Need Help on SMPS Power Supply Load Testing Failure..:(

[dennis_elx33]

Hi Guys!

I am a beginner in electronics repair. This time I tried to repair an SMPS power supply with 4 outputs 24VDC (10A), 5VDC(10A), 12VDC (4A) and 12VDC (4A) due to intermittent power on/off. After series of checking, I found some few capacitors with high ESR and replaced those with good spares. Now the power is stable and output voltages are all good. But whenever I load test atleast 2A, the voltage drop on outputs 5VDC, 12VDC, 12VDC respectively. The original 5VDC when there is a 2A load becomes 1.3VDC, and the 2 outputs with 12VDC becomes 5.4VDC. However, when I load 8A on 24VDC it drops to 23.6VDC and I think it has no issue. I already have replaced the 2 PWM UC3854BN and all the electrolytic capacitors but still failing. I have removed and measured all the resistors but cannot find abnormal reading. I also have checked the MOSFETs and proactively replaced it with same exact part number. I also have changed the 3-pin voltage reference ICs TL431C but with same result. The problem is I don't have any schematic diagram. Any idea on this is highly appreciated.

PSU Model: PFC375-4200

[sam_sam_sam]

Re: Need Help on SMPS Power Supply Load Testing Failure..

Please post a few pictures front and back of this switching power supply so we can see what you are dealing with

Please make sure that your pictures are very clear and sharp and in focus so the part numbers of devices can be read

[eccerr0r]

Re: Need Help on SMPS Power Supply Load Testing Failure..

When all else fails, replace everything including the optoisolator