Abstract: This design note shows how a 15V input flyback converter generates -185V at 50mA. The circuit uses the MAX668 step-up controller and a transformer to make a flyback converter.
Figure 1. 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 circuit above generates -185V at 50mA from a +15V input. Based on a MAX668 transformer flyback application, the circuit employs a custom transformer to generate -185V 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 185V + (6 × VIN) + flyback due to leakage inductance, a 600V rating is necessary. C3 is chosen as a film capacitor to get high voltage rating and high ripple-current capacity. A secondary LC (L1 and C8) reduces output ripple from 100mVP-P to less than 5mVP-P.
