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In this paper, a new and practical measurement method is proposed to characterize the planar transformer copper loss operating in a high-frequency switching mode power supply (SMPS). The scheme is easy to set up, and it provides an equivalent winding alternating current resistance, which is the result of all the field effects on the transformer windings to achieve more accurate copper loss characterization. A detailed error analysis for the proposed copper loss measurement method is conducted. The analysis results can provide useful guidelines on the SMPS transformer copper loss measurement scheme design. Measurement results on the copper loss of a planar transformer in a high-frequency dc/dc converter are presented. In order to verify the measurement results, a time-domain finite-element analysis transient solver is adopted to analyze the transformer copper loss. Good matching between the simulation and measurement results is achieved.