ABSTRACT: In this technical report, a minimum loss design of a 100 kHz inductor with litz wire and foil conductor is presented. The finite element analysis has been performed to investigate the effects of the air-gap length and the number of turns on the field distribution, operating flux density, leakage flux and core losses.
A considerable effort has been directed towards high-frequency inductor and transformer design. A minimum loss design of a 100 kHz inductor with foil windings and Ferrite core has been performed by A. Nysveen and M. Hernes. In technical report, a minimum loss design of a 100 kHz inductor with both litz wire and foil conductor and Permalloy80 (Cut-C core) is presented.
In high-frequency inductors, winding losses are normally kept under control by using foil conductors or multi-strand litz wire. When such conductors are properly designed, additional losses defined as skin effect and proximity effect (eddy currents) can be kept at very low values. Both foil conductors and litz wire have their advantages and draw backs. Litz wire is fairly easy to wind up and is adaptable to various window geometries; on the other hand, litz wire is expensive and gives poor fill factor (area of the bare conductor to the area of the wire). Foil conductors are fairly easy to produce in a variety of heights and weights, and if optimum foil height for a given number of turns is used, the winding becomes compact and has a potential for low losses. However, the optimal foil winding tends to become very wide where high current, high frequency and a large number of turns winding are presented at the same time. The additional losses are induced by the fringe flux near air-gaps. Fringe field causes eddy currents in the foil winding when trying to penetrate the winding. A quasi-distributed air-gap, built by a high number of small discrete air-gaps, is used to avoid this problem.
The objectives of this study are to develop an iterative design procedure and to minimize the core and winding losses of an inductor using the finite element method.
The two fundamental issues in the design of any high-power high-frequency inductor are minimum losses and low leakage flux. Core losses and winding losses are strongly related to the frequency. The core loss, for a given frequency and flux density, is material dependent. Therefore, as a first step in the design process, an investigation of various high frequency core materials is essential. The copper loss in the inductor is extremely sensitive to the leakage flux distribution in the window area, which in turn is dependent on the core and winding geometry.
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