Overlapping in converter results in

 31. In a 3-phase inverter, 120° mode of conduction means

A) Each switch conducts for 120°
B) Two switches conduct for 120°
C) Each switch conducts for 60°
D) All switches conduct simultaneously
Answer: A) Each switch conducts for 120°

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32. The major cause of thermal runaway in a BJT is

A) Negative temperature coefficient
B) Positive temperature coefficient
C) High β
D) Low base resistance
Answer: B) Positive temperature coefficient

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33. The gate triggering circuit for SCR should provide

A) High voltage pulses
B) Sharp and narrow pulses
C) Continuous DC gate voltage
D) Constant amplitude sine signal
Answer: B) Sharp and narrow pulses

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34. The advantage of MOSFET over IGBT is

A) High current handling
B) High switching frequency capability
C) Less on-state drop
D) High voltage blocking
Answer: B) High switching frequency capability

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35. In a buck converter operating in continuous mode, the average output voltage is

A) $V_o = D V_s$
B) $V_o = V_s / D$
C) $V_o = D^2 V_s$
D) $V_o = V_s / (1-D)$
Answer: A) $V_o = D V_s$

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36. For a boost converter, duty cycle $D$ and output voltage $V_o$ relation is

A) $V_o = V_s(1-D)$
B) $V_o = V_s / (1-D)$
C) $V_o = D V_s$
D) $V_o = V_s D^2$
Answer: B) $V_o = V_s / (1-D)$

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37. The device suitable for high voltage and low frequency applications is

A) MOSFET
B) BJT
C) IGBT
D) GTO
Answer: D) GTO

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38. The IGBT combines

A) MOSFET input, BJT output
B) BJT input, MOSFET output
C) SCR input, BJT output
D) FET input, SCR output
Answer: A) MOSFET input, BJT output

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39. The principle of operation of cycloconverter is based on

A) Phase control
B) Frequency synthesis
C) Load commutation
D) Forced commutation
Answer: A) Phase control

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40. The frequency of output voltage in a step-down cycloconverter is

A) Greater than input
B) Equal to input
C) Less than input
D) Twice the input
Answer: C) Less than input

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41. The maximum efficiency of a chopper is achieved when

A) Duty cycle = 0.5
B) ON and OFF times are equal
C) Switching losses are minimized
D) Output current is constant
Answer: C) Switching losses are minimized

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42. In a 3-phase converter, overlap angle increases with

A) Increase in load current
B) Decrease in supply voltage
C) Increase in commutation inductance
D) Both A and C
Answer: D) Both A and C

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43. In a single-phase full converter feeding R-L load, the current is

A) Discontinuous for all α
B) Continuous for α < 90°
C) Continuous for α > 90°
D) Continuous for all α
Answer: B) Continuous for α < 90°

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44. The function of freewheeling diode in converter is to

A) Increase current
B) Decrease firing angle
C) Provide current path during negative voltage
D) Improve power factor
Answer: C) Provide current path during negative voltage

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45. The average current through freewheeling diode is

A) Equal to average load current
B) Half of load current
C) Zero
D) Depends on firing angle
Answer: D) Depends on firing angle

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46. A dual converter operates in

A) Two quadrants
B) One quadrant
C) Three quadrants
D) Four quadrants
Answer: A) Two quadrants

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47. In class-D commutation, energy stored in the commutating components is

A) Dissipated in resistor
B) Reused in next cycle
C) Lost in diode
D) Converted into heat
Answer: B) Reused in next cycle

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48. The main disadvantage of a cycloconverter is

A) Poor output waveform
B) Low efficiency
C) High switching losses
D) Difficult control
Answer: A) Poor output waveform

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49. In current source inverter, the commutation failure is due to

A) Inductive load
B) Capacitive load
C) Voltage reversal
D) Current reversal
Answer: D) Current reversal

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50. The reverse recovery time of a diode affects

A) Forward conduction loss
B) Switching loss
C) Voltage drop
D) Breakdown voltage
Answer: B) Switching loss

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51. In high-frequency applications, which device is most suitable?

