microwave oven inverter P.S. revisited
 

Ugh! It's very similar to a power supply I worked on in the
mid-1970's for a tacan pulse power calibration system. The
scars had almost healed. There's not much advantage in it
over a vintage blood and thunder supply beyond improved
regulation. No reduction in the amount of steel or output
capacitance needed... lots of stored energy.

Though I did learn a lot about PUT's and SCR's in the process
so overall it was fun. It's also when I learned that power
supply engineers always run Towards the explosion.


steve H
http://www.realhamradio.com/hvreg.pdf

for a schematic.

microwave oven inverter P.S. revisited
 

On Sep 15, 8:09*pm, Grumpy The Mule wrote:
Ugh! *It's very similar to a power supply I worked on in the
mid-1970's for a tacan pulse power calibration system. *The
scars had almost healed. *There's not much advantage in it
over a vintage blood and thunder supply beyond improved
regulation. *No reduction in the amount of steel or output
capacitance needed... lots of stored energy.

Though I did learn a lot about PUT's and SCR's in the process
so overall it was fun. It's also when I learned that power
supply engineers always run Towards the explosion. *

steve H



http://www.realhamradio.com/hvreg.pdf

for a schematic.- Hide quoted text -

- Show quoted text -

I dont suppose there is an SMPS pwer supply design book for dummies
ou t there somwhere.
Seriously what would be a good book for transformer design. I
downloaded the info ferroxcube had. I was hoping they had someready
made transformers. I know they have made some for the military/gov,
but I didnt see anything off the shelf available except their cores.


Jimmie.

microwave oven inverter P.S. revisited
 

Howdy,


It's a tough choice, most of the books in the subject have at
least a couple of strong points or useful tricks.

If I could have only one book and it had to cover all aspects of
SMPS design I'd chose "Switching Power Supply Design" by Abraham
L. Pressman.

There are many individual books devoted to topology convolution,
wound magnetics design, control systems engineering and the care and
feeding of semiconductor switches. So no one book covers evey
aspect thoughoughly but Abe's book is pretty good.

He also wrote "Switching And Linear Power Supply Design, power
Converter Design", which was his first book. Worth having for
sometimes you might need a big linear supply.

"Switchmode Power Supply Handbook" by Keith Billings contains many
practical tips.

For magetics "Transformer And Inductor Design Handbook" by Colonel Wm.
T. McLyman is good. Another good book is Magnetics Engineering
Fundamentals and Computer Aided Design" by Fred M. Lillenstein.

"Transformers for Electronic Circuits" by Nathan Grossner is good
too, especially for heavy metal. There is a copy of it online
at some tube audio site but I can't find the link off hand.

Also check the free software and app notes at Magnetics Inc.
And at Micrometals Inc. too.

Some decent vendors of needed odds and ends are Pleo, lodestone
pacific, Robison Ind.,Dexter, MH&W. CWS Coil Winding Services
is worth checking out too.

For built transformers you'll mostly find low voltage and
telecom stuff off the shelf. Can't recall seeing any high
power (more than a couple of hundred watts) high voltage
transformers off the shelf. Even low power HV stuff is rare
unless you count the measly few hundreds of volts for EL
and neon lighting.

I really think this is a roll your own kind of affair. Furukawa
TEX-E triple insulated wire might be handy without a segmented
secondary bobbin.

I hope you find some of this useful. There are a LOT of books on
these subjects. Enough to cause my house to sag under the weight.



I dont suppose there is an SMPS pwer supply design book for dummies
ou t there somwhere.
Seriously what would be a good book for transformer design. I
downloaded the info ferroxcube had. I was hoping they had someready
made transformers. I know they have made some for the military/gov,
but I didnt see anything off the shelf available except their cores.


Jimmie.

microwave oven inverter P.S. revisited
 

On Sep 16, 8:15*pm, Grumpy The Mule wrote:
Howdy,

It's a tough choice, most of the books in the subject have at
least a couple of strong points or useful tricks.

