This 1.2W isolated reference design provides approximately 24V out at 50mA from a 12V automotive power source. The output voltage is split using a Zener regulator to +18V, –5V for an insulated-gate bipolar transistor (IGBT) gate drive. The circuit uses the UCC25800-Q1 half-bridge LLC transformer driver operating at a nominal switching frequency of 500kHz. The secondary-side resonant tank frequency is set slightly above the switching frequency to produce a flat gain. For this open-loop design, the output voltage is proportional to the input voltage by the transformer turns ratio. The transformer windings are integrated into the printed circuit board and sector wound for no overlap. The result is very low interwinding capacitance and common-mode current, along with high isolation voltage.