PicDude wrote:

>Hi all,
>
>Late last night, while building a 3-output power supply, a capacitor burned 
>out (it just made a poof sound and the electrolyte leaked out) and it's 
>totally baffling me.
>
>I built the 12V stage, tested it under a load, and it worked fine.  Then I 
>built the 5V stage, and that worked fine under load.  When I built the final, 
>3.3V stage and was testing it, the input capacitor for the 5V stage died.  
>This time though, the 5V stage was NOT under load.  I immediately thought 
>that the output capacitor must've died since it was not under load and 
>perhaps the output voltage went too high, but it was actually the input 
>capacitor (Nichicon 330uf, 35V, low-ESR electrolytic).
>
>In more detail, the input voltage (which in this case was 14V from a switching 
>power supply) is split in 3 directions, each going to a separate SB540 
>schottky diode (for reverse-voltage protection), then to the switching 
>circuitry for each stage.  The 5V switcher is an OnSemi MC33167, with a 330uf 
>35V input capacitor.  Because there are reverse-protection diodes on there, I 
>can't see how anything happening on either of the other stages could cause a 
>reverse voltage to the 330uf cap.  And since my input is 14V, that cap was 
>receiving a supply of ~13.5V (after the reverse-protection diode).
>  
>

I'll assume your 5v switcher is a buck topology - what frequency is it
and what size inductor did you use before the cap?
--
Martin Klingensmith
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