Analysis of technical advantages of LED for LCD backlight

LEDs are gradually replacing traditional cold cathode fluorescent lamps ( CCFL ) in more and more applications due to their advantages such as high brightness , long life, uniform color and space saving. From small mobile phone liquid crystal displays ( LCD ) to large 70-inch LCD TV panels , LEDs are rapidly becoming the preferred backlight technology.

Historically, LCD backlighting has used different light sources, including incandescent, fluorescent, and LED. The selection criteria that OEMs use to decide the best backlight technology for their applications include cost, brightness, uniformity, efficacy, lamp life, ruggedness, size, integration, and, more recently, environmental factors.

LEDs have been the backlighting technology of choice for small LCDs for many years, especially for handheld devices with a diagonal of less than 5 inches, such as cell phones, PDAs, and MP3 players. For larger screen sizes, CCFLs have traditionally been more cost-effective and have been backlighting most medium- and large-sized industrial displays. However, the transition to LEDs is underway, due to factors such as the increasing performance and steadily decreasing costs of high-brightness LEDs (HB LEDs), which are now comparable in brightness to CCFLs for backlighting larger displays.

Advantages of HB LEDs

HB LEDs have many important advantages over CCFLs in backlighting applications. HB LEDs can provide higher brightness than CCFLs, and, when properly integrated into the system, the LED backlight has a longer life. In addition, HB LEDs can operate efficiently over a wider temperature range, especially at low temperatures. In applications where it is difficult to provide the high voltage required by CCFLs, LEDs have a unique advantage because they can operate normally at low DC voltages. Other advantages of HB LEDs include increasingly higher light output per unit of electrical energy input and the ability to optimize the color gamut . Finally, the wide dimming capability of LEDs is also an important advantage in certain applications.

As the price of high-brightness HB LEDs continues to fall due to increased usage, many experts predict that HB LEDs will eventually reach the same price as CCFLs. This cost balance has prompted OEMs to choose HB LEDs for larger displays, such as GPS systems, portable DVD players, notebook computers, desktop monitors, and 6.5-20.1-inch LCDs used in industry. LEDs are also being used in large flat-panel LCD TVs ranging in size from 32 inches to 70 inches. Table 1 shows a comparison of LEDs with other commonly used light sources.

Control uniformity

When multiple LEDs are used to backlight larger LCDs, the light from the LED source must be spread over a larger area, and it is important to ensure that the brightness and wavelength of each color remain uniform to avoid bright spots and dark blocks. This is especially critical for direct projection LED backlight units (BLUs), where even small color differences can reduce the uniformity of the display . To better control uniformity, the wavelengths of each color of each backlight must be closely matched, with green and blue at 5nm, as they are the two most important colors.

During LED manufacturing, performance may vary slightly from the average values ​​given in the datasheet. For this reason, LED manufacturers group components by luminous flux , color, and forward voltage (V1) . Fine grouping of brightness and color can be used to achieve better consistency. For brightness, 1/4 grouping and 15% spectrum expansion per tube is the standard for brightness.

Optimizing LED lifespan

Proper thermal management of LED BLUs is important because the efficiency of the LED drops rapidly as the drive current and junction temperature increase, reducing brightness and shortening the life of the LED. Higher currents will cause the junction temperature to rise, and if the current is not limited, the junction will eventually fail due to high temperatures, a phenomenon sometimes referred to as thermal runaway. Therefore, it is common practice to mount the LED on a metal PCB to quickly conduct the heat away from the LED. Depending on the overall design, direct cooling of the LED backplane (such as using a cooling fan) may be required.

It is also important to maintain brightness and color consistency over the life of the LED. The three primary colors of LEDs (red, green, and blue) have different brightness decay rates. Using a closed-loop control system to maintain the individual brightness of the three colors is the best solution for achieving optimal brightness and correct color balance over a long period of time and a wide operating temperature range. Some commercially available three-color optical sensors can achieve this function together with appropriate control circuitry.

Advances in LED Manufacturing Technology

The semiconductor technology used to make HB LEDs is a key factor in determining the overall performance of the final product. Conventional HB LEDs are usually made with substrates of silicon carbide, sapphire or other materials. These substrates absorb some of the photons generated by the LED , which reduces efficiency.

To date, HB LEDs are generally made with two main technologies: InGaN and AllInGaP (also known as InGaAIP). Different colors can be achieved with these two main technologies. With an applied forward voltage of around 1.8V-2.3V, AlInGaP can emit light from green (570nm) to blood red (632nm). InGaN is used to emit light from blue (460nm) to emerald green (528nm) and phosphor-based colors such as white light (usually 3250K or usually 5600K). InGaN has a higher forward voltage of about 3.2V to 3.8V, depending on the color.

