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For a three-phase buck-type PWM rectifier input stage of a high-power telecommunications power supply module a differential mode EMC filter is designed which ensures compliance to EN 55022 class B. The design is based on a harmonic analysis of the converter input current and a mathematical model of the measurement procedure including the LISN and the test receiver. Guidelines for a successful filter design are given and components for a 5 kW prototype of the rectifier system are selected. There, the damping of filter resonance is optimized for given attenuation in order to facilitate a higher stability margin for the system control. Furthermore, the dependency of the filter input and output impedances and attenuation characteristic on the inner mains impedance is discussed. Finally, the theoretical considerations are verified by conducted emission measurements and the high quality sinusoidal shape of the resulting mains current is demonstrated.