Active EMI Filter with Switch-Mode Amplifier for High Efficiency

Power converters often require EMI filtering which usually involves bulky passive components. Active EMI filters (AEF) can reduce size but incur heavy losses in their linear amplifiers when designed to filter large ripple currents. Here we develop an active EMI filter with a switch-mode amplifier for reduced size and low loss penalty. The switching amplifier of the proposed AEF operates at 31 MHz to keep its own EMI out of the regulated EMI range. We further use fractional-order filtering in the compensation loop to achieve high loop gain and thus high current attenuation. We design the proposed AEF for a boundary conduction mode (BCM) boost converter operated at 150 kHz; we present simulation and experimental results which show 30 dB attenuation from the active circuit at the boost fundamental frequency while consuming only 1 W for 120 W output power. The proposed AEF is compared to a passive LC filter for the same application and is shown to have 8 times smaller volume than the size-optimized LC filter.