This paper describes the application of multidisciplinary design optimization to the power stage design of a power converter. Multidisciplinary design is used to integrate the electrical, loss, and thermal analyses into one system problem. The Sequential Global Approximation method, a non-hierarchic algorithm, is used to optimize the power stage design problem. The code used for the thermal analysis, COSMOS/M, runs externally to the Sequential Global Approximation algorithm. A comparison of the results of the non-hierarchic formulation and the non-decomposed formulation shows a 67 percent decrease in total system iterations and a 12 percent decrease in total finite element analyses required.