Design And Research Of Synchronous Rectifier Chip For Switching Power Supply

With the development of electronic products and the gradual increase of market demand for electronic products,the operating frequency of switching power supplies is getting higher and higher.In order to reduce power consumption,the operating voltage of the power supply needs to be reduced.In this way,under the condition of low-voltage and high-current output of the switching power supply,the proportion of the secondary rectifier diode loss will rise rapidly.In order to improve the conversion efficiency of the power supply,output rectification using a low on-resistance MOSFET will greatly reduce the rectification loss and improve the power supply efficiency.Therefore,this paper designs a synchronous rectification control chip which can be applied to the secondary side of flyback switching power supplyBased on the analysis of the working principle,driving mode and common topology of the synchronous rectifier power tube,the design scheme of the secondary-side synchronous rectification control chip for the flyback converter is proposed.In this paper,the voltage self-driving method is used to drive the synchronous rectifier.For the Discontinuous Conduction Mode(DCM),a reasonable turn-off detection point is designed.The threshold voltage is-350mV and the shutdown threshold voltage is-5mV.By sampling the drain-source voltage of the synchronous rectifier,when V ds<-350mV,the synchronous rectifier is turned on.When V_ds>-5mV,the driving voltage is quickly pulled down to the ground,and the synchronous rectifier is turned off.In this paper,the minimum on-time and minimum off-time circuit are set to shield the false trigger signal caused by the instantaneous voltage surge of the synchronous rectifier switch,to ensure the reliable operation of the system,and to provide different shielding time by adjusting the resistance of the external resistor to meet Different topology circuit requirements.An improved power supply module,driver module,and under-voltage protection module are set inside the chip to simplify the chip's peripheral circuits.Based on the 0.25um BCD process,the design and simulation of each sub-module circuit and the overall simulation of the chip are completed,and the chip and package are carried out.Finally,the physical test is carried out in the actual flyback conversion system.The test results show that the chip can The synchronous rectifier turns on and off the threshold point to reliably control the opening of the rectifier,and achieve 5 V/3 A output,and the rectification efficiency can reach 87%.