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Winfield Hill
Guest

Sat Jan 12, 2019 9:45 pm   



dagmargoodboat_at_yahoo.com wrote...
Quote:

Winfield Hill wrote:

There's very little in the signal path. And the 20ns delay to
*the MOSFET output* is about as fast as you'll find. My only
complaint is that the FETs are too big, too much capacitance.
When switching a 50-ohm cable-matching output resistor, a low
70-milli-ohm Ron is serious overkill.

Hey Win, here's a smaller-sized version of the same concept,
driver-plus-GaN on a chip:
https://media.digikey.com/pdf/Data%20Sheets/Navitas%20Semi%20PDFs/NV6113%20Datasheet%20(FINAL)%208-28-18.pdf


Wow, Navitas NV6113, GaN, 200V/ns, only $3.38 at Digi-Key. And
Octopart doesn't even know about them yet! It looks hard to get
heat out of the package, they say limited to 2MHz switching rate.


--
Thanks,
- Win

Winfield Hill
Guest

Sat Jan 12, 2019 9:45 pm   



Winfield Hill wrote...
Quote:

dagmargoodboat_at_yahoo.com wrote...

Winfield Hill wrote:

There's very little in the signal path. And the 20ns delay to
*the MOSFET output* is about as fast as you'll find. My only
complaint is that the FETs are too big, too much capacitance.
When switching a 50-ohm cable-matching output resistor, a low
70-milli-ohm Ron is serious overkill.

Hey Win, here's a smaller-sized version of the same concept,
driver-plus-GaN on a chip:
https://media.digikey.com/pdf/Data%20Sheets/Navitas%20Semi%20PDFs/NV6113%20Datasheet%20(FINAL)%208-28-18.pdf

Wow, Navitas NV6113, GaN, 200V/ns, only $3.38 at Digi-Key. And
Octopart doesn't even know about them yet! It looks hard to get
heat out of the package, they say limited to 2MHz switching rate.


Hah, you have to register, complete with password, just to see
a product list. Well, at least Digi-Key shows three single-FET
switches available now. But their two-FET half-bridge would be
far more interesting to those of us who like to make HV pulsers.

BTW, WRT their stated 2MHz operation spec, the Co(er) spec, which
is 16pF at 400V, that's only P = C V^2 f = 5 watts at 2MHz. But,
oops, with Rth-ja = 50C/W on 1-in-sq copper, that's dT = 250C.


--
Thanks,
- Win


Guest

Sun Jan 13, 2019 1:45 am   



On Saturday, January 12, 2019 at 3:20:53 PM UTC-5, Winfield Hill wrote:
Quote:
Winfield Hill wrote...

dagmargoodboat_at_yahoo.com wrote...

Winfield Hill wrote:

There's very little in the signal path. And the 20ns delay to
*the MOSFET output* is about as fast as you'll find. My only
complaint is that the FETs are too big, too much capacitance.
When switching a 50-ohm cable-matching output resistor, a low
70-milli-ohm Ron is serious overkill.

Hey Win, here's a smaller-sized version of the same concept,
driver-plus-GaN on a chip:
https://media.digikey.com/pdf/Data%20Sheets/Navitas%20Semi%20PDFs/NV6113%20Datasheet%20(FINAL)%208-28-18.pdf

Wow, Navitas NV6113, GaN, 200V/ns, only $3.38 at Digi-Key. And
Octopart doesn't even know about them yet! It looks hard to get
heat out of the package, they say limited to 2MHz switching rate.

Hah, you have to register, complete with password, just to see
a product list. Well, at least Digi-Key shows three single-FET
switches available now. But their two-FET half-bridge would be
far more interesting to those of us who like to make HV pulsers.

BTW, WRT their stated 2MHz operation spec, the Co(er) spec, which
is 16pF at 400V, that's only P = C V^2 f = 5 watts at 2MHz. But,
oops, with Rth-ja = 50C/W on 1-in-sq copper, that's dT = 250C.


--
Thanks,
- Win


I get 16pF x 1 MHz x 400V^2 = 2.56W

But, that's assuming the driver dissipates power in both directions,
which isn't true. When the FET is off and something else is charging
Co, the driver dissipation is essentially zero. So, the actual 2MHz /
400V dynamic dissipation, unloaded, should be about 1.3 watts, right?

