Weak current College】 【LED test method】.

<br> <br> 1. preface <br> <br> Semiconductor light-emitting diode (LED) has been widely used in led lights, signal lights, instrument displays, mobile phones back light, car lights, etc, white LED technology .is constantly developing, LED lighting applications more and more widely. In the past, for the test does not have LED a comprehensive national standards and industry standards, in production can only be based on the relative parameters, of different manufacturers, users, research organizations .on this controversial, lead to the development of domestic led industry is greatly affected. In the domestic and international test methods on LED on the latest standards, based on LED various application areas, the author from electrical characteristics, optical characteristics, switching characteristics, color .properties, thermal characteristics, reliability, etc. <br> <br> 2. electrical characteristics test method <br> <br> LED is a semiconductor inorganic material consisting of unipolar pn junction diode, its voltage and current-voltage characteristics of the relationship .is called. LED electric characteristic parameters including forward current, forward voltage and reverse current and reverse voltage, LED must be in the right current and voltage driven to work properly (as shown in Figure 1). Electrical characteristics through the LED test can be LED .by maximum forward voltage, forward current and reverse voltage, current, and in addition you can also work for the determination of the best power LED. <br> <br> LED electric characteristics of the test is generally the appropriate constant current and constant voltage .source power supply, voltage meter testing. <br> <br> 3. optical characteristics test <br> <br> Similar to other light, LED light characteristics of the test mainly consists of the luminous flux, luminous, radiant flux, radiation efficiency ., light intensity, light distribution characteristics and spectral parameters, etc. <br> <br> 3.1 luminous flux and luminous efficiency <br> <br> Flux test has two methods, namely the integrating sphere method and variable angle spectrophotometer method. Variable angle .spectrophotometer method is to test the light flux most precise method, but because it takes a long time, so I generally use the integrating sphere method to test the light flux. Existing integrating sphere measuring the LED luminous flux in two test structure, one is LED .to be logging are placed in the sphere and the other is placed in the sphere <br> <br> Integrating sphere measuring the LED luminous flux <br> <br> As a result of the integrating sphere method to test the luminous flux, light .of light self absorption will have an impact on the test results. Therefore, often a need to introduce auxiliary lamp method to eliminate the effects of self absorption <br> <br> The measured luminous flux, combining electrical parameter measuring instrument can measure the efficiency .of the LED. The radiation flux and radiation efficiency measurement methods, similar to the luminous flux and luminous efficiency of testing. <br> <br> 3.2 light intensity and light intensity distribution <br> <br> Light intensity testing issues <br> .<br> Point source of light intensity in various directions in space evenly, in different distance with a different receive aperture probes received test results will not change. However, the LED light intensity distribution because it is inconsistent, making the test results with the test .distance and detector aperture. Therefore, the presented two CIE-127 recommend testing conditions enabling the LED in the same conditions of light intensity test and evaluation as shown in Figure 5. Currently, CIE-127 recommended test condition has been the LED manufacturer and inspection .agencies. <br> <br> Recommended LED light intensity test conditions <br> <br> 3.3 spectral parameters <br> <br> LED spectral characteristics of the main parameters include peak emission wavelength, spectral radiation spectral bandwidth and power distribution, etc .. Monochrome LED spectral characteristics for a single wave soldering, to peak wavelength and bandwidth, and white led spectral synthesis by many monochrome spectrum. All led spectral characteristics are expressed by the spectral power distribution, white led spectral power distribution is shown in Figure 6. .By LED spectral power distributions can also be computed color parameters. <br> <br> Spectral power distribution of the test require light to light the various color light from mixed case out for determination, in General, you can use Prism and grating spectrometer. .<br> <br> White led spectral power distribution <br> <br> 4. LED switching characteristics test <br> <br> LED switching characteristics is LED in powerup and powerdown moments of light, electricity, color variation. Through LED switching .characteristics test can be LED in flux-off moment working state, material properties, such as changes, this will not only learn through power loss to the LED, can also be used to guide the design of LED driver modules, etc. <br> .<br> 5, color characteristics <br> <br> LED color characteristics include chromaticity coordinates, dominant wavelength, color purity, color temperature and color, the color of the LED on the white led is particularly important. <br> <br> .The existing color of the test method has a Spectrophotometric and integral method. Spectrophotometric method is through the monochromator spectral measurements LED spectral power distribution, and use of chroma weighted function integral access to corresponding colorimetric parameters; integral method is to use a specific filter with photoelectric .detector directly measured color. Photometric accuracy are significantly higher than the integration method. <br> <br> Color characteristics test method <br> <br> 6, thermal characteristics <br> <br> LED by the thermal characteristics of the main means .of thermal resistance and junction temperature. Thermal resistance is along the heat flow channel temperature difference and channel the ratio of the power dissipation; junction temperature is the temperature of the pn junction LED. LED junction temperature of thermal resistance and is led optoelectronic properties of important .factors. <br> <br> Existing on LED junction temperature testing in general there are two methods: one is the use of infrared microscope or miniature thermocouple measurements of surface temperature of LED chip and depending on whether they are LED by TJ, but accuracy .is not enough; the other is used to determine current under the forward bias and junction temperature changes in the relationship between an inverse ratio to determine the junction temperature of LED. <br> <br> 7, reliability <br> <br> LED .reliability including electrostatic sensitivity characteristics, life, environmental characteristics, etc. Electrostatic sensitivity characteristics is the LED can withstand electrostatic discharge voltage. Some LED higher by resistivity, and positive and negative electrode distance is short, when both ends of the electrostatic charge build-up .to a certain value, a static voltage breakdown pn junction, serious when PN breakdown causes LED failure. Therefore must be LED by electrostatic sensitivity characteristics test for electrostatic discharge fault LED critical voltage. At present, the general use of the human body model, the .machine mode, device charging mode to simulate real-life electrostatic discharge phenomena. <br> <br> In order to observe the time LED to long-term continuous use of the performance, you need to test the sample, LED through the permanent observation .and statistics to be LED by the life argument. <br> <br> For LED environment characteristics of often used simulation LED in the application of various natural invasion, generally have a high-temperature impact test, humidity cycle test, humidity test, salt .spray test, test, irradiation test dust, vibration and shock testing, drop test, centrifugal acceleration test, etc. <br> <br> Summing up the above test method, semiconductor light-emitting diode test method standards on LED electric characteristics, optical .characteristics, thermal characteristics, static characteristics and life tests for the corresponding regulations. <br> <br> For testing electrical properties, standard respectively provides LED forward voltage and reverse voltage and reverse current test block; for luminous flux testing, standards with 2 π .solid angle test structure; the intensity of the test, the standard refers to the recommended conditions CIE-127; in addition, the spectral measurement, thermal characteristics testing, ESD sensitivity test, test, and so life was clearly defined. <br> < .br> Conclusions of the <br> <br> National standards-setting summarizes existing LED test method, it will be one of the scientific method applied to the standard test method for upgrading, it is a great way to eliminate all the led test area .differences and make the results more truly reflect our overall level of the LED industry. However, in view of the LED technology also is constantly evolving, the formulation of national standards and not once and for all, should always have the latest and most appropriate testing .technology into standards. <br> <br>.