Research On DCM Buck-Buck/Boost PFC Converter With Segmented Fixed Duty-cycle Control

Buck power factor correction（PFC）converter is widely used in the AC/DC conversion field of low power and low output voltage because of its step-down characteristics.However,there is inherent dead zone in the input current of Buck PFC converter,which results in large current harmonics and low power factor（PF）at low input voltage.Meanwhile,owing to the double-line frequency component in the output voltage of PFC converter,the bandwidth of the designed conventional PI compensation network needs to be lower than double-line frequency to prevent distortion of the input current waveform,and this inevitably gives rise to poor dynamic response.In order to solve the input current dead zone problem of Buck PFC converter,this paper adopts Buck-Buck/Boost topology to compensate the dead zone,and analyzes the working principle of DCM Buck-Buck/Boost PFC converter with constant duty-cycle control（CDCC）in detail.A segmented fixed duty-cycle control（SFDCC）method is proposed to reduce the input current harmonics and increase the PF.A fixed duty ratio control（FDCRC）method is further proposed for simplifying the control system.Aiming at the poor dynamic response caused by the narrow bandwidth of conventional PI compensation networks,this paper proposes two types of smooth and fast dynamic response control to improve the dynamic response performance when the input voltage changes and the load changes.The first one is variable carrier frequency type,and the second one is variable modulation wave amplitude type.With variable carrier frequency control,the function of switching frequency on the input voltage and power is analyzed.And,with variable modulation wave amplitude control,the function of v_preal on input voltage and power is analyzed,which is multiplied by the compensation network error signal.The power components are determined by calculating parameters.The main circuits in the control system are designed,and the software flowchart is given.Finally,a 120W prototype is built up to verify the effectiveness of the proposed two control methods.