Introduction & Help for a newbie on a simple SMPS - Page 1
Forum section: electronics / beginners

author: jheatac
author_rank: Regular Contributor
timestamp: June 10, 2024, 04:45:27 pm
content: Hello to all.Just a little bit of a background. I have tinkered with electronics for 30 years but am very much a newbie still. I have basic knowledge of components and function (resistors, diodes, capacitors, etc) but can't seem to pull it all together to where I can troubleshoot.I have a nice multimeter, older 4 channel digital scope, variable CC/CV power supply, variable CC/CV load, soldering and desoldering equipment.I usually tinker with something (power supplies, car amps, surround sound units, etc) until I let enough magic smoke out that it is no longer a viable venture.The task at hand this time is an extremely simple SMPS from my daughter's mini fridge (peltier) It is so simple that I refuse to let it beat me. I am going to conquer this one.Board is Hanny 0757-83835908Unit was blinking when you turned it on, so I figured it'd just be a capacitor. Sure enough the 100uf on the bridge rectifier was dead. Replaced it and found little difference.As of right now, it has a ~12v output with no load and can only carry 0.1a until it drops down to 11.9v and 0.2a drops to 10.5v. Output capacitors and output diode check fine.Optocoupler: EL817 - I replaced with no changeRegulator - H431Mosfet - JCS9N50FCOscillator/Ticker? - 33052 - I cannot find anything on this chip - 6 pinsMy mechanical knowledge would point me to the 33052 being bad. My logic is with the failed bridge rectifier capacitor, it had to crank the frequency full blast to attempt to get the right output.The voltage divider feeding the regulator does drop below 2.5v, so that voltage divider seems to be functioning.I'm sort of at a loss until I remove everything from the board, chip by chip, until I find the failed component.Thank you in advance to anyone who can point me where to test.

author: Gyro
author_rank: Super Contributor
timestamp: June 10, 2024, 05:35:32 pm
content: QuoteIn a case where the primary side reservoir cap has dried out and the PSU is only capable of putting out very limited current, my bets would definitely have been on the secondary side output caps. In most SMPSs, they almost inevitably fail before the primary one due to higher current stress (and being snugged up to the heatsink). You say you have checked them - is that for ESR or just capacitance?Another cheap possibility is the little switcher chip supply/startup cap, C8. Being small, these tend to dry out too and it's in a warm place too. It's worth checking before starting to pull other bits. The fact that it is running at all would tend to rule out most active parts.

author: jheatac
author_rank: Regular Contributor
timestamp: June 10, 2024, 05:48:43 pm
content: Thank you very much for taking the time to reply.I did pull both secondary caps and checked ESR and uF. Capacitance is all I paid any attention to. I will pull them, test them, and report back.I will also pull the C8 and check it. I haven't checked it at all.

author: jheatac
author_rank: Regular Contributor
timestamp: June 10, 2024, 06:50:36 pm
content: Thank you for the direction.The two secondary capacitors were about 930uf when I originally checked them. I don't know if it was the heat of reinstalling them or what but here are the current values(all tested at 1kHz)#1 25v 1000uf - 824uf 0.05 ohm#2 25v 1000uf - 879uf 0.04 ohm#3 50v 10uf - 2.21uf 186 ohm (Likely the problem?)#1 replacement 1,048uf 0.02 ohm#2 replacement 1,045uf 0.02 ohm#3 replacment 9.39uf 2.8ohm - I may keep digging through my draw for a better one.

author: Gyro
author_rank: Super Contributor
timestamp: June 10, 2024, 07:45:04 pm
content: That was quick!Yes, the output caps are certainly down a bit, but not to the extent that I would have expected a problem - they don't show any signs of venting due to over-dissipation.The 10uF capacitor (presumably C8) does stick out like a sore thumb. It won't need to be anything special but the lost value and 186R ESR is a definite fail (it's as dry as a bone!), I would think this is the cause of the problem. Look for something 125'C rated if possible for a permanent replacement but anything will do for a test, ESR isn't going to be that critical [Edit, the one you found is good enough for that].The primary end of that SMPS must run hot.

author: tautech
author_rank: Super Contributor
timestamp: June 10, 2024, 08:35:05 pm
content: Quote from: jheatac on June 10, 2024, 06:50:36 pmAlmost certainly, a common failure point in SMPS is the controller IC's VCC cap.It initially is charged via dropper resistors from HV DC until UVLockout is surpassed for the controller to start where VCC then becomes supplied from the flyback winding via a diode.

author: MrAl
author_rank: Super Contributor
timestamp: June 10, 2024, 10:37:01 pm
content: Quote from: jheatac on June 10, 2024, 04:45:27 pmHi,I would say the most common parts to go are the larger electrolytic caps. I've had several power supplies go bad because of just that alone. I do not see any evidence of this from your picture though.The next thing is the driver transistors(s). The driver transistors blow because of overvoltage or overcurrent. That could be caused by a bad snubber though so if you change the transistor(s) it may blow again. The driver transistors could take out the driver chip or driver transistors or both, so they would be suspect too.It would be rarer to see a resistor burn out unless something else burnt out first like a transistor. A visual inspection would be good.Also rare to see an inductor or transformer burn out.If you could check the controller/driver chip(s) that might tell you if either of those went bad. Might need an oscilloscope for that.

author: jheatac
author_rank: Regular Contributor
timestamp: June 10, 2024, 10:50:34 pm
content: Thank you for the help.The unit is now has the rated output at 3.6a while maintaining 12.25vJust out of curiosity - can anyone find the 33052 IC used to pulse the signal? I came up with a zero.

author: MrAl
author_rank: Super Contributor
timestamp: June 11, 2024, 08:11:44 am
content: Quote from: jheatac on June 10, 2024, 10:50:34 pmHi,Sorry I do not know that part number. Closest I can think of might be the MC33063 or similar which was a very popular controller chip.If you can reverse engineer the circuit diagram we can almost certainly figure out what chip it uses or a replacement. This involves some work on your part, but it's been done a lot in the past. I had to follow many circuit board traces around in the past to figure out how to fix something. It takes time and effort but if you want to fix it you sometimes have to do more work. I take pictures of both sides to start with, then try to follow all the traces around on both sides and label all the parts, then draw a schematic. Sometimes you have to use an Ohmmeter to help figure out where all the traces go because they go under parts too.

author: Gyro
author_rank: Super Contributor
timestamp: June 11, 2024, 09:44:34 am
content: Quote from: jheatac on June 10, 2024, 10:50:34 pmGood to hear that the little capacitor fixed it. It's surprising how often people concentrate on the big ones and miss that one. It's under very little electrical stress, but being of very small physical volume and typically being close to Mosfet heatsink and other hot parts, they just dry out. It's sometimes worth using a higher voltage rated replacement simply to force it to a larger can size.P.S. No idea on the chip ID either (it's probably something Chinese), sorry.

author: MrAl
author_rank: Super Contributor
timestamp: June 12, 2024, 03:46:47 pm
content: Quote from: Gyro on June 11, 2024, 09:44:34 amI usually go a little higher in voltage for replacement electrolytics too when space permits.I found a part in an older Volvo that had an electrolytic rated for just 16 volts, and it was really bad with high ESR. I replaced it with a 25v unit.