An improved non-isolated resonant converter with integrated magnetics is proposed to achieve high step-down voltage conversion ratio for 48 V data center applications. The non-isolated resonant converter is composed of autotransformer cell and LLC resonant cell. The outputs of the two cells are in parallel to achieve higher output current. By adopting a symmetrical half-bridge circuit structure, the risk of DC bias in high-frequency transformers is addressed, and balanced current stress on synchronous rectifier switches is achieved. Magnetic integration of the autotransformer and high frequency transformer is proposed to achieve higher power density and higher efficiency. Topology, operation principle, integrated magnetic design and implementation of the proposed non-isolated resonant converter are analyzed in detail. Experimental prototype with 8:1 conversion ratio is developed for the front-end bus converter of 48 V data-center power systems. The power density of the prototype is 1.734 kW/in3, and the maximum efficiency is 98.2%.