Study on the properties of power LED junction temperature and thermal resistance at different currents

introduction

The Global Lighting Association said that in the near future, high- power light- 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, car taillights, LCD backlights, etc.). In recent years, the development of LED technology has been changing with each passing day. The improvement of its light efficiency and the reduction of device costs follow the Haitz law similar to Moore's law, that is, the price of LEDs is reduced to 1/10 of the original price every 10 years, and the performance is improved by 20 times.

Internationally, LED technology is developing towards 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 power of LEDs increases, excessively high junction temperature will affect the life and reliability of LEDs, and the heat dissipation problem becomes increasingly severe. Therefore, it is particularly important to understand the changing characteristics of power LED junction temperature and thermal resistance. This paper studies the changing characteristics of power LED junction temperature and thermal resistance with current through the forward voltage method and infrared thermal imaging method.

1. Power LED junction temperature measurement method

According to the standard, the general definition of thermal resistance is: under the condition of thermal equilibrium, the ratio of the temperature difference between two specified points (or areas) to the dissipated power that produces the temperature difference between the two points (unit: °C/W or K/W). The size of thermal resistance directly affects the life, light output rate, luminous intensity , etc. of LED. For LED, since the heat source is at the pn junction , its highest temperature usually refers to the temperature of the pn junction, that is, the junction temperature Tj, which is also an important parameter affecting the reliability of LED. At present, the more mature junction temperature measurement methods are infrared thermal imaging method and forward voltage method (also known as standard electrical method). The infrared thermal imaging method measures the infrared radiation on the chip surface when the device is working to give the two-dimensional temperature distribution on the chip surface, so as to characterize the junction temperature and its distribution. This method can only measure unpackaged devices, and finished devices need to be unsealed for measurement. 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 samples tested are all power LEDs for street lamps and night scene lighting, including 1W InGaN blue and green LEDs, 1W Al-GaInP red and orange LEDs, and 1W and 3W sapphire substrate InGaN white LEDs. All color chips use metal aluminum as the heat dissipation substrate material. The 1W sample is a 1mm×1mm chip. The 3WLED is a parallel structure of two 1W chips. The white light is achieved by coating the surface of the In-GaN blue LED with YAG phosphor.

3. Experiment and result analysis

During the test, the ambient temperature was set to 25°C, and the drive current increased from 100mA to 1A in increments of 100mA.

3.1 Analysis of thermal resistance measured by forward voltage method

Figure 1 is a trend chart of the thermal resistance of 1W AlGaInP red and orange LEDs with driving current at an ambient temperature of 25°C. As shown in Figure 1, the thermal resistance of 1W AlGaInP red and orange LEDs increases with the increase of driving current. Under the same driving current, the thermal resistance of orange AlGaInP LED is higher than that of red LED. During the change of driving current, the thermal resistance of orange LED increases from 10.28°C· W -1 to 15.05°C·W-1, and the thermal resistance of red LED increases from 9.85°C·W-1 to 13.25°C·W-1. The reason for this difference is that under the same input power, the electro-optical conversion efficiency of orange LED is lower than that of red LED, that is, under the same injection current, AlGaInP orange LED has a higher junction temperature than red LED.

Figure 1: AlGaInP red and orange LED thermal resistance trend

Figure 2 is a graph showing the thermal resistance of 1W InGaN green and blue LEDs changing with the driving current at an ambient temperature of 25°C. As can be seen from the figure, the thermal resistance of InGaN green and blue LEDs increases with the increase of driving current. The thermal resistance of blue LEDs increases from 10.02°C·W-1 to 21.57°C·W-1, while the thermal resistance of green LEDs increases from 13.74°C·W-1 to 17.68°C·W-1, and the change range is smaller than that of blue LEDs. When the blue LED is operated at a driving current greater than the rated operating current of 350mA, the change in thermal resistance tends to be moderate. Since the various defects inside the device and the mismatch of materials reach a stable value when the device is operated at a current greater than the rated current, the effect of the increase in current on them is not as obvious as in the low current stage (unless the current is added to a level sufficient to cause the internal electrode of the LED to warp and the gold wire to fuse), resulting in the internal obstacles of the device that hinder the conduction of heat flow to the outside not changing much with the increase in driving current. The article believes that the increase in thermal resistance may be due to the current crowding effect caused by large current, which in turn leads to a decrease in electro-optical conversion efficiency (reduction in radiation recombination area). Although the input electrical power increases, the output optical power decreases as the current increases, which ultimately leads to an increase in thermal resistance.