If I could have only one book and it had to cover all aspects of
SMPS design I'd chose "Switching Power Supply Design" by Abraham
L. Pressman. *

There are many individual books devoted to topology convolution,
wound magnetics design, control systems engineering and the care and
feeding of semiconductor switches. *So no one book covers evey
aspect thoughoughly but Abe's book is pretty good. *

He also wrote "Switching And Linear Power Supply Design, power
Converter Design", which was his first book. *Worth having for
sometimes you might need a big linear supply.

"Switchmode Power Supply Handbook" by Keith Billings contains many
practical tips.

For magetics "Transformer And Inductor Design Handbook" by Colonel Wm.
T. McLyman is good. *Another good book is Magnetics Engineering
Fundamentals and Computer Aided Design" by Fred M. Lillenstein.

"Transformers for Electronic Circuits" by Nathan Grossner is good
too, especially for heavy metal. *There is a copy of it online
at some tube audio site but I can't find the link off hand.

Also check the free software and app notes at Magnetics Inc.
And at Micrometals Inc. too. *

Some decent vendors of needed odds and ends are Pleo, lodestone
pacific, Robison Ind.,Dexter, MH&W. CWS *Coil Winding Services
is worth checking out too.

For built transformers you'll mostly find low voltage and
telecom stuff off the shelf. *Can't recall seeing any high
power (more than a couple of hundred watts) high voltage
transformers off the shelf. *Even low power HV stuff is rare
unless you count the measly few hundreds of volts for EL
and neon lighting.

I really think this is a roll your own kind of affair. Furukawa
TEX-E triple insulated wire might be handy without a segmented
secondary bobbin.

I hope you find some of this useful. *There are a LOT of books on
these subjects. *Enough to cause my house to sag under the weight.



I dont suppose there is an SMPS pwer supply design book for dummies
ou t there somwhere.
Seriously what would be a good book for transformer design. I
downloaded the info ferroxcube had. I was hoping they had someready
made transformers. I know they have made some for the military/gov,
but I didnt see anything off the shelf available except their cores.

Jimmie.- Hide quoted text -

- Show quoted text -

My big problem at the moment is designinging a transformer. after much
reading in the last couple of days I am begining to think it is not
the big deal I thought it was. Since I am not trying to build 50k
units I dont have to use the cheapest thing I can get by with and can
overdesign a bit. I can see that just a few cores may take care of all
of my needs. Oh well its back to work so it will be a few days before
i get to piddle with this anymore. One of my jobs this week is to go
through all the old UPSs we have in the store room and find out which
ones are good and chunk the rest.

Jimmie

microwave oven inverter P.S. revisited
 

Howdy,

I found the URL for Grossner's book online.
http://tinyurl.com/3z7mxv


It seems to me that using the inverter oven transformer
and IGBT with a flyback controller like UC3842 might work.

Just a straight enough, hard-switched, flyback with single
ended output. You'd "only" get 1.5-2KV out less output power
maybe one half to three quarters as much... but that's not bad.

Mmm! Old UPS's have lots of good parts in them, especially
the ones with big 60Hz transformers and big heatsinks. Chuck
them into the trunk of your automobile.




My big problem at the moment is designinging a transformer. after much
reading in the last couple of days I am begining to think it is not
the big deal I thought it was. Since I am not trying to build 50k
units I dont have to use the cheapest thing I can get by with and can
overdesign a bit. I can see that just a few cores may take care of all
of my needs. Oh well its back to work so it will be a few days before
i get to piddle with this anymore. One of my jobs this week is to go
through all the old UPSs we have in the store room and find out which
ones are good and chunk the rest.

Jimmie

microwave oven inverter P.S. revisited
 

On Sep 17, 7:59*pm, Grumpy The Mule wrote:
Howdy,

I found the URL for Grossner's book online.http://tinyurl.com/3z7mxv

It seems to me that using the inverter oven transformer
and IGBT with a flyback controller like UC3842 might work.

