LED testing methods and formulation of national standards (Figure)

1. Introduction

Semiconductor light emitting diodes (LEDs) have been widely used in indicator lights, signal lights, instrument displays, mobile phone backlights, vehicle-mounted light sources, etc., especially with the development of white light LED technology, the application of LEDs in the field of lighting is becoming more and more extensive. However, in the past, there were no comprehensive national standards and industry standards for LED testing. In production practice, only relative parameters could be used as the basis. Different manufacturers, users, and research institutions had great disputes over this, which seriously affected the development of the domestic LED industry. Therefore, the national standard for semiconductor light emitting diode testing 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, etc.

1. Electrical characteristics

LED is a unipolar PN junction diode made of semiconductor inorganic materials. It is a type of semiconductor PN junction diode. The relationship between its voltage and current is called the volt-ampere characteristic. As shown in Figure 1, the LED electrical characteristic parameters include forward current, forward voltage, reverse current and reverse voltage. The LED must be driven by a suitable current and voltage to work properly. Through the test of LED electrical characteristics, the maximum allowable forward voltage, forward current, reverse voltage and current of the LED can be obtained. In addition, the optimal working power of the LED can also be determined.

Figure 1 LED volt-ampere characteristic curve

The test of LED electrical characteristics is generally carried out using a voltage and current meter under the power supply of a corresponding constant current and constant voltage source.

Figure 2 Integrating sphere method to measure LED luminous flux

In addition, since the self-absorption of light by the light source will affect the test results when the integrating sphere method is used to test the luminous flux, an auxiliary lamp is often introduced, as shown in FIG3 .

Figure 3 Auxiliary lamp method to eliminate the influence of self-absorption

After measuring the luminous flux, the luminous efficiency of the LED can be measured with an electrical parameter tester. The test methods for radiant flux and radiant efficiency are similar to those for luminous flux and luminous efficiency.

(2) Light intensity and light intensity distribution characteristics

Figure 4 Problems in LED light intensity testing

As shown in Figure 4, the light intensity of a point light source is evenly distributed in all directions in space. The test results obtained by using detectors with different receiving apertures at different distances will not change. However, due to the inconsistency of the light intensity distribution of LEDs, the test results vary with the test distance and the detector aperture. Therefore, CIE-127 proposes two recommended test conditions to allow each LED to be tested and evaluated under the same conditions. Currently, CIE-127 conditions have been cited by various LED manufacturers and testing agencies.

Figure 5 CIE-127 recommended LED light intensity test conditions

(3) Spectral parameters

The spectral characteristic parameters of LED mainly include peak emission wavelength, spectral radiation bandwidth and spectral power distribution. The spectrum of monochromatic LED is a single peak, and the characteristics are expressed by peak wavelength and bandwidth, while the spectrum of white light LED is synthesized by multiple monochromatic spectra. The spectral characteristics of all LEDs can be expressed by spectral power distribution, and the chromaticity parameters can also be calculated from the spectral power distribution of LED.

The test of spectral power distribution needs to be carried out by spectrometry, which separates each color of light from the mixed light for measurement. Generally, prisms and gratings can be used to achieve separation.

Figure 6 White LED spectral power distribution

3. Switching characteristics

LED switching characteristics refer to the light, electricity, and color change characteristics of LEDs when they are powered on and off. Through the test of LED switching characteristics, the changing rules of the working state and material properties of LEDs at the moment of power on and off can be obtained. This can not only understand the loss of LEDs caused by power on and off, but also guide the design of LED driver modules.

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 testing methods include spectrophotometry and integration. As shown in Figure 7: The spectrophotometry method is to measure the LED spectral power distribution through monochromator spectroscopy, and then use the chromaticity weighted function integration to obtain the corresponding chromaticity parameters; the integration method is to use a specific color filter in combination with a photodetector to directly measure the chromaticity parameters; the accuracy of the spectrophotometry method is much higher than that of the integration method.

Figure 7 LED color characteristics 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. Junction temperature refers to the PN junction temperature of the LED. The thermal resistance and junction temperature of the LED are important factors affecting the optoelectronic performance of the LED.

There are generally two methods for testing the junction temperature of the LED: one is to use an infrared temperature measuring microscope or a micro thermocouple to measure the temperature on the surface of the LED chip and regard it as the junction temperature of the LED, but the accuracy is not enough; the other is to use the inverse relationship between the forward bias voltage and the junction temperature under a certain current to determine the junction temperature of the LED.

6. Reliability

The reliability of LED includes electrostatic sensitivity characteristics, life, environmental characteristics, etc.

Electrostatic sensitivity characteristics refer to the electrostatic discharge voltage that the LED can withstand. Some LEDs have a 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 be broken down and the LED fails. Therefore, the electrostatic sensitivity characteristics of the LED must be tested to obtain the critical voltage of the electrostatic discharge failure of the LED. At present, human body mode, machine mode and device charging mode are generally used to simulate the electrostatic discharge phenomenon in real life.

In order to observe the change law of the light performance of LED under long-term continuous use, it is necessary to conduct sampling tests on LEDs and obtain LED life parameters through long-term observation and statistics.

Tests on the environmental characteristics of LEDs often use various natural invasions that simulate LEDs in applications, generally including: high and low temperature impact tests, humidity cycle tests, moisture tests, salt spray tests, dust tests, irradiation tests, vibration and impact tests, drop tests, centrifugal acceleration tests, etc.

3. Formulation of national standards

Summarizing the above test methods, the national standard for semiconductor light-emitting diode test methods has made corresponding provisions for LED electrical characteristics, optical characteristics, thermal characteristics, electrostatic characteristics and life tests.

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 stipulates the use of a 2π solid angle test structure; for light intensity testing, the standard quotes the recommended conditions of CIE-127; in addition, clear provisions are made for spectrum testing, thermal characteristics testing, electrostatic discharge sensitivity testing, life testing, etc.

IV. Conclusion

The formulation of national standards summarizes the existing LED test methods, upgrades the scientific and applicable methods into standard test methods, effectively eliminates the differences among various circles in the field of LED testing, and makes the test results more truly reflect the overall level of China's LED industry. However, given that LED technology is still in the process of continuous development, the formulation of national standards is not a one-time solution, and the latest and most appropriate test technologies should always be introduced into the standards.