Cooling High Power LEDs

Most LEDs are designed in SMT (surface mount technology) or COB (chip-on-board) packages. In the new 1~8W range of surface mount power LED packages, the heat flux at the devices thermal interface can range from 5 to 20 W/cm2. These AllnGaP and InGaN semiconductors have physical properties and limits similar to other transistors or ASICs (application specific integrated circuit). While the heat of filament lights can be removed by infrared radiation, LEDs rely on conductive heat transfer for effective cooling.

As higher powers are dissipated from LED leads and central thermal slugs, boards have changed to move this heat appropriately. Standard FR-4 technology boards can still be used for LEDs with up to 0.5 W of dissipation, but metallic substrates are required for higher levels. A metal core printed circuit board (MCPCB), also known as an insulated metal substrate (IMS) board, is often used underneath 1W and larger devices. These boards typically have a 1.6 mm (1/16 inch) base layer of aluminum with a dielectric layer attached. Copper traces and solder masks are added subsequently. The aluminum base allows the heat to move efficiently away from the LED to the system.

Increasing power density, a higher demand for light output, and space constraints are leading to more advanced cooling solutions. High-efficiency heat sinks, optimized for convection and radiation within a specific application, will become more and more important.

As with any semiconductor package, thermal resistance plays a significant role in the thermal management of LEDs. The highest thermal resistance in the heat transfer path is the junction-to-board thermal resistance (Rj-b) of the package [2]. Spreading resistance is also an important issue. Thermally enhanced spreader materials, such as metal core PCBs, cold plates, and vapor chambers for very high heat flux applications are viable systems to reduce spreading resistance. [3]

Linear heat sinks are available specifically for LED strips, such as OSRAM SYLVANIA’s DRAGONstick® linear LED strips, which are widely used in architectural lighting. For example,the maxiFLOW linear heat sink from Advanced Thermal Solutions, Inc., has a patented spread fin array that maximizes surface area for more effective convection (air) cooling, particularly when air flow is limited, such as inside display cases.

Round heat sinks are available specifically for round LED boards, which are used to replace halogen light bulbs, in applications such as spotlights and down lighting. A typical LED spotlight is shown in Figure 2 [5]. Here, a round QooLED© heat sink from Advanced Thermal Solutions is used for cooling three LEDs. The round heat sink has a special star-shaped profile fin design that maximizes surface area for more effective convection (air) and radiation cooling in the vertical mounting orientation, e.g., inside ceilings.

Active thermal management systems can be used for high-flux power LED applications. These include water cooling, two-phase cooling, and fans. Although active cooling methods may not be energy-justifiable for LEDs, reasons for using them include ensuring lumen output or maintenance-free operation, or to meet specific wavelength requirements.

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