Optimal damping of multi-stage EMI filters

Electromagnetic interference (EMI) and its filtering are important considerations in switching power converter design. It has been recognized that an EMI filter must be sufficiently damped in order to avoid undesirable interactions with the converter or the source, which otherwise may lead to instability and degraded control performance. Optimal damping of single-stage LC filters that achieves maximal damping effects with minimal additional components and power losses has been reported. Multi-stage EMI filters, which are preferred over single-stage designs when high attenuation is required, can be damped in different ways, making their design optimization very complicated. This paper presents an optimal damper design method for multi-stage EMI filters that separates the optimization of the blocking capacitors or inductors from that of damping resistors. Several design examples are presented to demonstrate the design process and effects of optimization. Experimental results are also presented to demonstrate the importance of damping and the benefits of the proposed design method.