Design and implementation of LED backlight module

introduction

The LCD screen itself does not emit light. In order to clearly see the content of the LED display, a certain amount of white light backlight is required. The backlight is an optical component inside the LCD display, which is composed of a light source and necessary optical auxiliary components. If the LED backlight is increased by contrast, regional control and other means, the performance is far better than the CCFL backlight. At present, LED backlights have been popularized in small and medium-sized panels, such as mobile phones, digital photo frames, etc. With the development of LED technology and the continuous maturity of LED chips, LED LCD TVs may gradually replace traditional CCFL LCD TVs.

Backlight sources are divided into direct-lit backlight sources and edge-lit backlight sources according to the position of the light source. In direct-lit backlight sources, the LED light source is placed directly below the light-emitting surface. The light emitted by the light source is diffused and mixed by the diffusion plate over a certain distance and then emitted as a surface light source. Direct-lit backlight sources require a certain light mixing distance. The edge-lit backlight structure is more advantageous for ultra-thin module design. With the rise of the "ultra-thin trend", large-size ultra-thin edge-lit LED backlight sources have become a research hotspot for major TV manufacturers and upstream companies.

1 Structural design

The LED backlight designed in this paper is an ultra-thin side-lit structure. The entire structure includes: LED light strip, driver board, film material, light guide plate, heat sink, upper frame, and backboard. The backlight uses white light LED. The entire structural design takes the center point of the Active Area as the design center of all components, and designs other dimensions based on the size of the LCD screen used in LCD TVs. The structure is designed by comprehensively considering the requirements of circuit design and optical design. The structural design starts with the UⅣOUT layout diagram to express the overall mechanism and the assembly relationship between each component, and then proceeds to the parts drawing structure design. The upper frame adopts a segmented design, which is segmented in the length direction to avoid excessive length. The outlet ends of the light strip are set at the four corners of the backboard to reduce the winding length.

2 Optical Design

The function of the backlight module is to transform the light emitted by a point light source into a surface light source through diffuse reflection. In order to obtain a qualified surface light source, the first thing to do is to select a suitable LED. This design uses a white light dual-chip LED. By presetting the white field photometric index and combining the research and analysis of influencing factors such as the LCD screen and optical film, the calculation of the luminous flux required for the entire backlight source is completed. Based on the calculated luminous flux, combined with the optical characteristics of the LED, the number of LEDs required is calculated. Table 1 shows the electrical and optical characteristics of the selected LEDs.

Table 1 LED electrical and optical characteristics

In order to improve the brightness of the backlight source, the film material structure is matched as follows: a layer of diffuser + a layer of BEF + a layer of DBEF. Among them, the role of the diffuser is to atomize the light by refraction and reflection of the diffuser material, so that the emitted light is more uniform: the role of BEF is to gather the light so that it enters the liquid crystal module vertically to increase the brightness: DBEF uses 50% of the light originally absorbed by the traditional absorption polarizer to increase the brightness. The diameter of the light guide plate mesh point is 0.54~1.55mm. Figure 1 shows the distribution diagram of some mesh points of the light guide plate.

Figure 1 Light guide plate dot distribution diagram

3 Circuit Design

The LED backlight circuit design mainly includes the light bar design and the drive control circuit design. Figure 2 shows the structural block diagram of the entire circuit part.

Figure 2 Circuit structure diagram