Summary:
The traditional LED and its module light, color, and electrical parameters are detected by electric pulse driving, CCD fast spectroscopy, and under certain conditions, after thermal equilibrium measurement, but the measurement conditions and results of these methods and LED enter lighting The actual working conditions in the appliances are very different. The article introduces the measurement of the Vf-TJ curve and controls the LED to measure its light, color and electrical parameters at the controlled junction temperature. It not only provides a target limit for how to ensure the LED working junction temperature of LED lighting fixtures. It also makes the measurement parameters of the light, color and electrical parameters of the LED and its module closer to the actual application conditions. The article also describes the use of LED lighting fixtures such as measuring the junction temperature of the LED and determining the limit temperature of the LED reference point as a function of junction temperature. This provides an effective way to quickly assess the operating status and service life of LED lighting fixtures.
First, the preface
For an emerging product, the development of the product itself always precedes the product standard and testing method. Although the standards and testing methods of the products cannot be prior to the development of the products, the standards and testing methods of the products should be as close as possible to the progress of product design and development, because the standard of the products and the method of testing the testing process itself is the product development. Review and summary of the process, as long as the conditions are basically mature, the more timely the product standards and testing methods are formulated, the more blind the product development process can be. The LED lighting industry has developed to the present, and we have basically come back to the review and summary of LED lighting product standards and testing methods.
Second, the current status and improvement methods of photoelectric parameters and detection methods of LED modules
1. Traditional LED module detection method
At present, there are two main methods for detecting traditional LED modules. The first one is to use pulse measurement method, which is to fix the illumination LED module on the measuring device (such as the measuring position of the integrating sphere, etc.), using a pulse constant current Power Supply and Simultaneously measuring the synchronous linkage of the spectrometer, that is, simultaneously emitting the pulse current of the constant current of tens of milliseconds to several milliseconds to the LED, simultaneously opening the shutter of the instantaneous measuring spectrometer, and performing the light parameters (light flux, light color parameter, etc.) emitted by the LED. Quick detection, meanwhile, also synchronously collect parameters such as forward voltage drop and power of the LED. Since the junction temperature of the LED is almost equal to the room temperature during the detection process, the light effect of the measurement result is high, and the light color and the electrical parameter are significantly different from the actual use condition, which is generally produced by the LED chip (device). The rapid detection method adopted by the quotient is not comparable to the state in which the LED is actually applied in the final lighting fixture.
The second test method is to install the LED module on the detection device, possibly with a fixed heat sink (may also have a base temperature control function), apply the claimed working current to the LED, and be detected by the traditional illumination source. The effect of the method is also to wait until the LED reaches thermal equilibrium before starting to measure its photoelectric parameters. This method seems to be more rigorous, but in fact, its thermal equilibrium conditions and working conditions are still not well correlated with the state of such LEDs into the final lighting fixture, so the measured photoelectric parameters and future practical applications The parameters of the state are still not comparable. The GB/T24824-2009/CIE 127-2007NEQ "Measurement Method for Basic Performance of LED Modules for General Lighting" has been promulgated in this respect: "The LED module should work under thermal equilibrium during test or measurement. While monitoring the ambient temperature, it is best to monitor the operating temperature of the LED module to ensure the reproducibility of the test. If it is possible to monitor the LED module junction voltage, the peak voltage should be monitored first. Otherwise, the LED module should be monitored. Temperature measurement point temperature". It can be seen that measuring the photoelectric parameters of the LED module under the condition of monitoring the junction voltage is the preferred solution for ensuring the reproducibility of the detection. However, the standard does not specify the light, color and electrical parameters of the LED module under the condition of simulating the actual use of the junction temperature.
2. Improvement of measurement method of LED module
As we all know, the light and electrical parameters of LEDs are closely related to the junction temperature during operation. The same LED product does not cause significant difference in junction temperature, which also causes the same LED light, color and electrical parameters. The apparent inconsistency of the measurement results, so the measurement of the photoelectric parameters of the LED should first be considered under the conditions of the set operating junction temperature. In addition, because of the difference in packaging process and materials, the claimed maximum operating junction temperature is significantly different. In order to ensure the high efficiency and longevity of LED lighting products, the actual working junction temperature of LED should be significantly lower than the highest working junction temperature. . For example, at present, a large number of LED packaging methods and technologies are used, and in the face of LED illumination, there are coating layers of polymer silica gel and phosphor. Practice has proved that for such LED lighting fixtures, the time to 70% of the lumen maintenance rate should be ≥ 60,000 hours, and the operating junction temperature must be kept below 70 °C ~ 75 °C. From the perspective of improving light efficiency and service life, it is better to keep the working junction temperature of LEDs below 60 °C, but in terms of the shape, volume and cost performance of lighting fixtures, it should be controlled to achieve the desired light efficiency and On the basis of the service life, it is most suitable to control the maximum operating junction temperature of the LED at 70 ° C to 75 ° C. In order to make the detection of the light, color and electrical parameters of the LED and its module as close as possible to the junction temperature of the actual application, it is necessary to solve how to measure the junction temperature of the LED and to perform light, color and electricity at this junction temperature. Detection of parameters.
(1) At present, the junction temperature measurement method of LED is probably
1) Determine the junction temperature by measuring the pin temperature and chip dissipation power and thermal resistance coefficient. However, because of the inaccurate power dissipation and thermal resistance coefficient, the measurement accuracy is relatively low.
2) Infrared thermal imaging method, using infrared non-contact temperature meter to directly measure the temperature of the LED chip, but requires the device under test to be in an unpackaged state, and additionally has special requirements on the refractive index of the LED packaging material, otherwise it cannot be accurately measured, and the measurement accuracy is compared. low.
3) Using the peak shift of the luminescence spectrum to measure the junction temperature is also a non-contact measurement method. The band gap temperature is directly determined from the luminescence spectrum to determine the junction temperature. This method requires higher resolution accuracy of the spectral test instrument. The accuracy of the bit is difficult to measure, and the error variation of the spectral peak displacement of 1 nm corresponds to a change in the measured junction temperature of about 30 degrees, so the measurement accuracy and repeatability are relatively low.
4) Nematic liquid crystal thermal imaging technology requires high resolution of the instrument. It can only measure a single bare chip that is not packaged, and cannot measure the LED after packaging.
5) Measure the junction temperature of the LED using the Vf-TJ curve of the diode PN junction voltage and junction temperature.
From the various LED junction temperature measurement methods described above, it can be seen that the method of estimating the junction temperature by using the change of the monitor diode PN junction voltage is the most feasible and the measurement accuracy is the highest, so in many integrated IC circuits, in order to detect The operating junction temperature of the IC chip is often carved or valued by one or several diodes, and the junction temperature of the chip is measured by measuring the change in forward voltage drop.