Self oscillating flyback converter

Diode Dl prevents negative spikes from occurring at the MOSFET gate, the 100 ohm resistor is a parasitic suppressor, and Z1 serves as a dissipative voltage regulator for the output and also clips the drain voltage to a level below the rated power FET breakdown voltage.
Self oscillating flyback converter - schematic

Low-power converter uses the core characteristics to determine frequency. With the transformer shown, operating frequency is 250 kHz.

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