Change output voltage of SMPS - Page 1
Forum section: electronics / projects

author: ajp8868
author_rank: Regular Contributor
timestamp: January 27, 2023, 05:46:24 pm
content: Hi All,Kaihui sell this power supply:https://m.alibaba.com/product/60421519548/2018-KAIHUI-switching-ac-dc-led.htmlWhich is the 12/24v version of a power supply I need (12v 4a, 28v 2.1a). It's the exact same layout and I would guess they just use a different set of resistors with the reg ic?My question is, how easy would it be to get this to output 28v on that rail? Could someone help me do it once it arrives?Thanks!

author: magic
author_rank: Super Contributor
timestamp: January 27, 2023, 06:56:54 pm
content: This PSU looks too simple for independent regulation of both output rails. It probably only regulates one, and the other rail is determined by 2:1 turns ratio of the secondary windings on the transformer.It might be possible to mod 24V up to 28V, but then 12V will become 14V, most likely.Another issue is that the same transformer usually has yet another "secondary" winding located on the primary side, which provides low voltage power to the switching controller. This voltage will also increase by 16%, which would probably be harmless, but it's something to keep in mind when playing with those things.A hypothetical 28V output version of this PSU would likely use a different transformer, particularly if the lower rail must be 12V exactly.They say that they can produce other voltage versions, maybe it's an option if you are ordering more than a few units.

author: ajp8868
author_rank: Regular Contributor
timestamp: January 27, 2023, 07:20:07 pm
content: Ah thank you for that detailed explanation, makes perfect sense.They do a 12/28 version but it's OEM (I'm guessing they only produced it for Chauvet).Do you think it would be possible to get a transformer with the correct ratio to get the voltages I need..?

author: magic
author_rank: Super Contributor
timestamp: January 27, 2023, 07:28:54 pm
content: I suppose it would be hard, you have no idea who makes this transformer and what's inside and it could be more or less a custom order for this particular PSU manufacturer.Perhaps 13V/26V would be close enough?I suppose your problem is finding replacement for a broken PSU in some commercial product?

author: Bud
author_rank: Super Contributor
timestamp: January 27, 2023, 07:32:38 pm
content: I've seen a trickier voltage divider feedback loop, where both output voltages were fed to the resistor divider in a proportion, so changes in any of the rails resulted in adjustment to restore the balance.

author: ajp8868
author_rank: Regular Contributor
timestamp: January 27, 2023, 07:34:01 pm
content: To be honest it might be close enough! Though it might cause a problem with the led if it's trying to draw more current than it can handle?Yep it's exactly that, I can get a replacement through the manufacturer, but it's more than the light's worth so I'm happy to try other routes before biting the bullet.I really wanted to repair the one I've got, but it's blown up a couple of tracks and surface mount resistors so I can't even read them to get replacements. So at this point I'd be spending more on components lol

author: magic
author_rank: Super Contributor
timestamp: January 27, 2023, 07:42:59 pm
content: Post pics of the disaster, no harm in trying.Quote from: Bud on January 27, 2023, 07:32:38 pmIt doesn't matter. OP want to change the ratio of output voltages, and this ratio is solely determined by turns ratio. (OK, I'm ignoring voltage drops of secondary rectifying diodes, but the principle stands: increase one rail and the other increases too).For better regulation, the PSU would need two transformers and two independent controllers, or some sort of "buck" converter on the secondary for the lower rail like in ATX PSUs.editAny chance that the 12V rail is barely used at all? If so, it could work with any 28V PSU and some step down converter to power the logic or whatever else needs 12V.

author: ajp8868
author_rank: Regular Contributor
timestamp: January 27, 2023, 08:12:35 pm
content: It might work but it would need a few amps at least as it runs about 6 stepper motors and the main board.But I guess if I can get a powerful enough 28v psu and a buck that can handle it, it should workHow can I calculate what wattage 28v psu I'd need?

author: tunk
author_rank: Super Contributor
timestamp: January 27, 2023, 09:02:05 pm
content: Calculate the total wattage needed (assuming ~80% efficiency of the converter),and then divide it by 28V to get current. Or use the PSU in your first post, adjustit to 28V and use 2-3 high current diodes to drop the voltage to ~12V.

