Research and application of low-cost direct-lit TV backlight system

1 Development Background

With the rapid development of the LCD industry in recent years, the field of liquid crystal display has continuously proposed technical solutions to reduce costs, and the direct-type display technology (backlight solution) of various TV manufacturers has become less and less differentiated. With the development of LED technology, the power of a single lamp has been continuously improved, and the backlight system has also been continuously optimizing costs by reducing the number of backlight lamp beads and diaphragms used. Taking the 55-inch OD35 as an example, the number of light bars has been reduced from 11×3 to the current 13×2, and the diaphragm remains DOP. According to statistics, the current 32~55-inch direct-type OD35 basically adopts a multi-light bar backlight system, and the diaphragm adopts a single diffuser or DOP solution. From the perspective of the development of display industry technology, the cost optimization of the direct-type backlight system is still developing in the direction of reducing LED lamps and diaphragms. The cost of the light bar is mainly composed of three parts: LED lamp beads, LENS and PCB. If only the number of LED lamps is reduced, the cost of PCB cannot be reduced, and the cost reduction space is limited. Therefore, we optimized costs by reducing the number of light strips, and developed a new single-light-strip direct-lit backlight system by improving the power of single lamps and conducting related technical research on new LENS. The single-light-strip backlight system was expanded from three sizes of 32, 43, and 55 inches to 40 inches and 50 inches, achieving the closed-loop goal of the technical development of single-light-strip backlight systems for direct-lit OD35 products below 55 inches.

With the development of optical LENS backlight technology, OD38/OD35 have tended to be the same backlight solution, and the price difference of the system has tended to be the same. Compared with OD35, OD28 backlight technology requires more LEDs and LENS than OD35 under the same optoelectronic parameters and subjective evaluation standards (the luminous efficiency of the refractive LENS of the same solution is significantly better than that of the reflective LENS). The final evaluation result is that OD35 has a significant price advantage. The LED lamp beads currently used in the industry are EMC3030 small chip lamp beads with a driving current of about 600 mA, but Korean brands have been continuously developing and innovating high-power chips and commercializing them, especially in new materials, new processes and other technologies. There has been a significant improvement in its reliability comparison. The following table collects optoelectronic solutions for backlight systems.

In the mid-to-low-end market, products tend to be homogenized, and product prices are the main means of competition among manufacturers. In the future, the direction of competition in the mid-to-low-end market will still be price competition. The application of the new backlight system can provide strong support for product cost reduction, and at the same time provide technical cost reduction support for the development of a new generation of mid-to-low-end products. The application of the new backlight system effectively reduces the material of the light bar, facilitates supply chain material management and production line assembly, and improves system efficiency. The new backlight system targets a single light bar, which reduces the amount of PCB, lamp beads and LENS compared to the current products, effectively reduces the material and production costs of the backlight module, and realizes the design concept of energy saving, emission reduction and green lighting. The new backlight system can meet the ultimate design of the back panel edge, achieve the effect of visual thinness (Figure 1), and can be applied to the design of a new generation of mid-to-low-end products, bringing changes in shape. At the same time, the single light bar backlight system has also increased the cabinet loading volume of exported products accordingly. The back panel's gentle edge design can lower the movement/power board by a certain distance, reducing the thickness of the whole machine.

Figure 1 Schematic diagram of the new backlight system backplane

The new backlight system is a major technological innovation in the industry, breaking through the current technology in the field of LCD backlighting on the market. At present, the backlight system of 32-inch TV OD35 basically uses a 6×2 two-light bar solution, and achieves the product brightness index and uniformity index requirements through the diffusion of LENS. Through preliminary research and analysis, the main difficulties of the single-light bar backlight system are two points: ① The LED power reaches the limit, and the brightness index requirements of the product cannot be met after reducing the lights; ② LENS cannot solve the problem of uneven visual effects of the backlight after reducing the lights.

The preliminary research of the new backlight system focuses on the improvement of LED luminous power and the improvement of the uniformity of the new LENS. In the direction of improving the LED luminous power, we jointly carry out relevant technical research with LED/chip manufacturers to develop new high-luminous power LEDs. Through development cooperation with upstream manufacturers, the foundation of LED light efficiency is laid. At the same time, the new high-luminous power LED can meet the needs of mid- and low-end products for reducing lamp costs, and has obvious cost optimization advantages for current products. In the development of new LENS, through optical simulation and test mold verification, the problem of visual uniformity of single light strips is solved with optimized LEDs, and the development of single light strip backlight systems is realized.

The single light bar backlight system reduces the number of LEDs and the area of ​​PCB used in terms of materials, making an important contribution to green environmental protection. At the same time, the single light bar backlight system effectively improves the competitiveness of our mid- and low-end products in terms of cost and appearance. Through this pre-research project, the team can master the key technologies of high-power LEDs and new LENS design and development technologies, which will effectively guarantee the company's future product design improvements.

2 Research items

2.1 Study on the photothermal performance of a new flip-chip high-power LED backlight

There are three ways to dissipate heat: convection, radiation and conduction. The LED package in the direct-type LED backlight module studied in this study is fixed between the lens and the PCB board, and there is almost no convection heat dissipation. Its main heat dissipation paths are conduction and radiation. For SMC LED packages, the heat generated by the chip is mainly dissipated by conduction to the lower substrate, except for a small amount of heat radiated outward through the phosphor layer. Therefore, the main research and comparison considers the impact of heat conduction.

Formal LED Chip

Flip Chip LED

Figure 2 LED comparison heat distribution diagram

From the LED comparison thermal distribution diagram, it can be seen that the thermal distribution of the flip-chip LED chip is significantly better than that of the face-mounted LED chip. The comparison of the photothermal characteristics of the two different packaging processes is shown in Figure 3.

Figure 3 Comparison of photothermal characteristics of two different packaging processes

2.2 Design of new large-angle optical LENS solution and verification and confirmation of test model samples

2.2.1 LENS design and experimental model verification

Optical lenses involve light source selection, optical lens optical/structural design, material selection, ultra-precision mold design/production, precision molding and mass production quality assurance. Each link is directly related to the high quality assurance of the product and requires repeated testing and modification.

2.2.2 Optical lens design and manufacturing

In terms of optical lens design, it is necessary to realize the transformation from LED point light source to uniform surface light source through optical simulation. Due to the short optical distance, in order to make the light-entering surface receive a large amount of available light source, different Fresnel features need to be made. Since these features are all microstructures, it is necessary to find industry optical-grade ultra-high precision mold manufacturing equipment, and repeated experiments and tests are required to achieve the best results. Due to the short optical distance, in order to achieve a large angle, different optical microstructures must also be added to the light-emitting surface to achieve a single-module light source with the best uniformity. Material selection is directly related to the injection molding quality of the product. The light-emitting quality of the optical lens is tested through different brands, different optical grades and grades. In order to increase the uniformity, the material model is finally confirmed through repeated testing and comparison.

The focus of the research is on mold making. Since it involves production capacity/quality, etc., it mainly evaluates and improves the mold opening drawings based on the design party's drawings, such as the mold release angle, parting surface setting, gate/cavity number arrangement, cavity number position, allowable deviation value, process radius, packaging and loading methods, direction identification, etc.

In the later stage, a comprehensive evaluation of the mass production of the supporting lenses will be carried out, including mold design and production, packaging design and production, injection molding machine tonnage, process matching and planning, and pre-production matching verification of mass production molds.