Research On DCM Buck PFC Converter With Adequate Usage Rate Control Of Switching Cycles

Buck power factor correction(PFC)converter is widely used in low power AC-DC conversion applications due to its advantages of low dc output voltage,low voltage stress of switch,little change of efficiency in wide input voltage range and etc.When the converter operates in discontinuous conduction mode(DCM),during the switching cycle,there is a state in which the inductor current is zero,and there is no power transmission between the input and output.Therefore,the peak and RMS current values of power components such as inductor and switch are high,and so do the switching loss and conduction loss.Aiming at the problem,a method called adequate usage rate control of switching cycles(AURCSC)is proposed in this paper to improve the energy transmission.The basic working principle of DCM Buck PFC converter is introduced in this paper.Under the fixed duty ratio control(FDRC),the expressions of the inductor current,duty ratio and input PF are derived.The concept of switching cycle’s utilization rate is proposed,and its curves during a half line cycle at different input voltages are plotted.It is found that the utilization rate always achieves the maximum at π/2 and the minimum at θ and π-θ.The essential reason is that the duty ratio is fixed in a line cycle,thus the angle where the inductor current operates in critical conduction mode(CRM)or quasi-CRM is always π/2,leading to the imbalance of utilization in a line cycle.Based on this,the AURCSC is put forward.Compared with FDRC,AURCSC achieves the following effects:(1)The critical inductance is improved,and the utilization rate in [θ,π-θ] is significantly improved except for that near π/2.(2)The RMS value of the inductor current is reduced over the entire input voltage range and the peak value of the inductor current is greatly reduced at π/2,resulting in the reduction of the power components’ losses and the decrease of the current stress.(3)The PF is increased at low input voltages and decreased at high input voltages.(4)The output voltage ripple is reduced,especially at high input voltages.According to the theoretical calculation,the quota and selection of the power components are determined,and the main power components’ losses of the converter with the two control methods are analyzed.The control circuit is designed and the prototype is fabricated.The experimental results verify the effectiveness of the proposed method.