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Reliability of very low voltage PSU caps

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    Reliability of very low voltage PSU caps

    I was fixing up a friends Dell tower and the crummy CapXon output caps in the PSU were bad. These were 2200uf/6.3V jobs. My experience, having fixed a huge amount of test equipment over the decades, is that electrolytic caps with less than a 15V or so rating are invariably the first to fail. I can't imagine designing 6.3V caps into anything!

    Not having a suitable replacement on hand, and knowing that low ESR is really the primary consideration in a switching supply, not value, and suspecting that the CapXon part has no special abilities in that area, I installed a couple 150uF OSCONs. These have ultra low ESR and huge ripple current capabilities. So far the PSU is working like a champ.

    Still, I wish the value were higher, though design-wise I'm not sure that anything justifies it. Just curious, what do other think about what I did? Also, does anybody have actual specifications on the CapXon junk?

    Best,
    Conrad

    #2
    Re: Reliability of very low voltage PSU caps

    Watch it. Uf value is very important, only 300uf will cause quick voltage drops due to bad transient response. Esr is important too, you want to stay as close as possible to the original value as the pi filters are "in tune" so to speak with the esr. Straying too far down or up can cause an increase in ripple. The increase in ripple current dosen't hurt though.

    The 6.3v caps filter the 3.3v rail so they are more than adequate.

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      #3
      Re: Reliability of very low voltage PSU caps

      Yes, those unusually tall and slim Capxon 2200uf 6.3v caps are always the first to go.
      Got that in quite a few 250W Delta units.
      Latest victim is a horrendously skimpy 400w FSP. Good job the liquid cooling system failed first :P

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