Active Filtering for Differential Mode EMI

Switch-mode power converters operate at frequencies ranging from tens to hundreds of kilohertz and tend to generate significant conducted EMI within the regulated frequency band of 0.15-30 MHz. Converters typically require an input filter to comply with electromagnetic compatibility standards to prevent high-frequency currents from traveling through the power source conductors. The traditional EMI filters are usually made of passive components, which are substantial in size, sometimes occupying as much as one-third to one-fourth of the total volume, limiting the power density of the power converters. An alternative to bulky passive EMI filters is the utilization of active electronics, which inject voltages or currents to counteract the interference signal. This work introduces a boost converter in conjunction with a synchronized switch mode Active Electromagnetic Interference Filter (AEF), which reduces energy storage requirements compared to passive EMI filters. The proposed AEF operating at a frequency of 30 MHz effectively mitigates additional EMI into the system as its operational frequency lies beyond the typical range of conducted EMI. The AEF is realized using a synchronous buck converter with a series resonant tank and the current configuration is designed to counteract the ripple component of the boost converter. Firstly, this work presents the comprehensive analytical modeling of the AEF circuit consisting of a series resonant tank to determine the variation of AEF current magnitude to circuit parameters, and duty-controlled switching in the proposed AEF is implemented using high-speed analog circuits to generate pulse width modulated (PWM) signals for the filter. The proposed methodology controls the magnitude of AEF current to a desired value in an open loop to reduce the complexity of the circuit. Further, the AEF is employed in a (6-12)V-to-24V boost converter switching at 150 kHz and has been found to attenuate the fundamental ripple component. From the experimental results, an attenuation of 23dB is achieved using the proposed AEF and a reduction of LC product by a factor of 16 in the AEF and the effective volume of the AEF by 47% compared to that of a single-stage fully passive LC filter.