Bridgeless Isolated Buck-Boost PFC Converter

Power factor correction(PFC)technology has been widely researched and applied,because it can improve the power factor of the switching power supply and reduce the harmonic pollution caused to the power grid.Compared with the traditional two-stage PFC converter,the isolated single-stage PFC converter can realize power factor correction,low voltage output and electrical isolation in the single-stage converter.Therefore,it has the advantages of fewer components,simple structure,and high efficiency.However,the flyback PFC converter,as a typical isolated single-stage PFC converter,is often restricted by power levels in practical applications.Therefore,this paper proposes a new type of bridgeless isolated Buck-Boost PFC converter,which can be used for higher power levels.This paper also conducts a detailed analysis and research on its different working modes,and gives the corresponding design methods.First,this article analyzes the bidirectional switch bridgeless PFC converter,cascaded Buck-Boost converter and push-pull converter.On this basis,a bridgeless isolated Buck-Boost PFC converter is proposed.The basic working principles of the converter in the Continuous Conduction Mode(CCM)Buck-Boost mode,the Discontinuous Conduction Mode(DCM)Buck-Boost mode and the CCM Buck mode of the converter are analyzed.Using the state-space average modeling method,the DC steady-state gain expression and the control-output transfer function of the power stage are obtained for the analysis and design of subsequent chapters.Secondly,the control strategy of the converter in DCM Buck-Boost mode is analyzed in detail.The steady-state characteristic relationship is deduced,and the key operating waveforms are given.An active clamping method is studied,which can better solve the problem of magnetizing inductance discharge in DCM mode.In order to guide the design of the experimental prototype,the design and selection methods of the main components of the main circuit are analyzed.A precise digital control loop compensator for the voltage loop of the converter is designed.The correctness of the theoretical analysis is verified by simulation analysis and experiment.Third,in order to make the converter suitable for higher power levels,a detailed analysis and research on the converter working in CCM mode is carried out.Starting from the principle of single-cycle control,a OCC method suitable for CCM Buck-Boost working mode is derived,and an analog control block diagram is built based on the OCC equation.Combined with the control block diagram,the working process of the converter is analyzed.For the purpose of guiding the loop design,this paper draws on the analytical ideas of peak current control,and merges the OCC current loop with the original power stage to become the so-called equivalent power stage,and its low-frequency small-signal circuit model is also established.The detailed design process of the main circuit and digital control system of the converter is given.Finally,in order to further improve the efficiency of the converter,make the converter work in Buck mode and Buck-Boost mode in sections,which is called CCM Buck+Buck-Boost working mode.The second OCC method is derived for the working mode,which can eliminate the current distortion during the mode switching of the converter.Analysis and design similar to CCM Buck-Boost mode are given.The correctness of the theoretical analysis is verified by simulation and experiment.