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Thermal Management of LED Devices

ActiveLED light fixtures will not burn your hand like some other light sources, but they do produce small amounts of heat. Thermal management is arguably the most important aspect of successful LED fixture design.

Excess heat directly affects both short-term and long-term LED performance. The short-term (reversible) effects are color shift and reduced light output while the long-term effect is accelerated lumen depreciation and thus shortened useful life.

The light output of different colored LEDs responds differently to temperature changes, with amber and red the most sensitive, and blue the least. These unique temperature response rates can result in noticeable color shifts in RGB-based white light systems if operating junction temperature differs from the design parameters.

LED manufacturers test and sort (or "bin") their products for luminous flux and color based on a 15-20 millisecond power pulse, at a fixed junction temperature of 25°C (77°F). Under constant current operation at room temperatures and with engineered heat mitigation mechanisms, the junction temperature of products is typically 60°C or greater.

Therefore white LEDs will provide at least 10% less light than the manufacturer's rating, and the reduction in light output for products with inadequate thermal design can be significantly higher.

Continuous operation at elevated temperature dramatically accelerates lumen depreciation resulting in shortened useful life.

Why is "Junction Temperature" important?

If an LED can be driven to operate at a low junction temperature (below 85° C) its livetime can be well in excess of 20 years when run continuously. Three things affect the junction temperature of an LED:

  • Power
  • Thermal Path
  • Ambient Temperature

In general, the higher the wattage, the greater the heat generated at the die. Heat must be moved away from the die in order to maintain expected light output, life and color. The amount of heat that can be removed depends upon the ambient temperature and the design of the thermal path from the die to the surroundings.

The typical high-flux LED module is comprised of an emitter, a metal-core printed circuit board, and some form of external heat sink. The emitter houses the die, optics or phosphor, encapsulant, and heat sink interface used to draw heat away from the die. The circuit board has a dielectric layer bonded to a metal substrate, usually from aluminium. The circuit board assembly is then mechanically attached to a heat sink which can be a dedicated device integrated into the design of the luminaire or, in some cases, the body of the luminaire itself.

Heat management and an awareness of the operating environment are critical considerations to the design and application of LED luminaires for general illumination. The size of the heat sink and with it the size of the fixture depends upon the amount of heat to be dissipated and the materials' thermal properties.

ActiveLED uses superior heat sink designs and its patented heat transfer technology to dissipate heat, and minimize junction temperature. Keeping the junction temperature as low as possible and within manufacturer specifications is necessary in order to maximize the performance potential of LEDs in terms of their efficiency and lifetime. Long lifetime means automatically higher efficieny and vice versa.