In order to achieve higher conversion efficiency for buck DC-DC converter under light and heavy load conditions, a segmented structure and an NMOS transistor with smaller conduction resistance were used as input stage, and a PWM/PFM dual modulation mode was used. To solve the switching problem of PWM/PFM modulation signals, the zero current detection method was adopted for switching. By using the difference between (DCM) and (CCM) inductance voltages when the low-side NMOS was turned on to detect the period when the inductance voltage of the low-side NMOS was greater than zero when it was turned on. When the period of inductance voltage greater than zero was greater than 2, it was in DCM mode, and the circuit automatically used PFM modulation and turned off a part of the power MOSFET to reduce the switching frequency and parasitic capacitance of the power MOSFET to optimize the light-load efficiency. Otherwise, it was in CCM mode, and the circuit used PWM modulation. The simulation results show that the DC-DC converter can switch smoothly between the two modulation modes in the range of 10-1 000 mA of load current, and the peak efficiency can be increased to more than 96% at 800 mA.