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200V N-channel FET 200V with nsec speed?

J

Joerg

Jan 1, 1970
0
Hello Folks,

Is there an n-channel FET that can stomach 180V or more yet switch in
nanoseconds? I am looking for around 3-4 nsec or less for rise time,
fall time, toff delay and ton delay. The Zetex ZXMN10 does that nicely
but it's only 100V.
 
Joerg said:
Hello Folks,

Is there an n-channel FET that can stomach 180V or more yet switch in
nanoseconds? I am looking for around 3-4 nsec or less for rise time,
fall time, toff delay and ton delay. The Zetex ZXMN10 does that nicely
but it's only 100V.

I don't know what happens today, but Zetex used to just second source
the Supertex line

http://www.supertex.com/products/selector_guides/101/100

They certainly have parts that can switch 180V, in of the order of
10nsec - the speed that you can get does depend on the current sinking
and source capacity of your driver.

Back when I was interested - a long time ago now - Supertex were about
twice as fast as the competition. They'd found some different way of
doing things, which has probably long since lost patent protection.
 
J

Joerg

Jan 1, 1970
0
Hello Bill,
I don't know what happens today, but Zetex used to just second source
the Supertex line

http://www.supertex.com/products/selector_guides/101/100

They certainly have parts that can switch 180V, in of the order of
10nsec - the speed that you can get does depend on the current sinking
and source capacity of your driver.

Back when I was interested - a long time ago now - Supertex were about
twice as fast as the competition. They'd found some different way of
doing things, which has probably long since lost patent protection.

That picture seems to have changed quite significantly. Nowadays Zetex
builds parts that are hardly rivaled by anyone else.
 
F

Fred Bartoli

Jan 1, 1970
0
Joerg a écrit :
Hello Folks,

Is there an n-channel FET that can stomach 180V or more yet switch in
nanoseconds? I am looking for around 3-4 nsec or less for rise time,
fall time, toff delay and ton delay. The Zetex ZXMN10 does that nicely
but it's only 100V.

How much current do you need?
Otherwise, an obvious and really fast way would be going cascode.
 
H

Henry Kiefer

Jan 1, 1970
0
Hi Joerg -

Have you verified the ZXMN10 in a practical design?

Generally speaken, I think the problem is not the FET. A FET should switch
in under 1ns. The problem is how to control current spread across the die in
value and time. The die is large for PowerMOSFET and the gate is made of
silicon and not thick metal - i think. Additional you have a big case with
much inductance.

I would like to make a Class E amplifier in the 100 to 200MHz range. 100V
would be enough.

Good olle Siliconix.

Maybe you find one at www.ixysrf.com


regards -
Henry
 
C

colin

Jan 1, 1970
0
I don't know what happens today, but Zetex used to just second source
the Supertex line

http://www.supertex.com/products/selector_guides/101/100

They certainly have parts that can switch 180V, in of the order of
10nsec - the speed that you can get does depend on the current sinking
and source capacity of your driver.

Back when I was interested - a long time ago now - Supertex were about
twice as fast as the competition. They'd found some different way of
doing things, which has probably long since lost patent protection.

My gues is its the gate spreading resistance v capacitance that holds the
key.
(just like in a bjt)

Colin =^.^=
 
W

Winfield Hill

Jan 1, 1970
0
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.
 
B

Boris Mohar

Jan 1, 1970
0
Hello Folks,

Is there an n-channel FET that can stomach 180V or more yet switch in
nanoseconds? I am looking for around 3-4 nsec or less for rise time,
fall time, toff delay and ton delay. The Zetex ZXMN10 does that nicely
but it's only 100V.

Zetex ZVN4525
 
H

Henry Kiefer

Jan 1, 1970
0
I think the current delaying within the die is a typical device failure
mechanism. I've seen a MOSFET making cracking noise every time I switched on
the device at 400V and about 300A discharching a flashbulb. That was a more
experimental device made by Harris in 1997 but nether reached market
penetration. Lost my memory what exactly part number.

BTW: The maximum drain current is bounded to V(GS)! So, if you think of
having the die made of thousands of small MOSFETs, all switching at
different time, you never can go beyound a specific drain current
saturation. The RC-lowpass delay across the die gives f(t) max. The die
structure is very different between manufacturers AND R-gate too.

Interesting to see the difference between devices.

- Henry
 
J

Joerg

Jan 1, 1970
0
Hello Win,

Fred, it's around 4A peaks. Very short pulses (less than 50nsec) and a
duty cycle under 0.1%.
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?

Win, I can't disclose the application but it would be ok if the FET
leaves the last 10-20V "on the table". There could be a few pulses in a
row, like 10nsec on, 10nsec off and so on but the total duty cycle would
be next to nothing. Also, it's ok if the FET cannot stomach the full 4A
since we can parallel some. Cost is not the critical parameter here
(isn't that nice for a change?).