Cheers,
James Arthur

George Herold
Guest

Sun Jan 13, 2019 4:45 am   



On Saturday, January 12, 2019 at 12:42:03 PM UTC-5, dagmarg...@yahoo.com wrote:
Quote:
On Saturday, January 12, 2019 at 11:00:55 AM UTC-5, George Herold wrote:
On Saturday, January 12, 2019 at 12:58:21 AM UTC-5, dagmarg...@yahoo.com wrote:
On Friday, January 11, 2019 at 8:53:41 PM UTC-5, George Herold wrote:
On Friday, January 11, 2019 at 3:05:14 PM UTC-5, dagmarg...@yahoo.com wrote:
On Friday, January 11, 2019 at 12:44:48 AM UTC-5, Phil Hobbs wrote:
On 1/10/19 11:46 PM, dagmargoodboat_at_yahoo.com wrote:
RF MOSFETs. They're built for linear operation. At a glance, they seem

Welcome back! You've been a bit scarce since last spring, iirc.

Cheers

Phil Hobbs

Hi Phil!

I've been doing some pretty fun things, but hobby-level
electronics, nothing I thought would be very interesting
for sed. I do still peek in to see what's happening.
James!, As a tadpole here on SED I'd love to hear/see any
of your hobby level 'fun' stuff.
The fare has been a little thin here.

I'm doing this high side switch driving a heater to ground.
Looking at the circuit I got to work, I've got low pass filters
everywhere, with little understanding... which happens sometimes
when you're just trying to get something working.

I think my circuit was missing some resistance on the inverting input.
(I tried to do some asci art, but too much of a Friday night buzz. :^)

High side GS resistor (on pfet) that is turned off by
level sift 'grounded' base npn, with emitter resistor
driven by opamp. A resistor divider senses drain/load
voltage-compares that to error (in opamp) which drives
emitter R.

George H.

Like this?

Q1
V+ >--+-----+-. .-------+--------.
| | ^ | | |
.-. .- - - | |
Rgs | | | .-. |
'-' | | | Rload |
| | '-' |
'----+ | |
| | |
|/ === |
+5v >--| Q2 |
|>. .-.
| | | R1
.-. '-'
Re | | /| |
'-' /+|-----------+
| / | |
'-----< | .-.
\ | | | R2
\-|-- Vref '-'
\| |
===

That looks like an oscillator. :-)

Cheers,
James Arthur

Prezactly! I killed the oscillation with a nF across R2(10k) and
100 nF! of feedback C.
Q1
V+ >--+-----+-. .-------+---------------.
| | ^ | | |
.-. .- - - | |
Rgs | | | .-. |
'-' | | | Rload |
| | '-' |
'----+ | |
| | |
|/ === |
+5v >--| Q2 |
|>. .-.
| | | R1
.-. '-'
Re | | /| |
'-' /+|------------------+-------.
| / | | |
+-----< | .-. ___
| \ | Rcomp | | R2 ___ C2 1nF
| \-|--+-/\/\/-- Vrf '-' |
| \| | | ====
'-----||-----' ===
100nF

Is there some better way to compensate?
It's a heater thing so slow is fine.

Thanks for the nice pic, I certainly miss your presence.

George H.

C2 across R2 is anti-helpful. The problem, in lay terms,
is that your op-amp is reacting very quickly, but to old news.

That is, it's over-reacting to feedback that is delayed in
time. So you're creating a situation where you are already
driving Q2 appropriately, but Q1's output hasn't moved yet,
and your op-amp then tries to drive Q2 even harder even though
its existing drive level was already perfect (if it had only
waited long enough to see). C2 makes that worse.

The easy solution is to eliminate C2 and add in Rcomp. Rcomp,
sufficiently large, slows the op-amp response until the
feedback delay is inconsequential in comparison to the op-amp's
now-gradual corrections. In that way, the op-amp is reacting
to a realistic representation of the results of that op-amp's
last output, and can make appropriate new adjustments.

