A Valley-Locking Control Scheme for an Audible Noise-Free Valley-Skip-Mode Flyback Converter

In a traditional quasi-resonant (QR) control flyback converter, the switching frequency of the system will increase with the decrease of the load, so the switching loss under the light load becomes high, which greatly affects the average efficiency of the system in the whole load range. An improved QR control method called valley skip (VS) control can reduce the switching loss by decreasing the frequency of the system under the light load. However, the switching frequency of the conventional VS controller may change suddenly at some load points, and this sudden frequency hopping causes serious audible noise in the transformer. In this article, we propose a valley-locking control scheme to overcome the shortcomings of the traditional VS control technology. It can eliminate the audio noise caused by the frequency hopping in VS control. In order to verify the feasibility of the proposed method, a controller IC is fabricated with the proposed method and tested by a prototype experimental board of 24 W/12 V.