Design and implementation of direct LED backlight

         LCD display is mainly composed of LCD (liquid crystal panel) and backlight. Because the LCD itself does not emit light, the LCD needs to be transmitted or reflected by an external light source. Most of the existing LCD is a transmission device with a backlight, and the luminous characteristics of the backlight will directly affect the picture quality of the LCD display device, so the backlight is a very important component of the liquid crystal display.

    The low-priced, mature CCFL (cold cathode light tube) is still the main light source of the backlight of existing liquid crystal display devices. However, LED (light-emitting diode) as a display device application field is also more and more extensive, especially in the LCD TV function is getting stronger and stronger, the structure is more and more beautiful situation, LED is used in various parts of LCD TV, and in the rapid update, high brightness, high quality, multi-function is a main purpose of the current new products.

      LED backlights can currently be roughly divided into two types: direct and edge-lighted. The straight down type is designed with LED lights on the entire back of the LCD display: The edge-lit type is designed with LED lights on the edge of the LCD display. In general, a direct backlight is applied in a large-size LCD TV of 30in or more, and a side-lit backlight can be used below 30in.

        The direct LED backlight designed in this article, with a size of 47in, involves the backlight structure and circuit design, including the design of the arrangement of the LED lamp, which is based on the circuit board. At the same time, the size, shape, and component arrangement of the circuit board are determined according to the structure of the backlight. After mounting the circuit board, cover it with the required optical film and install various components to assemble a complete backlight.

1 Comparative analysis with CCFL technology

1.1 Features of CCFL

  CCFL is a sealed glass tube filled with an inert gas, and when a high voltage is applied to the tube, the gas ionization produces ultraviolet (UV) light, which excites the internal phosphor powder coating, producing visible light.

  Features of CCFL include white light source, low cost, long life (>25,000 hours), high voltage drive, and mercury containing toxic gases.

  At present, using CCFL as a backlight is the mainstream of LCD TVs. However, CCFL has an essential shortcoming, that is, the color gamut (that is, the color performance range of the TV) can be achieved, and ordinary CCFL can generally only reach

72% of the NTSC (National Television System Committee) color gamut standard, and it contains mercury, a toxic gas, which does not meet the requirements of green environmental protection.

1.2 LED features

  LED has many advantages as a new light source, such as energy saving, environmental protection, small size, fast response, long life, etc.

  The most significant advantage of LED as a backlight is that it can provide unprecedented color reproduction, by selecting a suitable short-wavelength LED and a matching color film, the color reproduction range of the LED backlight can reach 105% of the National Television System Committee standard, and can even reach more than 1 20%. In terms of picture quality, the LED backlight is displayed more vividly and brightly.

  But LED also has some disadvantages, first of all, the price, in a large size range, the price of LED backlight is about 3~5 times that of CCFL backlight: followed by luminous efficiency, the highest luminous efficiency of commercial LEDs is about 60~801m/W, and the luminous efficiency of CCFL is about 80~1001m/W; Finally, heat dissipation, for large-size backlights, LEDs will emit a lot of heat when working.

  However, with the development of technology, the shortcomings of LEDs are gradually improving:

  1) Luminous efficiency. Since CCFL is cylindrical, the light it emits is directional. When it is placed under the LCD screen, despite the semi-circular mirror, quite a part of the light is still wasted. LEDs usually use side light radiation, and its light energy is more effectively used. Therefore, on the whole, its efficiency is still higher than CCFL, and the luminous efficiency of LED is still improving year by year;

      2) The proposal of dynamic backlight technology not only solves the problem of heat dissipation, but also obtains a series of benefits such as uniform brightness and reduced power consumption.