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George Herold
Guest

Sat Jan 12, 2019 3:45 am   



On Friday, January 11, 2019 at 3:05:14 PM UTC-5, dagmarg...@yahoo.com wrote:
Quote:
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.


Quote:

Cheers!
James



Guest

Sat Jan 12, 2019 6:45 am   



On Friday, January 11, 2019 at 5:37:21 PM UTC-5, Tim Williams wrote:
Quote:
dagmargoodboat_at_yahoo.com> wrote in message
news:7d99192c-fcde-4176-95d4-819c25ad0b6e_at_googlegroups.com...
Since I'm wrestling wire-bond parasitics trying to get my
FETs /moving/, passivated die with solder bumps sounds fantastic.
I just wish they'd start delivering them!

??? EPC eGaN FETs have been around Digikey for half a decade IIRC!

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/


You didn't read the OP, did you?

That's okay, we all put our foot in it sometimes. :-)

Cheers,
James Arthur


Guest

Sat Jan 12, 2019 6:45 am   



On Friday, January 11, 2019 at 11:55:01 PM UTC-5, Clifford Heath wrote:

Quote:
The parasitics of a package that can produce 1800W CW over 1.8-400MHz
must surely be rather small?

https://www.nxp.com/products/rf/rf-power/rf-ism-and-broadcast/1-600-mhz-broadcast-and-ism/1800-w-cw-over-1.8-400-mhz-65-v-wideband-rf-power-ldmos-transistor:MRFX1K80H

Clifford Heath.


Excellent point. Yes, you'd think their parasitics would have to be
small. "Unmatched" seems a promising magic keyword for pulse applications.

I skimmed through the listings, but I didn't notice any that switch
hundreds of volts, not even the RF Cooking transistors. I need that,
at the moment.
(I've made some of my own R.F. 'cooking transistors,' including my first
and only +23dBm 920MHz p.a.)(Of which about 2e6 were ultimately produced.)

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.

Cheers,
James Arthur


Guest

Sat Jan 12, 2019 6:45 am   



On Friday, January 11, 2019 at 6:58:36 PM UTC-5, John Larkin wrote:
Quote:
On Fri, 11 Jan 2019 13:40:22 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:

On Friday, January 11, 2019 at 2:23:15 PM UTC-5, Gerhard Hoffmann wrote:
Am 11.01.19 um 02:58 schrieb dagmargoodboat_at_yahoo.com:


This little guy has Rth(jc) = .25K/W--pretty impressive.
http://ixapps.ixys.com/DataSheet/de275_501n16a.pdf

That's probably what has become of Directed Energy.
They bought die from APT IIRC and put them into their
freshly developed stripline case.

They were the bee's knees some 15 years ago in ham radio
amplifiers but seem to have faded away, or I did not follow it
since my interest shifted to VHF+.

cheers, Gerhard

Cool. An internet search for "vhf fet PA power amplifier"
brings up some interesting designs.

Here's one: n=77% @ 2 meters, 500W
https://www.researchgate.net/publication/309046370_ACKA_500_watt_power_amplifier_for_a_144_MHZ_channel_based_on_a_VHF_push-pull_power_mos_transistor

As expected, RF guys just resonate out bond wire inductances, something
I can't do when making fast pulses.

Cheers,
James Arthur

Right. That's why RF dmos fets aren't good for pulses. Some are
optimized for a specific RF band.

2 GHz is easy. DC to 2 GHz isn't as easy.


But I called out one, above, that's spec'd to tr=3nS @ 400V. That
looked promising.

Quote:
"N-channel enhancement mode linear RF power MOSFET
Ideal for class AB and C industrial, scientific, medical, and
commercial applications.
VDSS = 500 V ID25= 10 A"
https://media.digikey.com/pdf/Data%20Sheets/IXYS%20PDFs/IXZH10N50L2(A,B).pdf


Cheers,
James

Clifford Heath
Guest

Sat Jan 12, 2019 6:45 am   



On 12/1/19 8:40 am, dagmargoodboat_at_yahoo.com wrote:
Quote:
On Friday, January 11, 2019 at 2:23:15 PM UTC-5, Gerhard Hoffmann wrote:
Am 11.01.19 um 02:58 schrieb dagmargoodboat_at_yahoo.com:


This little guy has Rth(jc) = .25K/W--pretty impressive.
http://ixapps.ixys.com/DataSheet/de275_501n16a.pdf

That's probably what has become of Directed Energy.
They bought die from APT IIRC and put them into their
freshly developed stripline case.

