@spec Thank you.
No probs- apologies for not correcting the obvious opamp phase error sooner.
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 ?
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.