Change an SMPS overvoltage protection threshold (or disable it?) - Page 1
Forum section: electronics / beginners

author: dazz
author_rank: Frequent Contributor
timestamp: July 02, 2021, 01:38:28 am
content: Hey guys & gals. I just purchased a 13.8V SMPS, model MW ADD-155Ahttps://eu.mouser.com/ProductDetail/MEAN-WELL/ADD-155A?qs=l0g2inPJSHPfd47aHG%2F72A%3D%3Ddatasheet:https://www.meanwell.com/webapp/product/search.aspx?prod=ADD-155I want to increase the output voltage to some 20-24V, maybe make it adjustable. The output is floating so I may get another one and build a bench power supply, using one unit for the positive rail and the other one for the negative rail. In that case I may want to make it adjustable up to 36V or something. The output electrolytic caps are rated at 25V and will be replacing them for 35V or 50V ones.I have already tweaked the feedback resistor divider to make it go up to some 24V, but my problem now is that the overvoltage protection kicks in above 17.5VIs there a fool proof way to increase the overvoltage protection threshold so I can use this thing at at least 24V? Or a way to disable it altogether if that's OK?Attached a few pics, hope that helps. I believe the second one is where the overvoltage circuitry is.Thanks.EDIT: I won't let me attach more than one pic, here's the other one.

author: dazz
author_rank: Frequent Contributor
timestamp: July 02, 2021, 02:18:52 am
content: Apparently it uses this FAN4830 PWM controller:https://www.mouser.com/datasheet/2/308/FAN4803-1-1191942.pdf

author: Gyro
author_rank: Super Contributor
timestamp: July 02, 2021, 08:18:10 am
content: It is usually possible to push the output voltage of a decent SMPS up by 10-20% without too much drama, but you're looking at doubling it! That sort of change is likely to run you into all sorts of problems - fundamentally, the switching transformer has the wrong turns ratio for 24V operation and there's nothing you can do about that.You are likely to run into issues with pwm duty cycle and switching transistor overcurrent protection (maybe that's already kicking in) as the poor thing tries to meet your excessive demands, particularly if you start pulling any significant current. Even if successful, the maximum output current would have to be at least halved (you can't suddenly double the wattage of the PSU), probably much worse.As the saying goes, you didn't want to start from here! You would be far better starting with the ADD-155B24V27.6V version of the same supply and adjusting its voltage upwards and downwards from there.As you are asking "Is there a fool proof way to increase the overvoltage protection threshold so I can use this thing at at least 24V?" then, I'm sorry, the answer has to be no.

author: dazz
author_rank: Frequent Contributor
timestamp: July 02, 2021, 08:30:06 am
content: Quote from: Gyro on July 02, 2021, 08:18:10 amOh, the current can be halved, no problem with that, but I see there's a host of other issues I won't be able to overcome. No problem then, I'll leave it at 15-16V. Thanks Gyro.One more quick question. How can I figure out whether it's safe to ground the output so that it's not floating anymore?

author: Gyro
author_rank: Super Contributor
timestamp: July 02, 2021, 12:22:55 pm
content: You're welcome dazz,If is designed as a floating output PSU then it should be fine to ground it. They designed it that way for flexibility. If you check the block diagram in the datasheet (linked on the Mouser page), you can see that both output terminals are only connected to the frame ground (chassis) by an EMC suppression capacitor, so you can ground either of them. At least Meanwell are a respectable brand.Sorry I couldn't give you better news, I very vaguely remember a YouTube video on modifying MW PSUs for extended range variable output (downwards), irrc, the poster had added a voltage display too. You may want to scan through the available videos. It would almost certainly have been a different model (controller IC?) though.

author: dazz
author_rank: Frequent Contributor
timestamp: July 02, 2021, 03:31:25 pm
content: Quote from: Gyro on July 02, 2021, 12:22:55 pmI think I'm going to grab another one of these, so I can build that bench PSU. Then I can go up to 30V with both of them in series or -15V / 15V since both are floating. Changing the feedback trimpots to a higher value to make it adjustable to a much lower voltage should be doable for me. Or at least I think so, haha.