Abstract: A bidirectional resonant converter with a wide-input and wide-output voltage range is proposed for the energy storage system required by the energy internet, the electric vehicles, etc. Based on the bidirectional LLC resonant converter topology, the converter adds the extra auxiliary switches on the secondary side. Adopting the fixed frequency operation, the gain of the converter is adjusted by the cooperation of switching between the full bridge and half bridge operation modes at the primary side and the PWM (pulse width modulation) control at the secondary side, which allows the converter to work in the range of a four-times voltage gain. The power devices work under the soft switching, which is benefit to improve the efficiency of the proposed converter. The constant frequency operation of the converter is conducive to the design of the transformer. This paper analyzes the bidirectional working principle and modulation strategy of the converter. By establishing a 3kW prototype, the experiment of bidirectional power flow between 400V DC bus and 105V-420V battery is realized. The experimental results verify that the converter achieves the bidirectional power flow with wide voltage gain range and high efficiency.