I am thinking of using a bank of lower voltage devices like the ZXMN10
that can do these speeds and transform up. But ferrite core transformers
are often frowned upon because they become custom devices.

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.

That sounds quite impressive. We will order HV FETs this morning so we
can give it a shot despite of what the datasheets say.

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.

Yeah, those datasheets. Sometimes it doesn't seem to make sense. One FET
was listed with just a couple nsec turn-on but over 10nsec turn off.
That appears to be too extreme but sez so on the datasheet. I wish they
had a wee bit more detail. Let's see how those devices fare when Fedex
brings them.

Now if I was allowed to design in a tube I'd be home already :)))
 
J

Joerg

Jan 1, 1970
0
Hello Henry,

Have you verified the ZXMN10 in a practical design?

Yes. Works like a champ but won't take more than 100V.

Generally speaken, I think the problem is not the FET. A FET should switch
in under 1ns. The problem is how to control current spread across the die in
value and time. The die is large for PowerMOSFET and the gate is made of
silicon and not thick metal - i think. Additional you have a big case with
much inductance.

I would like to make a Class E amplifier in the 100 to 200MHz range. 100V
would be enough.

Make sure this won't get you into trouble with the Federales (FCC)...

Good olle Siliconix.

Maybe you find one at www.ixysrf.com

Tried them, as well as all the other cutting-edge manufacturers. No luck
but we'll try actual devices now because the datasheet values seem to be
a bit arbitrary in some areas.
 
J

Joerg

Jan 1, 1970
0
Hello Colin,

My gues is its the gate spreading resistance v capacitance that holds the
key.
(just like in a bjt)

But that wouldn't explain a more than 5:1 ratio between tdoff and tdon,
with tdoff being the larger value.
 
H

Henry Kiefer

Jan 1, 1970
0
Joerg said:
Hello Henry,



Yes. Works like a champ but won't take more than 100V.

Interesting. I stored the datasheet in the high-speed folder immediately...
Make sure this won't get you into trouble with the Federales (FCC)...

Class E uses a resonance circuit at the output. At least in my circuit.
Tried them, as well as all the other cutting-edge manufacturers. No luck
but we'll try actual devices now because the datasheet values seem to be
a bit arbitrary in some areas.

Hm. Seems that money is of no problem? Hey, give me a project job. You can
reach me at [email protected] (No joke) I like analog-digital-software mixed
systems and speak german.

What is about using an avalanche device? Should be possible because of the
very low duty-cycle. Zetex makes such devices. I heard they bought them in
russia. Ordinary small bjt should also work in avalanche mode. But that
should you try first and is far beyond the datasheet. Gold-doped transistors
have a extremely short carrier-lifetime. I guess carrier-lifetime is the
reason for your larger toff?? MOSFET includes intrinsic BJTs.


- Henry
 
C

colin

Jan 1, 1970
0
Joerg said:
Hello Colin,



But that wouldn't explain a more than 5:1 ratio between tdoff and tdon,
with tdoff being the larger value.

Thats a good point but wait a minute, it might do for low voltage turn on
devices where the test gate drive is from 0-12v.
If you drove the gate symetricaly about the turn on point maybe tdoff would
get closer to tdon ?
It obviously has An effect, Win elaborated on it brilliantly (as with most
things).

Maybe parallel lower current mosfets would be better ?
presumably they have smaller dies so less far for the gate drive to travel
accros the chip.
could you use a differential set up ? so you would only need 100v devices,
and time would be average of tdon/tdoff?

Of course there are probably other mechanisms wich limit the speed than just
the gate drive voltage propogating accors the die
but im far from being a semiconductor expert;

Colin =^.^=
 
W

Winfield Hill

Jan 1, 1970
0
Joerg wrote...
Hello Win,


Fred, it's around 4A peaks. Very short pulses (less than 50nsec)
and a duty cycle under 0.1%.

Win, I can't disclose the application but it would be ok if the FET
leaves the last 10-20V "on the table". There could be a few pulses in a
row, like 10nsec on, 10nsec off and so on but the total duty cycle would
be next to nothing. Also, it's ok if the FET cannot stomach the full 4A
since we can parallel some. Cost is not the critical parameter here
(isn't that nice for a change?).

It's been my experience you can rapidly finesse the MOSFET on and
off, well beyond the ordinary rules, for a time or two, using care
for the turn-on charge, and turnoff charge, but things add up and
its hard to do as well repeatedly, even for a short time.
I am thinking of using a bank of lower voltage devices like the ZXMN10
that can do these speeds and transform up. But ferrite core transformers
are often frowned upon because they become custom devices.

