Paul Burridge wrote in message . ..
....
It'd be fun to see just how fast you can force current in and out of
an ordinary, GP power MOS FET gate to get it switching as fast as
possible, I reckon, before pressing ahead with the dedicated devices.

Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating. I
was seeing close to 100V in about 5nsec just before the
self-destruction as I recall. And the Siliconix V-mos transistors
were good for RF power back in that era.

If you're thinking driving it "digitally", that's probably the wrong
answer. Resonate the input and output capacitances, and life will be
much easier. Charging and discharging capacitance through a resistive
source is quite inefficient. Remember, too, that tuned triode valve
amplifiers are generally neutralized. Be VERY careful to not exceed
the gate-source voltage rating! Have fun playing, but expect some
"surprises."

Cheers,
Tom

On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!

John

"John Larkin" a écrit dans
le message news: ...
On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!


If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?

Thanks,
Fred.

On Thu, 6 May 2004 17:43:10 +0200, "Fred Bartoli"
r_AndThisToo wrote:


"John Larkin" a écrit dans
le message news: ...
On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!


If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?


I think that most mosfet substrate diodes are now designed to have
soft recovery, so that they won't snap and make a horrible dv/dt. I
blew up a lot of early Motorola mosfets in an h-bridge motor driver...
had to switch to darlingtons, and only figured it out later.

Anything with a p-i-n structure has a chance of being a snap diode. I
think it needs a hyperbolic doping profile or something to work well.
1N4005-7 types work, but usually only after a brief forward bias, not
DC. Somebody told me that many varicaps snap, but I haven't verified
that. Specifically-designed PIN diodes (the kind used in RF switches
and attenuators) don't snap, as they are doped to have very long
recovery times.

We tested over 60 different TO-220 power diodes to find the best
high-voltage drift step-recovery part. Then we found something else,
much better and more repeatable, but that's still a secret.

John

On Thu, 6 May 2004 17:43:10 +0200, "Fred Bartoli"
r_AndThisToo wrote:


"John Larkin" a écrit dans
le message news: ...
On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!


If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?


I think that most mosfet substrate diodes are now designed to have
soft recovery, so that they won't snap and make a horrible dv/dt. I
blew up a lot of early Motorola mosfets in an h-bridge motor driver...
had to switch to darlingtons, and only figured it out later.

Anything with a p-i-n structure has a chance of being a snap diode. I
think it needs a hyperbolic doping profile or something to work well.
1N4005-7 types work, but usually only after a brief forward bias, not
DC. Somebody told me that many varicaps snap, but I haven't verified
that. Specifically-designed PIN diodes (the kind used in RF switches
and attenuators) don't snap, as they are doped to have very long
recovery times.

We tested over 60 different TO-220 power diodes to find the best
high-voltage drift step-recovery part. Then we found something else,
much better and more repeatable, but that's still a secret.

John

"John Larkin" a écrit dans
le message news: ...
On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!


If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?

Thanks,
Fred.

On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!

John