Active Current Balancing for Paralleled SiC Semiconductors in Time-Staggered Switching Mode

The steep voltage slopes of today's wide-bandgap, fast-switching power semiconductors — like SiC and GaN — lead, among other challenges, to reflection on long cables and thereby overvoltage at inductive loads. To apply this technology to motor drives without specially reinforced insulation the voltage slopes need to be reduced. This article presents a new low-loss countermeasure, which involves the parallel connection of two half-bridges via an interphase transformer. Undesired components in the cross-current of the interphase transformer occur due to non-idealities in the voltage symmetry. The resulting additional magnetization can cause the core to saturate. Influences on the converter voltage due to various parasitic influences are described. Especially aspects relevant for dimensioning the magnetic core of the interphase transformer are emphasized. Also, this article describes an approach to deal with the undesired cross-current components using two different closed-loop control concepts. The control uses a special differential-mode shunt for a dedicated measurement of the cross-current mean value. Both control concepts are verified and compared on a 500-kVA SiC inverter test bench, using measurements of the controlled current.