Power Electronics SCR and BJT MCQ Questions

Power Electronics

Questions

ER. FARUK BIN POYEN

ASST. PROFESSOR

DEPT. OF AEIE

[email protected]

mailto:[email protected]

Set I

1. What is holding current in SCR?

It is the minimum current required to hold the SCR in forward conduction state.

When the forward current becomes less than holding current, SCR turns from forward

conduction state to forward blocking state.

2. What is latching current in SCR?

It is the minimum current required to latch(turn on) the SCR from forward blocking state

to forward conduction state.

3. What are the different turn on methods of SCR?

Forward voltage triggering

Gate Triggering

dv/dt triggering

Temperature triggering

Light triggering

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Set I

4. What is snubber circuit?

The snubber circuit is used for the dv/dt protection of the SCR.

It is a series combination of a resistor and a capacitor in parallel with the SCR.

5. What is hard switching of the thyristor?

When gate current is several times higher than the required gate current, the SCR is said to

be hard fired.

It reduces the turn ON time and enhances the di/dt capability.

6. What is firing angle?

The angle between the zero crossing of the input voltage and the instant the SCR is fired is

called as delay angle or firing angle.

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Set I

7. What is meant by SOA?

SOA - Safe Operating Area determines the voltage and current boundary within which the

Power Device can be operated without destructive failure.

8. What are the main components used for isolating the Power Circuits, Power

Semiconductor from the low-power circuit?

Opto-Couplers, Transformers

9. Name some of the current controlled (current driven) devices.

SCR, GTO, GTR

10. Name some of the voltage driven ( Voltage controlled) devices.

IGBT, MCT, IGCT, SIT

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Set I

11. What is duty cycle?

It is the ratio of the ON time of the chopper to total time period of the chopper.

D = Ton / [Ton + Toff]

12. Can fuses with an AC voltage rating be used in a DC applications?

Fuses must be rated for the voltage AC or DC in which they will be used.

Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating.

13. What are the characteristics of ideal Op-Amp?

Infinite open loop voltage gain

Infinite input impedance

Zero output impedance

Infinite Bandwidth

Zero offset voltage

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Set I

14. For High voltage applications will you prefer MOSFET or IGBT?

For High voltage applications we have to use IGBT.

Because MOSFETs are low voltage devices. ie, Their voltage rating is lesser than IGBT.

General rule is MOSFETs are suitable for applications which has breakdown voltage less

than 250V.

The IGBTs are suitable for applications which has breakdown voltage up to 1000V.

15. For High frequency applications will you prefer MOSFET or IGBT? Why?

For High frequency applications, MOSFET is the right choice of the device.

Because MOSFET has low switching losses compare to that of IGBT.

General rule of thumb is for low-frequency applications having frequency range upto

20kHz, we have to use IGBT.

For high frequency applications having frequency range of more than 200kHz, we have to

use MOSFET.

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Set II

1. Draw the buck topology.

The output voltage is of the opposite polarity than the input. This is a switched-mode

power supply with a similar circuit topology to the boost converter and the buck converter.

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Set II

2. Which is the good choice among the 2 options mentioned below to get the 12V as an

output? Why?

Option I: Input voltage = 48V

Option II: Input voltage = 24V

Duty cycle D = Vout / VInput

For case I: D = 12/48 = 0.25

For case II: D = 12/24 = 0.5

So for case II, the duty cycle is more than case I.

It means device is turned ON more time in the second case. while designing SMPS we have

to consider the principle mentioned in Q-3.

3. What is effect of having more duty cycle and less duty cycle? (continuation Q - 2)

If the duty cycle D is greater than 0.5, the core would not be completely demagnetized at

the end of the off-period, and a DC magnetization of the core would build up, resulting in

core saturation.

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Set II

4. To design the buck converter, what are basic & essential information (parameters) we

need to get from the Customer?

