A Natural Bidirectional Input-Series-Output-Parallel LLC-DCX Converter with Automatic Power Sharing and Power Limitation Capability for Li-Ion Battery Formation and Grading System

For the high-power Li-Ion battery (LIB) Formation and Grading System using multi-stage conversion structure, bidirectional isolated DC-DC converter with high input voltage and large output current is needed to achieve isolation and relatively fixed voltage conversion gain. This paper proposes a modulation strategy and modifies the topology to achieve naturally bidirectional power transfer, automatic power sharing and power limitation for an Input-series-output-parallel (ISOP) LLC-DCX converter. Firstly, an active Pulse Width Modulation (PWM) strategy is proposed to avoid synchronous rectifier (SR) sensing and to achieve the nature bidirectional power flow for each sub-module. The resonant current always keeps discontinuous, and the conversion gain can keep constant if ignoring the conduction resistance. Secondly, clamping diodes are added in the conventional LLC converter to limit the inrush current during start-up, and to achieve constant power limitation under over load condition. Thirdly, with the proposed modulation, the power sharing of two-module ISOP LLC converter is analyzed and compared with the conventional ISOP LLC converter. Finally, experiments based on an 8kW (output:15V/533A) prototype composed of two-module ISOP LLC converter are conducted to verify the effectiveness of the proposed solution.