Re: OLTC II Initial calculations

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

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 16:50 08/05/03 -0600, you wrote:
>Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
>
>
>Yep!  But be careful of how giant that gate current needs to be.  We need 
>to slam it on, but we also need to keep the current through Cgc from 
>driving up the gate voltage during turn on too.  Just imagine adding a 24 
>NF cap between the gate and the collector.  That IS exactly the situation 
>we have with these big puppies...

That's the Miller effect right? As the gate voltage goes up, the collector 
voltage goes down. So the current through the Cgc will be fighting against 
the gate drive signal and driving the gate voltage down. Not up. In 
practice this just means we need to use even more gate drive current <evil 
cackle>

According to the Powerex datasheet, and guesstimating up to Vcc=1kV and 
Vge= 30V the total turn-on charge is around 6000nC (about half of which 
went into the Miller capacitance, that's the flat portion of the graph) so 
to turn it on in 0.5us we require I=Q/T = 6e-6/0.5e-6 = 12 amps gate drive 
roughly.

That's not really as bad as I thought. In practice, we might get away with 
less, because the tank circuit limits dIc/dt. At .5us after turn-on, the 
collector current will be no more than 500A, which according to the 
datasheet, needs about 10V gate voltage hence 2400nC of charge. That 
corresponds to 4.8A gate drive. I think I'll be going for 12A anyway.

Steve C.