Output Voltage VOUT

Input Voltage VIN

Output Current (load current) IOUT

Maximum Ripple voltage allowed at the output side

Efficiency of the converter

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Set II

5. To design the buck converter, what are basic & essential information (parameters) we need toget from the Customer?

Voltage across Inductor VL = L di/dt

Rewriting the above equation as follows L = V L. dt/di

VL = Maximum voltage appears across the Inductor

dt= The time during which the maximum voltage appears across the inductor; di = Ripplecurrent

VL The maximum voltage appears across the inductor when the power switch is turned on.During that time the voltage will be

VL = VOUT - VIN

dt = The time during which the maximum voltage appears across the inductor = D/FS = Dutycycle/Switching frequency

di = Ripple current = Generally it is assumed in between 10% to 30% of the load current

By knowing all these values we can get the required inductor value for the buck converter.

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Set II

6. What are the various parameters we have to consider, while selecting IGBT?

In IGBT datasheet, we have to focus on following parameters

Collector to emitter voltage VCES

Collector Current IC

Collector Dissipation PC

Junction Temperature Tj

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Set II

7. For Selecting MOSFET, what are the major parameters we have to consider in the

datasheet?

In MOSFET datasheet, we have to focus on following parameters:

Drain to Source Voltage VDSS

On state Drain to Source Resistance RDS(ON)

Reverse Recovery Current IRR

Reverse Recovery Charge QRR

Gate Charge QG

Body Diode Characteristics

ID at ambient temperature

Channel power dissipation

Channel temperature

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Set III

8. What are the advantages of free wheeling diode in rectifier circuit?

The input power factor is improved.

It prevents the output voltage from becoming negative.

The Load current waveform is improved.

9. What is meant by commutation?

The process of changing the direction of current flow in a particular path of the circuit. It

is used to turn off the SCR.

10. What are the types of commutation?

Natural commutation

Forced commutation

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Set III

11. What is natural commutation?

The process of the current flowing through the thyristor goes through a natural zero and

enable the thyristor to turn off is called as natural commutation.

12. What is forced commutation?

The process of the current flowing through the thyristor is forced to become zero by

external circuitry is called as forced commutation.

13. What are the types of commutation with respect to commutation process?

Voltage commutated chopper

Current commutated chopper

Load commutated chopper

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Set III

14. What is meant by cyclo-converter?

It is also known as frequency changer. It converts input power at one frequency to output

power at another frequency with one stage conversion.

15. What are the types of cyclo-converters?

Step up cyclo-converter

Step down cyclo-converter.

16. What is step down cyclo-converter?

It is the converter whose output frequency is less than the input frequency.

17. What is step up cyclo-converter ?

It is the converter whose output frequency is more than the input frequency.

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Set III

18. What does the Voltmeter in AC mode show? Is it RMS value or peak value?

Multimeter in AC mode shows RMS value of the voltage or current. Also when it is DC modeit will show the RMS value only.

19. What is the necessity to use the special machines?

General purpose motors (Induction motors, synchronous motors) are neither precision speednor precision position motors. For many automated systems require high precise speed andhigh precise positioning motors. In such cases special purpose motors like stepper motors,PMDC motors etc. are used.

20. What are the control strategies of chopper?

The control strategies of chopper are

1. Pulse width modulation PWM (Variable TON, Constant frequency)

2. Frequency modulation (Constant TON or TOFF, Variable frequency)

3. Current Limit Control (CLC)

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Set III

21. What is delay angle or what is firing angle of phase controlled rectifier?

The delay angle is the angle at which thyristors are triggered after zero crossing.

After zero crossing of supply voltage, one pair of thyristors is forward biased. ie, After

delay angle(α) these SCRs are triggered.

22. What is Universal Motor?

It is defined as a motor which can be operated either on DC or single-phase AC supply at

approximately the same speed and output.

The universal motor is built exactly like a series DC motor. But a series DC motor cannot

be run as a universal motor, even though both motors look the same internally and

externally.

We cannot use these motors in the industrial applications due to the low efficiency (25% -

35%). It has high starting torque and a variable speed characteristic. It runs at dangerously

high-speed on no load.

