This chapter presents an LLC resonant converter with constant frequency. Since LLC resonant converters possess the advantages of high efficiency and low electromagnetic interference, they are widely used in many applications. However, the EMI filter design becomes more complex because of the variable-frequency operation of LLC resonant converter. Therefore, in this chapter, a circuit configuration of a buck converter cascade an LLC resonant converter, buck-fed LLC resonant converter, is proposed to complete constant-frequency operation and simplify the EMI filter design. The buck converter is applied in the proposed converter as the input source for the LLC resonant converter. In addition, by adjusting the output voltage of the buck converter, the output regulation and constant-frequency operation of the proposed converter can be achieved. Finally, a prototype of the buck-fed LLC resonant converter with input voltage 380-400 V and output 24 V/10 A is implemented. The experimental results demonstrate the feasibility of the proposed converter.