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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: PSU ripple current
Date: Wed, 15 Jan 2003 13:39:29 -0500
NNTP-Posting-Date: Wed, 15 Jan 2003 13:38:57 EST
Organization: Bell Sympatico
John Woodgate wrote:
> I read in sci.electronics.design that Mike Monett
> wrote (in <3E256FD4.3B35@sneakemail.com>)
> about 'PSU ripple current', on Wed, 15 Jan 2003:
> >The harmonics are a substantial percent of the fundamental. You cannot
> >ignore them and expect to get accurate answers for ripple current, which
> >is what this thread is about.
> Well, why do we want to know the ripple current? To determine whether
> the I^2R loss in the capacitor will overheat it (even if we do that by
> comparing our ripple current figure with that in the cap manufacturer's
> If we have 20% third harmonic content in the current waveform, it
> contributes 4% of the power.
> The rounded tip on the positive-going 'tooth' of the (blunt) saw-tooth
> waveform actually represents a lower content of very high order
> harmonics than are contained in the approximative (sharp) saw-tooth
> The conduction angle (part of the half-cycle during which the diodes
> conduct) of a typical rectifier is 55 degrees (less for a direct on-line
> rectifier, more for one fed from a low-power transformer). From this,
> the amplitudes of the fundamental and harmonics of the (sharp) saw-tooth
> voltage waveform, and of the capacitor current waveform, can be
> Regards, John Woodgate
The conduction angle changes dramatically with source and load impedance.
This changes the harmonic content.
The harmonics add to the peak current, which is an important limit in
There is little reason to use approximation methods and rule of thumb
calculations when SPICE is freely available.
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