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: "rheidlebaugh by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>
> 
> What I am trying to say is the input must be biased at center potential to
> turn the n chan on as the p chan is turned off. if the bias is not set
> correctly to the incoming signal the results will be zero . Bias is
> everything. If your input is not swinging each side of a common reference the
> gates will not be turned on as the other is turned off giving a voltage
> swing. The starting point must be 6v with no signal at the capacitor as you
> are using only one power supply. The input must be biased at a mid point +.
>    Robert  H
> 
> > From: "Tesla list" <tesla-at-pupman-dot-com>
> > Date: Tue, 04 Jun 2002 22:58:03 -0600
> > To: tesla-at-pupman-dot-com
> > Subject: Re: SSTC, xfmr gate drive oddity
> > Resent-From: tesla-at-pupman-dot-com
> > Resent-Date: Tue, 4 Jun 2002 23:18:45 -0600
> >
> > 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.
> >
> > --
> > -----------------------------
> > The Numeric Python EM Project
> >
> > www.pythonemproject-dot-com
> >
> >
> >

Are you using a transformer to feed the FET gates?  One very bad side
effect of driving a lot of FETs directly with one driver, is that when
one FET fails, its likely to take out every bit of your circuitry,
driver and all.  I know as I had it happen before.  A well designed gate
drive transformer is a godsend. (Mine is 5 turns primary and secondary
on 77 type material toroid) If you can still see 8-12V p-p on the
secondary side while shunted with a 5-10ohm resistor, you've got a good
isolated driver circuit.  That is- if it still looks like a square wave
:)   Rob.

-- 
-----------------------------
The Numeric Python EM Project

www.pythonemproject-dot-com