Another way is to accelerate the feedback to the op-amp
so that the op-amp's information isn't so stale, such
as with feed-forward compensation. Here, that would mean
putting C2 across R1. But if you don't need the maximum
speed possible, just slow down the op-amp. It's less ticklish.

Slowed down, the op-amp will make corrections gradually, then
have plenty of time to see the effects of those corrections,
then make even more corrections. Tada! that's also known as
'closed-loop feedback'. :-)

Did that make sense?

Yeah thanks James. I thought I needed some inverting R.
I need something for the feedback cap to 'work' against.

George H.
Quote:

Cheers,
James Arthur


George Herold
Guest

Sun Jan 13, 2019 5:45 am   



On Saturday, January 12, 2019 at 1:38:33 PM UTC-5, John Larkin wrote:
Quote:
On Sat, 12 Jan 2019 10:17:47 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:

On Saturday, January 12, 2019 at 11:57:16 AM UTC-5, John Larkin wrote:
On Fri, 11 Jan 2019 21:58:18 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:

On Friday, January 11, 2019 at 8:53:41 PM UTC-5, George Herold wrote:
On Friday, January 11, 2019 at 3:05:14 PM UTC-5, dagmarg...@yahoo.com wrote:
On Friday, January 11, 2019 at 12:44:48 AM UTC-5, Phil Hobbs wrote:
On 1/10/19 11:46 PM, dagmargoodboat_at_yahoo.com wrote:
RF MOSFETs. They're built for linear operation. At a glance, they seem

Welcome back! You've been a bit scarce since last spring, iirc.

Cheers

Phil Hobbs

Hi Phil!

I've been doing some pretty fun things, but hobby-level
electronics, nothing I thought would be very interesting
for sed. I do still peek in to see what's happening.
James!, As a tadpole here on SED I'd love to hear/see any
of your hobby level 'fun' stuff.
The fare has been a little thin here.

I'm doing this high side switch driving a heater to ground.
Looking at the circuit I got to work, I've got low pass filters
everywhere, with little understanding... which happens sometimes
when you're just trying to get something working.

I think my circuit was missing some resistance on the inverting input.
(I tried to do some asci art, but too much of a Friday night buzz. :^)

High side GS resistor (on pfet) that is turned off by
level sift 'grounded' base npn, with emitter resistor
driven by opamp. A resistor divider senses drain/load
voltage-compares that to error (in opamp) which drives
emitter R.

George H.

Like this?

Q1
V+ >--+-----+-. .-------+--------.
| | ^ | | |
.-. .- - - | |
Rgs | | | .-. |
'-' | | | Rload |
| | '-' |
'----+ | |
| | |
|/ === |
+5v >--| Q2 |
|>. .-.
| | | R1
.-. '-'
Re | | /| |
'-' /+|-----------+
| / | |
'-----< | .-.
\ | | | R2
\-|-- Vref '-'
\| |
===

That looks like an oscillator. :-)

Cheers,
James Arthur

Is that a linear voltage regulator? There was mention of a highside
switch.

I took it as a linear system but you're right, George said "switch."

If he literally meant hard-switching Q1, that's a different kettle
of fish. A PWM scheme with a slow feedback loop might make more
sense for that.

Cheers,
James

Driving heaters from linear amps is messy. There's the square-law
linearity issue, and general inefficiency.

PWM is better for heater control.

Right, this is going to run at a constant temp
against a mostly constant background... So only some limited power
range. (at least that's my hope. :^)

George H.
Quote:


--

John Larkin Highland Technology, Inc

lunatic fringe electronics



Guest

Mon Jan 14, 2019 12:45 am   



On Sunday, January 13, 2019 at 6:13:19 PM UTC-5, Clifford Heath wrote:
Quote:
On 13/1/19 6:55 am, Winfield Hill wrote:
dagmargoodboat_at_yahoo.com wrote...

Winfield Hill wrote:

There's very little in the signal path. And the 20ns delay to
*the MOSFET output* is about as fast as you'll find. My only
complaint is that the FETs are too big, too much capacitance.
When switching a 50-ohm cable-matching output resistor, a low
70-milli-ohm Ron is serious overkill.

