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A planar coil plays the role of inductance in the resonant circuit of the resonant converter in induction cookers. Generally, this coil is subjected to very high magnitude currents. Furthermore this coil should be constructed as compact as possible in order to increase its efficiency. Such a structure causes big losses in the coil. In induction cookers, to reduce these losses Litz wires are used in the coil construction. In this paper, two main causes of the big losses i.e. skin effect and proximity effect at high frequency current excitations in the output coil are analyzed, the both effects are simulated and described in sample cases. The analysis is generalized to the real case i.e. planar coil. For this purpose a planar coil and its load (a metallic pan) of an induction cooker is simulated by FEM (finite element method) based software. The current densities are studied in two cases: planar coil with conventional wire and Litz wire. The presented analysis is used for the design and selection of Litz wire for the planar coil of an induction cooker.