Research And Design On A New LED Driver

As the4thgreen light source, Light Emitting Diode (LED) obtains many outstandingmerits, such as energy saving, green, safety, and longevity. It has a great trend to replacetraditional light sources. The development of LED lighting technology needs high reliability,high power factor, high efficiency,long lifetime and low cost LED drivers. This thesis isdedicated to the design of a novel electrolytic capacitor-less AC-powered LED driver.To meet the actual requirement, a two-stage LED driver circuit with a high power factorand accurate drive current is proposed in this paper. The sepic converter under DCM isselected as the prior circuit to achieve power factor correction(PFC).And the buck converterunder CCM is selected as the back circuit to realize constant current driving.In the prior PFC circuit, In order to balance the difference between the instantaneousinput and output power, a large storage capacitor is required, and electrolytic capacitor isoften used. But the lifetime of electrolytic capacitor is much lower than the lifetime of LED,and electrolytic capacitor is the main component that limits the lifetime of the LED driver. Soelectrolytic capacitor should be removed. This thesis proposes a bridgeless and electrolyticcapacitor-less PFC circuit based on sepic converter which operates in special DCM. Theproposed circuit is able to eliminate the electrolytic capacitor while maintaining high powerfactor and high efficiency. Then deeply analyses its operating principles and its designing oncircuits parameters, also verifies the correctness on circuits theory via saber software.The back constant current driving circuit is mostly switching power supply with highconversion efficiency. But switching power supply has large output current ripple resulting inunfavorable effect on performances of LED, such as luminous efficiency, lifetime. So itmainly uses electrolytic capacitors for filtering to reduce current ripple. However, it willseverely limit the reliability of LED driver for the lifetime of the capacitor is short. In order tosolve the problems above, this thesis proposes a design method of electrolytic capacitor-lessconstant current driving circuit based on buck converter, which combined switching powersupply and linear power supply together. It transfers energy efficiently by using the switchingpower supply, and could attenuate the output current ripple by the linear power supply insteadof using electrolytic capacitors as well. A buck LED driver based on hysteresis current control with the function of output current ripple cancellation is proposed in this paper. Then deeplyanalyses its operating principles and designing on circuits parameters, also verifies thecorrectness on circuits theory via saber software.A31W bridgeless and electrolytic capacitor-less prototype has been built and tested inthe lab. the entire circuit is able to eliminate the electrolytic capacitor while maintaining highpower factor and efficiency, also can achieve accurate constant output current. Theexperimental results are presented and verify the feasibility of the proposed methods.