Author Topic: TL431 linear power supply  (Read 6130 times)

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Offline mike_mike

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TL431 linear power supply
« on: December 12, 2018, 02:16:45 am »
Hello, I built the power supply from the attached image but I found that it is oscillating. What should I do to prevent the oscillations that appeared on the scope at the output of the power supply as a sine wave with vpp amplitude of about a few tens of volts ?
While the power supply was oscillating, the voltage drop on the 1N4148 diode was 4V instead of 0.7V and there was a high frequency noise.
Also, the load was about 6-7A at 25Vdc output.

What should I do to eliminate the oscillations ?

*in the tested schematic, R6 was 2 0.1R in parallel resistors.
 

Online xavier60

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Re: TL431 linear power supply
« Reply #1 on: December 12, 2018, 10:25:37 am »
Try adding some frequency compensation to the TL431. Start with a series 1K and 1uF between K and R.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu, New! FY6600 function gen.
 

Offline AngraMelo

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Re: TL431 linear power supply
« Reply #2 on: December 12, 2018, 10:52:23 am »
What is the purpose of R4 and R12?
« Last Edit: December 12, 2018, 10:59:04 am by AngraMelo »
 

Online Zero999

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Re: TL431 linear power supply
« Reply #3 on: December 12, 2018, 12:47:52 pm »
What is the purpose of R4 and R12?
They improve the transient response by discharging the base-emitter capacitance, when the base current decreases.
 
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Online David Hess

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Re: TL431 linear power supply
« Reply #4 on: December 12, 2018, 01:54:28 pm »
What is the frequency of oscillation?

Get rid of C2.  It belongs either at the load or where the leads enter the chassis.

AC bypass R18 with a low value capacitor.

Swap the positions of R8 and R9.  Then AC bypass the top fixed resistor with a capacitor or series capacitor and resistor.  This adds phase lead and works in combination with xavier60's suggestion.

Lower the value of R4.
 

Offline imo

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Re: TL431 linear power supply
« Reply #5 on: December 12, 2018, 08:48:18 pm »
Also the anode of the TL431 and the cold side of R8 should be wired close to the LSP4, and the hot side of R9 wired close to the LSP3 (better do it with a separate wires/tracks).
At 7A current even <1milliOhm wire resistance could cause a voltage drop creating oscillation in the control loop. Therefore you must separate "power" signal paths from "sensing" paths (wires) - a "star" config.
« Last Edit: December 12, 2018, 09:24:55 pm by imo »
 

Offline mike_mike

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Re: TL431 linear power supply
« Reply #6 on: December 13, 2018, 03:21:07 am »
I found a power supply schematic and I would like to use it to power my equipment.
Do you know if it will oscillate or not ?
 

Offline spec

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Re: TL431 linear power supply
« Reply #7 on: December 13, 2018, 03:39:47 am »
Hi mike_mike,

Not surprising that you have frequency instability problems with a TIL431. Take a look at the frequency stability charts on the TL431 data sheet.

On first inspection, the second PSU looks very hairy too with a load of gain, especially in the constant current mode which has two zeros in series. My advice would be to look elsewhere.
« Last Edit: December 13, 2018, 03:43:40 am by spec »
 

Offline mike_mike

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Re: TL431 linear power supply
« Reply #8 on: December 13, 2018, 04:12:51 am »
Thanks for the reply.
Please recommend a good power supply schematic.
I need the following performance
1. min 5A output
2. 3-25V output voltage
3. to not oscillate
4. to have a simple schematic
 

Offline spec

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Re: TL431 linear power supply
« Reply #9 on: December 13, 2018, 04:21:36 am »
No probs :)

I will have a look for a suitable PSU, but I am quite busy at the moment. A number of members are experienced in PSUs, perhaps they could recommend some suitable designs as well.

