A high step-down ratio hybrid resonant converter is investigated for 48V data center power systems applications. The hybrid resonant converter is derived by integrating a switch-capacitor cell, an autotransformer, and an LLC resonant cell. High step-down ratio is achieved easily while soft-switching of all switches can be guaranteed. The output of these circuit cells are in parallel, which makes the converter suitable for lower output voltage and higher output current applications. A high-density integration method is further studied. Higher power density and higher efficiency is achieved through magnetic flux cancellation and decoupling integration of the LLC transformer and autotransformer. A one-eighth brick power module with 8:1 conversion ratio is developed for the front-end bus converter of 48V data-center power systems. The power density reaches 1.8 kW/in³ and the maximum efficiency is 98.2%.