Design and Manufacturing Considerations for LED Backlights

Direct backlighting is today's norm for LED backlights used in LCD TVs, but the addition of light-extracting light guides will allow for thinner backlights with fewer LEDs.

by David DeAgazio

LIGHT-EMITTING-DIODE (LED) backlights are being increasingly utilized in backlighting applications, such as keypads and flat-panel displays (FPDs). This comes as no surprise for handheld-sized liquid-crystal displays (LCDs), but there is now considerable excitement about the use of LED backlights in mid-sized and larger LCDs, which have conventionally been backlit by cold-cathode fluorescent lamps (CCFLs).

Designers of LCDs and developers of keypads are always looking for ways to significantly enhance performance and manufacturability. The targeted design and performance parameters include

• Brightness
• Uniformity
• Thinness
• Design flexibility
• Efficiency
• Power consumption
• Connectivity
• Ease of assembly.

For custom designs, one or more of these parameters will have to be given priority, as deemed appropriate by the designer.

Backlighting Keypads

When backlighting keypads in devices such as smart phones and personal digital assistants (PDAs) – whether they are of the conductive silicone rubber, film-on-rubber, or polycarbonate-hardtop type – designers are trying to increase brightness and uniformity, eliminate electronic noise, minimize the number of secondary components (such as inverters), reduce cost, and obtain longer lifetime.

 

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Fig. 1: In keyboard backlighting, the use of a molded LED light guide placed between the PCB and keypad can tightly focus the light as it enters the through-holes to the keypad (bottom). Compared to direct backlighting (top), this allows for a substantial reduction in the number of LEDs.

 

Electroluminescent (EL) panels, while they are thin and have very good uniformity, are susceptible to noise (requiring shielding), require an inverter (another secondary component), degrade in brightness over time more quickly than LEDs (especially when white is desired), and are generally more expensive. As a result, LED-based backlighting is becoming increasingly popular for use in keypads.

Many designers of PDAs, wireless handhelds, and Gen 3 smart telephones have been using direct LED backlighting. This method eliminates noise and the need for an inverter because the LEDs are powered directly from the batteries, thus providing more design flexibility. Direct LED backlighting employs top-firing surface-mounted non-focused (120° viewing angle) LEDs placed between the keys (Fig. 1, top). The brightness is sufficient, but because there is little or no way for this technique to channel the light uniformly, combined with the diffusive properties of the keypad itself, many LEDs are required to achieve good uniformity. Because of the large number of LEDs used, the cost and the size of the printed-circuit-board (PCB) become key issues.

Another approach is molded-light-guide LED backlighting, in which side-firing LEDs that focus the light of the LEDs into a high-performance light guide (Fig. 1, bottom) are used. The current production keypad for a PDA, approximately 2.5 x 1.25 in. in size, will typically have from 30 to 50 through-holes in the backlight to allow the light to travel and actuate the domes on the circuitry layer.

The major problem with the molded-light-guide technique is that the light rays generally terminate at the edge of the first through-hole, thus making light uniformity extremely difficult to achieve. However, by using state-of-the-art optical simulations, some of today's keypad designers can actually use diversion features – which are molded directly into the light guide – to bend some of the light around the through-holes, resulting in very uniform backlighting (Fig. 2). One high-end designer/ manufacturer is able to achieve a uniformity better than 80% with far fewer LEDs than required by the direct-LED-backlighting technique – and there are fewer, if any, hot spots and dark areas.

Backlighting LCDs

It is not surprising that LED backlighting is becoming more popular than CCFL backlighting in smaller form factors with diagonals of 3.5 in. or less – the sizes used for PDAs, cellular telephones, and similar devices. But LEDs are now making inroads in mid-sized (3.5–7.0 in. on the diagonal) LCDs and even in the larger LCDs used in flat-panel TVs. The reasons for this include

• Smaller form factor
• Wider color gamut
• Longer lifetime
• DC power (no inverter)
• Greater design flexibility
• Lower cost.

For all applications, increasing the brightness and reducing the parts count are critical factors.

Full-color displays require white LED backlighting that must be more efficient than ever before. High efficiency, high performance, low cost, and ease of assembly are the goals. As LEDs become brighter, designers must look for ways to spread the illumination from an LED light source across a larger area while maximizing brightness and uniformity and minimizing hot spots and dark areas. LCD-panel manufacturers agree that, as far as color reproduction is concerned, LEDs offer the best performance of any of the available backlighting techniques.

For larger panel sizes – 32 in. and larger – being used in LCD TVs, a new generation of advanced high-brightness white-LED light sources is making it possible to achieve color-reproduction ranges in excess of 100% of the National Television Standards Committee (NTSC) specification and to freely adjust white-color expression.

Not only do white backlights using RGB LEDs offer a significantly wider color reproduction range than conventional CCFL backlights, they also make it much easier to reproduce colors that are difficult to achieve with phosphors in cathode-ray-tube (CRT), plasma-display-panel (PDP), or surface-conduction electron-emitter display (SED) technologies.

