Can Junction Temperature Alone Characterize Thermal Performance of White LED Emitters?

Thermal performance of phosphor-based white light emitting diodes (LEDs) under an input current of 350 mA is investigated by finite-element simulation in which the thermal and optical interactions are considered. It is demonstrated that the temperature of the phosphor particles, regardless of phosphor placement, is always higher than the junction temperature. It is concluded that the junction temperature, which characterizes the thermal behavior of monotonic color LED emitters, cannot be used alone for characterizing the thermal behavior of white LED emitters. In fact, the phosphor temperature is critical in determining the lumen performance and reliability of white LED emitters. In addition, the phosphor temperature is effectively reduced by coating the phosphors directly on the chip and maintaining a relatively higher phosphor concentration (above 60 wt.%) in the phosphor-silicone mixture layer.