Abstract: Semiconductor light-emitting diode (LED) is a new type of light-emitting body with high electro-optical efficiency, small size, long life, low voltage, energy saving and environmental protection. It is an ideal lighting device for the next generation. LED photoelectric test is an important and only method to test the photoelectric performance of LED. The corresponding test results are the basis for evaluating and reflecting the current development level of my country's LED industry. The development of standards for LED photoelectric test methods is an important way to uniformly measure the photoelectric performance of LED products, and is a prerequisite for the test results to truly reflect the development level of my country's LED industry. This article combines the latest national standards for LED test methods to introduce several main aspects of the photoelectric performance test of LEDs.
Semiconductor light-emitting diodes (LEDs) have been widely used in indicator lights, signal lights, instrument displays, mobile phone backlights, automotive light sources, etc., especially with the development of white light LED technology, and LEDs are becoming more and more widely used in the field of lighting. However, in the past, there was no comprehensive national standard and industry standard for LED testing. In production practice, only relative parameters can be used as the basis. Different manufacturers, users, and research institutions have a lot of disputes about this, which has caused serious development of the domestic LED industry. influences. Therefore, national standards for semiconductor light-emitting diode test methods came into being.
2. LED test method
Based on the actual needs of various LED application fields, LED testing needs to include many aspects, including: electrical characteristics, optical characteristics, switching characteristics, color characteristics, thermal characteristics, reliability, and so on.
1. Electrical characteristics
LED is a unipolar PN junction diode composed of semiconductor inorganic materials. It is a type of semiconductor PN junction diode. The relationship between voltage and current is called volt-ampere characteristics. It can be seen from Figure 1 that the electrical characteristic parameters of the LED include forward current, forward voltage, reverse current and reverse voltage. The LED must be driven by a suitable current and voltage to work normally. Through the test of the electrical characteristics of the LED, the maximum allowable forward voltage, forward current, reverse voltage and current of the LED can be obtained, and the best working electrical power of the LED can also be determined.
Figure 1 LED volt-ampere characteristic curve
The electrical characteristics of LEDs are generally tested using a voltage and current meter under the power of a corresponding constant current and constant voltage source.
2. Light characteristics
Similar to other light sources, the test of LED light characteristics mainly includes luminous flux and luminous efficiency, radiant flux and radiant efficiency, light intensity and light intensity distribution characteristics, and spectral parameters.
(1) Luminous flux and light effect
There are two methods that can be used to test the luminous flux, the integrating sphere method and the variable angle photometer method. The variable-angle photometer method is the most accurate method to test the luminous flux, but because it takes a long time, the integrating sphere method is generally used to test the luminous flux. As shown in Figure 2, there are two test structures in the existing integrating sphere method for measuring LED luminous flux. One is to place the tested LED on the center of the sphere, and the other is to place the tested LED on the wall of the sphere.
Figure 2 Integrating sphere method to measure LED luminous flux
In addition, since the light source's self-absorption of light will affect the test results when the integrating sphere method measures the luminous flux, auxiliary lights are often introduced, as shown in Figure 3.
Figure 3 The auxiliary lamp method eliminates the influence of self-absorption
After the luminous flux is measured, the luminous efficiency of the LED can be measured with an electrical parameter tester. The test method of radiant flux and radiant efficiency is similar to that of luminous flux and luminous efficiency.
(2) Light intensity and light intensity distribution characteristics
Figure 4 Problems in LED light intensity test
As shown in Figure 4, the light intensity of the point light source is uniformly distributed in all directions in space, and the test results received by detectors with different receiving apertures at different distances will not change, but the LEDs are due to the inconsistency of their light intensity distributions. It varies with test distance and detector aperture. Therefore, CIE-127 has proposed two recommended test conditions to enable each LED to perform light intensity testing and evaluation under the same conditions. At present, CIE-127 conditions have been cited by various LED manufacturers and testing organizations.
Figure 5 CIE-127 recommended LED light intensity test conditions
(3) Spectral parameters
The spectral characteristic parameters of LEDs mainly include peak emission wavelength, spectral radiation bandwidth and spectral power distribution. The spectrum of a monochromatic LED is a single peak, and its characteristics are expressed in terms of peak wavelength and bandwidth, while the spectrum of a white LED is composed of multiple monochromatic spectra. The spectral characteristics of all LEDs can be represented by the spectral power distribution, and the chromaticity parameters can also be calculated from the spectral power distribution of the LED.
