Was Optocoupler now PWM--Plan B Skip to plan B - Page 1
Forum section: electronics / repair

author: status1
author_rank: Contributor
timestamp: December 21, 2020, 11:47:38 pm
content: This is just a shot in the darkI bought this power supply on ebay It is 115v input and 24v output at 5AmpsAfter I plugged it in it turned on for a second and that it went out than I tapped with my hand and I heard some arcing inside and started smokingAfter arguing with the seller because there was no visible damage on the outside he agreed to let me open it and showed the part that smoked He identified the part as an optocoupler so he refunded my moneyI am just curious if I can find this part and make the power supply work againThe part is an 8 pin ic with pcb mount pins Unfortunately there was no writing on top of the ic so not sure of the part number The only readable part was on the bottom "7a85" which is probably meaninglessI was wondering if it would be possible to somehow reverse engineer and find a part that would be appropriate for the designFrom what I have seen most of them contain just one optocoupler and some of the pins are not used but from the traces I looked on the bottom it seems that all the pins go to different places making me think that maybe there are 2 optocouplers in this one 8 pin ICI was wondering if that maybe narrows down the field of optocoupler types

author: WattsThat
author_rank: Frequent Contributor
timestamp: December 22, 2020, 12:17:28 am
content: Nope, doesn’t narrow the field at all. There is something that caused the coupler to fail, it’s a symptom, not a cause. Replace it and it and it will go poof a second time. Not worth the time or effort.Recycle time.

author: bob91343
author_rank: Super Contributor
timestamp: December 22, 2020, 06:53:58 am
content: Most optocouplers have only four pins. If this has 8 it may be something else.Post a pic of the circuit.

author: status1
author_rank: Contributor
timestamp: December 22, 2020, 02:08:44 pm
content: You are correct that this is a symptomAfter removing it from the bottom part of the plastic case and looking at the solder joints I found a few pins of a TO-220 part where the solder was cracked I assume this caused the intermittent connection which I am guessing caused a reaction in the so called optocoupler One of the leads where the solder was cracked goes through a resistor to pin 8 of the optocoupler

author: CaptDon
author_rank: Super Contributor
timestamp: December 22, 2020, 02:36:58 pm
content: Optocouplers commonly have 4, 6 or 8 pins. 4 and 6 is common forsingle devices and 8 is common for more complex single devicesand very common for dual devices. So someone saying it probablyisn't an opto because it has 8 pins was not providing accurateinformation. A vast majority of SMPS units use 6 pin devices whichvery often are in a white plastic/epoxy encapsulation making themvery easy to spot on the circuit board.

author: status1
author_rank: Contributor
timestamp: December 22, 2020, 04:09:00 pm
content: Thanks for the informationIn a dual opto configuration is that arranged in a certain standard or not necessarily ?I have seen some where there are some diodes from pins 1 to 4 and 5 to 7 would be the other sideI attached a few pictures in case it helps to identify the device that should be thereIn the Ic blown picture you can see the device that was blownin opto 004 you see the pcb after I removed the part and you can also see Q3 which is the TO-220 part I mentioned before This one I was able to read it and it says Magnachip MDF12N50 and it is an N channel mosfetin opto 003 you see the solder side of the pcb At the top you see the solder that is cracked on the TO-220 pins of Q3The one on the right is the Gate and that goes to the Diode D4148 which was also blown and I removed itIt also goes to that resistor marked 220 that looks ok but I measured it and it is openThe other side of the diode and resistor goes to pin 8 of the optoIt also goes through resistor R303 to the Source of Q3The source of Q3 goes directly to pin 6 of the optoThis pin 6 was the most burned a melted away from the body of the icNot sure if all that helps to narrow down the deviceI can provide more pictures if that helps

author: CJay
author_rank: Super Contributor
timestamp: December 22, 2020, 04:15:15 pm
content: That's not an optocoupler.It looks like it's the PWM controller and there's a mad selection of terrible soldering in the pictures you've attached.

author: Alti
author_rank: Frequent Contributor
timestamp: December 22, 2020, 04:21:11 pm
content: That is not an optocoupler but a MOS gate driver, possibly with controller built-in.

