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Overview of Power Electronic Devices ET502/Unit 1

UNIT 1 OVERVIEW of POWER ELECTRONIC DEVICES

OBJECTIVES

General Objective: Explain the operation and use of basic electronic devices

Specific Objectives: At the end of this chapter, you should be able to

1.7 Study other types of semiconductor devices:

1.7.1 Draw the symbols of Bidirectional diode or DIAC, Zener diode, Photo diode,

Photo transistor, Opto-coupler, Photo thyristor, Programmable Unijunction

Transistor (UJTs) and Operational Amplifier (Op-Amp)

1.7.2 Identify the terminals of devices in 1.7.1

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INPUT

1.0 Introduction

1. As previously discussed in particular order, the thyristor family is the manufacturers’

ultimate choice concerning electronic switches. They are a perfect switch for AC regulators,

lighting and heating applications, DC motor drives, large power supplies and electronic

circuits. In addition, the availability of these devices in high voltage and current ratings are

abundant.

2. Apart from that, the FET family is seconds away from being the top manufacturers’ choice

as their applications are mainly used for low power application such as in AC and DC low

power drives, inverters and choppers, microprocessors, memories, calculators and Logic

Gates.

3. But few others electronic components also made the list such as the Bidirectional diode or

DIAC, Programmable Unijunction Transistor (PUT), Opto-coupler, Zener diode, the

Photo family - Photo transistor, Photo thyristor, Photo diode; and Operational Amplifier

(Op-Amp).

4. In this final chapter, the discussions are merely based on their building structure, symbol,

static characteristics and utilization.

~ Colleges are places where pebbles are polished and diamonds are dimmed. ~ Robert G. Ingersoll

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2.0 Bidirectional diode (DIAC)

1. Bidirectional diode is a diode for the AC application, hence the name DIAC. Through the

material construction Figure 1, you can see that this semiconductor falls in the

thyristor family1. But the gate terminal is missing. This will become clearer after you

have gone through the rest of the chapter.

2. It is a trigger diode that conducts current only after its

breakdown voltage has been exceeded momentarily. DIAC is called

symmetrical trigger diodes due to the symmetry of its

characteristic curve.

3. The operation of the DIAC can be easily understood by referring

to Figure 2. As T1 is made positive with respect to T2, the layers p-n-p-n starts conducting.

This will only happens when voltage of T1 is more than break over voltage VB01.

4. And once the conduction starts, the current through the DIAC becomes very large and has

to be limited by an external resistance in the circuit. But, when T2 is made positive with

respect to T1 the layers p-n-p-n’ conducts (refer to Figure 1).

5. And as in T1 operation, this will only happens when the voltage of T2 exceeds break over

voltage VBO2.

6. In both the cases the current during blocking regions are small leakage currents. The

behaviour in both the directions are is similar because doping level is same in all the layers in

two directions. The break over voltage for commonly used diac is about 30V.

Life is about trusting your feelings and taking chances, losing and finding

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1Arjun. (2010). Diac (Bidirectional Thyristor diode). Reterieved on 26th November 2010 from

http://electricalandelectronics.org/2008/10/19/diacbidirectional-thyristor-diode/

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Figure 1: The DIAC material construction (left) and symbol (right)1

Figure 2: DIAC V-I static characteristic1

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7. On the other hand, when T1 is positive and voltage is less

than VBO1 only a small leakage current flows through the device.

But when that voltage exceeds VBO1, it will start to conduct and

current becomes large1.

8. And as the current increases, the voltage drop across DIAC

decreases. Thus it exhibit negative resistance characteristics. The characteristic in the

reverse direction (when T2 is positive) lie in the third quadrant and is exactly similar to that

in the first quadrant. The break over voltage VBO1 and VBO2 are exactly equal in magnitude. In

both cases, the device exhibits negative resistance behaviour during conduction region.

9. The DIAC like was mention before has no gate electrode, unlike some other thyristors that

are commonly used to trigger, such as TRIACs or SCRs. Nonetheless,

its application is still important.

10. This DIAC is mainly used for triggering TRIAC, because the TRIAC

requires either positive or negative gate pulse for turning it on. And

some TRIACs contain a built-in DIAC in series with their Gate terminal

for this purpose. The example of the triggering circuit can be

explained below2:

Figure 3: DIAC as a triggering device to TRIAC

2 Retrieved on 25th November 2010 from http://www.cavehill.uwi.edu/fpas/cmp/online/el10c/gibbs/Thyristors.htm

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11. The DIAC is used in conjunction with the Triac to provide a full-wave control of AC signals.

These devices are commonly used together to control the speed and direction of electric

motors or control the temperature of heating elements.

12. They are used in air conditioning, heating, and ventilation systems

and they can even be used in electronic garage-door systems.

They may also be used to simply switch or control the amount of

power applied to individual lamps or complete lighting systems.

