Hi Bob:
> A company I represent has a problem with their switching power supply.
> A single point failure in the regulator can cause the output voltage to
> rise higher than the voltage ratings of the electrolytic capacitors. As
> a result, the capacitors are overstressed and fail in a variety of ways
> depending upon the capacitor venting method. Sometimes the vent will
> pop and thats the end of it. Other times, the capacitor will overheat
> and expel ethylene glycol in the form of vapor or liquid. In some
> cases, the conductive fluid will bridge the primary circuits to earth
> causing a failure of the hypot test. This is unacceptable for IEC950
> compliance.
In my experience, the power supply is so designed that a failure does
NOT cause the output voltage to exceed the voltage rating of the
electrolytic capacitors.
> I have spoken to several power supply designers and they inform me that
> it is common practice not to provide overvoltage protection. Of course,
> if that is true, then there appears to be a lot of power supplies in
> the world, perhaps including the one in my pc, that may or may not pass
> the fault testing of IEC950 depending on how the electrolytic
> capacitors fail. Obviously, my client does not want the extra cost of
> adding overvoltage protection when it appears that others dont do it.
I've not seen power supplies with specific overvoltage protection
for the electrolytic capacitors. We typically spec an output voltage
range that includes maximum voltage under single-fault conditions. By
so doing, the regulation scheme, including regulation under fault
conditions, provides protection of the electrolytic caps.
Your client will have to fix the power supply. (Others DO provide
for protection against the effects of capacitor venting under fault
conditions!)
Best regards,
Rich
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