Adjusting a SMPS output

[Patman]

Hello everybody on diyAudio,
I've got a challenge for the electrical engineers on the forum.

I've pulled a switch mode power supply from an old lexmark printer (got it a few years ago) and I'm trying to incorporate it into a small amp-on-a-chip project. I'm set on this ps because it outputs both 24v and 3.3v, perfect for driving the amp and some decorating LEDs.
Here's the problem: this amp supposedly runs best at 24v, but can operate between 8v-28v. I already know that if I lower the input voltage (say, to 12v) it'll give less gain, which I need for this particular application. This will also allow me to run the case cooling fan (an old CPU cooler) without risking burning it out.

Is there a way (and if so, how?) to lower the 24v output section of this supply, and maintain the 3.3v rail intact? I've been following the traces on the board, and I'm taking a rough guess that if I would be able to modify the driver circuit for the driver ICs, the output could be adjusted.

Also, should I be shielding the enclosure to prevent EMI?

As for filtering, I've noticed one particular fluorescent fixture in my house (on the same breaker as my workbench) generates a lot of buzzing in the speakers when switched on. Is there anything I could/should do about this? The PS has pretty good filtering as it is, I think.

I'd love any help I could get. I need to finish this one quick!

[bob91343]

You can't adjust the outputs of the power supply separately. They are in a fixed ratio, determined by the turns ratio of the power transformer. Having said that, however, there are some supplies that have separate feedback paths and if you are lucky, you have one of those. Rare, though.

Shielding is a good idea; those units are practically radio transmitters.

The fluorescent lamp can be a problem; I doubt there is much you can do with it. If the EMI is line propagated you can try ferrites on the power leads. But if it's radiated from the lamp itself you are out of luck, short of screening it with some mesh.

[luka]

you could do it, but you would need to redo the transformer..

[Patman]

Originally Posted by bob91343

You can't adjust the outputs of the power supply separately.

Remember Bob, this is a switching power supply, not a linear one. I should upload pictures of it, but it's controlled by two small 8-pin ICs. One that (I assume) controls the main power MOSFET (i.e. the PWM cycle) for the 24v system and the other (that I assume) controls a secondary MOSFET [positioned after the first] for the 3.3v output.

[bob91343]

I would imagine the main FET controls all the output. Each output has its own winding on the transformer and the ratio between the outputs is fixed. The secondary FET might be for the standby supply, which these beasts all have, so it can know when to switch on. The green wire needs to be grounded to make the supply start up.

[Patman]

I'll post pictures, they should help decipher this.
From what I see, the main FET goes to the only active coil on the transformer, which supplies the other half of the board. from there it goes to the other IC and eventually the second FET, which runs to the 3.3v.

[wwenze]

SMPS providing multiple output voltages, for some the rails are group regulated, for others individually regulated. Or the power source for the lower voltage rail may be taken from the higher voltage rail.

If you can manage to google the function of the chips that should answer the questions.

[Patman]

Yes, this ps takes the low voltage from high, but has an SMD IC regulating it via a secondary MOSFET (I think...)

[wwenze]

Try cross-loading.

[Patman]

Cross-loading? explain.