Application of random PWM technique for reducing the conducted electromagnetic a emissions in active filters

A pulsewidth modulation (PWM)-controlled active filter generates harmonics at the switching frequency and its multiples in the RF range, and the concentrated power spectrum may produce electromagnetic interference (EMI) problems. In this paper, the random PWM (RPWM) technique is applied in order to spread the noise spectrum over a wide range, thus, considerably reducing the amplitudes of these harmonics and the consequent EMI problems. To study the operation of active filters, the case of an ac/dc converter along with a power-factor corrector is considered as a nonlinear load and a series active filter, respectively. A line impedance stabilization network is used to study the RF noise emanating from the converter. A noise model to study the EMI emission is presented and used in this paper. Theoretical analysis of the RE noise power spectrum is carried out in order to demonstrate the advantages of the RPWM technique over conventional PWM. Experimental results confirm the validity of the theoretical calculations and simulation results, and demonstrate the effectiveness of applying the RPWM technique in reducing the RE noise level.