This circuit uses the MAX668 and a transformer (which will need about a one-to-six turns ratio) to make a flyback converter. The op amp inverts the feedback from the negative output voltage.
The above circuit generates -165V at 100mA from a +12V input. Based on a MAX668 transformer flyback application, the circuit employs a custom transformer to generate -165V through a combination of turns ratio and duty cycle. The transformer is designed on an SG5-size bobbin to facilitate the winding and to decrease the leakage inductance.
D2 is chosen as a fast-recovery diode to moderate the cost for a high-breakdown voltage part. Because the diode will see 165V + (6 x Vin) + flyback due to leakage inductance, a 600V rating is necessary. C3 is selected as a film capacitor to get high voltage rating and high ripple-current capacity.