Design Of Mixed-modulation Primary-sidefeedback AC-DC Control Chip

With the rapid development of various electronic products,our power requirements for these products are also getting higher and higher.Traditional AC-DC converters mostly use flyback secondary feedback topology,and the control mode mostly uses PWM.Control mode,the secondary side feedback switching power supply uses the optocoupler device to directly sample the output voltage,so the secondary side feedback power supply has the advantage of high output voltage accuracy,which is mostly used in medium power occasions,but this structure increases the optocoupler device and TL431 The reference voltage source will increase the application area of the system.At the same time,the optocoupler device is susceptible to interference and cannot work in a high-temperature environment,resulting in an increased probability of system failure.Its extra power consumption also makes the secondary feedback unsuitable for small power power supply.The PWM control mode of the switching power supply adopts a fixed switching frequency to adjust the conduction time of the switching tube.The efficiency is very high under heavy load and the frequency hopping control method is used in the standby state,the output ripple will be very large.Therefore,it is difficult to realize the design of a low-power switching power supply.In order to meet the charging requirements of small power consumption and small volume of electronic products,this paper designs a PFM type switching power supply control chip,which can be used by adapters and chargers of mobile devices such as mobile phones,PDAs,and LEDs.This chip adopts the primary feedback type flyback topology,eliminating the secondary side control circuit and optocoupler device,which can not only reduce the area but also improve the stability and reliability of the entire system.In order to improve system efficiency and reduce switching losses,the chip adopts PFM control mode and quasi-resonant control mode when it is heavily loaded,which can achieve soft switching at the bottom of the quasi-resonant voltage of the switch tube while suppressing chip EMI interference.The PFM control is adopted at light load,which greatly reduces the operating frequency of the system and thus reduces the standby power consumption of the system.Compared with the PWM control method,it has unparalleled advantages,so it has important research significance for low-power switching power supply.According to the division of functions in the chip,this chip has designed a power supply module,three control loop modules,various protection circuits and output drive circuits, and has built-in output cable compensation technology and line voltage compensation technology in the CC and CV loops.Make the power supply output accurate constant current and constant voltage.In the CS loop,the chip can switch the CS comparison voltage under light load to reduce the audio noise of PFM under light load.The designed chip can be applied to the isolation scheme below 12 W.After that,the sub-circuit-level module and system-level module of the AC / DC controller were simulated and verified under the Candence Spectre environment.The simulation results show that the load output current can reach 2.4A when the system is in constant current mode,and under heavy load The dual modulation of PFM and quasi-resonance has been realized.In constant voltage mode,the load voltage is 5V,the system is modulated by PFM,the maximum conversion efficiency is about 74%,and other performances also meet the chip design indicators.Finally,in the environment of Candence Virtuoso,FMIC’s 1.5 micron BCD process was used to complete the layout design of the chip,and Calibre back-end tools were used to check the layout design rules and circuit consistency of the layout to meet the chip’s tape requirements.