Calculation of Parasitic Self- and Mutual-Inductances of Thin-Film Capacitors for Power Line Filters

This work presents a simple and accurate method for the calculation of both the self-inductance and the mutual inductance between thin-film capacitors, placed in close proximity in electromagnetic interference filters. From physical considerations regarding the impact of both skin and proximity effects, we approximate the currents flowing inside the capacitors at high frequencies by strip currents. This hypothesis is supported by electromagnetic simulations and measurements in this paper. On the basis of this model, we use analytical expressions to calculate both the self-inductance and the mutual inductance as a function of purely geometrical parameters such as the size of the capacitors, their orientation, and the distances between them. Because no previous impedance or network parameter measurements are required to estimate these inductances, this technique is suitable for the virtual prototyping design. We have constructed and measured several test boards to check the accuracy of the method and to identify its scope and limits. Also, in this work, we employ this technique to study the applicability to thin-film capacitors of a design strategy originally conceived to reduce the negative effects of the inductive parasitics of surface-mount capacitors. We demonstrate that this design strategy significantly improves the attenuation provided by filters with thin-film capacitors at high frequencies.