author: ajp8868
author_rank: Regular Contributor
timestamp: January 27, 2023, 10:11:20 pm
content: Thank you!So damage report: there's a couple tracks blown which I've soldered a couple and replaced one with a wire (not currently connected at one end)It blew up a 0.15ohm 2w resistor Which was on the high voltage side of the optocoupler. I didn't have any of that value so I've used 1.5ohm - will that work?It blew up a couple tracks and resistors on the smaller PCB which has those 5 pins on the high voltage side. I thought this was the voltage regulation circuit but I'm not sure?It just blows the fuse instantly when plugged in but I can't find any obvious shorts, I'm running out of ideas to fix it!Could the transformer cause a short?

author: wasedadoc
author_rank: Super Contributor
timestamp: January 27, 2023, 10:47:55 pm
content: A resistor as low as 0.15 Ohm could be a current sensing resistor in the emitter or source of the switching device. If so, replacing with a resistor 10 times the value is going to severely reduce the power that the supply can deliver.

author: ajp8868
author_rank: Regular Contributor
timestamp: January 27, 2023, 11:09:26 pm
content: Quote from: wasedadoc on January 27, 2023, 10:47:55 pmAh ok, best change that then. If it's only current sensing, is the 2w one in place actually overkill or is there a reason for it?

author: magic
author_rank: Super Contributor
timestamp: January 27, 2023, 11:20:46 pm
content: This looks like the primary switching FET is shorted. It often happens when they fail that all three terminals get shorted together and then high voltage passes through the transformer (very low resistance), through the shorted MOSFET and blows up the current sense resistor (0.15Ω) and often also the gate drive circuit - some resistors, maybe a diode and probably the UC3843 chip too.UC3843 is a very common and well known controller, you can easily find datasheets, application notes and any other information. Also replacement chips, if necessary. This PSU should be repairable, but there are many components which may have been exposed to high voltage and destroyed, and failing to identify and repair just a few could lead to another bang and wasted time.Don't try to turn it on to avoid further damage. Make sure all caps are discharged and see if you can detect a short between the high voltage rails on the primary with DMM. Remove the 1.5Ω resistor - it's wrong value anyway. With this removed, test for drain-source short in the MOSFET. If it appears shorted in absence of the 0.15/1.5 resistor, it's 99% sure the MOSFET is shorted internally - desolder it and test out of circuit. It shouldn't have a short between gate and source, and if you short gate to source, it shouldn't conduct from drain to source.If bad FET is confirmed, test all resistors on the UC3843 board. Check for internal shorts in UC3843, like FET drive pin to GND or VCC.

author: ajp8868
author_rank: Regular Contributor
timestamp: January 27, 2023, 11:57:35 pm
content: Magic, I think you are spot on. I've already replaced the MOSFET as it was indeed shorted.I stupidly plugged it in after replacing the known bad capacitor (but before realising the MOSFET was bad) and it went bang again. I think this is what blew the 3843.With not being able to identify at least one resistor on the board with the 3843, is it even worth going through it all? I would have liked to save it from the scrap heap, but it might also be useful for spares - well, whatever's not fried!Potentially if I could somehow identify that resistor and replace everything found to be bad on that board, I think it would work. But without having a big enough supply of spare parts, I think I'm out of luck/budget!Thanks very much for your advice, I appreciate it. Trying to learn as I go, but even after so many years there's still new challenges!

author: magic
author_rank: Super Contributor
timestamp: January 28, 2023, 11:38:32 am
content: The blown resistor probably just connected UC3843 with the MOSFET so it could be replaced with some ballpark value.To repair this, you would need to produce schematic of this small board and figure out which parts are doing what. This gives some idea what their typical value might be and whether they were at risk of damage.When I dealt with a UC384x PSU with this exact same kind of damage, the blown parts were:- the FET- the current sense resistor (your 0.15Ω)- resistors and a diode which connect the FET to the chip- the chip, it was visibly cracked and smoked in my case- some transistor in the chip's low voltage power supplyI replaced all of that and also tested most other components (it was a valuable PSU so I was quite paranoid) and it came back to life.

author: ajp8868
author_rank: Regular Contributor
timestamp: January 28, 2023, 03:04:08 pm
content: Those are the exact same symptoms, right down to the mosfet being cracked! I've got this so far, but am I going in the right direction? The resistors coming from pin 6 are the ones I need to identify - that are connected to the gate of the mosfet.How far do i need to go with the schematic?

