An efficient, meanwhile simple optimization routine is presented for design of high frequency power transformers. Minimizing the product of power loss, core cross section and winding area is chosen as the optimization goal. This object function leads to a closed form solution, which reduces the computation time and the number of iterations. A CAD tool has been developed according to this method, which is addressed in this paper. By this tool the optimized core which fulfills the temperature rise constraint is determined. If the area product of the selected core is bigger than the optimum area product, the user can modify his design further by minimizing the power loss. The validation of the method is verified by a sample design example.