High frequency EMI filter parasitic characterization

Design of electrical EMI filters requires an in-depth understanding of the desired frequency response. While low frequency filter performance (below 2-5 MHz) often matches the expected response, at higher frequencies the actual filter attenuation is generally much worse than expected. Traditional explanations involve the self parasitics of the reactive devices, equivalent series inductance (ESL) of capacitors, and equivalent parallel capacitance (EPC) of inductors. This paper demonstrates that the parasitic mutual coupling across successive filter stages plays an important role in degrading filter attenuation performance at high frequencies. An example filter is constructed, and it is shown that the performance is heavily influenced by the mutual inductance and mutual capacitance between the components. Techniques to measure these parasitic parameters are provided, as well as methods to reduce these parasitics and improve the filter attenuation. Circuit simulation results show excellent agreement with the measured filter performance, and demonstrate the filter degradation due to the self and mutual parasitics.