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