From: Chris Carlen
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Subject: Re: My flyback experiment is on the web
References: <3DAA0563.9040507@BOGUS.earthlink.net> <firstname.lastname@example.org>
Date: Tue, 15 Oct 2002 03:09:17 GMT
NNTP-Posting-Date: Mon, 14 Oct 2002 20:09:17 PDT
John Larkin wrote:
> is the torroid core yellow? Sort of looks like that in the photo. If
> so, it's probably a dreadful Micrometals powdered-iron core.
> It also looks, from the waveforms, like you may have a lot of fet
> drain capacitance... I'm too lazy to look it up. That will waste
> energy, specifically 1/2 * Cd * Vp^2 every cycle. This is a, maybe
> *the* serious problem in high-step-ratio converters like this; at some
> point, all the energy charges the fet and vout becomes limited to the
> flyback peak. Then you need a transformer.
Hi. Thanks for the input.
Yes, I have deduced from the main ring-down frequency and the inductor
that the capacitance around the inductor is 320pF, with about 170pF
coming from the IRF730 (datasheet). So that gives 6.4uJ of energy left
in the capacitance per cycle. At 50kHz that's a waste of 0.32W, which
when added to the FET Ron=1ohm loss (plus 1ohm current measuring
resistor) of about (0.33A)^2*(2ohm)=0.22W, for a sum of 0.53W from these
But I am pumping in (18V)(0.33A)=6W, and getting out 4W, so the total
power loss is 2W. Subtracting what we've accounted for in the FET
resistance and leftover in the Cd, that's 2W - 0.53W = 1.47W.
I seems pretty likely that the toasty warm inductor core is where that
remaining 1.47W is going. The inductor sure feels a lot warmer than
just 0.32W from the ring down, which probably all gets eaten by the
inductor as well.
I think it's time to take Bob's advice and buy real cores.
Christopher R. Carlen
Suse 7.3 Linux 2.4.10