Fred Bartoli wrote...
Joerg a écrit :
How much current do you need?
Otherwise, an obvious and really fast way would be going cascode.
I'm not sure cascode would be useful for fast turnoff.
We need to know more to better answer Joerg's question, for example,
is he thinking of completely switching the MOSFET in 4ns, or is he
thinking of obtaining a say 180V swing in 5ns, say not including the
last 20V of a 200V load? Is he doing this once every now and then,
or is he seeking to repeat this every 5 to 10ns, as Henry would like
to do? Joerg, tell us, what are you working on?
I'll venture the observation that many, if not most, high-voltage
MOSFETs can be made to switch ON in 5 to 10ns if enough gate current
is applied. At least the near portion of the die will switch; there
may be issues getting all of the MOSFET's area to rapidly turn on,
and it may not achieve its low long-term Rds(on) value until many
more ns have passed. If you look at a typical spice MOSFET model,
you'll usually see a series gate "spreading" resistor, often 50 ohms.
I have found these apparently arbitrarily-picked values to be far too
high when compared to the performance of an actual part on the bench.
A better model would divide the MOSFET into several portions, with a
very low gate spreading resistor for one portion, etc.
With respect to fast turn-on, for small output load-current swings
the gate scene described above is the issue, but for high currents,
driving capacitive loads, etc, where a high dV/dt means a high I/C,
the raw current capability of the MOSFET can become the determining
issue. For example, I just completed a fast 1.2kV cable pulser in
which I used a single FET switch to drive a 50-ohm coax through a
50-ohm source resistor. My MOSFET needed to sink I = 1kV/100 = 10A
in driving the 50-ohm resistor in series with the 50-ohm coax. I
observed that even though it could sink 5A to make a 500V pulse in
10ns, when making a 1kV 10A pulse it slowed to about 15ns. There's
a set of electrodes at the far end of the coax, which sees a nice
-1kV pulse with 15ns risetime. Happily that was good enough for us.
Joerg mentioned fast rise and fall time. While it's easy to rapidly
turn off a portion of the area in a typical power MOSFET, portions
that have high series gate resistance will stay on until later, so
that a gradual rather than abrupt complete turnoff is experienced.
This can last as long as 50ns or even 100ns in severe cases. This
is assuming the turn-on gate drive lasted long enough to reach the
far parts of the MOSFET. I've found wide variations in this effect
from one manufacturer's part to another, and in one type to another.
Joerg, you'll need to experiment with many different MOSFETs to see
how they fare. Sadly, these issues are not covered in the datasheet.