Light load efficiency enhancement of a LLC resonant converter

Zero Voltage Switching (ZVS) technique minimizes the turn-on loss of the MOSFETs in LLC resonant converter structure. However, as switching frequency increases, the converter efficiency will suffer due to the increased turn-off loss and gate charge loss, and these effects are especially problematic at light load conditions, where the switching frequency has to be further increased in order to regulate the output. A new hysteretic control scheme is proposed in this paper. By fixing the switching frequency equal to the resonant frequency all the time and skipping pulses proportional to the load condition, the switching losses are reduced and the light load efficiency is improved. The experimental results prove the functionality of the proposed hysteretic control scheme.