Suggestions for stabilising a TL431 + MOSFET current sink

Cap across it, from high side to ground.

John

Oh, a capacitor across the whole thing, +24 to ground, might help too.

John

I recommend a cap from K to R on the TL431. 10 nF is probably more than enough.

Your MOSFET is shown as a P type?? 33N10 certainly isn't..

Compensation across the '431 is the way to go. Besides 10nF, you may = need series resistance to its input, to set the RC time constant. I = don't know what R5 and R6 are; R7 is small enough at 150 ohms to be = tough to drive. It's not like the input takes much current, 10k ohms is = plenty.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

Oops, I do mean N type, the part number is correct, last time I was drawing MOSFETs, they were high side switching with P-channels, didn't notice I drew the arrow wrong way round here.

Your eyes probably didn't believe the numbers 20, 12, 12 on the diagram for trimpot, R5 and R6, the values are in Ohms. I did mention 17mA going down that part. Yes, it low, I wanted fairly high current through there to swamp the 20 Ohm trimpot's lower spec tolerance so the adjustment range worked as expected (it did :)

Not from the ~2.5 Ohm source, it isn't ;)

Well, I put 200 Ohms in there between R9 and TL431.R, to get something to work against, then added 1.0nF from R to K, plus

Just come back to the PC now to see if anyone replied to my post, after adding some caps and stuff to the circuit. Funny thing is that after writing up the problem, I had to problem guesstimating a set of values that stabilised the thing first try, when I went back to the soldering iron.

I think it was a confidence issue, because last time I tried to stabilise a linear MOSFET power amp (8 n-channels in parallel) the thing simply didn't work out for me. That was 18 months or two years ago, and I think I was given some logic threshold devices to work with.

This current sink is now over-compensated (I think), but I can't tell until I make a little load switcher to give the thing a step change so I can watch the response and fine tune it.

Drifts only about 1mV when it warms up, another mV variation from short circuit to 20 Ohms load. So I think it will be okay for what I want.

I'm expecting to trim compound power resistors to about one percent accuracy with this current source, so it is good enough for that.

The compound resistors I'll be trimming only need to be accurate enough for monotonic operation over a 6 bit range, so aiming for

Grant.

...

Thanks, see my reply to Tim, I put 1.0 nF in there, after adding 200 Ohm to R9 for the cap to act against.

Grant.

And if it's not, you can always go cascode.

You could even make a "corrected cascode", so an op-amp drives the = cascoding transistor so as to keep its source at a constant voltage. = Something you're already doing with this transistor, so you just do it = twice. That should essentially eliminate early effect from = consideration. As is, TL431's gain should be more than high enough to = not care about this yet.

I'm sure you've taken stock of such things as tempco? A fan might not = be a bad idea; turbulence is supposed to make things much noisier, but = that should still be less error than the error due to tempco, if the = resistors are getting very warm. The resistors down inside that monster = stack will run slightly warmer without forced air. Maybe not by much.

The TL431 itself is only about 1% (2.475-2.525V), so you'd need a much = better reference to do anything more accurate anyway. At this point you = start getting into precision *everything*, calibration procedures and, = ehh maybe not as far as tracable references...

Tim

--=20 Deep Friar: a very philosophical monk. Website:

MOSFET has Early effect? Or, do you mean the TL431?

I was concerned the TL431 wouldn't have enough gain, but it seems good enough for this task.

No, such things I measure or get a feel for once built ;)

The resistors sharing 2.5 Watts, 104mW each. Yes, they feel a little hotter than the heatsink, maybe a fan _is_ good idea, particularly if I'm using the thing in summer, it gets to 40'C in here. At the moment, my heater is struggling to get the place up to 20'C ;)

The controller is bolted really close to the heatsink, there is a noticeable drift on warmup, but only a mV or so. And, I measure shunt voltage at the sensing end, not the where the current is being carried -- four wire measurement.

Texas TL431B I'm using here is 2.495V +/- 12mV, 0.5% initial accuracy, (TL431A is 1%, standard part is 2% -- TI datasheet) plus I trim the current via voltage across the 0.1% sense resistor, so circuit accuracy depends on the 0.1% shunt resistor and tolerance of Fluke 4 1/2 digit multimeter (it _is_ quite an old meter though, Fluke Model 8050A).

The circuit drifts about 1mV across the shunt for warmup, another mV for load varying between near zero to 20 Ohms, and another mV for line regulation (24.8 to 29V on charge, but it's not normal to charge the batteries whilst using them for this sort of thing, also not good for the batteries either as the charger not smart enough to compensate for a load).

