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test of chip junction temperature for led package device

At present, LED is its excellent electro optical conversion efficiency and light efficiency of the industry’s full recognition. But we all know that LED in addition to its own functions light, there is an important issue that can not be ignored is the LED fever.

At present, LED is its excellent electro optical conversion efficiency and light efficiency of the industry’s full recognition. But we all know that LED in addition to its own functions light, there is an important issue that can not be ignored is the LED fever. After 10 years of development, although the LED’s electro optical conversion efficiency has reached 40% to 60%, but there are a lot of energy is distributed through the form of heat out. Although people are wishful thinking hope that a light source can directly convert all the energy into light energy radiation, want to burn excess calories, but Chenqie can not do ah.

The theme of this article is to let you know how to conduct LED thermal test.

First from the packaging point of view, the current LED development model and the development of electronic packaging come down in one continuous line, the difference is nothing more than functional electronic chips can be replaced by LED chips to light, and the opaque plastic packaging replaced silicone or Epoxy resin and other light transmitting material. Therefore, it can be found that the LED thermal conduction path of this package circuit is also consistent with most of the chip type electronic components, can be simplified as a one dimensional heat dissipation path from the chip to the substrate.

Figure 1 LED one dimensional heat conduction path

LED fever is mainly derived from its chip, now that the heat from the first non radiative carrier composite, and second, from the carrier composite photon generation and can not be effectively fired out. If the chip is solid crystal in the bracket, there is no lens or fluorescent adhesive coverage, this time can be observed through the infrared thermal imaging chip surface temperature. In general this situation can be obtained with the infrared surface temperature of the chip. In particular, the surface temperature of the light emitting surface can not be measured with a thermocouple. Although the thermocouple is a very convenient thermometer, but for both a luminous and fever of the LED chip, the thermocouple because of the absorption of the optical radiation generated considerable error, the closer to the light emitting surface, the thermocouple measured temperature error Will be greater.

led lights at home depot t8 led tube

Figure 2 LED chip infrared photography

But our question is, if it is already packaged a good chip, how do we measure the chip junction temperature it? Said infrared camera can only measure the surface of the object (infrared transmittance materials, except strong) temperature, and can not be taken to be covered by the lens or fluorescent adhesive chip junction temperature. In fact, LED, because of the particularity of its diode, its own can be used as a characterization of the temperature sensor. Standard JESD51 1 there have said that the diode terminal voltage will change with the PN junction temperature changes, and the terminal voltage and junction temperature is very close to a linear change! Since this is the case, then we can not use the voltage to monitor the internal PN junction temperature of the chip it! In fact JESD51 1 is to introduce this feasible and accurate method.

Figure 3 K factor test example

Here we briefly introduce the JESD51 1 standard mentioned in the test method.

First of all, we put the above mentioned voltage temperature curve of the slope? V ? T is called the K factor of the semiconductor chip. Each chip has its own K factor, which is the chip’s own PN junction decision, belongs to one of its own characteristics. Figure 3 for the K coefficient test examples.

K factor measurement is a very small test current Isssss (such as 1mA) to read the voltage at different ambient temperature, and as long as enough time we can think of the chip is equal to the actual temperature of the ambient temperature. That test current Isssss Why use a small current? Because the current is large, the current will cause the chip heat, the chip fever and the actual temperature on the ambient temperature is relatively large difference, and ambient temperature is the parameter we can control, so our test current Isssss to use a small current.

test of chip junction temperature for led package device

After getting this K factor is not to know the junction temperature? Since there is a voltage and temperature of the corresponding relationship, then we can not read the normal working LED voltage to reverse the chip junction temperature it? However, things are not as simple as this (unless your home’s LED is operating at ~ 1mA). On the single chip LED, the normal working current Idddds has now reached several hundred mA, or even more than 1A, we want to know is that LED operating current in this junction temperature. What about that? Since the K factor, then we can use a jump method to test?

For example, the LED operating current Idddds light for some time, that the LED thermal balance, the work current suddenly jump to the test current Isssss, as long as the current transition time fast enough, as long as the acquisition voltage can keep up the speed, And the test accuracy is also leveraged, then we can measure jump into the test current after the instant the voltage V1. In order to be able to use the K factor, we let the LED work in the test current until the thermal equilibrium with the ambient temperature, this time and then measured a voltage value of V2. Because these two voltage values ​​are measured with Isssss, then the two voltage can be used to correspond to the K factor temperature, with the two voltage values ​​?? change? V divided by the value of K can be a temperature change? T.

Figure 4 current transient voltage and temperature characteristics

Smart readers can find that this temperature change value? T is actually the difference between ambient temperature and junction temperature, so only need to put? T with the ambient temperature can be the chip’s junction temperature. Because the acquisition time is short enough, so short can be considered that the heat has not time to dissipate immediately, measured the first voltage value corresponding to the temperature can be considered in the LED current corresponding to the temperature value. JESD51 1 standard which is described in this method, which has a strict proof, the author here is a simple description of the test method.

The problem is, since there are such advanced testing methods, there are equipment to meet the high speed switching and high precision requirements? Of course, the JEDEC committee in this standard set specifically to do a set of fully in line with this standard test equipment, that is, the industry famous T3Ste (r thermal transient tester ThermalTransientTester), whether diodes, transistors, field effect transistor Or IGBT, the junction temperature and thermal resistance of these semiconductor devices can be measured using T3ster. But also after the mathematical operation of the heat transfer path on each layer of the structure of the thermal resistance analysis out to find the cooling bottleneck.

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