Would it be feasible to modify an PSU to use 12V?

[gee]

There's a *LOT* of design work that has to be done to make a 12 volt powered PSU. You can't just 'change parts' in a ATX PSU - the entire input section of the supply has to be different, which involves a completely different feedback section, controller IC, etc. You're best off designing something from scratch.

Possible? yes. Feasible? no.

If the input/output sides of the supply don't need to be isolated, it's best to use a non-isolated, synchronous-buck stage. I recently made a supply for work that does 28V -> +-15V, 3.3V and 5V, and it's easily modified to work at 12V.

I'm using a 'LT1628' 2-phase synchronous DC/DC buck converter to do 28V->3.3V/5V, and National makes a LM5642 which is basically an identical chip. Essentially you just pick transistors/inductors/capacitors, some extremely-low-resistance current sense resistors, and do a bit of MATLAB to design a compensation network for the supply. Or if you're feeling lazy, build a clone of their evaluation board...

I've seen power supplies sold for Mini-ITX boards (namely a certain 55W supply) that use more or less this design.

I'm using a monolithic adjustable buck regulator from On Semi to do +15V from +28V, with an auxiliary winding on the inductor to produce -15V. For higher current 12V->12V, it's probably best to build a synchronous flyback or SEPIC converter, or a split-winding buck.

 

[Elledan]

Thank you for the detailed and very informative post, gee!

I do get the feeling that I lack the necessary knowledge and experience to even think of designing a PSU from scratch, though

Time to search for a schematic I can use, I guess