I have a number of PC power supplies that have failed. They are easy enough to replace, but the reasons for failure are unclear. So I decided to tear one down to investigate.
Enermax Noisetaker 420W power supply (designed and manufactured in 2004-2005) looks nice on the outside, but it failed shortly and generated no output. I opened it up before and found the fuse blown. So I replaced the fuse (250V 10A), it was blown violently as soon as I plugged in the AC. So there is a short somewhere. Despite of the massive heatsinks, the one-layer power supply PCB is surprisingly easy to desolder. I find inside the familiar forward converter topology. There are two separately transformers, one for main power and the other for standby power (5V). Two FET's in parallel drive the primary, controlled by UC3842 with opto feedback. Schottky diode pairs are on the secondary. A magnetic amplifier is there to improve regulation. A 7912 is used for -12V. One IC monitors the outputs. There is one custom IC with Enermax logo; it appears to be one to throttle the fans depending the temperature.
Not long I found the culprit, a diode (HER208) connected to the reset coil of the primary failed short, effectively shorted the rectified DC bus. A nice power supply is brought down by a lowly diode. Then how did the diode fail? HER208 is rated 1000V 30A peak. There is no obvious damage on the outside.
Now I have in hand a nice collection of capacitors, inductors, transformers, FETs and diodes.
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