Why did this repair my switch mode power supply? - Page 1
Forum section: electronics / beginners

author: canadaboy25
author_rank: Regular Contributor
timestamp: September 20, 2023, 05:12:44 am
content: I have a laptop power brick that quit working. I have other units but figured I would fix it for the education.When plugged into AC power, there would be a short burst of voltage on the output and then it would bleed off to 0V. Nothing else would happen until the primary side filter capacitor was discharged and the supply was re-energized.I checked everything obvious and finally ended up reverse engineering the primary side. I have omitted the input filtering and rectification from my schematic.Upon startup, I would get 1 burst of pulses on the IC drive pin and then nothing. All voltages on the other IC pins seemed pretty reasonable and from the datasheet of the NCP1203, it should have been continuously trying to restart.I removed R25 to isolate the drive pin of the IC. With R25 removed, the IC kept continuously sending bursts of drive pulses just as pictured on page 9 of the datasheet. This seemed right, except the moment I connect R25, the pulses stop.I thought perhaps the cycle skipping feature was causing issues so I grounded pin 1. The datasheet states this will disable the cycle skipping feature and that should have made it easier to troubleshoot.I plugged it in and the secondary side filter caps forcefully vented their electrolyte. Putting a meter on the output and briefly applying power showed over 40V. It should be 19V.So clearly the supply was not regulating. I applied voltage to the secondary side output with it unplugged to see if the optocoupler was getting a voltage across it. There was no voltage. This is when I discovered a nearly invisible crack in the PCB, disconnecting the optocoupler from circuit.I repaired the traces, replaced the caps, and the supply is back to working normally.I am left with some questions:1: Why did the supply not overvolt and blow the capacitors when the trace initially cracked? From the point of view of the controller, the output was low so it should've been outputting drive pulses and trying to start. My only guess is that there is some hidden logic inside the controller IC that detects no change on the optocoupler after the first burst of drive pulses and then latches in a failed state. However, the datasheet makes no mention of such a feature.2: Why did pulling Adj low cause the supply to start? I see no way that the open trace on the input of the optocoupler could affect the circuitry connected to the Adj pin.3: What is the circuitry on the left of the schematic connected to the Adj pin doing? It seems like a lot of complexity to set a threshold voltage for the cycle skipping feature.4: What is the purpose of R25, R20, and Q2? Why the very specific resistor values?I appreciate anyone taking the time to help shed some more light on the topic for me.

author: magic
author_rank: Super Contributor
timestamp: September 20, 2023, 05:58:47 am
content: There are errors here.This Drv circuit makes no sense, there is no way to turn on the FET with reasonable speed.The 1MΩ resistors should be pulling up VCC, not CS.As for why it didn't blow up right away, I don't know. Some controllers have overvoltage protection by monitoring AUX winding voltage (which is proportional to secondary voltage). Maybe circuits on the left implement similar protection and command the chip to shut down when AUX is suspiciously high. At a quick glance, it looks like ZD1, Q4, Q3 are pulling ADJ up when AUX voltage crosses some threshold.

author: canadaboy25
author_rank: Regular Contributor
timestamp: September 20, 2023, 06:24:07 am
content: Quote from: magic on September 20, 2023, 05:58:47 amYes, you're right, R20 should be 31.6Ω, not 31.6K. I must've missed deleting the K when entering the value.Same with R19. The schematic editor defaults resistors to 1K so I have to remember to manually delete the K each time.I will update the schematic in my first post.Quote from: magic on September 20, 2023, 05:58:47 amFor R3A and R3B, I am quite confident on the connections. I have attached the images I used for the reverse engineering. I was very careful buzzing the connections out under the chip and double checked my results. Looking at the layout of the PCB, I cannot see any way they connect to VCC.The controller IC initially supplies voltage to the VCC line from the HV connection on pin 1 until the supply starts. Is this the reason for suspecting VCC should be pulled up to HV?Quote from: magic on September 20, 2023, 05:58:47 amThat makes sense that the feedback would be through the AUX winding. Looking at the internal diagram of the chip in the datasheet, the output will shut down if Adj > FB. However, pulling Adj low after a failed startup did not make it retry, even after the initial charge on the secondary side bled off. So there must be some sort of fault latching in the chip that is not described in the datasheet.Thanks for the reply.

author: EPAIII
author_rank: Super Contributor
timestamp: September 20, 2023, 06:42:28 am
content: Am I the only one who finds the text (part numbers and values) far too small to read?

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2023, 06:59:43 am
content: There is an empty footprint for a resistor between the Collector of Q3 and the Base of Q4 which would cause the transistors to latch as suggested in the data PDF.This could be an undocumented variant of the NCP1203 that has the latching added internally.

author: canadaboy25
author_rank: Regular Contributor
timestamp: September 20, 2023, 07:10:29 am
content: Quote from: xavier60 on September 20, 2023, 06:59:43 amWow, excellent catch. That looks like it's exactly what's going on.The marking on the chip is DAP8A. The NCP1203 datasheet came up when I searched the chip number and seemed to match.I just did another search and found DAP008 which is a customer specific device with the suspected latch integrated to the Adj pin.This makes much more sense and explains the latching behavior I was seeing. I wish I had found that originally, would've saved me a lot of time.The datasheet for the DAP chip also details how switching frequency jitter is intentionally introduced to reduce EMI signature. The plain NCP1203 does not seem to do this. I was seeing jitter on the scope and didn't know why. That explains it.

author: soldar
author_rank: Super Contributor
timestamp: September 20, 2023, 08:24:04 am
content: Quote from: EPAIII on September 20, 2023, 06:42:28 amIf you download the file you get it in the original size and not in reduced size.Try this: