A Work In Research And Design Of Multi-mode Off-line Flyback Converter

With the development and improvement of information technology, switching power supplies are more widely used. The off-line flyback converter is one kind of it, which is commonly used at the 50 to 100W power range because of its merits, such as simple circuit topologies, electrical isolation between input and output, without filter inductance at the output terminal, wide range of output voltage. In order to have better adapt in multi-function portable of miniaturized device, switching power supply continuously weeds through the old to bring forth the new, towards the direction of high efficiency, low voltage, high current and high frequency.In this paper, a multi-mode off-line flyback power supply chip is designed. It can work in four modes, which maximizes efficiency at all loads; Integrated soft-start circuit reduces the output overshoot and minimizes start-up stress; Improved auto-restart effectively limits power dissipation of the chip by reducing duty cycle to typically 2% in short circuit and open loop fault conditions; Dual slope line feed-forward improves the line ripple rejection; Frequency jittering reduces EMI filter cost.The design integrates multi-mode control system in a chip, as well as protection circuit, the power MOSFET, which makes the chip have small size, low cost and improved self-protection. The main work of this paper as follows: (1) Analysis static and dynamic characteristics of flyback converter, and propose the structure design of the whole circuit. (2) Design multi-mode control circuit, and then derive the range of feedback current in any mode. (3) Based on 3um BCD process, the whole circuit are simulated by EDA software, HSPICE. (4) Layout and tape-out of the chip is successfully completed. (5)Test of the key parameters is finished, which provides test pattern and data to verify the design of the chip.The problem of smooth transition between operating modes is successfully resolved through continually improvement and innovation. The final test results show good performance and high yield about this chip, which proved the circuit design is successful.