by Rainer Huber, Osram Opto Semiconductor GmbH (LpR Issue 04)

The application field of Solid state lighting, in the form of light emitting diodes (LEDs) has changed from traditional indicator functions to illuminator applications. With traditional light sources such as incandescent lighting, much of the energy required to generate visible light is dissipated as heat in the radiant beam of light. LED light sources do not radiate heat energy within the light beam. The heat generated by the chip heats up the device itself. This phenomenon of self-heating affects the performance of the LED regarding light output and reliability. To achieve reliability and optimal performance of LED light sources like the Golden Dragon®, proper thermal management is necessary.

Influence of Junction Temperature
Basically, the maximum allowable junction temperature should not be exceeded, since this can lead to irreversible damage to the LED and spontaneous failure. Due to the fundamental physical interdependencies which arise during the operation of light emitting diodes, changes in the junction temperature TJ within the allowable temperature range have an effect on several LED parameters. These effects are reversible in nature. The forward voltage, luminous flux, wavelength (color) and lifetime of the LED are all influenced by the junction temperature.
Depending on the given requirements, this can finally affect the application.

• Influence of Forward Voltage Vf and Luminous Flux Φv
For LEDs, an increase in junction temperature leads to a decrease in the forward voltage VF (Figure 1), as well as a reduction in luminous flux Φv (Figure 2). This physical effect on the light output is known as degradation; the amount of degradation strongly depends on the LED technology employed.

In this case, the resulting changes are reversible. That is, the output values return to their original level when the temperature change is reversed.
For the application, this means that the light output increases with a decrease in the junction temperature TJ.

• Wavelength (λpeak and λdom) and Color Coordinates (x, y)
The influence on the wavelength of saturated LEDs and respectively color coordinates of white LEDs by a change in junction temperature appears as a reversible shift in the output values. The amount of the shift can be calculated by means of the respective temperature coefficients (Figure 3).

Figure 3: Example of junction temperature influence on dominant wavelenght.

An increase in junction temperature of 40°C, for example, leads to a shift of the dominant wavelengths in the red range of nearly 3 nm from 617 nm to 620 nm. This shift leads to a change in the color settings and appearance, and can have an influence on the application, depending on the given requirements. Depending on the application, it must be determined if this shift can be tolerated, or that appropriate measures can be taken to avoid or at least compensate for temperature related effects.
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