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August 10, 2001

Trouble Shooting Power Supplies

Question: How do you troubleshoot a switching-mode power supply?

Original Question: How do you troubleshoot and repair a switching-mode power supply?

Answer: Over the years I have had dozens of electronic technicians work for me. By far the best at troubleshooting switching-mode power supplies had a very simple approach. He looked at it. Under a magnifying stand with good light he examined the circuitry with infinite patience. What he was looking for were opens in the conductors, open fuses, shorts, bad solder joints, signs of overheating such as discolored components, circuit boards, melted solder joints, etc. Suspicious areas were checked with an ohm-meter or continuity checker. Before ever applying power to the circuitry, he found 80% of the problems.

When it comes to applying power to the board, it gets a little tricky. First, you should have a good schematic of the circuit -- which may be the most difficult part. Judging from questions I receive from those trying to repair their computer power supply, getting a schematic is almost impossible. And the official schematic may be less than useful. Almost every power supply designer I know has a hand-drawn schematic they use for troubleshooting, almost never using the official schematic. With the schematic as a guide, you should slowly apply power to the board and make sure voltages are appearing where they should be. This will take some analysis of the schematic. If some voltage does not appear, this is probably the problem or at least part of it. The tricky part is that switching-mode power supplies do not like low voltages and are often damaged by them -- hence you have a risk management problem in troubleshooting. If you get this far, look at the reference and its divider network. A properly functioning power supply just regulates to the reference voltage or a ratio of it. If the reference commands the wrong voltage, the output will be wrong.

Obviously, this is too complex a subject to cover in a short email, but there is help on the web.

For more details, you might want to look at Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies

Posted by Jerrold Foutz at August 10, 2001 04:27 PM