Figure 2: InGaN green and blue LED thermal resistance trend

Figure 3 is a graph showing the thermal resistance of 1W InGaN white and blue LEDs versus driving current at an ambient temperature of 25°C. Although white LEDs have an extra layer of YAG phosphor than blue LEDs, as shown in Figure 3, the thermal resistance values ​​of the two are not much different, indicating that the YAG phosphor does not seriously affect the heat dissipation of 1W white LEDs. The internal heat of power LEDs is rarely dissipated by radiation, and is mainly dissipated to the outside by conduction from the chip to the substrate, and from the substrate to the aluminum substrate.

Figure 3: InGaN-based white and blue LED thermal resistance trend

Figure 4 is a trend chart of the thermal resistance of 3W white light LEDs changing with driving current, where Figure 4(a) is a trend chart of the thermal resistance of 3W white light LEDs measured by Jayasinghe et al. of the American Lighting Research Center at an ambient temperature of 25°C under different driving currents, and Figure 4(b) is a trend chart of the thermal resistance of 3W InGaN-based white light LEDs measured at the same ambient temperature. The LED chips used in the two experiments are of the same size, but the tube measured by the American Lighting Research Center is larger than the author's package. In Figure 4(a), when the driving current changes from 100 to 800mA, the thermal resistance value increases from 8°C·W-1 to 15°C·W-1. In the same current change range, the thermal resistance value in Figure 4(b) increases from 7.5°C·W-1 to 19°C·W-1, with a small difference, indicating that China's high-power white light LEDs are developing rapidly and their heat dissipation performance is already relatively good.

Figure 4: (a) The thermal resistance of a 3W white LED measured by the Lighting Research Center of the United States varies with current; (b) The thermal resistance of a 3W white LED varies with input current

3.2 Junction temperature measurement analysis using the forward voltage method

Table 1 shows the junction temperature of 1W power LEDs of different colors at the corresponding current when the ambient temperature is 25°C and the driving current varies from 100 to 1000mA. As can be seen from the table, the junction temperature of power LEDs of various colors increases with the increase of driving current. Analysis shows that as the driving current increases, a current crowding effect will occur inside the LED. Current crowding will lead to a decrease in light output efficiency (reduced radiation recombination), thus causing the junction temperature to rise. The increase in junction temperature will lead to changes in the thermal conductivity of the LED material. Some groups have found that the thermal conductivity of GaN decreases from 2.50W/(cm·K) to 1.75W/(cm·K) at 25 to 175°C[4]; others have found that the thermal conductivity of GaN decreases from 2.0W/(cm·K) to 1.6W/(cm·K) at a temperature of 25 to 125°C[5]. In turn, the decrease in the thermal conductivity of the material will restrict the thermal conduction of the LED, further increasing the junction temperature of the LED. This mutual restriction may even form a vicious cycle. In addition, excessive current will also lead to changes in the mismatch between the contact layers of the LED, degradation of the solder, etc., and will also cause the LED temperature to rise.