author: CaptDon
author_rank: Super Contributor
timestamp: December 22, 2020, 05:04:06 pm
content: Yes, in this case you can see it drives the mosfetand would be some sort of PWM controller. Also,It is very seldom to see a failed optocoupler witha ruptured case!!!

author: status1
author_rank: Contributor
timestamp: December 22, 2020, 11:31:13 pm
content: Thanks for the informationI only said optocoupler because when I showed the burned part to the seller he said"ok, the optical coupler was been burned"The soldering is not too bad but the picture is bad because of the low resolution I had to use to attach it hereIt's probably typical as it comes out of the wave solder the leads are somewhat on the long side but it is mounted in a plastic box so it's not going to short outThe biggest problem is the poor design or attachment of the TO-220 transistor It's just screwed to a 1/8" aluminum plate with just one pin on the opposite end There is nothing else holding the plate in place so of course the plate will move around during handling and shipping and after a lot of vibrating and flexing I am not surprised the the solder crackedSo the consensus is that this is some kind of mosfet driver ?Well at least that is more than I knew beforeIs there a way to narrow down the list of these mosfet driver types ?

author: CJay
author_rank: Super Contributor
timestamp: December 23, 2020, 08:06:33 am
content: Quote from: status1 on December 22, 2020, 11:31:13 pmD4148 upper right corner appears to have been snapped off the board, the resistor marked 104 on the mid left appears to have little to no solder on the right hand side, it's awful.Yes, you can make an educated guess as to the type of chip (it's not a MOSFET driver, it's a PWM SMPS controller) but you need to draw out the circuit or at least show better pics of a larger section if not the whole of of the board.Are you absolutely sure there's not even partial markings on the chip?

author: status1
author_rank: Contributor
timestamp: December 23, 2020, 02:54:20 pm
content: D4148 was blown as I mentioned it in the explanation of the picture so I removed it It was not snapped offResistor 104 I agree it has a slight angle and it may appear that it does not have enough solder but that is because of low resolution I looked at it with a loupe and it is well solderedI am sure there are no markings at all on the top side just on the bottom as I mentioned earlier but I don't think that is helpfulI started looking around for this PWM SMPS controler I am just not sure what to look forDoes it have to have certain frequency or other characteristics to match the mosfet or I could use any PWM that would match the pin out ?

author: status1
author_rank: Contributor
timestamp: December 27, 2020, 04:42:32 pm
content: PLAN BI did not have any luck finding a replacement pwm so I figured maybe I try to make something out of it with some known parts This is more like an experiment nowI attached a schematic that uses the UC3842N pwm and I am trying to see if it's possible to install it without changing too many parts I understand that I will have to change the pinout of the pwm I am just trying to identify the pins at the momentThe marks in red are the existing parts and pins of the ic as best as I can identify themThe marks in green are the substitute parts or proposed pinsThe marks in orange are some of the unknown and not yet identified pinsFor example on my board I had a diode in parallel with the 22 ohm resistor that was going from the gate of the mosfet to pin 8 of the old pwm now pin 6 of the new pwmIs there any purpose for that diode ? Any problems if I leave it out ?I plugged in the ac power and measured 168v at the cap and at the drain of Q1 so I figured the input section is working so if I get the switching circuit to work I should be able to get something out of itThere is still pins 5 and 7 that I am not sure where they are supposed to be connectedI am open to all comments suggestions or feedbackI am interested if someone can look at the schematic and perhaps comment on the unknown parts in orange

author: status1
author_rank: Contributor
timestamp: December 27, 2020, 10:48:04 pm
content: Before going to Plan B I think I may have stumbled on the PWM ic that I need for this supplyI was watching a youtube video from a guy with a Russian accent reviewing a 24v 4A supply and as he was turning it around I was able to read the part number on the ic It was CR6841 The data sheet for that was in Chinese so I was not able to read it but I think I found an equivalent SG6841 which was in english and both of them matches my pin out so I ordered an SG6841DZ and should get it in a couple of weeksI attached the pin out sheetIt has the gate going to pin 8 which is exactly what my supply has so hopefully I could install it and not have to change anything around it

author: Alti
author_rank: Frequent Contributor
timestamp: December 28, 2020, 10:00:38 am
content: This is most likely referenced to mains (it is on primary side of a converter). With differential isolated probe, experience and knowlege finding susbstitute of the noname chip would cost you 10x the price of off-the-shelf converter of similar parameters.

