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 all,
> 
> What I tried now was switching places of mosfets in the old compl. source
> follower circuit, as shown in the CD4049 datasheet "internal schematic".
> Now the p channel pulls up to 13.6V. The n channel pulls down to GND.
> Common-drain connection. This did help _hugely_ to make overcome the
> mosfets V_th threshold voltage problem and make the output rail-to-rail
> and work like a charm waveform wise, but now there's new grief with
> cross-conduction. About 20A peak during the ~50ns switching time =>
> mosfets rather hot. A small series inductor would restrict this current,
> but sadly the gate xfmr peak output current too => not a very usable
> solution. :o(
> 
> >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.
> 
> Umm... not sure... my garbled-ASCII schematic is trying to depict a
> "complementary source follower" (as in "emitter follower") which isn't a
> "totem pole drive" and doesn't require an additional inverted signal.
> Until a year ago I too thought that "totem pole" is just a synonym for
> "complementary emitter follower". Ain't so. :)
> 
> But anyway, you did give a good idea here!
> Totem pole seems the way to go.
> 
> So then... My 50%-duty-always drive signal has to be split into two
> signals, inverted and non-inverted, with additional dead time in between
> to prevent this cross-conduction. That calls for an el-cheapo LM3525 or
> other PWM IC clocked by the existing circuit, right? And then finally let
> the PWM IC output drive two n channel 10A / 60A-pulsed mosfets in a totem
> pole setup.
> 
> Yeah talk about complicated...
> 
> Is there a simpler way?
> Any advice'd be highly welcome!
> 
> (Already considered push-pull drive for the gate drive xfmr but lack of
> dead-time would be a problem there too. Driver ICs with 6A or 9A peak
> "only" seem still poor wrt a 60A peak discrete driver... well maybe after
> a few hours more of trouble and pain such ICs seem a good deal after
> all... oh well let's see ;o)
> 
> many thanks,
> 
>  - Jan
> 
> --
> *************************************************
>  high voltage at http://www.hut.fi/~jwagner/tesla

See Alan Sharp's web page. Search for it on Google as I don't have the
URL handy.  He has given up on dead time and says it causes more
problems than it fixes.  He says the gate capacitance gives all the dead
time you need, and I believe him after seeing the pictures of his coils
in operation.  Rob.

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

www.pythonemproject-dot-com