Green Mode Flyback AC_DC Switch Off Power Supply System Design And Implementation

Switching power supply has been widely used in various fields because of its advantages, such as, low power loss, high conversion efficiency, ease of miniaturization, etc. Reducing power loss of switching power supply is crucial for further improving conversion efficiency. Currently, new devices, new topologies structure and new control mode of switching power supply can improve conversion efficiency of switching power supply. Offline fly back AC_DC converter can achieve multiple outputs and they don’t need filter inductors because of transformer isolation. Costs can be remarkably lessened and converter volume can become smaller. Fly back converter is suitable for low power occasion5W～150W.Green work mode AC_DC switching power supply chip designed in this thesis uses offline fly back topologies structure and PWM peak current-control mode. They have the characteristics of high performance, low standby power consumption and low cost. Application circuit of the chip can be directly grid supplied and the chip is suitable for low-power applications within30W. It can be used for battery chargers, power adapters, set-top box power supply, and other occasions.The system normally work under PWM current-mode, if it is under the condition of no load or light load, it enters burst mode. By reducing switching actions, switching losses can be lowered in order to get low standby power consumption and higher conversion efficiency. An RC oscillator design with low supply voltage sensitivity has been proposed and frequency shuffling module has been designed to make65KHz oscillator frequency have±2KHz shuffling in2ms cycles. It has effectively improved system EMI and reduced the cost of system EMI. The chip also has build-in slope compensation and LEB circuit. It also has perfect protection circuit, including VDD over-voltage clamping, under voltage lockout, over current protection, overload and over temperature protection. All these can make the chip have high security and reliability.Based on the0.5μm BICMOS process, EDA simulation tools have been used to do complete simulation analysis of sub-modules of the chip, then whole circuit of the chip should be verified through simulation, including electric start on the chip, normal working situation of the chip, burst mode function during light load, and the simulations of various protection functions. The theoretical analysis and simulation results show that the functions of the chip are normal under this green work and it meets design specifications. Finally, some designed parts of sub-module layouts and overall layout have been shown. Currently, the chip is being taped out.