Research On Losses Design And Winding Techniques For Planar Transformer

High frequency, high power density and low profile are the trends of theSwitching Mode Power Supply. Planar transformer is used more and more in thefield of high-frequency and high additional value due to its low profile, highpower density, good thermal characteristic and process consistence. How todesign planar transformer with high performance and low cost is an importantissue. Thus, the losses model, winding technique, design method and softwaredevelopment for planar transformer are mainly studied in the paper.The losses source, losses influencing factors and losses model of planartransformer were analyzed in the paper firstly. Then a new optimal design methodbased on temperature rising was proposed. The optimization of core material,core size and winding design were manifested in this new design method.The influence of winding structure on winding AC losses, leakage inductanceand parasitic capacitance were deeply analyzed in the paper. And techniques werestudied to reduce winding AC losses, leakage inductance and parasiticcapacitance.By the deep study on the MMF balance characteristics of harmonic excitationcurrents, the new winding loss model was built for central-tapped transformerwith four windings and its windings operate time-sharing. Base on the analysis,the total winding losses can be considered as the sum of the winding losses ofseveral non-tapped transformers with respect to the related harmonic excitations.Based on this model, AC winding resistance measuring schemes for eachharmonic were developed. The new model was verified by a400W planartransformer optimal design.By the deep analysis on winding eddy current loss mechanism of thetransformer in forward-flyback converter, a new idea is proposed that themagnetic field in winding window of the transformer can be considered as thecombination of the two components, that is, transformer field component andinductor field component. The orthogonal characteristics of the two magnetic field components in CCM (continuous conduction mode) are further revealed.Base on this knowledge, a new analytical model was developed for winding lossof the transformer in forward-flyback converter.Finally, a planar transformer design tool with friendly human-machineinterface was developed base on the loss models and the design method. Thedesign tool not only could provide various core specifications and its losscharacteristic curves base on the core database, but also suit for PCB windingsdesign of non-storage-energy type planar transformer with arbitrary number ofwinding and layer, and arbitrary connections. It is a good design tool for planartransformer with enough engineering precision, high performance and low cost.