Interactions between EMI Filters and Power Interconnects on EMI Filter's Performance
The output impedance of EMI filters is the function of parasitic parameters at high frequencies. When the EMI filter is connected to power electronics circuits, the output impedance can interact with the power interconnects such as DC bus bar, or power interconnect between LISNs and the EMI filters. The phenomenon is investigated using transmission line theory. When the length of the power interconnect is larger than 1/20 of the wave length of noise, the power interconnect is no longer a pure conductor. It has characteristic impedance with an impedance transformation function. Depending on the relationship of the characteristic impedance and the impedance on the other side of the power interconnect; the transformed impedance seen from EMI filter side could be capacitive or inductive. And this transformed impedance is the load of EMI filters. The parasitics in the output impedance of the EMI filters can interact with this transformed impedance. As a result, there are some peaks in the insertion gains of the EMI filter at certain frequencies. It may cause the noise problems at high frequencies.