Coupled Inductors for High-Frequency Drives With Parallel-Connected Inverter Legs

In this article, coupled inductors with cross-coupled windings are described and designed for voltage source converters with parallel-connected inverter legs in each phase. The nature of the cross-coupled windings produces a very low effective series output inductance, related to the intralimb leakage inductance rather than the higher interlimb leakage. The effective inductance between the parallel inverter output terminals can be made high to reduce circulating currents and is related to inductor magnetizing inductance. These two features make the use of parallel inverters attractive for generating multilevel high-frequency fundamental pulsewidth modulation output voltages with a very low fundamental voltage drop across the coupled inductors. Sample magnetic designs are presented for a three-phase system using three inverter legs connected in parallel in each phase. The effective output inductance of the cross-coupled winding arrangement is compared with that of the coupled windings on separate limbs without cross-coupling and is shown to be 97% smaller. The performance of the two inductor winding arrangements is demonstrated with simulation and experimental results of an 11 kW (300 Vdc, 208 Vac/30 A) laboratory prototype operating with fundamental frequencies in the kilohertz range up to 5 kHz.