Analysis and Design of a Practical Discontinuous-Conduction-Mode

Impending international standards on harmonic current levels drawn by single-phase mains-operated equipment have created a need for low-cost off-line power-factor-corrected switched-mode power supply topologies in the power range up to a few hundred watts. The boost integrated/flyback rectifier/energy storage/dc-dc converter (BIFRED) is one such topology which shows promise in this regard. In particular, the discontinuous- conduction-mode (DCM) BIFRED avoids the light-load high- voltage stress problem associated with the continuous-conduction- mode design, while still achieving the combined advantages of a low-cost single-stage topology with high displacement factor and low total harmonic distortion. In this paper, a practical DCM BIFRED converter with integrated low-loss snubber is investigated from both power and small-signal control perspec- tives. Design equations are given to ensure DCM operation under closed-loop output voltage control, in which switch duty cycle is varying. Experimental results on a prototype converter are also presented.