Traditional single-stage LLC converters may have problems such as wide switching frequency regulation range and significant impact from load fluctuations. In response to the shortcomings of LLC converters, this paper proposes a topology structure of LLC converters cascaded with Buck circuits, and enhances the converter’s ability to resist load disturbances through fixed frequency integral sliding mode control (PWM-ISMC). Firstly, the working principle of the LLC converter was analyzed and the differential equation of the Buck circuit was established. Based on the relationship between the kQ product and the effective value of the current, the resonant parameters for optimal efficiency were designed. The two-stage topology structure was used to improve the regulation ability of a wide range of inputs. At the same time, a fixed frequency integral sliding mode controller for the secondary Buck circuit was designed, and the control rate u was derived. Finally, through simulation and experimental verification, it has been proven that the proposed converter has the ability to adjust over a wide range of inputs and good robustness.