21. The turn-on time of an SCR depends on
A) Junction temperature
B) Gate current magnitude
C) Anode voltage
D) All of the above
Answer: D) All of the above
22. The latching current is
A) The minimum current required to keep SCR conducting immediately after triggering
B) The minimum current required to turn on SCR
C) The current at which SCR turns off
D) The leakage current
Answer: A)
23. A TRIAC can be triggered by
A) Positive gate current only
B) Negative gate current only
C) Both positive and negative gate currents
D) No gate current
Answer: C)
24. Which device is suitable for very high frequency switching?
A) SCR
B) BJT
C) MOSFET
D) GTO
Answer: C) MOSFET
25. A freewheeling diode is connected across
A) Supply
B) Load
C) SCR
D) Transformer
Answer: B) Load
26. The function of a snubber circuit is to
A) Reduce harmonics
B) Protect SCR from high dv/dt
C) Reduce conduction loss
D) Increase current
Answer: B)
27. The SCR conducts when
A) Forward biased and gate is triggered
B) Reverse biased and gate triggered
C) Forward biased only
D) Reverse biased only
Answer: A)
28. The dv/dt rating of an SCR indicates
A) Maximum rate of rise of anode current
B) Maximum rate of rise of anode voltage without triggering
C) Minimum holding current
D) Forward blocking voltage
Answer: B)
29. The firing angle in a full converter varies from
A) 0° to 90°
B) 0° to 150°
C) 0° to 180°
D) 0° to 120°
Answer: C)
30. The output voltage of a single-phase full converter is
A) Vdc = Vm/π
B) Vdc = (2Vm/π)cosα
C) Vdc = (Vm/2π)cosα
D) Vdc = Vm cosα
Answer: B)
31. In a three-phase full converter, the average output voltage is given by
A) (3√2 Vm/π)cosα
B) (3Vm/π)cosα
C) (1.35Vm)cosα
D) (1.17Vm)cosα
Answer: C)
32. The voltage across a conducting SCR is typically
A) 0 V
B) 0.7 V
C) 1 to 2 V
D) 5 to 10 V
Answer: C)
33. In Class A commutation, the SCR is turned off by
A) External circuit
B) Load current reversal
C) Resonance
D) Forced commutation
Answer: B)
34. In Class B commutation, the SCR is turned off by
A) Resonant current
B) Supply reversal
C) Load voltage
D) Pulse current
Answer: A)
35. A chopper operates as a
A) AC to DC converter
B) DC to DC converter
C) DC to AC converter
D) AC to AC converter
Answer: B)
36. The duty ratio of a chopper is defined as
A) Ton/T
B) Toff/T
C) T/Ton
D) Ton/Toff
Answer: A)
37. The ripple in DC output of a single-phase full converter is reduced by
A) Increasing firing angle
B) Increasing load inductance
C) Decreasing supply frequency
D) Reducing load resistance
Answer: B)
38. In a DC chopper, if the duty cycle is 0.5 and input is 200 V, output voltage is
A) 400 V
B) 100 V
C) 200 V
D) 0 V
Answer: B)
39. The main advantage of IGBT is
A) Low on-state voltage
B) High switching speed and voltage rating
C) High input impedance and high current capability
D) Both B and C
Answer: D)
40. The turn-off time of GTO is
A) Larger than SCR
B) Smaller than SCR
C) Equal to SCR
D) Infinite
Answer: B)
41. The IGBT stands for
A) Insulated Gate Bipolar Transistor
B) Integrated Gate Bipolar Transistor
C) Isolated Gate Base Transistor
D) Insulated Gate Base Transistor
Answer: A)
42. A DIAC is a
A) Two-terminal unidirectional device
B) Two-terminal bidirectional device
C) Three-terminal unidirectional device
D) Three-terminal bidirectional device
Answer: B)
43. The conduction angle in a half-wave rectifier with firing angle α is
A) α to 180°
B) 0° to α
C) α to π
D) Both A and C
Answer: D)
44. The turn-off process of SCR involves
A) Recombination of charge carriers
B) Diffusion of electrons
C) Latching
D) Injection
Answer: A)
45. Commutation in power electronics means
A) Turning ON SCR
