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

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

On Sep 8, 7:13*pm, Grumpy The Mule wrote:
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?..._hvpower&key=1...
046 *

Hey mule:
Your url got cropped.

The inverter PS I have can be fooled into thinking it has a load.

There is a current sense I found and a voltage sense
both on the primary side.
It's a piece of cake opto coupling a signal back
to those sense inputs. they are both straight voltage dividers
with Vrefs. And the current sense uses a 50K ohm trim pot for
adjustment.

They use a 100meg bleeder for a load. And the filament which is at
least
150 watts, loads the supply too.

I suspect my 120VAC inverter will work off of 240VAC

And if you take out the filament circuit that gives yoiu an extra 150
watts.
I suspect I can gits 1500 watts from my 1300 watt interer PS, with
input
filtering of the DC off the mains.

But the final report wont happen until i gits the feedback circuit
wired.
That will prevent it from trying to produce 1300 watts into no load.
With no load it will try to up the voltage to produce more current.
Which it can't
produce more current so it keeps increasing the voltage untill it lets
out the smoke,

When it hits 3000 volts I will fool the sense circuits: a voltage
divider for
the current sense and another divider for the voltage sense, into
seeing 1300 watts.

Another trick is to run the mains off a variac starting at 10 VAC very
slow. Also supply seperate power to the PWM IC circuit,
Just to see how far it will go, and not get it to go into run away.

I see the power trouble how can you gits 1300 watts out of that little
bitty core?
That's what they used to say about silicone diodes when they used to
replace a 5U4 tube.
They used to tell me how can a little piece of wire with a bead on it
replace a tube as big as your fist
no freaking way. I also made a living selling 2.5 amp 1000 piv diodes.

I mean the primary of this thing uses litz wire that looks like 10AWG
wire about 13 turns around the core
and they using 4 strands of 28 AWG to run the filament
The HV secondary measures only 6.8 ohms DC.


73 OM

n8zu

Ok, this should work. http://tinyurl.com/6at4zu

I mean the primary of this thing uses litz wire that looks like 10AWG
wire about 13 turns around the core
and they using 4 strands of 28 AWG to run the filament
The HV secondary measures only 6.8 ohms DC.


I'm glad you mentioned that. If the core is ferrite and it's gapped,
then the litz makes sense to reduce eddy current losses from the
fringing field sprayed out of the gap. A gapped core would also most
likely mean it is a flyback.

It really is amazing the power density SMPS can achieve. Though the
overload capacity available from big hunks of iron and copper isn't
to be discounted lightly. I had to pick up a 10KW autotransformer
today. I don't know how much it weighed but I'd guess 130lbs, or
there about. The two transformers in the 10KW SMPS are maybe four
pounds total.

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.

Oh!

I forgot to mention soft-switching, that's the
most common catch-all term.