DC-DC Flyback Converters in the Critical Conduction Mode: a Re-examination

The critical conduction mode for DC-DC flyback SMPS, in which the converter is forced to operate at the boundary between continuous and discontinuous conduction modes, represents an interesting alternative to the classical constant-frequency PWM technique. In fact, such an operating mode allows for a soft turn off of the freewheeling diode, zero voltage commutations of the switch and a reduction of the generated EMI. In this paper, this operating mode is re-examined with the aim of accurately predict switching frequency variation and component stresses in those applications in which the delay inserted between the turn off of the freewheeling diode and the turn on of the switch, used to achieve zero voltage commutations, cannot be neglected. The analysis presented allows for a correct prediction of the converter behavior in all operating conditions as well as for a proper design of the feedback loop through a suitable small-signal characterization. The theoretical forecasts are verified by means of a flyback prototype built using a new smart power IC developed by ST Microelectronics in VIPower(R) M3 technology.