Design of Quasi-resonant flyback converter control IC with DCM and CCM operation

Abstract

A Quasi-resonant (QR) flyback converter control IC is design and implemented. Flyback converter is applied in small to medium power rating applications mostly, due to low cost and simple circuit topology. The conventional fix frequency operated flyback converter is the most adapted power converter, but the switching losses cause lower conversion efficiency. The flyback converter can be operated in BCM and DCM by quasi-resonant control. The quasi-resonant control utilizes the resonant action between magnetize inductor of transformer and parasitic capacitor of power MOSFET to achieve valley voltage turn-on and reduce the turn on losses increasing the conversion efficiency. The switching frequency increases with reducing load, and the switching losses will cause poor efficiency. Flyback converter operated in BCM and DCM in heavy load condition will lead to higher conduction losses. To improve the conversion efficiency at light load and heavy load condition, the proposed control circuit is integrated with frequency clamp and maximum off-time limit functions.