Just a straight enough, hard-switched, flyback with single
ended output. *You'd "only" get 1.5-2KV out less output power
maybe one half to three quarters as much... but that's not bad.

Mmm! *Old UPS's have lots of good parts in them, especially
the ones with big 60Hz transformers and big heatsinks. *Chuck
them into the trunk of your automobile.



My big problem at the moment is designinging a transformer. after much
reading in the last couple of days I am begining to think it is not
the big deal I thought it was. Since I am not trying to build 50k
units I dont have to use the cheapest thing I can get by with and can
overdesign a bit. I can see that just a few cores may take care of all
of my needs. Oh well its back to work so it will be a few days before
i get to piddle with this anymore. One of my jobs this week is to go
through all the old UPSs we have in the store room and find out which
ones are good and chunk the rest.

Jimmie- Hide quoted text -

- Show quoted text -



I was reading up on push-pull topology of switching power supplies
and see that they have problems with flux imbalance. I used to work on
some power supplies that were push pull when I was in the USAF and the
driver transistors were always failing, now I know why. I see that
this isnt as much of a problem for FETs, How about IGBTs.


Jimmie

microwave oven inverter P.S. revisited
 

Short answer, IGBTs are much better but not as good as MOSFETs.
In many cases they're good enough that the turn-off tail can be
ignored with very minor accommodation if any. In some cases an
IGBT may be superior than a MOSFET because you don't have to worry
about the switches RDSon drifting apart with temperature.

Though now days you can buy a 75V 120A 0.0004 Ohm MOSFET in a
T0-220 package... I guess RDSon isn't much of an issue.

The main problem with both IGBT and regular old bipolar transistors
in a push-pull circuit is the turn-off tail. The MOSFET does not
have a turn-off tail. There are two classes of IGBT, punch-though
and non-punch through. The punch-trough devices have better turn-off
times but are more fragile. Lately I've been using Trench Field Stop
IGBT's and they're very good.

There are several means of preventing imbalance of the transformer
in driven (not self oscillating) converters. The PWM will adjust
the on-time to compensate for the tail as it regulates the output
voltage. You can sense the differences with a circuit that converts
time to voltage (a capacitor and a current source) then make the
correction.

There are clever flux balancing windings that can be added. One
Unitrode app note describes how this can be done in the course of
presenting a half-bridge power converter. I can't recall the
document number.

In the half bridge and full bridge sometimes a capacitor in series
with the primary wdinding prevents saturation. I think you could
build a two capacitor divider across the input voltage and at the
center connect your transformer centertap lead. Then as the
imbalance increases the voltage at the center tap with shift
to compensate for it. I've seen half bridges built this way...
might work for a push pull... just guessing as I've never tried it.

A very small gap (0.001-0.003") will prevent saturation if the
imbalance is minor and not decrease the magnetizing inductance
too much. Sometimes any decrease is unwelcome though.
A distributed gap material like powdered-iron, koolmu, MPP
or sendust, might be useful if you expect to have flux imbalance
problems.

Pulse by pulse current limiting will mask the problem, so the
transformer is in saturation but not far into it and the current
limit keeps it from destroying the switches. Kinda risky to rely
on this alone but it's helpful combined with other measures.

In a current fed converter the transformer may saturate and then
switches are effectively connected directly to the current source.
No harm done! A current fed push-pull is a rugged topology. The
VAX8800 computer uses one for its control and start-up power supply.


I was reading up on push-pull topology of switching power supplies
and see that they have problems with flux imbalance. I used to work on
some power supplies that were push pull when I was in the USAF and the
driver transistors were always failing, now I know why. I see that
this isnt as much of a problem for FETs, How about IGBTs.


