Bright Future for Demanding Applications
In addition to using heat pipe TGPs for newly designed electrical components and device packages, designers can improve existing LED electronic components that need to fit in tightly constrained spaces such as small, hand-held devices, avionics and laptop computers. Insertion of TGPs into existing systems can overcome the constraints of size and power while simultaneously reducing the operating temperature of the LED electronic components, or improving the performance of the current thermal management system for handling increased thermal dissipation. In essence, the cost savings is in the extension of the current product life without having to significantly reengineer it for increased power.
Another Cooling Alternative
Encapsulated annealed pyrolytic graphite (APG) such as the k-Core® system is another LED thermal management option. APG is hermetically sealed within a structural shell, often referred to as the encapsulant, made of traditional materials like aluminum, copper, beryllium, ceramics or composites, and is three to five times as conductive as copper with less density (g/cm3) than aluminum.
Encapsulated APG solutions have no moving parts, so they are compatible with standard finishing and processing manufacturing steps, giving thermal engineers design flexibility while addressing concerns about durability and maintenance. Unlike heat pipes that rely on two-phase heat transfer using a working fluid, APG is a system that relies on conduction heat transfer. APG is an excellent solid conduction material and has an in-plane conductivity of 1700 W/m·K which is greater than 4 times that of copper (~390 W/m·K).
The coefficient of thermal expansion (CTE) offered by encapsulated APG can be tailored or matched to a specific application, allowing dissipation of dramatically increased heat fluxes by allowing direct attachment to the LED and minimizing thermal resistance.
Figure 3 shows the results of using encapsulated APG inside a printed wiring board to create a high-conductivity PWB (HCPWB). At 100 mA (13.2 watts) the HCPWB is showing almost two times the Lux readings of the standard PWB. The standard PCB shows the Lux readings peaking and then falling off at the highest current levels, indicating that the excessive heat is preventing the additional current from creating additional light.
In general, encapsulated APG lowered LED package temperature by nearly 20°C, allowing the LED array to run significantly cooler than a standard printed wiring board (PWB). With a 17°C/W resistance to the junction temperature of the individual LEDs, the standard PWB configuration would require significant device de-rating to avoid overheating and/or short-life of the devices. Copper-clad APG provides a highly conductive core (up to 1,200 W/m·K) that can serve as a ground or power plane. When integrated within a PWB, this will result in a PWB with thermal conductivity greater than 300 W/m·K (15 times a standard PWB).
Potential Defense Application
Figure 4 shows a unique potential application of LEDs in defense – replacing standard chemical flares with advanced LED lighting technology. A chemical flare is a type of pyrotechnic that produces brilliant light without an explosion. These traditional flares face reliability issues, especially after long-term storage. Battery-powered LED flares offer higher reliability and provide a lot of light in minimal space. When deployed from a parachute or from a helicopter, the LEDs illuminate a wide area of the ground, offering an important benefit in military applications. Plus, the LED is recoverable and reusable. In this case, the thermal solution would be similar to that shown in Figure 1. The heat pipes contact the LEDs directly and transport their heat to natural convection fins. As the flare is dropping, the air circulates through the fins and removes the heat.
Because LEDs can be designed to operate on various wavelengths, they can also be used for infrared lighting for night vision goggles, vehicle lighting or security lighting, offering a valuable tactical advantage in military applications.
Another potential use of LEDs is on the ultraviolet wavelength, where the light can be used for water treatment devices or sterilization of surgical equipment for field units.
Because of the long list of engineering, quality and financial concerns and considerations when designing LEDbased lighting applications – such as power consumption, operating efficiency, product reliability, operating life, size and cost – there is no one-size-fits- all thermal management technique. A number of thermal technology options and materials must be explored, with the specific application requirements driving selection and implementation. For many high-power, space-constrained military LED lighting applications, custom thermal management solutions using passive thermal management technologies are an ideal option. Passive thermal technologies allow design engineers the flexibility to pack more power into a smaller footprint, thanks to lightweight, flexible designs resulting in higher quality, more reliable and longer lasting lights.
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