AN ISOLATED MOSFET GATE DRIVER

Traditional methods of isolated MOSFET/IGBT gate drive are presented, and their pros and cons assessed. The best options are chosen to meet our objective — a small, high speed, low cost, low power isolated gate drive module. Two small ferrite bead transformers are used for isolation, one transmits power at 2.5MHz, the other sends narrow set reset pulses. On the secondary these pulses drive a transistor totem pole to ensure high current drive, and the value is held by CMOS buffers with positive feedback. An alternative design for driving logic level devices uses only an HC buffer on the secondary. Double sided SMD construction (primary one side, secondary on the other) yields an upright module 40x18x5mm. Propagation delay was 20ns, and rise/fall time 15ns with a 1nF load. The design places no limits on frequency of operation or duty cycle. Power supply requirements were 5V @ 20mA for operation below 100kHz, dominated by magnetising current.