Power Supply Design & Repair Flashcards
7 cards from real CET practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Power Supply Design & Repair flashcards as text
What is the function of the opto-isolator in a typical isolated SMPS feedback loop?
Answer: Transfer the error signal from secondary to primary while maintaining galvanic isolation
The opto-isolator couples the voltage error signal from the isolated secondary side back to the primary-side PWM controller without a direct electrical connection.
Which type of capacitor is preferred for high-frequency decoupling on the output of an SMPS due to its low ESR and ESL?
Answer: Ceramic (MLCC)
Multilayer ceramic capacitors (MLCCs) have very low ESR and ESL, making them ideal for high-frequency bypassing on switching supply outputs.
When a diode in a full-wave bridge rectifier is open (failed), the output will exhibit:
Answer: Full-frequency ripple with reduced DC output and increased ripple
One open diode converts the bridge to a half-wave rectifier, resulting in larger ripple at the input frequency and a lower average DC output voltage.
What does 'power factor correction (PFC)' accomplish in a modern SMPS design?
Answer: Makes the input current waveform more sinusoidal to reduce reactive power draw
PFC circuits shape the input current to be nearly sinusoidal and in phase with the AC voltage, minimizing reactive power and meeting regulatory requirements.
A technician finds a MOSFET in an SMPS shorted drain-to-source. What is the most probable root cause?
Answer: Overvoltage spike exceeding MOSFET Vds rating
Voltage spikes from transformer leakage inductance or poor snubbing can exceed the MOSFET's drain-to-source voltage rating, causing avalanche breakdown and short-circuit failure.
What is the purpose of a 'soft-start' feature in a power supply controller?
Answer: Gradually ramp up duty cycle at startup to prevent inrush current and component stress
Soft-start slowly increases the PWM duty cycle during startup, limiting inrush current and avoiding transformer saturation or output overshoot.
For a bench power supply, what does the 'CV/CC' mode indicator mean?
Answer: Constant Voltage / Constant Current operating modes
CV (Constant Voltage) mode maintains set voltage; the supply switches to CC (Constant Current) mode when load current reaches the current limit set point.