Power Supply Blown, but not the fuse - Page 1
Forum section: electronics / projects

author: ajp8868
author_rank: Regular Contributor
timestamp: February 16, 2023, 03:07:15 pm
content: Hi All,Hoping you can help, I've got another power supply that I'm struggling with. It originally blew the power transistors (D207L) which were both shorted.I purchased the only replacements I could find, D209L and then checked for shorts which were now gone.When it blew, it took out a 2.4ohm (r42) and a 10ohm (r40) and resistor. I only had 2.2ohm replacements but thought they'd be close enough for this situation.For some reason, when plugging in, it doesn't blow the fuse (I guess might be too high of a rating) but it does go pop, and blows those 2 resistors again.I can't find any shorts and I'm struggling to find any issues.I've ordered a pwm chip TL494 replacement in case it's that.. but where else should I be looking?There's no apparent damage to the secondary side.Thanks

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 16, 2023, 04:26:16 pm
content: Hi ajp8868,The TL494 PWM IC is certainly a good candidate for this kind of failure, and replacing it is a good idea.You might also have a shorted winding on the primary side of the flyback transformer. This decreases the primary inductance to a very low value, so that the current rises that fast that it blows the transistors. This can happen so fast (<< 1ms) that it does not blow the fuse, because once the transistors are dead/vaporized, no more current can flow.It's a pretty usual situation : the semiconductor is actually protecting the fuse...Usually, you cannot check the transformer with a multimeter.The easiest way is to take the transformer from a known good power supply (same model of course) and put it in the DUT, to give it a try.For your next test, do not connect your SMPS directly to the mains : put a 40-75W vintage incandescent light bulb IN SERIES. If the light shines bright, your device has still an issue. And most of the time, this test setup avoids to kill your transistors.

author: strawberry
author_rank: Super Contributor
timestamp: February 16, 2023, 04:39:09 pm
content: half bridge SMPSchinese fuses are the most durabletransformer inductance will reveal transformer failure(sometimes they put so thin enamel or someone had stronger hands when winding)could be too powerful rating for small SMPS

author: ajp8868
author_rank: Regular Contributor
timestamp: February 16, 2023, 05:00:45 pm
content: Thanks both, I'll wait for the chip to arrive and then give it a test with a bulb in series if I can find one.Weirdly neither FET is shorted this time, just the blown resistors. And for some reason these are only blowing on one side.One of the caps looks a bit off but it's the one on the opposite FET to the one that's blowing. If these aren't charging equally, could that be a potential cause?

author: strawberry
author_rank: Super Contributor
timestamp: February 17, 2023, 08:32:23 am
content: in theory film capacitors block DCthis circuit need little DC current kick on power transistors to bring some power to TL494. if TL494 circuit is not working(not generating PWM with low power loss) it will blow transistors

author: jonpaul
author_rank: Super Contributor
timestamp: February 17, 2023, 10:01:43 am
content: Repair of off line SMPS requires specialized equipment, isolation transformers, etc.poorly designed ChiCom junk, worth perhaps £3impossible to debug or, to repair, even if you are very experienced.just replace, it save your timej

author: wraper
author_rank: Unknown
timestamp: February 17, 2023, 10:24:43 am
content: Check large red film capacitor if it has not shorted and still has capacitance.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 17, 2023, 10:59:32 am
content: Thanks I'll remove and test those parts too!

author: ajp8868
author_rank: Regular Contributor
timestamp: February 17, 2023, 11:00:27 am
content: Quote from: jonpaul on February 17, 2023, 10:01:43 amSeems a bit of a poor attitude to have in an electronics repair forum..I'll try what others have suggested first.It's not worth £3 because it's a specific output and design that outputs 5v 18a and 12v 4a - can't find anything else like that, that will actually fit in the space available.

author: jonpaul
author_rank: Super Contributor
timestamp: February 17, 2023, 01:22:11 pm
content: Aha! ACK!In decades of SMPS design/consult and repair I have found most blowups of the switch transistors/FETs are only the end point of a long chain: The power fault kicks back into the driver, control IC and service power.Thus troubleshooting is usually off the mains, with a separate variable lab PS on the control/driver power bus and a se[parate HV supply 0..360 VDC for example on the power DC bus.Enjoy!Jon

author: mariush
author_rank: Super Contributor
timestamp: February 17, 2023, 01:36:51 pm
content: I would always check diodes to make sure they're conducting only in one direction instead of being open or conducting in both directions.Check for stuff that may have overheated ... for example see D14 and D15 with the tiny balls of solder, as if they got too hot and solder melted.probably not an issue but trim the leads ... see left of R39 (2nd picture) that lead seems to be too long.check for cold solder joints... for example above R10 looks a bit iffy to me.The incandescent bulb in series is very good advice...

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 17, 2023, 11:40:57 pm
content: Hi ajp8868,Quote from: ajp8868 on February 17, 2023, 11:00:27 amAs this seems not to be a vanilla model and space is too sparse to replace it with two off-the-shelf SMPS (5V & 12V), it's definitely worth repairing your unit.It is true is that such kinds of repair can be demanding and, using adequate TE (= many dedicated tools, sometimes expensive) slightly raises the success rate.Any failure or even weakness of 80% of the parts on the board can lead to severe malfunction and/or magic smoke release.All of the parts you cannot test thoroughly should be replaced.If you're not familiar with the realm, you might have a look to those videos :As @mariush suggested, redo all of the solder joints.Check out the secondary side of your unitUnplug the SMPS from the mains.By using a lab PS, set to 5 / 12 VDC with current limitation at 100mA, connect the lab PS to the output terminals of your SMPS to check if everything's fine. After a 5 to 30 seconds, the current should drop to a few mA. If the lab PS still limits at 100mA, increase to 200-300mA and try to locate which part gets hot by spraying some Isopropyl alcohol on the parts. You'll see rather easily where the alcohol evaporates first.Of course, at this point, using a first tier thermal cam changes your life...If your unit cannot be repaired, you may perhaps find enough space for a single 12V 150W SMPS followed by a20A step-down converterto get the 5V rail.

author: ajp8868
author_rank: Regular Contributor
timestamp: February 18, 2023, 06:13:51 pm
content: Thanks for all the advice, I've replaced the pwm controller, those blown resistors and reflowed a couple contacts (the FET contacts didn't look great) and it's now working like a charm!Thanks again - much appreciated

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 18, 2023, 07:09:32 pm
content: Quote from: ajp8868 on February 18, 2023, 06:13:51 pmHi ajp8868,Good news that you have successfully repaired your SMPS !