For the high-power Li-ion battery (LIB) formation and grading system, using a multistage conversion structure, an isolated bidirectional dc–dc converter with a high input voltage, and a large output current is needed to achieve isolation and a relatively fixed voltage conversion gain. This article proposes a modulation strategy and modifies the topology to achieve natural bidirectional power transfer, automatic power sharing, and power limitation for an input-series–output-parallel (ISOP) LLC-DC transformer (DCX) converter. First, an active pulsewidth modulation (PWM) strategy is proposed to avoid synchronous rectifier (SR) sensing and to achieve the natural bidirectional power flow for each submodule. The resonant current always keeps discontinuous, and the conversion gain can keep constant if ignoring the conductive resistance. Second, clamping diodes are added in the conventional LLC converter to limit the inrush current during startup, and to achieve a constant power limitation under an overload condition. Third, with the proposed modulation, the power sharing of the two-module ISOP LLC converter is analyzed and compared with the conventional ISOP LLC converter. Finally, experiments based on an 8-kW (output: 15 V/533 A) prototype composed of the two-module ISOP LLC converter are conducted to verify the effectiveness of the proposed solution.