The measurement of the spectral power distribution needs to be carried out by light splitting, and the light of each color is distinguished from the mixed light for measurement. Generally, prisms and gratings can be used to achieve light splitting.
Figure 6 White LED spectral power distribution
3. Switching characteristics
LED switching characteristics refer to the light, electricity and color change characteristics of the LED at the moment of power-on and power-off. Through the test of the LED switching characteristics, the changing laws of the working state and material properties of the LED at the moment of power-on and power-off can be obtained, which can not only understand the loss of the LED on power-on and power-off, but also can be used to guide the design of the LED drive module.
4. Color characteristics
The color characteristics of LEDs mainly include chromaticity coordinates, dominant wavelength, color purity, color temperature, and color rendering. The color characteristics of LEDs are particularly important for white light LEDs.
The existing color characteristic test methods include spectrophotometry and integral method. As shown in Figure 7: The spectrophotometric method is to measure the LED spectral power distribution by a monochromator, and then use the chromaticity weighting function to integrate to obtain the corresponding chromaticity parameters; the integral method is to use a specific color filter and a photodetector to directly measure Chromaticity parameters; the accuracy of the spectrophotometric method is much higher than that of the integral method.
Figure 7 LED color characteristic test method
5. Thermal characteristics
The thermal characteristics of LED mainly refer to thermal resistance and junction temperature. Thermal resistance refers to the ratio of the temperature difference along the heat flow channel to the power dissipated in the channel. The junction temperature refers to the PN junction temperature of the LED. The thermal resistance and junction temperature of the LED are important factors that affect the photoelectric performance of the LED.
There are generally two methods for testing the junction temperature of LEDs: one is to measure the surface temperature of the LED chip using an infrared temperature measuring microscope or a miniature thermocouple and regard it as the junction temperature of the LED, but the accuracy is not enough; the other is One is to determine the junction temperature of the LED using the inverse relationship between the forward bias voltage and the junction temperature under a certain current.
The reliability of LEDs includes electrostatic sensitivity characteristics, lifetime, environmental characteristics, and so on.
The electrostatic sensitivity characteristic refers to the electrostatic discharge voltage that the LED can withstand. Some LEDs have high resistivity and a short distance between the positive and negative electrodes. If the electrostatic charge at both ends accumulates to a certain value, this electrostatic voltage will break down the PN junction. In severe cases, the PN junction can break down and cause the LED to fail. Therefore, the electrostatic sensitivity characteristics of the LED must be tested to obtain the critical voltage of the LED's electrostatic discharge failure. At present, the human body model, machine model, and device charging model are generally used to simulate the electrostatic discharge phenomenon in real life.
In order to observe the change law of the light performance of the LED under the condition of long-term continuous use, it is necessary to conduct a sampling test on the LED, and obtain the LED life parameters through long-term observation and statistics.
For the test of LED environmental characteristics, it is often used to simulate various natural attacks encountered by LEDs in the application. Generally, there are: high and low temperature impact test, humidity cycle test, humidity test, salt spray test, sand dust test, radiation test, vibration and Impact test, drop test, centrifugal acceleration test, etc.
3. Formulation of National Standards
Summarizing the above test methods, the national standards for semiconductor light-emitting diode test methods have made corresponding provisions on the electrical characteristics, optical characteristics, thermal characteristics, electrostatic characteristics and life tests of LEDs.
For electrical characteristics testing, the standard specifies the test block diagrams for LED forward voltage, reverse voltage, and reverse current; for luminous flux testing, the standard specifies the use of a 2π solid angle test structure; for light intensity testing, the standard quotes CIE-127 Recommended conditions; in addition, clear regulations have been made for spectrum testing, thermal characteristics testing, electrostatic discharge sensitivity testing, and life testing.
The formulation of the national standard summarizes the existing LED test methods, and upgrades the scientifically applicable methods to the standard test methods, which eliminates the differences in the field of LED testing from all walks of life, and makes the test results more truly reflect my country’s LEDs. The overall level of the industry. However, in view of the continuous development of LED technology, the formulation of national standards is not done once and for all, and the latest and most appropriate test technology should be introduced into the standard at all times.