# Power Electronics Questions and Answers – Choppers-2

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Choppers-2″.

1. The below given figure is that of a _________ (IGBT is used as a chopper switch) a) step-up/step-down chopper
b) step-down chopper
c) step-up chopper
d) none of the mentioned

Explanation: When the switch is closed current flows through the load and also charges the inductor L, when the switch is off, the current freewheels through L-R-FD-L and voltage is zero because the FD short circuits the load.

2. In the below given circuit, when switch (SW) is on a) voltage is non-zero and current is rising
b) voltage is non-zero and current is decaying
c) voltage is zero and current is rising
d) voltage is zero and current is decaying

Explanation: When the switch is closed current flows through the load and also charges the inductor L, hence voltage is Vo and current is rising.

3. In the below given circuit, when switch (SW) is off a) voltage is non-zero and current is rising
b) voltage is non-zero and current is decaying
c) voltage is zero and current is rising
d) voltage is zero and current is decaying

Explanation: When the switch is off, the current freewheels through L-R-FD-L and voltage is zero because the FD short circuits the load.

4. 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

Explanation: Vo = Duty cycle x Vs = 0.75 x 100 = 75 V.

5. If Vo is the output voltage and Vs is the input DC voltage, then for the below given circuit. a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) Insufficient information

Explanation: The given circuit is that of a step-up chopper. For a step up chopper the output voltage is always (even if duty cycle is very very small) greater than the input voltage.
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6. In the below given circuit, when switch (SW) is on a) voltage is non-zero and current is rising
b) voltage is zero and current is decaying
c) voltage and current both are non-zero
d) voltage and current both are zero

Explanation: When the switch is on, the current flows from Vs-L-SW-Vs and charges the inductor (L), hence voltage and current both are zero.

7. In the below given circuit, when switch (SW) is first switched closed and then opened the a) voltage is non-zero and current is rising
b) voltage is zero and current is decaying
c) voltage is non-zero and current is decaying
d) voltage is zero and current is rising

Explanation: When the switch is off, the inductor is has stored energy from the earlier chopper on operation. The current starts to flow from Vs-L-D-Load-Vs, hence current is decaying as the inductor is discharging and the voltage is Vs + Vl (inductor voltage) (apply KVL).

8. What is the voltage across the load when the SW is first closed and then opened? a) Vs
b) duty cycle x Vs
c) Vs + L (di/dt)
d) zero

Explanation: When the switch is off, the inductor is has stored energy from the earlier chopper on operation. The current starts to flow from Vs-L-D-Load-Vs, hence current is decaying as the inductor is discharging and the load voltage is Vs + Vl (inductor voltage)
Vl = L(di/dt)
Vo = Vs + L(di/dt).

9. 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

Explanation: During Ton, the energy stored in the inductor is
Win = Vs x Ton x (I1+I2)/2
During Toff, the energy released by the inductor is
Wout = (Vo – Vs) x Toff x (I1+I2)/2
Considering ideal condition,
Win = Wout
Vo = Vs x (T/Toff)
α = Ton/T
Vo = T/T-Ton = Vs/1-α.

10. 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

Explanation: Vo = Vs/1-α.

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