Power Electronics SCR and BJT MCQ Questions

Power Electronics
Questions
ER. FARUK BIN POYEN
ASST. PROFESSOR
DEPT. OF AEIE
FARUK.POYEN@GMAIL.COM

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
2

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.
3

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
4

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
5

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.
6

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.
7

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.
8

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
9

Set II
5. To design the buck converter, what are basic & essential information (parameters) we need to
get 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 = Ripple
current
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 = Duty
cycle/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.
10

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
11

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
12

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
13

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
14

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.
15

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 mode
it 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 speed
nor precision position motors. For many automated systems require high precise speed and
high 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)
16

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.
17

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 is
provided 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.
18

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
19

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 energy
into mechanical energy, which supports its motion. In braking, it works as a generator converting
mechanical energy into electrical energy, and thus, opposes the motion. Motor can provide
motoring and braking operations for both forward and reverse directions.
20

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
21

Set IV
33. What is brushless DC motor?
An inverter fed trapezoidal permanent magnet AC motor drive operating in self-controlled
mode is called brushless dc motor.
34. What is a PMSM?
PMSM = Permanent Magnet Synchronous Motor. In medium and small size synchronous
motors 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
22

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.
23

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
24

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.
25

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
26

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
27

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.
28

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
29

51. What are the various parts of electrical drives?
 Electrical motors and load
 Power Modulator
 Sources
 Control Unit
 Sensing unit
30

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.
31

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
32

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
33

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.
34

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
35

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
36

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
37

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
38

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
39
(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
41

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
42

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
43

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
44

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
45

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
46

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)
47

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
48

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.
49

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.
50

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
51

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.
52

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.
53

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
54

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: 𝐼 𝐶 = 𝛽𝐼 𝐵 + 𝛽 + 1 𝐼 𝐶𝑆 = 𝛽 𝐼 𝐵 + 𝐼 𝐶𝑆 + 𝐼 𝐶𝑆 ≫ 𝛽 = 7.95
55

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.
56

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
57

Power Electronics SCR and BJT MCQ Questions

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