Using LEDs for backlight displays can provide performance improvements, increased durability, and reduced toxicity
BY JOO-HEE LEE
Samsung Electro-Mechanics
Korea
Now that flat-panel LCD and plasma displays have all but pushed CRTs out of the market, manufacturers of the devices are under intense price competition. Picture quality, power consumption, and display thickness are all key factors contributing to the competitive situation.
Recent advances in solid-state lighting (SSL) have allowed LCD and projector TV designers to use an LED backlight unit (BLU) or projector light engine (PLE) in large-screen displays for consumer television applications.
Using LEDs for large-screen backlighting is a new application area with new requirements for core knowledge at the manufacturing level. Since LEDs are more efficient, more rugged, and last longer than traditional CCFLs they provide a positive value to the customer.
Light and color
SSL light sources such as LEDs offer multiple benefits in LCD backlighting over the CCFL illumination currently used, from durability to performance in not only color intensity, but also because they have a faster response time than CCFLs.
The fast response time allows an LED-based BLU to virtually eliminate the motion blur in rapid action video by strobing light in time with the switching of the liquid crystals. In addition to improved response, using a synchronized LED backlight improves the contrast ratio while reducing power consumption.
The fast response time of LEDs can virtually eliminate the motion blur in rapid action video in LCDs.
There are two means of creating the white light required for LCD and projection TV. Blue, UV, or violet LEDs doped with phosphors can now emit white light of sufficient quality for large-screen TV applications, or white light can come from arrays of red, green, and blue (RGB) emitters.
A significant performance advantage with an RGB LED backlight is in the color gamut of the display, as CCFLs do not provide light of sufficiently spread spectrum to render accurate colors. Because such LED light sources have color values at the extremes of the chromaticity chart, a display based on RGB LEDs can exceed the NTSC color gamut by as much as 15%. For example, a Samsung rear-projection monitor with an RBG LED-based PLE won “Best of Show” at the 2006 Consumer Electronics Show, beating out both plasma and CCFL-driven LCDs, and their 40-in. LCD TV with LED BLU unit won the European Imaging and Sound Association (EISA) Award last year.
There is also development ongoing to further improve motion performance in very large screens by better addressing and timing of the LEDs involved. For example, Samsung Electro-Mechanics is addressing the motion blur problem experienced with TVs measuring approximately 70 in. or so by dividing the screen into sectional units and applying an impulse-driven scanning method to better sync with the display pixels. In addition, the LED BLU automatically turns off and on according to the brightness or darkness of the image signal. This “local dimming” technology elevates the contrast ratio to 10,000:1, enabling the LCD TV to provide far better image clarity.
There are a number of techniques available to both the supplier and the designer to improve the image quality of high-end TVs and monitors. Now standard procedure, “LED binning” is the sorting of the red, green, and blue diodes, which have minute differences in brightness and wavelength, to create the cleanest possible white screen.
The LED array needs to contain the best ratio of red, green, and blue emitters to provide the white light required. In a measure intended to reduce parts count and cost, manufacturers such as Samsung Electro-Mechanics have added red and green phosphors to the blue LED chip rather than use a backlight unit with separate red, green and blue LEDs. This technology still generates white light with an excellent color range as it is still created by the mixing of red, green, and blue light.
Drivers and packaging
LEDs need an enclosure to hold them, electronics to provide their precise power needs, and heat-management systems, as anything handling power gets hot. This creates a cross-disciplinary marketplace where knowing how to handle the heat or hold the device or focus the light best is as valuable as knowing how to make the light itself.
The power system of an LED backlight should do more than provide energy. Proper design of the driver will improve efficiency, reduce thermal loading, and increase reliability.
One issue of LEDs is drop-off of light over time, with each color exhibiting its own derating curve. Using an optical sensor to provide feedback on the color intensity of the various emitters system in the driver circuit will enable the display to maintain its color balance for the life of the device.
Proper packaging can also solve the heat emission issue at the LED package and module assembly stage, enabling a slimmer TV form factor and preventing heat buildup after prolonged use. The LED packaging also contains the optical diffusors and light guides that enable an array of single emitters to illuminate a flat surface in a uniform manner.
Edge lighting with LEDs, used regularly with LCD backlights in portable devices, is another technique to reduce cost and device complexity. An edge-driven BLU uses a flat light guide to direct and control the light to reduce the number of LEDs needed and with it the power requirements and related component count.
An edge-driven LED BLU can reduce the cost, thickness, and power consumption over direct illumination.
LEDs offer environmental benefits as well. They do not contain hazardous substances such as mercury, allowing them to comply with today’s stricter environmental regulations for both manufacture and disposal. This is in stark contrast to CCFL-based BLUs that contain toxic materials that are easily released due to the fragile nature of the CCFL’s glass tube.
The market for LEDs
The LED market has been growing fast recently and is expected to become a major segment within the electronics industry. In 2006, global LED demand totaled US$6.3 billion, and annual growth is averaging 15%. Some US$11 billion worth of LEDs are expected to be sold in 2010, an amount that would be equivalent to 90% of the NAND flash memory and 40% of the DRAM markets.
LED BLUs cost more than the CCFL versions, so despite their advantages and technological sophistication, their use is still not widespread. As the SSL market matures, more users will appreciate the cost-of-ownership advantages and long-term savings of having a backlight system that can last for tens of thousands of hours of operation.
Educating customers on the why of LEDs and how they work in LCD backlights while demonstrating how the various factors of power, thermal management, and packaging are involved can provide solid results.
Samsung Electro-Mechanics continues to expand its involvement in the LED backlight unit business. The company’s diverse and innovative technology improvements can be applied to displays of all sizes, responding to the availability of ever-larger models as well as to steadily diversifying LCD applications. Samsung Electro-Mechanics’ LED for lighting applications include white LEDs color with temperatures ranging from 2,000K to 15,000K and LEDs in the full color range (green, blue, red, and amber).
“Sunnix” is the name of Samsung’s high-power LED brand, and the Sunnix platform currently covers two lineups–1W and 3W. Samsung has the package technology to lower the thermal resistance coefficient to 8°C/W, laying the groundwork for increasing productivity while minimizing thermal resistance and optical properties. Therefore, when large modules are made, thermal resistance is lower than that for other LEDs, allowing for greater design flexibility.
The white Sunnix LED line offers diverse color temperatures from warm to cool white, with luminous intensities ranging from 60 1umens/80 1umens (@350mA) to 130 1umens (@700mA). The viewing angle is 120 degrees and ESD protection is 5 kV and higher.
Samsung Electro-Mechanics directly sells over 80 percent of its own products, operates manufacturing facilities, R&D distribution and sales centers, and offices in the U.S., Europe, Japan, China, and Southeast Asia.
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