A Study of a Hybrid Filter

This paper presents results of a comparative study of two possible hybrid filter topologies, comprised of a passive and active stage, which can be implemented in any general dc supply distribution system. The main filter task is to mitigate current dynamics in the dc distribution system in order to prolong the operational life of delicate dc supplies, i.e., fuel cells, and to reduce the electromagnetic interferences between sensitive electronic circuits connected to the distribution net. The active stage is incorporated into the passive part in order to: 1) improve its insufficient attenuation in the low-frequency range and 2) source or sink any surplus energy flow between the dc source and load in case of low-frequency current dynamics. Two active stage topologies are proposed, analyzed, and evaluated in detail: 1) an active filter based on a single-leg inverter and 2) an active filter based on an electronic smoothing inductor. A special focus is on achieving attenuation at different voltage levels of the auxiliary supply and at different coupling inductances. The impact of the current ripple generated by the switching mode of the active filter operation is discussed as well. The active filter based on the single-leg inverter offers superior attenuation, particularly in the low-frequency range where attenuation is improved nearly for 15 dB compared to the passive filter.