Re: fet rise/fall times

Winfield Hill wrote:
Winfield wrote:
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?

I can give you a couple of examples from instruments
I've made lately.

First, an ON Semi MTW6n100E, rated 1kV 6A 1.3-ohms,
driven from a TC4429 mosfet driver with a 1-ohm gate
resistor (estimated 5A Miller gate current), switches
1.1kV and 11A into a 100-ohm resistive load (50-ohm
source termination + 50-ohm coax) with a 15ns turn-on
risetime. Turn-off time wasn't measured (note, this
can be a problem, if active methods aren't employed).

Second, a pair of Fairchild FQA10n80 mosfets, rated at
800V 10A, driven by an ir2113-2 driver IC with a flying
high-side driver, with 3.3-ohm gate resistors (estimated
gate current, 1.3A), switch 600V 6A into 100 ohms (see
above) with 30ns risetime and 30ns falltime. The ir2113
driver IC provides about 30ns of delay after turning off
one mosfet, before turning on the other. Even so, there
probably was some rail-rail shoot-through current, which
could have been reduced or eliminated by adding Schottky
diodes across the 3.3 ohms to speed mosfet gate turnoff.

From these numbers we can see that actual power mosfet
gate spreading resistances are smaller than the values
you usually see in the manufacturer's spice models, etc.
However, modeling a power mosfet as a single element,
with one spreading resistance is a serious mistake, as
I've written about here in the past.

Thanks, I guess the data*** rise/fall times won't trick
me into buying a part that shows 5nS rise/fall now :)

I never heard of anyone using a 1 ohm gate resistor before,
heard of people having difficulty with ringing using low gate
resistances though :) Any tips on how to get away with such
low gate drive resistance?

I am making a SMPS, and using overkill gate drivers (TC4452
12Amps!!) I have paralleled fets on each leg of the bridges,
not sure if I will try to get away with 1 gate resistor for
all the fets per leg or if its better to use 1 resistor right
at each fets gate. Thanks! :)

cheers,
Jamie




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