On Sep 8, 1:13 am, Grumpy The Mule wrote:
If it's a flyback then push-pull drive won't
provide enough voltage. In a flyback the E*T
products of each half cycle have to be equal
or the transformer saturates. You see a flyback
transformer isn't really a transformer. It's
a coupled inductor. The switch charges the core
then the core discharges into the secondary.
Current doesn't flow in both primary and secondary
at the same time.

So the output voltage isn't a function of the
turns ratio but of the ratio of switch's on and
off times. A flyback with voltage feedback control
regulates the output voltage regardless of the
turns ratio. Other considerations demand more
turns for high output voltages, mainly the
secondary voltage is reflected to the primary
and added to the input bus increasing voltage
stress on the switch. The primary turns are
determined by the factors effecting core saturation.

All this leads me back around to thinking it's a single
ended forward converter with resonant reset into the
seconday.

Even if it is some form of ZVS flyback it may be possible
to convert the oven power supply to a voltage regulated
converter with either a voltage divider on the output
or a voltage sensing winding with fewer turns. Because
the volts per turn in the secondaries are fixed by the
control this works well. If the sense winding has 100
times fewer turns the voltage developed by it will be 100
times less. There are some errors due to imperfect
coupling between the windings and the rectifiers Vf but
these can be tweaked out enough to get the regulation
down about 3% with some reasonable minimim load maintained.

There is another topology called a fly-forward that
transfers energy during both the off and on times of
the switch to the secondary. It's bascially a single
ended forward converter that resets the magnetizing
energy into the secondary in flyback mode. I'm
convinced the oven uses resonant switching though so
I doubt that's being used here.

The reason for the very small input filter capacitor
is power factor. With the small filter capacitor the
line current follows the envelop of the switch current.
If the duty cycle is constant over a half cycle then
the current waveform is determined by the line voltage
and then you have nice haversines of current drawn from
the line. If the voltage control circuit response is
fast, then the current will be distorted. With a larger
input filter capacitor the current will look like the
typical uncorrected SMPS with a capacitor input and draw
huge current spikes at the peak of the line as the capacitor
is charged. Then you must draw less power from the line
to account for the low power factor. The input rectifier
might have to be beefed up as well and more agressive inrush
limiting would be needed too.

I hope it's a forward converter then the transformer would
be very handy. I may have to start snagging ovens from the
curb side on trash day. I have a bucket full of 4CX250B
(really, a bucket full) waiting for a home.

And yes, IGBT's rock.

I agree it's not a flyback.

I don't think it's resonant converter because you looky at VK3HZ's
data on loading it, the duty cycle would take it way out of resonant.

As for power factor with lots of input capacitor filtering, a double
pi AC filter
with toroids would smooth the fawk out of any huge SMPS input
capacitor filter.
When I put a triple pi toroid on the AC line to my 500 watt SMPS
computer
tower, what a big difference, knocked out all kinds of harmonics and
spikes.
And reduce my electric bill. Is that like stealing from the electric
co?

73
n8zu