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

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

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>

Thanks.  Actually, I think the problem may be that I do not have enough
coupling capacitance as I was only trying about 0.1uF.
Without the coupling capacitor and the uneven duty cycle (25%), i was
definitely getting some DC into the transformer and saturating it.
Tomorrow I'll try some larger values of coupling capacitance as well as
start from scratch winding a new gate transformer and actually testing it
with different number of turns
and a capacitor on the output to simulate the actual FET gate capacitance.

Dan


 > Dan,
 >
 > If you can, put a small gap in the core of the transformers.  This would
 > help prevent saturation in the transformers from the DC.  You should then
be
 > able to remove the caps (which would block the DC).
 >
 > David L. McKinnon
 > D&M's High Voltage
 >
 >
 >
 >  > Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>"
 > <dhmccauley-at-spacecatlighting-dot-com>
 >  >
 >  > 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 TL494 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
the
 >  > gate 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
 >  >
 >  >
 >
 >
 >
 >
 >