But I can recommend a complete lab PSU that will do your job for around £47UK, including shipping (the transformer for a DIY PSU would cost more than that). The lab PSU provides 0V to 30V and 0A to 10A, with digital readouts for both voltage and current. There are controls for coarse and fine voltage output and the same for current. I have three of these and use them for electronics, battery charging, you name it.

https://www.ebay.co.uk/p/Adjustable-DC-Power-Supply-30v-10a-Variable-Precision-Digital-Lab-PSU-Clip-Cable/23025566378

You can probably get this generic lab PSU delivered to Romania.
« Last Edit: December 13, 2018, 04:49:08 am by spec »
 

Online T3sl4co1l

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Re: TL431 linear power supply
« Reply #10 on: December 13, 2018, 04:52:26 am »
Simplest possible supply that meets those requirements:
- Phase controller or VARIAC
- 18 to 20VAC transformer, rated 8-10A RMS
- Rectifier, capacitor filter

You did not specify anything further in depth like regulation or noise, or adjustable current limiting, or cost, so #4 is easiest to optimize.  This uses about six parts (including fuse and power switch), hard to beat. :)

Tim
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Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline mike_mike

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Re: TL431 linear power supply
« Reply #11 on: December 13, 2018, 04:58:49 am »
Thank you.
But I need a power supply made by me. I am a kind of maniac in electronics.
I need that the power supply to be variable via potentiometer and stabilized.
 

Online T3sl4co1l

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Re: TL431 linear power supply
« Reply #12 on: December 13, 2018, 05:11:00 am »
Literally just the first schematic, with an R+C connected between TL431 ref and cathode.  Say 10k and 1nF for starters.  Adjust values to give good step response.

Tim
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Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Online xavier60

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Re: TL431 linear power supply
« Reply #13 on: December 13, 2018, 09:19:33 am »
Thank you.
But I need a power supply made by me. I am a kind of maniac in electronics.
I need that the power supply to be variable via potentiometer and stabilized.
You have not mention if you have tried adding compensation to the TL341 or not.
That first power supply circuit has the best chance of working properly than most others you are likely to find.
There are a few more small improvements that can be made. For example the overload LED can be moved to a better spot.
The BD140 isn't necessary.
« Last Edit: December 13, 2018, 02:33:31 pm by xavier60 »
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu, New! FY6600 function gen.
 

Offline spec

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Re: TL431 linear power supply
« Reply #14 on: December 13, 2018, 01:13:51 pm »
Thank you.
But I need a power supply made by me. I am a kind of maniac in electronics.
I need that the power supply to be variable via potentiometer and stabilized.
:)
 

Offline spec

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Re: TL431 linear power supply
« Reply #15 on: December 13, 2018, 01:21:28 pm »
This is a genuine question asked out of interest.

Has anybody ever built a variable power supply that has actually worked using a TL431, not simulated but in the flesh. My experience with the TL431, and family, is that unless they are in a nice benign position, like the reference for a switch mode PSU, they are not happy.
 

Offline spec

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Re: TL431 linear power supply
« Reply #16 on: December 13, 2018, 01:30:51 pm »
mike-mike

I have done a sketch for a PSU that replaces the TL431 with an opamp, but otherwise uses the architecture and components of the TL431 circuit. Hopefully I will be able to post the schematic in about 17 hours from now.

By the way, there seems to be an uneasy relationship between the voltage regulation and the current limiting function.

If you like, just remove the current limiting LED indicator from the circuit and see if the PSU settles down (the current limiting function will be completely disconnected and not involved in the circuit).
« Last Edit: December 14, 2018, 07:12:00 pm by spec »
 

Online David Hess

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Re: TL431 linear power supply
« Reply #17 on: December 13, 2018, 03:28:48 pm »
The only reason I have not used the TL431 that way is why would I bother when better performing and easier to use operational amplifiers are plentiful?  It is routine in isolated switching power supplies however.

The problem is a little like using the 723 or most switching regulator controllers where stabilizing feedback to the inverting input comes from the transconductance output instead of the low impedance buffered output.  The TL431 has a transconductance output, despite what the datasheet specifications show, so gain varies considerably from device to device and with load.  If you build a series regulator with it, then the variable biasing conditions are altering the transconductance unless steps are taken to prevent it so the frequency compensation becomes difficult to control.

