Thermal Characterization of LED Packages Covered with Wavelength Converting Phosphor Over a Large Area

Decreasing trend of thermal resistance was observed during the lifetime test of 2835 type mid-power LEDs. According to the results, the decrease in the radiant flux of the LEDs is mainly caused by the degradation of the wavelength converting phosphor. No significant changes could be observed in the electrical parameters, while the degradation of the main heat conduction path is not justified by any aging conditions. Moreover, the decrease in the value of the thermal resistance has a moderate correlation (with the value of 0.57) with the decrease in the radiant flux. In such a case, a suspicion arises that there could be a systematic error in the evaluation of the measurement results. This paper deals with investigation, revelation and possible elimination of this issue.