Analysis and Design of EMI Filters for DC-DC Converters Using Chain Parameters

Switch-mode power converters (SMPCs) are known as one of the greatest sources of electromagnetic interference (EMI) in electronic equipments. In order to comply with the strict electromagnetic compatibility (EMC) requirements, an EMI filter is needed at the input of a SMPC. The design of EMI filters aims at achieving required insertion loss (IL), i.e. attenuation of the power of the unwanted electromagnetic emissions (EME) from a switch-mode dc-dc converter. However, the EMI filter affects converter’s dynamics. This paper discusses how chain parameters can be used to calculate the IL provided by converter’s input filter and also in the input system stability analysis. The stability analysis shows that relatively small resistances in series with the capacitor and inductor, which are closest to SMPC’s input, can compensate the effect of converter’s negative input impedance. Chain parameters are very useful in IL calculations as well. Comparison between LCand π-filter shows that if dc power line’s impedance is know to be very low, LC-filter will perform as π-filter. With unknown or high dc source impedance π-configuration should be used.