The wafer is flipped over and attached to a support plate that contains several elements that provide a highly reflective mirror surface. The initial substrate is then removed by peeling. Different peeling techniques are used depending on whether the substrate is AlInGaP or InGaN material. The compound wafer is surface treated using conventional metallization processes, then cut into individual LED chips and packaged. The resulting die has a light-emitting layer and no side emission, bringing efficiency to new levels.

Using light guide materials

When space is limited, the three-color LED and light guide material can be integrated into the design to accommodate the space constraints and keep the thickness of the BLU to a minimum. One solution is the 6-lead MULTILED from OSRAM Opto Semiconductor (Figure 1), which contains three thin-film chips for red (R, 625nm), turquoise (G, 528nm), and blue (B, 458nm). The three chips placed closely together in the same package provide optimal color mixing without the need for a large mixing area in the light guide material, and the LED provides high optical efficiency and extended service life (>50,000 hours). The 6-lead package allows each cathode and anode of each color to be electrically connected in series with the adjacent LED, simplifying the drive circuit. The desired white balance point can be achieved by adjusting the individual drive currents for each color.

Figure 1: 6-lead Multi LED (left) used as a 19-inch TFT LCD backlight

By selecting the right number of LEDs, displays up to 24" in size can be backlit using side-firing light guides. In the 19" display shown in Figure 2, 154 6-lead MULTILED units replace two CCFLs and are mounted on two insulated metal substrate (IMS) PCBs. In this example, the original housing, light guide, and optical film are retained without the need for redesign.

Direct LED backlighting for LCD TVs

For LCD TVs, direct LED backlighting is the first choice because it can provide higher system efficiency of up to 80%. A direct backlighting solution combines GoldenDRAGONARGUSLED with "thin film" technology with a lens designed for backlighting . The wide radiation characteristics of this LED show the advantages of uniform overlap of the three primary colors over a large range, achieving uniform color mixing within the minimum thickness.

To design a complete backlight unit, RGB Quad LEDs composed of Golden DRAGON ARGUSLEDs can be systematically arranged into a matrix. When testing the structure, for QuadLEDs with a pitch of 85mm, the typical height of the BLU can be 35~45mm.

To address the cost issues associated with secondary (mid-level) LCD TVs, one possible solution is to use multi-phosphor-converted white LEDs to produce a color gamut greater than 95% NTSC (the color standard set by the National Television Directors Committee in the United States), which is better than most CCFLs. Using fewer LEDs saves costs (compared to a mixture of red, green, and blue [RGB]), and only requires a single drive channel. Premium primary TVs such as the Sony 70XBR will still use RGB, with a color gamut greater than 105% NTSC, but the higher quality comes at a higher price (see Figure 2).

Figure 2 High-quality first-class TVs such as SONY70XBR will still use RGB, with a color gamut greater than 10590NTSC

Scalability to larger LCDs

As HB LEDs continue to move toward larger LCDs, the continued improvement in LED brightness and efficiency will reduce their use, thereby reducing the energy requirements for providing large LCD backlights, making HB LEDs a more viable option for larger applications. Currently, CCFLs remain a more cost-effective solution for certain backlighting needs, especially in industrial applications, where the advantages of LEDs are not yet needed. However, as HB LED performance continues to improve and prices continue to fall, cost will no longer be an obstacle in the design of HB LED BLUs, even in industrial applications.

Other advantages of LED backlight large screen LCD

Motion blur reduction, contrast enhancement - The fast switching speed of LEDs will enhance the performance of LCDs because "active driving" can be used to increase the dynamic range of brightness and contrast, and reduce the "motion blur" encountered when the video information exceeds the switching speed of the LCD. This is achieved by adjusting the brightness of the LED relative to the image information, which is difficult to achieve with CCFLs due to their slow switching times and limited brightness adjustment range.

It is foreseeable that in the near future, faster LCDs will appear that can use sequential color mode in which the three primary colors are turned on one by one. This will eliminate the need for color filters, which account for a large proportion of the cost structure of TFT color LCDs, and make the system cost of LEDBLUs lower than the backlight cost of CCFL-equipped displays.

Conclusion

HB LED technology has reached the level of backlight brightness and efficiency required for larger LCDs, while a wide color gamut, brightness uniformity, longer life and durability are also very important. The continuous improvement of HB LED brightness will reduce the number of LEDs required for BLU, reduce the necessary energy consumption, and increase the overall brightness of the LCD. As HB LEDs penetrate the market for high-volume applications such as general lighting products and consumer products using large and medium-sized LCDs, their costs will continue to decline. All signs indicate that more and more LEDs will replace CCFLs in the backlight of larger LCDs.