spirals413

Hi, I've just repaired one of these power supplies and if its not an issue with one of the output MLCC's failing short circuit on the modular interface then usually it is one of the minor rails which is not working/out of spec. First check for shorts on all rails (3.3V, 5V, 12V,-12V). If they are ok, then you can usually back feed the PSU with a 12V current limited power supply thereby bypassing the protection. I used one of the 8pin connectors to inject the voltage into. Make sure you get the polarity right or you will blow one or all of the electrolytic caps up! Obviously dont have the PSU plugged in to the mains when you do this. This will make the fan spin and will generate all the minor rails so you can test them with a multimeter and ensure they are present. In my case the -12V rail was at fault- no voltage present. The 0.1uF bootstrap capacitor (an MLCC) connected to pin 1 of TPS54231 failed short circuit and took out the step down converter IC itself and also the 4mohm current sense resistor blew open circuit. Replaced these parts and all is good again. Very rarely seen the primary side fail on these PSUs, usually it's always an MLCC causing the problems from my experience. Hope that helps someone! PS. Don't go opening PSU's up if you don't know what you're doing! Bulk caps can hold a lethal charge and need to be discharged safely with a resistor.