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From: "Phil Allison"
Subject: Re: PSU ripple current
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Tue, 14 Jan 2003 11:33:28 +1100
NNTP-Posting-Date: Tue, 14 Jan 2003 11:24:18 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"steve c l" wrote in message
> Phil Allison wrote in message
> > ** We cannot see your maths or any figures, you may be making an
> > Say the cap is 4700uF and the voltage is 50 volts DC average. A
> > typical ripple voltage is 10 % so 5 volts p-p - OK ? For a triangle
> > wave the conversion factor is 0.57 of the peak so 2.5 x 0.57 = 1.42
> > rms approx. The reactance of a 4700uF cap is 0.34 ohms at 100 Hz so
> > dividing gives 4.3 amps.
> > I = C dv/dt so a 5 volt drop in 5 mS gives I = 4.7 amps average
> > load current.
> > Is this what you did ?
> > ................ Phil
> Very glad you've come back on this, Phil. Bar the numbers, it's exactly
> what I did, and you're right - I've blundered somewhere in the arithmetic.
** That is why I was asking about how you got an rms voltage figure.
> The text I referred to took dt = 10 (so did I), not 5. If 10 were used in
> your example, then the ripple rating would be reasonably close to twice
> load current! My result was about 4 times the load, so I went adrift
> somewhere. I'll check it out later.
> Bearing in mind, you prefer as a rule to measure the ripple rather than
> attempt to calculate it, do you consider this approach unreliable?
** Measurements are always better IME since components like caps can
vary so much from the published nominal values.
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.
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