Thanks, though that scope is taken with it on max load.
The top FET is running hotter than the lower FET by a good 10-12 degrees C.
I suspect that the top FET somehow got damaged when the output freewheel diode blew up, in an earlier test session.
(a diode with too high trr was accidentally used and it overheated).
if you want symmetric waveforms across the mosfets
I think this is a topic worthy of serious lengthy research, because why would someone want the same gate drive to each FET?
(i guess to evenly distribute the switching losses, but that can still be done with differing waveforms to each)
This has been discussed previously on edaboard, but i cant now find it. If one fet has a faster drive, then it takes all the turn off losses, but less of the turn on losses....so it pretty much tends to even out the overall losses between the two fets, even if the fet drives are different.
I mean, having two coils in the GDT, for top and bottom fets, makes the design and manufac of that GDT more complicated, (to get nice equal and good coupling for each coil) and you also have more components, so why do it...why not just drive the bottom fet directly.?
I suspect the answer lies in Chinese imports of SMPS...where they deliberately make the transformers more awkward to wind, knowing that western fingers just wont be able to hack the extra winding awkwardness of a two coil output GDT......so the product just keeps getting ordered from China.....is this right?
I mean, if you wind a two coil output GDT, ___pri half 1, sec1, sec2 pri half 2____, (ie sandwiched) then you haven't got equal coupling
of the two output coils to the primary anyway....so why bother?
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As can be seen from the following attached, there is no problem with the fets of a 2 tran forward not
turning on at the same time. It really doesn't matter.
The mystery of why some drive both upper and lower fets from coils from the same gate drive transformer
also possibly harks back to the days of the half bridge.
Obviously in a half bridge, both must never turn on at the same time, and so if driven from the same transformer,
then it assured that one was always high when the other was low.
Why this then got carried over to the 2 tran forward is probably just a matter of "doing things the
same way"?
But its a waste of components and makes the gate drive transformer more complex than it needs to be.
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