author: Ian.M
author_rank: Super Contributor
timestamp: December 28, 2020, 11:09:52 am
content: After a blow-up like that, on the primary side, every semiconductor, low value resistor and small electrolytic cap is suspect. You also need to check the secondary side rectifier diodes. If you miss a faulty part on the primary side there's >50% odds the new parts will blow immediately on power-on. Also if the transformer has a shorted turn similar blow-ups are likely, so its very hard to determine if you've missed one, or if the transformer's bad and its B.E.R. so attempting to fix it can rapidly become a money pit.

author: status1
author_rank: Contributor
timestamp: December 28, 2020, 03:35:01 pm
content: I appreciate the warning and I will try to be careful when turning it onThere are not that many parts to be damaged on this supply and I think because of the intermittent connection to the fet it blew up quickly so I don't think it had a chance to blow up anything on the secondary side but I will keep that in mind in case it doesn't workI checked the regulator on the output with my meter on the diode scale and it seems to be okThe transformer I can't check it because my inductance meter is not working but it's not openThe list of parts that are known to be damaged are the pwn , mosfet , the 22 ohm resistor and diode in parallel that was going from the gate to pin 8 of the pwm and the 0.24 ohm resistor from source to gnd pretty much the entire switching circuit sectionThe schematic I showed is not for my power supply so a lot of those resistor caps and diodes are not on my board I only showed that in case I need to replace the pwm with another partBecause this sg6841 is called highly integrated I am guessing that's why it does not need all those other additional partsI am guessing the 168V value on the bulk cap seems to be about right for a 120v input so I assume the input section is working normallyI understand the repair cost value is an issue but for me this was more like a learning experience I learned a lot about how switching power supplies work This is just a one shot deal If replacing the blown parts does not fix it I will not go any further I will not be looking for a transformer if that turns out to be shortedActually the shipping cost more than the parts but you have to pay the shipping if you want to fix it

author: Ian.M
author_rank: Super Contributor
timestamp: December 28, 2020, 04:15:52 pm
content: https://www.repairfaq.org/sam/smpsfaq.htmis worth a *THOROUGH* read through, re-reading anything you don't understand and asking questions here if necessary(as the sci.electronics.* newsgroups have degenerated to pits full of trolls and spam), before starting your SMPS repair career with one with a fault that may be a tough challenge.The supply decoupling cap for the control IC is particularly suspect as the current sense resistor in series with the MOSFET source blew, which would have caused a large positive going transient, blowing the gate oxide and putting 168V straight into the control chip output, which would have dumped it onto its supply pin till the chip exploded. Small low voltage electrolytics don't like 168V, and if it had been sustained the cap would almost certainly have ruptured. As is, its seal and electrode separator may well be compromised, and as small electrolytics are too cheap to be worth removing to test, and as you probably don't have an ESR meter suitable for in-circuit testing or the experience to use it effectively and in-circuit capacitance value testing can be misleading, it should be replaced, with a low ESR 105 deg C rated part of the same capacitance and same or up to 60% higher voltage rating.

author: status1
author_rank: Contributor
timestamp: December 28, 2020, 11:58:11 pm
content: Thanks for the link I will read through that while I am waiting for the partsI am not familiar with the decoupling cap term Are you referring to the 22Uf cap coming from the smaller winding ?I only have 4 electrolytic caps on this supply 1 big cap 100Uf 400V 1 small cap 22uF that I mentioned and 2 medium sized ones on the output 680 and a 1000ufI attached a snapshot from the video which has a much closer resemblance although not an exact match to what I have The pin out of the ic is an exact matchFor example the 82Uf that the pen is near is the one I am referring to as my 100Uf 400v capOn the top where it shows 2 820k resistors in series I have 2 680k resistors in series going to pin 3The 27 ohm res that goes from the gate to pin 8 I have 22 ohm and D4148 diode in parallel with itOn pin 6 I have a direct connection to the drain I don't have that resistor in between however I have a 30k in between pin6 and 8 of the pwmOn pin 4 I have the same 24k resistor to gnd and pins 1 and 2 are also a match except I don't have that cap going from pin 2 to gndPin5 and 7 are a little different on my supply I have pin 5 going through a 100k resistor to the smaller winding on the primary winding where it also connects to one side the C22uF cap The other side of the cap goes to pin 7Would you able to point out on this schematic which is the decoupling cap that you mentioned ?

