Study Of Flyback Converter Based On Synchronous Rectification

Low-voltage and high-current DC-DC Switching Mode Power System(SMPS) takes up half of the market needs of the SMPS all the while, so the research of it's key technology has important value of applications. In these recent years, with the increasing popularization of the applications such as 3.3V microprocessor, digital signal processor and many kinds of special low-voltage integrated circuit. Low-voltage and high-current SMPS has become one of the most important studies, and the high efficiency of the SMPS has become one of the important objects the producers pursues.The traditional converter always adopts common diode or schottky diode to rectify, thanks to the great positive turn-on voltage of the diode, the power dissipation is considerably great, the rectifying dissipation turns to the main dissipation and can't content with the high efficiency and small volume of the low-voltage and high-current SMPS. In this situation, Synchronous rectification(SR) should be adopted. It adopts power MOSFET instead of traditional schottky diode and common diode to rectify. As a result of the low turn-on resistance, short switching time and high input resistance, the power dissipation of the switching converter has been greatly cut down between the rectifying process, the efficiency of the converter has also been increased, so the MOSFET becomes to the preferred rectifier of the low-voltage and high-current SMPS.Uniting the SR to some proper circuit topologies can receive low-cost and high-efficiency converters. Flyback converter has the excellence of the simple topology, the insulated input and output, the wide input voltage range and the easily using of multiply outputs, as a result of these, it has been widely used in the circuit of the high input voltage and low input power, especially widely used in the SMPS of 5~150 watts.This paper introduces the design of the flyback converter based on SR, compares the different working mode of the flyback converter, makes a research of the design of the snubber and the SR, discuss the working principle of the SR flyback converter in detail. After analyzing the SR, according to its character, we design proper gate-drive circuit fit for flyback converter and pay stress to the key points should be paid attention in the experimentation.This paper makes academic analysis of the flyback circuit both using schottky and MOSFET to rectify, founds the dissipation mode to compare, and design a flyback SMPS with 85~265V input voltage, 3.3V output voltage and 4A load. The circuit use UC2842 as its IC, The secondary side uses schottky and MOSFET to rectify separately, academic analysis and experimental results prove that the SR circuit has significant effect on increasing the efficiency of the SMPS with low voltage and high current, and the SR flyback converter has practical value in the application of the SMPS with low voltage and high current.