Power Supply Based On Buck Converter With Active Ripple Compensation For LED

Along with the technological breakthroughs of gallium nitride and the advent of high power white light-emitting diode(LED), LED is starting to be used in lighting and has broad prospects for development. The reliability and ripple characteristic of the power supply for LED have drawn more and more attentions of scholars all over the world. The ripple current has greater influence on the luminescent efficient and half-life of LED, the reliability of power supply is the short-board of the LED lamps lifetime. Switching power supply is the main driver for LED, its reliability and ripple characteristic is related to high capacity electrolytic capacitor which is widely used in the power supply for filtering. The driving power usually has large output ripple and large volume due to its own factor of electrolytic capacitor, but the more importance is that the reliability of driver is serious limited by the electrolytic capacitor. So adopting new circuit topology and high stability capacitor to alternative electrolytic capacitor filtering is significant for LED lighting.Considering the above-mentioned problems, a low ripple current output and high reliability LED driver is presented in this paper, which combines switching power supply with linear power supply and fully utilizes their advantages respectively. The topology achieves high conversion efficiencies by the switching power supply, at the same time, utilizes linear power supply to inhibit current ripple insteading of electrolytic capacitor filtering. Since this new driver does not employ the high capacity electrolytic capacitor and the low capacity capacitor, such as polyester capacitor, would be used to get a higher reliability.The main research work is introduced as followings:1. The structure and working principle of traditional Buck circuit were researched in this paper. How the output voltage ripple comes into being and how to reduce it were presented. Then the factors that influence the performances of electrolytic capacitors were analysed.2. A new circuit topology, based on Buck converter with active ripple compensation, was proposed for the first time. The circuit structure and working principle were introduced, the ripple current detection and compensation circuit were designed, the equivalent small signal circuit model was established, the transfer function of output current with input voltage and duty cycle about this model were analysed, the loss in compensation circuit and the output efficiency were discussed, the efficiency is higher when the circuit works in long duty cycle or adopts auxiliary power supply.3. Considering the negative temperature effect of the forward voltage in the LED, a new control method which called improved peak current control mode was proposed by using output current feedback instead of the output voltage feedback in the traditional peak current control mode. The LED driver based on Buck circuit with active ripple compensation under the new control mode was designed. Inductive current oscillation and slope compensation were discussed when the circuit works in long duty cycle. The feedforward action was deduced when the input-voltage was disturbed. The transfer function structure of the new control mode and the whole transfer function structure were founded finally.4. The designed circuit has been simulated by simulation software, which verifys the feasibility of the novel topology and the validity of the control method. In the end, a test circuit has been built, the experimental result accords with the theoretical analysis and simulation results mainly.The results of both simulation and experiments showed that the ripple current of inductor was whole compensated by active ripple compensation circuit and the output current is DC. There is no high capacity electrolytic capacitor in the new topology and the ripple compensation circuit can be integrate with control circuit together, therefore, it can avert the high failure rate by electrolytic capacitor. The output has no ripple current so it's the ideal driver for high power LED and it will have profound significance and wide application prospect for the LED lighting.