Sounds Like Worst-case, But Theres Danger Lurking in the Middle

Sounds Like Worst-case, But There s Danger Lurking in the Middle 

Sanjaya Maniktala describes a PC power supply failure that seemed widespread in Japan, but seldom occurred in the U.S. Was it something in the sushi he asks. It turns out, the failures are related to the way components are specified and tested. This amusing and instructive piece kicks off a new column on power supply design. Sanjaya s comments will appear monthly at Planet Analog online. 

In hindsight, this probably seemed routine enough to inspire some complacency. An OEM PC power supply was all set to go into full production. The Design Integrity Team put it through its paces. Maximum loads were applied at extreme ambients. Stress levels were verified, life predictions matched up, vibration testing, EMI, safety etc., were a cinch. The product was released and no problem was noticed in the several thousand units first shipped to the American market. Then the failures started to show up in droves. All came mysteriously from Japan. Must ve been something in the sushi  

What happened was actually so simple that several people must have kicked themselves (and each other). Turns out this power supply had not been tested in Standby mode Why test in Standby mode That s only a handful of watts compared to the 550 W max load. However, when fully operational, the power supply had a fan running off its main 12 V output. In Standby mode the fan stopped as all the outputs collapsed. Well, all outputs but one A small standby integrated converter was also present on-board delivering a low power 5 V 

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