Power Electronics

LED Developments Herald Changes in Power Management for LCD Backlighting

The emergence of LEDs bright enough to backlight large LCD displays promises to reshape power requirements in LCD TVs and monitors. Recent announcements and technology demonstrations provide further evidence that increases in LED light output are making these solid-state lamps viable replacements for the cold cathode fluorescent lamps (CCFLs) currently used in large LCD displays. The adoption of LED backlights in these applications promises to reduce the power consumed by backlighting (in the long run, if not always in the near-term LED backlight solutions.) At the same time, LED backlights should alter the backlight’s power supply requirements.

Although LEDs have become a dominant source of LCD backlighting in many handheld products, these devices have just begun to make inroads into backlighting applications in the larger LCD displays associated with computer monitors and televisions. One vendor, Cree, recently announced that it has significantly increased the brightness of its blue and green LED chip products targeted for the design of energy-saving LED backlighting solutions for larger LCD monitors and televisions.

For high-power LED-based backlight systems, Cree now is shipping blue XB 900 power LED chips that are 38% brighter, increasing the minimum radiant flux from 65 mW to 90 mW. The company also is shipping green XB900 chips that are 33% brighter, increasing the minimum radiant flux from 30 mW to 40 mW. For backlight systems using smaller chips, Cree has increased the brightness of its green XThin LED chips from 8 mW to 10 mW, for the top radiant flux bins, to better match the high brightness of the company’s blue XT-21 and XT-24 chips.

Cree’s brighter chips mean less power is needed to meet the brightness specifications for backlighting applications, significantly reducing energy consumption and the need for active cooling systems. Cree introduced its own backlighting solution at the SID International Symposium, held May 17-24 in Boston. The Cree LED backlighting solution consumes less energy than traditional CCFL-based solutions while demonstrating superior color rendering. For some applications, the improvement in color rendering may drive adoption of LED backlighting even where it does not reduce power consumption from that of CCFLs.

“LED-based backlighting for LCD monitors and televisions is expected to grow dramatically over the next several years, and we expect Cree LED chip products will be key enabling components for the development of next-generation backlighting solutions,” said Scott Schwab, Cree vice president and general manager of optoelectronics.

Another LED supplier, OSRAM Opto Semiconductors, also demonstrated the viability of LED backlighting in larger displays by presenting a prototype of an 82-in. backlighting system at the SID International Symposium. Using the company’s Golden DRAGON LEDs, the prototype demonstrates what the company calls “the largest LED backlighting system ever completed for an LCD display.” The backlighting system has a depth of just 40 mm, produces 10,000 cd/m² and has a life span of 50,000 hr. Its power consumption is 1000 W. The backlighting system is freely scalable, allowing it to be applied to displays of varying sizes.

These announcements follow earlier news from companies such as Lumileds Lighting, which has previously reported the use of its LEDs for backlighting LCD monitors. For example, last August, Lumileds reported that its high-power Luxeon series LEDs were being used to backlight Sony's 40-in. and 46-in. Qualia Series televisions. Lumileds and Sony collaborated to produce the LED-illuminated Triluminos backlight, which expanded the gamut of reproducible colors by as much as 45% when compared with existing light sources. More recently, Mitsubishi Electric publicized its use of Lumileds LEDs in a prototype 23-in. LCD monitor. As in the Sony monitor, superior color rendering and control were cited as the key benefits of the backlight design.

The replacement of CCFL backlights with LED backlights is likely to impact the backlight power supply design as it will alter voltage and current requirements for the supply. Depending on the approach taken in designing the LED backlight, a lower voltage supply may be possible using high brightness LEDs. It’s also possible that a high-voltage supply will still be required to drive long strings of LEDs in the backlight. In either case, the LED is a current controlled device that is capable of being fully dimmed without affecting the color it produces. Those characteristics likely will drive the development of more-sophisticated backlighting power supplies than are currently employed with conventional backlighting designs.

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