This article was published in the April/May 2012 issue of LEDs Magazine.
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Efficient removal of heat from LED light sources is a crucial factor in ensuring that an LED lamp or luminaire exhibits the expected levels of performance and lifetime. Conventional light sources release heat to the ambient through the emission of infrared radiation, and convection is also an important mechanism in the case of fluorescent lamps. These modes of heat transfer typically do not require advanced cooling methods to facilitate the process. However, LED sources lose heat through conduction, which requires the additional knowledge of how to efficiently carry waste heat out of the LED in order to prevent performance degradation and system failure.
Active cooling approaches include conventional fans and diaphragm-based forced air cooling. While enabling higher power dissipation, these generally produce noise and may have issues of reliability and performance degradation over time. They are not orientation-dependent, but add slightly to the operating cost since they consume electricity. In general, there is a split between the advantages and disadvantages of active and passive approaches.
Two-phase heat exchangers
Another passive cooling approach is the two-phase heat exchanger, which works on the same principle as a heat pipe. One example, the HPK-Fin from FrigoDynamics, is a two-phase heat exchanger that aims to combine the advantages of both active and passive systems.
The effective thermal conductivity across a two-phase heat-transfer device can be up to several hundred times greater than an equivalent aluminum or copper rod. The process can even work independently of the device’s orientation. The HPK-Fin coolers have a series of fins around the tubes to dissipate heat (see Fig. 2).
The effectiveness of the passive HPK-Fin two-phase heat exchangers was tested using an SC HPK-Fin 150 (fin diameter of 100 mm with 150-mm overall length and weight of 220g) in combination with a Xicato XSM 2000-lm, 36W LED module. A variety of orientations were tested.
The key measurement is the case temperature (Tc) of the LED itself. This is the temperature on the LED surface that makes contact with the HPK-Fin two-phase heat exchanger. For this particular LED module, the maximum allowable Tc is 90°C, in order to guarantee the specified lumen output and 50k hours of operation.
Passive cooling relies heavily on surface area for free convection to occur, in contrast to active cooling which places more emphasis on the forced air-flow rate. Therefore, it is crucial that the heat can be spread effectively across the available area. To illustrate this point, a longer SC HPK-Fin 230 (230 mm in length) was analyzed with an infrared (IR) thermal-imaging camera. Fig. 3 demonstrates that the heat is distributed very evenly, at a temperature difference of less than 2°C along the whole length and surface of the two-phase heat exchanger.
This is particularly notable when considering that a large amount of heat is coming from a relatively small footprint at the base of the two-phase heat exchanger. Using only solid extrusions or castings would lead to considerable thermal gradients when subjected to a concentrated heat load, resulting in less-effective heat dissipation at the areas far away from the heat source.
The configuration and layout of the two-phase heat exchangers and fins can be adapted and tailored to meet specific product-design embodiments. Two-phase heat exchangers are typically larger than active cooling devices, but are still low in weight. Given their performance, they are compact relative to other passive solutions.
One recent example can be seen in Fig. 4, where the SC110 FrigoDynamics HPK-Fin two-phase heat exchanger is integrated into the AlphaLED Metropole light fixture from Projection Lighting Ltd, enabling high performance in an entirely passive fashion and with a lightweight design.
In summary, the passive two-phase heat exchanger approach combines the advantages of other passive and active cooling systems, including high reliability, no sensitivity to orientation, and zero operating cost.