Jimmie

microwave oven inverter P.S. revisited
 

On Sep 18, 8:56*pm, Grumpy The Mule wrote:
Short answer, IGBTs are much better but not as good as MOSFETs.
In many cases they're good enough that the turn-off tail can be
ignored with very minor accommodation if any. *In some cases an
IGBT may be superior than a MOSFET because you don't have to worry
about the switches RDSon drifting apart with temperature. *

Though now days you can buy a 75V 120A 0.0004 Ohm MOSFET in a
T0-220 package... I guess RDSon isn't much of an issue.

The main problem with both IGBT and regular old bipolar transistors
in a push-pull circuit is the turn-off tail. *The MOSFET does not
have a turn-off tail. *There are two classes of IGBT, punch-though
and non-punch through. The punch-trough devices have better turn-off
times but are more fragile. *Lately I've been using Trench Field Stop
IGBT's and they're very good.

There are several means of preventing imbalance of the transformer
in driven (not self oscillating) converters. *The PWM will adjust
the on-time to compensate for the tail as it regulates the output
voltage. *You can sense the differences with a circuit that converts
time to voltage (a capacitor and a current source) then make the
correction.

There are clever flux balancing windings that can be added. *One
Unitrode app note describes how this can be done in the course of
presenting a half-bridge power converter. *I can't recall the
document number.

In the half bridge and full bridge sometimes a capacitor in series
with the primary wdinding *prevents saturation. *I think you could
build a two capacitor divider across the input voltage and at the
center connect your transformer centertap lead. *Then as the
imbalance increases the voltage at the center tap with shift
to compensate for it. I've seen half bridges built this way...
might work for a push pull... just guessing as I've never tried it.

A very small gap (0.001-0.003") will prevent saturation if the
imbalance is minor and not decrease the magnetizing inductance
too much. *Sometimes any decrease is unwelcome though. *
A distributed gap material like powdered-iron, koolmu, MPP
or sendust, might be useful if you expect to have flux imbalance
problems.

Pulse by pulse current limiting will mask the problem, so the
transformer is in saturation but not far into it and the current
limit keeps it from destroying the switches. *Kinda risky to rely
on this alone but it's helpful combined with other measures.

In a current fed converter the transformer may saturate and then
switches are effectively connected directly to the current source.
No harm done! *A current fed push-pull is a rugged topology. *The
VAX8800 computer uses one for its control and start-up power supply.



*I was reading up on push-pull topology of switching power supplies
and see that they have problems with flux imbalance. I used to work on
some power supplies that were push pull when I was in the USAF and the
driver transistors were always failing, now I know why. I see that
this isnt as much of a problem for FETs, How about IGBTs.

Jimmie- Hide quoted text -

- Show quoted text -

I work with a couple of pieces of equipment that synthesises a
repetitive waveform by playing back the waeform from a ROM. I thought
I could do something like this to control the on-off timing of the
IGBT. This would set a minimum time between turn on and turn off and
the rest would be controlled by PWM.

microwave oven inverter P.S. revisited
 

This is one advantage of the flyback and asymmetrical forward
converters. They will reset if there's enough dead time.
So in those cases your ROM circuit would do the trick.

Symmetrical topologies like push-pull, half-bridge and
full-bridge can saturate even if there is plenty of dead
time. The core is always being driven by the control
circuit, so it has no time to relax, If the drive isn't
equal and opposite for each half cycle there is an offset
which the core accumulates. Eventually it will saturate.
unless there is some means to compensate for the imbalance
of the drive (like a coupling capacitor.)
So here the ROM circuit will not help.



wrote in news:8e344554-a8d0-4796-ae20-
:

I work with a couple of pieces of equipment that synthesises a
repetitive waveform by playing back the waeform from a ROM. I thought
I could do something like this to control the on-off timing of the
IGBT. This would set a minimum time between turn on and turn off and
the rest would be controlled by PWM.

microwave oven inverter P.S. revisited
 

On Sep 19, 11:57*am, Grumpy The Mule wrote:
This is one advantage of the flyback and asymmetrical forward
converters. *They will reset if there's enough dead time.
So in those cases your ROM circuit would do the trick.

