Thermal management of high-performance light-emitting diodes

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The thermal management of high-power light-emitting diodes (high-power LED) describes technical processes to dissipate the power loss of light-emitting diodes (LED), which are primarily used as light sources , in order to avoid excessively high temperatures on the light-emitting diode and the associated damage. Most of the output of a light emitting diode, approx. 70%, is converted into heat loss , the rest of approx. 30% into visible light . If this heat cannot be dissipated appropriately, the efficiency drops and the service life of the light-emitting diode is shortened.

For a long service life of the light-emitting diode, it is necessary to limit the temperature of the barrier layer to a maximum value. The thermal management of high-power LEDs is an area of ​​research and development.

General

In order to maintain a low junction temperature that will maintain the high performance of an LED , every possibility of heat dissipation from LEDs should be considered. Heat conduction , heat dissipation through air (convection) and radiation are the three possibilities for heat transfer. Typically, LEDs are encapsulated in a transparent resin, which is a poor conductor of heat . Almost all of the heat generated is conducted through the back of the chip. Heat is generated from the pn junction by electrical energy that has not been converted into light. It travels a long distance from the connection point to the soldering point, from the soldering point to the circuit board and circuit board to the heat sink and is then conducted to the atmosphere of the external environment.

The junction temperature is lower when the thermal impedance is lower or the ambient temperature is lower. In order to maximize the usable ambient temperature range for a given power dissipation, the total thermal resistance from the connection point to the environment must be minimized. The values ​​for the thermal resistance vary greatly depending on the material and the adjacent components. For example, the thermal resistance of the barrier layer to the housing ranges from 2.6 ° C / W to 18 ° C / W, depending on the LED manufacturer. The thermal resistance of the materials used also varies depending on the type of material selected. Common materials include epoxy , thermal paste , pressure sensitive adhesive, and solder. High-power LEDs are often mounted on metal core printed circuit boards (MCPCB) that are attached to a heat sink. Heat that is conducted through the metal module plate and the heat-dissipating heat sink is then dissipated by convection and radiation. In addition to the design and construction of the heat sink, the surface evenness and quality of each component, the contact pressure, the contact surface, the type of interface material and its thickness. These are parameters for the heat resistance or the cooling of the LED through heat dissipation.

Passive cooling

Passive cooling factors for efficient thermal management of high-power LEDs are:

Thermal adhesive

Thermally conductive glue is usually used to connect LEDs to the board and then the board to the heat sink.

Heat sink

Heat sinks make a significant contribution to the removal of heat. It works as a heat conductor that conducts the heat from the LED source to the external medium. Heat sinks can dissipate energy in three ways: heat conduction (conduction: heat transfer within or from one solid to another), convection (heat transfer from a solid to a moving fluid, for most LED applications the fluid is the ambient air) or Radiation (heat transfer from two bodies with different surface temperatures through thermal radiation).

  • Material - The thermal conductivity of the material the heat sink is made of directly affects the power dissipation of heat conduction . Usually aluminum is used because of its excellent price-performance ratio. In the case of flat heat sinks, copper is also often used, despite the high purchase price. New materials include thermoplastics, which are used when the requirements for heat dissipation are lower than normal (e.g. often in the home) or complex shapes in the injection molding process make sense. Graphite solutions often have a more effective heat transfer (not heat conduction) than copper and are lighter than aluminum. Graphite is considered an exotic cooling solution and is more expensive to produce. Heat pipes can also be added to aluminum or copper heat sinks to reduce resistance to spread.
  • Shape - The heat transfer takes place on the surface of the heat sink. Therefore, heat sinks should be designed so that they have a large surface area. This can be achieved by using a large number of fine fins or by increasing the size of the heat sink itself.

Although a larger surface area leads to better cooling performance, there must be enough space between the fins to create a significant temperature difference between the cooling fin and the ambient air. If the fins are too close together, the air in between can be almost the same temperature as the fins, so there is no heat transfer. Therefore, more cooling fins do not necessarily lead to more cooling capacity.

