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.