Harmonic winding losses in the transformer of a forward pulse width modulated DC-DC converter for continuous conduction mode

This study presents general winding power loss expressions for a high-frequency (HF) three-winding forward transformer operated in continuous conduction mode (CCM), where the source of the transformer excitation is considered to be non-sinusoidal with many significant harmonics. The efficient operation of an isolated converter is dependent upon the performance of the transformer used. The expressions presented in this study take into account the eddy-current losses caused by current harmonics at HFs. The expressions are derived based on Fourier series of the winding current waveforms, and using Dowell´s equation, which considers both the skin and proximity effects caused by eddy currents. Harmonic winding loss factors for the three windings are introduced. In addition to the power loss expressions, a step-by-step procedure to design a three-winding transformer used in a single-switch forward converter operated in CCM is presented using the area product (A_p) method.