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A simple SMPS power supply which operates on the principle of Buck converter with coupled inductor providing an isolation from the mains. The self sustaining oscillation is actually achieved by producing a positive feedback through the 10nF capacitor at the secondary of the 1P2S Transformer (Assuming the two transformers have a common core, and multi-secondary winding).
The high frequency pulses are rectified and filtered at the secondary feeding the load resistor.
The regulation part is done by the transistor at the top which you can assume it as a photo-isolate...
