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Optoelectronics/Displays??

Design lighting systems with tunable WLEDs

Posted: 16 May 2008 ?? ?Print Version ?Bookmark and Share

Keywords:white LED? tunable WLED? solid-state lighting?

By Gopal Garg
Cypress Semiconductor Corp.

Jason Posselt
LedEngin Inc.

The adoption of solid-state lighting rapidly increases as performance and efficiency levels surpass those of conventional lighting technologies. However, simply replacing conventional sources with static white LEDs fails to unlock the true potential and differentiation that is achievable with solid-state lighting. LED-based lighting fixtures, supported by intelligent drivers, have the capability to not only replace white lighting, but also add new functionality which is difficult to achieve using conventional lighting technology.

It is easy to overlook the fact that "all colors include white, too." Different shades of white, ranging from Warm White (such as produced by an incandescent lamp) to Cool White (such as is produced by many of the early-generation indicator LEDs and power LEDs) are represented by Correlated Color Temperature, which ranges from 2700K to over 6000K for these two examples. Using conventional technologies, the only intelligence which has been added to white light is "dimming." With solid-state lighting, designers can enable new possibilities, such as tuning the color temperature of white light to alter the environment.

The benefits of this new functionality include enhanced productivity in the workplace, attraction and flow of customers in a retail environment, and providing a welcoming atmosphere in a hospitality setting. It is also possible to alter the lighting effect in a retail environment based on the season, merchandise, or time of day to increase sales "velocity", without incurring a major re-lamping expense. Tunable lighting is a feature offered by solid-state lighting implementations beyond the tactical economic benefits of energy reduction and extended service life that are already driving LED adoption in the lighting market.

It is important to first deliver the right light, prior to enhancing functionality. To accelerate adoption, it is necessary to respect conventional norms, and deliver the quality of light expected by the lighting designer, architect and end user. Implementations which are capable of delivering the right light and are scalable to add intelligence, while still delivering a commercial benefit, will revolutionize the lighting market. Performance (lumen output), quality (color temperature and color rendering) and illumination (beam angle and candela) are all critical parameters to consider. Color rendering, typically measured by the IEC-sanctioned procedure as Color Rendering Index (CRI), measures the ability to accurately reproduce illuminated color.

For example, a red car would appear as brown under low-pressure sodium light, which has an amber hue and low CRI. End users expect the benefits of increased lifetime and reduced energy and maintenance costs which come with LED-based lighting, but will not adopt the technology if it does not meet quality of light expectations. Once these performance levels are achieved, consideration can turn to enhanced functionality.

With ultimate control comes the challenge to deliver consistency and uniformity. Color consistency remains a factor when considering a controllable, solid-state lighting fixture, and variations in performance, whether lamp-to-lamp or deviations over time, temperature and set point, must be resolved. Although approaches exist to resolve these issues, developing the overall system can still present a challenge to the lighting industry. Modular or system-level implementations as described in this article can assist in solving these challenges. LED systems with suitable power and control technology which compliment, rather than clash, and which supplement, rather than displace, are critical to accelerate adoption.

Lighting challenges
To displace a small-form factor-halogen lamp requires 400 to 1000 lumens from the source. Although LED performance continues to improve significantly, high-flux-density sources capable of delivering this performance are not common. Single-chip LEDs exist that emit well over 100 lumens, but there are few options which can deliver performance comparable to 20-, 35-, or 50W halogen lamps. While it is possible to assemble an array of lower-performance LEDs, this can lead to challenges in optical control. Generating the light is one part of the challenge, directing the light to deliver the right effect must also be resolved. The creation of a single source which is capable of delivering the required lumens can simplify the optical design and enhance the efficiency of the lamp.

Thermal management is an additional challenge that cannot be ignored in any high-power solid-state application. Heat generated from the LED source must be properly managed in order to ensure adequate life in a useable operating environment. While this challenge may not be resolvable within the LED package, minimizing the thermal resistance assists in reducing challenges at the system level.

A fundamental factor in creating tunable white light is the need to have multiple color channels which can be individually controlled and mixed. Delivering a consistent color across the beam, without color fringing or shadowing, can be a complicated task. Reducing the optical-source size of the LED or LED system can improve the color mixing.

One approach is to create a multichip LED, locating the individual colors as close as possible and therefore reducing the mixing distance. Although various multichip packages exist, few exist that enable both high flux density and color mixing. Most multichip packages are limited to a low drive current, reducing the flux density. Alternatively, designers may use multiple, discrete, high-flux sources, but this increases both the optical-source size and color-mixing challenge.

Although not insurmountable, the combined requirements for flux density, tunability, color mixing and thermal management do present a technical challenge for the LED manufacturer. Addressing these issues within the LED, however, can simplify the challenges for the OEM or system integrator.

Figure 1: 10W tunable white lights (such this one from LedEngin) use an individually-addressable multichip package.

Tunable source
Multichip high-power LEDs have been developed to provide a source which is tunable to a "white point", which reduces color-mixing challenges, and enables miniaturized spot-lighting applications. These 10- to 15W packages are capable of delivering not only significant increases in light output, but also dramatic improvements in flux density, compared to conventional power LEDs and array assemblies. These LEDs are optimized for thermal performance and reliability, and enable increased drive currents which further improve the flux density. The optimization of material selection, packaging, and the assembly and die attach process, increases the ability to extract heat and enhance the usable light under application conditions.

