Power Electronics Questions and Answers – Choppers-3

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) 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
View Answer

Answer: c
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
View Answer

Answer: b
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.
Find the average value of output voltage of a basic step-down chopper, with duty cycle = α
a) I = Vs x α
b) I = (Vs x α)/R
c) I = 0
d) I = Vs/R
View Answer

Answer: b
Explanation: Vo = Vs x α
I = Vo/R.
advertisement

4. For the below given circuit, find the output current at the instant of commutation.
Find the output current at the instant of commutation in given circuit
a) Vo/R
b) Vs/R
c) 0
d) α/2
View Answer

Answer: b
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
View Answer

Answer: c
Explanation: [(Ton/T).Vs2]1/2 = √α x Vs.
Free 30-Day C Certification Bootcamp is Live. Join Now!

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
View Answer

Answer: 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
View Answer

Answer: d
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
View Answer

Answer: c
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
View Answer

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

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
View Answer

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

Sanfoundry Global Education & Learning Series – Power Electronics.

To practice all areas of Power Electronics, here is complete set of 1000+ Multiple Choice Questions and Answers.

advertisement
advertisement
Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
I’m Manish - Founder and CTO at Sanfoundry. I’ve been working in tech for over 25 years, with deep focus on Linux kernel, SAN technologies, Advanced C, Full Stack and Scalable website designs.

You can connect with me on LinkedIn, watch my Youtube Masterclasses, or join my Telegram tech discussions.

If you’re in your 40s–60s and exploring new directions in your career, I also offer mentoring. Learn more here.