An experimental and computational study on efficiency of white LED packages with a thermocaloric approach

Abstract

Thermal management of light-emitting diode (LED) chips is crucial for light extraction and lifetime. It is well known that the light output of an LED decreases with the elevated temperatures. For higher light extraction, the power input to the chip should be substantially high leading nonuniform current spreading and local joule heating at the chip active layers. However, this leads to a high amount of heat generation at the chip and a considerable amount of increase in the junction temperature. Besides shortening the lifetime of the chip, it also strongly affects the light output of the system. Although nominal driving currents for LEDs are around 350-400 mA, the ideal operating condition for the cost effectiveness at higher driving currents and corresponding efficiency of an LED chip is to be explored. In this paper, LED chips' thermal and optical behaviors were investigated for different driving conditions while the board temperature is controlled using a thermoelectric cooler and the input current to the chip. The system was numerically investigated using a computational fluid dynamics software and validated with experimental studies. Consequently, a correlation for efficiency covering a wide range of operating conditions is presented. The efficiency of the LED that is obtained for 30 °C is 42%, whereas it drops to 30% for 50 °C board temperature. If one assumes a logarithmic relationship between the efficiency and the board temperature, the efficiency is expected to be around 20% for a typical LED operating temperature of between 80 °C and 100 °C.