I used a MTW6n100E, capable of 18A at 1kV for a short time, and
drove its Ciss = 3nF gate with a TC4429, capable of delivering 6A,
through a 1-ohm 1/2W carbon resistor. If we say the gate current
is 4A, that'd only take 4.5ns to move the gate by 6 volts.

High gate drive is important - apply more and more until your
desired speed is obtained. But remove the gate drive rapidly
as well, to minimize charging and allow faster shutoff later.
You may need to use negative voltage to obtain a fast shutoff.
That sounds quite impressive. We will order HV FETs this morning
so we can give it a shot despite of what the datasheets say.

I like to use the smallest MOSFET that can handle the current
spike, to keep the capacitance down.
 
F

Fred Bartoli

Jan 1, 1970
0
Joerg a écrit :
Hello Win,


Fred, it's around 4A peaks. Very short pulses (less than 50nsec) and a
duty cycle under 0.1%.

Hmm, I assumed this was inductive switching, but now 4A-200V and short
current pulses is closer to 50R.
So, what's your load?
 
T

Tom Bruhns

Jan 1, 1970
0
Joerg wrote:
....
Fred, it's around 4A peaks. Very short pulses (less than 50nsec) and a
duty cycle under 0.1%.
....

FWIW...

Back during the early days of power mosfets, I used one in essentially
a forward-mode switching supply to drive a load that varied with time
(well, really with heating...a filament whose resistance increased
considerably as it got hot). I blew up quite a few mosfets figuring
out what was going on right at turn-on: that is, when the power to the
filament was turned on, not when the FET turned on each cycle. This
was before the time of digital scopes, and the fastest available
storage scope had some trouble keeping up. I was getting about 200V in
WELL under 10 nanoseconds on turnoff: it turned out to be because the
load, a transformer that drove the filament, was storing quite a bit of
inductive energy that wanted to come back out quickly. It was NOT
because of inductive kickback overvoltage driving the FET into
avalanche.

Might it be possible to use some inductance in the net load, to help
out?

Warning: at least with older power mosfet designs, too high a dv/dt
could cause failure of the mosfet.

Beware g-d capacitance: 50pF*100V/nsec = 5 amps, and that has to come
out of the gate. There may be some advantage to cascode. Also: if
you can account for delay time externally and only need a fast rise
time, it seems like that should be easier.

I'd hate to add capacitance to the output node you're driving, but
could a P-channel (or N-channel) pullup help? How about a current
from a higher voltage, with a clamp diode to keep it limited to
whatever you want?

Have you looked into power mosfets used in RF amplifiers? That's not
an area I'm very familiar with, but I think you can find them with 200V
ratings. Clearly a mosfet driving a load at 150MHz must be able to
deal with cycles shorter than your application. Normal power mosfets
as you'd use in a 100kHz to 1MHz switching power supply don't work well
for VHF power amplification, as compared with RF power mosfets designed
for the job. They'll be pricier.

Cheers,
Tom
 
H

Henry Kiefer

Jan 1, 1970
0
Tom Bruhns said:
Have you looked into power mosfets used in RF amplifiers? That's not
an area I'm very familiar with, but I think you can find them with 200V
ratings. Clearly a mosfet driving a load at 150MHz must be able to
deal with cycles shorter than your application. Normal power mosfets
as you'd use in a 100kHz to 1MHz switching power supply don't work well
for VHF power amplification, as compared with RF power mosfets designed
for the job. They'll be pricier.

He already played with www.ixysrf.com RF-MOSFETs.

- Henry
 
Joerg said:
I am thinking of using a bank of lower voltage devices like the ZXMN10
that can do these speeds and transform up. But ferrite core transformers
are often frowned upon because they become custom devices.

This jarred loose a stone in my memory: to quickly charge/discharge a
MOSFET gate
there was a trick using a ferrite bead as a saturating transformer,
connected
in positive feedback, primary in series with FET source and secondary
in series with
the gate drive. It was only capable of a limited charge output before
the small bead
saturated, but the effect was to pull the gate off very rapidly because
(until the protection diodes start conducting) the gate wasn't just
grounded,
it was pulled negative.

Modern drivers are better, but the trick might still work. As for
getting nanosecond
wiring into a suitable saturating ferrite, that's something an engineer
could probably
do...
 
R

Robert Baer

Jan 1, 1970
0
Joerg said:
Hello Folks,

Is there an n-channel FET that can stomach 180V or more yet switch in
nanoseconds? I am looking for around 3-4 nsec or less for rise time,
fall time, toff delay and ton delay. The Zetex ZXMN10 does that nicely
but it's only 100V.
What about the BSS131 by Infineon? rather small die size, rated to
240V and N-channel SOT-23 package).
 

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