Abstract

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 an 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 pulsewidth modulation schemes for the ac-ac stage and the dc-dc stage have been compared based on the model. 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.