A comprehensive investigation on conducted EMI reduction for variable switching frequency PWM

This paper investigates the conducted EMI reduction of variable switching frequency PWM (VSFPWM) method compared with constant switching frequency PWM (CSFPWM) for three-level inverters. Many factors such as DC bus voltage, switching frequency of CSFPWM method, and modulation index are taken into consideration to research their influence on reducing conducted EMI for VSFPWM. The DC bus voltage has nothing to do with the reduction of conducted EMI no matter how high the voltage is. However, original switching frequency and modulation index have great impact on conducted EMI reduction. The appropriate choice of carrier frequency and modulation index for CSFPWM can make corresponding VSFPWM achieve great reduction on conducted EMI. An optimal operating condition is designed to indicate the fact that the conducted EMI can be reduced by nearly 20dB. Relevant experimental results together with analysis validate that VSFPWM has huge advantage on reducing conducted EMI in comparison with CSFPWM.