Research On PWM And PFM Hybrid Controlled Bridgeless Dual Resonant PFC Converter

In order to reduce the harmonic pollution caused by the power electronic equipment to the power system,it is necessary to add the power factor correction(PFC)converter to the front-end of the electronic equipment.The power factor correction technique can reduce the input current harmonics and increase the power factor,so the power factor correction technique is widely used in power supply equipment.Due to the front-end diode rectifier bridge,the efficiency of the traditional PFC converter is reduced.In order to improve the efficiency of the PFC converter,the bridgeless PFC has been extensively studied.A resonant circuit is introduced into the traditional bridgeless Flyback PFC converter,and a bridgeless dual resonant PFC converter is proposed in this paper.The turn-off current of switch decreases due to the resonance between the leakage inductor of the transformer and the resonant capacitor.Therefore,the turn-off losses of switch are greatly reduced.The operation principle and operation characteristics of the bridgeless dual resonant PFC converter working in discontinuous conduction mode(DCM)are analyzed in detail.Based on the derived input current expression,the unit power factor is realized by variable duty ratio control.The working principle of the variable duty ratio control is introduced in detail.The influence of the magnetic inductor on the working mode of the converter is analyzed,which provides the basis for the design of the magnetic inductor.Finally,the theoretical analysis is verified by simulation and a 60W experimental prototype.Through the study,it is found that the bridgeless dual resonant PFC converter with variable duty ratio control can't be complete resonance at light load,leading to lower power factor.Aiming at the problem of variable duty ratio control,this paper proposes a control method of constant on-time(COT)to control the bridgeless dual resonant PFC converter,so that the complete resonance can be achieved and power factor is improved.The operation principle and characteristics of COT control are analyzed in detail.Finally,the theoretical analysis is verified by simulation and a 20W experimental prototype.Because the bridgeless dual resonant PFC converter with COT control can cause high switching frequency at heavy load,the switching losses is increased and the efficiency is reduced.Aiming at this problem,a control method of pulse width modulation(PWM)and pulse frequency modulation(PFM)hybrid control bridgeless dual resonant PFC converter is proposed in this paper,which makes the converter have high power factor and efficiency in the whole load range.The operation principle of PWM and PFM hybrid control is introduced in detail,and the design of magnetic inductor and conduction time are studied.Finally,the theoretical analysis is verified by simulation and a 60W experimental prototype.