From: Chris Carlen
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Subject: Re: 87% ... Design requirements are here
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Date: Fri, 27 Dec 2002 19:22:47 GMT
NNTP-Posting-Date: Fri, 27 Dec 2002 11:22:47 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Tony Williams wrote:
> Assuming I-coil discharges to zero every cycle.
[nice drawing snipped]
> Transformer= 1:1.9 stepup, with Lp = 44uH.
> An error amp compares Vout with Vref, and varies the frequency of the
> voltage-controlled oscillator over the range 0-75KHz. The oscillator
> flips an RS bistable to turn the FET On, and the Isense shunt/circuit
> flops the bistable Off whenever the coil current = 2.7A.
> ...and I'm not going to say where those design numbers came from...
> it involved various runic symbols and casting of entrails.
> Tickle the 2.7A up slightly for more power.
> The diode back over the top clamps the flyback-V on the FET due to
> leakage inductance.
The autotransformer is interesting. But since the leakage inductance
can send the drain voltage all the way up to Vout before it gets
clamped, what is the advantage over a plain inductor which has no
leakage in the first place? I mean, the FET voltage stress is the same
in both cases. Also, it would seem that the plain inductor approach has
lower winding capacitance (or does it, you probably know better than
me), so that less energy would be left over to ring after most had been
dumped through the diode into the output capacitor, leading to slightly
But the main drawback to a non-isolated output winding, and this is what
is making me lean strongly toward a flyback rather than simple boost
topology for the design, is that if someone shorts the output, your
circuit will draw a lot of current, either causing a fault in whatever
is providing the raw DC, or burning out the diode or transformer or both.
In order to handle the requirements of dealing appropriately with a
short circuited output, it seems there is no alternative but to have a
Thanks for the input.
Christopher R. Carlen
Suse 8.1 Linux 2.4.19