13. For example, they can be used to flash a lamp on and off as a

warning signal, to control traffic-signal lights which are

placed at roadway intersections to regulate traffic flow or

provide light dimming in theaters. DIAC-TRIAC circuits can

also be used to control power in three-phase systems by

using multiple TRIAC circuit arrangements.

14. The above mentioned applications are only a few of the

many ways in which DIAC-TRIAC circuits can be used to

control power. A complete list of applications would not be

practical or even possible at this time since new applications

are constantly being found for these relatively versatile

solid-state devices. Recent improvements in their design and

construction have greatly extended their power handling

abilities and increased their potential applications.

15. Although the DIAC is widely used to trigger the TRIAC in power control applications, it is

important to realize that other triggering devices are also available. Most of these

triggering devices have bidirectional switching characteristics that are closely related to

those of a DIAC although each device is constructed differently and each is represented by

a different schematic symbol. The DIAC just described serves as a typical example of a

modern solid-state triggering device.

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3.0 Programmable Unijunction Transistor (PUT)

1. The programmable unijunction transistor or simply PUT is also a family member to the

thyristor. As mention more than twice before, to be in the thyristor family the device’s

construction should consists of four layers of p-n. And unlike the DIAC,

PUT has both the anode and cathode and also the gate. And by referring

to the Figure 43 the gate seems to be attached to the N-type in

contrast to the SCR. This will become clearer as you move on to the rest

of the chapter.

2. Contrary to DIAC, PUT is most likely be made into a triggering device

for SCR. The SCR conduction as explained before can either be from a

forward breakdown voltage or its gate was given a short pulse. Through

the construction of the PUT triggering circuit or in the world of Electronic Engineering

called it Relaxation Oscillator, you can figure out why is PUT compatible as the SCR

triggering device and not to a TRIAC.

Figure 4: PUT material construction, symbol and V-I characteristic

3 Kuphaldt, T.R. Thyristors – Silicone controlled Switch (SCS). Electricly Free Electric and Electronic Tutorial. 2010.

Retrieved on 7th Dec 2010 from http://electricly.com/tag/transistor/page/2/

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3. The concept of applying the PUT as the Relaxation

oscillator is practically the same as applying a 555 timers, op-

amp oscillator or Transistor Transistor Logic (TTL) oscillator

as a triggering device, it’s like having a Honda or a Toyota as

your car-does not make your car having a difference sets of

tyres. Probably in terms of the engine capacity or interior is

differ4.

4. A relaxation oscillator is a circuit that repeatedly alternates between two states at with a

period that depends on the charging of a capacitor. The capacitor voltage may change

exponentially when charged or discharged through a resistor from a constant voltage, or

linearly when charged or discharged through a constant current source3.

5. With exponential charging, the timing is expressed in terms of time constants RC. In a basic

circuit, the capacitor will charge with time constant τ1 from a starting voltage V1 towards a

supply voltage Vu for a time t1. When it reaches some voltage V2, it will then discharge with

time constant τ2 towards a lower voltage Vd for a time t2, when the cycle will repeat.

Figure 5: PUT triggering circuit

4 Retrieved on 7th Dec 2010 from http://mysite.du.edu/~etuttle/electron/elect10.htm

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6. And with a PUT as the Relaxation oscillator, the concept explained above is not much

difference. By referring to the circuit in Figure 5 the resistor R charges the capacitor until

the peak point before being discharge to a heavy conduction

moving the operating point down the negative resistance slope

to the valley point (seen in Figure 4).

7. At the same time of the capacitor discharging, there is

a current spike flows through the cathode developing a voltage

spike across the cathode resistors. And once it discharge, the operating point resets back

to the slope up to the peak point.

8. The voltage spike in discussion is later be used as the triggering pulse to the SCR Gate

terminal seen in Figure 6, because once VC = VP, PUT will fires and as soon as it fires the

capacitor starts discharging rapidly through the low on-resistance of the PUT and RK. The

discharging process of the capacitor put off the PUT the charging cycle starts

all over again.

Figure 6: PUT as a relaxation oscillator in dimmer circuit

9. As this voltage spike depends on the time constant developed by the RC, it is wise to select

the value of the R so that it is small enough to supply enough current to raise the anode to

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VP the peak point while charging the capacitor. And during the capacitor discharge, the

resistor must be large enough so that it will never supply the high valley current IP.

10. And only if these criteria are fulfilled then the operating point would never reset back to

the high resistance condition to the left of the peak point (seen in Figure 4).

11. Figure 7 below shows the DIAC (left) and PUT (right) in a manufacturer product form.

Figure 7: DIAC (left) and PUT (right) in a manufacturer product form

Sometimes what you want isn't always what you get, but in the end what you get is so much better than what you wanted

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4.0 Opto-coupler

1. Apart from those two devices, an opto-coupler can also be used as a

triggering device. An opto-coupler is basically an interface between two

different circuits which operates at different voltage level. An example

of an opto-coupler is shown in Figure 85.

