I'm fairly sure is a zero voltage switching converter with active
voltage clamp. The series inductor and the 4uF capacitor form
a harmonic filter. The resonant switching circuit is the capacitor
in series with the upper switch, the primary inductance of the
transformer and the capacitor in parallel with the transformer.

So it can be devolved into a single switch feed forward converter
function from the lower switch and resonant energy recovery by the
upper switch... which results in a quasi push-pull output.

Search patents by E. Miyata in USPTO for more details.

Considering it's purpose, I'd expect this design's output power
is probably in the neighborhood of 750W. Don't confuse this with
the name plate rating. Of course the designer didn't consider
ICAS service so YMMV.

Search patents by E. Miyata in USPTO for more details.

I had a nagging feeling that I'd disremembered the circuit
Miyata developed and tried to look it up. Must be an IEEE
paper since there's no patent in his name that I can find.

But these patents may help: 6936803 and 6884977

This paper seems to describe a circuit close to that oven converter.
http://yakimov.homeftp.net/banditko/...ticles%20from%
20ieeexplore/01217760.pdf

It would be interesting to know the phasing of the
transformer windings.

Since it's voltage fed and there is no output inductor
I would have guessed it was a ZVS flyback supply with
active recovery of the leakage inductance energy. A
flyback is a constant power output topology by nature,
so it's a natural for powering a magnetron. But the
waveform in the pdf concering the oven looks like a
forward converter. Maybe it's the result of the active
clamp?

I've designed flyback converters with outputs of a couple
KW for TWT tubes but they were hard switched with multiple
rectified outputs on the transformer stacked series to
reduce the voltage stress. At low output voltages a flyback
is unsuitable for much power because of the high peak current
developed in the rectifiers and switches.

Sorry to babble on about this but I found the oven circuit
fascinating. The white goods guys are very clever.

I hope you can get it working as a power supply for your
amplifier.

On Sep 7, 10:18*pm, Grumpy The Mule wrote:
It would be interesting to know the phasing of the
transformer windings.

Since it's voltage fed and there is no output inductor
I would have guessed it was a ZVS flyback supply with
active recovery of the leakage inductance energy. *A
flyback is a constant power output topology by nature,
so it's a natural for powering a magnetron. *But the
waveform in the pdf concering the oven looks like a
forward converter. *Maybe it's the result of the active
clamp?

I've designed flyback converters with outputs of a couple
KW for TWT tubes but they were hard switched with multiple
rectified outputs on the transformer stacked series to
reduce the voltage stress. *At low output voltages a flyback
is unsuitable for much power because of the high peak current
developed in the rectifiers and switches.

Sorry to babble on about this but I found the oven circuit
fascinating. *The white goods guys are very clever.

I hope you can get it working as a power supply for your
amplifier.

I have my doubts about using it as is too, but I think there is a lot
to learn here.
For example I hae never used IGBTs before and already I hae begun to
learn. This may also be a chance for me to learn to design a
transformer that will work at 30Khz. I was thinking of using push-pull
driver on the primay of the transformer instead of the flyback
circuit. Something similar to a PC power supply. The Panasonic power
supply is definately designed for constant current. The input current
is monitored through a current transformer and this applied to the
contol circuit so that a decrease in input current results in a
greater output voltage through pulse width modulation. The plan is to
take this feedback from a coil on the output transformer instead of
the current transformer and apply it to the control circuit. I am
hoping this will turn the constant current power supply into a voltage
regulated power supply. Also the output rectifiers use a voltage
doubler. My plan is to change this to a full wave bridge, I think
David mentioned this too. I may not get full legal power out of it but
maybe I can power a couple of 4cx250s. That wouldnt be bad for a $50
power supply.

Jimmie

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.

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

raypsi wrote:

snip
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.


Thanks to jimmie for the PDF service manual; inspection of the
schematics shows differences between the block diagram (which
seems to show a resonant configuration) and the inverter detail
schematic (which shows a voltage doubler configuration). There
are also mistakes in wiring runs (circuit box outlines confused
as wires and having cross-connect dots misplaced, etc.). After
accounting for these, the above assumptions seem correct.

A quick web search for inverter microwave ovens seems to yield
only Panasonic models, and the cheapest in my area is $158.00
at Walmart. To me, this isn't really cost-effective, and they
are too new to be found in the alley. I intend to continue
to explore utilizing PC PSU parts to build an HV supply.

Michael

On Sep 8, 11:08*am, msg wrote:
raypsi wrote:

snip

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.

Thanks to jimmie for the PDF service manual; inspection of the
schematics shows differences between the block diagram (which
seems to show a resonant configuration) and the inverter detail
schematic (which shows a voltage doubler configuration). There
are also mistakes in wiring runs (circuit box outlines confused
as wires and having cross-connect dots misplaced, etc.). After
accounting for these, the above assumptions seem correct.

A quick web search for inverter microwave ovens seems to yield
only Panasonic models, and the cheapest in my area is $158.00
at Walmart. *To me, this isn't really cost-effective, and they
are too new to be found in the alley. *I intend to continue
to explore utilizing PC PSU parts to build an HV supply.

Michael

The part can be bought as a spare for about $40 or $50 dollars.
I have found a couple of them at Goodwill. I would have be tearing
into it but our 20 year old microwave finally bit the dust so my wife
put claim on this one.
I have also seen them setting on the curb on trash day. On was on my
way out and thought I would pick it up on my way back in. Didnt work
out. Since I started looking for them it seems there are two common
reasons people get rid of old microwaves. One is to have the latest ,
greatest model and the other is because the digital display quits
working. Any sort of failure out of warranty usually means a trip to
the curb.


THESE MUST BE OPERATED UNDER LOAD OR THEY WILL SMOKE.

At least that is what happend to mine.

Jimmie

raypsi wrote:

snip
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.


I'm sorry, I should have been more precise in my use of terms.

It's zero voltage switching or ZVS which is resonant transition.
The resonance is much higher than the swicthing frequency and it
isn't effected by the changes in duty cycle. Although it does
steal some on-time

Sometimes it's called quasi-resonant and there's a variant called
ZCS or zero current switching.

It's not a resonant converter like series or parallel resonant
converters where the output is controlled by shifting the frequency.

THESE MUST BE OPERATED UNDER LOAD OR THEY WILL SMOKE.

At least that is what happend to mine.

Jimmie


Oh, sorry to hear that. Yeah, it's a bad idea to operate
any switch mode power supply without a minimum load unless
the min. load is built-in or the thing is so inefficent that
its own losses make up the minimum load. The output diodes
probably didn't like that... just my guess.

I found some interesting stuff on the panasonic inverter.

On the fusor chat they claim it's 85% efficient and good for
1200W. The efficiency seems right, the 1200W seems... optimistic.

Also this, instructions on getting the inverter to play.
http://www.fusor.net/board/view.php?...er&key=1162079
046