Key Laboratory of Optoelectronic Devices and System of Ministry of Education and Guangdong Provice, College of Physics and Optoelectronics Engineering, Shenzhen University, Shenzhen 518060, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61974094), the Key R & D Program of Guangdong Province, China (Grant No. 2020B010169003), and the Science and Technology Innovation Commission of Shenzhen, China (Grant No. JCYJ20200109105413475)
Received Date:09 July 2020
Accepted Date:03 March 2021
Available Online:15 April 2021
Published Online:05 May 2021
Abstract:According to the luminous spectrum characteristics of white light emission diode (WLED) light emission spectrum, through the analysis of the intersection (the trough point of the whole spectrum) of blue light spectrum and yellow light spectrum generated by blue light excited yellow phosphor, in this paper we design an LED steady-state thermal resistance measurement system based on the spectroscopic method by using the conventional spectrometer, and we also use the normal driving current to fit the whole spectrum trough through a certain function algorithm. According to the temperature rise curve, we can calculate the temperature rise of the LED junction temperature relative to the substrate under any working condition, and combine the heat dissipation power of the LED to get the steady-state thermal resistance of the LED. This method avoids the limitation of a similar forward voltage drop method which uses the minimum current calibration and requires the modules of high-speed data acquisition and high-speed sampling conversion, thus making the equipment expensive. Therefore it is necessary to reduce its cost. Finally, the system designed in this paper and the T3Ster instrument of Mentor Graphics Corporation in the United States are both used to measure various LEDs and their results are compared with each other. The results show that the maximum deviation of steady-state thermal resistance is only 3.64%. It indicates that the system and method designed in this paper can achieve the same precision as T3Ster instrument of Mentor Graphics Corporation, demonstrating that the system and method designed in this paper can achieve the same precision as the T3Ster instrument of Mentor Graphics, under the condition without needing expensive equipment, Moreover, this method uses non-traditional spectral method to measure the junction temperature of LED, which has the characteristics of remote real-time online detection of LED junction temperature, low cost, and no restrictions on the LED packaging structure. Therefore, this method has a wider application range than the voltage method adopted by Mentor Graphics T3Ster equipment, and has a certain practical value. Keywords:light emission diode/ steady-state thermal resistance/ trough/ heat dissipation power
表2相对于基准状态下, 不同温度下高色温白光LED光谱波谷相对强度 Table2.Relative strength of spectral trough of white LED with high color temperature at different temperatures to reference state.
将与基准状态下对比后的相对光谱强度与结温间的关系进行拟合, 得到如图3所示的定标曲线, 并得到定标函数y = 13.624x – 0.2033, 其中线性相关系数R2 = 0.9992, 拟合出的线性度相当高, 此定标函数的系数13.624即为谷处归一化光谱强度-结温敏感系数K. 图 3 高色温LED定标函数曲线图 Figure3. Calibration function curve of high color temperature LED.
再通过恒温器将LED基底保持在25 ℃环境中, 以350 mA的驱动电流点亮LED, 每隔20 s测量其发光光谱, 得到光谱波谷处的相对光强, 将其与基准状态下的波谷处的相对光强对比后, 代入定标函数y = 13.624x – 0.2033, 便可得出采用光谱法所测得的LED结温随工作时间的变化曲线, 如图4所示. 发现当LED达到热平衡时, 结温Tj约为43.12 ℃. 同时测得LED正常工作时其输入电功率P为1235 mW, 发光光功率P0为326 mW, 得到LED工作时的热耗散功率Pt为909 mW, 根据公式${R_{{\rm{js}}}} = \dfrac{{{T_{\rm{j}}} - {T_{\rm{c}}}}}{{P - {P_{\rm{0}}}}}$得到此高色温大功率LED的稳态热阻为19.93 ℃/W. 图 4 采用光谱法测得LED结温随时间变化曲线 Figure4. Curve of LED junction temperature with time measured by spectrum method.
25.2.大功率低色温白光LED -->
5.2.大功率低色温白光LED
随机选取某款荧光粉含量较多的大功率低色温白光LED (相关色温Tc = 3240 K)作为研究对象, 采用图2所示的系统测量其稳态热阻, 得到其发光光谱如图5所示, 以25 ℃为基准状态得到相对光谱强度数据如表3所列. 图 5 低色温归一化光谱相对强度分布 Figure5. Relative intensity distribution of low color temperature normalized spectrum.
温度差?Tj/℃
–20
–15
–10
–5
0
5
10
15
20
25
30
35
40
波谷处归一化强度差?I /%
–1.20
–0.83
–0.54
–0.15
0
0.33
0.70
0.97
1.36
1.68
1.94
2.41
2.71
表3不同温度下低色温白光LED光谱波谷相对强度 Table3.Relative strength of spectral trough of white LED with high color temperature at different temperatures.
将表3中波谷处的归一化强度差?I与结温差?Tj通过二维坐标的形式呈现, 结果如图6所示, 并通过线性拟合得到归一化强度差?I与结温差?Tj得到其定标函数y = 15.538x – 1.2111, 其中线性相关系数R2为0.9977, 线性程度大于0.99, 表明低色温大功率白光LED波谷处的归一化强度差?I和结温差?Tj仍具有较好的线性相关性, 此定标函数系数15.538即为波谷处归一化光谱强度-结温敏感系数K. 图 6 低色温LED定标函数曲线 Figure6. Calibration function curve of low color temperature LED.