Chinese Switched Mode Power Supply : To Repair, or Not to Repair?

[SnipTheCat]

Hi,

  I would like some advice about repairing a power supply. I have heard many people (in real life) say that switched mode power supplies should not be repaired, that it is dangerous, and not worth the time/risk vs the cost to buy a new one. However I find it's a good learning exercise and it's a bit sad to throw it away if it can be repaired easily.

  I'm not an electronic engineer, at the base I'm an IT guy (with professionnal experience in microcrocontrollers), and I have about 2 years of practical experience (hobby) in electronics.

  I found this article which provides the schematics of my power supply, it's almost 100% identical except mine is 18V/30A has different power transistors and the clearance issue mentionned in the article seems to have been fixed on the PCB of my supply : http://imajeenyus.com/electronics/20151028_smps_variable_voltage/index.shtml

  - How did my supply die:

     I was using it to power a self made ZVS induction heater circuit (which works fine), the supply was outputting 19V/27A when there was an accidental short between 2 turns of the heating coil causing the drawn current to spike. The supply just stopped working (no bang, no smoke it just stopped), I thought it was in protection but it wasn't. After that the 220V power cables started overheating but the fuse didn't blow. (I realized later that the 220V cable I used, purchased locally, had tiny conductors and severely overheated at 3 places...). I retried with proper power cables (just in case the issue was in the cable) and there the fuse blown instantly.

  - First inspection:

    I opened the supply to take a look, beside the blown fused, there is no sign of anything burnt and no burnt smell, by looking at the schematic I assumed that the most likely parts to have failed were the power transistors, the power diodes and eventually the transformer. I desoldered the diodes and transistors, the diodes are fine, the transistors are both dead (2SC2625).


  What should I do? Should I attempt replacing the 2 transistors, or is it very likely a lot more has failed? How to test safely a repaired power supply, should I put a light bulb in serie with one of the mains wire?

Thanks for any tip or advice.
David.

[tautech]

You're very much on the right track. 
I've always found it useful to identify the SMPS controller and get that datasheet, in fact as many as I can find for the IC. Study of the "typical applications" can often have the same or very similar schematic and component values of the PSU you're working on.
This simple move alone can save a shite load of grief and work.

A scope can be invaluable but it's not always necessary as there are some basic design blocks that aren't hard to test with some basic tools.

Some tricks and points of failure (no particular order).
You can test for correct V+ to the IC when powered.
You can provide the IC with external power only and check it's output for pulses.
Check the AC input bridge.
Flyback diodes.
Output diodes.
HV DC dropper resistor/s

And yes, before applying power use a incandescent bulb in series with the mains.
Google "dim bulb tester".

[SnipTheCat]

Thanks for the tips ;-) I'll give it a try when I get my hand on replacement transistors.

[Rasz]

blowing fuse suggests a short on the ass end, check bridge rectifier, its probably also shorted

[CJay]

Unless the PSU is only a couple of dollars/pounds, whatever local currency, then it's always worth looking into and possibly attempting repair.

Plus, you learn practical stuff which is something you just cannot buy.

First port of call would be to look at the fuse, is it vaporised, coated inside with molten metal, exploded or has it just died gracefully?

[Jeff_Birt]

A good quality switch mode power supply should not blow up when the output is shorted. I have repeatedly shorted the output of MeanWell supplies to test them and they always come back to life. As with many cheap Chinese products the cheap switch mode supplies are most often dodgy and full of cheap 'One Hung Low' parts. I think it would be worth a look at fixing it as a learning experience but next time try to find a better brand that won't blow up on you

[PartialDischarge]

As with many cheap Chinese products the cheap switch mode supplies are most often dodgy and full of cheap 'One Hung Low' parts.

Reminds me of the fake pilots names:

[kripton2035]

all the method step by step : http://www.repairfaq.org/sam/smpsfaq.htm

[Rasz]

if you are more of a video guy you might like this smps tutorial series

[SnipTheCat]

Thanks everybody for your feedback.

I got the supply repaired, it was just the power transistors as I suspected. The light bulb in serie helped as I had mistakenly caused a solder bridge on pins of one of the dual power diode while putting it back (but the short circuit protection kicked in while the bulb flashed). After that mistake fixed, it's working like a charm ;-) In the process I also replaced the blown through hole soldered fuse by a proper fuse socket.

In the end I saved a 36$ supply from going to the trash with a 1.5$ repair ;-) And I got one more experience point.