Tunable LED lighting systems typically use red, green and blue mono-color LEDs which are mixed to create white light, also known as an RGB approach. While the RGB approach is capable of delivering a vast array of colors as well as tunable white light, their CRI is quite low. The typical color rendering achievable for such a system ranges between 40 and 60, depending on the targeted color temperature. Interior lighting typically requires a CRI in excess of 80, with higher values demanded for certain applications such as retail and museum lighting. Therefore, such a light may be well suited to a signaling or "color-washing" application, but is less suitable as a tunable white source for interior lighting.

One possible way to increase the CRI is to add additional color components. The addition of amber, creating an RGBA system, can significantly enhance the CRI while still maintaining the color point tunablility. Ultimate color flexibility is still achievable, but more importantly for interior lighting, it is possible to migrate the white point along the black-body line while still maintaining a CRI approaching 90, delivering excellent color rendering and a high-quality white light.

It is possible to combine these product aspects to achieve reasonable results. For example, 10W RGBA emitters are available (Figure 1) which can deliver high flux density, color control, mixing capabilities, and packaging optimized for thermal performance and reliability.

Using LEDs of this nature makes it feasible to develop a lamp which can deliver the right quantity and quality of light, and also the added feature of white-point tunability with an enhanced color rendering. The unit in the figure has a 7mm x 7mm footprint and is capable of sustained operation at up to 1000mA per chip, while dissipating 10W and producing over 350 lumens.

Multicolor LED emitters
Multicolor LED emitters also need intelligent and integrated controllers, for the following reasons:

Multiple channel drivers: For a given color temperature, a high-quality CRI (90+) would require multiple types of LEDs. CRI is a function of spectral power distribution (SPD) of the light source. SPD is the amount of radiation emitted by a light source at a given wavelength (frequency). Incandescent lamps with the best CRI (97+) have a short life (750 to 2500 Hours) and poor efficacy (10 to 17 lumens/W, according to the U.S. Deptartment of Energy). Figure 2 shows the SPD curve of an Incandescent lamp and compares it to that of an RGB source.

Figure 2: Spectral power distribution comparison

It is clear that an RGB source alone is not sufficient to create a high CRI. The designer needs to add more channels of colors to fill in the band and thus create a curve close to that of Incandescent lamp. Therefore, the device must have a minimum of a four-channel driver to drive red, green, blue, and amber or red, green, amber, and neutral white combinations. This controller should also be either factory programmable to the correct values for u and v components, or should be programmable dynamically over a communication link.

EMI Control: Because of high-speed switching regulators, high-brightness (HB) LED fixtures may create higher electromagnetic interference (EMI), which should be controlled through special design techniques. Implementation of density-modulation techniques, which depend on a stochastic process using a pseudorandom counter to generate a output function which does not have dominant periodic components, can generate a modulation output that is lower in noise and is easier to work with, from an EMI/EMC perspective.

Compensation: LED design requires a complex set of calculations to deliver consistent color. The two biggest problems that high-brightness LED engineers face today are having to account for differing LED-performance specifications based on manufacturing bins, and LED degradation (such as output flux and wavelength) over different temperatures. These fixtures require intelligent drivers which can ensure color and CRI consistency, over time and temperature, through predictive means. For certain applications, these drivers should also be able to accept real-time feedback data for color and temperature, to ensure fixture performance.

Dimming: LED-based fixtures should be easily dimmable with a resolution that can create deep colors and also avoid flickering at lower intensities.

Software support
To assist developers in managing multiple-LED channels and complex color design and management, several companies are offering hardware which is capable of driving many LEDs, along with software which is designed to simplify LED development and control, Figure 3.

Figure 3: LED application-specific development and control software, such as PSoC Express from Cypress, assist developers.

The ability to drive multiple channels with a single device (sixteen, in the case of EZ-Color Controllers from Cypress) can substantially reduce the number of controllers required in very large designs, thus decreasing design complexity, power consumption, and board-space footprint.

Additionally, software drivers are available off-the-shelf to significantly reduce low-frequency flicker and radiated EMI, both common problems with HB LED designs including architectural lighting, general signage, rechargeable flashlights, entertainment lighting, and emergency-vehicle lighting. Integrated development environments also simplify design through the use of embedded visual-design tools which enable even inexperienced engineers to choose specific colors for selected LEDs.

Advances in both LED technology and control technology continue at a rapid pace, with new LEDs which open the possibilities of enhancing lighting functionality, along with the expected quality of light. Although the complexity of developing such a lighting system is not trivial, manufacturers are working to resolve challenges to accelerate the adoption. High-power, multi-chip LEDs from vendors such as LedEngin, and LED application-specific control software can be combined today to create high-performance, high-quality, tunable-light fixtures capable of delivering equivalent illumination performance of conventional halogen sources.

About the authors
Gopal Garg
is managing director of Cypress Semiconductor Corp. power PSoC division.
Jason Posselt is director of product marketing at LedEngin Inc.





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