# Power Electronics Questions and Answers – Choppers-3

This set of Power Electronics Test focuses on “Choppers-3”.

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

Explanation: If it is said the a chopper is always kept off, that means the switch is always open. As such, Ton = 0
Duty cycle = 0
Vo = Vs/1-duty cycle . . . (for a step-up chopper).

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

Explanation: If it is always in then, Ton = T. Duty cycle = 1.
Vo = Vs/1-duty cycle = Vs/0
Therefore, Vo = ∞.

3. Find the average value of output voltage of a basic step-down chopper, with duty cycle = α and load = R Ω.

a) I = Vs x α
b) I = (Vs x α)/R
c) I = 0
d) I = Vs/R

Explanation: Vo = Vs x α
I = Vo/R.

4. For the below given circuit, find the output current at the instant of commutation.

a) Vo/R
b) Vs/R
c) 0
d) α/2

Explanation: The circuit is that of a step-down chopper. The output current is commutated by the switch(IGBT) at the instant t = Ton. Therefore, output current at the instant of commutation is Vs/R.

5. 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

Explanation: [(Ton/T).Vs2]1/2 = √α x Vs.
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6. 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

Explanation: The switch could be anything, a IGBT, a SCR, a GTO etc. The current through the switch is the same as the current through the load for a step down chopper.
Vo = Vs/1-α
α = 0.75
I = √α x Vs/R.

7. 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

Explanation: Effective input resistance of the chopper circuit = (dc source voltage)/(average source current) = (Vs.R)/(Vs.α) = R/α.

8. 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

Explanation: 660 = 220/1-α
α = 2/3 = Ton/T
Ton = 2T/3 = 100μs. This gives chopping period T = 100 x 3/2 = 150 μs
Toff = T – Ton = 150 – 100 = 50 μs.

9. For a step-up chopper, when the duty cycle is increased the average value of the output voltage
a) increases
b) decreases
c) remains the same
d) none of the mentioned

Explanation: Vo = Vs/1-duty cycle
Hence, as duty cycle increases the output voltage increases.

10. For a step-down chopper, when the duty cycle is increased the average value of the output voltage
a) increases
b) decreases
c) remains the same
d) none of the mentioned

Explanation: Vo = Duty cycle x Vs. Hence, output voltage is directly proportional to the duty cycle.

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