Diode replacement for my ATX PSU

Story : I transformed a ATX computer PSU into a benchtop PSU and while I was using it, I pulled off 6~8A from the -12V rail which is rated at 0.3A
After 2~3 seconds it shut down with some popping sound. I took a look inside it, and could identify that the FR154 fast recovery diode was broken down in half due to overcurrent.
And btw, the PSU shuts itself down in less than 1 second anytime I try to turn it off and on again. I Guess some kind of safety circuit is activated.

Q1. Can I replace the FR154 diode with a FR155 diode?? If so, would it just have a higher peak voltage or would the higher recovery time cause it to overheat and break again?
(DATASHEET : https://pdf1.alldatasheet.com/datasheet-pdf/view/288350/SYNSEMI/FR154.html)

Q2. What other components would or could have been broken due to overcurrent?? I don't think the diode itself is causing it to shut down itself.

Q3. I'm thinking of replacing some capacitors which I pulled off first in order to take a look at the diode. If I replace the capacitors, Higher voltage limit wouldn't make any problem right??(Just being sure.)

Hi,

0.3A to 8A, quite an achievement

Hope this helps...

Have you got a schematic and a photo of the actual circuit, thanks.

1) I see no reason why you shouldn't replace 154 with 155. WIthout knowing how the power supply operates (i.e. Is it an SMPS that operates at a higher frequency than the diode, and would that be relevant to the diode location and function in the circuit?)

2) Without seeing a schematic it's pointless conjecture. Maybe a shorted capacitor, maybe a shorted transistor, maybe a fault detector shuts off as there's a diode missing, ...maybe many things.

3) No, higher voltage limit isn't a problem.

Higher voltage capacitors isn't a problem but stay to the same values and use low ESR types. As it's inside a PSU, it is a good idea to use capacitors rated for 105C temperature too. The temperature can be higher inside the capacitor than ambient so don't be fooled by thinking the PSU never gets that hot so low temperature ones can be used instead.

Brian.

d123 said:

Hi,

0.3A to 8A, quite an achievement

Hope this helps...

Have you got a schematic and a photo of the actual circuit, thanks.

1) I see no reason why you shouldn't replace 154 with 155. WIthout knowing how the power supply operates (i.e. Is it an SMPS that operates at a higher frequency than the diode, and would that be relevant to the diode location and function in the circuit?)

2) Without seeing a schematic it's pointless conjecture. Maybe a shorted capacitor, maybe a shorted transistor, maybe a fault detector shuts off as there's a diode missing, ...maybe many things.

3) No, higher voltage limit isn't a problem.

Click to expand...

Thanks, that helped a lot.
This is indeed a cheap power supply so I couldn't find any schematics for this one.
But looking at my PSU's copper traces, the 12V output is connected to :
-broken diode
-a large inductor with 2 more copper windings for other rails
-to 2 seperate pieces of FR1??diode(I can't identify exact model number)
-to the 2 pins at the side of a F16C20 dual fast recovery power rectifier(This one is connected to a large heatsink)

This is the furthest as I can tell without any schematics. Can you give me any more advice? Or should I contacat the seller for schematics?

These are the pictures of actual circuits. I couldn't get any schematics.

Hi again,

I wouldn't like to say things that are completely wrong or ill-informed, if there's an inductor it could be a switching supply, it could be a filter inductor...

You could certainly try contacting the seller/manufacturer for a schematic, and depending on how pleasant they are they may reply and may send you a copy, or may just suggest you send it to them/an affiliated repair shop...

I won't even suggest troubleshooting with a DMM as it may prove unsafe or detrimental for the DMM if we do not know how the circuit functions, and if it shuts down immediately anyway you'd need to bypass the faulty part(s) with a jumper cable and I certainly don't recommend that unless you know what you are doing.

If you see bulging capacitor lids, that's one visual fault easy to identify, I have no idea how to check if an inductor is damaged, don't know what else to suggest.

Sorry about that.

(If it was cheap, get another one?)

I can guarantee it's a switching supply! They all are.

It looks like a standard output circuit, only the +5V or 3.3V outputs will be regulated, if they are correct it is assumed the rest will be too. The -12V will come from a secondary winding on the transformer, through the diode, then a Pi filter, a cap to ground, a filter inductor and another cap to ground.

The row of five holes will be under the transformer with the primary side on the right and the individual secondaries for each voltage on their left.

Brian.