Estimation Of Junction Temperature Of LED Based On Its Series Resistance

Because of the small size,high luminous efficiency,and long lifetime,LED has been widely used in the lighting and display applications.While the high current density chip technology advances,the efficiency and lifetime of high power LED will be enhanced furthermore,and the corresponding market will be rapidly penetrated.The main factor,determining the luminous efficiency and lifetime of a LED,is the junction temperature.The high power LED tends to have a high junction temperature due to its large power density,and in the later use,the LED will decay quickly and its lifetime is shortened.In order to improve the reliability,the junction temperature of LED must be measured and the LED products with high junction temperature must be eliminated.The present methods of measuring LED junction temperature are the spectrum method and the forward voltage method.These two methods are not applicable for the industry use because of the high cost of test equipment and the complicated operationsThe aim of this study is to develop a method of measuring LED junction temperature with acceptable accuracy,low cost and simple process,especially for LED driven by high current density.In this paper,we first study the abnormal I/V characteristics of LEDs due to the thermal defects.In these LEDs,contrary to expectations,the voltage difference between the initial state and the final stead state does not increase along current,but tends to saturate or even decrease.After rigorously examining the relationship between the temperature and the parameters in the LED I/V model,it is found that the series resistance term may be the cause of the abnormality.Thereafter,with considerations of the temperature dependent series resistance,a junction temperature measurement method based on series resistance is proposed.At the small current region,the measured data was used to fit the curve of the differential resistance of the ideal PN junction.Then,using the fitting data and the measured data at the high current region,it can be deduced that the series resistance changes with the current density at the high current region,and then,according to the functions of current density and junction temperature,the junction temperature can be obtained.The junction temperatures of six LED lamps are measured by the proposed method and the small current forward voltage method.The results show high confirmation.There is a certain error,but,as a production qualified method,the method is applicable for the industry user.