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) Current LED junction temperature measurement method
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.
(2) The current internationally advanced Vf-TJ measurement method
At present, the internationally advanced Vf-TJ measurement method is to connect the LED to be tested into the silicon cylinder, and then heat the silicon cylinder to bring the temperature of the silicone oil to about 140 °C, and then let the silicone oil in the cylinder cool naturally, as long as it is cooled. When the temperature of the silicone oil drops slowly enough, it can be considered that the junction temperature of the LED is basically the same as the temperature of the heat sink of the LED. In the process, according to the measured temperature of the silicone oil, each time the temperature drops by 2 ° C to 10 ° C The LED inputs the specified current pulse and measures its forward voltage drop at this temperature. The temperature and forward voltage drop of this measurement point are imported into the database of the computer software, starting from about 140 °C, with the temperature drop, The temperature of the heat sink and the forward voltage drop are measured every time a set aliquot temperature is dropped, and it is measured to about 25 ° C. When this set of measurement data is completed and imported into the database of the computer software, a Vf-TJ curve is generated by the software. . This method is a measure of temperature drop and is feasible for measurement, but because the ambient temperature of the test chamber is constant (typically 25 ° C), and the oil temperature of the silicon cylinder drops from high to low. This causes the cooling temperature to be faster when the oil temperature of the silicon cylinder is higher, because the temperature difference with the ambient temperature of the test chamber is large, and the temperature is high when the temperature of the silicon cylinder is high in order to ensure the accuracy of the measurement. The drop is not too fast, but when the temperature of the silicone cylinder is low, the cooling rate is too slow because the temperature difference with the room temperature is too small, which greatly prolongs the measurement time of this detection process. For the above reasons, the measurement method when this temperature is lowered is unlikely to be short in the calibration Vf-TJ process (about 4 to 5 hours), otherwise significant measurement errors will occur. In addition, the detection device cylinder is fixed, and the second group is measured, and the time is very slow. In addition, the above heating device is at the bottom of the outside of the silicon cylinder, and there is a significant hysteresis between the heating and the temperature control and the measured temperature, which also makes the accuracy of measuring the junction temperature by this method relatively poor.
(3) New Vf-TJ detection method
The detection method invented by the mechanism adopts the measurement method when the temperature rises, and adopts a computer-set PID (integration, differential plus heating and non-heating time proportional control) method to heat and control the temperature of the silicon cylinder, that is, heating in the silicon cylinder In the initial stage, the ratio of heating time to non-heating time is small and adjustable, so that the temperature rise rate of the silicon cylinder can ensure the consistency of the LED junction temperature, the heat sink and the temperature of the silicone oil, and the temperature of the silicone oil gradually rises. The temperature difference with room temperature also increases. At this time, the PID heating and temperature control system will automatically increase the ratio of heating time to non-heating time (actually increase the heating power per unit time), so it can ensure the inside of the silicone cylinder. The temperature rise rate of the silicone oil is always maintained at the set rate, and the rate at which the oil temperature rises does not differ due to the difference between the silicone oil temperature and the ambient temperature. The temperature of the silicone oil can be set at any temperature in the application temperature range, and the temperature rise rate of 0.1 ° C / min to 2 ° C / min can also be achieved.
After each temperature rise phase, there is a temperature control phase, that is, a temperature control phase and a temperature temperature phase form a stepped temperature control curve. As the temperature rises stepwise, the measured forward voltage can be set to be measured every 0.5 °C rise, and can be adjusted stepwise to every 10 °C at an interval of 0.5 °C. In order to ensure the temperature control and the timeliness of the measured temperature, the built-in heating is used. In addition, in order to ensure the consistency of the oil temperature in the silicone cylinder, a magnetically induced stirring rod is added at the bottom of the cylinder, which is rotated by the external motor and driven by magnetic induction. The stirring magnetic strip rotates in the cylinder, and the rotation speed is adjustable, thereby ensuring that the temperature difference of the silicone oil in the cylinder is maintained within 0.2 °C. The measuring device has almost the same rate of temperature rise of the silicone oil, and performs stepwise temperature rise and temperature control, thereby ensuring accurate detection results under a reasonable temperature rise rate, and the detection time (from 25 ° C to 140 ° C or about It is about 2.5 hours) and can be significantly lower than the measurement time of the existing international detection devices. At present, the detection device existing in the world is a single silicon cylinder structure. The measuring device adopts a double silicon cylinder structure. When the measurement of a group of samples is completed, the replacement of a silicon cylinder can immediately start the detection of the second group of LEDs. After measuring the temperature and LED forward voltage drop at each measured temperature point, the measuring device is imported into the database and the Vf-TJ curve is generated by the programmed software.
(4) Measurement of junction temperature of illumination LED and measurement of light, color and electrical parameters by using Vf-TJ relationship curve
After obtaining the curve of the Vf-TJ of the LED under test, the most important thing is to measure the light, color and electric parameters under the condition of the junction temperature. The detection system is shown in Figure 1. Fix the LED to be tested to the integrating sphere with temperature control/thermostother base, and apply the working current to the LED. After the LED is ignited for 15-20 minutes, it will quickly switch to the measured current (ie, the front calibration Vf-TJ curve). The measuring current) quickly measures the forward voltage Vf of the LED under test in a few milliseconds, and compares it with the Vf corresponding to the set junction temperature value in the Vf-TJ curve. If there is a difference from the target value, the control program will automatically adjust the thermostatic base. The temperature of the socket is such that the forward voltage Vf of the LED reaches the junction voltage corresponding to the target junction temperature value. After a quick measurement of Vf, the unit will automatically return to the state where the LED is energized. When the measured LED passes the working current and its junction temperature reaches the target value (ie, reaches the Vf value corresponding to the target junction temperature value) and the heat balance, the system will automatically start the spectrometer to measure the light and color parameters while reading its electrical parameters. .
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