author: magic
author_rank: Super Contributor
timestamp: January 28, 2023, 03:45:36 pm
content: Yes, I think the blown stuff in the corner is resistors coming from pin 6 to the gate. There is also a diode there, all these components may be blown. See how they are connected, measure all resistors, test the diode, read all markings that are still readable.There is a 1kΩ resistor which seems to have arced to a nearby capacitor. Both are connected to the main board, what these connections are?Possibly current sense (pin 3) may have received something nasty too, see where that signal comes from (ultimately - from the 0.15Ω resistor) and what components are along the way.Check the chip for shorts, particularly from pin 6 to power supplies or between VCC and GND.Check if the short circuit across high voltage rails on the main board is still there (if not, and 0.15Ω is removed, temporarily replace it with a jumper). Remove the bad FET. Confirm that the short goes away.

author: ajp8868
author_rank: Regular Contributor
timestamp: January 28, 2023, 04:19:57 pm
content: going to continue tomorrow so I'll put more detail in about those pins - this is what I have so far. Meant to attach this to the previous post..!

author: ajp8868
author_rank: Regular Contributor
timestamp: January 30, 2023, 07:19:44 pm
content: Tested all of those and I'm pretty certain all the components in the bottom right of the small board are blown - diode showing 0.07 drop both ways, and resistors are showing massive values.I can't detect any shorts on the main board or the 3843, so I'm hoping we're good on that side of things.One thing I'm wondering is if the transformer could be what's causing the fuses to blow, could this be a failure point if high enough voltage got to the wrong bit..?Is there enough info in the diagram above to guess what the destroyed resistor will be? The one that has part marking left is 250 (I'm not certain on the 5, but definitely 2#0), how do I work out the other?

author: drvtech
author_rank: Regular Contributor
timestamp: January 31, 2023, 11:20:04 am
content: To prevent further damage when testing blown up SMPSs one useful technique is to provide an external, current limited supply to the controller chip (UC 3843 in your case). That way you can verify that it is oscillating and driving the gate of your fet before putting high voltage on it. You can check the frequency and see it come on and off at its uvlo voltage (16V from memory?)Edit to add: that resistor is likely to be 22 ohm (220). Also I wouldn't put money on the UC3843 having survived. Enough voltage has been applied to those resistors to blow them up and, as the only path for that current was through the output of the chip, it will have been damaged. My suggested test with a bench supply powering the chip will demonstrate that. Just for the avoidance of doubt, that test is done with NO input supply (a.c.) connected. Although if you didn't realise that you shouldn't be anywhere near the innards of an SMPS!

author: ajp8868
author_rank: Regular Contributor
timestamp: January 31, 2023, 11:42:23 am
content: Thanks I will test that, but I'd guess I'll need a scope for showing that pwm output? I'm going to buy one at some point but I don't have one yet..With regards to the resistor, are you talking about the completely destroyed one or the one on the right with partial marking?

author: magic
author_rank: Super Contributor
timestamp: January 31, 2023, 03:48:42 pm
content: It looks like R6 on your schematic is the 2x0 in the corner and R2 is the completely charred one.The 2x0 could be 220 or 270, which is 22/27 ohm. The black one may be a few ohms. Was 4.7 on my PSU.These aren't very critical resistors, it should still work with them a little off.The diode is obviously dead.I also doubt that the chip is still good. Considering its cost, I would just order a replacement together with the FET (or even two or three of each).

author: ajp8868
author_rank: Regular Contributor
timestamp: January 31, 2023, 04:00:31 pm
content: Thanks Magic. I'll use some donor boards for the resistors in that case, I have some that are very close to those values and plenty of diodes.I'll let you know how I get on after replacing those parts and testing for shorts.

author: magic
author_rank: Super Contributor
timestamp: January 31, 2023, 04:24:24 pm
content: Before powering up with the FET installed it's good to have confidence that the chip is working correctly and it won't turn on the FET permanently or for too long. If that happens, you are back to square one.The chip must be good. The current sense circuit (pin 3) must be good.It would also be nice if the path from the optocoupler to pin 2 is functional, otherwise overvoltage may occur on the secondary.