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Set III

23. Give some examples of power electronics applications in the day-to-day life?

UPS - Uninterruptible Power Supply; SMPS - Switch Mode Power Supply

Speed Control of Motors; ICU

24. What is meant by PMDC?

PMDC stands for Permanent Magnet DC Motor

A Permanent Magnet DC Motor is similar to an ordinary dc shunt motor except that its field isprovided by permanent magnets instead of salient-pole wound field structure.

There are three types of permanent magnets used for such motors namely;

(i) Alnico Magnets

(ii) Ceramic magnets

(iii) Rare-earth magnets

The major advantages are low noise, small size, high-efficiency, low manufacturing cost.

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Set IV

25. What is meant by Electrical Drives?

Systems employed for motion control are called as "Drives“. Drives employing

electric motors are known as " electric drives".

26. What are the different factors for the selection of electrical drives?

Steady state operation requirements

Transient operation requirements

Requirements related to the source

Capital and running cost, maintenance needs, life

Space and weight restrictions

Environment and location

Reliability

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Set IV

27. What are the parts of electrical drives?

Electrical motors and load

Power Modulator

Sources

Control Unit

Sensing unit

28. What are modes of operation of electrical drives?

Steady state

Acceleration including starting

Deceleration including stopping

29. What is meant by four quadrant operation?

A motor operate in two modes-motoring and braking. In motoring, it converts electrical energyinto mechanical energy, which supports its motion. In braking, it works as a generator convertingmechanical energy into electrical energy, and thus, opposes the motion. Motor can providemotoring and braking operations for both forward and reverse directions.

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Set IV

30. What is meant by regenerative braking?

Regenerative braking occurs when the motor speed exceeds the synchronous speed. In this

case, the induction motor would run as the induction generator is converting the

mechanical power into electrical power, which is delivered back to the electrical system.

This method of braking is known as regenerative braking.

31. What is the function of the free wheeling diode in a phase controlled rectifier?

To improve input power factor

To make the load current continuous

32. What is meant by chopper?

Chopper converts fixed dc voltage into variable dc voltage

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Set IV

33. What is brushless DC motor?

An inverter fed trapezoidal permanent magnet AC motor drive operating in self-controlledmode is called brushless dc motor.

34. What is a PMSM?

PMSM = Permanent Magnet Synchronous Motor. In medium and small size synchronousmotors dc field can be produced by permanent magnets.

35. Classification of PMSM.

According to construction

1. Surface mounted PMSM

2. Interior mounted PMSM

According to the nature of voltage induced

1. Sinusoidally excited

2. Trapezoidally excited

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Set IV

36. What is the basic principle of an electric generator?

A basic principle of an electric generator is based on Faraday's law of electromagnetic

induction. ie, whenever a conductor is moved in the magnetic field, dynamically induced

emf is produced in the conductor.

37. What is the purpose of yoke in a dc machine?

It acts as a protected cover for the whole machine and provides mechanical support for

poles. It carries the magnetic flux produced by the poles. The flux per pole divides at the

yoke, so that the yoke carries only half the flux produced by each pole

38. What is the purpose of interpoles in modern dc machine?

In modern dc machines, commutating poles or interpoles are provided to improve the

commutation process.

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Set IV

39. Why the armature poles are made of laminations?

To reduce the eddy current losses.

40. What are the different types of DC generator?

Separately excited dc generator

Self excited dc generator

- Series generator

- Shunt generator

- Compound generator

1. Long shunt compound

2. Short shunt compound

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Set IV

41. Define armature reaction.

The term armature reaction means the effect of the mmf set up by the armature current on

the distribution of mmf under main poles of a dc machine.

42. What are the main effects of armature reaction?

Due to armature reaction:

(1) The main field flux gets weakened or gets demagnetized. This effect is called

Demagnetization effect.

(2) The main field flux gets distorted. This effect is called Cross-magnetization effect.

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43. Name some of the various methods of traction motor control?

