.29. It provides a local path for internal overvoltages caused by reverse recovery current. Snubber circuit is also helpful in damping overvoltage transient spikes and for limiting dv/dt across the thyristor. The capacitor charges at a slow rate and thus the rate of rise of forward voltage (dv/dt) across SCR is also reduced. The resistance Rs damps out the ringing oscillations between the snubber circuit and the stray circuit inductance. Snubber circuits are also connected across transformer secondary terminals to suppress overvoltage transients caused by switching on or switching off of the primary winding. As snubber circuits provide only partial protection to SCR against transient overvoltages, thyristor protection against such overvoltages can be upgraded. This is done with the help of voltage-clamping devices.

The RC snubber is not enough for overvoltage protection of SCR. In practice, therefore, a combined protection consisting of RC snubber and voltage clamping arrangement is provided to thyristors.

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OVERCURRENT PROTECTION:

Thyristors have small thermal time constants. Therefore, if a thyristor is subjected to overcurrent due to faults, short circuits or surge currents ; its junction temperature may exceed the rated value and the device may be damaged. There is thus a need for the overcurrent protection of SCRs. As in other electrical systems, overcurrent protection in thyristor circuits is achieved through the use of circuit breakers and fast-acting fuses.

The type of protection used against overcurrent depends upon whether the supply system is weak or stiff. In a weak supply network, fault current is limited by the source impedance below the multi-cycle surge current rating of the thyristor. In machine tool and excavator drives, if the motor stalls due to overloads, the current is limited by the source and motor impedances. The filter inductance commonly employed in dc and ac drives may limit the rate of rise of fault current below the multicycle surge current rating of the thyristor. For all such systems, overcurrent can be interrupted by conventional fuses and circuit breakers. However, proper co-ordination is essential to guarantee that (i) fault current is interrupted before the thyristor is damaged and (ii) only faulty branches of the network are isolated.

Conventional protective methods are, however, inadequate in electrical stiff supply networks. In such systems, magnitude and rate of rise of current is not limited because source has negligible impedance. As such, fault current and therefore junction temperature rise within a few milliseconds. Special fast-acting current-limiting fuses are, therefore, required for the protection of thyristors in these stiff supply networks.

These fuses and thyristors are found to have similar thermal properties, The current-limiting fuse consists of one or more fine silver ribbons having very short fusing time.

Proper co-ordination between fast-acting current-limiting fuse and thyristor is essential. A fuse carries the thyristor current as both are placed in series. Therefore, the fuse must be rated to carry full-load current plus a marginal overload current for an indefinite period. But the peak let through current of fuse must be less than the subcycle surge current rating of the SCR. The voltage across the fuse during arcing period is known as arcing, or recovery, voltage. This voltage is equal to the sum of source voltage and the emf induced in the circuit inductance during arcing time ta. If the fuse current is interrupted abruptly, induced e.m.f. may be high; as a result arcing voltage would be excessive. It should therefore be ensured during fuse design and co-ordination that arcing voltage is limited to less than twice the peak supply voltage. In case voltage rating of the fuse is far in excess of circuit voltage, an abrupt current interruption would lead to dangerous overvoltages.

When both circuit breaker and fast-acting current-limiting fuse are used for overcurrent protection of SCR. The faulty circuit must be cleared before any damage is done to the device. A circuit breaker has long tripping time, it is therefore generally used for protecting the semiconductor device against the continuous overloads or against surge currents of long duration. A fast-acting C.L. fuse is used for protecting thyristors against large surge currents of very short duration. The tripping time of the circuit breaker, the fusing-time of the fast-acting fuse must be properly co-ordinated with the rating of a thyristor.

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ELECTRONIC CROWBAR PROTECTION:

As thyristor possesses high surge current capability, it can be used in an electronic crowbar circuit for overcurrent protection of power converters using SCRs. An electronic crowbar protection provides rapid isolation of the power converter before any damage occurs

Fig. 4.28 illustrates the basic principle of electronic crowbar protection. A crowbar thyristor is connected across the input dc terminals. A current sensing resistor detects the value of converter current. If it exceeds preset value, gate circuit provides the signal to crowbar SCR and turns it on in a few microseconds. The input terminals are then short-circuited by crowbar SCR and it shunts away the converter overcurrent. The crowbar thyristor current depends upon the source voltage and its impedance. After some time, main fuse interrupts the fault current. The fuse may be replaced by a circuit breaker if SCR has adequate surge current rating.

