Capacitive Coupling in EMI Filters Containing T-Shaped Joint: Mechanism, Effects, and Mitigation

The attenuation performance of an electromagnetic interference filter can be significantly degraded by coupling, parasitics, and frequency-dependent nonlinearity, especially in high frequency (HF) range. This article reveals and investigates a mutual capacitive coupling effect in the popular filter structures with T-shaped joint. The mechanism is explained and the impact on filter attenuation is analyzed, which show this coupling is the dominant cause of performance degradation of T-shaped filters and a major cause for other T-shape-related filters. The effect patterns for both common-mode (CM) and differential-mode (DM) filters are analytically derived and further examined in multistage filter structures. Mitigation solutions using PCB slits and grounded shielding are proposed to improve filter transfer gain up to 30 dB in the HF range. A topological strategy is also presented, further enhancing filter attenuation. In addition, the impact of relative positions of the inductors on the coupling capacitance is discussed, and five positions are experimentally studied and compared. Experimental results obtained from three-phase LCL and LCLC filters verify the significance of this coupling and the effectiveness of the mitigation methods.