A Variable Switching Frequency PWM Technique to Reduce Conducted Emissions of SiC MOSFET Based Active Front End Converter

Wide band gap (WBG) device based power electronic converters are evolving nowadays due to their high switching frequency and high power density. The conducted emission noise due to the switching frequency of the converter violates electromagnetic interference (EMI) standards. Large passive EMI filters are typically used to comply with the standards. The present work proposes a variable switching frequency based modulation technique to attenuate the low-frequency conducted emission of an active front end (AFE) converter. The majority of frequency dithering methods ignore the issues related to the resolution band width (RBW) of the spectrum analyzer. RBW refers to the bandwidth of the band-pass filter of the spectrum analyzer, which accumulates the energies of all frequency components falling within this band and displays a single frequency spectrum. While measuring the noise, it must be fixed at 9kHz as specified by international regulations. This article addresses the impact of RBW on noise measurement, and a modification in the existing technique is also presented such that the noise spectrum of different frequencies does not overlap within a 9kHz bandwidth to obtain maximum noise reduction. The proposed method has been implemented on a 22kW, 800V, three-phase AFE converter through MATLAB Simulink. A reduction of 16dBμV and 5.5dBμV on conducted emission noises have been observed for 500Hz and 9kHz RBW, respectively, and the results are compared with those of constant switching frequency operation.