Calculation of ohmic losses in litz wires by coupling analytical and numerical methods

Up to now, litz wires with many (several hundreds) strands are simplified in complex systems such as transformers or coils to make simulations feasible. However, frequency dependent losses of litz wires and solid conductors differ strongly. In this paper, a coupled process of numerical simulation and analytical methods is described which allows efficient simulation of full 3D structures including resolving litz wire sub-structure. To achieve this, 3D simulation with a simplified conductor calculates the magnetic field in the entire system. Based on these data, a new analytical method determines the losses in the litz wire along the conductor for the large number of 2D cross-sections. The error of the analytical method is less than 5% at frequencies up to 1MHz and external magnetic fields up to 2500kA/m. Because of the significant reduction of calculation time, windings and ferrite structures can be efficiently optimized.