Variable On-time(VOT) Control For CRM Boost PFC Converters

Thanks to the soft-switching characteristics,critical conduction mode(CRM)boost power factor correction(PFC)converters are widely used in the low-to-medium power application.However,the soft-switching process also results in an obviously higher input current total harmonics distortion(THD),which limits the application of high-frequency CRM boost PFC converters.In order to reduce the input current THD,many kinds of digital-based variable on-time(VOT)control are developed.Unfortunately,the adaptability becomes an issue for conventional look-up-table(LUT)-based VOT control within the universal AC input and entire load range.The existing real-time calculation method for VOT control needs massive online calculation and thus requires a high-end MCU.In order to solve the adaptability of conventional LUT-based VOT control,this paper reveals mapping relationships of VOT curves under different input voltage and load,and then proposes an adaptive LUT-based VOT control,which combines advantages of conventional LUT method and real-time calculation method.Based on mapping relationships,only one VOT table is required to be preloaded in LUT,and its adaptability can be conveniently realized through a simple real-time mapping.The inherent voltage loop is utilized to simplify the mapping process,eliminating the dependence of the proposed adaptive control on the circuit parameters and reducing the system cost.Even though with the ±20% circuit parameters variations,the proposed control still evidently improves input current THD than the constant on-time(COT)control.The experimental results of an 160-W CRM boost PFC prototype are built to verify the effectiveness and advantage of the proposed control.The proposed control can reduce the system cost with high reliability and great adaptability.Compared with the existing VOT control,the proposed control can achieve the higher conversion efficiency and the lowest input current THD.In order to simplify the calculation process and shorten the computation time of existing real-time calculation method for VOT control,this paper proposes five VOT controls with approximate explicit functions of on-time.Based on the function approximation and wave approximation,four simplified VOT control with segmented forms are proposed.Their performance on the input current THD and calculation process are evaluated and compared.Two of them have obvious advantages for practice.In order to further simplify the above VOT control,a simple VOT control with the uniform function at valley switch(VS)and zero voltage switch(ZVS)condition is proposed to minimize the computation time.Importantly,the proposed uniform VOT control can be implemented by the cost-effective analog circuits and with a great potential to be integrated.The experimental results of an 160-W CRM boost PFC prototype are built to verify the effectiveness and advantage of the proposed control above.Compared with the existing VOT control based on the rea-time calculation method,the proposed controls can achieve simpler calculation process and lower input current THD.For CRM boost PFC converters with harmonics injection,the soft-switching process introduces the exceeding injected harmonics.In order to solve the problem,this paper proposes a novel VOT control to realize accurate harmonics injection without any trial-and-error design.The experimental results of an 120-W CRM boost PFC prototype are built to verify the effectiveness of the proposed control.With the proposed control,the actual injected ratios at 100 Vac input,full load are same with the expectation,and the output voltage ripple is reduced by 33% within EN61000-3-2 Class D limits.