CET Power Supply Design & Repair 5

Question 1

In power supply design, what does 'line regulation' specify?

Accepted Answer

Output voltage change with varying AC input voltage

Answer

Line regulation measures how much the output voltage changes when the AC input (line) voltage varies across its specified range.

Question 2

A power supply's output drops to zero when a certain load is connected, then recovers when the load is removed. This behavior indicates:

Accepted Answer

Overcurrent protection (OCP) or foldback current limiting is tripping

Answer

Overcurrent or foldback protection shuts down or reduces output when load current exceeds the set limit, recovering when the overload is removed.

Question 3

What is the advantage of a synchronous rectifier over a standard Schottky diode in a low-voltage SMPS output?

Accepted Answer

Lower forward voltage drop, reducing conduction losses and improving efficiency

Answer

A synchronous MOSFET rectifier has an effective forward drop of only millivolts (Rds_on × I) versus 0.3–0.5 V for a Schottky, significantly improving efficiency at high currents.

Question 4

When measuring transformer secondary winding continuity with an ohmmeter, a technician reads infinite resistance. This indicates:

Accepted Answer

Open winding — likely a blown fuse link or broken wire

Answer

An infinite resistance reading on a winding indicates a break in the wire, often an internal fuse link that opens under fault conditions.

Question 5

Which switching frequency range is most commonly used in modern SMPS designs as a balance between component size and switching losses?

Accepted Answer

50–500 kHz

Answer

Most SMPS designs operate between 50 kHz and 500 kHz, where transformer and capacitor sizes are manageable and switching losses are not yet dominant.

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CET Power Supply Design & Repair 5