Hey Win, here's a smaller-sized version of the same concept,
driver-plus-GaN on a chip:
https://media.digikey.com/pdf/Data%20Sheets/Navitas%20Semi%20PDFs/NV6113%20Datasheet%20(FINAL)%208-28-18.pdf

Wow, Navitas NV6113, GaN, 200V/ns, only $3.38 at Digi-Key. And
Octopart doesn't even know about them yet! It looks hard to get
heat out of the package, they say limited to 2MHz switching rate.

50C/W - ugly. That would practically limit you to about 2 amps average,
at 300mOhm. That's just not... special.


Au contraire! Two amps is way more than I need, and I much appreciate
the reduced capacitances. (Not all of us are trying to drive big metal
all the time, or launch EMI out into space. :)

There are lots of big wide-gap devices for big-power stuff. But we
don't have many choices yet when it comes to making a dinky lil'
signal generator.


Cheers,
James Arthur

Clifford Heath
Guest

Mon Jan 14, 2019 12:45 am   



On 13/1/19 6:55 am, Winfield Hill wrote:
Quote:
dagmargoodboat_at_yahoo.com wrote...

Winfield Hill wrote:

There's very little in the signal path. And the 20ns delay to
*the MOSFET output* is about as fast as you'll find. My only
complaint is that the FETs are too big, too much capacitance.
When switching a 50-ohm cable-matching output resistor, a low
70-milli-ohm Ron is serious overkill.

Hey Win, here's a smaller-sized version of the same concept,
driver-plus-GaN on a chip:
https://media.digikey.com/pdf/Data%20Sheets/Navitas%20Semi%20PDFs/NV6113%20Datasheet%20(FINAL)%208-28-18.pdf

Wow, Navitas NV6113, GaN, 200V/ns, only $3.38 at Digi-Key. And
Octopart doesn't even know about them yet! It looks hard to get
heat out of the package, they say limited to 2MHz switching rate.


50C/W - ugly. That would practically limit you to about 2 amps average,
at 300mOhm. That's just not... special.

John Larkin
Guest

Mon Jan 14, 2019 2:45 am   



On Mon, 14 Jan 2019 10:13:11 +1100, Clifford Heath
<no.spam_at_please.net> wrote:

Quote:
On 13/1/19 6:55 am, Winfield Hill wrote:
dagmargoodboat_at_yahoo.com wrote...

Winfield Hill wrote:

There's very little in the signal path. And the 20ns delay to
*the MOSFET output* is about as fast as you'll find. My only
complaint is that the FETs are too big, too much capacitance.
When switching a 50-ohm cable-matching output resistor, a low
70-milli-ohm Ron is serious overkill.

Hey Win, here's a smaller-sized version of the same concept,
driver-plus-GaN on a chip:
https://media.digikey.com/pdf/Data%20Sheets/Navitas%20Semi%20PDFs/NV6113%20Datasheet%20(FINAL)%208-28-18.pdf

Wow, Navitas NV6113, GaN, 200V/ns, only $3.38 at Digi-Key. And
Octopart doesn't even know about them yet! It looks hard to get
heat out of the package, they say limited to 2MHz switching rate.

50C/W - ugly. That would practically limit you to about 2 amps average,
at 300mOhm. That's just not... special.


It might make sense thermally to parallel a few smaller parts, EPC
GaNs or these things.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

Gerhard Hoffmann
Guest

Thu Jan 17, 2019 11:45 pm   



Am 12.01.19 um 06:39 schrieb dagmargoodboat_at_yahoo.com:

Quote:
One of my problems, currently, is that most of the high-voltage fancy
FETs are monstrous compared to my needs, so even though their figures-
of-merit Ron vs. Qg are worlds better than the older tech stuff, the
fact that they're 20x over-sized for my application makes them slower
than an appropriately-sized MOSFET.

Even the most nimble GaN garbage truck FET still isn't as gamely as a
silicon itty bitty unicycle FET.


A bit smaller than the other transistors that were mentioned,
but nevertheless DC to 6 GHz:

< https://www.mouser.de/datasheet/2/90/ghv27030s-947886.pdf >

A nice driver, at least :-)

Cheers,
Gerhard

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