Turning a fixed SMPS into a variable power supply. - Page 1
Forum section: electronics / projects

author: Yamin
author_rank: Frequent Contributor
timestamp: November 27, 2018, 02:40:27 pm
content: 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.shtmlBut 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 elektronayahttps://elektrotanya.com/meanwell_sp_320.pdf/download.htmlAgain thanks for the help

author: coromonadalix
author_rank: Super Contributor
timestamp: November 27, 2018, 03:33:38 pm
content: 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 chipsyou 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.

author: Yamin
author_rank: Frequent Contributor
timestamp: November 27, 2018, 06:52:28 pm
content: 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.

author: strawberry
author_rank: Super Contributor
timestamp: November 28, 2018, 12:19:22 pm
content: to regulate output voltage from 0..100%and at full current. and not run ML4800 into UVLO , power that chip from external power sourcebut dont run output voltages too high , othervise MOSFET in your FORWARD topology will overvoltage and blow

author: Yamin
author_rank: Frequent Contributor
timestamp: November 28, 2018, 01:07:48 pm
content: 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.

author: Yamin
author_rank: Frequent Contributor
timestamp: November 28, 2018, 08:07:59 pm
content: 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 2.5V2. 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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2018, 06:34:09 am
content: 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

author: xavier60
author_rank: Super Contributor
timestamp: November 29, 2018, 09:46:49 am
content: Quote from: Yamin on November 28, 2018, 08:07:59 pmMost 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.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 29, 2018, 01:56:43 pm
content: 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.

author: xavier60
author_rank: Super Contributor
timestamp: November 29, 2018, 09:38:38 pm
content: Quote from: strawberry on November 28, 2018, 12:19:22 pmThe 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..