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LED heat dissipation

LED performance strongly depends on the ambient temperature

Our LED manufacturers already contribute significantly to the thermal management by using packages that conduct the heat from the chip through thermal spreaders to the outside world. Very often these thermal spreaders have a metallic surface providing a very good thermal conduction to the PCB, bypassing the normal soldering process. Operating LED-based luminaires at high ambient temperatures without proper thermal design may overheat LED packages, eventually lead to short lifespan or device failure in the worst case. Well-designed cooling system is required to remove heat from the junction in order to maintain high efficacy, reliability and lifetime of LED-based light source. Basic parameters to evaluate LED performance are : Junction temperature – Tj and Thermal resistance – Rj-a. Another possibility is to manufacture a PCB (typically FR4 material) with many vias conducting the heat to the other side of the PCB which can then be attached to e.g. an aluminum heatsink. Heat pipes and larger heatsink profiles are also appropriate means of cooling, while thermoelectric cooling as well as fluid cooling are not very common in LED lighting applications so far. Very often it makes sense to use an appropriate thermal substrate technology with embedded metal cores in order to conduct the heat away from the LED. The products manufactured by DK-Thermal Solutions in particular, (available at EBV) solve many heat problems, because so-called starboards are available for almost every LED. Lamp manufacturers can use these boards with a properly mounted LED and place it in their design in many different ways and with plenty of freedom – almost like they used to place halogen incandescent lights.

Power conversion

All light sources convert electric power into light and heat in various proportions

Compared to the incandescent bulbs which emit mainly in infrared (IR) region with only approx. 8% of light emitted or fluorescents which emit a higher portion of light (21%) but also emit IR, UV, and heat - LEDs generate little IR and convert up to 40% of the electrical power into the light. The rest is converted to heat that must be conducted from the LED active area to the underlying printed circuit board, cooling system, housing, and atmosphere. Therefore to mantain longterm LEDs efficacy the most challenging part in a luminairie is the design of thermal management and a cooling system. A simple example illustrates the effect of current and temperature: There is a real-world LED with a median lifetime of 50,000 hours at a forward current IF = 250 mA and ambient temperatures TA = 25°C. When the same LED is operated at IF = 350 mA at TA = 85°C its median lifetime significantly decreases to 5,000 hours. Furthermore, the colour intensities decrease with increasing temperatures. At the same time, the emitted spectrum changes – an effect known as ‘colour shift’, that is especially visible with red LEDs. In fact, thermal management means nothing else but transferring the heat from the LED die to the surrounding environment by conduction, convection and/or radiation.

Active cooling systems

A very innovative and possibly the most effective way of cooling LEDs is by using ‘SynJet™ Fanless Air Cooling™’ from Nuventix

The Nuventix developed SynJet® module is a new air-based synthetic jet cooling technology that takes advantage of turbulent pulses of air generated from an electromagnetic actuator. SynJets operate almost silently with extremely high reliability matching the lifetimes of LED lighting products. These solutions are highly adaptable, quiet, reliable cooling devices that efficiently, effectively dissipate heat from any surface. Nuventix manufacturers both, SynJet Coolers and complementary heatsinks all available from EBV ELEKTRONIK. The SynJet Cooler is always mounted to the heat sink creating a thermal management solution. Modern electronics and LED lighting systems are full of hot components. All of that heat leads to a shorter lifespan for your product. SynJet modules are more thermally efficient at air flow than conventional air movers. The turbulent pulsating flow results in more efficient heat transfer from the heat source to the air. As a result you get more cooling with less air. The SynJet allows designers to fit more lumens into less space. It’s able to cool more effectively, allowing LED light engines to achieve two times the lumen output in the same size or to reduce the size of the LED’s heat sink by an astounding two thirds. Nuventix thermal solutions are compatible with a variety of light engines and can be adapted to any light engine or array. With the elimination of frictional parts common to fans and blowers, the potential failure modes are greatly reduced. SynJets are constructed out of robust materials for applications that operate in extreme environments. The list of SynJet reliability attributes goes on: a stunning five year warranty, 100 thousand hour L10 rated life, rated operating temperatures from -40 to +85°C, high resistance to dust, humidity, bumps and vibrations. SynJet modules produce airflow that is thermally efficient; therefore the amount of air flow needed is reduced. Lower flow rates mean lower acoustic emissions- less noise. In addition, by not having any bearings, brushes, or other frictional parts, the SynJet module eliminates the acoustic problems associated with these interfaces.

Passive cooling systems

The term “passive” means NO energy-consuming mechanical components incorporated like pumps, jets or fans

Passive heatsinks are the most commonly used for LED luminaires. Generally, heatsink has finned metal encasement that conducts accumulated heat away from the LED light source. Since heatsink does not consume any additional energy, it is the most energy-efficient cooling system. However, LED light sources with high power consumption require a large cooling area, i.e. complex shaped heatsink, which adversely influences luminaire design. Passive cooling is the most preferable cooling system for LED luminaries. During such a thermal design it is necessary to take into the account several factors such a spacing of LED light sources, material properties of materials used for luminaire construction, shape and surface finish of heatsink being designed and several others.