When the classical buck converter is applied with large step-down ratio, the operating duty cycle is too small, which leads to the decrease of conversion efficiency and even affects the control stability. In this paper, a wide output step-down converter with variable operating mode is proposed. According to the relationship between input and output voltage, the converter toggles between buck and D2buck mode, so that the operating duty cycle can be kept in a reasonable range. Based on one cycle control, a smooth mode switch control scheme is proposed. The key parameters of the converter with 100 V input and 5–50 V output are designed. The feasibility of the converter and control scheme are verified by PSIM simulation platform. The steady-state duty cycle is maintained between 0.22 and 0.54.