Design Of Synchronous Rectifier Controller For Flyback Converters

With the rapid development of high-speed ultra-large-scale integrated circuits,the size of semiconductor devices has been continuously reduced,power consumption has been continuously reduced,so the required supply voltage is also getting lower and lower,and the output current is getting larger and larger,under low-voltage,high-current output environments.The conventional diode rectification method in DC/DC converters is not applicable.Synchronous Rectification(SR)refers to a technology that uses a low on-resistance power MOSFET instead of a conventional rectifier diode to achieve rectification.Since the end of the last century,thanks to the great advances in MOSFET technology,synchronous rectification technology has been introduced into switching power supplies,reducing the losses in the rectifying part and improving the efficiency of switching power supplies.Based on the existing voltage type self-driven synchronous rectification technology scheme,this paper presents a synchronous rectification control circuit suitable for flyback converters,completes the circuit architecture design,and designs the sub-circuits,then verificated.For the Discontinuous Conduction Mode(DCM),a turn-off comparator with a very small turn-off threshold is designed,and a very close to zero negative level detection is achieved by “eliminating the random offset first and adding the fixed offset first”.The test results show that the turn-off detector greatly prolongs the on-time of the synchronous rectification channel in the demagnetization time,shortens the conduction time of the body diode,effectively reduces the conduction loss,and improves the rectification efficiency.At the same time,a synchronous shutdown comparator(SYNC Comp)was added to allow the chip to operate in Continuous Conduction Mode(CCM),broadening the application range of the synchronous rectifier chip.In addition,the light load mode was introduced to improve the system’s rectification efficiency under light load mode.An open detector with a small delay is designed to control the opening of the synchronous rectifier and reduce the on-time of the body diode in the opening stage.A resonance/reset detector is added to detect the starting point of each switching cycle,and at the same time,the generation of resonance is detected and the TOFF Blanking timing is turned on to prevent the synchronous rectifier tube from being opened by mistake during the dead time.The design of the reference source circuit is completed,providing the reference voltage or bias current for the internal sub-module.The circuit is simulated and verified based on the 0.5μm BCD process,and it is taped and packaged.Finally,it is mounted in a Flyback converter system for physical testing.The test results show that the synchronous rectification control chip designed in this paper can drive the Flyback converter well.The secondary synchronous rectifiers improve the synchronous rectification efficiency compared to the traditional diode rectification methods,and the functions of the various modes of the chip have been fully verified.