 

Fig. 2: Shown is an optical simulation of a molded-light-guide keypad backlight that uses diversion features to direct the path of the light rays around the through-holes. The result is a very uniform backlight that uses fewer LEDs.

 

As is the case for keyboards and small LCDs, designers of LCD TVs have a choice between direct LED backlighting and an LED-based light-extraction technique that uses a molded light guide with light-extraction features directly molded into it. The molded-light-guide approach produces bright and uniform light in a thinner form factor than the direct-lighting approach. The light produced with molded light guides is more specular (collimated) than diffuse because incident light rays are controlled by redirecting their emission angles from the light guide while minimizing scatter. The technique provides very high luminance – typically about 3600 cd/m2 in a 1.5-in.-diagonal backlight with two white LEDs – and requires the use of fewer LEDs.

For larger LCDs, such as those used in HDTVs, backlight manufacturers have been using CCFLs, achieving luminances up to about 500 cd/m2 for a high-end LCD TV using CCFLs.

Now, however, backlight manufacturers are beginning to incorporate state-of-the-art high-output white backlights made with separate red, green, and blue LEDs, such as the OSRAM Golden Dragon® or the Luxeon® Star from Lumileds Lighting, that deliver luminances as high as 500–600 cd/m2.

The increasing interest in LED backlights for larger LCDs is based on several significantfactors, including their wider color gamut, design flexibility for retrofits, ruggedness because they are solid-state devices, superior color mixing, faster response time, longer lifetime, and a versatility that makes customization easy.

There are ways to utilize these new high-brightness LEDs for large panels, such as those used in LCD TVs. The conventional approach is to mount the RGB LEDs to a heat-conducting circuit board with a distance of 30–50 mm between the LEDs and a light diffuser on the viewer's side of the backlight. This allows the light to emit over a wide angle so it can mix properly to create a uniform white light (Fig. 3, top). At least two manufacturers mount unpackaged LED chips directly to the circuit board and encapsulate them after mounting.

These related direct-firing approaches have the advantage of a relatively simple structure, and they offer excellent brightness, but a large number of LEDs are needed to ensure excellent uniformity.

At Global Lighting Technologies (GLT), we have adapted the patented light-mixing wave guides and MicroLens technology developed for smaller backlights in the development of a new, modular LCD-TV backlight that provides outstanding brightness and uniformity (Fig. 3, bottom).

This new wave-guide technology results in an LED backlight that is thinner than those based on conventional approaches, provides better color mixing, and allows for a wider LED spacing that reduces the overall LED count, and minimizes thermal concerns.

As LEDs get brighter, backlights will consist of fewer LEDs spread over a larger area; therefore, we believe that color-mixing wave guides will become increasingly important to LCD-TV backlighting designs. Eventually, designs may migrate to edge-lit technologies requiring even fewer LEDs.

In the current GLT design, the LEDs are nested in the viewing area via recesses in the color-mixing light guide. The LCD and light guide are separated to permit an even RGB mix. The light guides are modular and connected to each other so that backlights of various sizes can be made.

 

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Fig. 3: Current LCD TVs use LED direct lighting (top), which is simple and bright, but requires relatively close spacing of the LEDs and a significant distance between the prism film and diffuser and the backlights to produce good light mixing. The use of light-mixing wave guides and modern light-extraction technology reduces the thickness of the backlight and permits a wider LED spacing and fewer LEDs, with undiminished brightness and excellent uniformity.

 

Assembling the Backlight

Given the high-volume applications that are targeted by LED-backlight manufacturers, ease of assembly is critical. The assembly of today's CCFL backlights for TV applications is so labor intensive that it is routinely done in countries with low labor costs.

By using LED backlights, we have the opportunity to design for assembly as well as for performance and materials cost. In the GLT design, a housing is supplied that includes all the mechanical features required to connect the backlight (Fig. 4). An assembly might include the following components, from the bottom up: back reflector, metal or white-plastic housing, MicroLens light guide with LEDs on a flexible circuit or PCB, light-management films and diffusers, masks to shield any unwanted light, and protective films.

Two pins extruding from the side of the housing could be used to connect the assembly to a PCB; a flex circuit that could be easily connected via an elastomer or ZIF; or a regular wire-to-wire, wire-to-board, or board-to-board DIN connector, depending on how the device is assembled.

Conclusion

LEDs offer substantial improvements in performance and, increasingly, lower cost to LCD-module manufacturers. Brightness and uniformity are the most critical front-of-screen parameters, but luminous efficiency, power consumption, thermal management, and package thickness are also important. Backlight designs that utilize fewer LEDs and secondary components will be less expensive, and the differences will become more evident as even brighter LEDs become available. These brighter LEDs will permit luminance targets to be met using fewer devices, but only in backlight designs that deliver good color mixing when the LEDs are spaced increasingly farther apart. •

 

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Fig. 4: A complete LED backlight assembly can be encased in a housing that incorporates the mechanical features required for easy connectivity.

 


David DeAgazio is Director of Sales, Worldwide, at Global Lighting Technologies, 55 Andrews Circle, Brecksville, OH 44141; telephone 440/922-4584, fax 440/922-4585, e-mail: info@glthome.com, URL: www. glthome.com.