Effective design of active filters for specific applications poses several challenges to EMC engineers. If on the one hand, the theoretical considerations available in the literature are complete and exhaustive, on the other hand, when moving to actual filter implementation, some practical considerations on the final application are mandatory. The first aspect to be carefully accounted for regards the actual source and load impedances of the system where the filter is installed. Concerning this, it will be proven that not only the impedance magnitude but also the phase plays a significant role, since disregarding it may lead to undesired oscillations, noise amplification or saturation of the electronics. Also, the non ideal behavior of the involved circuitry has to be considered when predicting the insertion loss of an active EMI filter. An enhanced circuit model of the filter is proposed and its effectiveness for the prediction of the filter insertion loss is proven by measurements on a common mode active filter in the frequency interval from 10 kHz up to 108 MHz.
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