Thermal Impedance Measurement on Different Chip Arrangements for Various Multichip LEDs Application

The effect of thermal impedance of three chip arrangements in a multichip light-emitting diode (LED) package has been discussed with each chip attached to a heat sink with thermal interface material. Each orientation represents different existing lighting applications such as T8 tube, MR 16 down light, and spotlight. In this paper, the emitter and sensor current are constantly maintained. One LED of each orientation is driven, while other LEDs are sensed until all LEDs are driven with the emission current of all the orientations. Comparisons are made between the thermal impedance matrix pairs of each LED arrangement to illustrate the difference in the thermal dissipation. Thermal impedance curves show the fluctuation in the thermal flow inside the package, implying the reciprocal behavior of each orientation. This paper further explains the reciprocal behavior of the chips and the importance of the orientation based on junction temperature. Moreover, the principle of linear superposition is implemented to estimate the junction temperature matrix. The theoretical and the experimental results are compared using thermal transient tester. Among three orientations, the square-shaped LED arrangement shows an average junction temperature of 116 °C within the package while exerting the highest change of junction temperature of 4 °C, comparing with the linear superposition method.