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From: "Phil Allison"
Subject: Re: PSU ripple current
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Wed, 15 Jan 2003 18:41:48 +1100
NNTP-Posting-Date: Wed, 15 Jan 2003 18:30:57 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Mike Monett" wrote in message
> John Woodgate wrote:
> > I read in sci.electronics.design that Phil Allison
> > wrote (in ) about 'PSU
> > ripple current', on Tue, 14 Jan 2003:
> > >BTW If the ripple voltage were a really a symmetrical triangle wave
> > >by differentiation the ripple current in the cap *must* be a square
> > >value equal to the DC load current.
> > >
> > > The normal ripple wave is a sawtooth with steep rise and
> > >fall and this differentiates to a pluse wave with a mark space ratio of
> > >about 3:1. The average of this wave must be zero for the cap to stay
> > >the same voltage so the charge current must be 3 times the
scharge - ie
> > >+3 amps for 2.5mS followed by - 1 amp for 7.5mS.
> > >
> > > This would have an rms value of 1.73 amps.
> > He keeps doing this: super stuff. But WHY can't he refuse to get
> > involved in flame wars?
> I just put this in SPICE. The ripple current is similar to a half sine
> wave followed by a flat discharge.
** It is NOT half sine. The rises and falls are much steeper.
More like a U bend.
The shape of the half sine changes
> with any change to the circuit, including leakage inductance of the
> transformer, source resistance, load current, and of course, cap value.
> Although the charge current can change shape dramatically, it never
> resembles a square wave, or even comes close.
** It is close enough to a rectangular pulse, as far rms value is
> The rational for the estimate given above is faulty.
** The rationale is not flawed - the ripple voltage is just not exactly
The ripple current is estimated to within about 10 % with the dv/dt
figures read off a CRO screen.
>> Any similarity to the actual result is purely coincidental.
That is a massive overstatment - like your other remarks.
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