LED lights and sustained exposure to extreme conditions

Since LED are semiconductor devices, the various temperature-dependent parameters to consider include forward voltage, wavelength and light output. It is not so much the lifespan of the LEDs themselves that is compromised, but rather the integrated control electronics. Because LED chips, drivers, and electronic ballasts are thermo-sensitive components, thermal management is an important factor in luminaire design. The current crop of high-quality LED light fittings easily handles temperatures up to +60 °C. Things become critical if there is extreme heat in excess of +70 °C.

Power dissipation caused by the LEDs themselves is a major contributing factor. What is crucial is the temperature at the p-n junction of the integrated chips, which is commonly noted as T_junction (T_j). Since approximately 70 to 80% of the energy spent on making the light shine is converted into heat, the chip can quickly heat up to temperatures above +100 °C while in use. In the long run, this will severely affect the lifespan and light output of the luminaire.

According to a rule of thumb, a temperature exceeding the permissible maximum by 10 °Cwill cut the lifespan in half. In addition, LED technology is a driving force behind the construction of smaller and smaller, super-compact luminaires. Such designs obviously also shorten the thermal path to the electronics. However, thanks to optimised control electronics, high-end products like our 6036 series LED tubular light fittings now provide far better temperature stability.

Depending on the type of application, LED fittings can of course be exposed to relevant environmental temperatures – for instance, in machine and systems engineering when they are installed in generators. Such luminaires therefore require a particularly heat-resistant design to weather temperatures exceeding +70 °C.

And there are also geographical extremes that really test their durability. In process plants in Middle Eastern deserts, housings may heat up to well over +60 °C, especially in direct sunlight. For operation in such locations, the housing materials and gaskets must be designed to permanently withstand intense heat and especially extreme temperature fluctuations. Other environmental stress factors, such as high humidity or ultraviolet light, further add to the requirements.

Thermal management seeks to achieve the most effective heat dissipation through conduction and convection. Optimal heat transfer is best realised through direct physical contact between components, so that the dissipated heat can transfer from the junction through the LED housing and into the luminaire body or heat sink. For unobstructed heat dissipation, there should be no air gaps or pockets along the thermal conduction route. Convection then ensures that heat is dissipated via the external surface of the fitting into the surrounding atmosphere. Large surfaces and free air circulation are obviously helpful for quick heat dissipation by convection.

For powerful luminaires in the 150 W range and above that generate a lot of heat, cooling fins on the exterior are an option to increase the surface for heat dissipation. However, this can cause other problems in harsh industrial environments with a lot of dust or dirt. Dirt that accumulates between such fins again reduces proper heat dissipation and may require extra cleaning routines.

Active cooling via integrated fans is another option for light fittings. However, the necessary redundancy makes this a time-consuming and expensive solution especially in hazardous areas.

Besides consulting temperature resistance calculations, we perform various stress tests together with our partner companies. For instance, in order to determine vulnerabilities and exposure limits as early as possible during product development, we subject our equipment to an accelerated ageing process in accordance with HALT (Highly Accelerated Lifetime Test) provisions. As part of this procedure, the luminaires are exposed to rapid temperature changes and vibrations in a climatic test chamber. The failure rates of the critical elements are determined to define the life expectancy within the permissible temperature range.

Because LEDs, compared to other light sources, do not fail suddenly, but emit less and less light over time, their end-of-life is defined as a degradation of the luminosity to 70% of the initial value.