Power electronics: board exam questions with solutions
On the ECE exam under Electronics Engineering and the REE exam under Electrical Engineering Professional Subjects. Try each question before you open its solution, then practise with fresh numbers below: 3 computed question types on this topic.
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Build it in the lab
Buck converter from parts · Boost converter from parts · Linear supply: transformer, bridge, capacitor, regulator · LM317 adjustable regulator: the same circuits on a live bench, with meters you can move.
Questions
1. Once an SCR has been triggered into conduction, how is it normally turned off?
- By removing the gate pulse
- By reducing its anode current below the holding current
- By applying a positive gate pulse
- By increasing the anode voltage
Show the answer and solution
Answer: B. By reducing its anode current below the holding current
The SCR latches on; the gate loses control after turn-on, so the anode current must fall below the holding current (natural or forced commutation).
2. An ideal buck converter operating in continuous conduction has Vin = 24 V and duty cycle D = 0.375. What is Vout?
- 9.0 V
- 38.4 V
- 15.0 V
- 14.4 V
Show the answer and solution
Answer: A. 9.0 V
Vout = D × Vin = 0.375 × 24 = 9.0 V.
3. An ideal boost converter in continuous conduction has Vin = 12 V and D = 0.6. What is Vout?
- 7.2 V
- 20.0 V
- 18.0 V
- 30.0 V
Show the answer and solution
Answer: D. 30.0 V
Vout = Vin/(1 − D) = 12/0.4 = 30.0 V.
4. An ideal inverting buck-boost converter in continuous conduction has Vin = 12 V and D = 0.4. What is the magnitude of its output voltage?
- 20.0 V
- 4.8 V
- 8.0 V
- 18.0 V
Show the answer and solution
Answer: C. 8.0 V
|Vout| = Vin × D/(1 − D) = 12 × 0.4/0.6 = 8.0 V (opposite polarity).
5. A single-phase half-wave SCR rectifier feeds a resistive load from 230 V rms. What is the average load voltage at a firing angle of 60°?
- 155.3 V
- 77.7 V
- 103.5 V
- 54.9 V
Show the answer and solution
Answer: B. 77.7 V
Vm = 230√2 = 325.3 V; Vavg = (Vm/2π)(1 + cos α) = (325.3/2π)(1.5) = 77.7 V.
6. An uncontrolled single-phase full-wave bridge rectifier with a resistive load is fed from 230 V rms. Ignoring diode drops, what is the average output voltage?
- 207.1 V
- 103.5 V
- 325.3 V
- 230.0 V
Show the answer and solution
Answer: A. 207.1 V
Vdc = 2Vm/π = 2 × 325.3/π = 207.1 V (≈ 0.9 × 230).
7. Which power device combines a MOSFET-like insulated gate drive with the low conduction loss of a bipolar output stage?
- TRIAC
- GTO thyristor
- Power diode
- IGBT
Show the answer and solution
Answer: D. IGBT
The insulated-gate bipolar transistor has a MOS gate (voltage drive, negligible gate current) and bipolar conduction for low on-state voltage at high current.
8. A three-phase six-pulse diode bridge is fed from a 400 V (line-to-line) supply. Ignoring diode drops and overlap, what is the average DC output?
- 360 V
- 566 V
- 540 V
- 312 V
Show the answer and solution
Answer: C. 540 V
Vdc = 3√2 VL/π = 1.35 × 400 = 540.2 V.
Worked examples with computed numbers
These numbers are fixed on this page; in practice they change every time.
9. An ideal buck converter steps 48 V down to 3.3 V. What duty cycle is needed?
- 6.88%
- 93.1%
- 7.38%
- 6.43%
Show the answer and solution
Answer: A. 6.88%
An ideal buck in continuous conduction gives Vo = D·Vin, so D = 3.3/48 = 6.88%.
10. An ideal boost converter runs from 24 V at 20% duty cycle. What is its output voltage?
- 4.8 V
- 120 V
- 28.8 V
- 30 V
Show the answer and solution
Answer: D. 30 V
Ideal boost: Vo = Vin/(1 − D) = 24/(1 − 0.2) = 30 V.
11. A linear regulator gives 5 V at 0.1 A from a 24 V input. How much power does it turn into heat?
- 500 mW
- 1.9 W
- 2.4 W
- 1.45 W
Show the answer and solution
Answer: B. 1.9 W
A linear regulator drops the difference: P = (Vin − Vout)·I = (24 − 5) × 0.1 = 1.9 W.
Other topics in the same subjects
DC circuits · AC circuits · Transients (RC, RL, RLC) · Semiconductors and diodes · BJT and FET circuits · Operational amplifiers · Oscillators and feedback · Digital electronics · Microprocessors and microcontrollers · Control systems · Instrumentation and measurement · Three-phase systems · Transformers · DC machines · Induction motors · Synchronous machines · Power systems and transmission · Protection: fuses, breakers, relays · Illumination · Power plant engineering · PEC: branch circuits, ampacity and protection
An independent, free reviewer: not affiliated with the PRC or the Boards. Subject weights and passing rules are from the PRC examination programs and the laws (RA 9292, RA 7920); check the current PRC program before you file. Code questions follow the Philippine Electrical Code 2017 Part 1; check them against your own copy of the Code.