I trust you understand the dynamic load impedance on startup and the effect of any Remanence from abrupt shutdown can have on saturation.
Thankyou thankyou warpspeed for coming out with this, I have been saying words to this effect on this forum for ages and all have slammed me down.One simple thing you can try is to introduce a very small air gap into the transformer, say .001 inch (.025mm).
Its actually not too bad.Think how you do that with a push pull….there are effectively two primaries…….so you have to split both of those into two and interleave the respective secondary in between that lot…that’s a very complicated and messy transformer wind
I have not considered this and will look at this to see what I can get from the calculations and measurements.I trust you understand the dynamic load impedance on startup and the effect of any Remanence from abrupt shutdown can have on saturation.
Do you also have the auxiliary bias ckt (D12, D14, L4 in page 9)?
I did think that myself but wasnt to sure if you were meant to put an air gap with a push-pull supply, or how it would affect the operation in any way?One simple thing you can try is to introduce a very small air gap into the transformer, say .001 inch (.025mm).
Got the right snubbers on the fets and you o/p diodes...?
The push pull is good in that it allows all low side fet drive, but the fet off-state voltages are more than double what they would be with a full bridge or 2TF. The leakage spike has to be dissipated out, unless you have some very complicated active snubber. A full bridge or 2TF has no more than vin on the fets in the off time.
Adding a very small air gap is not a normal thing to do with a push pull forward converter, but it is sometimes necessary.I did think that myself but wasnt to sure if you were meant to put an air gap with a push-pull supply, or how it would affect the operation in any way?
Its actually not too bad.
The two foil primaries go on together at the same time.
You need two rolls of foil and two rolls of mylar, and wind all four on together.
Interleaving both halves of the primary that way can result in astonishingly low leakage inductance between primary halves. Both halves are pretty well matched for dc resistance as well.
Naturally half the secondary goes on first.
Then both primary halves are wound on together at the same time.
Then the other half of the secondary on top.
Adding a very small air gap is not a normal thing to do with a push pull forward converter, but it is sometimes necessary.
..so the above (from post#10) is wrong? , at least with regard to current mode SMPS's?What can happen s that when you switch off the power, the core can remain slightly magnetised (remnant flux). Next time the power is turned on, there is a 50/50 chance the core flux will be driven in the same direction as the stored flux that is already there.
This can very easily saturate the core and result in extremely high peak current.
Its called "flux doubling".
By introducing an extremely small air gap that greatly reduces the permanent magnetisation effect, and usually solves the problem.
Current mode control solves several related problems in a very neat way, highly recommended for push pull.A current mode controlled push pull does not require any gap.
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