Common mode EMC input filter design for a three-phase buck-type PWM rectifier system

The EMC input filter design for a three-phase PWM rectifier is usually separated into the design of the differential (DM) and of the common mode (CM) stage. While for the DM filter part design rules and procedures are well-known and the parameters are easier to derive, the CM filter design is often based on trial-and-error methods and/or on the experience of the designer. In this work, a comprehensive design procedure for a CM EMC input filter is performed exemplarily for a three-phase three-switch buck-type PWM rectifier with an integrated boost output stage. A simplified model of the CM noise propagation is developed and the relevant parasitic impedances are identified. A capacitive connection from the star-point of the DM input filter to the capacitive centre point of the rectifier output voltage is proposed and the effect of this measure concerning CM EMC is verified. Finally, a two stage CM filter is designed and the compliance to the conducted emission requirements of CISPR 22 Class A is verified through measurements on a 5 kW prototype.