# 100 Most Important MCQ Of Power electronics with answer & explanation

1. Which one of the following loads is suitable for load commutation in a Single-phase Half-bridge inverter?

B. R-L-C overdamped
C. R-L-C underdamped

R-L-C underdamped are leading power factor loads. They do not require any forced commutation technique. Anti-Parallel diodes help in the commutation process.

A. True
B. False

R-L-C overdamped loads are generally lagging power factor loads. They require forced commutation. Anti-Parallel diodes do not help in the commutation process.

3. A step-down chopper has an input voltage of 40 V and output voltage of 20 V. Calculate the value of the duty cycle.

A. 0.5
B. 0.2
C. 0.3
D. 0.4

The output voltage of the step-down chopper is Vo = Vin×(D).

The value of the duty cycle is less than one which makes the Vo < Vin. The step-down chopper is used to step down the voltage.

The value of the duty cycle is 20÷40 = .5.

4. Calculate the De-rating factor if the string efficiency is 20 %.

A. .8
B. .4
C. .5
D. .6

The de-rating factor is used to measure the reliability of a string. The value of the De-rating factor is 1-(string efficiency) = 1-.2 = .8.

5. Calculate the string efficiency if the de-rating factor is .48.

A. 48 %
B. 52 %
C. 50 %
D. 60 %

The string efficiency is calculated for the series and parallel connection of SCRs. The value of string efficiency is

1 – (De-rating factor) = 1-.48 = 52 %.

6. 400 V rated 5 SCRs are connected in series. The operation voltage of the string is 500. Calculate the string efficiency.

A. 25 %
B. 40 %
C. 38 %
D. 35 %

The string efficiency can be calculated using the formula operation

voltage÷(Number of SCRs×Rated voltage)
= 500÷(400×5) = 25 %.

7. 100 V rated 3 SCRs are connected in series. The operation voltage of the string is 200. Calculate the De-rating factor.

A. .35
B. .33
C. .38
D. .18

The string efficiency can be calculated using the formula operation

voltage÷(Number of SCRs×Rated voltage)
= 200÷(100×3) = .66.

The De-rating factor value is 1-.66 = .33.

8. Gate triggering is more reliable.

A. True
B. False

Gate triggering is more reliable than any other triggering method. The risk of false triggering of SCR reduces in the case of gate triggering.

9. The range of turn-off time is _______

A. .51-3 μsec
B. 3-101 μsec
C. 9-400 μsec
D. 7-787 μsec

The range of turn-off time is 3-101 μsec. The time is very low as SCR takes a low time to turn off. Commutation time should be greater than circuit turn-off time to avoid false triggering.

10. The range of On-State voltage drop is _______

A. 1-1.6 V
B. 0-1 V
C. 1.6-2 V
D. 2.4-3 V