For LED lighting fixtures, longevity is one of the most important issues to be concerned. In the early, LED technology was applied to flashlights and table lamps, the previous spanlife is not long. Nowadays, LED has been widely used in ourdoor and indoor LED systems, especially LED street lights & flood lights with high power, heat emission and long working hours, so to improve the working performances of LED lighting, some things follow should be taken into consideration.

 

1. Light failure

Light failure literately means that the brightness of light fixtures are getting dark more and more with use and time. Reducing the light failure of LED chips is mostly considered to improve the lifespan of LED. White color temperature is obtained by illuminating a yellow phosphor with a blue LED. There are two main reasons to cause the light failure, one is the light failure of the blue color temperature itself, fade of blue color is much faster than, yellow and green color temperatures. Another is the phosphor decay that it’s very serious at high temperature. Light failures of LED sourced from different manufactures are different, most of LED manufacturers usually offer their curve graph for testing light failure rate. For example, the follow one is offered by Cree.

 

From the graph above, we can see that the light failure is caused by its junction temperature that is generated by PN junction of semiconductor, the light decays sooner as junction temperature is higher, that is to say, its longevity is less. And we can notice that when the temperature goes up to 105°C, its brightness goes down to 70% and its lifespan is only 10,000 hours, it gets 20,000 hours when it goes to 95°C, 50,000 hours when it goes to 75°C, and gets 90,000 hours when it goes to 65°C. All the figures are offered by Gree. And we can also take a look at the chart showed by Lumiled & LuxeonK2 below:

 

The chart above shows that the junction temperature increases from 115°C to 135°C, and its lifespan is reduced from 50,000 hours to 20,000 hours.

 

2. How to last the lifespand of LED fixture

From the descriptions above, we can conclude that the key to prolonging its life is to reduce its junction temperature, so we need to select the material with excellent thermal conduction as LED light housing or heatsink or to cool down quickly the heat that LED chips release out. In fact, we can apply the right material for heatsink according to the level of junction temperature, if we can measure any one of the heat sink can reach the junction temperature, that not only can compare a variety of heat sink cooling effect, but also know the longevity of LED lighting fixtures. In addition to the material, the right LED housing manufacturers are also important to cause the quality of your LED fixtures.

 

3. How to last measure the junction temperature

Junction temperature is only the index and level of temperature, however, we can’t always use a thermometer to measure the temperature inside the LED fixtures. Of course, sometimes we can measure with a thermocouple to know its temperature, and then based on the given thermal resistance Rjc (junction to case), it can calculate its junction temperature. But after installing the heat sink, the problem becomes complicated again. As LED is typically soldered to an aluminum substrate and the aluminum substrate is mounted on a heat sink, if only the temperature of the heatsink housing can be measured, it is important to know a large number of thermal resistances to calculate the junction temperature. Including Rjc (junction to case), Rcm (case to aluminum plate, in fact, which should also include the thermal resistance of the film printed version), Rms (aluminum plate to the radiator), Rsa (radiator to air), as long as there is a Inaccurate data will affect the accuracy of the test. Figure 3 gives a schematic of the thermal resistance from LED to the heatsink. Which incorporates a lot of thermal resistance, making its accuracy even more limited. That is, it is even worse to guess the junction temperature from the measured surface heatsink temperature.

 

There is a way to indirectly measure the temperature, that is the voltage measurement. So which voltage is relative to the junction temperature? Let's start with the LED's volt-ampere characteristics.

 

4. The temperature coefficient of LED volt-ampere characteristics

We know that LED is a semiconductor diode just like other types of diodes, they all have volt-ampere characteristic which has temperature coefficient, when the temperature rises, the volt-ampere characteristic shifts to the left.

 

Assuming that the power of LED is supplied by IO constant current, and the voltage is V1, when the junction temperature goes up to T2, its volt-ampere characteristic shifts to the left, the IO constant current doesn’t change, the voltage goes up to V2. The two voltage differences are temperature removed, you can get the temperature coefficient, in mV/oC said. For normal silicon diodes, this temperature coefficient is about -2mV/oC. However, most of the LED is not made of silicon material, so its temperature coefficient to be measured separately. Fortunately, most of the LED manufacturers datasheet gives its temperature coefficient. For example, Cree XLamp7090XR-E high-power LED, the temperature coefficient of -4mV/oC. 2 times larger than normal silicon diodes. As for the United States Purui array LED (BXRA) gives more detailed data.

 

However, the figures showed above are too broad in scope to lose the value of their use. In any case, as long as you know the temperature coefficient of the LED it is easy to figure out the LED junction temperature from measuring the forward voltage of the LED.

 

5. How to acurately know the LED junction temperature

Now take Cree's XLamp7090XR-E as an example. To illustrate how to calculate the specific LED junction temperature. The LED has been installed into the LED light housing, and the constant current driver is used as the power supply. At the same time to connect to the LED leads to the two leads. Connect the voltmeter to the output (the positive and negative of the LED) before switching on the power and then turn the power on and read the voltmeter reading immediately before the LED has not been heated up, which is equivalent to the value of V1, and then wait At least 1 hour, wait until it has reached thermal equilibrium, and then test again, the voltage across the LED, the equivalent of V2. Subtract these two values to arrive at their difference. Then 4mV removed, you can draw the junction temperature. In fact, most of the LEDs are connected in series and parallel, which does not matter. The voltage difference at this time is contributed by many LEDs in series. Divide the voltage difference by the number of LEDs in series and remove the 4mV , You can get the junction temperature. For example, the LED is 10 strings and the first measured voltage is 33V, after the second thermal equilibrium measured voltage is 30V, the voltage difference is 3V. This figure is first divided by the number of LEDs in series (10), get 0.3V, and then divided by 4mV, you can get 75 degrees. Assuming the ambient temperature before boot is 20 degrees, then the junction temperature at this time should be 95 degrees. Junction temperature obtained by this method must be more accurate than using a thermocouple to measure the temperature of the heat sink and then calculate the junction temperature.

 

6. How to predict the lifespan of LED lighting fixture

From the junction temperature to speculate the lifespan of fixtures seems to be very simple, as long as check the curve in Figure 1, you can know that corresponds to 95 degrees when the junction temperature can get the life of LED 20,000 hours. However, this method has some credibility for indoor LED fixtures, and there are many uncertainties if applied to outdoor LED lamps, especially high-power LED street lamps. The biggest problem is the heat dissipation of LED street lamp cooling efficiency over time. This is due to the accumulation of dust, bird feces and make its cooling efficiency is reduced. Also because there is a very strong outdoor UV, LED will also reduce the life expectancy. UV is mainly on the encapsulation of epoxy resin plays a big role aging, if using silicone, can be improved. UV light on the aging of the phosphor has some bad effects, but not very serious. However, this method is used to compare the cooling effect of the two radiators is more effective. Obviously, the smaller the volt-ampere left radiator, the better the heat dissipation. In addition, the prediction of the life of indoor LED lighting is still a certain degree of accuracy.