Design Of Synchronous Rectification Control Strategy For MHz Flyback Quasi-resonant Converter

The flyback quasi-resonant converter has wide application prospects in the field of small and medium power sources such as mobile phone adapters because of safety and stability.It simplifies the topology,achieves valley conduction through resonance,and increases the switching frequency.Besides,the use of synchronous rectification technology can reduce the power consumption of the rectifier diode and improve the overall efficiency.However,due to the resonant change of the operating waveform of the flyback quasiresonant converter,it is difficult to find the sampling control point effectively,which hinders the application of synchronous rectification technology in the flyback quasi-resonant converter.In response to the above problems,a synchronous rectification control strategy is designed for a flyback quasi-resonant converter.Firstly,the working principle and related driving technology of the flyback quasiresonant converter under synchronous rectification control are analyzed.Then,the loss model of the synchronous rectification algorithm is established and analyzed.On this basis,synchronous rectification based on the continuous conduction mode(CCM)and discontinuous conduction mode(DCM)table look-up method are designed.According to the working characteristics of different modes,this strategy samples the excitation current value in this mode and determines the drive signal for synchronous rectification in the current cycle.An error analysis algorithm is designed to sample the amount of error and automatically adjust the corresponding table value to ensure its accuracy to achieve the adaptation of synchronous rectification.The RTL code is written according to the proposed control strategy,with the help of Simulink and Modelsim’s joint simulation verification,the PCB system board and control board were designed separately,and the test prototype was built.The FPGA verification platform is used to verify the effectiveness of the synchronous rectification algorithm.The simulation and actual measurement results show that the synchronous rectification control strategy designed in this paper realizes the self-adaptive switching time of the synchronous rectifier tube,and work effectively in both CCM mode and DCM mode.The full-load efficiency is 91.35%,which is higher than the non-synchronous rectification efficiency by 2%,and the constant voltage accuracy reaches ±1.5%,which meets the design specifications.