A Double Pulse Switching Pattern for LLC Resonant Converter With Noise Suppression Capability Under Extreme Light Load Operation

LLC resonant converters are widely used in wide power range applications. However, their switching frequency will be much higher than normal load when they are applied under extreme light load conditions. Therefore, burst mode control is introduced to reduce the high switching loss and increase the light load efficiency. By applying the trajectory theory, a three-pulse switching pattern is implemented, which reaches the state-of-art highest efficiency of burst mode strategies. Nevertheless, for extreme light load conditions (lower than 10%), the burst frequency introduced by this switching pattern will be much lower than the resonant frequency, which can be lower than 20 kHz and leads to the severe audible noise. To overcome this issue, a double pulse switching pattern is proposed in this article, which ensures a higher burst frequency, and thus, the audible noise can be eliminated in a much wider load range. A detailed analysis based on the trajectory theory is introduced in this article, which illustrates the high efficiency and the effectiveness of the audible noise elimination capability. Additionally, a 180 W LLC prototype is established to verify the proposed strategy. Compared with the three-pulse switching pattern, the efficiency of the proposed strategy is slightly lower, while the audible noise is further eliminated from the 13.5% load to 6.7% load. The output voltage ripple is nearly halved because only half of the power is transmitted during each burst cycle. The converter efficiency is increased by around 2% when compared with the traditional burst control strategy. Furthermore, the audible noise can be eliminated under 6.7% load by combining the constant burst frequency control, while the efficiency can be improved compared with the three-pulse switching pattern.