Analysis And Design Of Synchronous Rectifier Driver For The Output Of Flyback Converter

With the development of electronic technology,the supply voltage of the chip has become lower and lower.But the output part of the switching power supply usually adopts the diode rectifier.The efficiency of the switching power supply has been reduced greatly for its higher forward conduction voltage drop.The use of a power MOS with a small conduction resistance instead of a diode rectifier can avoid this problem.Because of its advantages of simple structure and low cost,flyback converter is widely used in the field of small power.Therefore,it is of great theoretical significance and engineering application to study the output synchronous rectifier driver for flyback converter.The design scheme of output synchronous rectifier driver for flyback converter is proposed based on the analysis of the working principle and working waveforms of flyback converter and synchronous rectification technology.The state of the original switch is identified by sampling the drain source voltage of the synchronous rectification MOS and the time to drive the output synchronous rectification MOS is determined.The synchronous rectification MOS is open when the drain source voltage is less than zero.The time to turn off the synchronous rectification is determined by the current flowing through it.The output synchronous rectification MOS is switched off when the current flowing through it is less than the turn off threshold current.Through the accessory power supply in the driver,the peripheral circuit of the driving chip is simplified.The accessory power supply in the driver is consist of capacitor charging module and LDO module.Based on the principle of band-gap reference,a low temperature coefficient band-gap reference voltage source without operational amplifier is designed.The temperature coefficient of the band-gap reference voltage source is 10.42ppm/?.Finally,the design of an output synchronous rectifier driver for flyback converter is completed.Based on the BCD simulation file of 0.35um 5V/60V,the key inner module circuits has been designed and verified by the tool of Cadence.The verification platform of the driver has been built based on the flyback converter by using the designed driver.The simulation results show the feasibility of the proposed system.Based on the BCD design technology file of 0.35um 5V/60V,the layout of the circuit is fulfillment by using Cadence software.Finally the layout of the circuit has been verified by DRC and which has successfully passed the verification.