Structural Design Method Of MHz Planar Transformer Based On Electromagnetic Analysis

The appearance of planar transformers conforms to the development trend of high frequency,planarization,integration and array of magnetic components.Reducing the volume of the planar transformer can be achieved by increasing the switching frequency of the switching power supply.However,when the switching frequency reaches the MHz level,the parasitic parameters and losses generated by the planar transformer under high frequency effects can seriously affect the performance of the power supply.The diversification of the planar transformer structure makes it difficult to control parasitic parameters and losses during the design process.Therefore,it is of great significance to study the relationship between the structure of the MHz-level planar transformer and the parasitic parameters and losses.Aiming at the parasitic parameters of planar transformers,based on the equivalent model of gyrator-capacitor circuit,this paper deduces the formula for calculating the leakage inductance of laminated non-cross windings and cross-winding plane transformers.The case of inter-layer capacitor energy storage between plane transformers is analyzed.According to the principle of equal energy,the equivalent interlayer capacitance,the interturn capacitance and the inter-group capacitance of the laminated planar transformer are given.For the planar transformer loss,the eddy current effect in the winding is analyzed,and the calculation formula of the high frequency loss of the winding is obtained.The influence of the thickness and frequency of the winding on the skin effect loss and the adjacent effect loss is analyzed.The influence of a layer of copper foil divided into multiple layers of copper foil on the loss is analyzed.It is concluded that the thick copper foil is divided into multiple layers of thin copper foil in parallel and can not reduce the winding loss.The influence of the cross-arrangement of the primary and secondary windings of the planar transformer on the loss is analyzed,and the conclusion that the cross-transposition technology can effectively reduce the winding loss is obtained.Aiming at the problem of equalizing the current of the secondary parallel winding of a single secondary winding planar transformer,a multi-secondary winding planar transformer and a planar transformer with a center tap,a winding arrangement method for equalizing the current in the secondary parallel windings is derived.The Maxwell 3D electromagnetic field analysis software is used to establish planar transformers with different winding structures.The leakage inductance and distributed capacitance of the planar transformer are simulated respectively,and the accuracy of the leakage inductance and winding distribution capacitance calculation method is verified.The winding eddy current loss of the planar transformer is simulated,and the correctness of the loss calculation method is verified.The simulation results show that the minimum eddy current loss is obtained when the total thickness of the secondary parallel winding of the planar transformer is π skin depth.It is verified that the conclusion that the thick copper foil is divided into thin copper foil can not reduce the winding eddy current loss when the layout of the planar transformer winding is kept unchanged;It is verified that the cross arrangement of the primary and secondary windings can reduce the eddy current loss of the winding.The current density of single secondary winding planar transformer,multi-secondary winding planar transformer and secondary winding with central tapping plane transformer is simulated and verified.The effectiveness of the method of equalizing the current parallel arrangement of the secondary parallel winding of planar transformer is verified.