Research On Segmented Constant On-time Control For CRM Buck-Buck/Boost PFC Converter

Buck power factor correction(PFC)converter has the advantages of low output voltage and low voltage stress of power devices,which is widely used in low-power AC-DC converter.However,it has an inherent defect that when the input voltage is lower than the output voltage,the converter does not work and the input current has a dead zone,resulting in low power factor(PF).In order to eliminate the dead time,Buck-Buck/Boost PFC converter is proposed.When the input voltage is lower than the output voltage,Buck/Boost converter works to make the input current continuous.Buck-Buck/Boost PFC converter works in critical conduction mode(CRM)with the advantages of zero-current turning on for the switch and no reverse recovery in diode.However,with traditional constant on-time(COT)control,the input current has high total harmonic distortion(THD)and low power factor.Therefore,segmented constant on-time(SCOT)control strategy is proposed in this paper to solve this problem.By varying the on-time of Buck mode and Buck/Boost mode,the PF can be raised to close to 1 over a wide input voltage range.In addition,compared with traditional control,this control can also reduce the peak value and root mean square value of the inductor current and improve the efficiency.Although the power factor can be greatly improved with SCOT,the range of switching frequency variation is large,which will lead to the increase of switching loss,inductance core loss and limitation of efficiency improvement.In order to solve this problem,variable inductor technology is introduced in this paper.By detecting the input voltage,the control circuit is designed to achieve the critical inductor value at each voltage over the wide input voltage range.With the introduction of variable inductance control,the converter can achieve larger inductor value when the input voltage is high,the switching frequency range is reduced generally,and the efficiency is also increased.Finally,the components are selected and the prototype is built for experimental verification.The experimental results and data validate the theoretical analysis.