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The EMC input filter design for a three-phase pulse width modulated (PWM) rectifier is usually separated into the design of the differential mode (DM) and common mode (CM) stages. While for the DM filter design rules and procedures are well known and easier to predict, the CM filter design is often based on trial-and-error methods and/or on the experience of the designer. In the present study, a comprehensive design procedure for a CM EMC input filter is performed for a three-phase three-switch buck-type PWM rectifier. A 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 is verified. Finally, a two-stage CM filter is designed and compliance to the conducted emission (CE) requirements of CISPR 22 Class B is verified through measurements on a 5 kW prototype.
Date of Publication: March 2010