Thermal management of high power LED

Thermal management of high power LED

With the coming of energy-saving era, high power light-emitting diodes (LEDs) are promising to replace other technologies such as incandescent and fluorescent bulbs in signaling, solid state lighting, and vehicle headlight applications due to improved luminescent efficiencies and extended lifetime. Power dissipation ratings ranging from 500mW to as much as 10 watts in a single package have become a standard and are expected to increase in the future. However, current packaging efficiencies clearly indicate that conventional packages are inadequate for the demands of any current and future applications. Heat accompanied by higher power not only causes efficiencies to lower down, but also influences long-term reliability of LED devices. Consequently, thermal management of high power LEDs is extremely crucial for proper operation and extended life. Optimal heat dissipating material and package method should be well designed to fit the growing power needs.

Heat transfer procedure

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. In LEDs, heat is also transmitted through conduction. 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. Thus, heat is generated from the PN junction 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.

The thermal resistance between two points is defined as the ratio of the difference in temperature to the power dissipated; the unit is oC/W. From the LED junction to the thermal contact at the bottom of package, the thermal resistance is governed by the package design. It is referred to as the thermal resistance between junction and ambient (RJA). Different components in the heat conduction path can be modeled as different thermal resistances. The total power dissipated by the LED (PLED) is the product of the forward voltage and the forward current of the LED, which can be modeled as a current source. The ambient temperature is modeled as a voltage source. Therefore, the junction temperature (TJ) is the sum of the ambient temperature (TA) and the product of the thermal resistance from junction to ambient and the power dissipated. By “thermic Ohm’s Law”, we have the equation as follows:TJ = TA + (RJA × PLED) , and RJA = RJC + RCB + RTIM + RH

Intuitively, you 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 2.6oC/W to 18oC/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 one thermal resistance 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.

Passive thermal designs

Here below lists some considerations for passive thermal designs to ensure good thermal management for high power LED operation.


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 sink

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).
* Material – Material selection of heat sinks directly affects the dissipation efficiency through conduction. Consequently, material with higher thermal conductivity is desired. The material normally used for heat sink construction is aluminum, although copper may be used with an advantage for flat-sheet heat sinks.
* Shape - Thermal transfer takes place at the surface of the heat sink. Therefore, heat sinks should be designed to have a large surface area. This goal can be reached by using a large number of fine fins or by increasing the size of the heat sink itself.
* Surface Finish - Thermal radiation of heat sinks is a function of surface finish, especially at higher temperatures. A painted surface will have a greater emissivity than a bright, unpainted one. The effect is most remarkable with flat-plate heat sinks, where about one-third of the heat is dissipated by radiation. Moreover, a perfectly flat contact area allows the use of a thinner layer of thermal compound, which will reduce the thermal resistance between the heat sink and LED source. On the other hand, anodizing or etching will also decreases the thermal resistance.
* Mounting method- Heat-sink mountings with screws or springs are often better than regular clips. Thermal conductive glue or sticky tape should only be used in situations where mounting with clips or screws is not possible.

PCB (Printed Circuit Board)

* MCPCB - MCPCB (Metal Core PCB) are those boards which incorporate a base metal material as heat spreader as an integral part of the circuit board. The metal core usually consists of aluminum alloy. Furthermore MCPCB can take advantage of incorporating a dielectric polymer layer with high thermal conductivity for lower thermal resistance.
* Separation - Separate the LED drive circuitry from the LED board so that the heat generated by the driver will not contribute to the LED junction temp.

Package type

* Flip chip - The concept is similar to flip-chip in package configuration widely used in the silicon integrated circuit industry. Briefly speaking, the LED die is assembled face down on the sub-mount, which is usually silicon or ceramic, acting as heat spreader and supporting substrate. The flip-chip joint can be eutectic, high-lead, lead-free solder or gold stub. The primary source of light comes from the backside of the LED chip, and there is usually a build-in reflective layer between the light emitter and the solder joints to reflect the light emitted downwards up. Commercially, several companies have adopted the flip-chip based approach to package their high-power LED. About 60% reduction in the thermal resistance of the LED is achieved while keeping its thermal reliability.

ee also

* LED lamp - solid state lighting (SSL)
* Thermal resistance in electronics
* Thermal management of electronic devices and systems
* Active cooling

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • LED lamp — An LED lamp (LED light bulb) is a solid state lamp that uses light emitting diodes (LEDs) as the source of light. The LEDs involved may be conventional semiconductor light emitting diodes, organic LEDs (OLED), or polymer light emitting diodes… …   Wikipedia

  • Thermal copper pillar bump — The Thermal Copper Pillar Bump, also known as the thermal bump , is a thermoelectric device made from thin film thermoelectric material embedded in flip chip interconnects (in particular copper pillar solder bumps) for use in electronics and… …   Wikipedia

  • Fusion power — The Sun is a natural fusion reactor. Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus (in contrast with fission power). In doing so they release a …   Wikipedia

  • Electric power transmission — Electric transmission redirects here. For vehicle transmissions, see diesel electric transmission. 400 kV high tension transmission lines near Madrid Electric power transmission or high voltage electric transmission is the bulk transfer of… …   Wikipedia

  • Wind power — Wind power: worldwide installed capacity [1] …   Wikipedia

  • Nuclear power — Atomic Power redirects here. For the film, see Atomic Power (film). This article is about the power source. For nation states that are nuclear powers, see List of states with nuclear weapons …   Wikipedia

  • Environmental impact of nuclear power — Nuclear power activities involving the environment; mining, enrichment, generation and geological disposal. The environmental impact of nuclear power results from the nuclear fuel cycle, operation, and the effects of nuclear accidents. The… …   Wikipedia

  • Nuclear power debate — For nuclear energy policies by nation, see Nuclear energy policy. For public protests about nuclear power, see Anti nuclear movement. Three of the reactors at Fukushima I overheated, causing meltdowns that eventually led to hydrogen explosions,… …   Wikipedia

  • Nuclear power plant — This article is about electricity generation from nuclear power. For the general topic of nuclear power, see Nuclear power. A nuclear power station. The nuclear reactor is contained inside the cylindrical containment buildings to the right left… …   Wikipedia

  • Solar power satellite — A solar power satellite, or SPS or Powersat, as originally proposed would be a satellite built in high Earth orbit that uses microwave power transmission to beam solar power to a very large antenna on Earth. Advantages of placing the solar… …   Wikipedia