Research On Control Technology Of High Power Density Active Clamp Flyback Converter

Under the demand of modern electronic equipment for miniaturized and energy-efficient power supply,with the current development of battery materials,it is difficult to increase the battery capacity within the limited battery volume limit.Based on this situation,the fast charging technology in power management came into being.The charging adapter of small and medium power portable electronic equipment is mainly a flyback converter that can realize electrical isolation.The key of fast charging technology is to increase the output power of the flyback converter system and improve the system efficiency at the same time.The development process of the flyback converter topology is studied in thesis.Combined with the current development trend of integrated circuits,the active clamp flyback converter（ACF）system that can fully utilize the leakage inductance energy of the transformer is selected as the research object.An efficient system control chip for high power density ACF system is designed.In thesis,the working principle and characteristics of the ACF system are deeply studied.Considering the parameters affecting the system efficiency such as on-resistance and junction capacitance of Si and Ga N devices,Ga N,which is more suitable for the ACF system,is selected as the control power transistor.After the basic structure of the system is determined.This thesis is based on the problem of power supply and startup of the control chip in the high-voltage system.A soft-start scheme of segmented charging and a chip power supply scheme using auxiliary windings are designed.Because the designed power supply will have periodic voltage ripple,this thesis specifically designs a high PSR bandgap circuit and an LDO with self-protection function as the internal power supply of the control chip.The key technology to improve system efficiency is to realize zero voltage switching（ZVS）of high voltage power transistors.Based on the research of the ACF system,this thesis designs the control circuit to realize the adaptive ZVS of the main switch of the system.The bus voltage V_BULK is a key parameter for the realization of adaptive ZVS,so this thesis designs a bus voltage sampling circuit in the control chip by using the auxiliary winding,which can accurately reflect the information of the bus voltage to the control chip.The dead time t _Z from the upper transistor off to the main switch transistor on and the upper transistor on time t _DM in the ACF system are the key technologies of the adaptive ZVS implementation.Based on the results of bus voltage sampling,this thesis designs optimized circuits those the control chip can adaptively adjust the dead time t _Z and the turn-on duration t _DM of the upper transistor according to different system application conditions.The above design schemes and circuits are designed based on the 0.18μm BCD process and are simulated and verified in the ACF system.The simulation results show that the control chip based on this thesis can realize the adaptive ZVS opening of the main switch transistor in the ACF system under the output power of 45W,and the calculated system efficiency is higher than 90%,which is in line with the design expectations.