Ul is a dual voltage comparator with open collector outputs. The A side is an oscillator operating at 100kHz, and the B side is part of the regulation circuit that compares a fraction of the output voltage to a reference generated by zener diode D2. The output of U1A is applied directly to the gate of Q1. During the positive half-cycle of the Q1 gate voltage, energy is stored in Ll; in the negative half, the energy is discharged into C2.
High-efficiency-flyback-voltage-converter - schematic

A portion of the output voltage is fed back to U1B to provide regulation. The output voltage is adjustable by changing feedback potentiometer R9. Using the component values shown will produce a nominal 300-V output from a 12-V source. However, the circuit maximum output voltage is limited by RlO; a lower value for R10 yields a higher output voltage. The output voltage is also limited by the breakdown of values Ql, L1, D1, and C2.

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