Table 1: Junction temperature values ​​of 1W power LEDs of various colors at different drive currents measured by the forward voltage method

Secondly, it can be seen from the table that the junction temperature of the red and orange LEDs made of AlGaInP material is not much different under the same driving current. The junction temperature of the blue, green and white LEDs made of InGaN material is also very similar, while the junction temperature of the LED made of AlGaInP material is much lower than that of the LED made of InGaN material. This is due to the difference in the bandgap width of the materials. Under the same input current, the voltage value of the LED made of InGaN material is higher than that of the red and orange LEDs made of AlGaInP material. Although the photoelectric conversion efficiency of the InGaN material LED is higher, the value of its electrical power converted into thermal power is still greater than that of the Al-GaInP red and orange LED. That is, under the same driving current, the thermal power generated by the In-GaN material LED is greater than that of the AlGaInP material red and orange LED. Moreover, since the P-type doping concentration of the InGaN material is lower than that of the AlGaInP material, the series ohmic resistance of the InGaN chip is greater than that of the AlGaInP material. The heat generated by the series ohmic resistance under high current conditions [7] is also an important factor leading to the different junction temperatures of the two chip LEDs.

Again, the junction temperature of the red LED made of AlGaInP material is lower than that of the orange LED made of the same chip material, which proves that
the explanation of Figure 2 in the article is reasonable.

3.3 Comparison between forward voltage method and infrared thermal imaging method

The forward voltage method and the infrared thermal imager method were used to compare the junction temperature measurement methods using a 1mm×1mm chip made in the laboratory. Figure 5 shows the junction temperature change curves of a 1W blue LED measured by the two methods at different drive currents. As can be seen from the figure, the junction temperature values ​​measured by the two methods are basically the same. Regardless of the method, the junction temperature increases with the increase of the drive current. The forward voltage method obtains the average temperature effect. In contrast, the infrared thermal imager method can quickly obtain the temperature distribution image on the surface of the device, show the overall overview of the chip quality, and clearly display the distribution density of the hot spots, which may be the main factor causing the thermal failure of the device. In particular, in recent years, through the combination of modern high-speed development of computer technology, microelectronics technology and image processing technology, the sensitivity, accuracy, stability and automation of optical temperature measurement technology have been greatly improved, and its application field has become more and more extensive. However, its disadvantage is that it can only measure unpackaged bare chips, and the packaged chips must be unpacked before measurement, and the measuring instrument is expensive.

Figure 5: Junction temperature of blue LED measured by forward voltage method and infrared thermal imaging method

Figure 6 is the surface temperature distribution of a blue LED measured by an infrared thermal imager when the driving current is 800mA. As can be seen from the figure, the temperature distribution of a large area of ​​this flip-chip structure is relatively uniform, with the highest temperature being 79.37°C, mainly concentrated in the P area near the N-type electrode soldering point. The lowest temperature is 70.43°C, and the temperature difference is small. The main reason is that this LED chip uses a ring-shaped interdigitated electrode structure to reduce the current expansion path, reduce the lateral resistance of the current flowing in the N-type area, and reduce the heat generated, so the device temperature rise is small.

Figure 6: Surface temperature distribution of 1W blue LED

4. Conclusion

By measuring the junction temperature and thermal resistance of LEDs of various colors under different driving currents, it is found that the thermal resistance of any color LED increases with the increase of driving current. When the blue and white LEDs made of InGaN material work at a current lower than the rated current, the thermal resistance increases rapidly; when the driving current is higher than the rated current, the rate of increase of thermal resistance slows down. The rate of change of thermal resistance of other color LEDs with driving current is basically unchanged. The junction temperature will also increase with the increase of driving current. Under the same driving current, the junction temperature of red and orange LEDs made of AlGaInP material is lower than that of blue, green and white LEDs made of In-GaN material. The junction temperature values ​​of blue LEDs measured by the forward voltage method and the infrared thermal imager method are compared, and the advantages and disadvantages of the two methods are analyzed. The results show that the infrared thermal imager method can intuitively reflect the highest temperature area of ​​the chip, and the failure of the device is ultimately determined by the highest temperature; but the junction temperature measured by the forward voltage drop method is not much different from that of the infrared method. As a quick, convenient and non-destructive method, it can be widely adopted first.