Turning a fixed SMPS into a variable power supply.

[Yamin]

Hi, I bought a broken 24V power supply and got it fixed, the model is Meanwell sp-320-24. I would like to turn it into a variable power supply which preferably goes from 5V to 24V.
I found a very interesting article which shows how to do it. http://www.imajeenyus.com/electronics/20151028_smps_variable_voltage/index.shtml
But it is not similar to the power supply that I have got. The power supply I have has got a AZ431 shunt voltage regulator (same as the TL431).

Here is what I have done so far:

I have tried changing the values of R74 and R73 no matter what I did the output voltage doesn't seem to go below 16.50V, even when I left R74 open the output was 16.50V . The reference for the regulator is 2.5V. Any idea what I could do?.
As it is the voltage could be changed from 18.4V to 27.2V by turning the 1K pot.

I have also attached a screenshot of the regulator section from a schematic I found online.

There is a full schematic downloadable from elektronaya
https://elektrotanya.com/meanwell_sp_320.pdf/download.html

Again thanks for the help

[coromonadalix]

I dont think it will be that easy,  you have hacked some feedback for the control smps ic`s,  but normally they have some values range in their feedback to add tolerance for the ovp protection if any implemented.

The tl431 or any variant is normally used in the feedback loop, not has the main regulator, it drives the feedback optocouplers to the main smps chips


you have to search the 4800 ic`s specs and see what can you hack,   sometimes disabling the ovp may help, but you`re loosing some protections too, unless you really know what you`re doing, i would not mod this,   use a variable linear regulator at the output instead.

Your modification link point to an based tl 494  smps ic, theses are more easily modifiable,  there is some google projects too, an 0-50vdc variable smps based on it.

[Yamin]

Thanks, you're right I've been looking for a ML4800 mod. Doesn't give too much results. What do you suggest I do then to vary the output voltage (any suggestion on how to build a variable regulated supply). Do you suggest on using a LM317?. I don't have a specific current requirement but the higher the better hehehe.

[strawberry]

to regulate output voltage from 0..100%and at full current. and not run ML4800 into UVLO , power that chip from external power source

but dont run output voltages too high , othervise MOSFET in your FORWARD topology will overvoltage and blow

[Yamin]

I got a suggestion from someone - he suggested that I remove ZD4, and link out ZD8 and ZD9. I'm trying to figure out the logic behind it.

[Yamin]

Just an update - I changed the ZD9 to a diode which has a Zener voltage of 6V (it was 12V before). I changed R73 and with the 1K VR - I get resistance from about  ~6K to 1K. I get the output voltage variation from 10V to about 14V. Not what I really wanted.

1. Does the reference pin of the 431 always have to have 2.5V
2. If so how can I design a feedback which can provide 2.5V for the reference pin  and also have a large variable option for the output?

Really need help on how to design a feedback. I don't mind the output going from roughly 12V to 24V.  This project is more about learning

[T3sl4co1l]

What's the rest of the SMPS?

If it's self-powered, it's not going to take more than about +/-30% range.  More and you overvolt the controller on the high end, and undervolt it (so it browns out and hiccups) at the low end.

If it has an aux supply, it should be able to go down to zero, but mind particulars about compensation and parasitics and minimum pulse width (min/max duty cycle, especially if bootstrap drivers are involved somewhere).

Tim

[xavier60]

Just an update - I changed the ZD9 to a diode which has a Zener voltage of 6V (it was 12V before). I changed R73 and with the 1K VR - I get resistance from about  ~6K to 1K. I get the output voltage variation from 10V to about 14V. Not what I really wanted.

1. Does the reference pin of the 431 always have to have 2.5V
2. If so how can I design a feedback which can provide 2.5V for the reference pin  and also have a large variable option for the output?

Really need help on how to design a feedback. I don't mind the output going from roughly 12V to 24V.  This project is more about learning

Most of the things that you are blindly trying such as removing ZD9 and R74 are  disabling the voltage regulation loop causing the controller to go to full duty cycle, limited to something less than 50% usually for Forward converters.
The output goes over-voltage causing the OVP circuitry  to repeatedly shutdown the controller. Forward converters generally tolerate output over-voltage.
R74 needs to be gradually deceased while monitoring C34's voltage if you have the equipment to do it safely.

[Conrad Hoffman]

It can be hard to make a SMPS cover a wide range without some performance hiccup. This is pretty stone age, but I'd just put a big linear regulator after it and adjust that. Job done.

[xavier60]

to regulate output voltage from 0..100%and at full current. and not run ML4800 into UVLO , power that chip from external power source

but dont run output voltages too high , othervise MOSFET in your FORWARD topology will overvoltage and blow

The presence of C7 means that the Reset winding has the same turns number as the Primary winding.
The MOSFET's Drain will be clamped at double HVDC voltage. At something less than 800V.
.