Buckboost-Flyback PFC Converter Based On Inductor Multiplexing

In order to improve the power utilization efficiency of the electrical equipment and reduce the damage caused to the power grid by the current harmonics generated,power factor correction(PFC)power converter is required between the AC power supply and the electrical equipment.The dynamic performance of PFC converter is poor because it needs a low crossover frequency.And the input power is two times the line voltage frequency pulsation,so for the load of constant power,the output will have a large low-frequency ripple,and it cannot make a fast dynamic adjustment.In order to obtain a more stable and high-performance DC power supply,it is necessary to cascade one stage of DC-DC converter behind the former PFC converter.Traditional two-level cascaded AC-DC PFC converters and special integrated switch single-stage PFC converters require two magnetic components,which limit the size and weight of the converters.For two-stage cascaded AC-DC PFC converters,the pre-stage Buckboost PFC converter in discontinuous mode can achieve a high power factor through a single voltage loop control,and the output voltage can be either higher or lower than the input voltage.Flyback converter has been widely used because of its simple structure,can realize the boost-buck conversion,input and output electrical isolation and other characteristics.Due to the above characteristics of Buckboost PFC converter and Flyback converter,Buckboost-Flyback cascade converter has high power factor,low voltage stress and other characteristics.However,conventional cascaded Buckboost-Flyback converters require two control systems,two control switches,and two inductors.The switch multiplexing of a conventional cascaded Buckboost-Flyback converter simplifies control and reduces switch,but also requires two inductors.Therefore,this paper proposes a Buckboost-Flyback PFC converter based on inductance multiplexing in order to reduce the magnetic components of the two-stage cascade AC-DC PFC converter.This converter can realize the time division multiplexing of the inductor elements of the pre-stage Buckboost converter and the post-stage Flyback converter,reducing one inductor element.According to the continuity of the inductance current in the Flyback transformer,the converter can be classified into Discontinuous Conduction Mode(DCM)and Critical Conduction Mode(CRM).This paper studies the two operation modes of the converter.Firstly,the principle of inductance multiplexed Buckboost-Flyback converter running in DCM is analyzed,and its operation mode and control methods are studied in detail.In order to make the converter run in DCM,optimize the circuit parameter design and improve the system efficiency,the parameter limit condition of its stable operation in DCM is analyzed,the current peak value and current RMS value on each branch of the converter are derived,and the loss analysis is carried out.In order to verify the feasibility of the converter and the correctness of the theoretical derivation,the simulation and experimental verification are carried out.The results show that the inductively reusable Buckboost-Flyback converter and control scheme proposed in this paper are feasible.DCM operation mode control circuit only needs zero crossing detection of transformer primary side current,and constant frequency control is adopted,so it is simpler than CRM variable frequency control.However,compared with CRM,DCM operation mode has a larger root mean square value of current in its circuit,so its efficiency is relatively low.In order to improve the system efficiency and improve the utilization rate of the transformer magnetic core,the improved CRM operation mode of DCM is adopted,which can realize the higher efficiency operation under the same output power condition.In this paper,the realization principle and control method of inductance multiplexed Buckboost-Flyback converter run by CRM are analyzed,the expression of switching frequency is derived,and the function of switching frequency is studied in detail,and the appropriate parameters are selected to control the switching frequency in a reasonable range.The current peaks and RMS values of each branch are derived in detail,and the converter losses are analyzed.Simulation verifies that the effective value and peak value of each branch current in CRM operation are reduced compared with that in DCM operation.Simulation results show that the root mean square value and peak value of each branch current in CRM operation are reduced compared with that in DCM operation,and the operating frequency of the converter is within the range deduced theoretically,which verifies the correctness of theoretical analysis.The results show that the proposed CRM inductance multiplexing converter and its control method are feasible and effective.