This is what I've come up with for a SMPS I need to power a large 3D printer I'm building.
It doesn't say on the diagram, but here we're outputting 25V AC on the transformer.
Intended Operation:
1) Transformer steps down AC to 25V.
2) BR1 rectifies AC into full wave DC.
3) C1 evens out full wave DC from BR1.
4) Voltage divider R1-R2 provides 5V to the astable multi-vibrator, with a max current draw from the vibrator of 5mA.
5) The next bit in the middle is an astable multivibrator with added diodes for improved rise time, and a P-Type signal mosfet substituted for one of the timing resistors. M1 recieves feedback from the load, and changes resistance based on the feedback, thus changing the duty cycle.
6) The bit on the right is a buck converter, converting the 25V into 12V, regardless of load.
7) D4 is a zener diode which acts as a feedback channel for the oscillator. It will only allow current through if the load exceeds or is at 12V. If it isn't at 12V M1 will have very little resistance, and the duty cycle will increase until it is at 12V.
I'm sure there are MANY fundamental mistakes in this circuit. But, this is what I've come up with. I think it would be best to get critique instead of starting electrical fires.
Thanks1