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Temperature influence on LED analysis (2)

In the last article we mentioned that high temperatures will cause permanent damage to automotive headlights and increase the temperature will shorten the life of automotive lamp bulbs. Next I continue to talk about the temperature of the LED lights.

3.Increase in temperature will reduce the luminous efficiency of the automotive headlights

The temperature affects the LED light efficiency due to the following aspects:

1)As the temperature increases, the electron and hole concentrations increase, the band gap decreases, and the electron mobility decreases.

2)As the temperature increases, the probability of radiative recombination of electrons and holes in the potential well decreases, resulting in non-radiative recombination (heat generation), thereby reducing the internal quantum efficiency of the LED.

3)The temperature rise causes the blue light wave peak of the chip to shift to the long wave direction, make the chip's emission wavelength and the excitation wavelength of the phosphor not match, can also cause the outside light extraction efficiency of the white LED to reduce.

4)As the temperature increases, the quantum efficiency of the phosphor decreases, and the light emission decreases, and the external light extraction efficiency of the LED decreases.

5)The performance of silica gel is greatly affected by the ambient temperature. As the temperature rises, the thermal stress inside the silica gel increases, causing the refractive index of the silica gel to decrease, thereby affecting the light efficiency of the LED.

Under normal circumstances, the effect of reducing luminous flux as the junction temperature increases is reversible. In other words, when the temperature returns to the initial temperature, the light output flux will have a recoverable increase. This is because some of the relevant parameters of the material will change with temperature, resulting in changes in the LED device parameters, affecting the light output of the LED. When the temperature is restored to the initial state, the LED device parameters change and the LED light output will return to the initial state value. In this regard, the luminous flux value of the LED is divided into "cold lumen" and "hot lumen", which respectively represent the light output of the LED junction at room temperature and a certain temperature.

In general, the relationship between LED luminous flux and junction temperature can be expressed by the following formula:

Among them,means the luminous flux(lm) at the junction temperature , means the luminuous flux9(lm at the junction temprture , means the temperature coefficient (1/°C),means the difference in the LED junction temperature,ie..

In general, the value can be determined experimentally. For example, the InGaAlP LED-related values are shown in the following table:

form 1:

LED material category Temperature Coefficient (1/℃)
InGaAlP/GaAs  Orange red 9.52×10-3
InGaAlP/GaAs Yellow 1.11×10-2
InGaAlP/GaP    bright red 9.52×10-3
InGaAlP/GaP    Yellow 9.52×10-2

above form shows Temperature coefficient of different material class LEDs

form 2:

Above shows Relationship between light output (percentage) and junction temperature of different k-value LEDs

It can be seen from form 2 that the temperature coefficient k of the LED light effect is preferably 2.0×10-3 or less, so that the decrease of the LED light output caused by the temperature will not be great. For example, the InGaN-based LED has a k-value of about 1.2×10-3, and the junction temperature of 125°C reduces the light output by about 11% when compared to the junction temperature of 25°C.

At present, the temperature coefficient of most used GaN-based white LEDs is mostly between 2.0×10-3 and 4.0×10-3, and some even reach 5.0×10-3. K-value LED is too large, but also pay attention to control the junction temperature.

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