Generation of Multiple Isolated Bias Rails for IGBT Inverters using Flyback/SEPIC/Cuk Combination

Abstract:
Gate drive bias for IGBT-based motor drives generally require one isolated power domain for each of the three high side IGBTs, plus one for the three low side IGBTs, or four total. In the case of a low-end drive, each domain typically needs +15V for gate drive. While these lowcurrent single-rail requirements may be satisfied by one of several low-cost non-transformer methods, mid-range or high-end drives often require more current as well as -7.5V for gate turnoff and +5V to power an isolated ADC. The added demands point to transformer-coupled topologies such as flyback. In order to provide the multiple rails per domain, most biasing methods require regulation or conversion after the transformer output and/or 3-4 transformer pins per domain. Multiple pins per domain results in 12-16 pins total; too many for the output of a small transformer. This paper describes capacitor-inductor techniques for producing 2, 3, or more rails using only 8 transformer pins total for 4 domains, with good efficiency, good voltage regulation, and without linear or switching post regulators. This technique has been demonstrated with 2 rails per domain in a complete motor drive, and with 3 rails per domain in a single inverter phase. The flyback converter uses a PQ2625 transformer and a "bobbin extender" pcb with 10 mm creepage and 8 output pins.