Volume minimization of the main DM/CM EMI filter stage of a bidirectional three-phase three-level PWM rectifier system

Three-phase voltage source AC-DC converters disclose a differential mode (DM) and a considerable common mode (CM) bridge-leg output voltage at the switching frequency, which need to be filtered by an EMI input filter to comply with conducted EMI standards (e.g. CISPR 11) at the mains terminal. Additionally, the EMI filter (which in this paper includes the boost inductor) should usually limit the maximum peak-to-peak bridge-leg output current ripple, to which a DM and CM component contribute, to 20%-40% of the nominal peak current. Typically, the DM current ripple is defined by the boost inductance L_{dm, 1} and the CM current ripple by the CM inductance L_{cm, 1}. Therefore, a degree of freedom in the filter design exists: different combinations of L_{dm, 1} and L_{cm, 1} lead to the same peak-to-peak bridge-leg output current ripple. Accordingly, this paper investigates the optimal ratio k_{L, opt} = L_{cm, 1}/L_{dm, 1}, which minimizes the volume of the main LC stage of the DM/CM EMI input filter. To achieve a compact filter, it is demonstrated for a 10 kW PWM rectifier system that for k_L ≥ 3 the boxed filter volume is almost constant. Additionally, k_{L, opt} = 14 minimizes the stray field of the CM choke L_{cm, 1} and leads to a minimal boxed filter volume of 0.43 ltr. for a filter cooled by natural convection.