  • Surface finish - Heat radiation from heat sinks is a function of the surface finish, especially at higher temperatures. A painted surface has a higher emissivity than a light, unpainted surface. The effect is most remarkable with flat heat sinks, where about a third of the heat is dissipated by radiation. In addition, an optimal flat contact surface enables the use of a thinner thermal paste , which reduces the thermal resistance between the heat sink and the LED source. On the other hand, anodizing or etching also reduces thermal resistance.
  • Mounting - Heat sink mounts with screws or springs are often better than traditional clips, thermal glue, or tape. For heat transfer between LED sources above 15 watts and LED coolers, it is recommended to use a highly thermally conductive interface material (TIM) that has a thermal resistance across the interface of less than 0.2 K / W. Currently, the most common method in use is a phase change material, which is applied in the form of a firm pad at room temperature, but then turns into a thick gelatinous liquid once it rises above 45 ° C.

Heat pipes and steam chambers

Heat pipes and steam chambers work passively and their thermal conductivity is very effective from 10,000 to 100,000 W / mK. They offer the following advantages in LED thermal management:

  • transports heat to another heat sink with minimal temperature drop
  • isothermalized by natural convection, a heat reduction, increases the efficiency and reduces its size. There is a known case where the addition of five heat pipes reduced the heat sink mass by 34% from 4.4 kg to 2.9 kg.
  • efficiently converts the high heat flux directly under an LED into a lower heat flux that can be dissipated more easily.

Printed circuit boards

MCPCB

MCPCB (Metal Core PCB) are circuit boards that contain a base metal material for heat distribution as an integral part of the circuit board. The metal core usually consists of an aluminum alloy. MCPCB has the advantage of a dielectric polymer layer with high thermal conductivity .

Separation

Separating the LED driver circuit from the LED board prevents the heat generated by the driver from increasing the LED junction temperature.

PCB coating

Additive process

During the manufacturing process of the printed circuit boards, conductive substances are applied to the carrier material to create a conductive structure surface. The conductor is only applied to the specified conductor track pattern. In contrast, this is etched away in the subtractive process. Basically, there is a direct connection to the aluminum heat sink; No additional material for the thermal connection is required for the circuit. This reduces the heat-conducting layers and heat surface. Processing steps, material types and material quantities are reduced. Aluminum PCB (also IMS PCB for Insulated Metal Substrate ) - It increases the thermal connection and offers a high dielectric breakdown voltage. Materials can withstand heat of up to 600 ° C. The circuits are mounted directly on aluminum substrates so no thermal interface materials are required. Thanks to the improved thermal connection, the junction temperature of the LED can be reduced by up to 10 ° C. This enables the designer to reduce the number of LEDs required on a board by increasing the power for each LED. The size of the substrate can also be reduced to accommodate dimensional constraints. It has been proven that reducing the transition temperature greatly increases the service life of the LED.

Housing shape

  • Flip-Chip - The LED chip is mounted face down on the holder, which is usually made of silicon or ceramic and is used as a heat spreader and carrier substrate. The flip chip assembly can be eutectic , lead rich, lead free or gold stub. The primary light source comes from the back of the LED chip. A reflective layer is usually built in between the light emitter and the soldering points to reflect the light emitted downwards. Several companies are using flip-chip packages for their high-power LEDs, which reduces the thermal resistance of the LED by around 60%. At the same time, the thermal reliability is maintained.

Individual evidence

  1. ↑ http://www.ledsmagazine.com/articles/2005/05/fact-or-fiction-leds-don-t-produce-heat.html
  2. ↑ http://www.ledjournal.com/main/wp-content/uploads/2012/05/Philips_Understanding-Power-LED-Lifetime-Analysis.pdf
  3. ↑ https://www.svetoch.eu/de/2018/05/25/kuehler-und-thermisches-management-von-high-power-leds/
  4. ↑ http://www.electronics-cooling.com/2013/09/heat-pipe-integration-strategies-for-led-applications/
  5. ^ Advanced Cooling Technologies, Inc .: ACT has solutions for Photonics Cooling Applications
  6. ↑ High Heat Flux Heat Pipes Embedded in Metal Core Printed Circuit Boards for LED Thermal Management