The brighter the LED chip, the greater the heat generation, or the darker the chip, the greater the heat generation? When encountering this problem, I believe many people will think that the darker the chip, the greater the heat generation, because more energy is converted into heat energy. However, this is not the case. The brighter the LED chip, the larger or smaller the heat generation may be. This is because the current density will determine the optical power and thermal power of the chip, and the temperature will also affect the luminous efficiency of the chip. So how to analyze the light-emitting and heat-generating performance of LED chips and make use of them? Next, we will use the Jinjian micro-photothermal distribution test system to test and analyze, and find out with everyone.
Chip Luminescence Is Not Directly Proportional To Heat Generation
The following is the test data of the same chip through the micro-photothermal distribution test system developed by Jinjian under different currents. As shown in the figure, the relationship between the brightness and temperature of the chip changes with the current. We found that the light emission of the chip is not directly proportional to the heat generation.
- With the increase of the loading current, the brightness of the chip increases gradually, and begins to decay after reaching the maximum value, but the temperature of the chip increases sharply with the increase of the current!
- Under high current, if the current is increased, the chip will not become brighter, and may even become darker. This is because the LED chip has the problem of “quantum efficiency drop” (Efficiency Droop) under high driving current. This is also a key bottleneck restricting the application of high-power LED lighting and the development of high-efficiency and high-power lighting technology.
- The rated current specified by the chip manufacturer is 450mA, and the chip will indeed be brighter when used with super current. However, excessive supercurrent will no longer produce light energy, but heat energy, which will greatly endanger the life of the LED chip.
Why Do Lighting Factories Like Super Current?
There are several reasons for this:
- In the past, the LED industry did not have micro-photothermal analysis equipment to study the microscopic relationship between current, luminescence and heat generation of the chip. No in-depth research has been done in this area, and there are only some empirical values based on feelings.
- The chip factory’s regulations on the rated current are based on the size of the chip, which is not scientific. It does not take into account that the current density of the chip will change with the epitaxy, chip process, and graphic design.
- The lamp factory pursues high brightness, and they found that the lamp does not seem to have any problems when the rated current of the chip is exceeded. Over time, the lighting factory no longer trusts the rated current parameters provided by the chip factory, and uses over-current at will. However, this kind of behavior also has potential safety hazards, because the overshoot is completely based on feeling, lacking the support of test data theory, so that excessive overcurrent leads to lamp burning quality accidents.
- It can be seen that using Jinjian microphotothermal analysis equipment to draw the relationship curve of LED chip luminous intensity, heat generation and lighting current can guide LED chip, packaging and lamp factories to develop in a standardized and healthy direction.
Case Study
The picture below is the photothermal distribution diagram of the same chip tested by the microphotothermal distribution test system under different currents. The current in Fig. 2 is selected to be the current value of 590mA at the highest brightness point and the same brightness of 450mA and 760mA on both sides of the highest point. From this, we can clearly observe that the LED light source produces heat but does not produce light when excessive supercurrent is used.
- The rated current of the LED chip is 450mA. When the super current is 590mA, the brightness increases by about 9.3%, and the chip temperature rises by 41.2°C, about 46.2%.
- When the overcurrent is 760mA, the brightness does not increase, and the chip temperature rises by 122.7°C, about 137.6%. Excessive supercurrent, the brightness remains unchanged, and the temperature rises more than 2 times!
- If the lighting factory wants to exceed the current, it needs to choose the range of 450~590mA, which is the high quantum efficiency of the chip, so as to avoid the phenomenon of heat but not light!
Through the microscopic light and heat distribution system, chip factories, packaging factories and lamp factories can locate the current and voltage with the highest efficiency, and avoid blind use of super current to cause lamp burning!
Ambient Temperature Affects The Luminescence And Heat Generation Performance Of The Chip
The photothermal performance of LED light sources is greatly affected by temperature, and the test results that are separated from the actual working temperature are less accurate or even meaningless. The following is a comparison of the photothermal distribution diagrams of the chip measured at different temperatures by the Jinjian microscopic photothermal distribution test system, and the change of the chip brightness at different temperatures is also given.
- The higher the temperature, the worse the chip light distribution uniformity. This is because the temperature affects the performance of the material, and the high temperature makes the current expansion capability of the chip worse.
- As the temperature increases, the luminous intensity decreases, and the higher the temperature, the greater the luminous decay. When the temperature of the bracket pins rises from 80°C to 120°C, the temperature of the LED chip rises by about 45°C, and the luminous intensity decays as high as 30.6%!
- Therefore, the test data in the non-actual use temperature environment is inaccurate!
The actual operating temperature is crucial to the photothermal performance of the LED chip! The microscopic heat distribution test system independently developed by Jinjian is equipped with a high and low temperature digital display precision temperature control platform, which can stably control the temperature of the lamp bead pin and the substrate temperature, and simulate the actual working temperature of the device for testing, providing more real and effective data . The test platform is also equipped with a water-cooling cooling system, which can reduce the temperature of the platform from 100°C to room temperature within 100s, which effectively solves the problem of difficult cooling of the sample stage. The system can also stably control the temperature of the sample stage at 0°C-room temperature. It is suitable for some devices that need to maintain low temperature operation.