author: ajp8868
author_rank: Regular Contributor
timestamp: January 31, 2023, 04:38:55 pm
content: I'll replace as much as I can and checking each individual part as I go.Also definitely replacing the chip as per your advice!I've replaced the FET already and no shorts there anymore. I have a spare in case it goes, but I'm certainly hoping it doesn't.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 06, 2023, 06:50:23 pm
content: Hi guys,I finally got the new chip and replaced. I tested everything and can't see any shorts.. however it blew the fuse as soon as I plugged it in again.What other components should I be looking at? I thought I'd checked everything that it could be by now. No shorts across the high voltage side, but I'm not sure if the transformer itself could be the culprit..?Attached some photos of the board

author: magic
author_rank: Super Contributor
timestamp: February 06, 2023, 08:19:59 pm
content: It could, if it's shorted and has insufficient inductance as a result. If you have an LC meter you could even measure it; I don't know what are typical values to expect for PSU with this kind of power rating but it should be googlable.That being said, I don't think it's a common failure.Did you figure out how the chip is supplied with power, are all the components there good? What's the job of that 18V zener, is it not open circuit?It seems unlikely, but it has occurred to me that perhaps something could happen to the snubber, rendering it ineffective and killing the FET each time.There should be a circuit consisting of a diode going from the drain to a small high voltage capacitor, and a resistor which discharges the capacitor.editWhich components are smoked this time?Does it blow the fuse if the FET is removed? It shouldn't, but maybe there is some stupid problem elsewhere and we are wasting time.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 06, 2023, 08:31:27 pm
content: I've checked all components on the chip board and they seem to test good (as far as I can tell..)Do you mean the vertical diode mounted in front of the fet? It tests ok in circuit iirc.By Snubber, do you mean the yellow rectangular capacitor? I'll test it out of circuit but it's not measuring a short.Weirdly nothing seems to be smoking and no blackened components, just the fuse. So maybe something must be shorted fairly early on?

author: magic
author_rank: Super Contributor
timestamp: February 06, 2023, 08:55:07 pm
content: There is an 18V SMD zener on the small board. It may be involved in controlling supply voltage of the UC3843. And there are a few transistors (MMBT3904?), at the very least they should have their BE/BC junctions checked for short/open with DMM diode test.The vertical diode near the FET and a blue capacitor and vertical resistor look like they are the snubber. Check if the resistor measures OK in circuit.The vertical red resistor behind the transformer is your new 0.15, right?I suspect that the FET is shorted again.editBTW, what's that black wire on your earlier photo of the bottom side? It doesn't look like this connection should be there.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 06, 2023, 09:34:04 pm
content: Ah yeah that's the one diode that tests differently 0.7 one way but 2.4 in reverse.. but it has a couple resistors in parallel so could be because of those?The resistors all seem to test fine in circuit and no short on the FET.I'm going to be honest I think this stretching my knowledge and I think it may be too complex for me to repair. They are available for ~£50 from the manufacturer so I'll probably go that route but I would have loved to be able to fix it!If you fancy a challenge I could send it to you...

author: ajp8868
author_rank: Regular Contributor
timestamp: February 06, 2023, 09:35:11 pm
content: The black wire is a jumper because it blew that track when the original bad FET was powered on

author: magic
author_rank: Super Contributor
timestamp: February 06, 2023, 10:20:04 pm
content: It looks wrong.I'm referring tothis image. The small board's pins, from left to right, appear to be:1. Connected to MOSFET source and 0.15Ω - this is current sense signal, it should go to pin 3 of the chip.2. Connected to the negative of a small electrolytic and a transformer winding - this definitely looks like "ground" of the UC3843 circuit. Should be connected to the negative of the rectifier.3. MOSFET gate drive.4. Power supply, I think. Seems connected to the other end of the same transformer winding as 2 through a diode and 1Ω. Typical circuit.5. Optocoupler feedback from the secondary.6. Startup power supply, should be connected to rectifier's positive through 100k~1MΩ resistance. Check if the resistor isn't shorted.It looks to me like your wire connects pin 2 with MOSFET source. Such connection shouldn't exist.I really can't see any damaged trace on this photo.

author: magic
author_rank: Super Contributor
timestamp: February 06, 2023, 10:42:13 pm
content: Isn't one of those big electrolytics installed backwards? I seem to recall you replaced them.I think they both should face negative towards the FET side of the board.Confirm using markings on the rectifier.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 06, 2023, 10:56:36 pm
content: OMG.. I've put the cap in backwards 🤦‍♂️That black wire isn't attached in the photo because it would connect to the pin of the bad trace (definitely a bad trace as the copper came out of it)I'll try swapping that cap round and see if that sorts it.. what a rookie error!

