Re: SSTC, xfmr gate drive oddity

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

Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rob-at-pythonemproject-dot-com>

Tesla list wrote:
> 
> 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
> 
> --
> *************************************************
>  high voltage at http://www.hut.fi/~jwagner/tesla

I would recommend a UC2710 from TI.  No sense fooling around with
discretes when you can get 6A peak drive from a $5 part.  My problem
with the device is that is has so much drive capability that I needed to
swamp the gate-source junction with a 15ohm resistor, otherwise there
was severe ringing from the wires connecting to the FET. Digikey sells
these drivers.  Rob.

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