Research And Design Of Primary-side Flyback Converter With High Performance And Wide Range

With the advancement of science and technology,electronic devices tends to portability,multi-function and intelligence,which also puts forward more stringent requirements for the power supply as the ‘heart’ of electronic products.Among different power supply topologies,flyback converters are widely used in portable,low-power,lowcost devices because of their simple structure,low cost,and nature isolation of input and output.Compared with the secondary-side feedback flyback converter,the primary-side feedback flyback converter eliminates the need for optocoupler components and TL431,which can promote the miniaturization of the converter and achieve better hightemperature performance.In this context,this thesis discusses the wide load range and high performance primary-side feedback flyback converter.First,this thesis introduces the principle of PSR converter from three aspects: working mode,feedback mode and modulation mode,and establishes its corresponding mathematical model for the four modulation modes,then completes the verification of the model.After that,the reasons for the limited load range of high-power and medium-and small-power converters is analyzed.Pulse frequency modulation is used to expand the heavy-load range of the high-power converter.And the frequency foldback method is used in the small-and medium-power converter to expand the light-load range.And the respective loop compensation schemes are determined respectively,and their stability is verified by simplis.In this thesis,three knee detection strategies are proposed for different application scenarios.In the differential delay method strategy,the knee voltage can be detected by the voltage difference between the feedback reference voltage and the output voltage of buffer.The feedback reference voltage is differentiated through the voltage-type and current-type differential knee detection strategies to obtain its differential voltage and differential current.When the feedback reference voltage reaches the knee point,the slope increases.The knee point position can be obtained by detecting the differential voltage and or the differential current.In order to achieve the accuracy of knee voltage sampling,this thesis proposes an adaptive pre-charging scheme for the problem that the secondary side conduction time is too short to charging of the delay capacitor and thus the sampling voltage is lower than practical value.The propoesd scheme can adaptively adjust the charging duration of the precharge current in the leading edge blanking time according to the feedback reference voltage,which can effectively avoid the overcharge problem caused by the delay of the comparator and the small negative slew rate of the buffer.At the same time,because the traditional fixed delay compensation scheme cannot accurately compensate for the changing resonance frequency caused by the selection of different peripheral devices,this thesis proposes an adaptive delay compensation scheme.The scheme generates a linear resistor whose resistance value is only related to the resonant frequency to complete the adaptive compensation for the changing resonant frequency.The adaptive compensation schemes can effectively improve the sampling accuracy.Finally,the whole circuit is simulated and verified based on the 0.18μm BCD process.The output voltage error of high-power and medium-and small-power converters are within 3% and 2% respectively,the minimum load of high-power converters can reach10% full load,and the low-power converters can reach 0.5% full load.