B) Turning OFF SCR
C) Current control
D) Voltage control
Answer: B)
46. In Class E chopper, the load current
A) Always positive
B) Always negative
C) May reverse
D) Zero
Answer: C)
47. The function of a pulse transformer in gate drive is
A) Amplify signal
B) Provide isolation
C) Reduce current
D) Increase current
Answer: B)
48. The instantaneous power in AC circuit with voltage and current out of phase by 90° is
A) Maximum
B) Zero
C) Minimum
D) Infinite
Answer: B)
49. The effect of increasing load inductance in a converter circuit is
A) Increase in current ripple
B) Decrease in current ripple
C) Increase in voltage ripple
D) Decrease in efficiency
Answer: B)
50. The safe operating area (SOA) of a power device defines
A) Current and voltage limits
B) Frequency range
C) Input voltage
D) Duty cycle
Answer: A)
51. In a single-phase inverter, the output frequency depends on
A) DC input
B) Firing angle
C) Switching frequency
D) Load
Answer: C)
52. A full-bridge inverter uses
A) 2 switches
B) 4 switches
C) 6 switches
D) 8 switches
Answer: B)
53. The harmonic distortion in inverter output can be reduced by
A) PWM technique
B) Square wave switching
C) Increasing DC voltage
D) Reducing load
Answer: A)
54. The primary purpose of PWM control in inverters is
A) Increase efficiency
B) Control output voltage and reduce harmonics
C) Control frequency only
D) Increase current
Answer: B)
55. The commutation circuit components are mainly
A) Inductors and capacitors
B) Resistors only
C) Diodes and transistors
D) Transformers only
Answer: A)
56. Forced commutation is used in
A) Line commutated converters
B) Inverters
C) Cycloconverters
D) DC choppers
Answer: D)
57. The voltage source inverter has
A) Constant input voltage
B) Constant input current
C) Variable input voltage
D) None
Answer: A)
58. The current source inverter has
A) Constant voltage input
B) Constant current input
C) Variable current input
D) None
Answer: B)
59. The main loss in power MOSFET is due to
A) On-state resistance
B) Turn-on delay
C) Gate current
D) Reverse recovery
Answer: A)
60. A step-up chopper can be used for
A) Boost converter operation
B) Buck converter operation
C) Inverter operation
D) Cycloconverter operation
Answer: A)
61. The output voltage of a boost converter is
A) Less than input
B) Equal to input
C) Greater than input
D) Zero
Answer: C)
62. The power electronic switch that can block both polarities of voltage and conduct both polarities of current is
A) SCR
B) TRIAC
C) MOSFET
D) IGBT
Answer: B)
63. The GTO is turned off by
A) Removing gate signal
B) Applying negative gate current
C) Forward biasing
D) Reverse biasing
Answer: B)
64. The switching losses occur in
A) ON state
B) OFF state
C) Transition period
D) Both ON and OFF states
Answer: C)
65. For low power applications, the preferred switch is
A) SCR
B) MOSFET
C) GTO
D) TRIAC
Answer: B)
66. The main reason for using heat sinks in power devices is to
A) Improve efficiency
B) Reduce junction temperature
C) Reduce current
D) Increase voltage rating
Answer: B)
67. A forced commutated SCR circuit requires
A) External LC circuit
B) Load reversal
C) Gate turn-off
D) Freewheeling diode only
Answer: A)
68. The conduction losses in a power device depend on
A) On-state voltage drop and current
B) Switching frequency
C) Duty cycle
D) Supply voltage
Answer: A)
69. The switching frequency in power converters is typically in
A) Hz
B) kHz
C) MHz
D) GHz
Answer: B)
70. The ripple content in converter output can be minimized using
A) Filters
B) Snubbers
C) Inverters
D) Choppers
Answer: A)
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