They were the bee's knees some 15 years ago in ham radio
amplifiers but seem to have faded away, or I did not follow it
since my interest shifted to VHF+.

cheers, Gerhard

Cool. An internet search for "vhf fet PA power amplifier"
brings up some interesting designs.

Here's one: n=77% @ 2 meters, 500W
https://www.researchgate.net/publication/309046370_ACKA_500_watt_power_amplifier_for_a_144_MHZ_channel_based_on_a_VHF_push-pull_power_mos_transistor

As expected, RF guys just resonate out bond wire inductances, something
I can't do when making fast pulses.


That paper, while only 5 years old, is using an LDMOS FET that's more
than 20 years old. The analog TV transmitters it was made for have been
retired locally after decades of service. Just sayin', our Indonesian
fraternity might not be up with the very latest... and they don't show
their circuits either.

NXP publish a lot of good app notes and reference designs. Some of the
RF power amps span well over a decade of bandwidth, limited mainly by
the bandwidth of the baluns they use to transform between 50 ohms and 6
ohms (input) and 2-3 ohms output impedances. That's not really
consistent with your claim that the lead inductances are just resonated
out. If you look closely at the geometry of these transistors and at the
reference designs themselves, in many cases the inductance starts to
dominate over the capacitance only at some hundreds of MHz. So I don't
think your complaint about RF FETs is entirely fair.

Google for "site:nxp.com RF power" to get all the good info.

The parasitics of a package that can produce 1800W CW over 1.8-400MHz
must surely be rather small?

<https://www.nxp.com/products/rf/rf-power/rf-ism-and-broadcast/1-600-mhz-broadcast-and-ism/1800-w-cw-over-1.8-400-mhz-65-v-wideband-rf-power-ldmos-transistor:MRFX1K80H>

Clifford Heath.


Guest

Sat Jan 12, 2019 7:45 am   



On Friday, January 11, 2019 at 8:53:41 PM UTC-5, George Herold wrote:
Quote:
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

Tim Williams
Guest

Sat Jan 12, 2019 3:45 pm   



<dagmargoodboat_at_yahoo.com> wrote in message
news:f043f83b-114b-43bd-962f-267e9638fb41_at_googlegroups.com...
Quote:
You didn't read the OP, did you?

That's okay, we all put our foot in it sometimes. Smile


What, the 350V one?

The 200V ones have been around a while; I guess they expect you to cascode
for more.

You never did say what voltage you're targetting. Don't put your foot in too
deep either :-)

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

George Herold
Guest

Sat Jan 12, 2019 5:45 pm   



On Saturday, January 12, 2019 at 12:58:21 AM UTC-5, dagmarg...@yahoo.com wrote:
Quote:
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.
Quote:
Q1
V+ >--+-----+-. .-------+--------.
| | ^ | | |
.-. .- - - | |
Rgs | | | .-. |
'-' | | | Rload |
| | '-' |
'----+ | |
| | |
|/ === |
+5v >--| Q2 |
|>. .-.
| | | R1
.-. '-'
Re | | /| |
'-' /+|-----------+-------.
| / | | |
+-----< | .-. ___
| \ | | | R2 ___ C2 1nF
| \-|--+- Vrf '-' |
| \| | | ====
.--C1 0.1uF--. ===


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.

Winfield Hill
Guest

Sat Jan 12, 2019 6:45 pm   



dagmargoodboat_at_yahoo.com wrote...
Quote:

Winfield Hill wrote:
dagmargoodboat_at_yahoo.com wrote...

