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hi,i think is difficult.years ago,my design need a PWM power converter,and some others.when i tried to use the PWM,it is difficult to use,because a PWM is noisy,especially you need a big current.it will easyly cause the CPU run away.so if you want your design stability,this method can not work well.
It is in my experience very feasible way to make a switchmode converter based on PIC. Naturally, as shanren points out, the noise from switching may be a problem, if not handled properly.
I have some years ago made a PIC based DC to AC converter - to be used as phone ringing voltage generator in a telecom unit. I used a MosFet transistor switch and basically a flyback topology. Generating the switcher control was only part of the story; to make a power supply needs "closing the loop" - measuring the output and regulating the switch process to get ot the wanted voltage. The performance of that regulating loop is critical for transient response and stability, so the design is not quite trivial - specially if one needs a good transient response.
Because it is so much easier to use dedicated switcher control chips for DC power supplies, I would not make a ordinary power regulator from a PIC, but use it only in similar tasks than that project: If there simply is no neat and ready-made solution for the task, PIC is very flexible.
Again - the noise must be handled, and grounding is the most fundamental single issue in a switch topology. (also for dedicated regulator chips!) The power return currents must be kept in their own loop, not passing through the control circuit grounding. One popular way to do it is single-point ground between the "power side" and the "control side" (PIC in this case).
Conclusion: In my opinion, provided correct design and correct PCB layout a PIC should be able to do the trick pretty well. But the task is not easy for beginners in power supply design.