Re: SSTC, xfmr gate drive oddity

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

Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

Hi Jan,

> The mosfets should be clamping point Y to 13.6V and 0V in turn, 
> right?

Yes. The upper MOSFET pulls it high, the lower MOSFET pulls it low.
Or it's supposed to. Active HI, Active LO, works much better than a
pullup (or pull-down) resistor and a single N or P channel MOSFET.
But I still don't like the circuit, it has never worked well for me.
I have definately experienced these same problems, I always have just
blamed it on different Rds(on) resistances and turn-on/turn-off
times. P and N-CH are completely different it seems. 

I think that scoping the drains at the same time would reveal
everything.

To get around this problem, I switched to gate drive IC's such as
MAX4420 or MAX4429 (TC4420/9's cheaper). 6A drive capability, 2500pF
load, FAST rise/fall times. MAX628 dual inv/non-inverting, 3A drive.

> But scoping at Y gives 4V and 9V levels for the square wave signal.

> Very
> very odd. 

Sounds similar to the problems I had, although I've gotten it to work
OK before. I think I had to use different gate resistor values.
Perhaps one of your MOSFET's is going bad...if you haven't already,
try swapping out both MOSFET's at the same time and have a look. 

Also, (you probably know this already, but) if the circuit is drawing
lots of current from somewhere, the supply voltage will drop and
result in the low transformer primary voltage swing. However, this
doesn't explain why the voltage isn't going from GND up, or Vs down.
Because the fact that it's riding somewhere in the middle (as if on a
DC bias), this is most likely a timing problem.

The secondary winding will (should) always relate to the pk-pk
primary voltage and turns ratio, sounds like you're okay here.

Check the FET's, change resistor values, scope drains, and post
results if possible. I'm interested to see what turns up, Good luck
to you.

Justin Hays
KC5PNP 
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com