Key Parameter Affecting LED Light Source Lifespan - Junction Temperature
LED light source, referring to Light Emitting Diode, is a type of light source utilizing light-emitting diodes. This kind of light source has advantages such as small size, long lifespan, and high efficiency, with continuous usage lasting up to 100,000 hours, making LED light sources increasingly prevalent in various industrial lighting fields.
Junction Temperature (TJ) is the actual operating temperature of the semiconductor in an electronic device. In operation, it is typically higher than the case temperature. The temperature difference equals the power dissipated multiplied by the thermal resistance. The maximum junction temperature is specified for a given component and given power dissipation, calculating the thermal resistance between the case and the environment. Conversely, it can help designers determine an appropriate heatsink.
Lifespan and Luminous Decay
LED luminous decay refers to the decrease in light intensity of an LED after a period of illumination, where the decreased part is the luminous decay of the LED. LED manufacturers typically conduct tests under laboratory conditions (at 25°C ambient temperature) by continuously illuminating the LED with 20mA of DC current for 1000 hours to compare the light intensity before and after illumination.
The luminous decay of LEDs is related to their junction temperature. Junction temperature refers to the temperature of the semiconductor PN junction. The higher the junction temperature, the earlier luminous decay occurs, indicating a shorter lifespan. For example, with a junction temperature of 105 degrees, the lifespan until brightness drops to 70% is only over 10,000 hours, while at 95 degrees, it is 20,000 hours. Lowering the junction temperature to 75 degrees extends the lifespan to 50,000 hours, and at 65 degrees, it can be extended to 90,000 hours. Therefore, the key to extending the lifespan is to lower the junction temperature. Different LED chips have different luminous decay curves, and their thermal resistance and heat dissipation properties also vary greatly.
Factors Contributing to LED Luminous Decay
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Quality issues with LED products: Poor-quality LED chip materials lead to rapid luminous decay; production processes have defects, and LED chip heat dissipation cannot be well exported from the PIN foot, resulting in increased chip decay due to high temperatures.
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Usage conditions: LEDs are constant current driven, and some LEDs use voltage driving, causing LEDs to decay too quickly; driving current exceeds rated driving conditions.
In fact, there are many reasons for LED luminous decay, but the most critical is the issue of heat. Although many manufacturers do not pay special attention to heat dissipation in secondary products, the luminous decay of these secondary LED products will be higher than that of LED products with attention to heat dissipation in the long term. The thermal resistance of the LED chip itself, the impact of silver paste, the heat dissipation effect of the substrate, and aspects of the gel and wire bonding are all related to luminous decay.
How to Extend the Lifespan of LEDs
From the figure, it can be concluded that the key to extending its lifespan is to lower its junction temperature. And the key to lowering the junction temperature is to have a good heatsink. It can dissipate the heat generated by the LED in a timely manner.
Here, we are not discussing how to design a heatsink but rather which heatsink has relatively good heat dissipation effects. In practice, this is a measurement problem of the junction temperature. If we can measure the junction temperature that any heatsink can achieve, then not only can we compare the heat dissipation effects of various heatsinks, but we can also know the LED lifespan that can be achieved after using this heatsink.
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