polashd
Member
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).
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).