Phosphor-Silicone Coating Thickness Dependence of the White LED Junction Temperature

The junction temperature of a phosphor based white LED (WLED)is investigated as a function of the thickness of phosphor-silicone mixture coated on the chip. It is demonstrated that, to the contrary of a common believe, for a given correlated color temperature (CCT) with a fixed amount of phosphor in the mixture, the WLED junction temperature increases with decreasing thickness of the silicone phosphor mixture coating thickness. This observation can be explained in terms of the different coating thickness dependence of thermal resistance and the amount of backward scattering of photons into the chip respectively. Although, as expected, the thermal resistance of the coating layer will decrease as the thickness decrease, which will contribute to an enhanced thermal dissipation from the chip and thus a reduced junction temperature, but a much larger increase is expected by the amount of backward scattering of photons into the chip, which results in an overall increase in the junction temperature as the coating thickness is reduced.