A better question might be *why* the TL431 operates so benignly when used as a simple shunt regulator.  It will normally have a relatively large load capacitance which rolls off the gain of its transconductance output for dominant pole compensation which is exactly how a transconductance amplifier is compensated.  But in a circuit where it controls a series regulator, that cannot be done if fast response is desired and instead feedback from the squirrely output to the input implements the frequency compensation because people are used to working with voltage feedback amplifiers.  It can work as the 723 shows but can also be hit or miss.

This brings up an alternate compensation scheme; place a series RC network from the TL431 output to ground.
 

Offline not1xor1

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Re: TL431 linear power supply
« Reply #18 on: December 13, 2018, 06:39:09 pm »
Thank you.
But I need a power supply made by me. I am a kind of maniac in electronics.
I need that the power supply to be variable via potentiometer and stabilized.
You have not mention if you have tried adding compensation to the TL341 or not.
That first power supply circuit has the best chance of working properly than most others you are likely to find.
There are a few more small improvements that can be made. For example the overload LED can be moved to a better spot.
The BD140 isn't necessary.

The BD140 BE resistor sets a coarsely constant current through the TL431.
BTW 1kΩ is too much. 330-470Ω would be more appropriate and might even help mitigate the self-oscillation trouble as 1mA is the rated minimum current for TL431.
The BD140 at the maximum voltage allowed for the TL431 would dissipate approximately 250mW. It might be replaced with a BC327 if the ambient temperature (inside the PSU case) is not too high.
« Last Edit: December 13, 2018, 06:52:14 pm by not1xor1 »
 

Offline not1xor1

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Re: TL431 linear power supply
« Reply #19 on: December 13, 2018, 06:58:37 pm »
No probs :)

I will have a look for a suitable PSU, but I am quite busy at the moment. A number of members are experienced in PSUs, perhaps they could recommend some suitable designs as well.

But I can recommend a complete lab PSU that will do your job for around £47UK, including shipping (the transformer for a DIY PSU would cost more than that). The lab PSU provides 0V to 30V and 0A to 10A, with digital readouts for both voltage and current. There are controls for coarse and fine voltage output and the same for current. I have three of these and use them for electronics, battery charging, you name it.

https://www.ebay.co.uk/p/Adjustable-DC-Power-Supply-30v-10a-Variable-Precision-Digital-Lab-PSU-Clip-Cable/23025566378

You can probably get this generic lab PSU delivered to Romania.

From the pictures (if you look through the case aeration slots it looks empty inside) I guess it is a switching PSU.
The linear ones are better as they can be easily improved by replacing the electrolytic caps (they are low quality and too low voltage) and adding a proper TO3 heatsink (as minimal cure).
 

Offline not1xor1

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Re: TL431 linear power supply
« Reply #20 on: December 13, 2018, 07:21:03 pm »
Thanks for the reply.
Please recommend a good power supply schematic.
I need the following performance
1. min 5A output
2. 3-25V output voltage
3. to not oscillate
4. to have a simple schematic

It would be better to use at least a center tapped transformer, switching the secondary winding appropriately to halve the power dissipation.
Do you already have a transformer?

It would be possible to modify the 431 psu, and run a few simulation to get a coarse idea of the compensation network, but IMHO a classical 2 opamp circuit, with fixed feedback network and variable references and both voltage and current regulation, is much better.
If you feel audacious I can provide you with a proper 2 opamps circuit, but just tested it in LTspice as my lab is not available at the moment.

Here is a proof of concept schematic, just to get a rough idea.
If you like it I can provide you with a complete and real world usable circuit diagram.

BTW what do you plan to use the PSU for?
 

