Modding an atx psu

[m4ge123]

I want to mod this atx psu I got (it's a logisys ps480d) to make a variable supply that outputs about +/- 5-25v at 15-20 amps. The schematic seems to be pretty similar to this. My plan is:

desolder basically the whole secondary side (including the main transformer),
reconnect the primary of the main transformer by soldering wires to the board and the transformer,
rectify and filter the output with properly rated schottky diodes and a pi filter,
make a voltage divider out of a 10k potentiometer and connect it to pin 1 of the tl494 (after desoldering the built-in voltage divider, of course).

I also plan to remove the lm339 because it wouldn't be serving any purpose (that I can see) after I basically redid the circuit with only feedback regulation.

I have several questions:
Do you think my plan would work?
Should I add input filtering/PFC or whatever? (This cheap piece of crap left out a lot of components.)
Are the main power transistors are good enough or should I replace them? Part number is D13007K.
What capacitances and inductance should I use in the pi filter?
Should I rewind the transformer? I'd rather not if I don't have to.
Am I correct in thinking that the main part that limits the maximum output is the transformer?

[kc8adu]

Re: Modding an atx psu

i would pitch it.
enough well made units to use as a base for your project.

[m4ge123]

Re: Modding an atx psu

I just bought it...

[ratdude747]

Re: Modding an atx psu

if you can, return it.

[kaboom]

Re: Modding an atx psu

Operating conditions are vastly different at 5V out compared to 25V out if you go for a single-output supply.

For starters, the output inductor alone will limit how low you can go before you get into problems with minimum duty cycle and dropping out of 'continuous current mode.'

You will be trying to draw all power from a single output- now you have to redo the feedback to not only monitor one output, but also completely disconnect the rectifier/cap/inductor for the others. And if you do that, you might as well rewind the TX so you've got enough copper in there to handle your full load current with little I2R heating.

Now suppose the transformer ratio is such that with 50% duty cycle on both sides (assuming TL494 half bridge) 180* out of phase and with normal line voltage, output voltage is around 28V. This is usually what I've found before the output inductor.

"But how can we have 28V at the rectifier side of the output inductor and only 12 at the filter caps?" The inductor integrates the duty cycle! And it also reduces the ripple the output caps see.

Let's say that you just run it flat out at 50/50% and use higher voltage caps. 28V, right? When you load it up, that will fall. Transformer impedance/coupling, voltage drop on primary side, etc. Even worse is if you drop the duty cycle way down to get 5V off the 12V output. Now the duty cycle is so low that the output inductor will need more inductance so as to keep the dI/dT manageable and to reduce ripple and keep the caps happy. There are probably four or five separate coils on the big toroid. They cannot just be paralleled! They all have a different number of turns and they will 'fight' or buck each other! You'll have to strip the old windings off and rewind it with several parallel strands. The number of strands/filliars depends on ampacity/circular mils per amp, number of turns depends on the mix and size of the toroid and inductance required.


This is nearly a total rebuild here, not just some 'mods.'

-Paul

[m4ge123]

Re: Modding an atx psu

Thank you kaboom, first useful reply...
I was actually planning to use the 5v winding too, to basically have dual outputs 5-10v and 12-25v. If I use the same number of turns on the output inductor as was originally used (except rewind with a few strands of 24gauge in parallel), should I be fine? And approximately what capacitance should the output caps be?
Now that I mentioned that I was planning to use both windings, is it really worth it to rewind the transformer?

I do intend to rebuild the whole secondary, to just have 5 wires coming out of the case and going to my rebuilt secondary: the 2 primary wires, a feedback wire going straight to pin1 of the tl494, a ground wire, and a wire going to pin12 of the tl494 (I will probably use a 7812 so I don't have to worry about popping caps/the tl494).

I'm just trying to make sure this will work and know any likely issues before I desolder like half the power supply.