A) BJT
B) SCR
C) MOSFET
D) TRIAC
Answer: C) MOSFET

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52. In GTO, turn-off is achieved by

A) Negative gate current
B) Positive gate voltage
C) Natural commutation
D) Forced commutation
Answer: A) Negative gate current

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53. The voltage stress on switch in buck converter is

A) Equal to output voltage
B) Equal to input voltage
C) Equal to input minus output
D) Double input voltage
Answer: B) Equal to input voltage

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54. The switching losses in MOSFET occur mainly due to

A) Channel resistance
B) Overlap of voltage and current
C) Gate charge
D) Drain resistance
Answer: B) Overlap of voltage and current

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55. In PWM technique, harmonic reduction depends on

A) Carrier frequency
B) Load resistance
C) Switching angle
D) Power factor
Answer: A) Carrier frequency

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56. The effective value of output voltage in a single-phase half bridge inverter is

A) $\frac{Vdc}{\sqrt{2}}$
B) $\frac{Vdc}{2}$
C) $\frac{Vdc}{\pi}$
D) $Vdc$
Answer: A) $\frac{Vdc}{\sqrt{2}}$

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57. In 3-phase 180° conduction mode inverter, at any instant

A) 2 switches conduct
B) 3 switches conduct
C) 4 switches conduct
D) 1 switch conducts
Answer: B) 3 switches conduct

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58. The main function of commutation circuit is to

A) Turn on SCR
B) Turn off SCR
C) Control voltage
D) Control frequency
Answer: B) Turn off SCR

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59. The output voltage ripple in a converter can be reduced by

A) Increasing load resistance
B) Increasing filter capacitance
C) Decreasing input voltage
D) Decreasing firing angle
Answer: B) Increasing filter capacitance

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60. In a buck-boost converter, when duty cycle D = 0.5, the output voltage is

A) Equal to input
B) Twice input
C) Negative of input
D) Negative and equal to input
Answer: D) Negative and equal to input

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61. The main advantage of resonant converter is

A) Zero-voltage or zero-current switching
B) High ripple
C) Complex control
D) High noise
Answer: A) Zero-voltage or zero-current switching

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62. The device most suited for HVDC transmission converter is

A) BJT
B) SCR
C) MOSFET
D) IGBT
Answer: B) SCR

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63. Commutation failure in HVDC occurs due to

A) Overvoltage
B) AC system fault
C) High current
D) Gate pulse error
Answer: B) AC system fault

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64. The switching frequency of IGBT is typically

A) 50 Hz
B) 100 Hz
C) 20 kHz
D) 100 kHz
Answer: C) 20 kHz

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65. In PWM inverter, modulation index $m_a$ is the ratio of

A) Carrier to reference amplitude
B) Reference to carrier amplitude
C) Carrier frequency to reference frequency
D) Reference voltage to DC voltage
Answer: B) Reference to carrier amplitude

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66. Overlapping in converter results in

A) Increase in output voltage
B) Decrease in output voltage
C) No effect
D) Increase in efficiency
Answer: B) Decrease in output voltage

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67. A snubber circuit consists of

A) Resistor only
B) Capacitor only
C) Resistor and capacitor
D) Inductor and resistor
Answer: C) Resistor and capacitor

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68. In a full converter with α = 180°, average output voltage is

A) Maximum
B) Zero
C) Minimum positive
D) Negative
Answer: B) Zero

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69. The conduction loss in power device depends on

A) Voltage drop and current
B) Switching frequency
C) Gate pulse
D) Firing angle
Answer: A) Voltage drop and current

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70. The delay angle in converter control is measured from

A) Zero crossing of input voltage
B) Zero current
C) Peak voltage
D) Average voltage
Answer: A) Zero crossing of input voltage