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PCB Winding Coupled Inductor Design and Common-Mode EMI Noise Reduction for Soft-Switching Three-Phase AC-DC Converter

Circuit topology
Fig. 1. Circuit topology for two-channel interleaved three-phase ac-dc converter with balance technique.
  Critical conduction mode (CRM)-based soft-switching three-phase ac-dc converter with SiC power devices can achieve high power density and high efficiency at hundreds of kHz switching frequency. One of the major concerns for the converter is the conducted electromagnetic inter-ference (EMI) noise. Because of the fast-switching characteristic of SiC power devices, di/dt and dv/dt of the switches are much higher than those with Si power devices. Especially, the high dv/dt at the switching node of phase legs is seen over the parasitic capacitance between the power circuit and the ground and generates substantial common mode (CM) EMI noise. A large CM filter is needed between the converter and the ac source in order to abide by electromagnetic compatibility (EMC) requirements. This depreciates the benefits from using SiC power devices with CRM at high switching frequency: the small size inductors for the power stage and the small DM filter.
  A balance technique can effectively lessen the CM noise. In Fig.1, the two-channel interleaved three-phase ac-dc converter with the balance technique is presented. The circuit topology requires the additional return path connecting from the middle point of the dc capacitors, m, to the neutral point of the ac filter capacitors, n. In the return path, additional inductors are placed to form a balance structure. The inductors on the return path are coupled with the inductors in the main circuit for better balance at high frequency.
  The converter is implemented with PCB winding magnetics. The PCB winding coupled inductor needs to be carefully designed by taking into account a variety of considerations: 1) Winding configuration to minimize the winding loss, 2) placement of the return path to maximize the cou-pling between the main inductors and the additional inductors, 3) coupling coefficient between main inductors to reduce switching frequency range, device loss, and core loss, 4) turns number of the main inductors to balance out the winding loss and the core loss. By properly design the PCB winding inductor, CM Noise can be effectively reduce up to almost 15 MHz as shown in Fig. 2.
Measured EMI noise
Fig. 2.Comparison of measured CM EMI Noise between litz-wire inductor without balance technique and PCB winding coupled inductor with balance technique

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