You are using an unlicensed and unsupported version of Evoq Basic. Please contact customersuccess@dnnsoftware.com for information on how to obtain a valid license.

Ultra-Slim LCD TV Enabled by an Edge-Lit LED Backlight System

As always, thin is in when it comes to flat-panel TVs. LCD designers and engineers have been striving to make their TV sets thinner and thinner, especially as the threat from OLED TV looms. This article examines the problems encountered when trying to develop an edge-lit LED backlight and how Samsung overcame these hurdles to introduce its 10-mm-thick 40-in.- diagonal LCD-TV prototype in late 2007.

by Taeseok Jang

THE ANSWER seemed to be so simple. LCD designers and engineers knew that they had to keep making their modules thinner and thinner, especially once it became apparent that OLED TV was on a path to commercialization. The backlight seemed to be the logical place to slim down the LCD-TV form factor without sacrificing image quality, and the best way to do this was to shift from a direct LED backlighting unit (BLU) to an edge-lit BLU, as had been done in the notebook-PC and desktop-monitor sectors.

However, it was quickly determined that such an edge-lit structure could not be directly applied to large-sized TVs without a number of careful considerations. Difficulties arise due to lowered mechanical rigidity and insufficient luminous flux. Samsung has overcome these issues, as demonstrated by their 10-mm-thick 40-in.-diagonal LCD TV with an LED edge-lit backlight system (Fig. 1, Table 1) that the company first demonstrated at FPD International 2007.

In this article, we will discuss these issues as they relate to ultra-slim technology for large-sized TVs.

Module Design and Thermal Management

When designing an extremely slim LCD-TV module, the first consideration must be the design of parts such as the top and bottom chassis, which are the external metal frames surrounding the panel, and all optical components because these can make the module more vulnerable to deformities such as bending and warping. When slimming down a large module, it is difficult to maintain a flat shape even in the absence of external force. This situation worsens when the power is on. Heat is generated from the LED light source and concentrated along the edges before being transferred to other parts such as the light-guide plate (LGP), optical sheets, and top/bottom chassis. In the steady state, a non-uniform temperature distribution is established, which causes local variations in thermal expansion. Eventually, this will result in complex mechanical distortion. A simulation showing the result of thermal distortion is shown in Fig. 2.

 

fig_1_tif

Fig. 1: The 10-mm-thick LCD-TV module.

 


Table 1: Key specifications of the LCD-TV module

Item Specification
Display size 40 in.
Resolution FHD (1920 x 1080)
Mode S-PVA
Module thickness 10 mm
Bezel size 14.6 mm
Luminance 450 nits
Color gamut (NTSC) 92% (CIE 1931)

 

For a rough estimate of the thermal deflection, consider a disk of radius R with a linear temperature gradient from T0 (center) to TR (edge). The amount of distortion (l) (depth of dish) can be calculated after some simplification as

l = πR[6ß(TRT0)]1/2

where ß is the thermal-expansion coefficient. This equation states that the distortion is proportional to the module size and the square root of both the material property (thermal coefficient) and the temperature difference. Usually, the larger the module size, the larger the variation in temperature, so overall distortion occurs readily with larger modules. As an example, Fig. 3 shows the LGP's temperature distribution when the LEDs are placed along the circumference of the LGP. The actual amount of module distortion is derived from the vector sum of each component, including the LGP, the bottom and top chassis, and the LCD panel.

It is very difficult to circumvent this problem with an acceptable increase in cost. For example, aluminum could be considered for use as the bottom chassis material because it has an higher thermal conductivity compared to electro-galvanized steel (SECC) and can provide benefits for heat dissipation. However, use of aluminum inevitably results in increased cost and reduced mechanical strength. Some auxiliary parts need to be added to supplement the mechanical strength while maintaining a high level of heat dissipation. Aside from the mechanical rigidity, a thermal spreader is of great help in preventing deterioration of the liquid crystal and for avoiding wrinkles in the optical sheets.

