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 can be taken in three different ways:
- Operate with the same power at a lower temperature, improving lifetime and reliability
- Operate at the same temperature with higher powers, increasing the optical output
- Reduce the 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. Additionally one can expect that the benefit will be more noticeable in natural convection heat sinks as the fan operation plays a major role in forced convection performance.
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.