Small-signal analysis of DCM flyback converter in frequency-foldback mode of operation

The predicted closed-loop bandwidth of the DCM flyback converter in frequency-foldback mode (FFM) of operation obtained by the conventional averaged small-signal model that models the voltage-controlled oscillator (VCO) with a constant gain does not change with the load. However, this prediction is not in agreement with the closed-loop measurements. In this paper, the discrepancy between the conventional averaged small-signal model and measurements is systematically investigated. It is found that the source of this discrepancy is the nonlinearity of the VCO that generates sideband frequency components and, therefore, cannot be modeled as a constant-gain transfer function. By including the effect of the sideband components in the closed loop, a multi-frequency averaged small-signal model of the DCM flyback converter in the FFM which accurately predicts the loop bandwidth behavior with the load is proposed.