Getting back to my induction heater, I, first I removed the two dead
transistors from last time I had it out, and then I got to work
investigating why they died. The circuit seems just peachy at 130VDC,
putting out a clean 230W into a light load (1uH coil, resonant around
35kHz). But hmm, something I hadn't noticed before, trash on the falling
edge. Let's take a closer look at that, hmm and only on the falling edge
you say?

Hey, the high side gate drive is flying with the output, isn't it. If
there's enough capacitive coupling in the pulse transformer -- and with a
40ns edge, there is -- it could turn on again. Or off, as the case may be.
Ya know, I recall the gate drive has a couple hundred ns propagation
delay...isn't that curiously near the bouncing frequency? Gack...

Pic 1: output. Looks pretty sharp overall. Huh, what's that jaggie at the
corner of the squarewave? Let's zoom in...
Pic 2: ...Eww, it's trash! What could that be from?...
Pic 3: Oscillator output (coupled to the gate driver via pulse transformer
for isolation). Relatively high impedance allows edge trash to bleed
through.

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms