Re: Frustrating Problems with Gate Drive Transformers on SSTC - Ahhghhghghg! !! ! !

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

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>

Hi Dan,

This reminds me of all the problems I've been having with DC biases on gate 
drive transformers ;-). If there shouldn't be a DC bias, then I would look 
into it, because that might be your problem. 0.1 uf seems a little small, 
have you looked at the voltage across it? I know you probably thought of 
this, but I figured I should throw it out there anyway.

Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "by way of Terry Fritz "

I'm having some seriously frustrating problems with the gate drive circuit
on my full-bridge SSTC.

To begin with, I took a perfectly working single-ended full-bridge
(utilizing one-output from TL494) SSTC and converted it to push-pull mode
(utilizing both outputs from
TL494) to experiment with some audio modulation. I decided to go bipolar
mode with the TL494 so I could utilize 50% of dead time to reduce output
power during CW
mode during operation.

So in operation, half of the full-bridge would be on 25% of the time, and
the other half of the full bridge on for 25% with 25% deadtime between each
cycle.

Performed my modifications, checked out the TL494 driver circuit - all good!

Rewired my gate transformer so that now Output A of TL! 494 controls upper
left and lower right quadrant and Output B of TL494 controls lower left, and
upper right quadrant of full-bridge.
Each control via their own gate drive transformer.

Problem is, whenever I apply power now, one MOSFET driver (usually the
TC4421 of the TC4421/TC4421 pair of each output) starts to get SMOKIN' HOT!
I tried disconnecting secondaries
on gate transformers leaving them open, replacing the TC442x chips a million
times, and nothing seems to help. If the TC442x chips are in the circuits
by themselves, they are fine, but as soon as you mix a pair of TC4421 and
TC4422 together (outputs of these go to gate transformer), then one get
SMOKIN HOT!

The only thing I could think of was now that there is 75% deadtime per
output (as opposed to 50%) before, that maybe i have some DC bias or
something on the transformer primary winding.
I tried adding a 0.1uF capacitor in series with the primary winding of 
thegate transformer and it fixed this problem. Now the TC442x run cool and
seem to work.

HOWEVER! Always a however . . . Now my gate drive signals are all messed
up - basically probably because of the series capacitor now.
They look a bit like this:

http://users.snip-dot-net/~liche/waveform.jpg

If anyone has any comments on what I may be doing wrong, or what I can
possibly do to fix this problem, I'd appreciate it.
I was expecting to do this modification in one hour and have it ready to run
and try audio modulating it this evening, but now 4 hours later, I'm about
ready to
throw the damn thing out the window. Oh well . . . it always works that
way!

My schematic is here, although the gate transformers are still wired for
previous working configuration, not the new push-pull arrangement.
http://www.spacecatlighting-dot-com/datasheets/sstc2-pwmdriver.pdf
http://www.spacecatlighting-dot-com/datasheets/sstc2-hbridge.pdf

Thanks
The Captain



Jimmy