Rheostatic control

Thyristor control

Metadyne control

Buck and boost method

Series parallel control

Field control

44. What are the various methods of applying electric traction?

There are three methods of applying electric braking. They are

Plugging or reverse current braking

Rheostatic braking

Regenerative braking

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45. What are the basic requirements of a braking system?

The basic requirements of a braking system is given below

Easy to use for driver to operate

It should be inexhaustible.

The maintenance should be a minimum

It should be simple, quick, robust and reliable in action

Kinetic energy of the train be storage during braking which could be used subsequently

during acceleration of the train.

46. Give some of advanced methods of speed control of traction motors.

Tap changer control

Thyristor control

Chopper control

Microprocessor control

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47. What are the advantages of microprocessor based control of traction motors?

High speed of response

High accuracy

Over voltage and over speed protection

Electronic interlocking

Less sensitive to temperature variations and drift

Numbers of components used are less

48. Define dead weight and adhesive weight.

Dead weight

The total weight of locomotive and train to be pulled by the locomotive is known as dead

weight.

Adhesive weight

The total weight to be carried on the driving wheels is known as the adhesive weight.

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49. What is tractive effect?

The effective force necessary to propel the train at the wheels of the locomotive to which

the motor is geared is called the tractive effect. It is measured in newtons and is tangential

to the driving wheels.

Total tractive effort required to run a train on tract = Tractive effort to produce acceleration

+ Tractive effort to overcome effort of gravity + Tractive effort to overcome train

resistance.

50. Define electric drives.

Systems employed for motion control are called as drives and drives employ any of the

prime movers such as diesel or petrol engines, gas or steam turbines, hydraulic motors and

electric motors for supplying mathematical energy for motion control. Drives employing

electric motion is called as electrical drives.44

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51. What are the various parts of electrical drives?

Electrical motors and load

Power Modulator

Sources

Control Unit

Sensing unit

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52. What are various advantages of electrical drives?

They are having flexible control characteristics. The study state and dynamic

characteristics of electrical drives can be shaped to satisfy load requirements.

Drives can be provided with automatic fault detection systems. PLCs and computers

can be employed to automatically control the drive operations in a desired sequence.

Drives are available in wide range of speed, power and torque.

Control gear required for speed control, starting and braking is usually simple and easy

to operate.

It can operate in all the four quadrants of speed - torque plane. Electric braking gives

smooth deceleration and increases life of the equipment compared to other forms of

braking.

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53. Why self commutated devices are preferred over thyristors for chopper circuits?

Self commutated devices such as power MOSFETs, power Transistors, IGBTs, GTOs and

IGCTs are preferred over thyristors for building choppers because they can be

commutated by a low power control signal and don't need commutation circuit.

54. Why thyristors are not preferred now a days for chopper fed DC drives?

Since it requires commutation circuit, thyristors are not preferred now a days for chopper

fed DC drives. The commutation circuit is complex for chopper circuits.

55. What is function of a free wheeling diode in a phase controlled rectifier?

Input power factor is improved

Make the load current continuous

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56. What causes poor input power factor in phase controlled DC drives?

The input power factor in the phase controlled rectifier is low when the output voltage is

less than the maximum, that is when the firing angle is large. When the firing angle

increases, the converter draws more lagging reactive power. So the input power is low.

57. What are the advantages of three-phase drives over single phase drives?

Reducing the armature ripple current

The filtering requirement is less

Armature current is mostly continuous

Motor performance is better

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58. What are the various control strategies for varying duty cycle of the chopper?

Time Ratio Control (TRC)

Current Limit Control (CLC)

59. What are the different methods of speed control of induction motors?

Stator voltage control

Supply frequency control

Rotor resistance control

Slip power recovery control

60. What are the advantages in operation of choppers at high frequency?

The operation at a high frequency improves motor performance by reducing current ripple

and eliminating discontinuous conduction.