GATE PROTECTION:

Gate circuit should also be protected against overvoltages and over currents. Overvoltages across the gate circuit can cause false triggering of the SCR. Overcurrent may raise junction temperature beyond specified limit leading to its damage. Protection against over-voltages is achieved by connecting a zener diode across the gate circuit. A resistor R2 connected in series with the gate circuit provides protection against overcurrents.

A common problem in thyristor circuits is that they suffer from spurious, or noise, firing. Turning-on or turning-off of an SCR may induce trigger pulses in a nearby SCR. Sometimes transients in a power circuit may also cause unwanted signal to appear across the gate of a neighbouring SCR. These undesirable trigger pulses may turn on the SCR leading to false operation of the main SCR. Gate protection against such spurious firing is obtained by using shielded cables or twisted gate leads. A varying flux caused by nearby transients cannot pass through twisted gate leads or shielded cables. As such no e.m.f. is induced in these cables and spurious firing of thyristors is thus minimised. A capacitor and a resistor are also connected across gate

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to cathode to bypass the noise signals, as shown in the figure below. The capacitor should be less than 0.1 µF and must not deteriorate the waveshape of the gate pulse.

TWO TRANSISTOR MODEL FOR A THYRISTOR

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The principle of thyristor operation can be explained with the use of its two-transistor model (or two-transistor analogy). Fig.4.15 below shows schematic diagram of a thyristor. From this figure, two-transistor model is obtained by bisecting the two middle layers, along the dotted line, in two separate halves as shown in the figure. In this figure, junctions J1 – J2 and J2 -J3 can be considered to constitute pnp and npn transistors separately. The circuit representation of the two-transistor model of a thyristor is shown in figure (c).

In the off-state of a transistor, collector current Ic is related to emitter current IE as

IC = αIE + ICBO

where α is the common-base current gain and ICB0 is the common-base leakage current of collector-base junction of a transistor.

For transistor Q1 in Fig. 4.15 (c), emitter current IE = anode current Ia and IC = collector current IC1. Therefore, for Q1

IC1 = α1 Ia +  ICBO1 ……..(4.3)

where      α1 = common-base current gain of Q1

and          ICBO1  = common-base leakage current of Q1

Similarly, for transistor Q2, the collector current IC2 is given by

IC2 = α2 Ik +  ICBO2 …(4.4)

where      α2 – common-base current gain of Q2,ICBO2 =common-base leakage current of Q2 and

Ik = emitter current of Q2.

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The sum of two collector currents given by Eqs. (4.3) and (4.4) is equal to the external circuit current   Iα entering at anode terminal A.

There fore   Ia = IC1 + IC2

Ia = α1 Ia + ICBO1+ α2 Ik +  ICBO2 …(4.5)

When gate current is applied, then Ik = Ia + Ig . Substituting this value of Ik in Eq. (4.5) gives

Ia = α1 Ia + ICBO1+ α2 (Ia + Ig ) +  ICBO2

or

Ia = α2 Ig + ICBO1 + ICBO2 /[1-( α1+ α2)]

For a silicon transistor, current gain α is very low at low emitter current. With an increase in emitter current, a builds up rapidly as shown in Fig. 4.16. With gate current Ig = 0 and with thyristor forward biased,( α1+ α2)is very low as per Eq (4.6) and forward leakage current somewhat more than ICBO1 + ICBO2 flows. If, by some means, the emitter current of two component transistors can be increased so that α1+ α2

approaches unity, then as per Eq. (4.6) Ia would tend to become infinity thereby turning-on the device. Actually, external load limits the anode current to a safe value after the thyristor begins conduction. The methods of turning-on a thyristor, in fact, are the methods of making α1+ α2 to approach unity. These 0.25 various mechanisms for turning-on a thyristor are now discussed below :

(i)         GATE Triggering : With anode positive with respect to cathode and with gate current Ig = 0, Eq. (4.6) shows that anode current, equal to the forward leakage current, is somewhat more than  ICBO1 + ICBO2,Under these conditions, the device is in the forward blocking state.

