Chip to System Levels Thermal Needs and Alternative Thermal Technologies for High Brightness LEDS

Abstract

Light emitting diodes (LEDs) historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs with white light and monochromatic colors has allowed them to penetrate specialty and general illumination applications. The increased electrical currents used to drive the LEDs have resulted in higher heat fluxes than those for average silicon integrated circuits (i.e., ICs). This has created a need to focus more attention on the thermal management engineering of LED power packages. The output of a typical commercial high brightness, 1mm2, LED has exceeded 100lm at drive levels approaching 3W. This corresponds to a heat flux of up to 300W∕cm2. Novel thermal solutions need to address system architectures, packaging, phosphors for light color conversion, and encapsulants and fillers for optical extraction. In this paper, the effect of thermal management on packaging architectures, phosphors, encapsulants, and system design are discussed. Additionally, discussions of microscopic defects due to packaging problems as well as chip active layer defects are presented through experimental and computational findings.

Environmental and economical impact of LED lighting systems and effect of thermal management

“…As getting the maximum lumens over the entire lamp lifetime is the primary goal, changes in the polymer optical properties due to high temperatures is not tolerable. As noted above, high power LED packages [8] and systems require materials that need to survive high temperatures (>1001C when in contact with the chip or as a phosphor binder) and optical fluxes (>100 W/cm 2 ) for tens of thousands of hours. Traditional epoxybased systems have difficulties in withstanding these conditions for typical 1 W power packages on the market, as long-term operation over 1001C combined with violet or blue irradiation causes yellowing and darkening of epoxies.…”

“…A detailed discussion of internal thermal resistance paths is given in references [7] and [8]. To summarize, conduction is the dominant cooling path in LED packages followed by convection and radiation, which is quite different than traditional incandescent or halogen lamps where radiation is the primary cooling method.…”

“…Therefore, the thermal management system has to be designed with all of these components in mind to optimize the overall system efficiency. A more detailed discussion of some of the issues involved with these specific components is given in [8].…”

Thermal management technology of high-power light-emitting diodes for automotive headlights

“…In this study, the thermal resistance is composed of the cross-ventilation of the entire radiator fin and ambient. The three-way LED shows a parallel relationship; the thermal resistance of a single light source is 3R 8 . The thermal resistance from a single light source to the surroundings can be expressed as…”

“…However, a low electro-optical conversion efficiency of LEDs can induce a high percentage of input power that transforms into redundant heat. This leads to an increase in the junction temperature [7,8]. It is well known that high junction temperature in the LED would lead to reliability problems such as low quantum efficiency, wavelength shifts, short lifetime, and even catastrophic failure.…”