author: magic
author_rank: Super Contributor
timestamp: February 06, 2023, 11:01:59 pm
content: Well, I still don't see which trace is bad.To me it looks like pin 1 of the small board goes to FET source (this connection seems OK?) and pin 2 should go to the negative rail, i.e. the other end of the 0.15Ω resistor.editTrace to control board pin 3 looks worst, but you bridged it with solder, this should be good enough.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 07, 2023, 09:36:29 am
content: Well it's working! Sort of..I promise that trace is completely blown, it's hard to see because it looks like it's there but the copper actually lifted and came out of it.So it does now power on with correct voltages, however.. the light it powers is flickering badly and the unit resets itself constantly.Now my thoughts are (correct me if I'm totally wrong)1 I replaced the diode on the output to the FET gate, is it possible it's not capable of switching fast enough..?2 the chip I purchased was a 3843B rather than 3843A3 an output cap gone bad? They all test ok, but I only have a dmm so can only test farads as far as I know..

author: magic
author_rank: Super Contributor
timestamp: February 07, 2023, 10:07:36 am
content: Did you observe correct output voltages without load or under load? I suspect that under load they drop down, or perhaps the PSU shuts down periodically for some reason. How fast is the blinking?Diodes on the control board are marked T4, which is supposed to be the common 1N4148 diode in SOD323 package.Did you check R4 on the main PCB and the diode near it?

author: ajp8868
author_rank: Regular Contributor
timestamp: February 07, 2023, 10:17:21 am
content: Without load, I'll check it with load if it stays on long enough..Yeah I couldn't find any T4 diodes so just used another in a similar package. I'll find some T4s and try that. Is it likely that could be a cause?Yes I replaced that resistor which blew and tested the diode to be good.

author: magic
author_rank: Super Contributor
timestamp: February 07, 2023, 10:23:31 am
content: Quote from: ajp8868 on February 07, 2023, 09:36:29 amGood question. I never noticed the difference and there doesn't seem to be much information on the net.ON semi offers two variants but they don't explain the difference clearly. Comparing specification tables, I see that B is supposed to have a bit better frequency accuracy and lower power consumption during startup. Dunno about other manufacturers. So far I see no evidence that going from A to B should cause any problem, although I may be wrong.https://www.onsemi.com/pdf/datasheet/uc3842a-d.pdfhttps://www.onsemi.com/pdf/datasheet/uc3842b-d.pdf

author: magic
author_rank: Super Contributor
timestamp: February 07, 2023, 10:29:04 am
content: One thing you could try is check if supply voltage of the chip is good and stable, with and without load.Solder temporary cables to GND/VCC, connect a DMM with alligator clips, set to 200V or similar range.Turn power on, take a reading, turn it off, dismantles the setup.Beware that the "ground" of the primary side is actually mains voltage for 50% of the time (negative half-cycles).

author: ajp8868
author_rank: Regular Contributor
timestamp: February 07, 2023, 10:40:19 am
content: Thank you I'll try that tonight after work.Oh wait, one other thing I thought of. I replaced the 0.15ohm with a 0.45ohm resistor in the opto feedback circuit. Could that be it?I'll have to buy one as I can't find one in any of my donor boards

author: magic
author_rank: Super Contributor
timestamp: February 07, 2023, 11:01:58 am
content: It's nothing to do with opto, it senses current of the primary winding (and FET).3x high value → the chip thinks the current is 3x higher than in reality.This can cause the PSU to fail to deliver its full rated power.Somebody mentioned it already when you tried 1.5Ω.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 07, 2023, 11:05:03 am
content: Ah yeah makes 'sense' 🤣. I'll get a correct one ordered.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 11, 2023, 11:26:55 am
content: I suspect you knew already but that current sense resistor was the issue! It's working perfectly now. Thank you for all your help on this!

author: magic
author_rank: Super Contributor
timestamp: February 11, 2023, 11:36:51 am
content: You're welcome.Quote from: ajp8868 on February 11, 2023, 11:26:55 amIt was likely to be a problem, but one can never be sure if it's the only problem. Glad it's working now