Talking about LED backlight technology from LED LCD TV

Recently, two types of flat-panel color TVs, LED LCD TV and LED TV, have appeared on the market, and their appearances are not much different. Since LED TV is a new generation of TV with lower energy consumption, thinner body and better image effect, many TV manufacturers promote LED LCD TV in the name of LED TV to confuse right and wrong. In fact, LED LCD TV is the abbreviation of "LED backlight LCD TV". As the name suggests, this type of TV is still a branch of LCD TV. It just replaces the CCFL backlight in ordinary LCD TV with LED backlight.

1. Brief description of LED TV features

LED TV uses LED to directly form images, using the principle of active light emission. The color LED display uses three LED tubes R/G/B to form a pixel, and the image is displayed by controlling the on/off of each pixel. Its imaging principle is very similar to that of plasma, which is also actively emitting light. There is no backlight circuit inside the LED TV, and the whole machine is very thin (about 2cm), as shown in Figure 1. At present, although the image effect and energy efficiency of LED TV are better than those of LCD TV, the price is higher, so the current sales volume is much smaller than that of LCD TV, and it will take some time for it to replace LCD TV.

2. What is LED backlight?

In principle, LCD is a bit like a street advertising light box. The light source behind it is diffused and evenly illuminates the transparent film with the picture drawn on it, thus transmitting a bright picture. The LCD panel is like that layer of film, except that its picture changes, as shown in Figure 2.

Traditional LCD display devices use CCFL (Cold Cathode Fluorescent Lamps) as the light source. Although the name is very professional, we understand it as a household fluorescent lamp, except that the lamp tube is very thin, as shown in Figure 3.

LED backlight is to use LED light-emitting diodes to replace traditional fluorescent lamps (CCFL) as the backlight of LCD TV. Simply put, the original fluorescent lamp filled with inert gas is replaced with a tube containing light-emitting diodes, as shown in Figure 4. Of course, in actual applications, light-emitting diodes do not have to be made into tubes, but can also be arranged in arrays, as shown in Figure 5.

3. Introduction to LED light emitting principle

LED is the abbreviation of Light Emitting Diode. Its basic structure is a piece of electroluminescent semiconductor material placed on a rack with leads (as shown in Figure 6). It is sealed with epoxy resin on all sides to protect the internal core wire, so LED has good shock resistance.

The core of the light-emitting diode is a chip composed of P and N type semiconductors. There is a transition layer between the P type semiconductor and the N type semiconductor, which is called the PIN junction. In the PN junction of some semiconductor materials, when the injected minority carriers recombine with the majority carriers, the excess energy will be released in the form of light, thereby directly converting electrical energy into light energy. When the PN junction is applied with a reverse voltage, it is difficult for minority carriers to be injected, so it does not emit light. This diode made using the principle of injection electroluminescence is called a light-emitting diode, or LED for short. When the LED is in the forward conduction state (that is, a forward voltage is applied to both ends), when the current flows from the LED anode to the cathode, the semiconductor crystal emits light of different colors from ultraviolet to infrared. The intensity of the light is related to the current.

In the video field, people generally use NTSC as a standard to measure the color reproduction characteristics of video equipment. This indicator refers to the saturation of various colors that the display device can display in the entire color space, that is, the degree of blue, green, and red that can be displayed. Traditional LCD TVs usually use cold cathode fluorescent lamps (CCFL) as light sources. In daily life, we often see that the colors of many objects under fluorescent lights are distorted. This is because the light emitted by fluorescent lamps is not truly white light, so the displayed colors are not rich enough, the color reproduction is poor, and the color gamut that can be covered is only 65% ​​to 75% of the NTSC standard. LED backlights can expand the color gamut of LCD TVs to 105% of the NTSC standard, and can basically reproduce the colors of natural scenes.

4. Composition of LED backlight source

When using LED as a backlight source, there are two options: one is to use RGB-LED, and the other is to use white light LED.

When using RGB-LED to form a backlight source, the number of LEDs is 1:1:1, that is, each unit is composed of 1 red, 1 blue and 2 green LEDs, as shown in Figure 7. At present, the most common solution is that each unit is composed of 1 red, 1 blue and 2 green LEDs. This combination can effectively avoid the difference in LED luminous efficiency.

