LED is widely used in lighting, display and backlight because of its high brightness, low energy consumption, small size, long life and safety. It is considered to be the fourth generation to replace incandescent, fluorescent and high-pressure gas discharge lamps. Light source [1 ~ 4]. However, high-power LEDs still have heat dissipation problems such as fading problems such as light decay and color decay, and low external quantum efficiency and large thermal resistance of package structure [5, 6]. The increase in junction temperature caused by heat dissipation will cause a decrease in luminous efficiency and a shortened life [7, 8]. Therefore, the junction temperature, which is closely related to LED aging and heat dissipation, has become the starting point of the whole problem.
The LED chip is the core of the entire LED and is closely related to temperature due to its own characteristics. Ambient temperature, drive current, package structure, chip power, and lighting time all contribute to significant changes in LED junction temperature. The change of PN junction temperature will affect the photoelectric parameters such as luminous flux, color coordinates, luminous efficiency, relative spectrum and forward voltage [9,10]. Therefore, how to conveniently measure the LED junction temperature has become a common concern. As the junction temperature increases, the LED luminous efficiency and lifetime will be greatly attenuated. Therefore, the research on the LED junction temperature is of great significance for the evaluation of LED light source, thermal design and life prediction.
2.1 Packaging materials and equipment
LED chip was purchased from Wuhan Diyuan Optoelectronics Technology Co., Ltd. The phosphor is a rainbow LED YAG MLY-02D type. Phosphor gel and encapsulant were respectively used in 8866AB and 5212AB silica gel of Guangzhou Jieguo Electronic Technology Co., Ltd. Crystal expander and wire bonder respectively
It is the SH2002 type and SH2012 type of Shenzhen Sanhe Lighting Equipment Co., Ltd. The vacuum drying oven is the DZF type of Beijing Kewei Yongxing Instrument Co., Ltd.
2.2 Preparation of integrated package LED
The experimental material is industrial pure aluminum, and the cutting size is 50 mm Ã— 50 mm Ã— 5 mm. A reflector groove is made on the surface and then oxidized and insulated. The treatment steps are: degreasingâ†’cold water washingâ†’alkali etching (NaOH concentration: 45g/L, 50Â°C, 30s)â†’hot water washingâ†’cold water washingâ†’surface photochemical (HNO3, 130g/L, room temperature, 3min). The electrode is fixed at the groove of the reflector, and the enamel wire is welded on the electrode to complete the preparation of the integrated package substrate. The integrated package substrate is used to replace the bracket, the aluminum substrate and the heat sink in the conventional package, and the LED chip is directly crystallized onto the integrated package substrate.
2.3 Device performance characterization
The device was tested at room temperature and the spectral characteristics were tested using the PMS-50 UV-Vis-NIR spectroscopy system produced by Hangzhou Yuanfang Optoelectronic Information Co., Ltd. The thermocouple is DM6801A type of Shenzhen Xinbao Instrument Instrument Development Center; the power supply is MATRIX MPS-3303L-3 linear DC power supply; the voltage regulator is TDGC-6 contact voltage regulator of Shanghai Voltage Regulator Factory; It is a TEC1-12706 type semiconductor refrigeration sheet for Tianjin Central Electronic Refrigeration Technology Application; the heater sheet uses Huatian CTC type 220V-41W heating sheet.
2.4 junction temperature measurement
The thermal resistance of the integrated package substrate is very low, which is equivalent to the thermal resistance of pure aluminum. Based on its fast heat conduction characteristics, the cooling fin is fixed on the LED light source. The thermocouple reading is stable within 5 to 10 minutes, and the LED light source is in thermal equilibrium state. At this time, the spectral characteristics are measured. Adjust the plate voltage to get multiple sets of data. When the temperature of the integrated package substrate is constant during heat balance, and the thermal resistance of the integrated package substrate is very low, and the LED junction temperature is dynamically balanced, it can be approximated that the integrated package substrate temperature is the LED junction temperature, and the error does not exceed 0.5 Â°C.
3 Results and discussion
Figure 1 shows the junction temperature vs. forward voltage of the device. The device is a constant current drive with a current of 0.34A. As can be seen from the figure, as the junction temperature increases, the forward voltage decreases approximately linearly, and the voltage drops from 3.428V to 3.179V. That is, the LED junction temperature and the forward voltage have a negative temperature coefficient characteristic. This is because, at high temperatures, junction defects and impurities multiply and aggregate, which causes an increase in the extra recombination current, which causes the forward voltage to drop .
Figure 2 shows the junction temperature and luminous efficiency and luminous flux of the device. The overall lower light efficiency is due to the absence of a lens in the device. It can be seen from the figure that as the junction temperature increases, both the light efficiency and the luminous flux decrease. The falling speed of the luminous flux is higher than the falling speed of the light effect, and as the junction temperature increases, the current does not change, and the forward voltage decreases, that is, the power also decreases. This shows that the rate of decline of power is less than the rate of decline of the luminous flux, that is, the rate of decline of the forward voltage is less than the rate of decline of the efficacy. The main reason for the decrease of the luminous flux as the junction temperature increases is that there is a certain lattice mismatch between the epitaxial layers, thereby forming a large number of structural defects such as dislocations at the interface. At high temperatures, these defects rapidly multiply and proliferate until they invade the luminescent region. The formation of a large number of non-radiative recombination centers will reduce the injection efficiency and luminous efficiency of the device [15,16].