Hi again, A problem in push-pull converters are to achieve exactly the same magnitude of switch current in both halves of the primaries to avoid "flux walking". Is there any other ways than go for current-mode, to achieve balance in this respect? One thing I though of; is it possible to "take a snapshot" of the primary-current through the "top" switch by a sample and hold circuit, and drive the "low" switch until its current is equal? This would mean that only the top switch is pure pulsewidth modulated while the low switch is current controlled with respect to the top switch. Regards, Stefan
Active balancing schemes are certainly possible, but the common solution is to put a capacitor in series with the primary, yo accumulate the difference and turn it into a voltage difference that forces a balance.
Ok, I realize the capacitor is possible to use for half and full bridge designs, but is it really applicable for a push pull design were power is applied on the center-tap of the transformer?
You didn't explain that one very well so JP got the wrong idea until you did. After all a push-pull convertor doesn't have a top switch...... Anyway, before I get all pedantic and start dreaming about Ms Frostrup, I've lowered my sights, there 'may' be a way.
During the switch dead time output current is isolated on the secondary side as it recirculates through the diode things. In the mean time magnetising current is flowing somewhere in the primary.... it's all idealised this is.
So, what you do is sample and integrate the sensed primary current during the dead time with appropriate steering to the appropriate integrator based on which switch was just turned off. Then you take the difference of that and use it to adjust the on time of the switches in some sort of way.....
Anyway, current mode control makes the 'problem' go away.
However..... it does not work in a current fed push-pull convertor (or other types of current fed convertors) because the input current is fixed so it cannot differentiate between output and magnetising current.
That was a source of a giggle for me because there are lots of mil-spec convertors out there that use current fed topologies and they are all basically buggered as a result, unless you do something about it which they don't.... so they are buggered. Don't tell anyone though and don't go flying in flying things ;-).
Hmmmm.. If you want to play and look then get into that LTspice stuff. Keep your models ideal and poke about, it can be very insightful.
Years ago that would have been done using two transformers with centre-tapped primaries in parallel (more or less).
There would be the main power transformer, using a soft magnetic material, and an auxiliary timing transformer wound on a hard (near square loop) material.
The timing transformer would be designed to saturate well before the main transformer and would run unloaded (usually base drive power only), so that that the centre-tap current was mainly mag current, allowing an easy sensing of the approaching Bsat.
It's volt-seconds that you're trying to equalize; differences in magnetizing current are the result, not the source of the problem.
The primary current reflected from the secondary will have a peak value that's roughly constant and determined by the output current through the output inductor. Magnetizing current is superimposed on this in the primary.
Gapping the core is often a sufficient precaution if the switches are resistive ie fets. Their increased voltage drop, to the increased peak-to-peak magnetizing current acts to balance the driven voltage.
The average voltage on each phase is measurable on a coupled winding - their difference can be used to affect phase duty cycle to force equality.
The instantaneous current is measurable on each phase, and differences can be used to modify duty cycle without going for complete curent-mode control.
Depending on your type of modulator, simply adding current information into either the voltage ramp or voltage feedback lines could suffice.
You have to ask youself, in the end, just how much fiddling around you are prepared to do with a circuit that may never have optimum characteristics for your specific application. Difficulties of modifying the control circuit to a current-mode version may be trivial in many cases, as the basic elements for doing so are already present in the voltage-mode control circuit.
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