What Design Engineers Need to Know About Solid State Lighting

Unlike the incandescent/fluorescent lamp world of today's electrical lighting fixtures, solid state lighting (SSL) requires technical expertise in several distinct, high-level areas. (See the figure on the right.) Knowledge in one area can be quite unique, and the successful application of that know-how must be combined at the systems level to achieve an optimum lighting solution that takes into account interaction among the technologies.

LEDs
High-power LEDs are offered in 1W to 5W encapsulated packages, as well as hybrid or chip-on-board/array packages that can exceed 50W levels. Key performance metrics for LEDs include luminous efficacy (the amount of light provided in lumens per watt of electricity consumed [lm/W]), total power consumption, maximum current capability, and the associated luminous output, as well as correlated color temperature (CCT), color rendering index (CRI), or color quality scale (CQS). LEDs are commonly binned to assure uniformity of performance.

Manufacturers continue to achieve ever-increasing performance from their LEDs. Based on the highly competitive environment and current market status, continued improvements in high-power LEDs are expected as the industry moves toward the maximum achievable range of 220lm/W to 250lm/W (versus the theoretical limit of 300lm/W).

Optics
Traditional light sources emit light in all directions. In contrast, LEDs are mounted on a flat surface and emit light from the top and sides in a hemispherical pattern. With just the primary optics to protect the LED chip in its device-level package, the LED's Lambertian and other light distributions make them less useful for lighting applications. Secondary optics are often employed to optimize the LED's light distribution for specific applications such as down lighting or broadly disbursed or focused lighting. Light efficiencies within the targeted illumination area can exceed 90 percent with properly designed optics.

Basic LED light patterns are improved using secondary optics.

In addition to the specific angle and lighting efficiency, other system design considerations for the secondary optics include diffusers, lenses, prisms, multiple LEDs, and the ability to accommodate unusual footprints and form factors.

Driving LEDs
The LED driver module must handle design challenges and tradeoffs such as efficiency and life expectancy as parts of its performance criteria. Though there are several off-the-shelf driver modules available to address some of the design requirements, lighting fixture design engineers need to be comfortable with and verify how well all these concerns are addressed at the system level. This includes matching the driver module with the interface requirements of the LED module and the appropriate thermal response, overvoltage, overtemperature, overcurrent, and surge protection. The driver module has two additional criteria. It must be small, and it must integrate easily into SSL fixtures to simplify the entire lighting design process.

The figure below shows an example of the functions provided by a typical LED driver module. The capabilities include features such as temperature protection, current detection, and power factor correction (PFC), as well as several system-level functions. Input control/communications provide the ability to interface with AC line or 0–10V DC dimmers, facility management systems (FMS), and other emerging electronic controls, such as daylight harvesting, occupancy detection, and ambient light sensing.

In addition to converting AC to DC, LED drivers perform a variety of control and monitoring functions.