From: "Christopher R. Carlen"
Subject: Re: 87% All that vector calculus paid off
Date: Mon, 23 Dec 2002 09:29:16 -0800
Organization: Sandia National Laboratories, Albuquerque, NM USA
References: <firstname.lastname@example.org> <3E06A99F.776F9A81@ieee.org>
NNTP-Posting-Date: Mon, 23 Dec 2002 16:27:44 +0000 (UTC)
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> This effect has been reported in the literature, and, yes, it can
> seriously disturb the control to output transfer function, such that
> the error amp will dither erratically up and down between the two
> nearest steps. Not to worry, though, there are several possible fixes.
> The ringing occurs as 1/2*C*V^2 energy sloshes back and forth between
> the turned-off FET drain, diode, and other stray capacitances and the
> inductor. As it rings down it is all dissipated uselessly anyway, so
> why not dampen out the ringing and avoid the "stair steps" altogether?
> The simplest way to do this is to add a series RC network between the
> FET drain and ground. The capacitance should be about three times the
> average capacitance on the drain node (it varies with voltage - check
> the data sheets for the FET and diode) and the resistor should be about
> the sqrt(L/C) where L is the value of your boost inductor and C is the
> drain node capacitance. From your scope images, unless I'm misreading
> the time base, it looks like the ringing frequency is about 1.6MHz.
> If you know the value of your inductor at this frequency you can use
> it and the resonant frequency to calculate the unknown node capacitance.
I was hoping to avoid a snubber, but I will try it and see how much it
decreased efficiency. I also wonder how this connects to the decision
making process about whether to use a "transformer" instead of just an
What's also strange is that I noticed on a previous experiment (the one
on my web site) with a not quite right inductor, the slope of the output
voltage vs. duty cycle was slightly stepped, but the effect was very
subtle and more importantly, while tuning the duty% I could smoothly
walk the cutoff point of the last ring cycle through any part of the
cycle, but with the present setup, the effect is almost "quantized", and
there is almost *no* change in output voltage, until the last ring cycle
either appears or disappears suddenly, then the voltage changes a very
I'd love to sit down and figure all this out someday in depth, but I
think for the short term, I'm just looking for a fix.
> This approach *will* work, but it adds to the f*C*V^2 losses, thus
> reducing efficiency somewhat. If you are willing to live with a
> variable operating frequency, you can actually avoid ringing
> altogether and *increase* efficiency. Interested?
Yes, please explain. Tony suggested a "constant inductor peak current"
method. Is this what your thinking too?
Thanks for the input.
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA