CET Power Supply Design & Repair 4

Question 1

What is the function of the opto-isolator in a typical isolated SMPS feedback loop?

Accepted Answer

Transfer the error signal from secondary to primary while maintaining galvanic isolation

Answer

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.

Question 2

Which type of capacitor is preferred for high-frequency decoupling on the output of an SMPS due to its low ESR and ESL?

Accepted Answer

Ceramic (MLCC)

Answer

Multilayer ceramic capacitors (MLCCs) have very low ESR and ESL, making them ideal for high-frequency bypassing on switching supply outputs.

Question 3

When a diode in a full-wave bridge rectifier is open (failed), the output will exhibit:

Accepted Answer

Full-frequency ripple with reduced DC output and increased ripple

Answer

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.

Question 4

What does 'power factor correction (PFC)' accomplish in a modern SMPS design?

Accepted Answer

Makes the input current waveform more sinusoidal to reduce reactive power draw

Answer

PFC circuits shape the input current to be nearly sinusoidal and in phase with the AC voltage, minimizing reactive power and meeting regulatory requirements.

Question 5

A technician finds a MOSFET in an SMPS shorted drain-to-source. What is the most probable root cause?

Accepted Answer

Overvoltage spike exceeding MOSFET Vds rating

Answer

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.

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CET Power Supply Design & Repair 4