This 650V part puts an enhancement-mode GaN FET in cascode with a
depletion-mode GaN JFET. It's real, and it's kind of interesting too:
https://www.transphormusa.com/en/document/650v-cascode-gan-fet-tph3206psb/

LMG3410R070, 600V 40A GaN, 100V/ns, integrated driver and protection.

I have to make accurate fast rectangular pulses with flat tops
and high edge timing accuracy, so for now I'm wary of having
that much complexity between my trigger and the output signal.


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.


--
Thanks,
- Win

John Larkin
Guest

Sat Jan 12, 2019 6:45 pm   



On Fri, 11 Jan 2019 21:58:18 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:

Quote:
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.




--

John Larkin Highland Technology, Inc

lunatic fringe electronics


Guest

Sat Jan 12, 2019 6:45 pm   



On Saturday, January 12, 2019 at 11:00:55 AM UTC-5, George Herold wrote:
Quote:
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?

Cheers,
James Arthur

John Larkin
Guest

Sat Jan 12, 2019 7:45 pm   



On Sat, 12 Jan 2019 10:17:47 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:

Quote:
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.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics


Guest

Sat Jan 12, 2019 7:45 pm   



On Saturday, January 12, 2019 at 12:37:54 PM UTC-5, Winfield Hill wrote:
Quote:
dagmargoodboat_at_yahoo.com wrote...

Winfield Hill wrote:
dagmargoodboat_at_yahoo.com wrote...

This 650V part puts an enhancement-mode GaN FET in cascode with a
depletion-mode GaN JFET. It's real, and it's kind of interesting too:
https://www.transphormusa.com/en/document/650v-cascode-gan-fet-tph3206psb/

LMG3410R070, 600V 40A GaN, 100V/ns, integrated driver and protection.

I have to make accurate fast rectangular pulses with flat tops
and high edge timing accuracy, so for now I'm wary of having
that much complexity between my trigger and the output signal.

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.


--
Thanks,
- Win


When you take tpd as of the driver+FET, 20nS isn't bad. But I
need sub-nS edge-timing accuracy, so 20nS and a resistor-set
slew rate are rather scary to me at a time when I'm worried
about logic threshold drift affecting my timings.

I think I can manage about half that delay using hand-picked
parts, without getting too fancy, and then having the whole
signal path available and under my control.

Also, mine is a totem-pole driver. I'd have to float the high-side
LMG3410, pass its logic commands through an isolator that can handle
insane slew rates, and power it all with a floating supply.

With the signal isolator, my delay would be approaching 30nS. I'd
have to use a similar isolator on the low side to match the high-side
delays.

All of that is do-able, naturally. But I'm trying to avoid it, here.

And I don't trust most of the digital isolators when it comes to
jitter--who knows how much those modulated transmission schemes
jitter? Excess jitter is something this application can't abide.

I do love the LMG3410 though--it is a very tempting concept, and a
tempting part.

Cheers,
James Arthur


Guest

Sat Jan 12, 2019 7:45 pm   



On Saturday, January 12, 2019 at 11:57:16 AM UTC-5, John Larkin wrote:
Quote:
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


Guest

Sat Jan 12, 2019 8:45 pm   



On Saturday, January 12, 2019 at 12:37:54 PM UTC-5, Winfield Hill wrote:
Quote:
dagmargoodboat_at_yahoo.com wrote...

Winfield Hill wrote:
dagmargoodboat_at_yahoo.com wrote...

This 650V part puts an enhancement-mode GaN FET in cascode with a
depletion-mode GaN JFET. It's real, and it's kind of interesting too:
https://www.transphormusa.com/en/document/650v-cascode-gan-fet-tph3206psb/

LMG3410R070, 600V 40A GaN, 100V/ns, integrated driver and protection.

I have to make accurate fast rectangular pulses with flat tops
and high edge timing accuracy, so for now I'm wary of having
that much complexity between my trigger and the output signal.

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.


--
Thanks,
- Win


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>

600V and 300 milliohms instead of 70 milliohms.

Cheers,
James Arthur

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