John Larkin's fast 50V pulse with a 2n7000.

[Winfield Hill]

John Larkin wrote...

See John's fast rise/fall-time 50V pulse with a 2n7000, on a.b.s.e.

That's pretty good, 1.9ns rise and 2.5ns fall, how'd you do that?
Hmm, how short can you make it (without using a PFN)?


--
Thanks,
- Win

 

[John Larkin]

On 30 Mar 2005 17:03:25 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

John Larkin wrote...

See John's fast rise/fall-time 50V pulse with a 2n7000, on a.b.s.e.

That's pretty good, 1.9ns rise and 2.5ns fall, how'd you do that?

The fets are pretty much infinitely fast inside, so it's just a matter
of gate drive. We're driving them with a bunch (emphasis bunch) of
fast TinyLogic gates. I did another circuit a couple years ago that
drove a pair of 2N7002's with a gaasfet... +12 volts into the gates in
about 400 ps. Turnon speed was awesome, but turnoff drive was passive
so the falling edge was rotten.

Hmm, how short can you make it (without using a PFN)?

This is the HV option on our benchtop pulse/delay generator, so we can
make the pulse width anything. Low end, it starts to lose swing at
about 3 ns width; for wider pulses, the transformer saturates at
something like 1 usec at 50 volts, 10 usec at 5 volts.

Nice looking pulse, huh? It almost looks faked.

John

 

[Winfield Hill]

John Larkin wrote...

Winfield Hill wrote:

See John's fast rise/fall-time 50V pulse with a 2n7000, on a.b.s.e.
That's pretty good, 1.9ns rise and 2.5ns fall, how'd you do that?

The fets are pretty much infinitely fast inside, so it's just a matter
of gate drive. We're driving them with a bunch (emphasis bunch) of
fast TinyLogic gates. ...

I assume you're switching one 2n7000 to ground. What's the load, a
50-ohm or other resistive pullup, with no coax cable, etc., just a
low-capacitance scope probe?

Nice looking pulse, huh? It almost looks faked.

A textbook pulse. One of the issues in fast-driving power MOSFETs is
their gate spreading resistance. Depending upon the structure, some
of the FET's area is quickly reached by the gate signal, for fast turn
on and turn off, but the remaining area is "further away," with an RC
delay. This portion won't turn on until a little later in the pulse,
an aspect that doesn't have much effect on the turn-on waveform unless
the FET is running at very high currents and the reduction of Ron with
time is apparent. But at turnoff the "near" portion can be turned off
quickly, just as it was turned on, but some of the "far" portion, which
sadly is in parallel to the output after all, is RC-delayed and turns
off slowly, and the waveform can have a gradual-recovery appearance.

BTW, this is another aspect that spice models fail to handle properly.

Your 2.5ns turnoff is impressive. Does this indicate the 2n7000 parts
you were using don't suffer from the gate-spreading resistance problem?
Which manufacturer's FET were you using? Also, don't you prefer 2n7002
SMT parts rather than 2n7000 TO-92 through-hole for your fast circuits?

The list of 2n7002 manufacturers and the variants they offer is quite
impressive (A, C, D, E, F, K, L, LT1, M, T). And the datasheet output
characteristics plots show even more variation than I saw in the 2n7000
families.

What's your experience with these? For example, Fairchild's 2n7002MTF
type says it has "Improved Inductive Ruggedness," which sounds useful,
although I'm not sure what that means. Better avalanche capability?
This transistor's online datasheet features nearly unreadable plots,
http://www.fairchildsemi.com/pf/2N/2N7002MTF.html but it appears it's
capable of switching 2.5A, more than most other 2n7000 or 2n7002 parts.
They charge more, 7.5 cents rather than 5.6 cents, qty 1000. :&gt


--
Thanks,
- Win

 

[Winfield Hill]

Winfield Hill wrote...

John Larkin wrote...

Winfield Hill wrote:

See John's fast rise/fall-time 50V pulse with a 2n7000, on a.b.s.e.
That's pretty good, 1.9ns rise and 2.5ns fall, how'd you do that?

