Linear power FETs

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/

Prezactly! I killed the oscillation with a nF across R2(10k) and

100 nF! of feedback C.

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.

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

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

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

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

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

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

Hey Win, here's a smaller-sized version of the same concept, driver-plus-GaN on a chip:

600V and 300 milliohms instead of 70 milliohms.

Cheers, James Arthur

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

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

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

George H.

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.

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

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

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Am 12.01.19 um 06:39 schrieb snipped-for-privacy@yahoo.com:

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

< >

A nice driver, at least :-)

Cheers, Gerhard

Wow, that's one beast of a driver ;-)

Cheers, James Arthur