Research On Comprehensively Optimal Design Of Planar Transformer

Planar transformer becoming more and more popular in high power density, high frequency SMPS in recent years due to the unique advantages including low profile structures, excellent thermal properties, good repetition and veracity, lower leakage inductance, and reduced wingding AC resistance. Core loss, winding loss, leakage inductance and stray capacitance are the important factors for planar transformer. In a typical power converter, leakage inductance has a direct effect on the switching spike, the resonance of soft-switching circuits and electromagnetic interference(EMI) problems. Loss, especially the winding loss, is the key factor that concerns the efficiency, temperature rise and reliability of the transformer and even the whole power device. Leakage inductance and winding loss are the kernel parameters of transformer in high power converter. This paper is focused on the design and optimization of leakage inductance and winding loss.In this paper, the structure and electrical specifications are firstly introduced. Then leakage inductance and AC resistance are theoretically analyzed and calculated. After that the design method on fundamental parameters of planar transformer is investigated. The design of winding mainly consists of parallel windings and interleaving arrangement of windings. The influence of winding parallel structure and arrangement on leakage inductance and AC resistance is deeply analyzed based on the magneto motive force(MMF) and current density distribution of a winding layer in planar transformer. The design of planar transformer can be verified and guided by electromagnetic finite element analysis(FEA) simulation. According to the basic theory of transformer, the leakage inductance and AC resistance equivalent to primary winding can be measured directly in the model with secondary winding shorted. The design method can be validated by comparing the measurement and simulation result.A detail analysis about the effect of the current distribution in a winding layer on leakage inductance and AC resistance of a planar transformer has been presented. Then a method to optimize the leakage inductance and AC resistance by controlling the current distribution in a winding layer has been proposed and verified by FEA simulation. The optimal design method can be realized by adjusting the width of conductors in a winding layer. Then on the basis of a practical core, a series of current density distribution scheme is given out and compared through FEA simulation to verify the optimal design method. Finally samples based on the simulation models are established and the optimal design method is further validated by comparing the leakage inductance and AC resistance of different samples.