[REPAIRED] Cheap 150W 48V SMPS repair (clicking noise) - Page 1
Forum section: electronics / repair

author: dominicM
author_rank: Regular Contributor
timestamp: November 10, 2016, 06:08:29 pm
content: I have a cheap chinese 150w 48V power supply which failed after little use. It is actually the second psu like this one to fail in the same way so I would like to find the cause of the failure. No visible damage or any noise/smell at failure time. The psu just keeps making clicking noise with output in the mV range. I also have a similar 24V 150W supply that I used as a comparison. The voltages are the same as the 24V psu up to R4 (~330V). Measuring voltages across the 2 electrolytic caps near D4 give 1V on the left one and 13-15V on the right one. 24V psu was a stable 15V for the latter. Both caps measure 10uF after desoldering (have no replacements so soldered them back in). Both diodes measure about 0.5V or OL depending on direction. All resistors are within spec also. Is it likely that the IC itself failed on both psus? Found no shorts and am out of ideas at this point, any suggestions?

author: retiredcaps
author_rank: Super Contributor
timestamp: November 10, 2016, 06:15:59 pm
content: In my limited experience, a clicking noise suggests one of the following:1) a dried up startup electrolytic cap with high ESR that is tied to the SMPS IC VCC pin2) a shorted component on the secondary side like a diode3) a shorted SMPS IC - lookup the datasheet and see if VCC is shorted to GND

author: dominicM
author_rank: Regular Contributor
timestamp: November 10, 2016, 06:24:42 pm
content: Quote from: retiredcaps on November 10, 2016, 06:15:59 pmIt being a cheap Chinese PSU there's no documentation and the IC markings are scratched off. I tested all diodes and there are no shorts. Is it likely that the cap is dried up/high ESR and still be within capacitance spec? There are also no shorts between any of the IC pins.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 10, 2016, 06:29:13 pm
content: Quote from: dominicM on November 10, 2016, 06:24:42 pmYes, a capacitor could measure within specification with regards to capacitance, but have high ESR.Some people suggest warming up the capacitor with a hair dryer to lower its ESR initially so the SMPS IC can start. Of course, long term, lots of heat kills the capacitor.

author: Seekonk
author_rank: Super Contributor
timestamp: November 10, 2016, 06:35:32 pm
content: There is a winding on the primary side that powers the chip once it starts. There is a high ohm resistor that feeds it so it initially turns on. The clicking means it is not getting powered by this winding. The winding may be open or the diode on it is bad. Since it tries to start the cap and zener are probably good.

author: dominicM
author_rank: Regular Contributor
timestamp: November 10, 2016, 06:38:47 pm
content: Will give it a shot, should the caps be heated before applying AC power or it doesn't matter if it's already clicking away when caps are heated? I can probably salvage some caps, can I replace the 10uF caps with lets say 100uf caps of same voltage? Which ones are worth replacing first/most likely to be faulty in your opinion?

author: dominicM
author_rank: Regular Contributor
timestamp: November 10, 2016, 06:45:04 pm
content: Quote from: dominicM on November 10, 2016, 06:38:47 pmThe D1 and D2 are both OL / 0.48V.R1 measures 0.9 (R value is 0.15).R3 measures 95k.L2 is actually replaced (with 22ohm) though I think it's the wrong value and I replaced it with the original since. The original resistor is brown, brown ,black, silver (or maybe red, red) which is 11ohms but it measures 0.7. Perhaps it's just my multimeter unable to measure it properly because the other failed 48V psu and the working 24V psu have identical values and measurements.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 11, 2016, 01:05:52 am
content: Quote from: dominicM on November 10, 2016, 06:38:47 pmI can't tell from the photo, but either C2 or C3 will be your startup capacitor.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 11, 2016, 01:06:53 am
content: Quote from: dominicM on November 10, 2016, 06:45:04 pmComponents that start will L are usually an inductor. Why did you replace it with a resistor?

author: dominicM
author_rank: Regular Contributor
timestamp: November 11, 2016, 11:00:54 am
content: Quote from: retiredcaps on November 11, 2016, 01:06:53 amI assume it was marked wrong on the pcb because all 3 psus have same marking and have a resistor there and not an inductor. It has 4 bands - brown, brown, black, silver.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: November 11, 2016, 11:10:20 am
content: There are small inductors that have the same ring markings as resistors.Sure it is a resistor?

author: dominicM
author_rank: Regular Contributor
timestamp: November 11, 2016, 11:21:24 am
content: Quote from: retiredcaps on November 11, 2016, 01:05:52 amOk, I swapped out C2 and C3 caps between working 24v psu and clicking 48v psu and still same, 24v is working but 48v is clicking so the caps must not be the issue.

