Three-Terminal Common-Mode EMI Model for EMI Generation, Propagation, and Mitigation in a Full-SiC Three-Phase UPS Module

With superior loss characteristics, wide bandgap devices such as silicon carbide (SiC) MOSFETs are expected to replace Si-IGBTs in grid-connected applications. Uninterruptible power supply (UPS) is an application in which low conduction-loss and switching-loss from SiC devices can largely improve the system efficiency. However, fast switching of a SiC MOSFET worsens the electromagnetic interference (EMI). In addition, the UPS is comprised of multiple converters wherein different combinations of the converters take part in power-transfer depending on the mode of operation. This complicates the prediction and strategies for noise, especially the common-mode (CM) part. Such complexity calls for deliberate strategies to be set before prototyping to contain and mitigate the CM noise. In this paper, a three-terminal CM circuit model is presented for a three-phase UPS with an active battery charger and a battery rack. The significance of a dc-dc converter on CM EMI generation and propagation has been analyzed based on the model. In a mode of operation where the dc-dc converter is active, a considerable amount of the CM noise is generated from the dc-dc converter. Also, the multiple resonances on the propagation path associated with dc inductors and the battery rack highly deteriorates CM EMI. As a mitigation strategy in the design phase, different topologies and PWM schemes for the ac-ac stage and the dc-dc stage have been compared based on the model. Furthermore, a 20 kW full-SiC UPS has been built and tested to experimentally verify the impact of the dc-dc converter operation on the noise and to validate the mitigation strategy.