With the development of big data, cloud computing, the energy consumption of data center as a key infrastructure of information technology is rapidly increasing. This is leading to a change in data center structure from 12 V to 48 V. The original single-stage converter can no longer meet the demand for high step-down ratio at low voltage and high current, so cascaded converters are required. The pre-stage operates as a pre-step-down stage in an open-loop condition to achieve fixed-ratio power conversion, and the post-stage operates as a regulator stage to achieve precise high-swing rate control. The LLC resonant converter is a favorable choice as pre-stage due to its advantages of Zero Voltage Switching (ZVS), high frequency, power density and electrical isolation. This paper proposes an LLC-TLVR cascade structure, focusing on the high power density design of the LLC pre-stage and briefly discussing the reasons why it is difficult to precisely adjust to the resonant point at high frequencies. Finally, a 600 W prototype is built for verification.