Controlled switching of FETs

I've got a TEC thermal control circuit where I had to run a separate power return ground line.

By hanging more C on the gate? a Tee RCR network?

George H. That's

Why don't people just ground things?

John

No, that will be low impedance at high frequency, which risks oscillation (forms a pi network with the gate bond wire inductance and the internal capacitances). I just increased the gate resistor to several K. Miller does the rest.

I thought about it long and hard first, and I think hi-z gate drive avoids the various oscillation modes. That disrupts all the possible tuned circuits at the gate that might provide phase shift, kills all the Q's, and prevents high currents flowing anywhere. Still, it was reassuring to try it in real life. It works.

-- Cheers, James Arthur

That's

In my case I had a monitor voltage that went of to another board. The TEC could draw an amp or two, which would put a bit of offset on the monitor signal... I don't recall how much... 100's of uVs? Running another power supply ground wire was an easy fix.

George H.

That's

I sure wish they would.

Here, they've got up to 20A in loads, which they didn't want polluting the a/d inputs on the logic board, I s'pose.

Part of my difficulty is that, if you imagine PWR_GND is an inductor, which it is, hanging a tank on the gate makes a classic RF oscillator. An ordinary resistive drive yields a lightly damped impulse. Big.

Okay, so, on top of that ringing, I have a source-current sensing resistor as part of protecting the FET, which I have to read bouncing @ 250Khz.

I think it's whipped now. Ring amplitude way less, damping much higher, settles faster. RFI reduced dramatically, too.

-- Cheers, James Arthur

=A0That's

This system puts 10-20A on that wire. The architect is long gone, so I can only guess his motivation was similar to yours. The logic board is some distance from the system power supply, so running the load currents through the logic board would put transients on the logic board ground. To avoid that, he added a separate return.

Given that, I'd float and galvanically isolate the drive and sensing circuits from the control logic, if it were up to me. Reference them all to the FET. Driving a logic-level FET with an inductive source load, from a driver on a different ground, is really asking for it.

Heavy damping / hi-z drive worked here, which is why I posted it--I've not had this problem before, or seen a solution.

-- Cheers, James Arthur

John

=A0That's

plug in something like this

-Lasse

Saw no mention of a local bypass cap AT the FET (source to +24V supply)..

Not enough dirt??

But that decreases the drive current and (as you know) decreases the turn-on / turn-off speeds.

=A0That's

In my published Peltier-based thermostat, we optically isolated the MOSFET drivers from the rest of the circuit just to avoid having to put the Peltier drive current through a shared ground return. Worked fine.

Sloman A.W., Buggs P., Molloy J., and Stewart D. =93A microcontroller- based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor=94 Measurement Science and Technology, 7 1653-64 (1996)

During bug-hunting, my successor (Paul Buggs - a friend) temporarily swapped out the switching drive for the Peltier junction and replaced it with a linear driver we'd put together when we charactering various candidate Peltier junctions and it didn't change the noise levels at all, which were still dominated by processor noise.

-- Bill Sloman, Nijmegen

=A0That's

pply)..

Define "local". Bypass to which ground? There's a spider-web of supply and ground wires. So, there's no way to "bypass" the loads to a common reference, only to / through yet another inductive ground wire.

Some of the loads have built-in bypass capacitors; that makes the load happy, but the ground transient when you turn them on worse, of course.

-- Cheers, James Arthur

he

Which was the goal. We usually switch FETs hard and fast; it was interesting doing a controlled slew rate. Making a Miller integrator makes a nice, clean, linear ramp.

This could find use in switchers, as per Jim Williams' LT1534; occasionally a 1-2uS switching time wouldn't hurt the FET, doesn't waste much power, and would clean up a lot of RFI.

-- Cheers, James Arthur

We place a ferrite ring directly on the lead of the FET.

I'm using some protected FETs on this very board, but I couldn't find suitable FETs for each load. I missed this one--Digikey has it under PMIC instead of MOSFETs.

It's hard to find it by function even now, when I know it's there, looking specifically for it. (I get a zillion hits under PMIC, and commonsense qualifiers eliminate good parts (including this one), for some reason.)

Nice part--thanks.

-- Cheers, James Arthur

this

Check out the stuff Zetex makes too.

=A0That's

That's certainly my preference. Here, I guess packaging is the limitation--the components are spread physically, sparsely sprinkled throughout the system, with a bunch of wires leading to a star ground(*) at the main power supply.

(*) Sort of. Most of the electronics & analogs star to the controller, which stars to the main.

-- Cheers, James Arthur

Parasitic choke inline with the DS.

Jamie