for Switching Power Supplies

Trace the problem quickly with just a scope and a DC meter.

by Ed F. Rice W9NGP

Ever bad trouble with your switching power supply, butdidn't know whereto start to find the problem? Well, the troubleshooting guide in Figure 4 presents a sequence of tests to follow when there is no output from a switching supply, It outlines strategic tests that quickly eliminate testing of whole sections of the circuitry at once, so the defect can be quickly isolated without the nee^j to test a large number of individual components.

Switching supplies are subject to all the failures ol conventional supplies: destruction of diodes in the input rectifier due to transients on the AC line; chokes, diodes, and transistors damaged by shorted load I because of the high efficiency of switching supplies, a shaded output results in a very high voltage elsewhere in the circuit); and overheating due to inadequate ventilation or skimpy heat sinks.

Switching Supply Components

The block diagram of a switching power supply in Figure t shows the overall relationships between the circuit divisions.

The input rectifier is often a simple Vi-wave system which rectifies the line voltage and sends low voltage DC to the sw itch signal generator and the AC power switch. A bridge rectifier is used in high power units.


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