Symmetrical topologies like push-pull, half-bridge and
full-bridge can saturate even if there is plenty of dead
time. * The core is always being driven by the control
circuit, so it has no time to relax, *If the drive isn't
equal and opposite for each half cycle there is an offset
which the core accumulates. *Eventually it will saturate.
unless there is some means to compensate for the imbalance
of the drive (like a coupling capacitor.)
So here the ROM circuit will not help.

wrote in news:8e344554-a8d0-4796-ae20-
:



I work with a couple of pieces of equipment that synthesises a
repetitive waveform by playing back the waeform from a ROM. I thought
I could do something like this to control the on-off timing of the
IGBT. This would set a minimum time between turn on and turn off and
the rest would be controlled by PWM.- Hide quoted text -

- Show quoted text -

I think I get it. Would this explain why in a push-pull topology after
the transistors have been replaced a couple of times the power supply
just keeps failing for no apparent reason?

Are you saying that in other topologies it doesnt saturate or that it
doesnt matter if it does?

Ordered Abe's book.

Jimmie

microwave oven inverter P.S. revisited
 

Hard to say. Sometimes other parts are wounded and cause the
transistor on-times to be slightly different. Often a base
drive component, usually a resistor, changes value. Some
lousy designs just won't work without selected transistors
required to match the on-times. It only requires a small
imbalance for the push-pull transformer to accumulate enough
flux to eventually saturate. If the core is steel or powdered
iron which can be magnetized by the fault current of the first
failure sometimes (rarely) that causes problems too.

The other topologies we've discussed are more forgiving. BUT
if the switch is on for too long, or the voltage applied to the
winding too high, causing the flux density that the transformer
can sustain to be exceeded, it will saturate. The current will
then rise quite rapidly the sparks will fly.

It's just that they're not senstive to slight variations in the
on-time of the switch. They reset the transformer completely
during the dead time, so they don't accumulate any flux from
on-time imbalances.

An exception is current fed symmetrical topologies which are
just as senstive to imbalance. If the transformer saturates
the fault current is controlled by the inductor feeding the
converter and the current gradually increases. So the control
circuit can catch the fault before the transistors are turned
into lumps of glass.

Some topologies rely on saturation in order to function, like
the royer (and some forms of blocking oscillator supplies) where
saturation removes the positive feedback base drive and allows
the switch to turn off. Nasty things but sometimes useful for
low parts count, low-power, converters.

I'm sure you'll find Abe's book helpful. I still re-read it from
time to time.



I think I get it. Would this explain why in a push-pull topology after
the transistors have been replaced a couple of times the power supply
just keeps failing for no apparent reason?

Are you saying that in other topologies it doesnt saturate or that it
doesnt matter if it does?

Ordered Abe's book.

Jimmie

microwave oven inverter P.S. revisited
 

The designer of this power supply did something very wrong.
Using the safety/chassis ground as a power connection to
run 120V loads is dangerous. If you need to power 120V
loads run a neutral line or use a 240/120 transformer.

Safety ground should only be connected to the chassis,
faraday screens and Y-rated capacitors.

Neutral and safety ground eventually tie together so
it seems like an OK thing. It's not. It's a violation
of the NEC, any applicable safety standards and might
on the off chance cause serious injury or death.


==============http://www.realhamradio.com/hvreg.pdf

microwave oven inverter P.S. revisited
 

On Sep 21, 10:03*am, Grumpy The Mule wrote:
The designer of this power supply did something very wrong.
Using the safety/chassis ground as a power connection to
run 120V loads is dangerous. *If you need to power 120V
loads run a neutral line or use a 240/120 transformer.

Safety ground should only be connected to the chassis,
faraday screens and Y-rated capacitors.

Neutral and safety ground eventually tie together so
it seems like an OK thing. *It's not. *It's a violation
of the NEC, any applicable safety standards and might
on the off chance cause serious injury or death.



