SSTC, xfmr gate drive oddity

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Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

there haven't been very many questions about solid state coils lately...
:) Anyway, as I got around to tinker with SSTCs again lately, here's a
tough one... I hope someone can help out?

I've a mosfet buffer (complementary source follower with an n and p
channel mosfet) driving a small pulse transformer with a series capacitor,
i.e. a circuit like this:

           13.6V            +-R-C-+
            |               |     |
         ||-+ drain         |     |
      +--||<- n channel     +LLLLL+
sig   |  ||-+ source        =======
      |     |           C1  =======
------+   Y +-----------||---LLLLL--+
 X    |     |                       |
      |  ||-+ source                |
      +--||>  p channel             |
         ||-+ drain                 |
            |                       |
           GND                     GND

The signal at X on the mosfet gates is a clean square 0V<->13.6V signal
like it is supposed to be. When the xfmr is not connected, the sources'
node at Y follows 0V<->12V too. 

But, as soon as I connect the xfmr, the square signal measured at Y "gets
smaller" in amplitude. The square wave centers around 1/2*13.6V = 6.8V,
but now goes only from 4V<->9V instead of 0V<->13.6V. I.e. peak to peak is
now only 5V instead of 13.6V. It is till a good square wave, though. But,
the 1:1 transformer output is now -2.5V<->+2.5V (and this is also seen on
the primary side of course). Not the expected -6.8V<->+6.8V.

Probably someone else has noticed this too in their gate drive setup?
Anyone have an idea where this comes from? It must be something simple but
it feels like banging my head on the wall, thinking&testing for 1 hour
already => no result. 

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

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

Speefing up local RF decoupling from 330uF tantal to additional
1000uF electrolytic didn't change anything. Also doubling the turns on the
toroidal ferrite core xfrm didn't help. Changing C1 from 330uF tantal to
10uF electrolytic didn't have any effect either. 

Sooo... I'm completely puzzled now... 

Any ideas, tips, etc, would be highly welcome!
That is how to get the full +-6.8V voltage swings accross the pulse
transformer primary? Because +-2.5V is really on the weak side.

many thanks,

 - Jan

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