From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: PSU ripple current
Date: Tue, 14 Jan 2003 22:18:25 -0500
NNTP-Posting-Date: Tue, 14 Jan 2003 22:17:54 EST
Organization: Bell Sympatico
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 then
> >by differentiation the ripple current in the cap *must* be a square wave of
> >value equal to the DC load current.
> > The normal ripple wave is a sawtooth with steep rise and slower
> >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 at
> >the same voltage so the charge current must be 3 times the discharge - 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?
> Regards, John Woodgate
I just put this in SPICE. The ripple current is similar to a half sine
wave followed by a flat discharge. 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.
The rational for the estimate given above is faulty. Any similarity to
the actual result is purely coincidental.