How to modify/ design error amplifier compensation circuit (with TL431) for variable flyback converter and make it stable.

[polashd]

I bought a power supply adopter (old, used, made for some device) of 24v 4.5A (from 230vac mains). It is a fixed output psu. My intention is to make it a variable psu of 30v to <=5v (as low as possible, and continuously variable). Schematic (simplified) of its error-amp circuitry is attached below.

I tried the following methods (varying different component values)-

1) Replaced R_lower2 with a 10k Pot (Rv_lower1 is a variable resistor of 2k- adjusted to ~1k by manufacturer).
2) Replaced R_upper1&2 with a 100k Pot.
3) Replaced R_comps with a 100k Pot.
4) Removed DZ1&2 and connected the err_amp to a 12v source (common ground -Gnd_B).

Whatever I did ends up to same situation- up to 16v it goes without any problem. But when I try to lower the voltage further the output becomes very unstable.

I searched internet (including youtube, chatgpt3.5) and got the idea that the problem is with the compensation network of the error amplifier.

I also got some good documentations on the compensation network which describe about – transfer function, bandwidth, frequency response, poles & zeros and many more things. As a simple hobbyist these terms and equations are very complicated for me. I don’t understand where to start, how to proceed and steps involved.

Can anyone please help me in this? What should be the values of different components, how do I calculate step by step (so that a hobbyist like me can do it, for continuously variable psu).

Note: a) in the schematic I added TL431, in actual circuit it’s AZ431.
b) I didn’t get symbol and .lib/ .sub for LD7575PS- so I edited symbol of another ic in ltspice.
c) as per the equation in the datasheet of PWM IC –for a 120k resistor from RT to ground the switching frequency sb around 54kHz.
d) If possible provide me a working simulation schematic (with symbols and .lib/ .sub of important components like TL431 (or AZ431), optocoupler (in the PCB it is EL817), etc).
e) Can I do it with an op-amp (without TL431, achieve even lower output voltage like <3v).

 

[Tony Stewart]

For a more stable adjustable regulator over wide Vo and load range, you need to add current feedback to predict load before the voltage changes or need to add an active 10% load and watch out for CCM mode instabilities.

I don't have any specific solution.

 

[ChrisP58]

One possible cause of the instability is that the input control circuitry is being starved for operating power.

Most such flyback converters have a input side secondary winding to provide operating power to the primary control and drive circuitry.

The output voltage of this winding will track that of the main secondary, and will vary by the same ratio as the turns ratio of the two windings.

As an example let's say that the primary side output was designed to be 12 volts when the main output was 24, or a 1:2 ratio.

What that means is that, as you lower the main output from 24 to 16 Volts, you're also lowering the primary side voltage from 12 to 8 Volts. That may not be enough to keep it alive and happy.