Visit：2631   Date：2023-11-13

Light decay is a common issue in the LED industry. The light decay of LEDs is related to their junction temperature, which is the temperature of semiconductor PN junctions. The higher the junction temperature, the earlier the light decay occurs, which means the shorter the lifespan. If the junction temperature is 105 degrees Celsius and the brightness drops to 70%, the lifespan is only over 10000 hours, while at 95 degrees Celsius it is 20000 hours. If the junction temperature drops to 75 degrees Celsius, the lifespan is 50000 hours, and at 65 degrees Celsius it can be extended to 90000 hours. Therefore, the key to extending lifespan is to reduce the junction temperature.

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How to extend the lifespan of LED: The key to extending its lifespan is to reduce its junction temperature. The key to reducing junction temperature is to have a good heat sink that can timely dissipate the heat generated by LEDs. If we can measure the junction temperature that any type of heat sink can achieve, not only can we compare the heat dissipation effects of various heat sinks, but we can also know the LED lifespan that can be achieved after using this type of heat sink.

How to measure junction temperature: Junction temperature may seem like a temperature measurement issue, but the junction temperature to be measured is inside the LED, and it is not possible to measure its temperature by placing a thermometer or thermocouple into the PN junction. Of course, its shell temperature can still be measured using thermocouples, and based on the given thermal resistance Rjc (connected to the shell), its junction temperature can be calculated. But after installing the radiator, the problem becomes more complex again. Because LED is usually welded to an aluminum substrate, which is installed on the radiator, if only the temperature of the radiator shell can be measured, many thermal resistance values must be known 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 heat sink), and Rsa (heat sink to air), any inaccurate data will affect the accuracy of the test.

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Temperature coefficient of LED volt ampere characteristics: We know that LED is a semiconductor diode, which, like all diodes, has a volt ampere characteristic 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. Assuming that the LED is powered by Io constant current, the voltage is V1 when the junction temperature is T1, and when the junction temperature rises to T2, the entire volt ampere characteristic shifts to the left, the current Io remains unchanged, and the voltage becomes V2. These two voltage differences are removed by temperature to obtain their temperature coefficient, expressed in mV/oC. For ordinary silicon diodes, this temperature coefficient is approximately -2mV/oC. However, most LEDs are not made of silicon materials, so their temperature coefficient also needs to be measured separately. Fortunately, most LED manufacturers' datasheets provide their temperature coefficients.

How to predict the lifespan of this lamp: The heat dissipation efficiency of the LED street lamp radiator decreases over time. This is due to the accumulation of dust and bird droppings, which reduces its heat dissipation efficiency. Also, due to the strong ultraviolet radiation outside, the lifespan of LEDs can be reduced. Ultraviolet radiation mainly plays a significant role in the aging of encapsulated epoxy resins, and if silicone is used, it can be improved. Ultraviolet radiation also has some negative effects on the aging of fluorescent powders, but it is not very serious. However, this method is relatively effective in comparing the heat dissipation effects of two types of radiators. It is obvious that the smaller the left shift of the volt ampere characteristic, the better the heat dissipation effect of the radiator. In addition, there is still a certain degree of accuracy in predicting the lifespan of indoor LED lighting fixtures.