Heat pipes have an effective thermal conductivity of 10,000 to 100,000 W/m K, compared with aluminum’s thermal conductivity of about 180 W/m K. Therefore, the performance of large aluminum heat sinks can be improved with embedded heat pipes. The high effective conductivity allows the heat pipes to spread the heat throughout the heat sink. This heat spreading reduces the thermal gradient and likewise reduces the max temperature at the LED source. Benefits include:
- Improved lifetime and reliability by operating with the same power at a lower temperature
- Increased optical output by operating at the same temperature with higher powers
- Reduced heat sink size and weight
Our testing and analysis has confirmed that the longer the extrusion, the more heat pipes improve performance. As seen in Figure 1, the percentage improvement in thermal resistance with heat pipes increases approximately linearly with increasing heat sink length. For example one can expect to see a 5% improvement in thermal resistance for a 5 cm long heat sink, increasing to 30% for a 30 cm long heat sink. Please note that one can expect the benefit will be more noticeable in natural convection heat sinks, as fan operation plays a major role in forced convection performance.
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Figure 1. The efficiency of an aluminum extrusion heat sink can be greatly improved by embedding heat pipes. The benefit is increased with longer heat sinks.
Figure 2 shows a photograph and an infrared (IR) image of a heat sink with embedded heat pipes. In this case heat pipes were embedded in a 200mm long radial heat sink that that was dissipating 100 watts of heat. The heat pipe improves the efficiency of the heat sink, by transferring heat with a very low ΔT from the LED at the bottom to the entire length of the heat sink.
Figure 3 shows calculated temperatures for identical heat sinks, with and without embedded heat pipes. The heat pipes decreased the LED maximum temperature by 10⁰C, which will help achieve long, reliable operation.
