This application note describes how to correctly design and optimize a three-phase power-line filter (PLF) by decomposing the conducted emissions into their common and differential modes (CM and DM1, DM2, and DM3 respectively). Using an actual test case as an example, the proposed solution is focused on minimizing the total cost and size of the power-line filter and optimizing its performance. This can only be done after measuring and detecting the dominant mode in the conducted emissions of the device under test (DUT).