Research On DCM-CRM Single Stage PFC Converter

In order to reduce the harmonic pollution of the power electronics converter to the power grid,the converter must have power factor correction(PFC)function.The single-stage PFC converter is composed by a cascade of input stage PFC converter and an output stage DC-DC converter.The PFC and constant output voltage/current functions can be realized simultaneously using only one switch and one control circuit.Compared with the two-stage PFC converter,it reduces the size and cost of the converter,and reduces the double power frequency output voltage/current ripple of the conventional PFC converter.When using single-switch single-stage PFC converters such as Buck-Flyback and Boost-Flyback,it has the advantages of electrical isolation,low cost,and easy control.In most single-stage PFC converters of existing literature research,the input stage PFC converter operates in a discontinuous conduction mode(DCM),and the output stage DC-DC converter also operates in DCM mode.In order to further improve the efficiency of the single-stage PFC converter,this thesis will study the DCM-CRM single-stage PFC converter which the input stage PFC converter operates in DCM mode,and the output stage DC-DC converter operates in critical conduction mode(CRM).The Buck-Flyback single-stage PFC converter has the advantage of low intermediate bus capacitor voltage,while the existing studies use DCM-DCM mode and cannot use primary side control.This paper will propose the primary side control DCM-CRM Buck-Flyback single-stage PFC converter,without the use of optocoupler and secondary current sampling circuit;the input stage Buck PFC converter works in DCM mode,the output stage Flyback DC-DC converter works in CRM mode,constant output current and PFC function can be achieved using only one set of control circuit on the primary side.The simulation and experimental results show that the input current of converter meets the relevant requirements of international harmonic standard IEC61000-3-2,and can achieve high efficiency,high power factor and constant output current.In the DCM-CRM Buck-Flyback single-stage PFC converter based on primary side control,when the input voltage instantaneous value is less than the intermediate capacitor voltage,the input current is zero,which limits the power factor value of the converter;Boost-Flyback single-stage PFC converter can avoid this problem.Most of the existing literatures only study the operating characteristics of Boost-Flyback single-stage PFC converters with constant switch frequency.In order to improve efficiency,this paper proposes and deeply studies the DCM-CRM Boost-Flyback single-stage PFC converter to make the input stage Boost PFC converter operate in DCM mode,and the output stage Flyback DC-DC converter operate in CRM mode.Finally,the research shows that the converter input current meets the relevant requirements of the international harmonic standard IEC61000-3-2,and high power factor,high efficiency,fast dynamic response and constant output voltage can be realized by using only one switch tube and one current mode controller.