author: Ian.M
author_rank: Super Contributor
timestamp: December 29, 2020, 12:25:20 am
content: The 22uF presumably electrolytic capacitor connected from pin 7 (Vdd) of the control IC to pin 1 (Gnd) is the one to be concerned about. However you've described different connections for the negative side of the capacitor in your circuit, so if we need to investigate further rather than simply replacing it as I would recommend, you'd need to draw up a more accurate schematic of that section of the primary side circuit. You can skip drawing everything before (and including) the bridge rectifier, and everything on the secondary side for now.

author: status1
author_rank: Contributor
timestamp: December 29, 2020, 02:36:13 pm
content: Thanks for identifying the capAs I mentioned that part of the schematic is a little different from what I haveOne side of the cap goes to pin 7 but the other side does not go directly to gnd it goes to a 100 k resistor and than to pin 5 and perhaps internally is connected to gnd The side that connects the cap and resistor also goes to the smaller winding of the transformerI attached a screenshot of the SG6841 block diagram to show what is inside this pwm in case that helps to clarify thingsWould the power supply work without that cap ?

author: Ian.M
author_rank: Super Contributor
timestamp: December 29, 2020, 03:10:23 pm
content: No. Its responsible for smoothing the controller's Vcc, and if its gone high ESR, low value or has blown up or rotted off due to leakage, the controller would loose power and reset while attempting to output the MOSFET drive pulse, resulting in weak decaying gate drive rapidly blowing the MOSFET.

author: status1
author_rank: Contributor
timestamp: December 29, 2020, 04:38:32 pm
content: Thanks I will try to get oneIs there a part or brand that you recommend ?The part that is in there is a Hyncdz brand 22Uf 50v 105Degree capYou said something about the ESR How important is that ? Could I get any cap with the same value or do I have to look more specifically about the low ESR ?

author: Ian.M
author_rank: Super Contributor
timestamp: December 29, 2020, 06:07:21 pm
content: It only has to handle the ripple on the controller operating current so a super-low ESR isn't needed. I'd prefer a 105 deg C cap because I don't want to have to re-do a repair in a couple of years, but an 85 deg C cap will do for testing. I'd use whatever I had handy that's nominally low ESR, either 50V or 63V, of the same value, and if I didn't have any, hit the parametric search and add a pack of 10 to my next order from any of the major distributors.If you cant source a cap reasonably quickly, *IF* you've got a full LCR meter or *both* a capacitance value meter (or capacitance range on a DMM) and a calibrated ESR meter, I might be tempted to leave it *IF* it shows no signs of distress or electrolyte leakage, its leakage current is insignificant, its value is within the range -10% to +20% (nom. +/-20% but I'm always suspicious that caps at the bottom end of the tolerance range may be on the way out) and its ESR is 'in the ballpark' for budget low ESR caps of its size, value and voltage rating (hit a distributors parametric search to find typical ESR values or trawl throughhttps://www.eevblog.com/forum/beginners/esr-values-for-electrolytic-caps/).

