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

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

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.

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

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

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

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

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

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

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