So recently after learning about switch mode power supplies, I'm now wondering if you could make one operate directly as transmitter without a power supply. Essentially you just make the switching frequency in the medium wave or HF bands and skip the part where they re-rectify the output to DC. It seems that some of these already switch in the medium wave band. This would make a very efficient transmitter. What's even more interesting to me is if you could find one with a medium wave switching and convert it into a TX by bypassing the re-rectification and filtering on the output. You could produce a sort of dooms day type transmitter easily.
Is this already a type of design that I'm not aware of or does this not work how I think it would?
I read about someone using common parts from a SMPS and doing this but I don't remember where I read it. Essentially you use the TL494 chip that many of those supplies have and use a much higher oscillator frequency and run modulation into pin 2. People have used these chips as very simple, albeit crude.. PWMs and class D audio amplifiers. The problem is they really are designed to run at 300kHz or lower. I'm not sure what the upper limit is of the built in oscillator but an external oscillator would probably allow higher frequencies to be used, or maybe some other variant of the TL494.
As far as the SMPS itself, they will be limited by their internal toroidal transformers and other parts. Changing out those parts would be more of a chore and far more dangerous than to simply convert a single part in them to do PWM voltage output and then simply power a MOSFET PA, which is just a handful of parts if you use a 4 pin oscillator can and a simple single ended PA output in class E, which would then be powered by the converted SMPS. There are people who have done just this and it works. The main issue is the limited voltage output since you want half the supply voltage without PWM modulation, limiting the PA to 6 volts when using a 12 volt supply. 24v and higher rated SMPS do exist though.
If you do have some soldering and electronics skills I do recommend scrapping the PWM chips out of power supplies. They are really fun parts to experiment with and by themselves and a few power FETs make a decent PWM supply on their own. Also make quick and dirty audio amplifiers for homemade powered speakers but don't expect HIFI grade
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~