These types of problems for a slim-edge module quickly become more serious for increasing screen sizes. An optimal corrective approach must be taken from a holistic-system perspective in order to pass the various types of reliability tests required for mass production. If possible, it is highly advisable to work with the set manufacturer from the beginning of the design stage. Thermal issues are closely related to overall light efficiency and the level of power consumption. Development and use of highly efficient LEDs and optical components, such as the LGP and the system's optical sheets, are key factors for guaranteeing a sufficient level of commercial quality. Active backlight driving is also useful for reducing power consumption and heat generation. These points are discussed below.

Light Source and Optical System Efficiencies

LEDs are selected as the optimal light source over conventional cold-cathode fluorescent lamps (CCFLs), which present serious electrical and optical challenges in an ultra-slim LCD-TV module. Unlike a conventional direct-lit-type module, the light sources in an ultra-slim panel can only be placed along the edges of the module. However, tight spacing of CCFLs causes current leakage and a resultant severe decrease in electrical/optical efficiency. Furthermore, discharge instability can result.

When applying LEDs to edge-lit large TV modules, efficiency must be the first criterion considered, due to space constraints for LED placement and potentially severe thermal issues. There are many factors affecting the efficiency of the LEDs, such as the LED die (size, manufacturer, design, etc.), the package design, and the opto-mechanical characteristics of the module. While LED manufacturers are working diligently to increase the performance of their LED chips, it must be noted that packaging technology also has significant influence on the performance of the device. A package with low thermal resistance is critical for best efficiency, control of color shift, increased reliability, and longest life. For edge-type applications, the slug-type package, in which the LED chip is mounted on a metal slug for heat removal, needs to be widely adopted. Packaging with ceramic material is also highly recommended, even in view of increased cost.

 

fig_2_tif

Fig. 2: Simulated thermal distortion of an edge-lit LED module.

 

fig_3_tif

Fig. 3: Temperature distribution of the LGP.

 

In addition, the method by which white color is produced is critical because this decision can have a significant effect on color gamut and light efficiency. Table 2 shows a rough estimate of the projected efficiency and color gamut derived from the various methods of producing white light. Note that the efficiency is normalized to the case of the blue chip plus yellow phosphor combination. Each method has its own strengths and weaknesses; therefore, selection should be based on the target application and required specifications. For the edge-type TV application, the designer's first priority should be in maximizing efficiency as emphasized above. Case 2 might be most common, due to its simple driving circuit and reasonable color gamut. However, Cases 3 and 4 represent better choices in terms of efficiency. Case 4 looks superior to Case 3 in that it has better efficiency and a wider-color-gamut. However, Case 4 may require a color-control circuit, as would Case 5 (RGB LEDs) because the blue and red chips age at different rates and the time required for blue versus red chips to reach steady-state emission levels differs significantly.

The same argument applies to the LCD's optical components including the LGP and optical sheets. The light-extraction pattern of the LGP is formed by a CO2 laser – an improvement compared to the typical "scattering ink" pattern in screen-printing. The resulting microgroove is carefully controlled to provide about 10% higher efficiency, due to its reduced optical loss. As shown in Fig. 4, the microgroove has a well-defined shape that reduces the scattering properties. The amount of light extraction is controlled by the pattern pitch, the duty ratio, and the width. Also, the angle of the groove will determine the angular profile of the extracted beam. The proper combination of optical sheets is followed by careful application of the LGP to maximize efficiency and to provide best overall appearance.

Global Dimming

As a non-emissive display device, an LCD backlight typically consumes constant (maximum) power over time and emits a constant amount of light regardless of the final image seen by the viewer. Even from an early stage, active driving of the backlight has drawn considerable attention, and it has already been adopted for use in some high-end TVs. This technology adjusts the backlight luminance according to the image depicted, which results in reduced power consumption and a great deal of enhancement to the contrast ratio. Active-driving technology can be classified by the control method as global dimming, 1-D local dimming, 2-D local dimming, or three-way local dimming (color dimming). For an ultra-slim module with an edge-lit backlight, global dimming is most appropriate due to its relatively simple dimming algorithm and driving circuit, in addition to consideration of the physical layout of the light source.