2. And what more convenient is that an opto-coupler offers an electrical isolation

between the input and output of the circuit. With an opto-coupler, the only contact between

these two is a beam of light.

3. With an advantage like that, the opto-coupler offers an insulation

resistance ranging up to megaohms between the two respective

circuits. In addition, this isolation prevents noise generated in one

circuit from being passed to the other circuit.

4. For example, noise generated from a high voltage information gathering

circuits can easily be interpreted as a triggering pulse in a low voltage digital logic circuits.

This can lead to a false triggering and as a result leads to SCR or TRIAC into conduction.

Figure 8: Opto-coupler in a manufacturer product form

5 Well Gain Electronics, Inc & Electronics EZ Find. Retrieved on 7th Dec 2010 from

http://www.wellgainelectronics.com/gimst59042.aspx

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5. An example of an opto-coupler is as shown in Figure 8 and Figure 96 illustrate the inside

view of an opto-coupler.

Figure 9: The inside view of an opto-coupler6

6. It consists of an infrared emitting LED chip that is optically in-line with a light-sensitive

silicon semiconductor chip, all enclosed in the same package. The silicon chip could be in the

form of a photo diode, photo transistor, photo Darlington, or photo SCR.

7. The operation of how an opto-coupler can trigger a SCR, TRIAC or FET can be easier

understood by giving an example. The example is shown through the flow diagram in Figure 107.

When life gives you a hundred reasons to cry, show life that you have a thousand

reasons to smile

6 Lakshmi A.K, (2010). OPTO-Coupler Retrieved on 7th Dec 2010 from http://www.ustudy.in/node/7519

7 Retrieved on 7th Dec 2010 from http://talkingelectronics.com/ChipDataEbook-1d/html/OptoList.html

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Figure 10: The flow diagram of the opto-coupler example7

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8. By going through the Figure 10, you see that once the

photo device sees illumination, the resistance between the

transistor C-E terminals reduces. This reduced resistance

can activate another circuit.

9. The LED requires 1mA to 15mA. If the current-limiting

resistor connected to the LED has a high value, only a

small current will flow through the LED and it will not glow

very brightly, thus unable to trigger the photo transistor.

10. The transistor will not turn on very much and the resistance between the collector-emitter

terminals will be fairly HIGH. The output voltage will remain fairly HIGH.

11. As the current through the LED increases, (the current-limiting resistor is reduced in value)

the LED will glow brighter and the transistor will turn on harder. The output voltage will

reduce. If the current through the LED is allowed to rise and fall, the output voltage of the

circuit above will fall and rise. In this way the opto-coupler will pass AUDIO.

12. If the current through the LED changes instantly from zero to say 15mA, the output will

change from HIGH to LOW. This is the principle of SWITCHING or passing a DIGITAL

SIGNAL.

13. The application of the opto-coupler can be seen in the Figure 1168.

Be who you are and say what you feel because those who mind don't matter and those who matter don't mind.

8 Admin. (2010). Telephone Light. Retrieved on 7th Dec 2010 from http://www.engineeringslash.com/digital-ic/telephone-

light.html

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Figure 11: The opto-coupler application

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5.0 Zener Diode

1. Zener diode is practically a diode but a special kind of diode. It can permit current to flow

in the forward direction and also allow it to flow in the reverse direction through with a precise

breakdown voltage known as the Zener voltage9.

2. Zener diode is shown in Figure 12 (left) and a view of the two diodes (standard and zener)

in comparison is on the right hand side of Figure 12.

Figure 12: Zener Diode (left) and the comparison between zener diode and a standard

zener (right)10

3. If a standard diode is to conduct in the reverse conduction, the diode will

permanently damage but the zener diode is an exceptional.

4. In order for the diode to works in the reverse direction; it should have a

lower reverse breakdown voltage that is a lower zener voltage – for example

2.4V. So when a reverse current is above the zener voltage, it will pass

through the zener diode with a controlled breakdown which does not damage

9 Neil. (2009). What is a Zener diode.Retrieved on 15th Dec 2010 from http://www.reuk.co.uk/What-is-a-Zener-Diode.htm 10 Reterieved on 15th Dec 2010 from http://www.flickr.com/photos/goa2008/3764752095/

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the diode9.

5. Thus the voltage drop across the zener diode will be equal to the zener voltage of that

diode no matter how high the reverse bias voltage is above the zener voltage9.

6. The V-I characteristic is shown in the Figure 13 below.

Figure 13: V-I characteristic of the Zener Diode

7. By referring to Figure 13, if the zener diode is connected in the forward direction, it will

behaves exactly like a standard diode with a typical voltage drop of 0.3 – 0.7V.

8. But in the reverse direction there will be a very small leakage current between 0 – Vz, and

once the voltage reaches the breakdown voltage Vz, a sudden current flow freely through it.

9. And since the voltage dropped across the diode is a known voltage and fixed value, zener

diode is typically used as a voltage regulator in electric circuits.

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