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Now a sufficient gate-drive current between gate and cathode of the transistor is applied. This gate-drive current is equal to base current IB2 = Ig and emitter current Ik of transistor Q2. With the establishment of emitter current Ik of Q2, current gain α2 of Q2

increases and base current IB2 causes the existence of collector current IC2 = β2IB2 = β2

Ig. This amplified current IC2 serves as the base current IB1 of transistor Q1 With the flow of IB1 collector current IC1 = β1 IB1 = β1 β2 Ig of Q1 comes into existence. Currents IB1 and IC1 lead to the establishment of emitter current Ia of Q1 and this causes current gain α1 to rise as desired. Now current Ig + ICI = (1 + β1 β2) Ig acts as the base current of Q2 and therefore its emitter current Ik = ICI + Ig With the rise in emitter current Ik α2

of Q2 increases and this further causes IC2 = P2 (1 + β1 β2) Ig to rise. As amplified collector current IC2 is equal to the base current of Q1 current gain α1 eventually rises further. There is thus established a regenerative action internal to the device. This regenerative or positive feedback effect causes α1+ α2 to grow towards unity. As a consequence, anode current begins to grow towards a larger value limited only by load impedance external to the device. When regeneration has grown sufficiently, gate current can be withdrawn. Even after Ig is removed, regeneration continues. This characteristic of the thyristor makes it suitable for pulse triggering. Note that thyristor is a latching device

After thyristor is turned on, all the four layers are filled with carriers and all junctions are forward biased. Under these conditions, thyristor has very low impedance and is in the forward on-state.

(ii) Forward-voltage triggering : If the forward anode to cathode voltage is increased, the collector to emitter voltages of both the transistors are also increased. As a result, the leakage current at the middle junction J2 of thyristor increases, which is also the collector current of Q2 as well as Q1 With increase in collector currents IC1

and IC2 due to avalanche effect, the emitter currents of the two transistors also increase causing α1+ α2 to approach unity. This leads to switching action of the device due to regenerative action. The forward-voltage triggering for turning-on a thyristor may be destructive and should therefore be avoided.

(iii) dv/dt triggering : The reversed biased junction J2 behaves like a capacitor because of the space-charge present there. Let the capacitance of this junction be Cj. For any capacitor, i = C dv/dt.In case it is assumed that entire forward voltage va

appears across reverse biased junction J2 then charging current across the junction is given by

i = Cj dva /dt

This charging or displacement current across junction J2 is collector currents of Q2 and Q1 Currents IC2, IC1 will induce emitter current in Q2, Q1 In case rate of rise of anode voltage is large, the emitter currents will be large and as a result, α1+ α2 will approach unity leading to eventual switching action of the thyristor.

(iid Temperature triggering : At high temperature, the forward leakage current across junction J2 rises. This leakage current serves as the collector junction current of the component transistors Q1 and Q2. Therefore, an increase in leakage current ICI, IC2

leads to an increase in the emitter currents of Ql Q2. As a result, (α1+ α2) approaches unity. Consequently, switching action of thyristor takes place.

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(v) Light triggering : When light is thrown on silicon, the electron-hole pairs increase. In the forward-biased thyristor, leakage current across J2 increases which eventually increases α1+ α2 to unity as explained before and switching action of thyristor occurs.

As stated before, gate-triggering is the most common method for turning-on a thyristor. Light-triggered thyristors are used in HVDC applications.

The operational differences between thyristor-family and transistor family of devices may now be summarised as under :

i) Once a thyristor is turned on by a gate signal, it remains latched in on-state due to internal regenerative action. However, a transistor must be given a continuous base signal to remain in on-state.

ii) In order to turn-off a thyristor, a reverse voltage must be applied across its anode-cathode terminals. However, a transistor turns off when its base signal is removed.

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THYRISTOR PROTECTION: Reliable operation of a thyristor demands that its specified ratings are not exceeded. In practice, a thyristor may be subjected to overvoltages or overcurrents. During SCR turn-on, di/dt may be prohibitively large. There may be false triggering of SCR by high value of dv/dt. A spurious signal across gate-cathode terminals may lead to unwanted turn-on. A thyristor must be protected against all such abnormal conditions for satisfactory and reliable operation of SCR circuit and the equipment. SCRs are very delicate devices, their protection against abnormal operating conditions is, therefore, essential. The object of this section is to discuss various techniques adopted for the protection of SCRs.

