LED Backlight Technology From LED LCD TV

1. Brief description of the characteristics of LED TV

LED TVs are directly imaged by LEDs, using the principle of active light emission. The color LED display screen uses R/G/B three LED tubes to form a pixel point, and realizes the image display by controlling the on/off of each pixel point. The imaging principle is very similar to the active luminous plasma. 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 TVs are better than those of LCD TVs, the price is higher, so the current sales volume is much smaller than that of LCD TVs, and it will take some time for them to replace LCD TVs.

2. What is LED backlight

From the principle point of view, the liquid crystal display is somewhat like a street advertising light box. The light source behind it is diffused to a certain extent and irradiated evenly on the transparent film with the picture, thus transmitting a bright picture. The LCD panel is like that layer of film, but its picture is changing, as shown in Figure 2.

Traditional liquid crystal 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, but the tube is very thin, as shown in Figure 3.

LED backlight is to use LED light-emitting diodes instead of traditional fluorescent lamps (CCFL), so as to be the backlight of LCD TVs. To put it simply, 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 practical applications, the light-emitting diodes are not necessarily made into lamp tubes, but can also be arranged in arrays. As shown in Figure 5.

3. Introduction to the principle of LED lighting

LED is the abbreviation of English Light Emitting Diode (light emitting diode), its basic structure is a piece of electroluminescent semiconductor material, placed on a leaded shelf (as shown in Figure 6), sealed with epoxy resin around, To protect the internal core wire, so the LED has good seismic performance.

The core of the light-emitting diode is a wafer 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 a PIN junction. In the PN junction of certain semiconductor materials, when the injected minority carriers and the majority carriers recombine, the excess energy is released in the form of light, thereby directly converting electrical energy into light energy. With reverse voltage applied to the PN junction, it is difficult to inject minority carriers, so it does not emit light. This kind of diode made by the principle of injection electroluminescence is called light-emitting diode, or LED for short. When the LED is in the forward conduction state (that is, the positive voltage is applied to both ends), and the current flows from the anode to the cathode of the LED, the semiconductor crystal emits light of different colors from ultraviolet to infrared. II. 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 devices. This indicator refers to the degree of saturation that the display device can display on various colors in the entire color space, that is, to what degree of blue, green, and red can be displayed. Traditional LCD TVs usually use cold cathode fluorescent lamps (CCFL) as the light source. In daily life, it is often seen that the colors of many objects under fluorescent lamps are distorted. This is because the light emitted by fluorescent lamps is not white light in the true sense, so the display colors are not rich enough, the color reproduction is poor, and the color gamut that can be covered The range is only 65% to 75% of the NTSC standard. The LED backlight can extend the color gamut of LCD TVs to 105% of the NTSC standard, and can basically reproduce the colors of natural scenes.

Fourth, the composition scheme of LED backlight

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

When RGB-LED is used to compose the backlight source, it is partially in accordance with the number of 1:1:1, that is, each unit is composed of one red, blue and green LED, as shown in Figure 7. At present, the most common solution is that each unit consists of 1 red, 1 blue and 2 green LEDs. Because this combination method can effectively avoid the difference in LED luminous efficiency.

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

Although the performance of RGB-LED TV is good, it also has the following disadvantages: one is that there is no advantage in cost; the other is that a separate dimming circuit and a better heat dissipation structure are required, which will also lead to a complicated TV structure to a certain extent. In addition, if one of the backlight sources is broken, the image brightness at that point is obviously abnormal.

White LED is much simpler than RGB-LED. It replaces CCFL fluorescent tube with LED light source that can only emit white light. Since white LEDs do not require dimming of the backlight source like RGB-LEDs, the requirements on the circuit structure are relatively low. Although the color performance of white light LED is not as good as RGB-LED, it is worse than CCFL tube. In addition, each LED can complement each other in light emission. Even if a certain LED is damaged, it can basically ensure the uniformity of brightness. In addition, it has a cost advantage. White LEDs have become the preferred solution for popular LED LCD TVs.

5. How to install LED backlight

In the installation of LED backlight, according to the position of the person shooting, it can be divided into two categories: direct type (replace the entire back of the display screen with LED) and side person type (LED on the periphery).

The direct-lit backlight 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 on the bottom and sides, and then uniformly emitted through the diffuser and optical module on the surface, as shown in Figure 8.

The thickness of the direct-lit backlight is determined by the distance between the bottom of the light box and the diffuser. Generally, the thicker the thickness, the better the light uniformity of the backlight. When the backlight is thinner, the uniformity of color and brightness becomes the key technology of the direct backlight. The light field distribution type of the LED lamp plays an important role in the color and brightness uniformity of the backlight. Currently, LED lamps used in backlight sources usually have two types: Lambertian type and edge-emitting type. Since edge-emitting LEDs are more conducive to the light uniformity of the backlight, most direct-lit backlights use edge-emitting LEDs, but it is not suitable to use large-angle side-emitting LEDs. This is because the light intensity is low, which is easy to cause dark spots. Affect the uniformity of the backlight.

Side entry type means that the LED is installed 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 reflective sheet is needed at the back, so that the TV can be made very thin, but the light source utilization rate of this method is lower, and the thinner the light source, the lower the utilization rate, the maximum is about 50% .

Six, the advantages of LED backlight

1. Low working voltage

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

2. High efficiency and low consumption

The power of LCD color TVs using LED backlights is 1/3 or more lower than that of the same size LCD color TVs using CCFL backlights, which is incomparable to plasma color TVs. If a 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 LED has a lifespan of up to 100,000 hours (light decay is 50% of the initial), high stability, and no toxic substances such as mercury, which is environmentally friendly.

3. Short response time and good shock resistance

Traditional CCFL tubes have a low luminous frequency, and dynamic scenes may cause picture jumps. LED backlights can flexibly adjust the luminous frequency, and the frequency is much higher than that of CCFL, so it can perfectly present moving pictures. In addition, the planar structure allows the LED to have a stable internal structure with extremely high seismic performance, which is much stronger than the CCFL tube.

4. Wide adjustment range of brightness and contrast

LED brightness adjustment range is large, unlike CCFL, there is a threshold for the lowest brightness. The LED backlight white balance can ensure the overall contrast and brightness at the same time. LED is a flat light source. The most basic light-emitting unit is a small square piece with a side length of 3mm ~ 5mm, which can be easily combined to form a surface light source of a given area, and its brightness has a high uniformity, and at the same time, it needs auxiliary The optical components are also very simple.