A Digitally Controlled Soft Valley Change Technique for a Flyback Converter

In a flyback converter using the conventional valley-switching method, the switching frequency under the light load condition is higher than that under the heavy load condition, which increases the switching loss. A valley skip (VS) technique can reduce the switching frequency under the light load condition. Although it helps to reduce the switching loss, it makes the uncomfortable humming sound caused by a sudden change in switching frequency during valley change. To minimize the audible noise, a new digitally controlled soft valley change (SVC) technique is proposed in this paper. It changes the turn-on instant among valleys gradationally during several switching cycles under any transient situation. The SVC also helps to spread switching frequency so that electromagnetic interference can be reduced. The proposed technique is suitable for the applications that operate under the dynamic load condition such as an audio amplifier. The proposed technique is implemented in an integrated circuit and verified by a prototype experimental test board of 60 W/12 V.