RE: SSTC questions

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

Original poster: "Mccauley, Daniel H by way of Terry Fritz <teslalist-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>



(If no path is provided for the free-wheeling load current when both
switches are open, then a large voltage pulse is developed. This is often
referred to as "back-EMF" and is encountered with many inductive loads such
as motors and solenoids, when the current is interrupted abruptly.) The
action of the free-wheel diodes can be thought of as clamping the back EMF
to the supply rails.


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.

 >>> They're not really disabled.  The schotkey diodes in series will conduct
before the internal FET diodes because they
are faster and have a lower forward voltage drop.  What happens is that with
an inductive load, if you interrupt the current abruptly, you will induce a
large voltage pulse (reverse-EMF)in your circuit.  These added freewheel
diodes will effectively clamp this pulse to the supply rails.  With full
rectified DC, you'll be conducting even more through these diodes.

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

 >>> Thats a tough one.  A lot of work has gone into getting a good model of
a SSTC resonator, but none yet as proven to
be effective in pspice modeling for a good load.  Just for comparison, I
draw almost 1-2kW when running at half-rectified AC at 220VAC input into my
SSTC.  Almost double with DC input.  However, at DC input, the circuit gets
extremely hot and can only run for maybe a minute before I shut it down.

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?

 >>>I use stranded wire.  Either stranded or solid wire works fine here.
Since this SSTC doesn't have a primary resonant tank circuit, you don't have
to worry about number of turns for tuning the primary coil.  The SSTC
full-bridge circuit output frequency directly excites the secondary coil.
However, you'll need to experimentally play with the number of turns,
coupling, height in relation to secondary, to find best performance.


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).

 >>>>>>For a typical ferrite core, 15 turns should be just fine for
100-300kHz.  Ringing is normal and with
my SSTCs found it doesn't affect performance too much.

I'm
using TC4420 and TC4421 FET driver chips since the gate capacitance of the
IGBTs is relatively high (~13 nF).

 >>> Those TC442x FET driver ICs will NOT work for you unless you plan to run
at really low frequencies.  13nF is
a pretty big gate capacitance and you'll burn those FET drivers up extremely
quickly especially when running over 200kHz.
My FET drivers on my IXFN24N100 FETs which have a gate capacitance of about
700pF (i think) get extremely hot during operation. (200-300kHz) You should
look into having another transistor drive switch in there to get more
current capability.


The Captain