Research on Low-Voltage High-Current Synchronous Rectification Control for LLC Converter Considering Secondary-Side Parasitic Inductors

Synchronous rectifier (SR) control is used to reduce the losses in LLC resonant converters, in which sensorless detection is the mainstream SR method. However, due to the existence of secondary side parasitic inductor in low voltage and high current operations, SR control based on the traditional ideal modeling leads to delayed turn-ON and premature turn-OFF of the switch. To solve this problem, this paper establishes a full load time domain model considering the secondary side parasitic inductor. Based on this, the effect of parasitic inductor on the operating state of the circuit is analyzed, and bilateral modulation synchronous rectification control strategy is proposed. This strategy effectively avoids power dissipation issues caused by nonideal turn-ON/OFF, which are induced by resonant frequency shifts across the full load range. Finally, a 1.5 kW prototype LLC resonant converter with 10-15 V output validates the control strategy's effectiveness. The results show that, compared with the model without considering parasitic inductors, the proposed synchronous rectification model improves turn-ON and turn-OFF accuracy up to 13.3% and 34.1%. Additionally, the efficiency of the proposed control strategy is improved by 0.27% and 0.86% at light load and full load, the corresponding total losses are reduced by 1.83% and 9.77%.