CET Power Supply Design & Repair 3 — Questions and Answers
Question 1: What is the purpose of a snubber circuit across a switching transistor in an SMPS?
- Increase switching frequency
- Suppress voltage spikes caused by transformer leakage inductance (Correct answer)
- Boost output current capacity
- Provide soft-start capability
Correct answer: Suppress voltage spikes caused by transformer leakage inductance
Snubber circuits (RC or RCD networks) clamp the voltage transient that occurs when the switching transistor turns off and the transformer leakage inductance rings.
Question 2: Which measurement tool is best suited for evaluating the ESR of electrolytic capacitors in a power supply?
- Standard DMM on capacitance range
- ESR meter or LCR meter with ESR function (Correct answer)
- Oscilloscope only
- Simple ohmmeter
Correct answer: ESR meter or LCR meter with ESR function
An ESR meter measures equivalent series resistance of capacitors in-circuit or out, which a standard DMM capacitance function cannot accurately determine.
Question 3: A 78xx series three-terminal regulator requires a minimum input-to-output voltage differential to regulate properly. This is called the:
- Ripple rejection ratio
- Dropout voltage (Correct answer)
- Line regulation
- Load regulation
Correct answer: Dropout voltage
Dropout voltage is the minimum voltage difference between input and output required for a linear regulator to maintain regulation; for 78xx regulators it is typically 2–3 V.
Question 4: When testing a power supply under load and the output voltage drops significantly, which parameter is being evaluated?
- Line regulation
- Load regulation (Correct answer)
- Ripple rejection
- Transient response
Correct answer: Load regulation
Load regulation measures how much the output voltage changes as load current varies from minimum to maximum.
Question 5: What is the primary safety hazard when working on the primary side of a plugged-in switching power supply?
- Low-voltage DC shock
- High-voltage AC/DC present even when power switch is off (Correct answer)
- RF interference
- Overheating of output inductors
Correct answer: High-voltage AC/DC present even when power switch is off
Large filter capacitors on the primary side retain dangerous voltages (300–400 V DC) even after the unit is unplugged and the power switch is off.
Question 6: In a flyback converter, energy is stored in the transformer core during which switch state?
- When the switch is OFF
- When the switch is ON (Correct answer)
- During the blanking time
- Only at startup
Correct answer: When the switch is ON
In a flyback converter, energy is stored in the transformer's gapped core while the primary switch is ON, then transferred to the secondary when the switch turns OFF.
Question 7: A power supply produces a loud high-pitched whine under load. What is the most likely cause?
- Faulty output rectifier
- Loose or saturating transformer/inductor core (Correct answer)
- Open input fuse
- Failed PWM IC
Correct answer: Loose or saturating transformer/inductor core
Magnetostriction in transformer or inductor cores, especially when a core is saturating or mounting is loose, causes audible noise at the switching frequency.
CET Power Supply Design & Repair 3
What is the purpose of a snubber circuit across a switching transistor in an SMPS?