SSTC questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Sean Taylor by way of Terry Fritz <teslalist-at-qwest-dot-net>" <seantaylor-at-attbi-dot-com>

Hey all you SSTCers, I have a few questions . ..

I'm working on building a high power SSTC with a full H bridge of IGBTs that
happen to have body diodes.  It will run off fully rectified & filtered 120
V mains.  The IGBTs are rated at 70 A, 600 V, and have a Tr of 350 nS, and a
Tf of 300 nS.  Pretty slow, I know, but I plan on making a fairly low
frequency coil, and with current being at zero, or close to during
switching, it shouldn't be much of a problem.

So the questions I have are:

1) Why do people "disable" the body diodes of the FETs?  With a full bridge,
there shouldn't be a problem with too much current through the diodes, next
to none if the switching of the FETs is done correctly and there is at least
one rectifying diode on the input.

2) How do anticipate how much current I'll be drawing?

3) What should I use for the primary?  I was planning on using 8 or 10 AWG
fine stranded (cause thats what I have laying around) wire in a solenoid,
but should I use something fairly "coarse" stranded for lower RF resistance,
and how many turns approximately?

4) The inductance of just the primary is fairly low, so why is it that the
current draw isn't that great, despite the output being loaded down,
especially if an arc is drawn to ground?

5) For gate drive transformers, what will the waveform look like if I start
getting too few turns?  I have about 15 turns right now, and get a decent
amount of ringing on the output (when connected to a capacitive load).  I'm
using TC4420 and TC4421 FET driver chips since the gate capacitance of the
IGBTs is relatively high (~13 nF).  Also, should I allow Vge to be +/-Vg, or
use a cap and clamp it to -0.7 V at the lowest?  I know letting Vge get
lower will turn off the IGBT faster, but I'd rather have more voltage in
order to turn the IGBT on better.  Maybe clamp the lowest point to -10 V,
and let it get up to 20 V, which is the "maximum" according to the data
sheet, but I know several people have ran IGBTs much higher than that and
not had a problem, at least in the short run . . .

Thanks for the help!

Sean Taylor