feedback and stability

Yes. I've seen the idea all along, just not sure what was connected to where. Doesn't change my earlier answer.

Check the diagram I drew showing the 431 as an op amp & output xsistor, with an internal 2.5 V ref. The 431 works as a comparator driving a transistor, and your cap feeds the output back into the op amp input, causing oscillation.

Your TL431 will operate (turn on or off) in response to a voltage change at the battery. The current through R will suddenly change when the 431 switches on or off, changing the drop across R. *That* change will be fed back by the cap into the input of the 431 at the ref pin as a pulse, and the 431 will react to that pulse. That feedback path is causing your problem. You do not want

Ed

TL431's don't SWITCH. They're more akin to an OpAmp.

To do it correctly you want a FIXED FREQUENCY oscillator with variable duty cycle. Have the TL431 vary the duty cycle. ...Jim Thompson

Strictly speaking, no, but for usage here, it's simplest to think of them switching. The switching is like what you see on a typical zener curve where V goes from sloped to flat when the zener avalanches and conducts in the reverse direction. So they can be thought of as on or off.

Ed

Repeat my last paragraph over and over until it sinks in. Don't be a hacker, do it right ;-) ...Jim Thompson

snip

Or just save some components and time and pick from one of the hundreds of PWM controllers available.

This is begining to sound like one of them chinese smps's anything to save 2 cents.

I guess it makes sense labour is cheap in China.

Alternator SMPS ?? ...Jim Thompson

Maybe I missed something but isn't this about a regulator that goes haywire when he placed a cap across the Cathode ref of the TL431.

From the OP.

"I built a voltage regulator for an old truck I own. The circuit uses a voltage controlled oscillator with a square wave output to drive the gate of a mosfet that controls the alternator's field. The voltage regulation feedback loop uses a voltage divider from the alternator output/vehicle battery, connected to the reference pin of a TL431. The TL431 controls the vco."

I'm just assuming it's a hack PFM using a TL431 and maybe a KA331 VCO.

It looks like the C is leading in the circuit, If that is what's going on, you'll get an oscillating effect.

That's just my observations.

Try putting a R in series with the C or lower the value of C or both.

reference pin=20

the duty=20

alternator.

EUWWW. According to the datasheet is supposed to be used as a=20 linear (read non-switching) device.

duty

I hate to have to keep repeating myself... a TL431 should be considered as an OpAmp with VOS = VREF and with an open collector output. Is that too complicated for some of you to grasp ?:-) ...Jim Thompson

From the datasheet: "Active output circuitry provides a very sharp turn-on characteristic making these devices excellent replacements for Zener diodes..."

Where do you see the datasheet indicating it "is supposed to be used as a linear (read non-switching) device" ? How do you define linear as non-switching?

I'm sure you have something in mind here, but I'm not getting it. Vin vs Vout is anything but linear when the device is turned on. So you must have something different in mind. As to switching, the datasheet specifies a very sharp turn-on and refers to a zener. How does my description of that fail to capture the gist?

Ed

They also make good cheap comparators. I've used them for that purpose and it's also in the data sheet under application examples.

Once it starts to conduct it IS LINEAR... it's like a railed OpAmp. Believe me, I know, I was using them as OpAmps back in the late '70's. ...Jim Thompson

pin

duty

alternator.

Yes, I think it is, for me. I think you see the implications of that clearly, while for me it is likely less clear.

This is what I see: I look at the op amp input and see large gain, the output and see large (vs op amp output) current capability with the need for a load. I see the Vref input. In his circuit, I see the cap on the output which can cause instability, and can only guess at the load, but I assume it's small enough to draw a few mA. Whatever else I should be considering, I'm missing.

Ed

pin

duty

alternator.

I'll draw up a pretty picture tomorrow... wine time right now ;-) ...Jim Thompson

variable

I thought i read something like walking back and forth around the=20 knee of the regulation curve. Did i?

What "knee"? TL431's are quite sharp. Treat it as an OpAmp... drawing showing how sometime today. ...Jim Thompson

te:

ence pin

the duty

alternator.

=A0 =A0 =A0...Jim Thompson

=A0 =A0 ...Jim Thompson

The load is simply a pull-up resistor. The voltage at the cathode is the control for the oscillator.

pin

duty

alternator.

Take a look at this...

The snag is that the OpAmp IS compensated for unity feedback, for who-knows-what resistive and capacitive load.

So some caution is advised. You can, for instance, apply the usual feed-forward/feed-around schemes to isolate the load capacitance form the feedback loop.

I have, in the past, used these device as OpAmps, driving matched photo-couplers, to get a quite linear _analog_ photo-link ;-) ...Jim Thompson