Light load efficiency improvement for high frequency LLC converters with Simplified Optimal Trajectory Control (SOTC)

The light load efficiency has drawn more and more attention for the LLC resonant converters. And the high frequency LLC converters are emerging in recent years. The conventional burst mode control cannot fix to the efficiency-optimal state during the burst-on time. Optimal Trajectory Control (OTC) for burst mode can solve this problem by using fixed 3-pulse pattern; however, the burst operation range of OTC is limited when it is applied to the high frequency LLC converters due to the fixed 3-pulse pattern and digital delay. To extend the burst operation range for the high frequency LLC converters, Simplified Optimal Trajectory Control (SOTC) for burst mode with adaptive burst on-power and adaptive multi-step are proposed in this paper. The burst mode operation range can be extended by increasing either the burst on-power or the burst on-time. The experimental results are demonstrated on a 500kHz 1kW 400V/12V LLC converter with 92.5% efficiency at 10% load and 91.8% efficiency at 2.4% load.