Winding losses caused by harmonics in high-frequency flyback transformers for pulse-width modulated dc-dc converters in discontinuous conduction mode

High-frequency transformers used in pulse-width modulated (PWM) converters conduct periodic non-sinusoidal currents, which give rise to additional winding losses because of harmonics. This study presents expressions for winding power losses in a two-winding transformer subject to non-sinusoidal excitation operated in discontinuous conduction mode (DCM). Dowell´s equation is used to determine the winding resistances because of eddy currents as a function of frequency. Both skin and proximity effects are taken into account. Fourier series of the primary and secondary triangular current waveforms in isolated dc´dc power converters and the primary and secondary winding resistances are used to determine the primary and secondary winding power losses at various harmonics, respectively. The harmonic winding loss factors F_Rph and F_Rsh are introduced. The theory is illustrated by the case study of the two-winding flyback converter operating in DCM. Plots of the winding power losses are given as functions of the output power and the dc input voltage.