Optimal Damping of Multistage EMI Filters

Electromagnetic interference (EMI) filter is an integral part of most switching power converter design. An EMI filter must be sufficiently damped in order to avoid undesirable interactions with the converter input impedance or source output impedance, which otherwise may lead to instability and degraded control performance. Optimal damping of single-stage LC filters that achieves maximal damping effects with minimal damping component size and power loss has been reported. Multistage EMI filters are preferred over single-stage designs when high attenuation is required. This paper presents an optimal damper design method for multistage EMI filters that separates the optimization of the blocking capacitor or bypassing inductor from that of the damping resistor. Such separation simplifies the optimization problem and reduces the required solution time. More importantly, it provides much more insight into the operation of different damping circuits and how their design is affected by system damping requirements. Several design examples and experimental results are presented to demonstrate the design process and the effects of optimization.