Re: SSTC, xfmr gate drive oddity

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

Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>


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

Like this?
                   Vdd
                    |
                ||--+ source
    +--+--|>|---||     p-channel
    |  |   Z1   ||--+ drain
    |  ^            |
    |  - Z2         |
    |  |            |
----+  |            |
    |  |            |
    |  - Z3         |
    |  V            |
    |  |   Z4   ||--+ drain
    +--+--|>|---||     n channel
                ||--+ source
                    |
                   GND

(hope it doesn't get garbled up... use a fixed font to view!)

Hmm the zeners there are probably all the wrong way round...? Or are they?
Also, what v_br voltage should the zeners have? around 5V?

> Bipolars will not exhibit this problem in 
> the original arrangement with their bases tied together.

I tried looking for fast, high current, low voltage, low V_ce_sat & low
V_be bipolars. 

There are many "selection guide" lists around, but all the more commonly
available bipolar transistors for this task seem to have microsecond range
rise&fall times, i.e. much too slow to drive fets? Also, horrible V_ce_sat
voltages...
T.ex. the NPN BD909 seems otherwise nice and common but V_ce_sat~=3V and
for BD910 (NPN) V_ce_sat~=6V.

Maybe someone who has already built a high peak current gate drive
with bipolars could help me out and suggest some non-exotic
bipolars that would be suitable?

I.e. ~24A peak >40V NPN+PNP transistors that could switch at <=300kHz
"cleanly" with <100ns rise and fall times?

Though I'm starting to doubt this is possible at all... ?


many thanks,

 - Jan

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