Offline mike_mike

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Re: TL431 linear power supply
« Reply #21 on: December 13, 2018, 08:27:46 pm »
Currently I am waiting for a response from @spec. The current limiting led is not necessary. If you will use an operational amplifier, then please use a common operational amplifier for example LM358 (I have many of them). The input of the supply will be maximum 45Vdc, so there needs to be made something in order to power the op amp.
I need the power supply to power leds, Arduino, motors, relays etc.
« Last Edit: December 13, 2018, 08:47:03 pm by mike_mike »
 

Offline not1xor1

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Re: TL431 linear power supply
« Reply #22 on: December 13, 2018, 11:38:37 pm »
Currently I am waiting for a response from @spec. The current limiting led is not necessary. If you will use an operational amplifier, then please use a common operational amplifier for example LM358 (I have many of them). The input of the supply will be maximum 45Vdc, so there needs to be made something in order to power the op amp.
I need the power supply to power leds, Arduino, motors, relays etc.

What does it mean 45VDC?
i.e. is that just transformer secondary rmsV * sqrt(2) or an AC/DC switching module?

LM358 might still work up to 24-25 VDC out with a single transformer. But you should use a 24VAC (or better 12+12VAC) 8+8A transformer to get a regulated 24-25V 5A DC output.

The best opamp IMHO is LT1013 due to a high supply voltage, limited cost and wide IO range (and much better features than LM358). A cheaper alternative is MC33171/2/4
Better opamps are more expensive and harder to find.

Another solution is a voltage boost stage (common base or common emitter) at the expense of higher chances of self-oscillations.
The Harrison topology would be better (i.e. floating control circuit) requiring just a further small power (few VAs) transformer and is usually more stable.
 

Offline mike_mike

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Re: TL431 linear power supply
« Reply #23 on: December 14, 2018, 12:25:36 am »
45Vdc means the voltage after rectifier and filtering.
I would still like to remain to LM358 since my electronic components vendor does not have this part number (the one with LT).
I just need something like a error amplifier controlling one or more transistors (npns), a ref voltage, a current limit (not variable). Also I want to have a output current of about max 5A and a voltage of about 3-25Vdc. A overload protection led should be very useful, but only if it is simple to implement.
I can use a 30vca 10A transformer or an 24vca, 6A transformer.
And I want to be very stable and I want to NOT oscillate... this is the hardest thing for me.

Could you please help me with such a schematic ?
« Last Edit: December 14, 2018, 02:45:04 am by mike_mike »
 

Offline spec

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Re: TL431 linear power supply
« Reply #24 on: December 14, 2018, 05:43:48 pm »
No probs :)

I will have a look for a suitable PSU, but I am quite busy at the moment. A number of members are experienced in PSUs, perhaps they could recommend some suitable designs as well.

But I can recommend a complete lab PSU that will do your job for around £47UK, including shipping (the transformer for a DIY PSU would cost more than that). The lab PSU provides 0V to 30V and 0A to 10A, with digital readouts for both voltage and current. There are controls for coarse and fine voltage output and the same for current. I have three of these and use them for electronics, battery charging, you name it.

https://www.ebay.co.uk/p/Adjustable-DC-Power-Supply-30v-10a-Variable-Precision-Digital-Lab-PSU-Clip-Cable/23025566378

You can probably get this generic lab PSU delivered to Romania.

From the pictures (if you look through the case aeration slots it looks empty inside) I guess it is a switching PSU.
The linear ones are better as they can be easily improved by replacing the electrolytic caps (they are low quality and too low voltage) and adding a proper TO3 heatsink (as minimal cure).

UPDATE #1  2018_12_14  not1xor1 is correct, my PSUs are in fact switching- I just had the covers off
[/quote] It is a linear PSU, with a very low level noise output of 3mV, why do any guessing.

Although these PSUs work well as is, there are a few things that you can do to enhance them.
[1] Check the quality of solder joints and general workmanship (my three are fine, but the quality can vary according to some reviewers)
(2) Fit an output rail on/off switch
(3) Fit better quality potentiometers, say ten turn for the fine controls.

Incidentally these power supplies, at the price, can just be bought for the parts. The transformer itself would cost you over £50, and then there is the chassis, voltage and current meters, 2N3055s, heatsinks, fan, knobs, caps, resistors, diodes ...

I am thinking of converting one into a precision current source.
« Last Edit: December 14, 2018, 08:40:09 pm by spec »
 


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