This issue of CTO technology is aimed at these issues. Interview with Xie Qitang, deputy general manager of Juhong Electronic Optoelectronics Group, he believes that in design, the primary condition for optimal color mixing must be achieved. This is to be overcome by optical design techniques; Generally, high power and low power LEDs are used, each with some disadvantages. However, if a middle power LED is used in conjunction with the lens design, it is a compromise solution, which will be described in detail below. In addition, he also believes that because the price is still too high, the era of LED backlights for large-size LCD TVs will not come so soon, and everyone may be too optimistic.
The price is too high, the LED backlight grows up and waits for 2 years.
The brightness of LEDs continues to increase, with a multiplier increase every 23 years, which has led to the trend of LEDs as a variety of light sources today, such as lamps, outdoor large advertising light boxes, or LCD backlights in this article.
In terms of luminous efficiency, LEDs are lower than the cold cathode tubes (CCFL" target="_blank">CCFL) used in the current stage, and large-size LCD panels are actually facing competition from other flat-panel displays such as PDPs; and CCFLs are not There is no future at all. Although the EU has imposed stricter restrictions on environmental protection considerations, the mercury content below 5mg can still be input. Therefore, LED as a backlight is not as optimistic as the market speculation.
The most important problem is the price. In the past six months, the RGB LED used in the LCD TV backlight has no price, mainly because the demand has not yet increased. Based on the experience of contact with major LCD TV manufacturers in Japan and South Korea, most Japanese and Korean TV manufacturers hope to have LED-backlit TVs close to mass production specifications at the end of the year. This shows that LED backlights are the mainstream of TV in the future, but the market takes a while to form. And the amount gradually increases in order to reduce costs and further drive the market; it is generally estimated that the starting power of the real growth of the LED backlight TV market should be two years later.
Multiple chip improves luminous efficiency
LED luminous efficiency is poor, for example, CCFL lamp luminous efficiency is about 60100lm / W, white LED luminous efficiency, if Luminade 1W can drive 40lm / W luminous efficiency, 3W can drive 65lm / W luminous efficiency, you can see At present, the luminous efficiency of white LEDs is still inferior to CCFL.
However, this can also be improved by changing the chip architecture. For example, Citizen uses a multiple chip design to make 1.2W/output 84 lm, and 3W can even drive 210 lm/W in different packaging modes. This technology is expected to be used in the R, G, and B architectures in the future.
What kind of technical architecture is the multiple chip? The main method is to cut the original chip into small pieces, each of which gives less power, so that the individual power consumption is lower, the heat generation is smaller, and the luminous efficiency can be effectively improved. Citizen's design is to cut into about 8 small chips of 0.2mm × 0.2mm, and use four in the middle, four outer corners to surround the four, and then packaged into an LED module.
The luminous efficiency of white LEDs is lower than that of CCFL. The average luminous efficiency of R, G, and B LEDs is lower than that of white light. Therefore, improving luminous efficiency has also become the direction of research and development.
Heat dissipation problem can be solved by color mixing technology
In addition to the low luminous efficiency, heat dissipation is another problem to be solved, but this should not be a serious problem. It can be solved by the existing off-the-shelf heat dissipation technology, but there is a heat distribution with high temperature at the heat source and low peripheral temperature. If the problem is uneven, if the loop heat pipe is used, the heat can be evenly distributed. However, if used in large-size TVs, the amount of LHP used will double, and the price is too high, which is another problem that needs to be faced.
As for color mixing, it is a difficult technical obstacle, and optical must be done very well. Because basically, the design of the LED backlight board does not want to use too many numbers, but if less light sources are used, the brightness will be uneven. In this case, optical technology is used to mix the light, so that the light can be expanded. It is open and evenly covers a larger area of ​​illumination.
There are also a variety of ways to mix light. Some designs use a combination of methods to first mix light at the source, but doing so wastes energy. It is better to let the light shine directly, and then optically do the mixing.
It is also worth mentioning that the LED backlights mentioned here all use direct-type light source technology instead of side-light technology. The main purpose is to use the light guide plate to convert the point light source into a surface light source, and the energy consumption is large. It is not suitable for TV panels of large size, and the technical ability is not high when using sidelights. However, the use of direct light sources can not only achieve the high brightness of LEDs, but also the output brightness is sufficient for large LCD TVs.
Low power LED circuit design is not easy to control
LED backlight panel design, most of the LEDs used are low power and high power. At present, Taiwanese and Korean manufacturers use more low power, while Japanese manufacturers use more power. Both have their own advantages and disadvantages, as detailed below.
The low-power LED is driven by 20mA/chip. Usually, the R, G, and B LED chips are packaged together into a single module. The number of low-power LEDs must be increased. For example, a 40-inch panel requires 10002,000. Due to the high density of LED arrays, thinner thicknesses can be achieved without special optical design, and 2.53 cm is not a problem.
The disadvantage of Low power LED is that in large-size TV applications, the number of pixels is too large, the circuit control is not easy, and the area of ​​illumination of each LED does not overlap. If a single LED has problems in light intensity or hue, it looks like It will be very obvious, so the requirements for LED quality and stability are quite high.
High power LEDs usually use G, R, B, and G to form a module with fewer numbers, easier circuit design, and better control. With specially designed optical components on the panel, it can produce overlapping effects. The adjacent light source modules can cover each other. Therefore, the requirements for LED quality and stability are relatively low, even once there is one LED. The problem will not be seen too clearly.
In terms of disadvantages, because it is driven by 1W, the luminous efficiency is poor, and it is easy to locally overheat, and heat treatment is required; in addition, the thickness is not easily reduced to within 3 cm.
Shown is a backlight board using high-power LEDs, in which the left half is a complete backlight, showing uniform color mixing, and the right half is a partial view without diffusing plates, showing the internal design architecture. The figure on the right shows the thickness of the ruler, which is 34 cm. (Source: Juhong Electronics)
Zone control is a big goal for LED backlights
In addition to the high brightness, color saturation, mercury-free and long life, the LED backlight has an important technological development. It must be realized by using LED as a backlight. It is regional control. ), this technology is also a big significance to promote LED backlights. Nowadays, Japanese and Korean manufacturers are moving towards the development of LED backlights.
The area control should be able to be controlled finely, the LED density must be high, and the high power LED is more dispersed. If the brightness of the light source is changed as the picture changes, the affected area may be too large, which is another disadvantage of high power.
From the area control fineness, circuit control complexity and backlight thickness evaluation, in fact, using the driver power of 0.5W / piece of middle power is a compromise method, the use of the number does not need to be as much as low power, plus The design of the upper optical lens can be like the high power method, which can make the light to overlap, improve the LED quality tolerance, and has the advantages of thinning the backlight like low power.
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