Repairing power supply for fun

[smashrc]

Thank you for reading this post

I know nothing about electronic but I am very interested to repair my borken power supply. This power supply is a 2 in 1 design that means it has 2 power supplies built in the same case. You can see the picture, left and right sides are the same. The output of this power supply is 12v 22A each channel.

One day it stopped working. So, I opened the case and see which part is broken. I found the 3 parts are broken

300v 460uf Capacitor is puffed
5D-15 NTC Thermistor is broken
Fuse is gone

Then I ordered those 3 parts online and replaced myself. Turned on the power supply with no loading, the screen (yes it has lcd screen) said one channel is over load and one is normal. After a few seconds, I hear some clicking sound inside. Then I off the power supply and opened the case for a check up. This time same thing happen, the Capacitor is puffed, the 5D-15 NTC Thermistor was very hot, the fuse almost gone. It means that the problem is not solved, it must be something else got wrong to make those 3 parts keep blowing again. As I have no electronic knowledge, I would like to ask what parts should I replace\check to make this power supply back to life. I understand I may not provide enough information here asking for help. So, please feel free to ask me any information if needed.

Last, thanks so much for your help.

 

[(*steve*)]

I would next be looking at the mosfets and diodes attached to that heatsink.

If both halves of the power supply are similar then you can compare the result you get on one side with those from the other.

You will initially me looking for physical damage to the mosfets (small holes or chips, and possibly a smokey residue) and then for short circuit between source and drain. Beware that you may need to remove the mosfet to do the latter.

 

[(*steve*)]

In this case it is the input capacitors that are being destroyed, not the output capacitors.

Having said that, the cause is similar -- excessive ripple current.

 

[Elecbegginner]

In this case it is the input capacitors that are being destroyed, not the output capacitors.

Having said that, the cause is similar -- excessive ripple current.

How this excessive ripple is generated ?

 

[(*steve*)]

How this excessive ripple is generated ?

By the ripple voltage.

 

[Elecbegginner]

Could this PSU contain a Full-wave rectifier ??

 

[KrisBlueNZ]

I'd suspect the bridge rectifier at the input. A very leaky diode in the bridge would cause those symptoms. It could also be something downstream, as Steve suggested - a MOSFET or a high-speed diode. But I suggest you replace the bridge first; they're not expensive.

Edit: You could also check the suspect bridge rectifier with an ohmmeter. You will probably get valid results testing it in-circuit.

 

[Elecbegginner]

I also suspect the Mosfets causing this , since they operate heavily on current .

 

[smashrc]

Thank you Steve, Elecbegginner, and KrisBlueNZ for helping me grasshopper.

I have inspected those mosftets, diodes, hi-speed diodes by my eyes. There is no holes, chips or smokey residue. Also no short circult on the board.

For the bridge rectifier (still no holes, chips or smokey residue), I don't have ohmmeter. If I buy the ohmmeter and how can I check to tell if the bridge rectifier is good or bad? The bridge rectifier is really low price maybe I just replace it. The cost is way lower then buying the ohmmeter I believe.

For the Mosfets (still no holes, chips or smokey residue), how can I check if they are good? I only have a voltmeter here.

The capacitor on the left, the top area already puffed a little. I want to know should I replace it or I can still use?

 

[smashrc]

More pictures

 

[(*steve*)]

I also suspect the Mosfets causing this , since they operate heavily on current .

Elecbeginner, you're way out of your depth.

Based on what Kris said in the post immediately prior to yours, a person would probably start looking at the bridge BEFORE looking at the mosfets.

However I do recall a lot of discussion about a SMPS fault (endemic in a particular device) which caused the input capacitor to fail. As I recall, it was difficult to fix (as it involved the original fault taking out a controller chip as well. If I recall correctly, that device had active power factor control and I suspect this power supply does not.

 

[(*steve*)]

Smashrc, If you can take similar photos, this time without flash (outdoors in shade is often best) then we should be able to read the part numbers on the ICs.

Also, an image of the copper side is often advantageous.

 

[Elecbegginner]

to check mosfets youll need a DMM with diode test mode at least , it will include all other function too

 

[(*steve*)]

Elecbeginner, stop giving bad advice.

 

[KrisBlueNZ]

I agree Steve, I don't see any power factor correction stage there.

smashrc, I suggest you replace the bridge rectifier on suspicion, and replace the electrolytic, before you test again.

 

[Elecbegginner]

Elecbeginner, stop giving bad advice.

Sorry , i just did previously some Mosfet checks only with the DMM's diode test mode to see if there is some short circuit in them , but i you think its a bad advice , you're the MOD anyways .

 

[(*steve*)]

Elecbeginner, I probably should move this elsewhere, but parts of it may help the OP so I won't...

Firstly, you asserted

to check mosfets youll need a DMM with diode test mode at least , it will include all other function too

You don't NEED a diode test function, and I probably wouldn't use it on my meter anyway. If you don't have it, then you can make a very simple circuit to use with any multimeter anyway.

Also, the fact that it has a diode test function doesn't mean it includes all the other functions. For example, the meter may not have capacitance or inductance modes -- both of which might be useful in testing parts in the SMPS.

I would hate to think that someone might be influenced to choose a multimeter on such a limited basis (diode check = good meter).

Secondly, your posts are essentially "me too" posts. Whilst I normally don't mind flattery, if someone were to decide on the best advice based on the number of posts suggesting something, they would place less weight on the excellent advice given by Kris (which incidentally disagrees with my post, and which I actually agree with)

Thirdly, you are offering opinions for the cause of a fault when you clearly don't understand the mechanisms of failure of one of the components.

Fourthly, you'll find that another moderator has removed one of your posts in this thread with the reason "Really bad info".

By all means, jump in when you think you can help, but if you venture beyond your level of expertise, expect that if you are wrong, each response to one of your posts is likely to escalate if you show no signs of getting the point.

Everyone here must be prepared to learn. Do it with good grace. Do it without disrupting someone else's thread. I post a lot here, and it means I probably make the most mistakes I try to take any correction addressed to me as a learning experience.

Read this post and remember it.

Everyone should be allowed to make mistakes and not have them preserved forever. This post and possibly others in this thread may be deleted in a few days because they're not helpful to this thread, nor to anyone else...

 

[elebish]

If the cap is blown, you probably have a shorted power supply diode feeding ac directly to the cap. Many of those diodes are a bridge type. If any ohm checks (use diode mode) checks shorted, replace. You might also check for shorted MOV's or other safety components used. Ed.