==============http://www.realhamradio.com/hvreg.pdf- Hide quoted text -

- Show quoted text -

Yeah its a fool killer, Did you notice how they develop the Vcc.
Im sure under the conditions
its designed to be used its safe enough but if you get enough
hams tinkering with it someone might get killed.
David Smith kind of backed out of the project because of this.

I got sidetracked on this project a bit remodeling my bathrooms
but while prowling through my storage shed(junk box) I came across an
old SMPS, it s rated 48 volts at 50 amps. I was going to build a
big FET amp around it but never did. The output of it is just
rectifiers and filters connected to the transformer. There
is a second winding that is rectified and appears to develop
feedback voltage. I was thinking of building a step up transformer
and connecting it back to back with the transformer on
this power supply. I was also having thoughts of just replacing the
transformer with one designed for HV.

microwave oven inverter P.S. revisited
 

On Sep 21, 10:03*am, Grumpy The Mule wrote:
The designer of this power supply did something very wrong.
Using the safety/chassis ground as a power connection to
run 120V loads is dangerous. *If you need to power 120V
loads run a neutral line or use a 240/120 transformer.

Safety ground should only be connected to the chassis,
faraday screens and Y-rated capacitors.

Neutral and safety ground eventually tie together so
it seems like an OK thing. *It's not. *It's a violation
of the NEC, any applicable safety standards and might
on the off chance cause serious injury or death.



==============http://www.realhamradio.com/hvreg.pdf- Hide quoted text -

- Show quoted text -

I never saw where the neutral and ground tied together. The closest
thing I could see was on the HV side of the transformer where one side
is tied to ground. This is OK. I did find in another service manual
where it warned about not having the mounting screws to the power
supply fastened tightly while servicing. Thats almost funny. Probably
a good idea to use this on a GFCI circuit.

Anyway I came across some 120/240 to 240/480 volt 2KVA transformers I
had. This got me thinking that it may be at lot of fun building a
switcher when you had 600VDC at a few amps to switch. Coming up with a
transformer may be fun.

Jimmie

microwave oven inverter P.S. revisited
 

I meant in this design...
http://www.realhamradio.com/hvreg.pdf

A safety ground connection is used for
120V loads because there's no neutral
taken from the AC line connection.

They should have used a four pole connector
and run a neutral as well as ground to the
power supply.

The ground and neutral are tied together
at your service entry.

You could connect two 120/240 transformers with
the primaries in parallel and the secondaries in
series. Then you'd have 480AC source which makes
a decent input to a doubler for about 1200VDC output.

There's nothing wrong with blood and thunder supplies
except the weight and the stored energy. If the parts
are available the cost factor might win out.


I never saw where the neutral and ground tied together. The closest
thing I could see was on the HV side of the transformer where one side
is tied to ground. This is OK. I did find in another service manual
where it warned about not having the mounting screws to the power
supply fastened tightly while servicing. Thats almost funny. Probably
a good idea to use this on a GFCI circuit.

Anyway I came across some 120/240 to 240/480 volt 2KVA transformers I
had. This got me thinking that it may be at lot of fun building a
switcher when you had 600VDC at a few amps to switch. Coming up with a
transformer may be fun.

Jimmie

microwave oven inverter P.S. revisited
 

On Sep 25, 7:38*pm, Grumpy The Mule wrote:
I meant in this design...http://www.realhamradio.com/hvreg.pdf

A safety ground connection is used for
120V loads because there's no neutral
taken from the AC line connection.

They should have used a four pole connector
and run a neutral as well as ground to the
power supply.

The ground and neutral are tied together
at your service entry.

You could connect two 120/240 transformers with
the primaries in parallel and the secondaries in
series. *Then you'd have 480AC source which makes
a decent input to a doubler for about 1200VDC output.

There's nothing wrong with blood and thunder supplies
except the weight and the stored energy. *If the parts
are available the cost factor might win out.



I never saw where the neutral and ground tied together. The closest
thing I could see was on the HV side of the transformer where one side
is tied to ground. This is OK. I did find in another service manual
where it warned about not having the mounting screws to the power
supply fastened tightly while servicing. Thats almost funny. Probably
a good idea to use this on a GFCI circuit.

