TL431 driven regulator - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: August 28, 2018, 02:46:20 pm
content: I have made a circuit using the below. It works but I still have oscillation.The lower TL431 seems to be OK, as can be expected it is the top one that is doing the work that has some ripple on it. Any suggestions?Capacitors in parallel sound like a bad idea and only certain values may be used. the datasheet has a similar BJT based regulator with a 10nF capacitor between the feedback pin and the cathode. Does his sound like a good idea?My first thought was that given there is no load across it perhaps some resistance like 10K would help give it something better to do than overreact and oscillate.The input capacitance of a single mosfet measures at 2.85nF so maybe the 3 with some cabling is making up enough capacitance to cause it to oscillate?

author: Simon
author_rank: Unknown
timestamp: August 28, 2018, 02:51:16 pm
content: I have taken a look at the 36V supply. This comes from a charge pump. It has a lot of noise on it looking very much like the noise on the TL431 and the output. So it would appear that some filtering is in order.

author: xavier60
author_rank: Super Contributor
timestamp: August 28, 2018, 03:02:00 pm
content: Also it is common practice to put a resistor in series with the Gate of each MOSFET to prevent possible oscillation with tack reactance.And compensation for the top TL431 by providing some negative feedback from K to R. Usually with a resistor and series capacitor.

author: Simon
author_rank: Unknown
timestamp: August 28, 2018, 03:44:53 pm
content: OK, something to put on the next version. I have also had a look at my bench supply and it looks ropy, I thing a lot of the noise comes from there and the TL431 + mosfets are amplifying it a bit.I could try some gate resistance, these mosfets are on flying leads so that is the easiest.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2018, 06:36:39 pm
content: TL431 is an open-collector op-amp with one input tied to ground and the other input having a massive yet curiously stable offset voltage.Putting capacitance on its output would seem ill advised, and not having a local feedback (compensation) path, equally so.This smacks of X-Y problem; what is the nature of the 30V supply, and load?Tim

author: Simon
author_rank: Unknown
timestamp: August 28, 2018, 06:45:28 pm
content: Currently the supply is a bench power supply. The load is two little motors running at 40'000 rpm with 15 amps between them. Ultimately the supply will be a vehicle, tomorrow I'll try batteries.

author: Simon
author_rank: Unknown
timestamp: August 28, 2018, 06:46:09 pm
content: There is a low pass filter inline with the power supply.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2018, 07:15:17 pm
content: Why would you even do that mess with two TL431's stacked. If you need more voltage rating and/or dissipation at HV, just put it in cascode with a suitable N-MOSFET rated for linear operation, with its gate driven by a tap in the Ref divider chain chosen to limit Vs, and thus the TL431 cathode voltage to about 6V.https://www.eevblog.com/forum/projects/tl431-common-base-amplifier-stability/is worth a look - especially its comments on modelling a TL431 in LTspice.A MOSFET cascode as I described above adds quite a bit of negative feedback via the gate to channel capacitance, so should be more stable than an uncompensated TL431.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 28, 2018, 07:23:02 pm
content: The loop around the upper TL431 will see 2 stages of low-pass filtering:1) the gate capacitance2) the output caps (3x220 µF + 3 µF) together with a limited trans-conductance.So that MOSFET stage is way to slow to close the loop.It would likely need some local compensating feedback. Chances are it gets rather slow for the TL431 to drive 3 MOSFET gates.With a filter at the input side, there should be sufficient capacitance on the drain side of the MOSFETs too.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 28, 2018, 07:34:37 pm
content: Hah !! I fell into the same pit.Cascading TL431s is unstable, I learned the hard way.What works is a Zener and just *one* TL431 at the bottom. with a divider over the whole thing.RegardsWolfgang

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2018, 07:46:13 pm
content: Quote from: Simon on August 28, 2018, 06:45:28 pmWhat, you're regulating the voltage tomotors?!I don't even?Tim

author: Simon
author_rank: Unknown
timestamp: August 28, 2018, 08:25:52 pm
content: Quote from: Wolfgang on August 28, 2018, 07:34:37 pmYes I decided on the same solution for a next version. The bottom one is actually stable, The top one is not bad but incoming noise is a problem.

author: Simon
author_rank: Unknown
timestamp: August 28, 2018, 08:27:00 pm
content: Quote from: Kleinstein on August 28, 2018, 07:23:02 pm220µF and 3x1µF ceramics, the inductor is 1µH I think.