author: status1
author_rank: Contributor
timestamp: December 29, 2020, 11:56:57 pm
content: Thanks for clarifying about the ESRI actually remembered that I got an old lc meter a while back from work that was getting thrown out so I took it home to see if I can fix it I found the on/off switch was somewhat loose so it was intermittently turning on and off but I fixed that and I put it in the basement since I did not have any caps to measure at the timeSo I dug it out and measured the cap on the board and I got 22.2uf so I figure that's close enough just to see if it worksLater on I found the exact one on ebayhttps://www.ebay.com/itm/2-5-10-20-50-100-Piece-22uF-50V-105C-Radial-Electrolytic-Capacitor-5x11mm/123976893765?hash=item1cdd994545:g:sNAAAOSwl8VdzwfRSo I ordered 5 pcs just to have some on hand in case I need it

author: Ian.M
author_rank: Super Contributor
timestamp: December 30, 2020, 12:41:32 am
content: You need to figure out how to use your LCR meter to check the ESR! It should be under 2 ohms, and under 1.5 ohms if its a regular low ESR cap, maybe under 1 ohm if its a high quality low ESR cap. If a cap's been stressed by overvoltage and has lost some electrolyte, as long as the oxide film on the surface of the anode foil is still wet all over with electrolyte the capacitance will remain pretty close to nominal, but if the porous paper seperator has had much of the electrolyte boiled out of it, its resistance will have increased significantly, increasiung the measured ESR. High ESR==Bad Cap, that cant do its job of adsorbing and releasing current pulses quickly, which is essential for most capacitors in SMPSUs.For comparison, I've just grabbed a fairly crappy 'jamicon' 22uF 50V 85 deg C electrolytic that I bought about ten years ago (so pretty much date expired), and it reads as 21.1uF, 2.1 ohms ESR. I certainly would want something better at that location in a SMPSU.

author: status1
author_rank: Contributor
timestamp: December 30, 2020, 02:36:51 pm
content: I understand your concern but I don't have an ESR meter so I have to take a chance with either the existing cap or the new oneThis is just a one shot deal if it works great if not than it was a good learning experience who knows maybe it's something else that is damaged that is not readily apparentThe cap does not look damaged in any way no bulges and no leakingIs there a way to turn it on slowly or perhaps place a momentary switch between output of the pwm and the gate just as a way to perhaps catch it before it starts to blow up and maybe cause a fire ?

author: Ian.M
author_rank: Super Contributor
timestamp: December 30, 2020, 03:30:05 pm
content: Not really. Most SMPSUs misbehave more and even blow up due to over-current if you attempt to 'soft start' them by ramping the voltage up slowly after disconnecting/disabling the UVLO circuit. You *may* get somewhere putting an incandescent lightbulb in series with the transformer primary as a current limiter. You can check the startup circuit and MOSFET gate for signs of life with the feed to the transformer disconnected, but that wont help at all with assessing the 'health' of the capacitor in question.See the link to "Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies" I posted back in reply #17 for more ideas.

author: status1
author_rank: Contributor
timestamp: December 30, 2020, 05:08:43 pm
content: I understandIn that case I will just place it on a concrete floor keeping it at lest 6ft away from everything and just spin the wheel of destiny and see what happensI will report back in either case

author: status1
author_rank: Contributor
timestamp: January 09, 2021, 05:36:04 pm
content: Good news bad newsGood news I replaced all the bad parts and it did not blow up, bad news it still does not workI measured some voltages around the pwm and a lot of them are bouncing between 11-15V not sure if it's because it's not the exact pwm and it may need different parts for it to work or maybe I have another bad part somewhereI am still waiting for the filter cap but I don't think that will make any difference That 11-15 Voltage is also across the filter capSo looks like I will be placing this in the junk pile in case I need some parts for another project

author: status1
author_rank: Contributor
timestamp: January 11, 2021, 02:48:21 pm
content: Possible fix ?I watched a couple of videos where a guy is testing the feedback circuit by applying the dc power to the output without any ac going into the input and he is looking at a change in voltage at the output of the optocouplerCan all optocopler be tested in that way or it just worked for that particular power supply ?I gave it a try but I was not getting anything on the output of the optocouplerSo I am guessing the optocoupler was blown when the pwm blew up so there is nothing going to pin 2 of the pwm from the feedback circuit and maybe that is why the power supply is not workingIs that a possibility ?

