From: Tony Williams
Subject: Re: 87% ... Design requirements are here
Date: Sat, 28 Dec 2002 09:23:41 +0000 (GMT)
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NNTP-Posting-Date: Sat, 28 Dec 2002 09:36:11 +0000 (UTC)
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In article ,
Chris Carlen wrote:
> But what is interesting me now, is the idea of zero voltage switching,
> or constant off time switching (variable frequency with off time tuned
> to put the switch-on moment at the zero voltage point of the ring down
> period with inductor current flowing in the optimum direction). It
> seems that for a variable output voltage and variable load supply, that
> constant off time can't work, because the flyback time is uncertain.
> Thus it would seem necessary to implement a circuit that implements
> dynamic zero voltage switching, always turning the FET back on at the
> first zero voltage after the flyback.
Zero voltage switching has been used for years,
particularly on old-fashioned self-timed push-pull
transistor saturating dc-dc converters, and to a
certain extent in transistor blocking oscillators.
In a self-timed system the feedback winding starts
to bring the transistor On at the first inductor-
-driven downward swing. In addition to that though,
when the inductor drives the collector negative it
inherently pre- forward biasses the base, ready for
the official On-signal from the feedback winding, a
uS or so later.
That was a useful speed-enhancer for transistors, not
neccessarily important to MOSFETs.
> But then there is the question, which zero voltage point
> is the right one?
The flyback dumps energy into the output cap at 350V
and at the very end of the stroke the diode turns Off,
leaving the Cstray on the end of the inductor charged
up to 350V. The energy stored there is 0.5CV-squared,
or about 6uJ for every 100pF at 350V. That energy is
lost during the subsequent damped L-C ringings.
So if you are going to recover that energy it would be
best done on the first ring, at the negative peak on
the coil, when the voltage across Cstray is zero.
How that could be done is a subject for someone far more
expert than me.