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

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

Original poster: "Malcolm Watts by way of Terry Fritz <teslalist-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Dan,
         Looking at your schematic, I see there is no pulldown of any
sort on the TL494 emitters. You need at least resistors but better is
an active pulldown. I think from the pinout you are using the chip
output transistors as emitter-followers? I will shortly draw a little
circuit I've used in the past and send it to you direct.

Malcolm


On 27 May 2003, at 20:26, Tesla list wrote:

 > 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
 >
 >
 >