Anyway I came across some 120/240 to 240/480 volt 2KVA transformers I
had. This got me thinking that it may be at lot of fun building a
switcher when you had 600VDC at a few amps to switch. Coming up with a
transformer may be fun.

Jimmie- Hide quoted text -

- Show quoted text -

I agree, I understand the commercial version of this has been
redesigned hopefully addressing some of the issues you mentioned.

I had given some thought to using the transformers as you discribe but
for right now my purpose is to learn abut and build a switching power
supply.

Jimmie

microwave oven inverter P.S. revisited
 

I think I get it. Would this explain why in a push-pull topology after
the transistors have been replaced a couple of times the power supply
just keeps failing for no apparent reason?

Are you saying that in other topologies it doesnt saturate or that it
doesnt matter if it does?

Ordered Abe's book.

Jimmie

Well I remember repairing Sony TV push pull SMPS I made a living from
replacing many a blown PP pair. Sony then went to PP pair in a single
package. That reduced the business for me but I had much experience
repairing them already. Trick with the Sony push pull was 2% timing
components. The PP had to be within 2% of 50% duty cycle. They used a
self starting multi vibrator design. After replacing the blown parts
I'd power the input up at 20vac and use a 12vdc supply for the start
up circuit, Then check the waveform on a scope to make sure it was
with 2% fo 50% duty cycle. There was no dead time in the Sony's. They
just varied the frequency to regulate the voltage.

73

n8zu

microwave oven inverter P.S. revisited
 

Topologies like push-pull, half-bridge and full bridge don't
require dead time to reset the transformer core. Though
that doesn't mean the switch's conduction times can overlap
which causes shoot-though current.

I think what kills the push-pull in this case is overlapping
conduction times not core saturation.


For amplifier power supplies it would simplify things to
do the regulation at a lower voltage and keep the HV parts
at a minimum. Phase controlled 60Hz switching is ok but
this might be a better way.

There's a use for push-pull or half-bridge or full-bridge
where the switches duty cycles are not modulated and the
frequency is fixed. The switches run as close to 50% duty
cycle as possible without overlap. It's called a "DC
Transformer." It's one of the building blocks of compound
converter topologies. Handy because it offers isolation
and a fixed ratio of step-up or step-down with a DC input
and output.

Since there's minimal dead-time and no output inductor is
required. The efficiency can be very high. The control
circuit is an oscillator running at 2F (Like a 555,) followed
by a flip-flop and a couple of gates to insure there is never
overlap. When using MOSFETs an RCD network on their gates
will work, though I favor using logic gates.

So if you built one of these with a 10:1 ratio you could
put 200VDC in and get 2000VDC out. Any regulation or
protection would be done to the 200VDC input. This might
not be a bad idea... your 2KV output stage now consists
of only rectifiers and a capacitor.

Doesn't have to be 200V, pick the voltage that makes it easy.

Just a thought.



raypsi wrote in news:f60045d7-f5a2-4dbe-a850-
:

Well I remember repairing Sony TV push pull SMPS I made a living from
replacing many a blown PP pair. Sony then went to PP pair in a single
package. That reduced the business for me but I had much experience
repairing them already. Trick with the Sony push pull was 2% timing
components. The PP had to be within 2% of 50% duty cycle. They used a
self starting multi vibrator design. After replacing the blown parts
I'd power the input up at 20vac and use a 12vdc supply for the start
up circuit, Then check the waveform on a scope to make sure it was
with 2% fo 50% duty cycle. There was no dead time in the Sony's. They
just varied the frequency to regulate the voltage.

73

n8zu

microwave oven inverter P.S. revisited
 

On Sep 10, 8:05*pm, Grumpy The Mule wrote:
Exactly right it is a gapped core. Once I git's a scope on I'll see.

