Solid-state lighting (SSL) is rapidly emerging as a major segment of the lighting fixture industry. What was once a simple electrical device has now been transformed into a complex electronic system. As light fixture manufacturers transition their existing incandescent and fluorescent fixture designs and create new SSL luminaires, they face a broad array of issues.
Even though light-emitting diodes (LEDs) are produced in very sophisticated processing facilities, there are many variations among the LEDs produced, even during the same run. Variations in forward voltage occur throughout the production simply due to normal statistical distributions inherent in all products. To provide repeatable and dependable systems, the LEDs must be electrically sorted based on this forward voltage. A similar sorting is done to accommodate variations in the color. This sorting results in the “bins” in which all LEDs are offered.
A quality producer of LED systems will take all of these differences into account as the LEDs are matched for consistent color and performance. A luminaire manufacturer must select the proper mix of LEDs to arrive at a consistent color rendition or they must depend on suppliers that make this complex issue transparent to them.
The LED Light Module
An LED light module (LLM) or light engine is a sub-assembly that includes an LED light board (printed circuit board with LEDs mounted), thermal interface, socket (if required), electrical connections, and matching optics as depicted in Figure 1.
Primary & Secondary Optics
Traditional light sources emit light in all directions. As a result, the optics for redistributing the light from these sources are typically less efficient because some light bounces within the optical components or luminaires’ bodies. In contrast, LEDs are mounted on a flat surface and emit light from the top and sides in a hemispherical pattern. In many applications, the LED’s inherent directional light pattern adds to its lighting efficiency.
In addition to the primary optics that protect the LED chip in its device-level package, secondary optics provide greater functionality at the system level. Secondary optics optimize the distribution of the LED light for specific applications such as down lighting, broadly disbursed, or focused lighting. Some system manufacturers offer easily interchanged optics to achieve different distributions within the same package that allow the user to adjust the system for the specific application (see Figure 2).
Figure 3 shows an example of the functions provided by a typical LED driver module. The primary function of SSL driver electronics is supplying a controlled power level over the operating temperature range for the LED or LEDs in order to maintain a consistent light output. Other capabilities include protection features such as temperature protection, current detection, and power factor correction (PFC), as well as several system-level functions. Input control/communications provides the ability to interface with AC line or 0-10 VDC dimmers as well as facility management systems (FMS) and other emerging electronic controls such as daylight harvesting, occupancy detection, ambient light sensing, thermal monitoring, and more.
Adequate thermal protection for temperature sensitive LEDs requires localized temperature measurements to provide a sufficiently quick response to limit excessive temperature and shut down if necessary, to avoid stressing LEDs or contributing to early-life failure. In systems with several LED sub-assemblies, each LED subassembly must be accounted for as a single point of potential failure if its localized temperature exceeds the maximum safe operating level.
LED driver module design must handle design challenges and tradeoffs such as efficiency and life expectancy as part of its performance criteria. All of these design aspects are addressed in a driver module assembly to avoid burdening lighting fixture companies with electronic circuit design and system concerns (see Figure 4).
Excessive temperature is detrimental to the performance, lifetime, and reliability of LEDs. High temperature also affects nearby circuitry and connectors. To keep LED temperatures within safe operating limits, large heat sinks are sometimes required. However, heat sinks are just one aspect of an SSL’s thermal design.
Advanced SSL lighting designs incorporate a comprehensive thermal management system to monitor, detect faults, and protect the light source. The LED’s critical junction temperature is deep within the package — far from the surface of the chip. Since this is not easily measured, a package (or case) temperature is normally read and the actual junction temperature is calculated.
In addition to reducing the operating life, increasing temperature reduces the performance of LEDs. For example, relative luminous flux (light output) reduces with increasing temperature. At a case temperature of 130°C, the output is reduced about 80% or more depending on the color temperature rating. Increased temperature causes other performance changes including increases in dominant and peak wavelengths and color temperature shifts.
A NEMA article, “Recommendations for Solid State Lighting Sub-Assembly Interfaces for Luminaires,” provides detailed recommendations for temperature test points for LEDs. Figure 5 shows an example of the acceptable location for temperature sensors in a multiple LED design. With the importance of temperature to the overall system design and complexity of making accurate temperature measurements, specially-designed test tools that accurately record temperatures can simplify the task of verifying that the temperature design has been done correctly.