The Design Of Primary Side Flyback Conveter Digital Control Loop

By eliminating the isolation optocoupler circuit components, and reducing the circuit complexity as well as the cost, the PSR flyback converter is widely used in the field of small power adapter in recent years. The PSR converter for digital composite multi-mode control loop which can be more flexible and effective in achieving high average efficiency, high dynamic response and high precision performance, has become the trend. But the digital composite multi-mode control loop is also problematic, because it inlcludes the sampling circuit area, and the mode switch is not smooth, as well as the PFM control is inaccurate. This can say that it has been the academic research focus today.In this thesis, according to the PSR flyback converter, a new type of digital composite multi-mode control loop strategy has been designed, which can achieve PWM/PFM/DPWM/DPFM/DDPFM mode switch in the full load range. Firstly, small-signal model for DCM flyback converter was constructed in it, providing theoretical basis for digital PI compensator. Then, a primary digital sampling technology was designed in it, which can eliminate the cost of additional circuitry to save the power consumption and the circuit area. What’s more, the constant voltage module algorithm, especially the smooth switching between modes and PFM control technology in it, has been designed.This can make a low perturbation mode switch and a precise frequency modulation in PFM mode to be possible and improve the system dynamic response speed. Next a constant current algorithm and valley switching algorithm was designed in it, and then propose a new type of shutdown restart circuit, which can effectively protect the circuit. Finally, the control strategy and the effectiveness of the algorithm are verified in a 5V/1A AC-DC power system.According to the test results, the constant voltage accuracy can reach±1% in the whole load range, as well as the constant current accuracy is ±4.8%. In additional, the overshoot peak value is 1.28V for dynamically switch between full load and no-load and the average efficiency is 79.26%. What’s more, the valley switching algorithm can improve the efficiency of the system more than 0.5%, and achieve smooth switching between modes. All these can make the algorithm work, so the request of the thesis can be reached.