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Sounds like worst-case, but there’s danger lurking in the middle

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 550W max load. However, when fully operational, the power supply had a fan running off its main 12V 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 5V housekeeping rail. Unfortunately, it was left freestanding by an otherwise experienced engineer who thought its low power wasn't worth his while. Its temperature in Standby mode thus made history. But we also learned that the Japanese actually initiate the Standby function of their PCs rather than leaving the computer idle, as Americans would).

Also remember that a buck converter's input capacitor sees the maximum RMS current at D=0.5. If your input voltage range is say 15V to 48V, and you test it “diligently” at both the input corners, you still may never know how long your capacitor will really last. Or take an interleaved buck converter with two independent channels running out of phase. Here we are actually relying on both channels being fully loaded to reduce the input capacitor's RMS current. But in fact this current can be even higher if one channel is unloaded. Coming to magnetics, we also now know that the surface temperature of a core-loss dominated choke means nothing if there is a variable-speed fan present. Remember a few years ago when major manufacturers had field returns, in which the powdered iron chokes had virtually returned to their original powdered form because of prolonged core temperatures. This had then prompted vendors to scramble to characterize life expectancy. Try www.micrometals.com for more information.




Sanjaya Maniktala, author of the popular Planet Analog series on EMI in power supplies, has over 15 years of design experience with companies like Artesyn Technologies in the Bay Area and Siemens AG in Leipzig. He holds graduate degrees the Indian Institute of Technology, Bombay and Northwestern University, Illinois, as well as patents on the Floating Buck Regulator topology. This article kicks off a regular column at PlanetAnalog online. “Write me at sanjaya.maniktala@nsc.com with your favorite bedtime story!” he bids.

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