Since current is measured at about 2.5 mV per mA through the shunt, the circuit is accurate to something better than 2mA plus 0.1% shunt and tolerance for the meter used to adjust the shunt voltage to match the calculated value (2.535V).

I don't know how to tell LTSpice how to model the TL431 and trust I'd get an accurate simulation. So it's the old soldering iron method for me. I think it doesn't do trimpots either, but it's been a while since I use LTSpice. Might have a go at it later.

Grant.

... It's now compensated and on overnight test :) New circuit is up too.

or, see

Restoring C1 across the 13V zener had no effect, adding supply bypass and the usual feedback compensation components worked first try. The circuit may be under-damped, I need to make a load step switcher so I can see how the circuit responds for dynamic response, another day.

Makes small sparks when shorting the output with a nickel plated alligator clip to stainless steel resistor lug. Would you expect that from a one Amp constant current sink running from 24V?

Power supply is fairly stiff, a pair of 12V, 100AH SLA batteries through a 35A auto spade style fuse and a couple or three metres of cable.

Grant.

I hate that mosfet symbol. I draw mosfets as

| |-

--------| | |>

and

| |<

--------| | |- |

which is much more obvious.

John

I use the old symbol. I don't like it, but there it is.

Yours looks a little like an IGBT. Might that cause confusion?

James

(snip)

Use the LT1431 which comes with LTSpice. Look in the Op-Amps folder.

Cheers, John

Would a small cap between GND and the junction of R4 and The mosfet gate be of any use? SOmetimes a stopper resistor (R4) is not enough on its own.

It doesn't confuse me! Besides, my source and drain thingies come out horizontal, and an IGBT uses angles, and a funny emitter thing.

And I don't use IGBTs!

The symbol lets you know, intuitively, which direction the current flows, and that the gate is insulated, namely not a BJT. After that, you have to look up the datasheet.

I've seen "my" symbol used in some IC equivalent schematics. I've also seen

which I don't like much.

I draw mesfets and phemts just like jfets, because they behave that way.

Oh, I put a dot in the middle of schottky diodes...

|\ | | \ |

----------|* \|-------- | / | |/ |

That's the hot carrier.

John

You could eliminate D3, R3, C2, C3, and R10. But it would be fairly slow. A fast load box probably needs opamps.

John

(1) Insert moderately high value resistor between pot ("VR1") tap and "R" terminal of TL431

(2) Then a cap "K" to ""R"

The TL431 is, after all, an OpAmp of sorts :-)

Caution! Still a potential for oscillation... the cap from "K" to "R" may need a small series resistor to introduce a "zero"/phase-lead, because of the MOSFET capacitances. ...Jim Thompson

"John Larkin" wrote in = message news: snipped-for-privacy@4ax.com...

Well gee, that's just patently wrong. The arrow is the substrate, so = you've drawn a P type substrate follower. Or something. Which has = absolutely no physical meaning whatsoever, and is forward biased, which = is completely useless.

On the other hand, this symbol is EXACTLY what it is.

I don't suppose you use the symbols that show the extra zener as well? = I've always been tempted to rebel against that symbol by drawing the = substrate 'dash' with zener wings.

And you could draw a MESFET with a schottky "long S" gate, same as = schottky BJTs. It would even be acceptable to use one line instead of a = gate and three dashes, since they're pretty much all the same hunk of = material, no insulators.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

I did in new circuit, 200 Ohm, seems low but then there is 17mA going past the 'R' terminal.

Yes, I tried the opamp classic style controller feedback: small cap plus larger cap with series resistor, surprisingly it worked on first try values, but might be underdamped. I'll get back to that when I make a load delta switcher, just an oscillator driving a big MOSFET switching in 5 or 10 Ohms extra load. Need one of them anyway for testing responses, last one I made added voltage into feedback loop, hardly reusable.

I don't see any oscillation, I've got a noisy mains ground here so there's much noise on the CRO even when the thing is powered off :( And the thing is powered from batteries isolated from earth.

I'm renting, so there's not much I can do about the bad earth -- did ask for an electrician to check it years ago, he pulled earth wire out of the neutral bar, measured resistance between neutral and earth and pronounced it okay! Ignoring the fact two other units on same earth point were looping earth via their neutral blocks.

Hardly a proper earthing test, and we have odd law here that says bad earthing that met standards in place when the units were built is okay -- even though running an earth connected to water pipe, and going through taps before the pipe goes underground breaks modern rules.

Grant.

Thanks.