(a) di/dt protection. When a thyristor is forward biased and is turned on by a gate pulse, conduction of anode current begins in the immediate neighbourhood of the gate-cathode junction, Fig. 4.6 (a). Thereafter, the current spreads across the whole area of junction. The thyristor design permits the spread of conduction to the whole junction area as rapidly as possible. However, if the rate of rise of anode current, i.e. di/dt, is large as compared to the spread velocity of carriers, local hot spots will be formed near the gate connection on account of high current density. This localised heating may destroy the thyristor. Therefore, the rate of rise of anode current at the time of turn-on must be kept below the specified limiting value. The value of di/dt can be maintained below acceptable limit by using a small inductor, called di/dt inductor, in series with the anode circuit. Typical di/dt limit values of SCRs are 20-500 A/µ sec. Local spot heating can also be avoided by ensuring that the conduction spreads to the whole area as rapidly as possible. This can be achieved by applying a gate current nearer to (but never greater than) the maximum specified gate current.

dv/dt protection. With forward voltage across the anode and cathode of a thyristor, the two outer junctions are forward biased but the inner junction is reverse biased. This reverse biased junction J2, Fig. 4.3 (b), has the characteristics of a capacitor due to charges existing across the junction. In other words, space-charges exist in the depletion region around junction J2 and therefore junction J2 behaves like a capacitance. If the entire anode to cathode forward voltage Va appears across J2 junction and the charge is denoted by Q, then a charging current i given by Eq. (4.6) flows

i = dQ/dt =d(Cj Va )/dt

=Cj (d Va /dt) + Va(d Cj /dt) …………..(4.6 a)

As Cj the capacitance of junction J2 is almost constant, the current is given by

i = Cj (d Va /dt) …………..(4.6 b)

If the rate of rise of forward voltage dVa/dt is high, the charging current i will be more.This charging current plays the role of gate current and turns on the SCR even when gate signal is zero. Such phenomena of turning-on a thyristor, called dv/dt turn-on must be avoided as it leads to false operation of the thyristor circuit. For controllable operation of the thyristor, the rate of rise of forward anode to cathode voltage dVa/dt must be kept below the specified rated limit. Typical values of dv/dt are 20 – 500 V/µsec. False turn-on of a thyristor by large dv/dt can be prevented by using a snubber circuit in parallel with the device.

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INDUCTION HEATING OF CONDUCTING MATERIALS

Induction heating is a non-contact heating process. It uses high frequency electricity to heat materials that are electrically conductive. Since it is non-contact, the heating process does not contaminate the material being heated. It is also very efficient since the heat is actually generated inside the work piece. This can be contrasted with other heating methods where heat is generated in a flame or heating element, which is then applied to the work piece. For these reasons Induction Heating lends itself to some unique applications in industry. A source of high frequency electricity is used to drive a large alternating current through a coil. This coil is known as the work coil.

The passage of current through this coil generates a very intense and rapidly changing magnetic field in the space within the work coil. The work piece to be heated is placed within this intense alternating magnetic field.

Depending on the nature of the work piece material, a number of things happen...

The alternating magnetic field induces a current flow in the conductive work piece. The arrangement of the work coil and the work piece can be thought of as an electrical transformer. The work coil is like the primary where electrical energy is fed in, and the work piece is like a single turn secondary that is short-circuited. This causes tremendous currents to flow through the work piece. These are known as eddy currents.

In addition to this, the high frequency used in induction heating applications gives rise to a phenomenon called skin effect. This skin effect forces the alternating current to flow in a thin layer towards the surface of the work piece. The skin effect increases the effective resistance of the metal to the passage of the large current. Therefore it greatly increases the heating effect caused by the current induced in the work piece.

(Although the heating due to eddy currents is desirable in this application, it is interesting to note that transformer manufacturers go to great lengths to avoid this phenomenon in their transformers. Laminated transformer cores, powdered iron cores and ferrites are all used to prevent eddy currents from flowing inside transformer cores. Inside a transformer the passage of eddy currents is highly undesirable because it causes heating of the magnetic core and represents power that is wasted.)

