Small-signal model of variable-frequency flyback converter

An accurate, straight-forward, small-signal model of the variable-frequency flyback converter is presented. The model, which replaces the switching devices with dependent sources representing average terminal currents, is similar to the discontinuous-conduction-mode, three-terminal switch model presented in the past. However, since the converter always operates at the boundary of discontinuous and continuous-conduction-modes under both steady state and transient conditions, the inductor can be removed from the average large-signal model. In fact, the control-to-output transfer function can be accurately described with its current loop closed at frequencies less than half of the switching frequency. The model is extended to the ringing-choke converter, which is a common type of variable-frequency flyback converter used in many cost-sensitive applications. The small signal model is validated with measurements made on an offline, 5-V/2-A experimental prototype.