Thermal analysis of light-emitting diodes based on photo-electro-thermal relationship

Thermal characteristics of light emitting diode (LED) devices directly effects on the luminous efficiency and long term reliability of LEDs. Junction temperature and thermal resistance are the main parameter that reflection of the heat dissipation capability of the LED device. In this study, relationships between thermal resistance and luminous flux and electrical power are simplified based on the photo-electrothermal theory. A new method using the optical-electrical characteristic of the LED is proposed to measure thermal resistance and junction temperature. The calculation of thermal resistance of the LED utilizes the reduction rate of luminous efficacy with the increasing temperature k_e, heat dissipation coefficient k_h, heat sink temperature T_hs and the electric power corresponding to maximum luminous flux P_d*. The thermal resistances of different brands and different power of LED devices are measured; the measuring results are highly consistent with nominal thermal resistance, which confirms the feasibility and generality of the proposed method. This method is simple and credible, and requires no expensive thermal measuring instruments, making it valuable for engineering application.