author: status1
author_rank: Contributor
timestamp: February 09, 2021, 05:40:41 pm
content: I am back to working on this powers supply again just to see what else I can do with itI tried that trick with the optocoupler but that did not work but than I measured the output and I was getting about 90 ohms across the pins so that did not look good so I ordered a replacement but that still did not work or maybe the PWM blew up for a different reasonSo I am trying to do a more systematic testing of each partI hooked up the n channel mosfet to my arduino that I got recently and I was able to get an LED to flash so I would say the mosfet is switchingThan I turned my attention to the PWM and this is where I hit a snag and I could use some helpI removed the PWM from the board just to find out what is the voltage going into itPin 1 of the PWM is GND and it goes to the negative side of the bulk capPin 3 is supposed to be the VIN and goes through 2 680K resistors in series so 1.36M ohm to the plus side of the bulk capSo when I apply power I am getting the 168V across the bulk cap which I had before but I am getting 147V at pin 3 and according to the PWM spec the max voltage should be only 36VIs there something wrong there or maybe it cannot be measured that way ?The voltage only drops 20V when it goes through 1.36M ohms ?I am afraid that if I install another PWM it is going to blow up or at least not work because the voltage is too high

author: status1
author_rank: Contributor
timestamp: February 09, 2021, 10:53:29 pm
content: Before installing the PWM into the bd I thought I would use the breadboard from the arduino and just hook it up with wires that way in case it blows up it's easier to change plus it's easier to take voltage measurementsSo I guess that 147v on pin 3 was misleading because when the PWM is hooked up to the bd it just bounces around 19V so I am not sure what the 36V from the spec means Interestingly if I remove the power the voltage stabilizes ataround 19V and stays there quite while I guess the bulk cap is holding it charged upThe other measurement was on pin7 VDD which stabilized around 15V after removing the powerAt this point I only had 6 wires hooked up I did not hook up the FB and RT (Feedback and Temperature)I reconnected the diode over the opto pins and somehow this made the voltages stable or at least I thought it did even though there was no wire going to pin 2 FBI have to recreate this condition if I can to verifySo I installed the PWM into the bd but the oscillations cam back on pins 3 and 7 so I am back to the same problemBefore mounting the PWM into the bd I took some resistance measurements and I have about 90K between pins 1 and 2 and about 300k between pin 1 and 8After installing the PWM into the bd I measured the same points just to see if something changed like blown up perhaps and for some reason I measured close to 0 ohms across pins 1 and 8Pin 1 is GND and goes to the source of the mosfet and pin 8 goes to the gate of the mosfet so I thought maybe the mosfet got shorted but after a while the resistance increased to 27KAfter looking at the bd there is a 30K resistor between the source and gate so that is what I was measuring but I wasn't sure where the 0 ohms was coming from so after removing the mosfet and turning on the power it looks like pin 8 is pulled to GND for some reason until the bulk cap discharges and than it opens and I can measure the 30k that is in parallel with pins 1 and 8So that is the next mystery to solve

author: status1
author_rank: Contributor
timestamp: February 10, 2021, 09:27:29 pm
content: I verified that the diode on the opto did not make a difference I think I did not turn on the power so that is why it looked like it was stableAfter doing that experiment I wanted to see what happens if I take out the ic from the breadboard while it was still showing about 19V on between 1 and 3 of the PWM with the power off and it was still holdingSo that is basically the bulk cap voltage however if I apply power without the pwm it goes up to 147V so the spec makes no sense with the 36V I suspect the PWM has some resistor inside to pull it further down to 19V so I am not sure how that is supposed to workWhile I was there I also experimented with pin 7 VDD of the PWMIf I remove pin 7 connection from the PWM the voltage on pins 1 and 3 becomes stable and if I put it back it starts bouncing again so that may be a clue that needs further investigationIf I measure pin 7 from the bd without being connected to the PWM I see the voltage on the filter cap about 11V but it's dropping meaning it is not getting any power from the second winding of the transformerSo where does the voltage come from ?Well it is actually coming from the PWM pin 7 if you can believe thatSo that's another thing that doesn't make sense to meVDD is supposed to be supplied by the second winding to the PWM which it does not but at the same time there is already voltage coming out of pin 7 of the PWMThis thing looks like a catch 22 to meIn order for the PWM to work it has to start switching the fet through the primary of the transformer to get any movement on the second winding to power the PWM but it cannot power the PWM because the PWM is not switching