A boost half-bridge LC resonant dc-dc converter is proposed in this paper. The proposed converter adds two power switches to the conventional half-bridge (HB) LLC converter, which forms the boost-bridge-arm. This converter can work in high-voltage-gain (HG) mode and low-voltage-gain (LG) mode according to whether the boost-bridge-arm works. In the HG mode, the converter can work with boost energy storage through the boost-bridge-arm, which makes the proposed converter acquire higher voltage gain than the conventional HB LLC converter. In the LG mode, the operating principle is close to the conventional HB LLC converter, but the proposed converter has a smaller circulating current. Finally, a 400 V input/180–300 V output experimental prototype was built to prove the theoretical analysis.