Thyristors and Other DevicesRev1

8/11/2019 Thyristors and Other DevicesRev1

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THYRISTORS & OTHER

DEVICES


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Thyristors are a class of semiconductor devices characterized by 4-

layers of alternating p- and n-material. Four-layer devices acts aeither open or closed switches, for this reason, they are most

frequently used in control applications

Some thyristors and their symbols are:

Introduction

Shockley diode

silicon-controlled rectifier


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Introduction continue

They stay on once they are triggered, and will go off only

if current is too low or when triggered off.

Applications: lamp dimmers, motor speed controls,

ignition systems, charging circuits, etc.

The 4-layer diode (or Shockley diode) is a type of

thyristor thatacts something like an ordinary diode but

conducts in the forward direction only after a certain

anode to cathode voltage called the forward-

breakover voltage is reached..

It does not conduct when it is reverse-biased.


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Advantages and disadvantages

Advantages Disadvantages

Thyristors have low on-state

conduction losses

Higher power handling capabilitythan transistor

High blocking voltage rating

Worse switching performances than

transistors.

Thyristor heating due to currentflowing through them represents

additional power losses in the

distribution system

The thyristor system may also have a

higher equipment cost


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The Basic Four Layer DeviceThe 4-layer diode (or Shockley diode) is a type of thyristor that acts

something like an ordinary diode but conducts in the forward direction onlyafter a certain anode to cathode voltage called the forward-breakover voltage is

reached.

The 4-layer diodehas two leads, labeled the anode (A) and the cathode

(K). The symbol reminds you that it acts like a diode. It does not conduct

when it is reverse-biased.

Equivalent circuit


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The Basic Four Layer Device

It will not conduct when

reverse biased and will not

conduct when forward biased

until the forward breakover

voltage (VBRF)is reached. Itwill continue to conduct as long

as the holding current(IH) is

maintained.

Forward-

conduction

region

Forward-blocking

region

Normal diode

characteristics

When FB, current is

exponential function

of voltage


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Equivalent Circuits(4 Layers diode)

The concept of 4-layer devices is

usually shown as an equivalent

circuit of a pnp and an npn

transistor. Ideally, these devices

would not conduct, but whenforward biased, if there is sufficient

leakage current in the upper pnp

device, it can acts as base current

to the lower npndevice causing itto conduct and bringing both

transistors into saturation.


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Silicon-Controlled Rectifier (SCR)

SCR is another four-layerpnpn device.

Has 3 terminals: anode, cathode, and gate.

In off state, it has a very high resistance.

In on state, there is a small on (forward)

resistance.

Applications: motor controls, time-delay

circuits, heater controls, phase controls, etc.


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The Silicon-Controlled Rectifier

The silicon-controlled rectifier (SCR) is a four layer device with threeterminals, the anode, cathode, and gate.

Equivalent Circuit


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Silicon-Controlled Rectifier

(SCR)The SCR had its roots in the 4-layer diode. By adding a gate connection, the

SCR could be triggered into conduction. This improvement made a much more

useful device than the 4-layer diode.

The SCR can be turned on by exceeding the forward breakover voltage or by

gate current. Notice that the gate current controls the amount of forwardbreakover voltage required for turning it on.


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Once an SCR is switched on, it

remains latched on, even when

the gate signal is removed.

Holding current (IH) is the

minimum required current fromanode to cathode

Reverse breakdown voltage is the

maximum reverse bias voltage for

the SCR

Operation

To switch on an SCR:

Forward bias the

anode-cathode

terminals (VF)AND

Apply sufficient gate

voltage (Vgate) and

gate current (IGT)

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The Silicon-Controlled Rectifier


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Another applications

Another application for

SCRs is in crowbar circuits

The purpose of a crowbar

circuit is to shut down apower supply in case of

over-voltage.

Once triggered, the SCR

latches on.

The SCR can handle a large

current, which causes the

fuse (or circuit breaker) to

open.


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SCSSilicon-Controlled Switch

An SCS is like an SCR, except that it has

two gates: a cathode gate and an anodegate.Operation

Either gate can fire the SCS

A positive pulse or voltage on the

cathode gate A negative pulse or voltage on the anode

gate

Either gate can switch off the SCS

A negative pulse or voltage on the

Cathode gate A positive pulse or voltage on the anode

gate

Note: The anode gate requires higher voltages than the cathode

gate.

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The Diac and Triac

The diacand triacunlike the SCR will conduct in both directions making it

ideal for ac applications. The diac turns on when the breakover voltage is

reached in either direction.


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The Diac and Triac

The triac is basically a diac with a gate terminal. The triac canbe turned by a pulse at the gate.


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The Diac and Triac

In this basic triac phase

control circuit R1

controls the trigger

point at which the triac

turns on for each half of

the cycle. The off timeis called the delay

angle and the on time

is called the conduction

angle.


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Unijunction Transistor (UJT)


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The Unijunction Transistor

(UJT)The UJT has one pn junction and is used mainly as a triggering devicein thyristor circuits and can also be used in oscillator circuits. Thesymbol is similar to a JFET. Note the angle of the emtter.The other

terminals are called base 1 and base 2. The characteristics are quite

different than any other transistor.


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The Unijunction Transistor

(UJT)The resistive equivalent circuit of a UJT shown makes it easier tounderstand its operation. The emitter current controls the value of rB1inversely. The total resistance or interbase resistance (rBB) equals the sum

of rB1and rB2. The standoff ratio() is the ratio rB1/rBB.


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The Programmable UnijunctionTransistor (PUT)

Although it has the same name as a UJT the programmable unijunction

transistor

sstructure is not the same. It is actually more similar to an

SCR. The anode to gate voltage is used to turn it off and on.


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The Programmable UnijunctionTransistor (PUT)

The PUTcan be programmedto turn on at a certain voltage by an

external voltage divider. This yields a curve similar to a UJT therefore it

can used in oscillator circuits like the UJT.

Thyristors and Other DevicesRev1

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