RE: SSTC, xfmr gate drive oddity

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

Original poster: "Aron Koscho by way of Terry Fritz <twftesla-at-qwest-dot-net>" <kc5uto-at-wt-dot-net>

Ok,

 Just of ye'ol bread board:

I have tried several configurations to achieve high current capabilities in
a gate drive circuit. I used a TL494 running at 350khz for all of these
tests. All gates and drive ICs have 15v zener diodes on the output for
protect.

1. A TC4420 was used to drive a 1:2 transformer to provide a bipolar square
wave of 30v pk-pk. A bipolar oscillator could be used as well. The output of
the transformer was then feed into a common drain (half-bridge)
configuration composed of an IRFZ20 and an IRFP540. These drove a 1:2
transformer with two secondary connected to IRFP460s. The resulting waveform
was a rounded off 30v pk-pk square wave with a 100ns rise time. The waveform
was defiantly usable.

2. Four TC4420s were paralleled for a peak output current of about 24a.
These ICs CAN NOT be directly paralled. There are minor differences in the
output phase and duty cycle of each IC (relative to the input). These
differences are enough to far exceeded the peak current ratings for a few ns
and kill the IC. Also, it is important to note that if the input is left
floating any noise it picks up is amplified and reflected to the output;
this too can kill these ICs. To over come the above problems a 1k resistor
was used to pull all the inputs low. Each IC has its input connected to the
oscillator with a 220 ohm resistor to balance the input across all four ICs.
The outputs are connect in the same way with 10 ohm resistors. This allows
that minor phase difference to be dissipated in the resistors and still
produces a nice clean 25ns square wave. This gave me a rounded squarewave
with a 250ns rise time on the gates of the IRFP460s. I would not use this
above 100khz.

Based off of these experiments and some others not mentioned I would
conclude that a half bridge of fets can be use to drive large captive loads
(many fets) IF the input is a perfect square wave with 50/50 duty cycle and
a significant voltage (+/-10v) above and below ground.

More to come as I test the many, many, many possible drive scenarios.

-Aron

kc5uto
Aron Koscho
www.hvguy-dot-com