At present, LED is fully recognized by the industry for its excellent electro-optical conversion efficiency and light efficiency. But everyone knows that in addition to its own function - LED, there is an important issue that can not be ignored, that is, the heat of LED. After more than a decade of development, although the electro-optical conversion efficiency of LEDs has reached 40% to 60%, there is still a lot of energy that is emitted through the form of heat. Although people are wishful thinking that a certain light source can directly convert all of the electric energy into light energy, and want to remove the excess heat, the court can't do it.
The topic of this article is to let everyone know how to perform thermal testing of LEDs.
First of all, from the packaging point of view, the current development mode of LED is in line with the development of electronic packaging. The difference is that the functional electronic chip is replaced by a light-emitting LED chip , and the opaque packaging plastic is replaced by silica gel or Light-transmitting material such as epoxy resin. Therefore, it can be found that the LED heat conduction path inherited by the package is also consistent with most of the chip-type electronic components, and can be simplified as a one-dimensional heat dissipation path from the chip to the substrate.
Figure 1 LED one-dimensional heat conduction path
The heat of LED is mainly derived from its chip. It is believed that the cause of heat generation is from the non-radiative carrier composite, and the second is that the photon generated from the carrier is not effectively emitted. If the chip is only solid crystal on the support, there is no lens or fluorescent glue on it. At this time, the surface temperature of the chip can be observed by an infrared camera. Generally, this case can be obtained by infrared imaging of the surface temperature of the chip. It is important to note that the surface temperature of the illuminating surface cannot be measured with a thermocouple. Although thermocouples are very convenient thermometers, for an LED chip that is both luminescent and hot, the thermocouple will generate considerable error due to the absorption of optical radiation. The closer to the illuminating surface, the temperature error measured by the thermocouple. It will be bigger.
Figure 2 LED chip infrared image
But our question is, if it is a packaged chip, how do we measure the junction temperature of the chip? The above mentioned infrared thermal imager can only measure the temperature of the surface of the object (except for materials with strong infrared transmittance), and can not capture the junction temperature of the chip covered by the lens or fluorescent glue. In fact, because of the particularity of its diode, LED itself can be used as a sensor to characterize temperature. As shown in the standard JESD51-1, the terminal voltage of the diode changes with the temperature of the PN junction, and the terminal voltage and junction temperature are very close to linear changes! Since this is the case, then we can not use voltage to monitor the temperature of the PN junction inside the chip! In fact, JESD51-1 is a description of this feasible and accurate method.
Figure 3 K coefficient test example
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