author: Simon
author_rank: Unknown
timestamp: August 28, 2018, 08:37:32 pm
content: Quote from: T3sl4co1l on August 28, 2018, 07:46:13 pmAh, you may remember a previous regulator I attempted and failed at. The motors have an absolute maximum of 40V, and I am using them on a vehicle. 33V is the top "normal" constant supply voltage with surges at 40V. With 40V being the voltage of the surges and the maximum the motor drivers are rated for (with a supplier that would rather see a stable 24V supply) the 7V does not provide enough headroom for TVS's. I still need to protect from the 250V spikes but any TVS has a 3V hysteresis so in theory the minimum cut in is 36V to make sure that at the end of any spike I am still not conducting at normal supply voltages and burn the TVS in a short out or blow fuses or at 36V there is no guarantee the voltage won't peak at over 50V damaging the motors electronics anyway. So I made a voltage regulator that under normal load will just sit fully open but will pull in at 29.5V and hold there for anything over that up to 33V non stop. Those are 3 TO247 package MOSFET's on an aluminium bar, bolted to the case. It gets warm but it can take it at 33V and is limited by ambient temperature.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2018, 10:08:23 pm
content: What the hell are the motors? BLDC and papier-mâché?If they're literally spinning copper and iron, seriously, don't worry about it, they can takewaymore overload than a couple transistors bolted to a metal bar.Tim

author: xavier60
author_rank: Super Contributor
timestamp: August 28, 2018, 10:46:47 pm
content: 0.5mA into K of the bottom TL431 should always keep it in conduction.Its divider resistors should be made larger so that they don't bypass too much current.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 28, 2018, 10:49:06 pm
content: ... for good regulation it should even be above 1mA

author: xavier60
author_rank: Super Contributor
timestamp: August 28, 2018, 11:05:34 pm
content: I think that it would have been better to have had the regulation feedback to the bottom TL431 and setup the top TL431 for a fixed voltage drop.The divider resistors for the top TL431 should be higher values to make the clamping current minimum.With the regulation feedback applied to the bottom TL431, the divider resistor calculation will be simpler also.

author: Simon
author_rank: Unknown
timestamp: August 29, 2018, 06:46:50 am
content: Quote from: T3sl4co1l on August 28, 2018, 10:08:23 pmPretty much. I don't have details on the internals but what i did manage to extract from the manufacturer was that the abs max rating of the parts is 40V. If look for motor driver chips you will find plenty that top out at 40V, fine for 12V auto but not 24V.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2018, 08:53:00 am
content: Quote from: Ian.M on August 28, 2018, 07:15:17 pmI bench tested that circuit in the end and it worked very well. No oscillation and the transient response was fine too.Are you talking about doing a similar thing, but with a MOSFET, with the gate tapped from the output divider?That should work and be easier, apart from working out the resistor values.

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2018, 11:27:52 am
content: Yep. Using your component values and Eugene Dvoskin's TL431 model from Audio-Perfection.com, and Tian et al's loop probing method*it gives the zero gain phase margin as almost 90 degs near 124KHz, and the -3dB corner frequency as 216Hz.Alternatively, in the time domain, a 5mA load step with a vicious 1ns rise time results in a spike of less than 430mV and about 2.3us wide, and less than 10mV offset between the steady state levels with and without the 5mA diverted from the shunt regulator.That's pretty good with no extra decoupling or compensation.Of course a 2N7002 isn't the best choice of MOSFET here - something slightly beefier would be better to handle the worst case dissipation.Choosing R2 is fairly easy - just make it near R1*Vgs_th_max*2/2.5. It's not too critical so there is an opportunity to trim the divider here by juggling R2 against R3 if you only stock 1% tolerance E24 series resistors.Unfortunately I don't have the facilities to test the frequency response easily in the lab with real components.* Michael Tian, V. Visvanathan, Jeffrey Hantgan, and Kenneth Kundert,"Striving for Small-Signal Stability", IEEE Circuits and Devices Magazine,vol. 17, no. 1, pp. 31-41, January 2001.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2018, 03:18:49 pm
content: To clarify what I said earlier: I have not bench tested the MOSFET variant of the circuit, only the BJT version, which was a success. I didn't do an AC test, but a transient response test and there wasn't any ringing or oscillation.It's good the MOSFET cascode also works. It can also be connected, as per the BJT in my circuit, at the cost of an additional resistor. Yes, a higher powered MOSFET is will be more suitable than the 2N2007, which was just the model I had to hand. I wouldn't go too over the top, because bigger MOSFETs have a higher gate capacitance.