author: dominicM
author_rank: Regular Contributor
timestamp: November 11, 2016, 11:26:58 am
content: Quote from: SaabFAN on November 11, 2016, 11:10:20 amYes, it does look like it's an inductor after allResistance reading was close to marked bands so I didn't even consider it at the time.

author: strawberry
author_rank: Super Contributor
timestamp: November 11, 2016, 03:21:35 pm
content: Replace D2

author: SeanB
author_rank: Super Contributor
timestamp: November 11, 2016, 06:54:12 pm
content: Resolder all the pins on the transformer, you probably have a dry joint on the feedback winding. The one joint does look pretty dodgy, and resoldering them will at least fix any dry joints there. Also resolder R15 and R7, the joints there look terrible, and they are also a part of the feedback.power supply for the chip.

author: dominicM
author_rank: Regular Contributor
timestamp: November 11, 2016, 07:09:53 pm
content: Quote from: strawberry on November 11, 2016, 03:21:35 pmNo change, what was the reason for this suggestion specifically?

author: dominicM
author_rank: Regular Contributor
timestamp: November 11, 2016, 07:12:18 pm
content: Quote from: SeanB on November 11, 2016, 06:54:12 pmDone with no change and I really don't see how a dry joint could be a cause of 2 psu's failure with no physical stresses

author: strawberry
author_rank: Super Contributor
timestamp: November 11, 2016, 08:36:03 pm
content: Quote from: dominicM on November 11, 2016, 07:09:53 pmSometime they use wrong parts.Once I had bias winding broken.

author: xavier60
author_rank: Super Contributor
timestamp: November 11, 2016, 08:48:47 pm
content: The ticking means that C2 is being charged up to startup threshold voltage for the IC but not being sustained. For the 48V output rail to be so low, there could be a short on it.

author: dominicM
author_rank: Regular Contributor
timestamp: November 11, 2016, 09:56:08 pm
content: Quote from: xavier60 on November 11, 2016, 08:48:47 pmI replaced C2 and C3 so the caps are fine and tested it to make sure - no shorts.

author: dominicM
author_rank: Regular Contributor
timestamp: November 11, 2016, 10:03:48 pm
content: The ticking also persists after ac power is removed for about a 2 minutes slowing down until it stops. Thought I might add that in case it provides any clues to someone.

author: xavier60
author_rank: Super Contributor
timestamp: November 11, 2016, 10:12:47 pm
content: Did you also check the 48V output for a short circuit? There can be can be other reasons like an open circuit in the secondary circuit.

author: xavier60
author_rank: Super Contributor
timestamp: November 11, 2016, 11:31:31 pm
content: It is also possible to have shorted windings in the transformer. This can be difficult to prove.

author: _Wim_
author_rank: Super Contributor
timestamp: November 12, 2016, 08:59:25 am
content: Quote from: dominicM on November 11, 2016, 07:09:53 pmDo not replace this one by a "normal" diode. You need to use a ultra fast recovery type in many circuit to work correctly. Also ensure is has a high enough reverse breakdown voltage. I had 2 SMPSes failing because the diode reverse breakdownvoltage was serverly limited (got to hot probably), but measured still ok with a multimeter.Some more info which type to select can be found here (altough I just picked one I had lying around from a previous project):https://www.maximintegrated.com/en/app-notes/index.mvp/id/849

author: kxenos
author_rank: Frequent Contributor
timestamp: November 12, 2016, 09:23:56 am
content: Is U2 LM431? Can you swap it with a good one?

author: Richard Head
author_rank: Frequent Contributor
timestamp: November 12, 2016, 11:15:42 am
content: Power the control IC from a separate bench power supply and measure its gate drive output.The bench PSU obviously has to be isolated! You will probably find that as soon as you provide external power to the chip that the PSU works perfectly. If that is the case then disconnect the external bench supply DC and measure the signal from the feedback winding when the unit is clicking. There should be a string of pulses from the feedback winding that is able to charge the bleed-on cap and power the chip.

author: dominicM
author_rank: Regular Contributor
timestamp: November 12, 2016, 01:22:35 pm
content: Quote from: xavier60 on November 11, 2016, 10:12:47 pmThere are no shorts between any output terminals.

author: dominicM
author_rank: Regular Contributor
timestamp: November 12, 2016, 01:23:53 pm
content: Quote from: xavier60 on November 11, 2016, 11:31:31 pmSeems kind of unlikely to me for it to happen to 2 psu's though, no?

author: dominicM
author_rank: Regular Contributor
timestamp: November 12, 2016, 01:26:17 pm
content: Quote from: _Wim_ on November 12, 2016, 08:59:25 amI replaced it with a working one from a similar 24V psu and there was no change, 24v worked while 48. clicked as before with D2 swapped out.

