EMI Filter Design for a 1 MHz, 10 kW Three-Phase/Level PWM Rectifier

The attenuation characteristics of electromagnetic interference (EMI) filters in practice often differ from theoretical predictions and minor changes can result in a significant improvement in performance. The performance of the differential-mode (DM) filter stage can usually be well predicted, but the common mode (CM) behavior is more difficult to handle. This is especially true for three-phase pulsewidth modulation (PWM) rectifier systems, which inherently show a large high-frequency CM voltage at the rectifier output. Possible CM noise current paths of a three-phase/level PWM rectifier are analyzed in this paper where parasitic capacitances to the heat sink and to earth are considered. In addition, a concept to significantly reduce CM emissions is discussed in detail. Based on the proposed models, an EMI filter design for a system with 1 MHz switching frequency is shown. Experimental verification of the designed EMI filter is presented by impedance and conducted emission (CE) measurements taken from a 10 kW prototype. Several practical aspects of filter implementation such as component arrangement, shielding layers, magnetic coupling, etc., are discussed and verified by measurements.