Re: SSTC, xfmr gate drive oddity

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

Original poster: "Alan Sharp by way of Terry Fritz <twftesla-at-qwest-dot-net>" <AlanSharp-at-compuserve-dot-com>

I've been following this thread with interest as I'm going to
have to design such a totem pole driver for my latest full
bridge. I had used  the TC4422cpa which is capable of 9A
but its expensive and my new power FETs have a higher gate charge.

A few thoughts:

To minimise the shoot through we need to minimise the switching time.
So you need fast FETs eg tr and tf <50nS. We also need FET's with a low
total gate charge - if your FET's have a combined gate charge of say 100nC
and you are driving them with a circuit that can supply 1A it will take
100nS
to switch.  So I'm going to use a 4427 to drive a totem pole of 
SPP18P06P + STP16NE06 combined gate charge is 40nC, the 4427 can supply
1.5A so in theory 26nS switching.  Worst case shoot through current is 48A.
I can make them switch off faster than they switch on by using two gate
resistors
with a diode across each one so that each FET is turned on via its resistor
(a few
ohms) and turned off via its diode.

BTW some small FETs that looked ideal had very high gate charges - 100nC +

While we want the driver to switch fast you may not want the FET's on the
half bridge
to switch at warp speed. Richie Burnett tried an ultra faster driver and
blew his fets
even faster than usual - switching a current of amps in a few ns induces
horrible voltages,
and ringing. 

have fun

Alan Sharp

www.alansharp.co.uk