mosfet intrinsic NPN transistor (paper)

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Original poster: "Jan Florian Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

just thought this could be of interest to the solid state coilers here...

 http://www.advancedpower-dot-com/TechSupp/App_Notes/apt9803/APT9803.pdf
 (and yes, solid state welding and solid state coiling are "similar"
 applications! :o)
 
The interesting stuff starts on page 7, "III. Failure Mechanism Theory".

The explanaiton is more of theoretical interest, i.e. the "why" part, but,
I guess it is pretty useful to know the "why" in any case... :-) As it
is not only the "slow reverse recovery of the intrinsic body diode
problem" thing!


An in-tune SSTC full/half bridge driver is a ZCS and ZVT resonant driver,
so I'd imagine that the paper applies, even though the circuit in the
paper is slightly different in that the resonant tank is actually on the
primary side. It could be just as well moved to the secondary side, like
in a SSTC.


The main problem, after the diode slow reverse recovery prob, seems to be
turn on of the bipolar transistor at a time when it really should be off.

Turn on can happen due to either high dV/dt (driver out of resonance or
too short duty cycle, say <40%), or due to the slow reverse recovery of
the diode and the associated large shoot-through current dI/dt (which in
turn happens because of too little dead time, i.e. too _large_ duty cycle,
say >40% ;o).
So this is the main reason for disabling the intrinsic diode/transistor
with a series schottky and parallel ultrafast recovery diode.

Just some thoughts.... 
Any comments?

cheers,
 - Jan

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