How to solve the problem of LED light failure

2022-08-23
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How to solve the problem of LED light failure

light failure is a common problem in the LED industry, which often makes people feel headache. What is the relationship between LED light failure and led bead life? How can we solve the problem of light decay? Finally, I found a better way to look at the flame resistance and chemical resistance of Plexiglas

the following is the light decay curve:

from the figure, it can be seen that the light decay of LED is related to its junction temperature. The so-called junction temperature is the temperature of semiconductor pn junction. The higher the junction temperature, the earlier the light decay occurs, that is, the shorter the service life. It can be seen from the figure that if the junction temperature is 105 degrees, the lifetime of brightness reduced to 70% is only more than 10000 hours, and there are 20000 hours at 95 degrees, while if the junction temperature is reduced to 75 degrees, the lifetime is 50000 hours, and it can be extended to 90000 hours at 65 degrees. Therefore, the key to prolong the service life is to reduce the junction temperature, but these data are only suitable for Cree led, not for LED of other companies

how to prolong the life of LED

so when we buy LED lamps (not specifically led spotlights), we must see whether its heat dissipation design is good

it can be concluded from the figure that the key to prolong its service life is to reduce its junction temperature. The key to reduce the junction temperature is to have a good radiator, which can emit the heat generated by the LED in time

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

how to measure junction temperature

junction temperature seems to be a temperature measurement problem, but the junction temperature to be measured is inside the LED, so you can't take a thermometer or thermocouple into the PN junction to measure its temperature. Of course, its shell temperature can still be measured by thermocouples, and then its junction temperature can be calculated according to the given thermal resistance RJC (junction to shell)

but after installing the radiator, the problem becomes complicated again. Because usually the LED is 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, you must know a lot of thermal resistance values to calculate the junction temperature. Including RJC (junction to shell), RCM (shell to aluminum substrate, which should also include the thermal resistance of thin-film printed boards), RMS (aluminum substrate to radiator), RSA (radiator to air). As long as there is an inaccurate data, it will affect the accuracy of the test

schematic diagram of each thermal resistance from led to radiator

fortunately, there is an indirect method of measuring temperature, that is, measuring voltage. So which voltage is related to the junction temperature? What about this relationship? We should start with the volt ampere characteristics of LED

temperature coefficient of volt ampere characteristics of LED

we know that led is a semiconductor diode, which has a volt ampere characteristic like all diodes, and also has a temperature characteristic like all semiconductor diodes. Its characteristic is that when the temperature rises, the volt ampere characteristic shifts to the left. The temperature characteristics of the volt ampere characteristics of LED are shown in the figure

assume that the LED is powered by IO constant current. When the junction temperature is T1, the voltage is V1. When the junction temperature rises to T2, the entire volt ampere characteristic shifts left, the Current IO remains unchanged, and the voltage becomes v2. The temperature coefficient can be obtained by removing the two voltage differences by temperature, which is expressed in mv/oc. For ordinary silicon diodes, this temperature coefficient is about -2mv/oc. However, most LEDs are not made of silicon, so its temperature coefficient should also be measured separately. Fortunately, most of the LED manufacturers' data sheets give their temperature coefficients. For example, for Cree's xlamp7090xr-e high-power LED, its temperature coefficient is -4mv/oc. It is 2 times larger than ordinary silicon diode

however, the range of data given by them is too broad, so that they lose the value of utilization. Anyway, as long as the temperature coefficient of LED is known, the junction temperature of LED can be easily calculated from the forward voltage of LED

how to predict the service life of this lamp

it seems that it should be very simple to infer the service life from the junction temperature in recent years. As long as you check the curve in Figure 1 below, you can know the service life corresponding to the junction temperature of 95 degrees, and you can get the service life of the LED of 20000 hours. However, this method is still reliable for 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 dissipation efficiency of LED street lamp radiator decreases with time. This is due to the accumulation of dust and bird droppings, which reduces the heat dissipation efficiency. Also because there is strong UV outside and the connector wire used for EV, the service life of LED will also be reduced. Ultraviolet light mainly plays a great role in the aging of the encapsulated epoxy resin. If silica gel is used, it can be improved. Ultraviolet light also has some bad effects on the aging of phosphors, but it is not very serious

however, this method is relatively effective for comparing the heat dissipation effects of the two radiators. Obviously, the smaller the volt ampere characteristic shifts to the left, the better the heat dissipation effect. In addition, there is a certain degree of accuracy for predicting the life of indoor LED lamps

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