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Heat dissipation design



As the fourth generation of lighting sources, LEDs received more and more attention in people's daily life, which were developed by improving luminous efficiency and integrating multi-chips within limited areas life. Compared to traditional incandescent lamps, LEDs demonstrate a number of advantages, such as long lifetime, energy efficiency, wide range of colors, durable, design flexibility, easily-dimmed, low-voltage power supply and environmental compatibility.     The LED-array-based high-power LED lighting has three main characteristics:


1)up to a 93% improvement in stability over other LED-based systems; 2) up to 100-fold increase in light output over single LED systems; and 3) millions of possible combinations of spectral output . However, the efficiency of photoelectric transformation is not good enough because only 15 to 25 percent of input power can be transformed into light and the rest of input power dissipate as heat, Especially, compact high power LED array is required to product high luminous output and not to induce the high junction temperature. The lumen output and the devices efficiency are strongly dependent on the heat dissipation capacity extracted from the package. Therefore, a qualified high-power LED lighting can dissipate heat to the ambient in quantity which is several times of the conventional lighting device as well as keep the LED junction temperature below 80°C to assure reliability and low optical decay. Heat dissipation is thus an important issue in high-power LED lighting technology.


In order to improve the performance of the LED lighting, two methods for heat dissipation should be taken into account: (1) increasing the air velocity with fan around the system; (2) improving the thermal performance of LED chips package; and reducing the thermal resistance from junction to ambient [5]. In this paper, we choose the first method to solve the heat dissipation problem. There are also two approaches to improve the thermal performance of LED array system. (1) Design a new substrate which reduces the average die temperature, and improves the heat dissipation capability of LED arrays, (2) Design new substrate which makes the junction temperature of the module uniform across the LED arrays, and equalizes temperatures between the central chips and the edge chips.