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.