Average and small-signal modeling of self-oscillating flyback converter with applied switching delay

This paper presents a consistent methodology to assess the effect of variable-frequency operation on the dynamics of a flyback converter used in low-cost, high-volume, battery-charger applications. The self-oscillation is typically implemented using peak-current control in boundary conduction mode with a deterministic delay before switching on the next cycle. The modeling is based on the modified state-space-averaging method, where the effect of the varying cycle time and peak-current-mode control is included using special cycle-time and on-time constraints derived from the inductor-current waveforms. The method leads to accurate full-order models, where the effect of the circuit parasitics and the switching delay may also be taken into account. In most of the cases, the effect of parasitics and switching delay is, however, minimal, and therefore, simple small-signal models may be used. The applied switching delay affects the dc operating point in terms of duty ratio and switching frequency, and shall be, therefore, carefully considered.