Printed Circuit Design & Fab - July 2008 - (Page 24)

ThERMAL ManaGEMEnt solviNg sYsTem level Thermal management Challenges Thermal management is critical to overall system performance so mechanical designers need to consider thermal issues from the earliest stages of the design. by Dr. JoHn Parry The United States Department of Energy (DoE) heralds LED lighting technology because it offers significant energy saving opportunities while enhancing the quality of the building environment. Thermal management is critical to LED performance because the useful service life of the LED is a function of junction temperature. LED system manufacturers are addressing this challenge by seeking out improved heatsink designs, high efficiency circuit boards, high thermal conductivity enclosures and other advanced thermal design techniques. The LED offers an ideal model of the need for system level co-design. When thermal considerations are so overwhelming a performance driver, every component needs to be carefully factored into a successful product design. To meet these objectives, thermal simulation is playing an increasingly important role because of its capability to evaluate alternatives and optimize the system-level design from a thermal standpoint prior to the prototype phase. to approximately $10.3 billion in 2012. Yole projects that high-brightness and ultrahigh-brightness LEDs sales will be responsible for about $4.45 billion of that total, over 5.5 times the $783 million market size in 2007. “These solid state lighting devices are rapidly becoming the lighting source of choice for diverse applications that include traffic signals, interior and exterior lighting in cars and trucks, large screen visual displays, and small LCD backlighting and decorative illumination,” a recent iSuppli report states1. “And new lighting applications are constantly being discovered.” Thermal challenges High-powered LEDs provide greater thermal challenges than most other light sources, largely because LEDs don’t generate infrared radiation. According to the U.S. Department of Energy, 75% to 85% of energy used to drive LEDs is converted to heat “… and must be conducted from the LED die to the underlying circuit board and heatsink, housings or luminaire frame elements.” The US DoE’s Office of Energy Efficiency & Renewable Energy has produced a fact sheet on “Thermal Management of White LEDs2”. In the short term, excess heat can reduce an LED’s light output and produces a color shift. However, another reason thermal management is so important is the long-term effects that include accelerated reduction in light output resulting in a shortened useful life. The DoE says that manufacturers normally test LEDs at a fixed junction temperature of 25oC. On the other hand, under constant operation the junction temperature is typically 60oC or greater and under these conditions the LEDs light output may be 10% or more below the rating, and could be signifcantly higher for products with inadequate thermal design. For tungsten light bulbs, the heat flow path moves JULY 2008 Emergence of LED lighting Solid-state lighting is a pivotal emerging technology that promises to fundamentally alter lighting options in the future. LEDs were originally designed to operate with less than 50 milliwatts of electric power. Over the last decade, LED power consumption has decreased dramatically, to 40 to 80 lm/watt. Besides energy efficiency, LEDs also offer longer life, and depending on the manufacturer and type, the useful life for white LEDs can range from approximately 6,000 hours to more than 50,000 hours. This compares to 30,000 hours for fluorescent tubes and less than 2,000 hours for incandescent bulbs. LEDs can also be manufactured to emit light of a specific color without the use of filters. Market analysis firm Yole Développement says that high power LEDs will drive growth in the solid state lighting market, starting at well under $1 billion in revenue in 2007 24 printEd circuit dESign & fAB

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Printed Circuit Design & Fab - July 2008

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