100 Most Important MCQ Of Power electronics with answer & explanation

7 Comments / MCQ / November 15, 2022

21. Calculate the De-rating factor if the string efficiency is 90 %.

A. .4
B. .1
C. .7
D. .8

Answer: A

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-.9=.1.   

 

22. Calculate the string efficiency if the de-rating factor is .22.

A. 78 %
B. 42 %
C. 20 %
D. 90 %

Answer: A

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

1-(De-rating factor)=1-.22=78 %.   

 

23. 25 V-rated n SCRs are connected in series. The operation voltage of the string is 300 V and the string efficiency is 40 %. Calculate the value of n.

A. 27
B. 30
C. 34
D. 36

Answer: B

The string efficiency can be calculated using the formula operation

voltage÷(Number of SCRs×Rated voltage).

The value of n is 300÷(25×.4)=30.   

 

24. The most reliable method for thyristor triggering is thermal triggering.

A. True
B. False

Answer: B

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

 

25. SCR is unipolar.

A. True
B. False

Answer: B

The SCR is a uni-directional, bipolar power electronics device. It is bipolar in nature because of its reverse and forward mode blocking capability.   

 

26. What is the formula for thermal voltage?

A. K×e÷T
B. K×T÷e
C. K2×e÷T
D. K×e2÷T

Answer: A

The formula for thermal voltage is K×e÷T

where

K is the Boltzmann constant
e is the charge on the electron
T is the absolute temperature.   

 

27. Device with highest d(i)÷d(t) and d(v)÷d(t) capability is _____________

A. GTO
B. BJT
C. SCR
D. SITH

Answer: D

SITH stands for static induction thyristor. It has a buried gate structure in which the gate electrode is placed in the n-base region. They have high d(i)÷d(t) and d(v)÷d(t) capability.   

 

28. ____________ is the switching frequency of SITH.

A. 55 kHz
B. 100 kHz
C. 91 kHz
D. 105 kHz

Answer: B

100 kHz is the switching frequency of the static induction thyristor. Static Induction Thyristor has a low frequency because of lower turn-off current gain.   

 

29. Calculate the value of the Crest factor if Vpeak=1.5 V and Vr.m.s=2.5 V.

A. .4
B. .6
C. .5
D. .7

Answer: B

The value of the crest factor is
Vpeak÷Vr.m.s=1.5÷2.5=.6 V.

It signifies the peak value is .6 times than the r.m.s value.   

 

30. Calculate the output voltage of the Boost converter if the supply voltage is 1 V and the duty cycle value is .9.

A. 10 V
B. 20 V
C. 40 V
D. 60 V

Answer: A

The output voltage of the buck converter is
Vo = Vin×(D)

The value of the duty cycle is less than one which makes the Vo < Vin. Vo = Vin×(1-D)=10 V.   

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7 thoughts on “100 Most Important MCQ Of Power electronics with answer & explanation”

  1. abhishek

    sir..apka 1st ans wrong h..SCR unijunction kaha hota h..it has 3 junction j1,j2 & j3.
    and sir 2nd que bhi doubt me h..thyrister family me diac triac scr sb aate h na….

    1. admin

      yes abhishek its unidirectional device corrected and SCR , LASCR, DIAC, TRIAC, SBS, ASCR. RCT, Gate all are the family of thyristor since SCR is most widely used thyristor, hence it is almost referred to as thyristor.

  3. Bangladesh Automation Technologies

    Hello Sir, You are information share very nice. Very half full question.

  4. Manu kumari

    sir please tell me about Q74
    Ans is incorrect
    it maybe
    medium input resistance and high output resistance
    please tell me its correct or not

    1. electricalexams

      The output resistance of CE configuration is around 10 kilo-ohms. Some reference book termed it high and other moderate. It is better the distinguish between CC, CB, CE as low, medium, high

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