From: Tony Williams
Subject: Re: 87% ... Design requirements are here
Date: Fri, 27 Dec 2002 22:52:03 +0000 (GMT)
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NNTP-Posting-Date: Fri, 27 Dec 2002 22:52:28 +0000 (UTC)
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In article ,
Chris Carlen wrote:
> 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.
I was being lazy..... because I already know that you
have a FET that can withstand 400V. A previous thread
some months ago suggested a FET clamping diode into
a cap for this purpose. This would limit V-FET to
120V (see below).
> 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
> better efficiency.
I stuck my neck out from this direction.........
At Vin=10V and Vout= 350V the duty cycle would
be 35:1, and/or T-flyback would be about 340ns,
which I thought was too short. So I set T-flyback
at 1uS minimum, and the rest followed (sort of)....
We know from your 1.8A/100uH measurements that the inductor
can charge/discharge 162uJ per with acceptable efficiency.
For 12W that means not less than 75000 charges plus
discharges per second.
At Vin=10V and Pout=12W there is no dead time.
This is an important design end-point.
The 75KHz cycle time (13.3us) is totally consumed
by (T-On and T-flyback). ie, 13.3 = (12.3 + 1) uS.
That's an On duty-cycle of 0.92.
Assume that at 10V the current-shunt and IR stray
drops add up to 0.5V loss at I-peak, or 0.25V mean.
At 12W and 9.75V, I-in = 1.23A Avg.
2*1.23/0.92 gives I-pk = 2.67A (2.7A).
Lp = 9.75V*12.3uS/2.7A = 44.4uH.
At the end-point, Vin*T-On = V-flyback*T-Off.
Vin*T-On is fixed at L*I-pk, and T-Off= 1uS.
This gives V-flyback = 120V for a single 44uH winding.
For 350V an extra 1.9x winding is req'd in series.
Note that the flyback is a const-I output, so at 90V
T-off automatically adjusts to about 4uS and the
feedback reduces the frequency as required.
> 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.
Stick a 2A fuse in series with Vin?