For ferrous metals like iron and some types of steel, there is an additional heating mechanism that takes place at the same time as the eddy currents mentioned above. The intense alternating magnetic field inside the work coil repeatedly magnetises and de-magnetises the iron crystals. This rapid flipping of the magnetic domains causes considerable friction and heating inside the material. Heating due to this mechanism is known as Hysteresis loss, and is greatest for materials that have a large area inside their B-H curve. This can be a large contributing factor to the heat generated during induction heating,

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but only takes place inside ferrous materials. For this reason ferrous materials lend themselves more easily to heating by induction than non-ferrous materials.

It is interesting to note that steel looses its magnetic properties when heated above approximately 700°C. This temperature is known as the Curie temperature. This means that above 700°C there can be no heating of the material due to hysteresis losses. Any further heating of the material must be due to induced eddy currents alone. This makes heating steel above 700°C more of a challenge for the induction heating systems. The fact that copper and Aluminium are both non-magnetic and very good electrical conductors, can also make these materials a challenge to heat efficiently. (We will see that the best course of action for these materials is to up the frequency to exaggerate losses due to the skin effect.)

In order to achieve uniform heating of conducting materials, electrical induction heating is a very effective method as compared to heating of such work peaces by direct heat or fuel fired furnaces. This method is faster and cleaner and also economical.

The frequency to be used for induction heating is determined by the size of the work piece and the depth of heating required, rather than the type of conducting material. Eddy currents, hysterisis and skin effect principles have to be borne in mind when deciding the power required as well as the frequency for induction heating.

For heating small work jobs, or for heat treatment for hardening the surface of the material, higher frequencies are used to produce greater “skin effect”. Due to the skin effect the induced currents in the work piece heat only about 5% of the outer surface of the work job above 1500 degrees F. The internal metal below is not greatly heated and hence it remains softer and tougher, thus providing the required strength. Much of the heat quickly gets transferred to the inner portion and also some heat is radiated. Hence, it is important that the surface is brought to a high temperature quickly in few seconds and quench or cool it before the heat has traveled to the inside of the work piece or is lost due to radiation. Since the time involved in the procedure is minimal, the cost is very much reduced. Also, with this method, the main part of the work job maintains its shape, thereby straightening of the piece is not necessary, which would have been required if the whole piece was heated.

The frequency to be used does not depend much on the type of conducting material, but depends more on the size and shape of the material, and also on the depth of heat penetration that is required. At high frequencies, high temperature is obtained quickly with lesser amount of energy input. The frequency used can vary between 10 KHz to 100 KHz from thicker rods to thinner rods for surface hardening. The depth of penetration of the heat decreases in proportion to the square root of the frequency approximately whereas the amount of power increases in proportion to the square root of the frequency. About 5 to 50 KW/square inch (of surface) input power is required for most metal hardening.

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Induction heating can be used for any application where we want to heat an electrically conductive material in a clean, efficient and controlled manner.

One of the most common applications is for sealing the anti-tamper seals that are stuck to the top of medicine and drinks bottles. A foil seal coated with "hot-melt glue" is inserted into the plastic cap and screwed onto the top of each bottle during manufacture. These foil seals are then rapidly heated as the bottles pass under an induction heater on the production line. The heat generated melts the glue and seals the foil onto the top of the bottle. When the cap is removed, the foil remains providing an airtight seal and preventing any tampering or contamination of the bottle's contents until the customer pierces the foil.

Another common application is "getter firing" to remove contamination from evacuated tubes such as TV picture tubes, vacuum tubes, and various gas discharge lamps. A ring of conductive material called a "getter" is placed inside the evacuated glass vessel. Since induction heating is a non-contact process it can be used to heat the getter that is already sealed inside a vessel. An induction work coil is located close to the getter on the outside of the vacuum tube and the AC source is turned on. Within seconds of starting the induction heater, the getter is heated white hot, and chemicals in its coating react with any gasses in the vacuum. The result is that the getter absorbs any last remaining traces of gas inside the vacuum tube and increases the purity of the vacuum.

