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TL431 linear power supply

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spec:

--- Quote from: blackdog on December 19, 2018, 12:44:43 pm ---But I get the impression that people are getting more and more lazy while the info with a little search is easy to find.
That's why I have to laugh and sometimes maybe cry a bit...
--- End quote ---
What a presumptuous, patronizing fellow you are.  :-//

Rather than being so opinionated perhaps it would be better is you read the previous posts on this thread.


--- Quote from: spec on December 18, 2018, 09:22:44 am ---You will have, no doubt read the posts about the compensation being too heavy. Don't worry about this, it is intentional, as I have previously stated. There is little point in fine tuning the compensation until the PSU has been prototyped and tested. Once that is done we can optimize the stabilization, if necessary, by a few simple modifications, mainly capacitor changes.

In common with many PSU designs, the approach is that the opamp voltage servo loop defines the absolute DC output voltage and also provides the very low frequency currents, the big electrolytic capacitor provides the medium frequency currents, and the solid capacitor (ceramic) provides the high frequency components. You can get a more in depth coverage of this area from the manufacturers application notes and from individual component data sheets.

--- End quote ---

As I said already, lets see your full circuit to meet the OP's request- or are you the one who is lazy?

spec:

--- Quote from: not1xor1 on December 19, 2018, 08:16:02 am ---
--- Quote from: spec on December 18, 2018, 02:19:56 am --- [...]

--- Quote from: not1xor1 on December 15, 2018, 09:28:39 am ---I'll see if I can simulate it.
--- End quote ---
Be great if you did do a bode plot of the circuit. :)

--- End quote ---

I simulated just AC. The bodeplot doesn't look so good. It needs some more work on compensation.



--- End quote ---
not1xor1
I had missed this simulation that you did for the pre reply #89 circuit. Thanks a lot- very interesting.  :)
But you have omitted one fundamentally important component: R19 (10K)
What is the battery on the non inverting input of the opamp doing there (the reference voltage is the variable).
Also in #89 revised schematic:
R13, R20, and R21 are 56R.
All small signal transistors are BC337/327
D1 (D5) has been removed and replaced by a trace
R4 has changed from 1k to 56R

not1xor1:

--- Quote from: spec on December 21, 2018, 05:24:24 am ---not1xor1
I had missed this simulation that you did for the pre reply #89 circuit. Thanks a lot- very interesting.  :)
But you have omitted one fundamentally important component: R19 (10K)
What is the battery on the non inverting input of the opamp doing there (the reference voltage is the variable).
Also in #89 revised schematic:
R13, R20, and R21 are 56R.
All small signal transistors are BC337/327
D1 (D5) has been removed and replaced by a trace
R4 has changed from 1k to 56R

--- End quote ---

You're right. I had overlooked the fact that the reference voltage was on the inverting input. Of course the local feedback is affected by the resistor in series with that input, but there is little difference between 10k and 11k (i.e. the pot at its minimum) of source resistance.
I also changed the transistors and the resistors, anyway the bodeplot is still ugly  :D.



I tried to compensate the circuit in a different way and it improved a little (phase margin in the 50-70° range according to the load). But I've no idea if that might work in the real world. Later I'll run a transient load simulation.

spec:

--- Quote from: not1xor1 on December 21, 2018, 08:19:02 am ---
--- Quote from: spec on December 21, 2018, 05:24:24 am ---not1xor1
I had missed this simulation that you did for the pre reply #89 circuit. Thanks a lot- very interesting.  :)
But you have omitted one fundamentally important component: R19 (10K)
What is the battery on the non inverting input of the opamp doing there (the reference voltage is the variable).
Also in #89 revised schematic:
R13, R20, and R21 are 56R.
All small signal transistors are BC337/327
D1 (D5) has been removed and replaced by a trace
R4 has changed from 1k to 56R

--- End quote ---

You're right. I had overlooked the fact that the reference voltage was on the inverting input. Of course the local feedback is affected by the resistor in series with that input, but there is little difference between 10k and 11k (i.e. the pot at its minimum) of source resistance.
I also changed the transistors and the resistors, anyway the bodeplot is still ugly  :D.



I tried to compensate the circuit in a different way and it improved a little (phase margin in the 50-70° range according to the load). But I've no idea if that might work in the real world. Later I'll run a transient load simulation.



--- End quote ---
Nice work- thanks. I am just digesting the sim results along with breakfast. :)

blackdog:
Hi Spec,

Thank you for the insight of my psyche, I didn't know that yet, keep up the good work!  :-DD


Hi not1xor1  :)

Your last scheme looks better with R7 1-Meg and C2 4N7.
But because of the amplification in Q6 it becomes difficult to get the whole stable again.
Because of this you need C7 1N and R14 1K.

I think it is useful to know, that for every ampare output current you need about 50uF over the output, as a rule of thumb.
The more phase margin the control loops have, the smaller this value can become. (and some other things)
With this kind of "standard" circuits as in this topic, use around 50uF/Ampere for good dynamic behavior.

Even if you have sufficient phase margin, the dynamic behaviour may not be sufficient. (not fast enough)
It becomes even more difficult when you hang the i loop in the circuit,

Fault peak current protection
I always use the BC337 as in your circuit Q9 to limit the peak current.
and then with an extra opamp make second loop for an adjustable current control.
R2 and Q9 then limit the current a little above the maximum current the power supply should supply.

Q9 is much faster in responding than an opamp loop.
So in case of a short circuit Q9 protects your power trasistoren and your D.U.T. better than the current loop.
But only if you make C7-1nF and especially the previous schematic C8 of 100uF that your surge protection with Q9 will not work!

The high value of R16, 240K can make a nice noise contribution.
C1 of 10pF is too small, I take depending on the total circuit setup here 10 to 100nF.
A larger value helps to reduce the noise contribution and parasitic capacities.
But an extra resistor is needed directly at the opamp input, this to protect the input during short circuits.
Then the energy in C1 is partly dumped in the opamp, one or two diodes also help to keep the opamp intact in these error conditions.
If the opamp itself already has diodes, take into account that you usually should not exceed the 10mA peak current in or out a opamp input!

Furthermore it is good to think about bias currents, the LM358 is in this respect an old opamp so that higher bias values are unfavourable for the noise contribution and offset drift specifications.

Most power supply circuits will exhibit undesirable behaviour in the transition area between CV and CC.
And then it also depends on the amplification of the circuit (R16 and R17) and the current on which the power supply is set.

That's the hardest part of designing power supplies.
Spyce helps, but you will also have to build a test circuit to run into things Spice can't do for you.

I hope this helps.

Kind regards,
Blackdog


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