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2017年9月15日
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How to predict led lifespan?

All things have a process of occurrence, development and extinction, LED is no exception, is a certain life. The early LED was just a flashlight, desk lamp and other gifts, which did not last long and did not have a prominent life span. But now LED has begun to be widely used in outdoor and indoor lighting, especially high-power LED street lamps, which have large power, high heating, long working time and outstanding life problems. The myth that LED life must be 100,000 hours seems to have been shattered. So what’s the problem?

Regardless of power and drive failures, the LED’s lifetime is characterized by its light decay, which means that over time, the brightness gets dimmer and dimmer until finally goes out. It is usually defined as a period of decay of 30%.

So can the life of leds be predicted? This question cannot be answered simply; it needs to be told from the beginning.

1: LED light decay

Most white leds are made by shining blue leds on yellow phosphors. There are two main reasons for LED light decay. One is the light decay of blue LED itself. The light decay of blue LED is much faster than that of red, yellow and green LED. There is also a phosphor light decay, phosphor at high temperature attenuation is very serious. Different brands of leds have different light decay. Usually LED manufacturers are able to produce a standard set of fading curves. For example, the optical decay curve of American Cree company is shown in figure

P1 CreeLED light decay curve

As can be seen from the figure, LED light decay is related to its junction temperature. The so-called junction temperature is the temperature of semiconductor PN junction. The higher the junction temperature is, the sooner the light decay will occur, which means the shorter the life. As can be seen from the figure, if the junction temperature is 105 degrees, the brightness down to 70% of the life only more than 10,000 hours, 95 degrees have 20,000 hours, and the junction temperature reduced to 75 degrees, the life is 50,000 hours, 65 degrees can be extended to 90,000 hours. So the key to prolonging life is to reduce junction temperature. But this data only applies to Cree’s leds. It is not suitable for other companies’ LED. For example, the optical decay curve of LuxeonK2 of Lumiled company is shown in figure 2.

P2 Lumiled LuxeonK2light decay curve

When the junction temperature is raised from 115℃ to 135℃, the service life is reduced from 50,000 hours to 20,000 hours. The decay curves of other companies should be available from the original factory.

2.How to extend the life of LED

It can be concluded from the figure that the key to prolong its life is to reduce its junction temperature. The key to reduce junction temperature is to have a good radiator. It can send out the heat generated by LED in time.

Here we are not going to discuss how to design the radiator, but to discuss which radiator is relatively good heat dissipation. In fact, this is a problem of junction temperature measurement. If we can measure the junction temperature achieved by any kind of radiator, we can not only compare the cooling effect of various radiators, but also know the LED life that can be realized after adopting this kind of radiator.

3.How to measure junction temperature

Junction temperature seems to be a temperature measurement problem, but the junction temperature to be measured in the LED internal, always can not take a thermometer or thermocouple into the PN junction to measure its temperature. Of course, its shell temperature can be measured by thermocouple, and then according to the given thermal resistance Rjc (junction to the shell), its junction temperature can be calculated. But once the radiators are installed, the problem becomes more complicated. Because the LED is usually welded to the aluminum substrate, and the aluminum substrate is installed on the radiator, if you can only measure the temperature of the radiator shell, then to calculate the temperature of the junction must know a lot of value of thermal resistance. These include Rjc (junction to housing), Rcm (enclosure to aluminum substrate, which should also include the thermal resistance of the thin-film printed version), Rms (aluminum substrate to radiator), Rsa (radiator to air), where just one inaccurate data will affect the accuracy of the test. Figure 3 shows the schematic diagram of each thermal resistance from LED to radiator. Many thermal resistances are incorporated, limiting their accuracy. In other words, it is even less accurate to infer the junction temperature from the measured surface temperature of the radiator.

P3 LED Schematic diagram of each thermal resistance to the radiator

Fortunately, there is an indirect way to measure temperature, and that is to measure voltage. So which voltage is related to the junction temperature? What about this relationship?

We start with the volt-ampere characteristics of leds.

  1. Temperature coefficient of LED volt ampere characteristic

We know that an LED is a semiconductor diode, and like all diodes, it has a volt ampere characteristic, and like all semiconductor diodes, it has a temperature characteristic. The characteristic is that as the temperature goes up, the volt ampere characteristic shifts to the left. The temperature characteristics of the volt-ampere characteristics of the LED are shown in figure 4.

P4.temperature characteristics of LED volt-ampere characteristics

When the junction temperature is T1, the voltage is V1. When the junction temperature rises to T2, the entire volt-ampere characteristic shifts to the left, the current Io remains unchanged, and the voltage changes to V2. The two voltage differences are removed by temperature, and the temperature coefficient, expressed in mV/oC, can be obtained. For ordinary silicon diodes, the temperature coefficient is approximately -2mv /oC. But most leds are not made of silicon, so their temperature coefficients have to be measured separately. Fortunately, most LED manufacturers’ data sheets show their temperature coefficients. For example, for Cree’s xlamp7090xr-e high power LED, the temperature coefficient was -4mv /oC. It’s twice as big as a regular silicon diode. More detailed data are available from the puri array LED (BXRA) in the United States.

But their figures are too broad to be useful.

However, knowing the temperature coefficient of the LED, it is easy to calculate the junction temperature of the LED from measuring the forward voltage of the LED.

  1. How to calculate the junction temperature of LED

Take the example of xlamp7090xr-e from Cree. To explain how to specifically calculate the junction temperature of LED. The LED has been installed in the radiator and the constant current driver is used as the power supply. At the same time to connect to the LED to the two wires out. Before electricity a voltmeter is connected to the output (LED the positive and negative), and then, turning on the power supply before LED is not hot, immediately read the voltmeter reading, which is equivalent to the value of the V1 and at least 1 hour, etc. It has reached thermal equilibrium, test again, the voltage across the LED equivalent of V2. Subtract these two values to get the difference. And then we can get the junction temperature by removing it by 4mV. In fact, most leds are connected in series and in parallel, which does not matter. At this time, the voltage difference is jointly contributed by many series leds. Therefore, the junction temperature can be obtained by dividing this voltage difference by the number of series leds and then dividing by 4mV. For example, the LED is 10 series of 2 and the voltage measured for the first time is 33V, the voltage measured after the second thermal balance is 30V, and the voltage difference is 3V. This number should be divided by the number of leds in series (10) to get 0.3v, and then divided by 4mV to get 75 degrees. Assuming that the ambient temperature before starting is 20 degrees, then the junction temperature should be 95 degrees.

The junction temperature obtained by this method is definitely much more accurate than that calculated by measuring the temperature of radiator with thermocouple.

6.How to predict the life of the lamp

It seems easy to infer the life from junction temperature. Just check the curve in figure 1 below to find out the life corresponding to the junction temperature of 95 degrees, and you can get the LED life of 20,000 hours. However, this method still has certain credibility when applied to indoor LED lamps. If it is applied to outdoor LED lamps, especially high-power LED street lamps, there are still many uncertain factors. The biggest problem is that the heat sink of LED street lamps decreases with time. This is due to the accumulation of dust, bird droppings and its heat dissipation efficiency is reduced. Also because there is a very strong ultraviolet outside, will also reduce the life of LED. Ultraviolet radiation is mainly used to encapsulate epoxy resin aging plays a great role, if using silica gel, can be improved. Ultraviolet radiation also has some bad effects on the aging of phosphors, but it is not very serious.

However, this method is more effective for comparing the heat dissipation effect of two kinds of radiators. Obviously, the smaller the volt-ampere characteristic moves to the left, the better the heat dissipation effect. In addition, there is a certain accuracy in predicting the life of indoor LED lamps.

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