An Improved LED Dimming Circuit To Reduce Line Current Distortion With High Input Voltage Or Light Load

At present people pay more and more attention on energy saving and environment protection because of increasing energy crisis around the world. Illumination is one important energy-savable aspect since it is a great deal of energy consumption. Under that circumstance, Light emitting diode (LED) lighting is considered as one of high technology with wide perspective of development in 21st century due to its low power consumption, free pollution, long lifetime and fast response speed.This research mainly focuses on resolving the problems existing in the design process of high power LED lighting system. One aspect is to analyze the effects of different dimming method on luminance efficiency, color temperature, color coordinates, dimming linearity, dimming range and dimming efficiency of light system. Based on that, it can be concluded that advantages and disadvantages of dimming performance with Analogy Modulation (AM) and Pulse-Width Modulation (PWM). It would be the fundament to study high efficiency, high quality dimming method for LED light system.The second aspect which is also the major point of this research is to analyze the reasons of line current distortion of Critical Conduction Mode Single-Stage Flyback Active Power Factor Correctors (CRM SS Flyabck APFC). According to previous analysis, an improved control method is introduced in to reduce line current harmonics by injecting square component of sin(θ). Injected ratio of square component of sin(θ) depends on the variable range of input voltage and output power, consequently it can be fixed in a reasonable range.On the basis of theoretical analysis, the improved control strategy with prop injecting ratio is proved effective on a 30W prototype. In accordance with experimental results, the improved control method with proper injecting ratio of sin(θ) square component, line current peak distortion can be greatly eliminated and the converters can reach lower THD to satisfy the line current harmonics standard of Solid-Stage Lighting(SSL) required by IEC61000-3-2 and Energy Star.