The Research On CRM Buck PFC Converter With Variable On-Time Control

In the production and the life,the power supply of power electronic equipments mostly are the direct current(DC),while the electricity of the power grid is usually the alternating current(AC),therefore we need to convert AC to DC.The earliest is the capacitor filter uncontrollable bridge rectifier circuit,the distortion of the network side input current waveform is serious and contains large number of harmonic components,so it need the power factor correction technology to reduce the harmonics and to improve the power factor.The switching frequency of the swith varies with the input voltage variations in the line cycle of Buck PFC converter with CRM mode,and a relatively large frequency range leads to a large of switching loss,also causing the EMI and other adverse effects.This paper proposed a variable on-time control strategy compared to the traditional peak current-control.Injecting a certain number of third harmonic into the the peak current reference signal,achieving the turn-on time,turn-off time and the switching cycles are variable in the line cycle.Finding the optimal amount of third harmonic to reduce the maximum switching frequency of the converter and obtain the minimum of switching frequency range,enhancing the application performance of the converter.The AC-DC Buck converter has a disadvantage,when the input voltage is less than the output voltage,the converter does not work,the input current has some dead zone,which causes the power factor is not as high as other topologies.This paper proposed a variable on-time control strategy compared to the traditional constant on-time control.Injecting a certain number of inverse third harmonic into the the peak current reference signal to offset the original content,got the functions of third harmonic and the conduction time when the PF achieved the maximum value,designing the circuit to realize the control process.The harmonics can be reduced and PF can be improved,enhancing its engineering applications.In order to verify the effectiveness of the theoretical analysis,experimental verification is done.A prototype of variable on-time to optimize the switching frequency for CRM Buck PFC and a prototype of variable on-time to improve the power factor for CRM Buck PFC are made.The experimental waveforms at different input voltages are given respectively,the experimental data and the analysis of the results verify the validity of the theoretical analysis.