Yet another common application for induction heating is a process called Zone purification used in the semiconductor manufacturing industry. This is a process in which silicon is purified by means of a moving zone of molten material. An Internet Search is sure to turn up more details on this process that I know little about.

Other applications include melting, welding and brazing or metals. Induction cooking hobs and rice cookers. Metal hardening of ammunition, gear teeth, saw blades and drive shafts, etc are also common applications because the induction process heats the surface of the metal very rapidly. Therefore it can be used for surface hardening, and hardening of localised areas of metallic parts by "outrunning" the thermal conduction of heat deeper into the part or to surrounding areas. The non contact nature of induction heating also means that it can be used to heat materials in analytical applications without risk of contaminating the specimen. Similiarly, metal medical instruments may be sterilised by heating them to high temperatures whilst they are still sealed inside a known sterile environment, in order to kill germs.

In theory only 3 things are essential to implement induction heating:

1. A source of High Frequency electrical power, 2. A work coil to generate the alternating magnetic field, 3. An electrically conductive work piece to be heated,

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In practice the work coil is usually incorporated into a resonant tank circuit. This has a number of advantages. Firstly, it makes either the current or the voltage waveform become sinusoidal. This minimises losses in the inverter by allowing it to benefit from either zero-voltage-switching or zero-current-switching depending on the exact arrangement chosen. The sinusoidal waveform at the work coil also represents a more pure signal and causes less Radio Frequency Interference to nearby equipment. This later point becoming very important in high-powered systems.

The system schematic above shows the simplest inverter driving its LCLR work coil arrangement.

Note that this schematic DOES NOT SHOW the MOSFET gate-drive circuitry and control electronics!

The inverter in this demonstration prototype was a simple half-bridge consisting of two MTW14N50 MOSFETs made my On-semiconductor (formerly Motorola.) It is fed from a smoothed DC supply with decoupling capacitor across the rails to support the AC current demands of the inverter. However, it should be realised that the quality and regulation of the power supply for induction heating applications is not critical. Full-wave rectified (but un-smoothed) mains can work as well as smoothed and regulated DC when it comes to heating metal, but peak currents are higher for the same average heating power. There are many arguments for keeping the size of the DC bus capacitor down to a minimum. In particular it improves the power factor of current drawn from the mains supply via a rectifier, and it also minimises stored energy in case of fault conditions within the inverter.

The DC-blocking capacitor is used merely to stop the DC output from the half-bridge inverter from causing current flow through the work coil. It is sized sufficiently large that it does not take part in the impedance matching, and does not adversely effect the operation of the LCLR work coil arrangement.

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In high power designs it is common to use a full-bridge (H-bridge) of 4 or more switching devices. In such designs the matching inductance is usually split equally between the two bridge legs so that the drive voltage waveforms are balanced with respect to ground. The DC-blocking capacitor can also be eliminated if current mode control is used to ensure that no net DC flows between the bridge legs. (If both legs of the H-bridge can be controlled independently then there is scope for controlling power throughput using phase-shift control.

At still higher powers it is possible to use several separate inverters effectively connected in parallel to meet the high load-current demands. However, the

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separate inverters are not directly tied in parallel at the output terminals of their H-bridges. Each of the distributed inverters is connected to the remote work coil via its own pair of matching inductors which ensure that the total load is spread evenly among all of the inverters.

These matching inductors also provide a number of additional benefits when inverters are paralleled in this way. Firstly, the impedance BETWEEN any two inverter outputs is equal to twice the value of the matching inductance. This inductive impedance limits the "shoot between" current that flows between paralleled inverters if their switching instants are not perfectly synchronised. Secondly, this same inductive reactance between inverters limits the rate at which fault current rises if one of the inverters exhibits a device failure, potentially eliminating failure of further devices. Finally, since all distributed inverters are already connected via inductors, any additional inductance between the inverters merely adds to this impedance and only has the effect of slightly degrading current sharing. Therefore the distributed inverters for induction heating need not necessarily be located physically close to each other. If isolation transformers are included in the designs then they need not even run from the same supply!

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QUESTIONS:

1) State and explain the principle involved in induction heating procedure.

2) What are the advantages of induction heating?

3) List applications of induction heating.

4) Why is induction heating more preferable than conventional furnace heating methods?

