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've not re-connected it for testing as I'd like to know what I should 
check/test for.  So far, I've verified that that cap and other components 
were soldered in with the correct polarity, etc.  Also, can/should I test 
with the existing (blown) cap in place, or should I replace it first?  I 
assume the latter.  I know that switchers need a minimum load, and I did make 
provisions for this on the PCB, but do not have a min load resistor on there 
yet.  How would I determine the min load required?  The datasheet does not 
indicate how to determine this.  Let me know if you need other info/circuit 
diagrams, etc.

Much thanks,
-Neil.



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