| In recent years,with the rapid development of consumer electronics products and the popularization of mobile Internet applications,the global market demand for switching power supplies,especially AC / DC converters,is also increasing rapidly.In addition,with the significant increase in public awareness of environmental protection,people are increasingly paying attention to green power,which has brought tremendous commercial potential for energy conservation issues.The valley switching with the quasiresonant(QR)control can be an attractive solution to improve the power efficiency.However,the traditional QR control continues to maintain the first valley switching under light load conditions,the switching frequency may increase because the conduction time becomes very small,resulting in greater switching losses and poor light load efficiency.Aiming at the current market requirements for power adapters,this thesis proposes a quasi-resonant PSR flyback CV&CC converter with light load efficiency optimization under constant voltage(CV)mode.The converter adopts the QR control scheme to switch in the valley mode either in CV or CC operation,which greatly reduces the switching loss.In addition,aiming at the problem of poor light load efficiency of traditional quasi-resonant converter,the converter adopts an adaptive frequency control scheme in CV mode,and adaptively adjusts the switching frequency according to the load condition to optimize the efficiency.As the load becomes lighter,adaptively adjusts the switching frequency to reduce switching losses,thereby achieving better light load efficiency and improving the overall average efficiency of the system.The proposed converter adopts the primary-side regulation(PSR)structure,and the output information is fed back through the auxiliary winding,which saves the peripheral circuit and reduces the cost.In CV mode,the converter samples the voltage of the auxiliary winding at 2/3 demagnetization time and keeps it until the next sampling period.Compare the sampled voltage with the internal reference voltage and amplify the error.The obtained error voltage not only determines the switching frequency,but also determines the peak current of the primary inductor,thereby controlling the on-time to achieve a constant voltage output.In the constant current(CC)mode,the converter detects the demagnetization time of the converter through the auxiliary winding,and automatically adjusts the primary inductor peak current of the next cycle according to the primary sampling resistor peak voltage and demagnetization time of the previous cycle,so that the product of the peak voltage of the sampling resistor and the demagnetization time is a constant value,thus realizing the constant current output.The proposed QR converter was implemented using the HHGRACE 0.35μm BCD process and verified in a 5 V/1A circuit prototype.The test results show that the maximum maximum standby power consumption of the converter is 94 m W at 90 ～ 265 V AC input voltage.In the CV mode,the output voltage accuracy is±2.2%,the linear adjustment rate is 1.37%,the load adjustment rate is 0.83%.In the CC mode,the output current accuracy is ±2.8%,the linear adjustment rate is 2.44%,the load adjustment rate is 1.22%.The designed converter can realize adaptive frequency control according to load conditions in CV mode to optimize system efficiency.The minimum efficiency can reach about 77.5% under light load.The maximum efficiency can reach about 80.5% under heavy load.Experimental results show that the light-load efficiency of the proposed converter is improved by up to 4% when compared to the conventional QR controller.In addition,the valley switching is realized under different load conditions,which improves the system efficiency in the entire range of output load.It can be seen that the control chip proposed in this paper has a good application prospect in low-power QR flyback AC-DC cconverter. |
