Abstract: By measuring the junction temperature and thermal resistance of various color LEDs under different driving currents, it is found that the thermal resistance values ​​of LEDs of various colors become larger with the increase of driving current. Among them, blue and white LEDs based on InGaN materials work less than rated. When the current is under, the thermal resistance rises rapidly; when the driving current is greater than the rated current, the thermal resistance rise rate becomes slower. Other color LED thermal resistances change substantially with the rate of change of the drive current. The junction temperature also increases as the drive current increases. At the same drive current, the junction temperature of 1W red and orange LEDs based on AlGaInP material is lower than the junction temperature of blue, green, and white LEDs based on InGaN materials. The junction temperature of the self-made 1mm×1mm blue chip was measured by forward voltage method and infrared thermal imager respectively, and the advantages and disadvantages of the two methods were compared. The results show that the electrical measurement is simple and fast, and the measurement results can meet the requirements.
introduction
The Global Lighting Association said that in the near future, Powerlight-emitting diodes will play a vital role in general lighting. Since 1994, high-power LEDs have developed rapidly and have replaced traditional light sources in many fields (such as street lights, automotive taillights, LCD backlights, etc.). In recent years, the development of LED technology is changing with each passing day. The improvement of light efficiency and the decline of device cost are subject to Haitz's law similar to Moore's Law, that is, the price of LED is reduced to 1/10 per 10 years, and the performance is Increase by 20 times.
Internationally, LED technology is developing in the direction of high power, high brightness, high efficiency and low cost. The optical and electrical properties of power LEDs are strongly dependent on junction temperature. As the LED power increases, too high a junction temperature will affect the life and reliability of the LED, and the heat dissipation problem becomes increasingly severe. Therefore, it is especially important to understand the variation characteristics of the power LED junction temperature and thermal resistance. In this paper, the forward voltage method and the infrared thermal imager method are used to study the junction temperature and thermal resistance of the power LED with the change characteristics of the current.
1, power LED junction temperature measurement method
According to the standard, the general definition of thermal resistance is the ratio of the temperature difference between two specified points (or regions) and the dissipated power that produces the temperature difference between the two points (in °C/W or K/W) under thermal equilibrium conditions. The size of the thermal resistance directly affects the life, light output, and luminous intensity of the LED. For LEDs, since the heat source is at the pn junction, its maximum temperature is usually referred to as the temperature of the pn junction, that is, the junction temperature Tj, which is also an important parameter affecting the reliability of the LED. At present, the more mature junction temperature measurement methods are infrared camera method and forward voltage method (also known as standard electricity method). The infrared camera method provides a two-dimensional temperature distribution of the surface of the chip by measuring the infrared radiation on the surface of the chip during operation, thereby characterizing the junction temperature and its distribution. This method can only measure unpackaged devices, and for finished devices. Need to open the seal to measure. The forward voltage method is a non-destructive chip temperature measurement method. Compared with the infrared thermal imaging method, the forward voltage method has the advantages of high sensitivity, rapid measurement, and low test cost.
2, experimental samples
The tested samples are power LEDs for streetlights and night lighting, including 1WInGaN blue, green LED, 1WAl-GaInP red, orange LED, and 1W, 3W sapphire substrate InGaN white LED. All color chips are cooled with metal aluminum. Substrate material. The 1W sample is a 1 mm x 1 mm chip. The 3W LED is a parallel structure of two 1W chips, and white light is realized by coating YAG phosphor powder on the surface of the In-GaN blue LED.
3. Analysis of the experiment and results
The ambient temperature was set to 25 ° C during the test, the drive current was increased from 100 mA to 1 A, and the growth interval was 100 mA.
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