5) Can induction heating be used for non conductive materials like plastic? Justify your answer.

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DIELECTRIC HEATING OF NONCONDUCTING MATERIALS

A non-conducting material generates heat when subjected to an alternating electric field. Dielectric heating is the result of dielectric loss in the material to be heated. The heat produced will depend on the dielectric strength of the material to be heated. This method of heating is widely used in wood and plastic industry. For heating and gluing / joining / bonding multiple layers of plywood, this is a very effective method, since the heat that is generated is uniform throughout the material, providing certain conditions are adhered to.

The other areas where this method is used are textile, rubber, food and chemical industries. The job to be heated is place between two electrodes which are connected to a very high frequency supply. The basic arrangement is as shown in the diagram. In very simple terms a capacitance is formed between the two electrodes and the job or work piece, where the two electrodes are as good as two plates of a capacitor and job is the dielectric material between the two electrodes. The current flowing in the circuit is given by Ic = E / Xc, where Ic is the current flowing through the capacitor in amperes, E is the magnitude of the high frequency voltage applied to the two plates and Xc is the reactance of the capacitance in also. Also we know that Xc = 1 / 2¶ √LC.

Because of the charging and discharging of the capacitance, the molecular arrangement in the job changes due to the continuous stress caused by the alternating electric field. This causes heat to be produced.

Thus dielectric heating is based on the principle of converting high frequency electric energy into heat energy. The block diagram shown represents the basic principle of operation. An oscillator is used to provide the high frequency. Generally radio frequency oscillators are used in this process. The amount of heat generated depends upon and is directly proportional to the following:

1) frequency2) capacity of the job3) square of the supply voltage4) power factor of t he load5) area of the electrode plates, and

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6) dielectric constant of the job or work piece.

Note that the heat produced is inversely proportional to the distance between the two plates.

The frequency to be used will depend upon the size / dimensions of the job and also on the high frequency electrical power output. For lesser power outputs, of 100 watts to 1000 watts frequencies in the range of 200 MHz maybe used, whereas, as the power increases to about 40 – 50 K-watts, the frequency used is less at about 30 MHz and yet, for still higher electrical outputs in the range of 200 K-watts, the frequency used may be in the range of 10 -15 MHz. The dielectric constants for most materials generally range between 2 and 17, with a few exceptions like, in case of gases it can be as low as 0.5 and for ceramics it can be as high as 1000 or more.

For uniform heating of the material there are two important and absolutely essential conditions, namely, the size of the electrodes that are used as plates of the capacitor should be larger or greater than the size or dimensions of the job, and, the length of the electrodes or plates should be greater than the distance between the two plates. Also note that the maximum value of the voltage applied to the electrodes should not be more than 15,000 volts so as to ensure that arcing or corona effects do not takes place between the electrodes or plates. Yet one more consideration is that there should not be any air gap between the plates and the job, which should be contact with both the plates so that series capacitance due to air is not introduced.

One important point to be noted is that, any other method of heating of non-conductive materials will not produce uniform heating or rise in temperature and hence will take longer duration to heat.

Specific applications of dielectric heating are:

1) Drying and heat treatment of textile goods such as rayon, nylon, terylene, etc.

2) Processing of chemicals during manufacturing process.3) Gluing, drying and curing of wood. The glue between the wood

layers can be dried using dielectric heating in the manufacture of plywood.

4) Preheating and curing of plastics. A “perform” of a biscuit sized material is placed in a dielectric heater for a minute and then it is passed on to a press that moulds it in the required shape. Approximately 2000 watts of power will heat the plastic perform of 1 pound to about 300 degrees F. in just one minute.

5) Processing rubber and other synthetic materials.6) Processing and manufacture of semiconductor devices.7) Sterilization of food and medical supplies.8) Curing of resin based adhesives and sand cores in foundries.9) Sewing of seam in plastic materials. (Plastic sheets are joined

together using combination of heat and pressure.)

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10) A V.H.F. band oscillator can b used in the medical field for heating the human body, which produces “artificial fever” when desired for medical treatment. When the V.H.F. electricity flows in wires or electrodes several inches away from the body, heat is produced inside the blood vessels.

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VI. EQUIPMENTS/APPARATUS REQUIRED

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