Design Of Multi-Mode PWM Fly-back SMPS

Recently, the request for power supply becomes more and more strict, especially in low power and low EMI noise. And fly-back DC-DC converter is widely used in high voltage offline application. But in fly-back, the voltage stress is high across the power switch for the leakage on transformer and the drain/source capacitor on switch. And the power efficiency will further decrease in media/light load condition. So this paper designed a DC-DC fly-back multi-mode converter based on that.The system adopted quasi-resonant mode in the condition of heavy load to improve power efficiency and reduce the EMI. Because the power efficiency of quasi-resonant is low in moderate load, this paper adopted constant on time mode which is also called frequency fold back mode. In light and no load conditions, the BURST mode is adopted. This paper designed the system and the module circuit and tested the taped-out chip .The controller contains both feed-forward and feedback. The feed-forward is used to reduce the input to output modulation and improve the dynamic response. The feedback is used to improve the output voltage precision. The peak current detecting method is adopted. The system controlling is all DCM so the slope compensation is not needed. The small signal transfer function is derived by State-Space Averaging method and the PI compensate network is chosen.The feature of the chip designed includes line and load over-voltage, line over-current protection and UVLO.The flyback converter designed by this paper provides a 19.4-V regulated output at 6.2 A of load current, operating from a universal ac input. The chip has been taped-out in abcd150 process of NSC. The test result shows that the system can keep stable in both heavy and light load conditions and all the function excepted has been realized. The efficiency of the converter can reach 70% from heavy load to no load.