Hello,
I stumbled on this thread by accident, and it fits perfectly with some vague plans I have to build a micro-controlled PSU.
This means that (eventually) i would like to make some changes, but to prepare for that, I want to understand the design.
So, I started from the Kicad files posted recently by Mike_mike (many thanks for that!), and first rearranged the schematics
with the purpose to make it simpler to follow the flow of how it operates, and so give a better overview.
In particular, the two opamps now point from left to right, reducing the number of wires going the wrong way.
This makes the schematics more tidy. See for yourself in the attached screenshot. (I have preserved the component designators.)
I like the simplicity of the CV/CC controls, and I will certainly keep the shutdown connections.
Admittedly, I have made a first change: replacing the TL431 reference by a REF02.
This change is not because of the accuracy: in a later step, the two pots will be replaced by DACs,
such that a micro can control the voltage and current settings. The PSU can then be calibrated in software.
The change is intended to increase stability of the output voltage over temperature and age.
It may be necessary to raise the 8V supply by a few volts to accommodate the new reference.
At this point I have the following questions regarding the design, I hope you can help to answer them.
(I have learned a lot from scanning the previous 47 pages of posts, but may have missed an answer or two.)
1) In the local feedback loop of U1b (CV output), R5 (4k7) is connected so at to include D1 in the loop.
Why is D1 included in the local feedback loop? (Alternatively, R5 could be connected to the output of U1b.)
2) What is the purpose of R7A1 (10k)? (And why does it have an unusual designator?)
3) What is the purpose of R10? Especially when Q1 starts drawing current (depending on the power stage),
it causes the reference for Q1 (emitter voltage) to rise, making life more difficult for the opamps.
4) What is the purpose of R41 (10M)?
In the new schematics, I have drawn boxes around subsections of the circuit that are interesting for playing around with.
The box around the reference and the two pots is clear: this is where the microcontroller should hook into.
(I don't trust a micro to control the pass stage directly; too many uncertainties.
But it is good for controlling the setpoints, sensing user controls (except shutdown), and displaying information.)
There is also a box around the resistor divider providing the feedback signal for CV control.
It may be advantageous to use special resistors to match the stability of the reference and pots (or DACs).
Also here, accuracy is less of a concern.
Finally, I have drawn a box around the pass-transistor stage and its driver.
The pass stage should behave like a PNP transistor. In case I understand the CV/CC control loop well enough,
connecting a power PNP transistor (2955 or so) to the three terminals should produce a working PSU. (Is that correct?)
To increase output power, it is then possible to hook up instead a Sziklai pair consisting of a smaller PNP
that drives one (or multiple) larger NPN power transistors to do the heavy lifting.
But the behavior of the attached circuit should resemble the behavior of a PNP transistor
(The emitter sinks current, the base and collector source current. This is why I have relabeled
the connection terminals to indicate their function according to the single PNP transistor configuration.)
So far, so good (I hope), and all feedback is highly appreciated: comments, corrections, and suggestions.
In case there is any interest in the new schematic and/or planned modifications, I can post the Kicad files as well.
Kind regards, Evert-Jan