Research On Variable On-time Controlled Boost PFC Converter Based On Variable Slope Sawtooth Wave

Power factor correction(PFC)technology can improve the power factor and eliminate the harmonic pollution of the power grid.In recent years,PFC technology has been received extensive attention and research.There are many types of PFC converters,and their functions and performances are also different.Among them,Boost PFC converter can realize the boost function,and it can be seen in many applications.According to whether the inductor current is continuous,it is divided into continuous current mode(CCM),critical continuous mode(CRM),and discontinuous current mode(DCM).The DCM Boost PFC converter has many advantages such as zero current switching of the switch,no reverse recovery of the diode and simple control.It is suitable for small and medium power applications.But it can only maintain high input power factor(PF)at low input voltage,but at high input voltage,the power factor will be obvious reduce.This paper designs a new variable on-time(VOT)control strategy based on variable slope sawtooth wave.The auxiliary winding detects the zero-crossing time of the boost inductor current,so that the capacitor starts charging when the switch is on,and stops charging when the inductor current crosses zero.The slope of the sawtooth wave is controlled by the on-time of the main switch,the discharging time of the inductor,and the switching period.The variable on-time control can be achieved because of the variable-slope sawtooth wave,thereby the unit power factor will be achieved.The test results based on the 120 W principle prototype show that the proposed VOT control strategy based on variable slope sawtooth wave has a significantly improved PF value,a significantly lower THD,and a lower inductance than the traditional control method.The current peak value,the effective value of the switch and diode current,and the output voltage ripple are all significantly reduced.CRM Boost PFC converter inherits the advantages of zero current switching of DCM Boost PFC converter and no reverse recovery of the diode,and it can achieve unity power factor in the full voltage range,which is more suitable for high-power applications.However,the switching range of the conventional CRM Boost PFC converter is too large,and the design of EMI filters is difficult.When the input voltage is high,the excessive switchi ng frequency causes excessive converter losses and reduces efficiency.This paper proposes a VOT-controlled DCM & CRM mixed-mode Boost PFC converter based on a variable-slope sawtooth wave,which enables the converter to switch to the constant-frequency DCM mode at load reduction or input voltage zero crossings,thereby limiting the maximum operating frequency and maintain unity power factor.Based on theoretical derivation and simulation verification,the switching frequency range of the proposed VOT controlled DCM&CRM mixed-mode Boost PFC converter is limited,and the unit power factor can still be achieved.Compared to DCM Boost PFC converter,the effective values of the switch and the freewheel diode currents of the proposed VOT-controlled DCM&CRM mixed-mode Boost PFC converter based on a variable-slope sawtooth wave are reduced.