Although LEDs have an unrivaled light output compared to conventional lighting technologies, they still face the challenge of efficiency. The highest efficiency LEDs currently available on the market convert only 60 percent of the electrical power into light. The rest of the energy is released as heat. When excess heat generated on LEDs cannot be removed, it accelerates the degradation process, affects optical performance, and can cause system malfunctions.
Although LEDs have an unrivaled light output compared to conventional lighting technologies, they still face the challenge of efficiency. The highest efficiency LEDs currently available on the market convert only 60 percent of the electrical power into light. The rest of the energy is released as heat. When excess heat generated on LEDs cannot be removed, it accelerates the degradation process, affects optical performance, and can cause system malfunctions.
When an LED is operated above the maximum allowable junction temperature, light output and luminous efficiency are significantly reduced. In InGaN LEDs, this reduction can reach up to 25 percent, while in AlGaInP LEDs the light output can drop up to 70 percent.
Although the temperature-dependent reduction in light output is a short-term and reversible effect, continuous operation at high temperature can significantly accelerate the gradual reduction of light output. Every 10°C increase in junction temperature will reduce the lifetime of the LED (L70 – the time the LED produces 70 percent of its initial light output) by 30 to 50 percent.
The frequency of light produced by LEDs is governed by the energy between the conduction band and the valence band inside the LED. The operating temperature of the LED is inversely proportional to the band gap of the semiconductor. Therefore, a noticeable color shift occurs when the operating temperature of the LED is greatly increased.
Operating LEDs outside of rated conditions can cause thermal degradation of materials used in LED packages, such as phosphors, capsules, and plastic lenses. This results in a permanent chromaticity shift and spectral change.
The change in the thermal expansion coefficient due to the increase in temperature will cause the epoxy capsule of the LED package to expand. This will cause premature wear and breakage of the binding wire in the LED package and will completely break the LED.
In systems where the LED module and LED driver are together, the thermal load from the LEDs may cause the driver to malfunction by drying the liquid electrolyte in the electrolytic capacitors on the LED driver circuit.
High temperature is the most important failure accelerator in LED lighting systems. Because of this; the cooling system, which ensures the distribution of thermal energy, is an integral part of the LED lighting system design.
