GaN基白光LED的结温测量

Methods for Determining Junction Temperature of GaN-based White LEDs

用正向电压法、管脚法和蓝白比法等三种方法测量GaN基白光LED的结温,获得了较为准确的结温,误差可以控制在4℃以内。正向电压法在恒定电流的条件下,得到了正向电压与结温的线性关系;蓝白比法在不同环境温度和不同注入电流两种情况下,都得到了蓝白比与结温较好的线性关系。提出了蓝白比法可能的物理机制,提高环境温度和增大注入电流都会使结温升高,蓝光峰值波长也会改变,这两个因素都会影响荧光粉的激发和发光效率。降低结温需要考虑的主要因素有白光LED的接触电阻、串联电阻和外量子效率,封装材料的热导率,反射杯和管脚的设计,以及空气散热部分的散热面积等。

Three methods, namely Vohage-Method, Pin-Method and Spectra-Method, for determining the junction temperature of GaN-based white LEDs were studied. In the Voltage-Method, we show that the forward voltage depends on the junction temperature linearly with uniform current. In the Spectra-Method, we show that the ratio R = W/B is strongly linear with the junction temperature as changing the ambient temperature or changing the forward current, where W and B are the entire and the blue part radiant power of the LED＇s emission, respectively. The physical mechanism of Spectra-Method and the factors of controlling the junction temperature were also discussed. Increasing either the ambient temperature or the forward current can lead to the rise of junction temperature and the shift of emission peak wavelength of GaN-based LED chips. These two factors can cause the decrease of excitation and emission efficiency of phosphor much greater than that of LED chips. To lower the junction temperature of white-LEDs, we need to consider the contact and series resistance, the emission efficiency, the thermal conductivity of encapsulation material, the design of reflector cup and pins, and the cooling surface area.