A Design Principle Ensuring Uniform Flux Density Distribution of the Two Middle Legs Planar Core for LLC Converter

The use of PCB winding planar magnetic elements is conducive to improve the power density of LLC converter. For the two middle legs planar core, this paper analyses the relationship between the flux density distribution and the feature of the core for the two common design models through the magnetic circuit model, and proposes the design principle which ensures the uniform flux distribution by setting an appropriate spacing between the middle legs. The two common design methods, uniform legs spacing model and uniform winding width model, both have the problem that the flux distribution is concentrated between the middle part of the plate, which increases the core loss. By properly designing the distance between middle legs to the sum of twice the middle-side legs spacing and the height of the middle legs, the core will have the uniform flux density and the least core loss. ANSYS Maxwell 3D FEA simulation proves the correctness of the analysis and the effectiveness of the proposed design principle.