Switch-mode active EMI filtering

Abstract

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. This work proposes two different approaches for active EMI filters with a switch-mode amplifier to achieve reduced size and low loss penalty: a high-frequency AEF and a synchronous AEF. The high frequency AEF features a switching amplifier operating at 31 MHz to keep its own EMI out of the regulated EMI range. A fractional-order filtering technique is used to design the feedback compensation loop, achieving a high loop gain and thus high current attenuation of 30 dB from the active circuit at the dc-dc boost fundamental frequency of 150 kHz, while consuming only 1W for an output power of 120 W. The proposed high frequency AEF is compared to a passive LC filter for the same application and is shown to have a volume that is eight times smaller than that of the size-optimized LC filter. The proposed synchronous AEF, in contrast to the high-frequency AEF and other typical AEF circuits, does not use feedback, thereby avoiding the bandwidth and attenuation limitations associated with feedback stability. This AEF also has very low energy storage requirements compared to passive EMI filters and achieves very high efficiency compared to typical AEF circuits with linear amplifiers. Furthermore, it can simplify circuitry by directly utilizing the same gate signals as the main power converter. Additionally, the AEF does not interfere with the closed-loop controller of the main converter, a common challenge in the design of passive EMI filters or feedback-based AEFs. We demonstrated the proposed synchronous AEF through simulations and hardware prototypes for both a boost power factor correction (PFC) and a dc-dc boost converter, operating at different current control modes and switching frequencies. The AEFs achieved high differential-mode current attenuation from 20 dB to 65 dB at different harmonic frequencies and provided significant common-mode current attenuation of over 29 dB by injecting a common-mode current that largely cancels the common-mode current generated by the boost PFC. Additionally, the volumes of the synchronous AEFs are significantly smaller than those of conventional passive LC filters — approximately 1/16 to 1/32 of the size-optimized LC filter. They also have very low power consumption, with a maximum efficiency penalty of less than < 0.7% when filtering high current ripple ratios of up to R = 100% from the boost PFC, in contrast to AEFs based on linear amplifiers. Both AEF proposals present very promising approach to (mostly) replace the conventional LC filter and linear-mode AEFs for smaller volume and high efficiency. At the end of this work, we present the design and implementation of two ultra-fast isolated gate drivers with 2-8 ns propagation delays, one of which will be used to design the gate driver of the high-frequency AEF. This will help improve the compensator bandwidth of the high-frequency AEF, which is limited by propagation delays primarily caused by the bootstrap gate driver and the comparator in the previous design.Electrical and Computer Engineerin