Re: SSTC, xfmr gate drive oddity

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

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

Hi Robert,

On 5 Jun 2002, at 8:37, Tesla list wrote:

> Original poster: "rheidlebaugh by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>
> 
> Looking at your drawing I dont see any thing to drive the gates out of
> phase. the voltage at the capacitor remains the same with out regards to
> current. The circuit requires the drive signal turn one on as the othes is
> turned off. I only see both driven at the same time. No voltage swing, only
> current change. The capacitor sees no change.
>   Robert  H

The circuit shows a classic totem pole with N and P devices, just 
like those in a CMOS gate circuit. They are driven out of phase by 
virtue of their opposite gate voltage requirements. A significant 
problem with it is that the output comes from the sources, not the 
drains. Remeber that the source is pegged below the required gate 
drive voltage. That circuit could never provide rail-rail drive. You 
actually want the drains providing the output and the position of the 
two reversed. There will be some conduction overlap if the gates are 
tied together. That is pretty much unavoidable with this simple 
arrangement. A better idea is to replace the FETs with bipolars as 
the circuit is drawn. Then you will get a drive maybe a volt or so 
below each rail.

Regards,
Malcolm

> > From: "Tesla list" <tesla-at-pupman-dot-com>
> > Date: Tue, 04 Jun 2002 22:58:55 -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 23:16:21 -0600
> > 
> > Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> > <m.j.watts-at-massey.ac.nz>
> > 
> > Hi Jan,
> > What MOSFETs are you using? Is the cap and transformer
> > primary resonant at your switching frequency?
> > 
> > Regards,
> > malcolm
> > 
> > On 4 Jun 2002, at 12:33, 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
> >> 
> >> 
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