| Nowadays, LED backlights are being utilized more and more to flatpanel displays–from small to mid-size to larger LCDs which havetraditionally utilized cold cathode fluorescent lamps (CCFLs). A LEDbacklight system has many well-known advantages, such as wide colorgamut, low instance turn on, mercury free, low power consumption…etc.Therefore it is no doubt that LED backlight will replace CCFL backlightfinally in the future’s marketplace.In this paper, we chose “Direct Illumination” model instead oftraditional side-emitting illumination for our LED backlight system.Based on the analysis of layout of LED matrix array, novel triangularLED array unit and pyramid LED array unit are proposed for optimizingthe luminance uniformity. With these novel structures well illustrated anddesigned, the comparison simulation between traditional and novel layoutusing Lighttools software is executed to prove that the novel designperformed better on the parameter of uniformity.We select National-Semiconductor chip LM3404HV to drive ourbacklight system: with each one of them driving two pyramid units (8 LEDs in all), a sum of42chips is used to drive the84pyramid unitsseparately, that’s336LEDs in total, and through the driving circuitdesign and simulation we ensure that the entire backlight system workproperly.Finally the PWM dimming method is adopted for LED brightnessadjustment to achieve better contrast ratio, in order to test how thismethod work out, we choose typical duty circle of PWM signal at100%,80%,50%and20%and make different backlight areas LEDs work onseparate PWM control signal, and the simulation results meet our designrequirements.This paper systematically analyzed the characteristics of directillumination model of LED backlight, proposed two innovative LEDlayout units for optimizing the luminance uniformity, and carried out aseries of hardware and optical simulations to verify our designing goalsas well. It provides practical significance for the design of directillumination LED backlight. |
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