In recent years, liquid crystal display technology has developed rapidly. The backlight technology, which is one of the core technologies of LCD display, has also been greatly developed. Whether on display or on television, LCD backlight products have rapidly gained popularity. In 2009, LED (light-emitting diode) backlight technology has reached a mature stage. Although LCD products with LED backlights are more expensive than LCD products with CCFL (cold cathode fluorescent lamp) backlights, the good picture quality is indeed the CCFL backlight LCD. Unmatched, only the naked eye can distinguish the obvious difference in image quality, plus LED backlight has a wide color gamut range, long life (up to 100,000 hours), large brightness adjustment range, low voltage operation The advantages of energy saving and environmental protection, LED era has completely replaced the CCFL era.
Regardless of the LED as a backlight or illumination source, it works most stably under constant current conditions according to the characteristics of LED operation [1]. At present, there are many circuits that supply LEDs. I only analyze the design of an LED backlight with a 66 cm LCD display.
1 LED backlight classification
1) Direct backlight
The direct-lit backlight is simple in process, does not require a light guide plate, and the LED array is placed at the bottom of the light box. The light emitted from the LED is reflected by the bottom surface and the side surface, and is evenly emitted through the diffusion plate and the optical module on the surface. The thickness of the direct-lit backlight is determined by the distance between the bottom of the light box and the diffuser plate. Generally, the thicker the thickness, the better the light uniformity of the backlight. In the case of a thin backlight, color and brightness uniformity become the key technology of direct-lit backlights [2]. The type of light field distribution of LED lamps plays an important role in the color and brightness uniformity of the backlight. At present, the LED lamps used in backlights usually have Lambertian and edge emission types, because edge-emitting LEDs are more beneficial to backlights. Light uniformity, direct-type backlights mostly use edge-emitting LEDs.
2) Sidelight backlight
The edge-lit backlight module is configured to set the point light source as a backlight on the side of the specially designed light guide plate, and the number of LEDs used is small; and because the light source is placed on the side of the light guide plate, the LED is limited by the number and brightness, and still needs to be When the light is evenly dispersed on the entire plane, the larger the panel area is, the more the brightness of the light source is applied. Because the LED backlight is placed on the side of the product, only the light guide plate needs to be added later to make the appearance thickness of the liquid crystal product Very thin, such as Sony's ZX1 series.
This design only analyzes the application of LED direct backlights.
2 Main technical requirements and design analysis
2. 1 Display related technical requirements
The machine is used in a semi-outdoor condition, which means that the video image can be clearly seen in the outdoor with the eaves and in the daytime.
1) Brightness index: 800 cd / m2;
2) Contrast: 800!1;
3) Signal input: AV, VGA;
4) Display resolution: 1 024 ∀ 768;
5) Single continuous working time: 20 h;
6) Temperature range: - 15 # ~ ​​45 # , (requires low temperature - 15 # once started);
2. 2 Design Analysis
According to the relevant technical requirements, to meet the requirements of brightness, continuous working time and operating temperature range, LEDs should be used as backlights, and switching power supplies with excellent performance should be used.
1) Select high-brightness LED chip, the main technical parameters are: working nominal voltage 3. 2 V, current 20 mA, luminous flux 80 lm /W. To achieve a brightness of 800 cd / m2, 1 728 LEDs are required for calculation. The LEDs are divided into 6 groups according to the selected power supply (24 V) (see Figure 1), where R 4 is the current limiting resistor. According to the 288 group arrangement, the direction of the line is 54 and the direction of the field is 32. The maximum drive power required is: P = 3.2 V ∀ 0. 02 A= 110. 59W.
2) Due to the small size LCD display, the design considers a single-group power supply constant current supply for the LED backlight. In order to ensure the reliability, the famous all-Han switching power supply, model number FSP205-4E01, is selected. The main electrical performance parameters are shown in Table 1 and Table 2. The power supply schematic is shown in Figure 2.
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