Mains-connected boost Power Factor Correction (PFC) circuits are employed in order to meet EMI standards. The EMI filter together with the boost inductor has to attenuate the High Frequency (HF) voltage component to the mains side of the converter. In this paper, it is how the switching frequency and the relative ripple of the boost inductor current affect the filter and boost inductor volume and losses. With it, the optimum switching frequency and ratio of the ripple and fundamental current amplitude with respect to volume and losses are found. procedure for designing AC line EMI filters is presented.