The transformer can be illuminated by the T-type equivalent circuit model. The primary and secondary leakage impedance characterizes the copper losses and the exciting impedance reflects iron-core losses. The iron-core losses and the copper losses can be obtained by open-circuit and short-circuit test. However, the copper losses of the primary winding and the secondary winding cannot be separated, which cannot satisfy the requirement of the cooling design of the high power transformer. The auxiliary winding method to measure the winding ac resistance of planar transformer in high-frequency switching converters is used in Reference [1], but the detailed arrangement of the auxiliary winding is not analyzed.