# Power Electronics Questions and Answers – Choppers-5

This set of tricky Power Electronics questions and answers focuses on “Choppers-5”.

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

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

Explanation: It is a step-up/step-down chopper, the C maintains the load voltage constant.

2. 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

Explanation: During Ton, current flows from Vs-CH-L-Vs. Current through L rises from I1 to I2 and Vs appears across the inductor and load as they are in parallel. Voltage drop at the diode has to be considered as zero.

3. 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 duty cycle is less than 0.5, the circuit behaves like a step down chopper. Hence, Vo < Vs.

4. 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
q4.png”>
a) Vs-CH-L-Vs
d) no current flows

Explanation: When the chopper switch is off, polarity of emf induced in the inductor L gets reversed biased. Emf induced in L forward biases the diode D and L starts to discharge from inductor-load-diode-inductor.

5. 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

Explanation: Iavg = [ (I1 + I2)/2 ] Energy stored = Win = Vs x [ (I1 + I2)/2 ] x Ton.
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6. A step-down chopper is also called as a

Explanation: It is called as a first quadrant chopper as the current always flows from source to load and the current and voltage both are always positive.

7. For the type-B chopper shown below

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

Explanation: During Ton, SW is on and vo = 0 as the switch S.C’s the load. E drives current through L and charges it.

8. For the type-B chopper shown below

a) during Toff, Vo = (E + Ldi/dt) and D is conducting
b) during Toff, Vo = (E + Ldi/dt) and D is not-conducting
c) during Toff, Vo = 0
d) during Toff, Vo = Vs

Explanation: When switch Is off, Vo = E + Ldi/dt exceeds source voltage Vs and D (diode) is forward biased and begins to conduct.

9. The below given chopper is that of a

a) step-up chopper
b) step-down chopper
c) step-up/step-down chopper
d) depends on the duty cycle

Explanation: The load voltage Vo = E + L(di/dt) is greater than Vs, hence it is a step-up chopper.

10. 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

Explanation: The type-C chopper is a two quadrant chopper, it operates in the 1st and the 2nd quadrant. It has type-A and type-B choppers connected in parallel.

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