 


Table 2: Comparison of light efficiency and color gamut for different methods of white-color production

Case
White LED
Efficiency
Color Gamut (CIE1931)
Main Applications
1
Blue chip + Yellow phosphor
100%
<70%
Notebook PC
2
Blue chip + Red/Green phosphor
~75%
~83%
TV
3
Blue chip + Green/Orange phosphor
>90%
~72%
4
Blue/Red chip + Green phosphor
>90%
~92%
5
Red/Green/Blue chip
~60%
>100%
Notebook PC, Monitor, TV

 

fig_4a_tif (a)  fig_4b_tif (b)

Fig. 4: Magnified view of the microgroove patterns of the LGP formed by a CO2 laser: (a) top view; (b) side view.

 

Figure 5 shows the working principle behind the global-dimming technique being used in our edge-lit TV module design. Figure 5(a) shows the gamma curve of conventional global dimming. Without dimming, the gamma curve follows the red curve at low gray levels due to considerable light leakage, which can cause significant loss of contrast ratio. Conventional global dimming works in the low gray region mainly for the purpose of increasing contrast ratio, so the amount of power savings is not significant overall.

With the advanced global dimming shown in Fig. 5(b), the input image data is converted and then transferred to the timing controller just as if the gamma curve were changed to, e.g., 1.8 (red curve) from the ideal value of 2.2 (blue curve). The ideal gamma level is achieved by backlight dimming over the entire gray-level range. The difference between the two curves corresponds directly to the amount of power reduction at a given gray level. When this technique is applied, the final gamma curve nearly coincides with the ideal (2.2) gamma curve and results in much higher (dynamic) contrast ratio. Compared to the pixel-compensated global-dimming algorithm, which needs memory to store the gray-level value for each pixel, this algorithm only requires gray-level averaging with some attention to maximum data level. Therefore, advanced global dimming provides a highly cost-effective performance-enhancing solution compared to the pixel-compensated global dimming technique. Of course, modifications may be needed to eliminate certain types of artifacts, including darker images in the low gray range and flickering due to abrupt changes in backlight luminance. Importantly, the power savings attained by the advanced global-dimming technique will also result in a substantial decrease in the temperature of the module.

Conclusion

Although edge-lit backlighting technology is mature in smaller-sized applications such as notebook PCs and desktop monitors, until now it has not been scalable to large-sized LCD TV. Significant technical barriers must be overcome in order to be able to mass-produce an edge-lit ultra-slim TV module. First, the module needs to attain a sufficient level of mechanical strength and thermal reliability. Accomplishing this will require panel makers to cooperate more closely with set makers. Secondly, the edge-type TV module must be energy efficient. To achieve this, it is more cost effective to employ an efficient light source and improved optical components than to try to solve the problem simply by way of thermal management. With this point in mind, the edge-lit ultra-slim TV can indeed contribute toward a green TV solution. Finally, the advanced global-dimming technique plays a particularly useful role in reducing overall power consumption, hence increasing efficiency and improving reliability. This technique also provides considerable enhancement to the contrast ratio and results in nearly ideal gamma characteristics. Therefore, it is expected that the ultra-slim edge-lit TV module will become increasingly popular with ongoing improvements being made in panel transmittance and LED efficiency. •

 

fig_5a_tif fig_5b_tif
(a) (b)
Fig. 5: Operating principle of the global-dimming technique: (a) conventional global dimming; (b) advanced global dimming.

 


Taeseok Jang is Vice-President of the Backlight Technology Team, LCD Business, Samsung Electronics Co. Ltd., Asan-City, Chungcheongnam-do, Korea 336-841; e-mail: jangt@samsung.com.