LLC resonant converter is widely used in switched mode power supplies. However, the present research on conditions of realizing soft switching and the design of dead time has obvious shortcomings. Therefore, the soft switching of LLC resonant converter is studied, and the soft switching boundary theory is proposed based on the precise equivalent model during the dead-time interval. According to the balance between the supplying charge during the dead-time and the required charge, together with the length relationship between the dead-time and the zero-crossing time of resonant current, the operation region of the converter can be divided into one soft switching operation region and three hard switching operation region, and the boundary curve of soft switching is concluded. The minimum dead-time for realizing soft switching even under the worst case is analyzed, and the mathematical equation of the minimum dead-time is derived. Finally, a 300 W prototype is build, and the accuracy of the theory is verified by the accordance between the theoretical design and the experimental results.