Re: An extremely good MOSFET driver

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



> Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<evp-at-pacbell-dot-net>
> > > > BTW, why would you want to turn on your MOSFET slowly???  Doing this
> > > will
> > > most likely put the FET in its linear operating range and start
> > > really
> > > heating it up.  You want something really fast.
> > >
> > > Dan
> >
> > [snipped]
> >
> > I'd love to turn them both off & on blazingly fast...except that if I do
> > that with any given pair of them, then both transistors of the pair find
> > themselves on at the same time while one is turning off & the other, on.
> > That shorts out the power supply during that instant of time.
>
> That's a NO NO!!!  A friend of mine who does power supply design for a
> living says you can get driver IC's which provide the necessary time
> delay between pulses, but I haven't gotten a typical part number as yet;
> will check with him and post that here if someone else hasn't done so
> already.  Same problem happens with push-pull self-excited transistor
> inverters, of course, and makes the transistors get HHOOTT!
>
> Ed
>
Even has a nifty name: "shoot through".. Mostly, it's not so much the timing
of the gate (or base) signals, its the fact that the devices turn off much
slower than they turn on, for a lot of reasons... Even if you clamp it to
ground with a low impedance driver, it still takes a while to get all that
charge off the gate (and the voltage is ever dropping, reducing the current
flow).  Another approach I've seen is to actually pull the gate negative for
a short time, then come back to zero.. Same sort of waveform you use to turn
it on.. spike and back porch.  The tricky thing is to not run negative too
long... or the device dies a horrible miserable death from the negative bias
(depending on particular devices..)  And the tricky part is that the
effective impedance in the discharge path changes with die temperature...