From: John Woodgate
Subject: Re: Strange failure mode in switchmode power supplies: Followup
Date: Fri, 20 Sep 2002 22:07:02 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Fri, 20 Sep 2002 21:11:27 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
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 Z
>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. http://www.jmwa.demon.co.uk
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