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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
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