Turning a fixed SMPS into a variable power supply.

Thread Starter

Yami

Joined Jan 18, 2016
337
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:
Schematic.jpg
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.

Thanks in advance for the help.

Here is a screenshot of the regulator section from a schematic I found online.

2.jpg

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

Again thanks for the help
 

Dodgydave

Joined Jun 22, 2012
9,912
You need to link out the zeners Zd8,9 and remove Zd4 , then you can alter the resistors R73, R74 and Vr1 , if your're going higher in voltage then you also need to change the output capacitors C52-60 , for higher voltage rating.
 

Thread Starter

Yami

Joined Jan 18, 2016
337
You need to link out the zeners Zd8,9 and remove Zd4 , then you can alter the resistors R73, R74 and Vr1 , if your're going higher in voltage then you also need to change the output capacitors C52-60 , for higher voltage rating.
Thanks Dave, What do you mean by "link out the ZD8 , ZD9"?. Bypass them completely or replace them with another value of zener or resistors? and could you please tell me why I should take out ZD4.
Thanks so much for your help
 

AlbertHall

Joined Jun 4, 2014
10,909
ZD4 sets an overvoltage for the Q3 supply. If you are not changing that you can leave ZD4 alone.
ZD7 and ZD8 set an overvoltage for the main output. If you want to increase the main output you will need to increase the voltage rating if these two zeners.
ZD9 will effectively limit the minimum output voltage you can set but shorting it to get lower voltages will increase the maximum current through that circuit, especially if you are also increasing the output voltage.
 

Thread Starter

Yami

Joined Jan 18, 2016
337
ZD4 sets an overvoltage for the Q3 supply. If you are not changing that you can leave ZD4 alone.
ZD7 and ZD8 set an overvoltage for the main output. If you want to increase the main output you will need to increase the voltage rating if these two zeners.
ZD9 will effectively limit the minimum output voltage you can set but shorting it to get lower voltages will increase the maximum current through that circuit, especially if you are also increasing the output voltage.
Thanks it does make sense. I want to go below the rated voltage would it make sense to replace the ZD9 with a variable resistor to figure out the limit?
 

AlbertHall

Joined Jun 4, 2014
10,909
I feel it would be better to use a lower voltage zener. In the 24V model ZD9 is 12.9V and that lets you get down to 16.5V.
So you want the output voltage to be 11.5V less than it is now which only leaves 1.4V for that zener.
It would seem that you need to short that zener to get down to 5V.

R75 is 1k so the current through it will be 24mA max at 24V output which doesn't sound too bad.
 

ebp

Joined Feb 8, 2018
2,332
Many switchers rely on a winding on the transformer to supply power for the primary side circuitry after initial start-up. If the voltage from this winding drops too low, the supply will shut down. This can make it impossible to convert a fixed voltage switcher to one that is variable over more than a small amount. You might get down to some fairly low fraction of nominal.

The schematic shown has an overvoltage shut-down feature - the series zeners and the optocoupler to the right, and a normal feedback circuit for regulation, the TL431-type device and its associated components. Because of the 431 and optocoupler circuit, it will be impossible to accomplish regulation at output voltage of less than about 3-4 V, regardless of any considerations with regard to supply voltage for the primary side.

When you start messing with the components surrounding the feedback circuit you mess with the frequency response (location of poles and zeros that compensated for the zeros and poles in the modulator, i.e. the power path) and in consequence can turn the thing from a good power supply into a supply with very poor dynamic response (overshoot and undershoot) or even a power oscillator. From what I've seen, the people who come up with these mods never look at anything but the average DC output and totally ignore dynamic performance.
 

Thread Starter

Yami

Joined Jan 18, 2016
337
Many switchers rely on a winding on the transformer to supply power for the primary side circuitry after initial start-up. If the voltage from this winding drops too low, the supply will shut down. This can make it impossible to convert a fixed voltage switcher to one that is variable over more than a small amount. You might get down to some fairly low fraction of nominal.

The schematic shown has an overvoltage shut-down feature - the series zeners and the optocoupler to the right, and a normal feedback circuit for regulation, the TL431-type device and its associated components. Because of the 431 and optocoupler circuit, it will be impossible to accomplish regulation at output voltage of less than about 3-4 V, regardless of any considerations with regard to supply voltage for the primary side.

When you start messing with the components surrounding the feedback circuit you mess with the frequency response (location of poles and zeros that compensated for the zeros and poles in the modulator, i.e. the power path) and in consequence can turn the thing from a good power supply into a supply with very poor dynamic response (overshoot and undershoot) or even a power oscillator. From what I've seen, the people who come up with these mods never look at anything but the average DC output and totally ignore dynamic performance.
Wow that's a very important point about the dynamic performance. I get quite fidgety when it come to measuring the response using a scope .
 

Thread Starter

Yami

Joined Jan 18, 2016
337
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 :)
 
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