Modelling of high frequency effects of laminated iron-core power inductors and reduction of parasitic capacitance

Abstract

With the increase of operating frequencies of switching devices, the size and weight of power converters are reduced. However, the impact of parasitic parameters becomes significant, which affects the performance of power converters. Compared with a simple equivalent circuit of filtering inductors, a detailed lumped circuit model is more suitable for the study of the behaviour of the inductor at high frequencies. The ac inductance, ac resistance and parasitic capacitance are studied respectively in this paper. The self-inductance and mutual inductance are measured to study the lumped circuit model of inductance. Because the iron core is laminated, the inductance is clearly decreased with the increase of the frequency. Meanwhile, the ac resistance is increased with the frequency because of the proximity effect and skin effect. The impact of the ac resistance and parasitic capacitance on the measured inductance are studied. The overall ac inductance is calculated and compared with the measured results to demonstrate the correctness of the analytical inductance. The ac resistance is obtained by theoretical calculation. The formulas of the turn-to-turn capacitance and turn-to-core capacitance are also derived. The overall parasitic capacitance is obtained by the theory of the electric network. Several inductors with the same inductance value but different configurations are built to investigate the overall parasitic capacitance. It has been found that the theoretical predictions agree well with the Finite Element predicted values and measured values regarding parasitic capacitance. The parasitic capacitance has negative effect on the performance of power converters. Therefore, the design parameters which can reduce the parasitic capacitance are also analysed by the theoretical predictions. The values of the influence factors and corresponding minimum parasitic capacitance are obtained by the parasitic capacitance formulas and verified by the experiment.