An Average Input Current Sensing Method of LLC Resonant Converters for Automatic Burst Mode Control

Burst mode control techniques have been studied for LLC resonant dc-dc converters to improve power conversion efficiencies at light load conditions. However, there are not many studies about how to detect the load conditions at burst mode not even at normal mode, at the primary side of LLC resonant dc-dc converters. In this paper, we propose and implement a lossless load detection technique for normal and burst modes by a novel average input current sensing method at the primary side of LLC resonant converters. Detailed theoretical analysis at normal and burst modes is presented and a load detection circuit is proposed with the consideration of integrated circuits. To verify the analysis, an LLC resonant offline converter with 400 V input and 16 V/10 A output is simulated using PSIM and a prototype board is built. The experimental results show that the proposed load detection technique is valid and practical. As a result, an automatic burst mode control for LLC resonant converters at light load conditions is realized with the proposed average input current sensing method.