S .N.D.

College of Engineering & Research Center, Babhulgaon

Tal- Yeola District- Nasik -423401
Department of Electrical Engineering

Power Electronics (PE) Multiple Choice Questions

Unit-2 Chopper
1. In the ___________ type of chopper, two stage conversions takes place.
a) AC-DC
b) AC link
c) DC link
d) None of the mentioned

2. Choppers converter
a) AC to DC
b) DC to AC
c) DC to DC
d) AC to AC

3. A chopper may be thought as a

a) Inverter with DC input
b) DC equivalent of an AC transformer
c) Diode rectifier
d) DC equivalent of an induction motor

4. Which device can be used in a chopper circuit?

a) BJT
b) MOSFET
c) GTO
d) All of the mentioned

5. A chopper is a
a) Time ratio controller
b) AC to DC converter
c) DC transformer
d) High speed semiconductor switch

6. What is the duty cycle of a chopper ?

a) Ton/Toff
b) Ton/T
c) T/Ton
d) Toff x Ton

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7. The load voltage of a chopper can be controlled by varying the
a) duty cycle
b) firing angle
c) reactor position
d) extinction angle

8. The values of duty cycle (α) lies between

a) 0<α<1
b) 0>α>-1
c) 0<=α<=1
d) 1<α<100

9. If T is the time period for a chopper circuit and α is its duty cycle, then the chopping frequency is
a) Ton/α
b) Toff/α
c) α/Toff
d) α/Ton

10. Find the output voltage expression for a step down chopper with Vs as the input voltage and α as
the duty cycle.
a) Vo = Vs/α
b) Vo = Vs x α
c) Vo = Vs2/α
d) Vo = 2Vs/απ

11. In a step down chopper, if Vs = 100 V and the chopper is operated at a duty cycle of 75 %. Find
the output voltage.
a) 100 V
b) 75 V
c) 25 V
d) none of the mentioned

12. Find the expression for output voltage for a step-up chopper, assume linear variation of load
current and α as the duty cycle.
a) Vs
b) Vs/α
c) Vs/(1-α)
d) Vs/√2

13. Find the output voltage for a step-up chopper when it is operated at a duty cycle of 50 % and Vs
= 240 V.
a) 240 V
b) 480 V
c) 560 V
d) 120 V

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14. If a step up chopper’s switch is always kept off then (ideally)
a) Vo = 0
b) Vo = ∞
c) Vo = Vs
d) Vo > Vs

15. If a step up chopper’s switch is always kept open then (ideally)

a) Vo = 0
b) Vo = ∞
c) Vo = Vs
d) Vo > Vs

16. For a step-down chopper, find the rms value of output voltage. Let α be the duty cycle
and Vs be the input voltage.
a) α x Vs
b) Vs/α
c) √α x Vs
d) Vs/2

17. A step down chopper is operated at 240V at duty cycle of 75%. Find the value of RMS
switch (IGBT/MOSFET) current. Take R = 10 Ω.
a) 2.07 A
b) 200 mA
c) 1.58 A
d) 2.4 A

18. Find the expression for effective input resistance of a step down chopper. With R load
and duty cycle = α.
a) R x α
b) R/2
c) 0
d) R/α

19. A step-up chopper has input voltage of 220 V and output voltage of 660 V. If the
conducting time of the IGBT based chopper is 100 μs, compute Toff width of the output
voltage pulse.
a) 100 μs
b) 150 μs
c) 50 μs
d) Insufficient data

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20. 1. For a step-down chopper shown in the figure below, dc source voltage = 230 V and the
voltage drop across the chopper = 2 V. Find the value of average output voltage for a duty cycle
of 0.4.

a) 230 V
b) 92 V
c) 90 V
d) 91.2 V

Explanation: The voltage drop across the switch occurs only when it is on.
Vo = (Vs – 2) x Duty cycle = 91.2 V.

21. For the step-down chopper shown in the figure below, dc source voltage = 230 V and the
voltage drop across the chopper = 2 V. Find the value of rms output voltage for a duty cycle of
0.4.

a) 91.2 V
b) 230 V
c) 144.2 V
d) 145 V

Explanation: The voltage drop across the switch will occurs only when it is on.
Vo (rms) = √0.4 x (230 – 2) = 144.2 V.

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22. For the step-down chopper shown in the figure below, dc source voltage = 230 V. Find the
power delivered to the load of R = 10 Ω. Duty cycle = 40 %. Take voltage drop at the switch to
be 2 V.

a) 1 kW
b) 2.08 kW
c) 569 W
d) 207 W

Explanation:
Vo (rms) = √0.4 x (230 – 2) = 144.2 V
P = 144.22/10 = 2079.3 W = 2.08 kW.

23. The below given chopper circuit is that of a

a) step-up chopper
b) step-down chopper
c) step-up/step-down chopper
d) none of the mentioned

24. The expression for a step-up/step-down chopper with α as the duty cycle and Vs as the dc
input voltage is
a) Vs/1 – α
b) α x Vs
c) Vs (α/1-α)
d) Vs (α/1+α)

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25. For a step-up/step-down chopper, if the duty cycle > 0.5 then
a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) None of the mentioned

26. A step down chopper has Vs = 230 V and R = 10 Ω. For a duty cycle of 0.4, the power taken
by the chopper is 2097 Watts. Find the chopper efficiency. Take the voltage drop across the
chopper switch as 2 V.
a) 98 %
b) 89.96 %
c) 99.14 %
d) 96.54 %

Explanation: Vo (rms) = √0.4 x (230 – 2) = 144.2 V

Po = 144.22/10 = 2079.3 Watts
Pi = 2097 Watts
η = Po/Pi = 99.14 %.