The fets are pretty much infinitely fast inside, so it's just a matter
of gate drive. We're driving them with a bunch (emphasis bunch) of
fast TinyLogic gates. ...

I assume you're switching one 2n7000 to ground. What's the load, a
50-ohm or other resistive pullup, with no coax cable, etc., just a
low-capacitance scope probe?

Oops, I missed your explanation in a.b.s.e.

"Here's a 50 volt pulse into a 50 ohm load. The output stage is two
2N7002's (the sot-23 version of the 2N7000) in parallel, driving a
transmission-line transformer for isolation. I wanted to go 75 volts,
but my guys started mumbling something about exceeding the abs max
specs or some wimpy nonsense like that.

"Still, that's a lot of slam for 50 cents worth of fets!"

Can you detail the transmission-line transformer connection?


--
Thanks,
- Win

 

[John Larkin]

On 31 Mar 2005 01:22:55 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Winfield Hill wrote...

John Larkin wrote...

Winfield Hill wrote:

See John's fast rise/fall-time 50V pulse with a 2n7000, on a.b.s.e.
That's pretty good, 1.9ns rise and 2.5ns fall, how'd you do that?

The fets are pretty much infinitely fast inside, so it's just a matter
of gate drive. We're driving them with a bunch (emphasis bunch) of
fast TinyLogic gates. ...

I assume you're switching one 2n7000 to ground. What's the load, a
50-ohm or other resistive pullup, with no coax cable, etc., just a
low-capacitance scope probe?

Oops, I missed your explanation in a.b.s.e.

"Here's a 50 volt pulse into a 50 ohm load. The output stage is two
2N7002's (the sot-23 version of the 2N7000) in parallel, driving a
transmission-line transformer for isolation. I wanted to go 75 volts,
but my guys started mumbling something about exceeding the abs max
specs or some wimpy nonsense like that.

"Still, that's a lot of slam for 50 cents worth of fets!"

Can you detail the transmission-line transformer connection?

It's just a few turns (4? I think) of micro-coax wound on a small
toroid. The braid is the primary and the inner is the secondary.
Probably a pot core would be more elegant, easier to wind and mount;
gotta try that some day.

John

 

[Phil Hobbs]

John Larkin wrote:

Can you detail the transmission-line transformer connection?



It's just a few turns (4? I think) of micro-coax wound on a small
toroid. The braid is the primary and the inner is the secondary.
Probably a pot core would be more elegant, easier to wind and mount;
gotta try that some day.

John



Higher leakage inductance, though.

Cheers,

Phil Hobbs

 

[John Larkin]

On Thu, 31 Mar 2005 11:05:41 -0500, Phil Hobbs
<pcdh@SpamMeSenseless.us.ibm.com> wrote:

John Larkin wrote:

Can you detail the transmission-line transformer connection?



It's just a few turns (4? I think) of micro-coax wound on a small
toroid. The braid is the primary and the inner is the secondary.
Probably a pot core would be more elegant, easier to wind and mount;
gotta try that some day.

John



Higher leakage inductance, though.

The secondary is *inside* the primary! The core's just for
low-frequency response.

John

 

[John Woodgate]

I read in sci.electronics.design that Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote (in
<d2gehu02r5i@drn.newsguy.com&gt about 'John Larkin's fast 50V pulse with
a 2n7000.', on Thu, 31 Mar 2005:

What's your experience with these? For example, Fairchild's 2n7002MTF
type says it has "Improved Inductive Ruggedness," which sounds useful,
although I'm not sure what that means.

Obvious; it's an L of a lot better than the others.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Winfield Hill]

John Woodgate wrote...

Winfield Hill wrote ...

What's your experience with these? For example, Fairchild's 2n7002MTF
type says it has "Improved Inductive Ruggedness," which sounds useful,
although I'm not sure what that means.

Obvious; it's an L of a lot better than the others.

ROFLOL.


--
Thanks,
- Win