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MCQ on Thyristor

1. The 4-layer (Shockley) diode can conduct current if

a. the anode-to-cathode voltage exceeds VBR

b. a current pulse is applied to the gate

c. both a and b are correct

d. none of the above

2. The SCR can conduct current if

a. the anode-to-cathode voltage exceeds VBR

b. a current pulse is applied to the gate

c. both a and b are correct


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MCQ on Thyristor

3. A bidirectional thyristor is the

a. 4-layer diode

b. SCR

c. TRIAC

d. silicon-controlled switch

4. A thyristor that looks like two back-to-back 4-layer diodes is the

a. SCR

b. TRIAC

c. SCS

d. DIAC

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MCQ on Thyristor

5. An SCR turns off when the

a. gate trigger current drops below a specified level

b. anode current drops below the holding current

c. both a and b are true


6. The purpose of a crowbar circuit is to protect a load from

a. excessive ripple

b. low-voltage

c. over-voltage

d. all of the above

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MCQ on Thyristor

7. A DIAC and TRIAC are similar in that both devices

a. can use break-over triggering

b. can be used in ac circuits

c. are bidirectional

d. all of the above

8. A device that has an unstable negative resistance region is the

a. UJT

b. DIAC

c. TRIAC

d. SCS

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MCQ on Thyristor

9. The symbol for a silicon-controlled switch (SCS) is

10. A programmable unijunction transistor (PUT) is “programmed” by choosing the

a. RC time constant

b. gate resistors

c. power supply voltage

d. cathode resistor

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(a) (b) (c) (d)

MCQ on Thyristor: Answers

Answers:

40

1. a

2. c

3. c

4. d

5. b

6. c

7. d

8. a

9. d

10. b

11. An SCR is a ...... triggered device

(1) Voltage

(2) Current

(3) Voltage as well as current

(4) None of the above

12. In an SCR circuit, the supply voltage is generally ......... that of breakover voltage

(1) Equal to

(2) Less than

(3) Greater than


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13. When an SCR is turned on, the voltage across it is about ..........

(1) zero

(2) 10 V

(3) 0.1 V

(4) 1V

14. . An SCR is turned off when

(1) Anode current is reduced to zero

(2) Gate voltage is reduced to zero

(3) Gate is reverse biased


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15. In an SCR circuit, the angle of conduction can be changed by

(1) Changing anode voltage

(2) Changing gate voltage

(3) Reverse biasing the gate


16. If firing angle is increased, then the output of an SCR

(1) Remains the same

(2) Is increased

(3) Is decreased


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17. If gate current is increased, then anode-cathode voltage at which SCR turns ON

(1) is decreased

(2) is increased

(3) remains the same

(4) none of the above

18. An SCR has ........... PN junctions

(1) Two

(2) Three

(3) Four


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19. An SCR has ........ semiconductor layers

(1) Two

(2) Three

(3) Four


20. In AC supply, an SCR can exercise control over

(1) Positive half-cycles only

(2) Negative half-cycles only

(3) Both positive and negative half-cycles

(4) Positive or negative half-cycles

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21. AC power in a load can be controlled by connecting

(1) Two SCRs in series

(2) Two SCRs in parallel

(3) Two SCRs in parallel opposition


22. When SCR starts conducting, which loses all control?

(1) Cathode

(2) Gate

(3) Anode


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MCQ on Thyristors: Answers

11. (2)

12. (2)

13. (4)

14. (1)

15. (2)

16. (3)

17. (1)

18. (2)

19. (3)

20. (4)

21. (3)

22. (2)

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BJT Questions:

1. Fill in the gaps

a) Under forward bias condition a large number of ___________________ carriers are

introduced in the base region.

b) Some minority charge carriers reaching base __________________ with majority

carriers there and the rest of them ___________________ to the collector.

c) When the base-emitter junction of a BJT is forward biased while the base-collector

junction is reverse biased the BJT is said to be in the _______________ region.

d) When both B-E & C-B junction of a BJT are reverse biased it is said to be in the

_________________ region.

e) When both B-E & C-B junction of a BJT are forward biased it is said to be in the

_______________ region.

Answer: (a) minority; (b) recombine, diffuse; (c) active; (d) cut-off; (e) saturation

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2. Fill in the gaps.

a) Doping density of the emitter of a Power BJT is several orders of magnitude

______________ than the base doping density.

b) Collector drift region is introduced in a Power BJT to block _______________ voltage.

c) Doping density of the base region in a power BJT is ________________.

d) Power BJT has ________________ DC current gain compared to signal level

transistors.

e) In a Power BJT multiple, narrow finger like distributed emitter structure is used to

avoid emitter ___________________.