27. A step down chopper has input dc voltage of 220 V and R = 10 Ω in series with L = 65 mH.
If the load current varies linearly between 11 A and 17 A, then find the duty cycle α.
a) 1
b) 0.4
c) 0.6
d) 0.7

Explanation: Average load current = 11+17 / 2 = 14A

Average load voltage = IxR = 14×10 = 140V
But, Vo = α x Vs
Therefore, α x 220 = 140, α = 0.7.

28. For a step-up/step-down chopper, if α (duty cycle) = 0.5 then

a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) none of the mentioned

Explanation:
Vo = Vs (α/1-α)
For α = 0.5
Vo = Vs x (0.5/0.5).

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29. In the figure shown below, the capacitor C is used to

a) maintain the load voltage constant

b) protect the load
c) protect the chopper switch (SW)
d) maintain the load current constant

30. In the below shown step-up/step-down chopper circuit, when the switch (SW) is on

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a) current through inductor rises and the load voltage is zero
b) current through inductor rises and the load voltage is Vs
c) current through inductor decays and the load voltage is zero
d) current through inductor decays and the load voltage is Vs

31. For a step-up/step-down chopper, if α (duty cycle) < 0.5 then

a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) none of the mentioned

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32. In the below shown step-up/step-down chopper circuit, when the chopper switch (SW) if first
on and then off, then the current flows from path

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a) Vs-CH-L-Vs
b) L-Load-D-L
c) L-D-Load-L
d) no current flows

33. For a step-up/step-down chopper, if current increases from I1 to I2 linearly during Ton, then
find the energy stored in the inductor during Ton
a) zero
b) Vs x (I1 + I2)
c) Vs x [ (I1 + I2)/2 ] x Ton
d) Vs x [ (I1 + I2)/2 ] x T

34. A step-down chopper is also called as a

a) first-quadrant chopper
b) second-quadrant chopper
c) third-quadrant chopper
d) fourth-quadrant chopper

35. For the type-B chopper shown below

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a) during Ton, Vo = 0 and L stores energy
b) during Ton, Vo is not 0 and L stores energy
c) during Ton, Vo = 0 and L releases energy
d) during Ton, Vo is not 0 and L releases energy

36. The type-C chopper or two quadrant type-A chopper has

a) type-A and type-B choppers in series
b) type-A and type-B choppers in parallel
c) two type-A choppers in series
d) two type-A choppers in parallel

37. In case of TRC (Time Ratio Control), _________ is varied

a) duty cycle
b) firing angle
c) supply frequency
d) supply voltage magnitude

38. In constant frequency TRC or pulse width modulation scheme, ________ is varied.
a) Vs
b) Ton
c) T
d) f

39. In case of variable frequency system __________ is varied

a) T
b) Ton
c) Toff
d) supply frequency

40. In pulse width modulation scheme, _________ is kept constant.

a) Vs
b) Ton
c) T
d) Toff

41. In case of a constant frequency system, Ton = (1/4)T. If the chopping frequency 2 kHz, find
the value of Toff.
a) (1/8) ms
b) (3/8) ms
c) (1/8) μs
d) (3/8) μs

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42. In case of frequency modulation system, ________ is kept constant.
a) T
b) Ton
c) Toff
d) Either Ton or Toff

43. The control strategy in which on and off time is guided by the pervious set of values of a
certain parameter is called as
a) time ratio control
b) pulse width modulation
c) current limit control
d) constant frequency scheme

44. In the current limit control method, the chopper is switched off when
a) load current reaches the lower limit
b) load current reaches the upper limit
c) load current falls to zero
d) none of the mentioned

45. Which of the following mentioned control strategy/strategies would require a feedback loop?
a) pwm
b) constant frequency system
c) current limit control
d) none of the mentioned

46. In the current limit control method, when the load current reaches a predefined lower value,
then
a) the chopper is switched off
b) the chopper is switched on
c) the source voltage is removed
d) load voltage goes to zero

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47. Name the below given circuit.

a) Type D chopper
b) Type B chopper
c) Two-quadrant type A chopper
d) Two-quadrant type B chopper

48. In the below given circuit output voltage will be 0 when

a) SW1 is on
b) FD is not conducting
c) D2 is conducting
d) SW2 is on

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49. In the below given circuit

a) current can never be zero

b) current can flow in both the directions of the load
c) voltage can never be zero
d) none of the mentioned

50. In the below given type C chopper circuit, _______ and _________ together operate like
type A chopper.

a) CH1, FD
b) CH2, FD
c) CH1, D2
d) CH2, D2

51. Which type of chopper is used in the regenerative braking of DC motors?

a) Type A
b) type B

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c) type C
d) type D

52. Type C chopper consists of

a) two diodes and two switches
b) one diode and one switch
c) one diode and three switches
d) three diodes and two switches

53. In a type C chopper, if only one switch is operated

a) only one quadrant operation will be obtained
b) two quadrant operation can be obtained
c) the chopper won’t work
d) none of the mentioned

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