From: "Harry Dellamano"
Subject: Re: Strange failure mode in switchmode power supplies: Followup
Date: Fri, 20 Sep 2002 15:56:28 -0700
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"John Woodgate" wrote in message
> I read in sci.electronics.design that Harry Dellamano
> wrote (in )
> about 'Strange failure mode in switchmode power supplies: Followup', on
> Fri, 20 Sep 2002:
> > Ok John,
> > Lets say the line is 1.6mH and the SMPS input EMI filter is .16uF. They
> >will resonate at 10KHz with a Z of 100 ohms which is less than the input
> >of the SMPS so we have a Q >1 and smoke.
> > What say John?
> I say 1.6 mH is far too big. European 230 V supplies, which are fed over
> long cables, come out around 800uH. US supplies, from relatively small
> pole-pig transformers and short supply cables, are undoubtedly a lot
> less inductive. I even have some figures, supplied by a US utility
> engineer for a 15 kVA transformer:
> R Ohms 0.03270
> X Ohms 0.02834
> Z Ohms 0.04328
> This is one 120 V secondary winding; the higher impedance of the two.
> 0.02834 ohms is 75 uH at 60 Hz. The service cable will add a bit to
> that, but not a lot.
> Regards, John Woodgate, OOO - Own Opinions Only.
Exactly, normal operation as you point out we have no problem but if the
this particular line may have gained some inductance then we have a problem.
This is not a normal line. We are looking for needles in haystacks,
something that is not on the original prints, parasitics. The chances of
this guess being true is probably one out of twenty but you have to guess at
something, following your nose with good measurements techniques until the
truth is learned. When we do learn the truth, we will all laugh at how
stupid we were and should have found it sooner.
Oh, by the way, if that Pole Pig is a step down then the primary is much
higher inductance and if one was left in series with this line then we are
I believe that I am preaching to the choir. You are a more learned man than