Research On Soft-switching Flyback LED Drive Power Supply

LED (Light Emitting Diode) has advantages of high luminous efficiency, long service life, environmental protection and energy conservation, is regarded as a new generation of lighting source. But the LED needs DC power supply, so the development of efficient, lightweight, durable power supply has become an important issue for the popularity of LED lighting. Switching power supply, which has advantages of high efficiency, high stability and design flexibility, has become the first choice of LED driving power supply. While switching frequency increases, the volume of power transformers, inductors and capacitors which are used in the power supply, decreases. In order to improve the power density and reduce the volume and weight of the power supply, high frequency has become an important development trend of switching power supply. But most of the present switching power supply uses semiconductor switching device, and the switching loss of semiconductor switching device increases with the increase of frequency. In order to solve this problem, soft switching technology is proposed.To achieve zero voltage switching (ZVS), reduce voltage stress and switching loss of switching device, the active clamp technique is applied to flyback converter in this paper, which can further improve the switch frequency and conversion efficiency.First of all, we introduce the current situation of LED lighting industry, especially the main technology of LED driver, and analyse its development trend. On this basis, we introduce the flyback converter, which is a common use for LED driving, point out its advantages and disadvantages, and then sum up the main role of the active clamp technology in the flyback converter. And then, we divide the active clamp flyback converter’s working process into eight stages, and analyse the energy conversion and the current’s and voltage’s changes in each stages in detail, and propose the ZVS conditions of switching devices, as well.Secondly, on the basis of analyzing the working principle of the active clamp flyback converter, a LED driving power supply is designed, which includes the power transformer, the clamping circuit, the control circuit and the protection circuit. In this paper, the UCC2894 chip is used as the controller. While the delay time (dead time) of the two PWM pulse output of the chip is properly set, the zero voltage switching of the main switch and auxiliary switch is realized. In order to further improve power efficiency, the synchronous rectification technology is used. And the delay time between the driving pulse of main switch and auxiliary switch are utilized to avoid the danger that the primary side and the secondary side of the flyback converter conducts at same time. In addition, this paper designs a EMI filter with surge protection function to improve the power supply’s reliability and reduce its transmission interference.Finally, based on the theoretical analysis and calculation, a LED driver prototype, which has an out power of 120W and a switching frequency of 300kHz and an input voltage that ranges from 180VAC to 240VAC, is made. The feasibility of the design scheme is verified by acquiring the voltage waveform of the key point of the prototype.