But the voltage doubler they using is full wave doubler. Which means
they taking the positive and negative waves *to get 4000 or so volts
DC
Now the positive half they use a .0082 mfd for a filter and the
negative half they use a .0056 mfd filter both at 3000wvdc.

my guess is the quasi-push pull output is due to the leakage energy
recovery circuit. *HV flyback transformers usually have very high
leakage inductance (the part of the magnetizng inductance not coupled
to the secondary is leakage inductance) because distance between
windings is a major cause of poor coupling and you need distance for
isolation. *

The leakage energy sloshes about in the primary causing all sorts of
mischief. *This circuit recovers it and dumps it back into the primary.
It's unusual since the leakage energy is usually dumped back into the
bulk storage capacitor on the primary side if it's recovered by a clamp
winding or active clamp circuit. *

A soft switching topology often uses the leakage inductance to reduce
transistion losses in *the switch. *Sometimes a discrete inductor is
added in series with the transformer primary to add to it. *Another
bonus for the lousy coupling of the HV flyback transformer... no
discrete inductor needed. *I think of this as electronic ju-jitsu.

The energy per half cycle (on the secondary) won't be equal so juggling
the capacitor values helps equalize the voltage stress.

Eh! *I might be completely wrong. *Simulating it would be the thing
but it's too much like work work and I'd need the transformer parameters.

At least that's how I think it works... Please let me know when you've
scoped the waveforms.

I was looking at some old UPSs without the big iron transformer and
was trying to figure how they get 60Hz 1KW out of that little
transformer.
The best I an figure is that it works somewhat like a class G
amplifier. In this case the pulse width of the 20Khz or so signal is
being PW
modulated so when the output is intergrated you get 60Hz. If this is
the case I am thinking that UPSs may be hacked into HV power supplies
a lot easier,
safer and better than microwave oven power supplies.

Jimmie

microwave oven inverter P.S. revisited
 

On Sep 30, 2:13*pm, wrote:
On Sep 10, 8:05*pm, Grumpy The Mule wrote:





Exactly right it is a gapped core. Once I git's a scope on I'll see.

But the voltage doubler they using is full wave doubler. Which means
they taking the positive and negative waves *to get 4000 or so volts
DC
Now the positive half they use a .0082 mfd for a filter and the
negative half they use a .0056 mfd filter both at 3000wvdc.

my guess is the quasi-push pull output is due to the leakage energy
recovery circuit. *HV flyback transformers usually have very high
leakage inductance (the part of the magnetizng inductance not coupled
to the secondary is leakage inductance) because distance between
windings is a major cause of poor coupling and you need distance for
isolation. *

The leakage energy sloshes about in the primary causing all sorts of
mischief. *This circuit recovers it and dumps it back into the primary.
It's unusual since the leakage energy is usually dumped back into the
bulk storage capacitor on the primary side if it's recovered by a clamp
winding or active clamp circuit. *

A soft switching topology often uses the leakage inductance to reduce
transistion losses in *the switch. *Sometimes a discrete inductor is
added in series with the transformer primary to add to it. *Another
bonus for the lousy coupling of the HV flyback transformer... no
discrete inductor needed. *I think of this as electronic ju-jitsu.

The energy per half cycle (on the secondary) won't be equal so juggling
the capacitor values helps equalize the voltage stress.

Eh! *I might be completely wrong. *Simulating it would be the thing
but it's too much like work work and I'd need the transformer parameters.

At least that's how I think it works... Please let me know when you've
scoped the waveforms.

I was looking at some old UPSs without the big iron transformer and
was trying to figure how they get 60Hz 1KW out of that little
transformer.
The best I an figure is that it works somewhat like a class G
amplifier. In this case the pulse width of the 20Khz or so signal is
being PW
modulated so when the output is intergrated you get 60Hz. If this is
the case I am thinking that UPSs may be hacked into HV power supplies
a lot easier,
safer and better than microwave oven power supplies.

Jimmie- Hide quoted text -

- Show quoted text -

Oooops, I meant class D instead of class G