The advantages of using RGB-LED backlight TVs are mainly in terms of color expression and contrast. This is because the use of independent RGB primary color light-emitting elements can exceed 120% of NTSC, and some models can reach 150%, which completely reaches or exceeds the level of plasma TVs. RGB-LED TVs support backlight area adjustment technology, and brightness adjustment is easy to achieve. Therefore, in terms of contrast, it can often achieve very high dynamic contrast, which can greatly improve image quality.

Although RGB-LED TVs have good performance, they also have the following disadvantages: first, they have no cost advantage; second, they require a separate dimming circuit and a better heat dissipation structure, which will also make the TV structure complex to a certain extent. In addition, if one of the backlight sources is broken, the image brightness at that point will be obviously abnormal.

White LED is much simpler than RGB-LED. It uses LED light source that can only emit white light instead of CCFL fluorescent tube. Since white LED does not need to involve backlight dimming like RGB-LED, the requirements for circuit structure are relatively low. Although the color performance of white LED is not as good as RGB-LED, it is worse than CCFL tube. In addition, each LED can complement each other in emitting light. Even if one LED is damaged, the brightness can basically be guaranteed to be uniform. In addition to the cost advantage, white LED has become the preferred solution for popular LED LCD TVs.

5. Installation method of LED backlight

In the installation of LED backlight sources, they can be divided into two categories according to the position of the light: direct-down type (the entire back of the display is replaced with LEDs) and side-down type (LEDs are placed around the periphery).

The direct-type backlight source is easy to install and does not require a light guide plate. The LED array is placed at the bottom of the light box. The light emitted from the LED is reflected by the bottom and sides, and then evenly emitted through the diffuser plate and optical module on the surface, as shown in Figure 8.

The thickness of the direct-type backlight source is determined by the distance between the bottom of the light box and the scattering plate. Generally, the thicker the thickness, the better the light uniformity of the backlight source. In the case of a thin backlight source, color and brightness uniformity become the technical key of the direct-type backlight source. The light field distribution type of the LED lamp plays an important role in the color and brightness uniformity of the backlight source. At present, the LED lamps used in the backlight source are usually Lambertian and edge-emitting. Since edge-emitting LEDs are more conducive to the light uniformity of the backlight source, direct-type backlight sources mostly use edge-emitting LEDs, but large-angle edge-emitting LEDs should not be used. This is because the light intensity is low, which is easy to cause dark spots and affect the uniformity of the backlight source.

The side-entry type refers to installing the LED on the side of a specially designed light guide plate, as shown in Figure 9. Because the light source is placed on the side of the light guide plate, only a reflector is needed at the back, so the TV can be made very thin, but the light source utilization rate of this method is low, and the thinner the TV is, the lower the utilization rate is, with a maximum of about 50%.

6. Advantages of LED backlight

1. Low operating voltage

LED uses a low voltage power supply, the supply voltage is between 5V and 24V, so the boost circuit of the cold cathode backlight is omitted, the safety factor of the power supply is greatly improved, and it is especially suitable for public places. At the same time, the design of its power supply module is also quite simple.

2. High efficiency and low consumption

The power consumption of LCD TVs with LED backlight is 1/3 or even lower than that of LCD TVs of the same size with CCFL backlight, which is unmatched by plasma TVs. If dynamic LED backlight is used, the minimum power consumption of a 47-inch LED backlight LCD TV can be as low as 10W.

In addition, the life of LED is up to 100,000 hours (light decay is 50% of the initial value), it is highly stable, and does not contain toxic substances such as mercury, making it green and environmentally friendly.

3. Short response time and good shock resistance

The light emission frequency of traditional CCFL tubes is low, and dynamic scenes may cause screen jitters, while LED backlights can flexibly adjust the light emission frequency, and the frequency is much higher than CCFL, so it can perfectly present motion pictures. In addition, the planar structure gives LEDs a stable internal structure and extremely high shock resistance, which is much stronger than CCFL tubes.

4. Wide adjustment range of brightness and contrast

LED brightness has a wide adjustment range, unlike CCFL which has a minimum brightness threshold. LED backlight white balance can well guarantee the overall contrast and brightness. LED is a planar light source, and the most basic light-emitting unit is a small square piece with a side length of 3mm to 5mm. It is easy to combine together to form a surface light source of a given area, and its brightness is highly uniform. At the same time, the auxiliary optical components required are also very simple.