What remains as the biggest challenge for current LED technology? (Part II)
Kaz Maruyama, Global Industry Director, LED Lighting, Dow Corning
The biggest challenge is that LED chips are quickly approaching their theoretical limits for efficient light output. As they do, incremental improvements to the LED die are providing diminishing returns on investment. Fortunately, innovations in optical silicone packaging materials are introducing new, more cost-effective ways to enhance efficiency (lm/W), as well as cost (lm/$) and light density (lm/cm2). Easily molded, and available as optically clear encapsulants or highly reflective materials, silicones are expanding design options for shaping light and improving LED output. Plus, as a class of materials, silicones also deliver comparatively high photo- and thermal-stability to support reliably consistent, long-term lumen maintenance in advanced high-intensity LED applications. Optical silicone technology continues to evolve. Several years ago, designers were confronted by the choice between more thermally stable methyl silicones and phenyl silicones, which offered a higher refractive index (RI). But recent advances in phenyl-based technology have produced optical silicone encapsulants that are suitable for today’s most advanced high-power general lighting applications, including 5W – 50W chip-on-board LED architecture. This is another significant step toward cost-effectively improving the efficiency and systems cost of next-generation LEDs: The RI for phenyl silicones reaches 1.54 vs. methyl’s RI of 1.41. Although this 0.13 difference may seem small, it can translate into about 7 percent more light output – independent of the LED chip, case or input power. As LED chips approach their physical limits to deliver more light, the potential of silicone packaging materials to enhance efficiency, value and performance comes as very good news to solid-state lighting manufacturers.
| Dr. Arun Dutta, Director R&D, LED Lamps Americas, OSRAM SYLVANIA
The biggest challenge for current LED lamp technology is the ability to efficiently extract heat from the LED source. Contrary to popular opinion, LEDs do generate heat with only about 20% or so of the input power being converted to light. A detectable increase in the overall heat transfer coefficient would have a significant positive impact on lamp efficacy (LPW) which directly translates to cost savings for the consumer. This thermal management becomes more severe in two conditions: (1) As the input power increases within the same fo rm factor, and (2) As we move to smaller and smaller form factors. For an example of condition (1), extraction of heat from a 100W equivalent A line LED lamp is really challenging if one needs to stay within the A19 form factor. This is why a there is a proliferation of LED lamps in the lower wattages, around 40W to 60W, and only a handful in the 75W and 100W category. For an example of condition (2), the lumen output in MR16 LED replacement lamps tend to be lower than halogen sources of the same small form factor. Inability to extract heat efficiently from the LEDs also decreases lamp life and affects color shift. Use of discrete LEDs in place of COB (chip on board) does help with thermal management. Use of special circuit board materials in place of the industry standard FR4/MCPCB boards also assists with thermal management. Advanced thermal interface materials and use of active cooling, phase change cooling and liquid convection cooling are some of the other methods being used by manufacturers of LED lamps for better thermal management. In addition, advanced heat sink design and new thermal modeling techniques greatly aid thermal management.
Steve Jackson, Business Development Manager of Thermal Management at Sapa Extrusions North America
Many in the industry would agree that the biggest challenge facing current LED technology is the thermal management required to keep LEDs cool. While LEDs have a long life cycle, this is greatly reduced at elevated temperatures. Heat generated by LEDs is detrimental not only to the life of the bulb, but also the quality of the light. Therefore, the challenge for engineers lies within designing high-quality lighting fixtures that are both visually appealing, as well as meet the thermal requirement needs.The proper use of heat sinks—a component used in electronic systems to dissipate heat—is paramount to thermal performance in LEDs. Heat sinks help decrease the heat, which in turn increases the life of the LEDs. A well-engineered design can encompass the heat sink into the lighting fixture. Different materials used to create heat sinks offer a broad range of thermal conductivity; however the lighting industry has predominantly used aluminum extrusions and castings for years. One advantage extrusions offer is a higher level of thermal conductivity, providing a porosity free product. Another important component is surface area. High ratio extruded heat sinks contain longer fins that increase surface are and thermal performance. These types of heat sinks are most often seen in street light LED applications. An additional benefit is the various sizes and finishing options that are available. Aluminum extrusions come in a range of can be bright-dipped, powder-coated or painted in a variety of colors that allow the architect to supply a range of finished products. Given the myriad of new applications for LED technology that are continuously developed, the thermal management challenge will continue to be a hot topic. To meet the design challenge of incorporating style and optimum thermal performance, engineers should consider aluminum extrusions as part of the design.
To view Part 1, click here.