A constant voltage source based on PWM control chip UC3846 is designed, which applies to the 175~265 V AC input voltage, and the rated output voltage is 24 V and output current is 2 A. The single stage PFC-three terminal correction technique is used to enhance the power factor of the system；meanwhile，the active clamping-soft switch technology is used to reduce the switch stress and improve the output efficiency of the power. The simulation and physical test shows that the input power factor of the constant voltage source is 0.992 and the system efficiency reaches 88% under the rated load.