Research On DCM And CRM Mixed Conduction Mode Flyback PFC Converter

In order to make electrical equipments meet domestic and international standards of the harmonic content of the input current,a power factor correction(PFC)converter must be used.With advantages of electrical isolation of input and output,ease of control and simple structure,Flyback PFC converters are widely used.Due to the diversification of electrical equipments,new requirements on the power supply has been imposed due to the wide range of load.In terms of flyback PFC converter,on the one hand,when it works in discontinuous conduction mode(DCM),the waveform of input current is an ideal sine wave.However,its output voltage ripple and the peak current of switch are large.It's applied to low-power occasions from now and then.On the other hand,when it works in critical conduction mode(CRM),its output voltage ripple is small.However,its input current is not an ideal sine wave.Furthermore,the switching frequency of the switch is significantly high near the zero crossing of the input voltage,which will increase the switching loss.It's mostly used in medium and high power situation.Therefore,by using mixed conduction mode(MCM),the converter can combine the advantages of DCM and CRM.The flyback PFC converter can work in DCM and CRM.Meanwhile,it can adjust the percentage that the converter works in either mode.It can improve various parameters of the flyback PFC converter,such as the waveform of input current,the output voltage ripple and power factor(PF).In this paper,the formulations of the MCM flyback PFC converter is derived in detail.Compared with conventional DCM and CRM flyback PFC converter in various parameters,its effect is displayed intuitively.Simulation and experimental results verify the converter is able to work in MCM.Compared with working in CRM,its waveform of input current is more similar to a sinusoidal wave.It reduces the switching loss by limiting the maxium frequency of the main switch aswell.Moreover,PF has been improved.In contrast to working in DCM,its output voltage ripple is smaller.The waveform of input current of conventional CRM flyback PFC converter is not an ideal sine wave.So a variable on time(VOT)control with a variable slope sawtooth generator is proposed.PF can reach 1 consequently.The on-time of the main switch is variable.The input current is an ideal sine wave as a result.By applying this variable slope sawtooth technique,the working effect of the MCM flyback PFC converter is further improved.In this paper,the formulations of the unit PF MCM flyback PFC converter is deduced in detail.Compared it with constant on time(COT)controll in various parameters,its effect is demonstrated intuitively.Through simulation verification,the converter can work in unit PF MCM.Compared with COT controll,the waveform of input current is an ideal sine wave,the value of PF is bigger and the value of THD is smaller.