Double-line-frequency Ripple Suppression Technology For Flyback PFC Converter

Due to the input power and output power do not match in real time,single-stage Power Factor Correction（PFC）converters have double-line-frequency（DLF）ripple（voltage/current）components at the output.For electronic equipment with high power quality requirements,it is not possible to directly use a single-stage PFC converter to power it.The quasi-single-stage（QSS）Flyback PFC converter is composed of one end of the dual output winding of the Flyback PFC converter connected in series with a Buck ripple suppressor（RS）,that is,the auxiliary output of the Flyback PFC converter is used as the input terminal of the Buck RS and the main output is connected in series with the Buck RS output.By controlling the Buck RS to generate a DLF ripple voltage with the same magnitude but 180°phase shifted voltage as the main output voltage,the purpose of suppressing the output DLF ripple is achieved.The circuit has the advantages of high power factor,high efficiency and low output ripple.Therefore,it is of great significance to study the QSS Flyback PFC converter.First,this paper takes QSS Flyback PFC converter as the research object,analyzes its working principle in detail,and establishes the equivalent DC steady-state model and AC small-signal model.The operating voltage range of Buck-type RS that realizes low output ripple and the critical inductance in continuous conduction mode（CCM）are derived.The efficiency of the two output windings with different power distribution is compared.A single-voltage loop control circuit of Proportional Integral（PI）compensation network structure is designed,and Buck RS is controlled by this method.A closed-loop small-signal model is established,and the relationship between the DLF ripple suppression performance and the loop gain is illustrated by the input-output transfer function.Studies have shown that the greater the loop gain and bandwidth,the better the output power frequency ripple suppression.Secondly,in order to further reduce the DLF ripple of output voltage and increase the load transient response speed,based on the constant on time（COT）control has the advantages of large bandwidth and fast speed,COT control combined with QSS Flyback PFC converter,the Buck RS is controlled by COT to suppress the converter output DLF ripple.The influence of the output capacitor equivalent series resistance（ESR）on the stability of the Buck RS controlled by COT is analyzed.A closed-loop small-signal model is established,and the audio susceptibility model is used to measure the effect of DLF disturbance of input voltage on the output voltage.The smaller the amplitude of the audio susceptibility at the DLF,the better the output DLF ripple suppression effect.The research results show that when the ESR is large,the Buck RS controlled by COT can work stably.In the equivalent circuit,the amplitude of the audio susceptibility of the two input voltages of the Buck-type RS to the output voltage at 100 Hz is very small,which can well suppress the output DLF ripple.The simulation results and experimental results verify the correctness of the theoretical analysis.It can also be seen from the experimental results that compared with the traditional voltage-type control,the COT control of the Buck RS improves the suppression effect of the converter’s output DLF ripple and has a faster dynamic response speed.Finally,the factors that affect the switching frequency of Buck RS with COT control in the QSS Flyback PFC converter are analyzed:the change in input voltage will cause the switching frequency to change.It is more difficult to design the EMI filter when the switching frequency changes greatly.To solve this problem,the input voltage feed-forward loop of the Buck RS is introduced into the on-timer to implement adaptive on time（AOT）control.A closed-loop small-signal model was established,and the audio susceptibility model was used to measure and compare the effect of DLF disturbance of the input voltage on the output voltage under AOT and COT control.The research results show that the use of AOT control eliminates the effect of input voltage changes on the switching frequency and reduces the switching frequency change.When AOT control is used,the audio susceptibility of input voltage v_o2 to v_o is less than that of COT control,indicating that AOT control can further improve the DLF ripple suppression performance of the converter output.The correctness of theoretical analysis is verified through simulation and experiment.Experimental results show that AOT control not only has the advantages of large bandwidth and fast speed of COT control,but also can further improve the DLF ripple suppression performance and efficiency of the converter.