Thermal Analysis of High Power White Light-emitting Diodes.

摘要

White light-emitting diodes (LEDs) can be readily produced by combing a monochromatic LED with phosphors. Heat is generated in the junction and packaging materials during the electrical-optical energy conversion process and light conversion/extraction process. Therefore, thermal management is important for high power white LEDs.;In the first part of this work, the influence of die attach adhesive (DAA) on thermal performance of high power LED emitter was studied. It shows that the junction temperature can be significantly reduced by increasing the thermal conductivity of DAA material (up to 20 W/mK), and/or enlarging the DAA area (up to the size of the die), and/or reducing the bond-line thickness of the DAA layer.;The following three parts investigated the factors that affect the thermal performances of a single-chip white LED package and a multi-chip white LED package by a combination of optical and thermal simulations. The simulation models were verified by corresponding experiments. It shows that the phosphor temperature is always higher than the junction temperature and should be evaluated for thermal design. At a given CCT of light output, the junction temperature decreases as phosphors are moving away from the chip (in the order of coating, incup, and remote) due to less absorption of backscattered light. However, the phosphor temperature decreases as phosphors are approaching the chip and/or as the concentration in the phosphor layer is increased (or the thickness of the phosphor layer is reduced) as a result of enhancement of heat conduction in the phosphor layer after formation of low resistance chains of phosphor particles. The phosphor temperature can be up to 211.4 degree Celsius in a 10 x 10 chip array (100 W) LED package with remote phosphor.;The last part focused on the cooling effect of high emissivity coatings. Under free convection, a reduction of 10.9 degree Celsius and 11 degree Celsius in the board temperature was observed when a 10% ZnO coating and 10% TiO 2 coating was applied on board, respectively. It is attributed to the enhancement of heat conduction and an increase in the effective surface area by nano-sized filler in silicone matrix.