Derivation and Validation of a Common-Mode Model for a Neutral Point Clamped Dual Active Bridge

This paper applies a common-mode modeling approach for a Silicon Carbide (SiC) based medium voltage neutral point clamped (NPC) dual active bridge (DAB) with a 2 level Full-Bridge (2L-FB) stage utilizing an electromagnetic interference (EMI) characterization testbed. A common-mode equivalent circuit model (CEM) for the system is derived, which accurately captures the effect of cross-mode coupling behavior between differential-mode and common-mode caused by circuit asymmetries, such as baseplate capacitance of multi-chip power modules or windings in the transformer. This cross-mode coupling effect is required to accurately model EMI at the higher frequencies of the conducted emissions standards. The derived CEM shows close agreement when compared to the mixed-mode simulation, verifying the model's efficacy. Additionally, baseplate current was shown to be minimized by tying the neutral point of the converter to the heatsink, where this result can be explained through the CEM. The CEM of the NPC DAB will be validated through empirical measurements on an EMI characterization testbed. The testbed features a copper ground plane and custom-built LISNs that can handle the unfiltered harmonic and EMI content of power electronic converters.