Electronics > Beginners
TL431 linear power supply
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xavier60:

--- Quote from: spec on December 14, 2018, 08:30:25 am ---
--- Quote from: xavier60 on December 14, 2018, 06:50:30 am ---
--- Quote from: mike_mike on December 13, 2018, 01:25:36 pm ---
And I want to be very stable and I want to NOT oscillate... this is the hardest thing for me.


--- End quote ---
The power supply circuit in your first post will work well  enough with compensation.

--- End quote ---
Can you show a circuit for this compensation using a TL431, especially one that you have built and tested successfully. Or perhaps know of such a PSU, that someone else has built and tested.

I am genuinely interested as stated before.

--- End quote ---
The only times that I have used a TL431 as an active part of the regulation loop rather than as the voltage reference, are as low voltage control rail supplies.  I used the same topology as the circuit in the first post, but with a medium power MOSFET or BJT acting as follower for the K voltage. Very easy to compensate with a capacitor between K and R.
Just like with the circuit  you posted, even if the feedback capacitor is oversize, the op-amp or TL431 becomes a very slow Miller integrator.
Transient response remains reasonable because all that the op-amp or TL431 needs do is compensate for small changes in B-E voltage as load current changes.
Response can optimized by reducing the size of the capacitor  and adding some series resistance.
spec:

--- Quote from: xavier60 on December 14, 2018, 09:29:48 am ---
--- Quote from: spec on December 14, 2018, 08:30:25 am ---
--- Quote from: xavier60 on December 14, 2018, 06:50:30 am ---
--- Quote from: mike_mike on December 13, 2018, 01:25:36 pm ---
And I want to be very stable and I want to NOT oscillate... this is the hardest thing for me.


--- End quote ---
The power supply circuit in your first post will work well  enough with compensation.

--- End quote ---
Can you show a circuit for this compensation using a TL431, especially one that you have built and tested successfully. Or perhaps know of such a PSU, that someone else has built and tested.

I am genuinely interested as stated before.

--- End quote ---
The only times that I have used a TL431 as an active part of the regulation loop rather than as the voltage reference, are as low voltage control rail supplies.  I used the same topology as the circuit in the first post, but with a medium power MOSFET or BJT acting as follower for the K voltage. Very easy to compensate with a capacitor between K and R.
Just like with the circuit  you posted, even if the feedback capacitor is oversize, the op-amp or TL431 becomes a very slow Miller integrator.
Transient response remains reasonable because all that the op-amp or TL431 needs do is compensate for small changes in B-E voltage as load current changes.
Response can optimized by reducing the size of the capacitor  and adding some series resistance.

--- End quote ---
Thanks- very informative.
spec:

--- Quote from: not1xor1 on December 14, 2018, 09:19:59 am ---
--- Quote from: spec on December 14, 2018, 09:01:14 am ---
--- Quote from: not1xor1 on December 14, 2018, 08:22:22 am ---
--- Quote from: spec on December 14, 2018, 07:11:32 am ---Hi mike-mike,

Attached below, is a schematic for your PSU as promised. Apologies for the delay, but it took longer to sort than expected, and other things got in the way too. >:(


--- End quote ---

you must be kidding  ;D
a 20V/µs opamp with a huge 1µF compensation capacitor...
and what would be the advantage versus a proven and foolproof design using the traditional LM723 ?

--- End quote ---
I said it was slugged. :)
I made no claim about any advantage over anything. Why didn't you post a suitable 723 circuit? You could have saved me a load of bother. >:D

--- End quote ---

there are loads of 723 circuit diagrams/projects on the net, and lots of them even in the datasheets (ST, NSC, etc). So what would be advantage of yet another (in this case untested) design ?  :)

--- End quote ---
As you say, there are stacks of 723 circuits around, some better than others, so I would suggest that linking to a suitable circuit would help the OP a lot.
spec:

--- Quote from: not1xor1 on December 14, 2018, 08:22:22 am ---you must be kidding  ;D  a 20V/µs opamp with a huge 1µF compensation capacitor...
--- End quote ---
I took this as a joke, but just in case, I chose those two opamps because of their low input ofset voltage, very low input bias current, high output current, RRO, RRI(100mV beyond) and the ability to drive a relatively large capacitor. The cost is reasonable too. The slew rate is not relevant in this application.

But, in general, there are endless situations, in fact most, where components are used way under their performance: 150mHz transistors used as audio amps, 20ns logic gates used to turn a LED on and off. A 10MIPS microprocessor used to alter the speed of a motor ...  :)
spec:
mike_mike,

Attached is a 723 PSU schematic which I found drifting around my laptop. This, is the kind of thing that not1xor1 and I were discussing.
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