author: dominicM
author_rank: Regular Contributor
timestamp: November 12, 2016, 01:52:32 pm
content: Quote from: kxenos on November 12, 2016, 09:23:56 amU2 is a TL431A. After swapping it works! So what was the logic behind this suggestion? The question now is why did it fail, it happened to 2 psus so I doubt it's just chance.Looking at the datasheet it seem like it's only rated up to 36V, am I missing something or is it a design flaw?

author: strawberry
author_rank: Super Contributor
timestamp: November 12, 2016, 02:51:27 pm
content: OB2269

author: kxenos
author_rank: Frequent Contributor
timestamp: November 12, 2016, 04:19:37 pm
content: No flaw. The 431 is probably in series with a resistor and optocoupler's LED.You said that after removing power the PS continued to make the noise for a while so there was no short-circuit in the output. So the next reason for the PS not to be able to start is if there is a problem in the feedback circuit. I looked at the photo and I saw a TO92 in the secondary side. This is a classic configuration and a classic problem in cheap PSUs. Probably it will be OK if you install a genuine part. If there are problems in the future again you will have to add a snubber RC parallel to the 431.

author: Bondguy
author_rank: Contributor
timestamp: November 12, 2016, 05:19:07 pm
content: @Richard Head: it is an interesting idea but what will be tested that way? feedback winding of primary transformer or control IC?Am I correct to think that by feedback winding you mean one of the primary winding that provides voltage for control IC when the power supply capacitor of control IC is being discharged?

author: dominicM
author_rank: Regular Contributor
timestamp: November 12, 2016, 09:18:52 pm
content: Quote from: kxenos on November 12, 2016, 04:19:37 pmWill get some from rs. Out of curiosity is it safe to assume that most parts like this one from china on ebay are fake?Would modifying the 24v ps to output 5v be feasible? It seems like it should be possible by changing resistors close to the 431. Don't want to blow it up just after repair

author: xavier60
author_rank: Super Contributor
timestamp: November 12, 2016, 10:04:59 pm
content: The output should have been pulsing up to about 1 volt during the clicks with the faulty TL431.Reducing the regulation voltage would also reduce the supply voltage to the control IC which would cause it to repeatedly shut down after it starts in this case.It would work in some designs where the IC is powered from somewhere else and not affected by the regulation voltage.

author: kxenos
author_rank: Frequent Contributor
timestamp: November 12, 2016, 10:14:01 pm
content: I don't know about ebay parts because I have never bought any but I guess if it's too good to be true it probably isn't. When you repair or build something you try to minimize uncertainty factors, so buying parts from well-known distributors ends up being cheaper IMO.About changing the output voltage, I'm sure you can. You will have to DaveCAD the secondary circuit around the 431 and replace a couple of resistors and maybe the output caps. The idea is that the output voltage controls the brightness of the optocoupler's LED. You will have to change the resistor values so that the LED has the same brightness (current through it)

author: dominicM
author_rank: Regular Contributor
timestamp: November 18, 2016, 07:28:44 pm
content: Well I did see a similar mod by someone somewhere and it worked fine, so it is feasible. In that case it was only maybe 10v lower than original supply so 24v to 5v may be bit too far. That being said 48v and 24v supplies look almost identical even most parts are identical or very close to.

author: dominicM
author_rank: Regular Contributor
timestamp: November 18, 2016, 08:08:58 pm
content: Got the parts but I think I may have gotten the wrong part. I ordered TL431C without really paying attention but the one in the PSU was TL431A. After replacing the clicking was still present though different sound. I can't really see the difference in the datasheet for these parts. What is the difference?

author: kxenos
author_rank: Frequent Contributor
timestamp: November 18, 2016, 09:49:00 pm
content: The difference is precision 1% for the A part vs 2% for the part you've got and also smaller operating temp. I don't think it matters much. Can you check the resistors around the 431 and install back the 431A part from the other PS and see what happens? Also can you place a small load in the output? Something like a 330 resistor

author: dominicM
author_rank: Regular Contributor
timestamp: November 19, 2016, 10:15:05 pm
content: Quote from: kxenos on November 18, 2016, 09:49:00 pmI replaced the TL431 with the original and it was the same thing. The new TL431 worked fine in 24v psu so the new part is fine. The 330 resistor made no difference. I remembered that I dropped one of the boards and after close examination 2 pins of D3 have snappedAn easy fix if nothing else is damaged.

author: dominicM
author_rank: Regular Contributor
timestamp: January 05, 2017, 12:47:22 pm
content: Just to say all the PSU's are repaired and 1 of them has been working for quite a while now so it seems the repair was completely successful!

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: January 06, 2017, 05:53:23 pm
content: That LM341 voltage reference could also be a (cheaper) Korean version KIA2431. Just saw that one in my Denon AVR-3312 SMPS...