Answer: (a) higher; (b) high reverse; (c) moderate; (d) low; (e) current crowding.

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a) In the “Cut off” region collector current of a Power Transistor is _____________.

b) In the __________________ region of a Power Transistor the dc current gain remains

fairly constant.

c) Saturation region of a Power Transistor can be divided into _______________ region

and ______________________ region.

d) Active region operation of a Power BJT is limited mostly by _______________

consideration.

e) “Second breakdown” in a Power BJT occurs due to ________________ of the collector

current distribution.

Answer: (a) negligible; (b) active; (c) Quasi saturation, hard saturation; (d) Power

dissipation; (e) non uniformity.

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a) An ideal switch can conduct current in ______________ directions. While a power

transistor conducts current in _______________ direction.

b) In power transistor there will be power loss due to ON state ________________ and

OFF state _________________.

c) Unlike an ideal switch the switching of a power transistor is not ____________.

Answer: (a) two, one; (b) voltage drop, leakage current; (c) instantaneous

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a) For faster switching of a BJT _______________ carriers are to be swept quickly from

the ________________ region.

b) The reverse biased base emitter junction can be represented as a ______________

dependent __________________.

c) In the quasi saturation region collector-emitter voltage falls at a ______________ rate.

d) Turn ON delay can be reduced by __________________ the rate of rise of the base

current.

Answer: (a) minority, base; (b) voltage, capacitor; (c) slow; (d) increasing.

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a) Turn OFF process in a BJT is associated with transition from the _______________

region to the ______________ region.

b) Negative _______________ current is required to remove excess charge carriers from

the ______________ region of a BJT during Turn OFF process.

c) VCE increases rapidly in the ________________ region.

Answer: (a) Saturation, Cut-off; (b) base, base; (c) active.

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a) BJT has large switching times, since it is a _________________ carrier device.

b) BJT has _______________ ON state voltage drop.

c) BJT is inefficient at ______________ switching frequencies.

d) Turn OFF snubber circuit is used to improve _______________ withstand capacity of a

BJT.

Answer: (a) minority; (b) low; (c) high; (d) voltage

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Q1. Why does the collector current of a BJT in the active region increases with increasing

collector voltage for a given base current.

Answer: In the active region as the VCE voltage is increased the depletion layer width at

the CB junction increases and the effective base width reduces. Therefore, for a given

VBE recombination of minority carriers in the base region reduces and base current also

reduces. In order to main constant base current with increasing VCE, VBE must increase.

Therefore, for a constant base current the number of minority carriers in the base region

will increase and consequently, collector current will increase.

Q2. A power BJT has I C = 20 A at I B = 2.5 A, I CS = 15 mA. Find out β.

Answer: 𝐼𝐶 = 𝛽𝐼𝐵 + 𝛽 + 1 𝐼𝐶𝑆 = 𝛽 𝐼𝐵 + 𝐼𝐶𝑆 + 𝐼𝐶𝑆 ≫ 𝛽 = 7.95

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Q3. Explain the importance of FBSOA.

Answer: FBOSOA compactly represents the safe operating limits of a power transistor in

terms of maximum forward current, maximum forward voltage, maximum average &

instantaneous power dissipation and second break down limits. It is most useful in

designing the switching trajectory of a power transistor.

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References:

Module 1 Power Semiconductor Devices Version 2 EE IIT, Kharagpur 1,

Lesson 3 Power Bipolar Junction Transistor (BJT)

Power Electronics A to Z/ INTERVIEW QUESTIONS

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Power Electronics SCR and BJT MCQ Questions

Engineering

Transcript of Power Electronics SCR and BJT MCQ Questions