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Optoelectronics/Displays??

LCDs turn to LEDs for lighting

Posted: 01 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:LCD? CCFL lighting system? white LEDs? backlighting?

Even if the next LCD you buy has pretty much the same characteristics as the last LCD you bought, a change is likely to have occurred behind the scenes. In display sizes below 15-inches, the tried-and-true cold-cathode fluorescent lamp (CCFL) lighting system for the LCD is rapidly being replaced by LEDs.

There are a number of reasons for the transition, and it has a number of ramifications. At the foundation of the change is that recent brightness improvements in white LEDs (WLEDs) have made them feasible for back- or edgelighting an LCD that's bigger than a 4- to 6-inch PDA-class display. Dropping prices are also a factor.

"CCFL will eventually go the way of the dinosaur," said Richard McKay, managing director at display VAR Apollo Display Systems, and a number of other sources agreed.

"LEDs will quickly take over LCD backlighting in sizes of 10inches and above," said Stewart Hough, VP for business development at Lynk Labs, a specialist in LED driver technology. Pretty much all the CCFL backlight suppliers, he said, "now also have LED backlights."

Environmental factors
One major reason for the ascent of LED backlights and edgelights is the European Union's RoHS initiative, which has sought to purge lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyl and polybrominated diphenyl ether from electronic equipment sold in EU member countries since July 1, 2006.

Mercury is a prominent component of CCFL tubes and other types of fluorescent lighting. Although vendors have reduced mercury levels in their fluorescent tubes, with mercury-free versions under discussion, additional benefits of the LED weigh in to make the CCFL look to be a relic in the making.

Eliminating mercury creates a whole new set of problems for CCFLs, according to David DeAgazio, director of sales for Global Lighting Technologies Inc., a specialist in LCD edgelighting.

"The global market is looking to go more and more greenas energy-saving and pollutant-free as possible," said McKay, "and that plays right into LED lighting."

Power issues
The green effect is one of the top two factors driving LCDs to LED lighting, according to Joe Maurer, display group marketing manager at LCD maker Kyocera Industrial Ceramics Corp. The other driver is voltage. Maurer pegs CCFLs' relatively high AC voltage requirements at between 1,500 and 1,600Vac at startup, settling down to 700 or 800Vac. They also need a power inverter to operate with a DC power supply, adding to the cost, real estate and weight of the application.

A typical LED lighting subsystem, in contrast, runs off 12-24Vdc or less, said Maurer, "and you can get rid of the inverter and eliminate EMI."

Dimming control is also easier with the lower-voltage LED technology, said Bob Joslyn, LCD applications engineer at Kyocera Corp.

An LED edgelight provides about 80 percent of the brightness at about 60 percent of the power of a CCFL for a typical Kyocera 5.7-inch VGA LCD.

Efficiency and lifetime
The de facto LCD backlight since the mid-1980s, the CCFL has made dramatic progress over the decades in efficiency and lifetime, but the prospects for major improvements in the future are dim for this aging technology. The LED alternative, in contrast, is still evolving fairly rapidly, and it has a clear-cut path to future generations of improvements to come.

Comparative specifications for CCFLs vs. LEDsbrightness, efficiency and lifevary tremendously from vendor to vendor. Cited lifetimes (to half-brightness) for CCFLs range from 8,000hrs to 100,000hrs, for example. Some sources give the efficiency edge to CCFL, others to LED.

But at this point, LEDs and CCFLs overall appear to be at near parity for all relevant parameters, such as luminous efficiency (in the range of 50-100lumens/W) and lifetime (25,000-75,000hrs). Some more subtle differences nonetheless exist. For sidelighting applications, for example, LEDs are a more natural fit than CCFLs, boosting efficiency. "A CCFL may produce more lumens/watt than an LED," said DeAgazio," but it's a diffuse light source, so you get a lot of wasted light." LEDs, in contrast, are highly directional and thus "easy to couple efficiently into a light guide."

As for lifetime, the time to half-brightness for both an LED and CCFL is highly dependent on how hard the device is driven. LEDs have an added complication here: Proper heat management is a critical factor in how long the devices last.

Items in the LED's favor include its superior durability and better color delivery compared with the glass-envelope CCFL. The broad light spectrum provided by CCFLs is about on a par with the light delivered by a WLED that is based on a blue-emitting diode in combination with a yellow phosphor coating. But WLEDs that are based on a trio of narrowband red, green and blue (RGB) LEDs provide the best match for the RGB color filters of an LCD and can generate the most saturated colors most efficiently.

LEDs couple light more efficiently than CCFLs into a light guide such as the MicroLens from Global Lighting Technologies.

Both CCFL tubes and LED chips can be used as backlights (placed behind the display) or edgelights (on the sides, top and/or bottom), and both can be used in multiples to increase brightness. LEDs are typically provided as strings of devices on a flexible substrate. The diodes are finding use in very high-end HDTVs with multiple hundreds of chips backlighting the display.

In less glamorous, more cost-sensitive applications, edgelighting helps keep LED counts as low as possible to minimize cost. As LED brightness has risen, pricing for the devices has droppednow reaching the point where an LED lighting subsystem equals the cost of a CCFL and its associated inverter, said Maurer.

A typical 320 x 240pixel, 5.7-inch color LCD today uses a single CCFL edgelight or multiple "strings" of LEDs, each string typically containing five or so devices.

In direct comparison, Kyocera passive LCD delivers 380nits of brightness for 2.8W using a CCFL or 245nits for 1.8W using three LED strings, each string with seven LEDs. For active-matrix models, the power dissipation remains the same, with the CCFL LCD shining 470nits bright and the LED LCD cranking out 350nits.

"In our original products, the LED strings pulled more power than CCFL," said Maurer. "We're now less by a significant amount."

- David Lieberman
EE Times




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