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Re: SSTC, xfmr gate drive oddity



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

Hi Jan,
        A post I just sent outlined the solution you now mention 
below.

On 5 Jun 2002, at 11:31, 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(

If you are really set on doing this with FETs, you will need to 
ensure that they cannot turn on together. You can approach this by 
using a 2-zener series string between the gates and driving the 
zeners at their junction. Bipolars will not exhibit this problem in 
the original arrangement with their bases tied together.

Regards,
malcolm

 
> >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
> 
> 
> 
> 
> 
>