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TL431 linear power supply
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spec:

--- Quote from: mike_mike on December 18, 2018, 04:34:23 am ---@spec Thank you.
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No probs- apologies for not correcting the obvious opamp phase error sooner. :)


--- Quote from: mike_mike on December 18, 2018, 04:34:23 am ---D5 should be a simple zener diode or it is better to use a zener diode and a npn transistor ? Or there can be used an LM317 ?

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You could do any of those things: it is not necessary though. But, if for example, you would rather use an LM317, just say and I will post a modified circuit for you.

D5 in the isue03 version is a simple zener diode and has two functions:
[1] To provide 15V Vcc for the opamp
[2] To provide the reference voltage for the PSU voltage stabilization loop.

Because of the architecture of the issue 3 PSU, where there is a large voltage difference between the raw supply voltage and 15V rail, a single zener will be quite adequate for your stated requirements. Bear in mind that this PSU has been designed specifically with your stated requirements that in mind. Given a free choice the PSU architecture would be different, as would a few other areas, including the opamp and driver transistor which would probably be a depletion NMOSFET. The 2N3055 transistors would also change to a transistor with a much lower thermal resistance, junction to case.

The other thing is that the LM358 current drawn from the 15V supply rail will be more or less constant at a maximum of 400uA,  as the LM358 is not sourcing any current, only sinking current. This is one of the handy characteristics of the LM358: a constant current drain, as opposed to a varying current drain, is beneficial for the voltage stability of the 15V rail.

But, I will post another PSU circuit to show how I would like to do the voltage reference using this architecture and if I have time, I will also post an LM317 reference version.

By the way, this PSU gives an output voltage of 0V to 25V at 0A to 5A and will current limit around 6A. But the OV needs to be qualified: exactly 0V output is impossible to achieve without extra circuitry. This problem holds for any linear PSU, not just this one. On balance, and in the interests of simplicity and your requirements, I omitted the extra circuitry. But if you would like true 0V just say.

The 0V ambiguity is due to leakage currents in the driver and output transistors, and there are effectively six transistors in parallel. Also the output transistors and driver will have a high junction temperature, especially with high currents and a low output voltage so the 0V potential will vary depending on usage. Temperature increases leakage exponentially.

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.

spec:

--- Quote from: Kleinstein on December 18, 2018, 06:49:31 am ---
--- Quote from: spec on December 18, 2018, 02:50:41 am ---mike-mike

I have been away from your PSU thread for a few days, but have now caught up with the latest posts.

Attached is a revised schematic for the 25V, 5A Version 3 PSU with the opamp phase error corrected and other modifications as follows.

...

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The regulator shown looks like it is rather slow with 1 µF at the OP and 100 µF after the transistor.  The more normal values are about a factor of 1000 smaller.  Is the stability (e.g. loop gain) check in a simulation ? With 2 low pass filtering stages and no phase boost there might be a stability issue, at least with a capacitive load.

Even if the BC546/BC556 are specified at 500 mW, a TO92 case still gets rather hot at 300 mW, if there is no fan.
So for the 10 mA current sink at the output the BD139 is a good idea. For the current source for the regulator one should get a way with a TO92 case transistor if the current is a little lower (e.g. 5 mA). This should be sufficient even for 5 A output.

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Both of these areas have already been discussed.

5mA Ik will not give sufficient current, worst case, to drive the output transistors.
mike_mike:
@spec
I need a final version of schematic, which uses LM317 instead of Zener diode and I will start drawing the layout.
Kleinstein:
With the extra charge pump stage as shown the voltage might get too high for an LM317.
So if at all one should use the charge pump to only give something like 2 times the normal voltage (diode to ground and not the normal positive supply).

I don't see a real need for the charge pump. Already just extra diodes and Filter capacitor would give some extra headroom for the current source, as with an extra capacitor one can have less ripple than the main supply. For the low current needed by the OP, the zener might be sufficient - the LM317 would give some extra loss in voltage, but could still work.
spec:

--- Quote from: mike_mike on December 18, 2018, 06:56:14 pm ---@spec
I need a final version of schematic, which uses LM317 instead of Zener diode and I will start drawing the layout.

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OK will do :)
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