| With the continuous development of integrated circuits,the market demand for power management chips is growing.In the field of switching power supply,how to improve efficiency is a very important research direction.In recent years,with the increasing scale of the circuit,the application of low-voltage and high-current becomes the trend of low-power switching power supply.The traditional diode rectifier will greatly reduce the power supply efficiency because of the large forward conduction voltage drop,so using synchronous rectifier technology,that is,using a power MOSFET with smaller turn-on resistance to replace the diode for rectification,can greatly reduce the rectifier loss and improve the conversion efficiency.This article has designed a synchronous rectification control chip for flyback AC-DC converter,which can work in discontinuous current mode.The voltage self-driving method is adopted,and the sampled voltage signal comes from the drain terminal of the synchronous rectifier MOSFET on the secondary side,without relying on the primary side signal.Therefore,the peripheral circuit has a simple structure and requires few components.The main work of this paper is as follows:1)A pre turn-off circuit is designed to drop the gate voltage of the synchronous rectifier MOSFET in advance to achieve the purpose of fast turnoff.Not only can reduce the body diode on-time to improve efficiency,but also reduce the rate of change of the drain voltage of the synchronous rectifier MOSFET and increase the turn-off threshold,and therefore reduce the impact of the comparator system offset voltage.Compared with the drive circuit without pre turn-off technology,the pull-down time can be saved by 70%.2)The integral shielding ring module is designed.When the synchronous rectifier is off,the system will ring due to the influence of parasitic parameters,which will cause the chip to turn on by mistake.The integral calculation of the area is used to distinguish the ringing signal,and the threshold is adjusted by an external capacitor to improve the reliability of the chip.3)In order to monitor the output voltage,this article adds a discharging current and primary side wake-up functions.When the load becomes smaller,the excess energy can be discharged through the built-in 100 m A current to prevent the output voltage from being too high.When the primary chip is in light load mode,a wake-up signal with a period of 30 us can be generated by the primary chip wake-up function to feed back the change of the load to the primary chip in time.4)A bandgap reference circuit without operational amplifier is designed.Under the TT simulation condition,the temperature coefficient is 28.94 ppm /℃,the power rejection ratio is 86 db,the linear adjustment rate is 0.027%,and the reference voltage is 1.3V,which provides the reference voltage for the chip.5)The internal LDO is used for power supply,the output voltage is 5.8V,no external power supply capacitor is needed,so that the chip can work in the voltage range of 5 ~ 25 V.At the same time,a floating ground potential generation circuit is integrated to generate a potential that is lower than the system voltage of 5V,which is used to reduce the use of high-voltage MOSFET in the drive circuit.This chip is designed and simulated based on nuvoton 0.35 um BCD process.And it is taped and packaged.The final test results show that this synchronous rectification chip can improve the system rectification efficiency by 2% compared with the traditional diode rectification scheme. |
PDF Full Text Request