Investigation of the high-frequency effects in Mn-Zn ferrites for EMI filter applications

This thesis focuses on the analysis, identification, and experimental investigation of the high-frequency effects encountered within the magnetic core when used in high-frequency, high-power electromagnetic interference (EMI) filters. These applications require cost-optimized, high-performance, and high-power-density magnetic components. As the manufacturers’ material specifications usually do not provide sufficient information to optimize the design, this thesis develops new methods to determine the high-frequency properties of Mn-Zn ferrites up to 20 MHz. Complex permeability and permittivity, as well as specific power loss, are typically provided as one value by the manufacturer, regardless of the core shape and size. Therefore, various magnetic materials are characterized for their complex permeability and permittivity. These two parameters are of differentiated physical origins, and so two independent measurement fixtures are developed and built. The impacts of physical size, temperature and force on complex permeability and permittivity are also considered. The detailed analysis of magnetic flux is introduced based on a 1-D analytical model, a novel shell-based transmission-line model, and finally, based on the FEM and Maxwell 3D eddy-current field solver. These models are used to calculate the complex permeability characteristics for various core sizes made of two materials: 3E10 and 3F36. A complete experimental validation is presented for the calculated values. The analytical methods show a very good correlation with the experimental measurements. The novel shell-based transmission-line model has the best accuracy, and the calculation can be implemented into simulation of a higher-order system or into any other magnetic component design algorithm. Flux verification methods are developed which use precisely-bored cores to accurately predict flux distribution. The results of the flux propagation, starting from the simple three-hole model up to the advanced four-section model confirm that the magnetic flux distribution is affected by frequency-dependent dynamic effects. Flux distributions was experimentally measured for T50 and T80 cores made of 3E10 and 3F36 material. Results are consistent with the FEM simulations and help in the development of a more accurate analytical model. A novel laminated-core common mode choke (CMC) is developed and presented in this thesis. The presented CMC core structure divides the conduction path into sub-regions which allowing for the reduction of the high-frequency effects. Laminated cores, made of several Mn-Zn ferrite materials, were tested and special attention is paid to the effect of magnetic material selection, core size and lamination thickness on the core high-frequency performance. Common mode insertion loss characteristic for the novel CMC shows that laminated ferrite structure give rise to significant attenuation improvement