Common-mode/differential-mode interactions in asymmetric converter structures

This paper presents a theoretical treatment of common-mode (CM) and differential-mode (DM) interaction that is known to exist in switch-mode converter systems. This analysis demonstrates that one important metric for evaluating this interaction (displacement currents through the module baseplate) can be quantitatively predicted through theoretical analysis of two types of asymmetry in the converter system: that arising through topological-induced current imbalances between phase legs, and that arising through parasitic imbalances within the structure of the multi-chip power module (MCPM). The latter has not been given analytical treatment in the literature to date, and represents an important degree of freedom for suppressing module baseplate current, which is a known contributor to CM behavior, and is also known to be exacerbated in the case of wide band-gap (WBG) converters due to fast signal edge rates. In this work, two studies are performed to explore the sensitivity of the baseplate current magnitude to two MCPM design parameters, namely the baseplate capacitance imbalance ratio, and the total baseplate capacitance value. These studies demonstrate that the output of the presented theoretical treatment agrees to a high degree of fidelity with time-domain simulation results. In addition, these studies reveal the unexpected prospect of eliminating the baseplate current due to the opposing influence of the two types of asymmetries considered.