Re: fet rise/fall times

Kevin Aylward wrote:
Jamie Morken wrote:

What determines a fet's ie. 650V n channel like this one:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=IPP6...
rise and fall times?

I usually look at total gate charge to find fet driver current
requirements for switching losses, and Rds(on) for on-time power
losses, but haven't figured out the relationship between rise/fall
times and these or other parameters.

I think rise and fall times must be proportional to the total gate
charge and the gate drive currents, but it seems that maybe some
fets have a minimum rise/fall time independent of the gate drive
currents maybe.

Sort of. If you drive them with an ideal zero source impedances, i.e. one
that can supply infinite current/charge, the fet will still have a mimimum
on/off time. 1st, there is internal resistance in the gate which will limit
the actuall maximum current possible, and hence speed, i.e. an Rgate.(Cgs +
gain.Cgd) network on the input. 2nd, the output current is limited to
Vgate.gm, and this finite current has to charge the output capacitances
of the device (Cgd, Cdb, Cds, Cload).

That said, many OF TODAY'S high-voltage power MOSFETs
will switch many amps over nearly a kV in 10 to 15ns,
with a high enough gate-drive current, which is pretty
impressive. The gate-spreading resistance info you
need to determine in advance how well a given type of
mosfet will do is not available. Jamie, you'll have
to measure this yourself. What are you working on?
.