High power light-emitting diodes (LEDs) are likely to replace other technologies such as incandescent and fluorescent bulbs in signaling, solid state lighting, and vehicle headlights because they save energy and extend the light's lifetime.[1] LEDs that use from 500 milliwatts to as much as 10 watts in a single package have become standard, and researchers expect to use even more power in the future. Some of the electricity in an LED becomes heat rather than light. If that heat is not removed, the LEDs run at high temperatures, which not only lowers their efficiency, but also makes the LED more dangerous and less reliable. Thus, thermal management of high power LEDs is a crucial area of research and development.
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In order to maintain a low junction temperature to keep good performance of an LED, every method of releasing heat from LEDs should be considered. Conduction, convection, and radiation are the three means of heat transfer. Typically, LEDs are encapsulated in a transparent resin, which is a poor thermal conductor. Nearly all heat produced is conducted through the back side of the chip. Heat is generated from the PN junction by electrical energy that was not converted to useful light, and conducted to outside ambience through a long and extensive path, from junction to solder point, solder point to board, and board to the heat sink and then to the atmosphere. The heat path of tungsten light bulbs is almost all straight into the atmosphere, starting from filament to the glass and ending with the thermal resistance from glass to the atmosphere. A typical LED side view and its thermal model are shown in the figures.
Intuitively, one can see that the junction temperature will be lower if the thermal impedance is smaller and likewise, with a lower ambient temperature. To maximize the useful ambient temperature range for a given power dissipation, the total thermal resistance from junction to ambient must be minimized. The values for the thermal resistance vary widely depending on the material or component supplier. For example, RJC will range from 2.6 °C/W to 18 °C/W, depending on the LED manufacturer. The thermal interface material’s (TIM) thermal resistance will also vary depending on the type of material selected. Common TIMs are epoxy, thermal grease, pressure sensitive adhesive and solder. In the most cases, power LEDs will be mounted on metal-core printed circuit boards (MCPCB), which will be attached to a heat sink. Heat flows from the LED junction through the MCPCB to the heat sink by way of conduction, and the heat sink diffuses heat to the ambient surroundings by convection. So, we can also add convection to the thermal model at the end of the heat transmission path. In the package design, the surface flatness and quality of each component, applied mounting pressure, contact area, the type of interface material and its thickness are all important parameters to thermal resistance design.
Some considerations for passive thermal designs to ensure good thermal management for high power LED operation include:
Adhesive is commonly used to bond LED and board, and board and heat sinks. Using a thermal conductive adhesive can further optimize the thermal performance.
Heat sinks provide a path for heat from the LED source to outside medium. Heat sinks can dissipate power in three ways: conduction (heat transfer from one solid to another), convection (heat transfer from a solid to a moving fluid, for most LED applications the fluid will be air), or radiation (heat transfer from two bodies of different surface temperatures through electromagnetic waves).