Strong temperature fluctuations of LEDs shorten the operating time considerably and exceeding the permissible chip temperature also has a direct effect on the luminous efficacy. Reducing the LED chip temperature by about 10 ° C can increase the life of the LED by about a factor of 3.5.
Not only the LED alone is responsible for the quality of the light, as it is made up of several function blocks. Among other things, the power supply of the LED, the optics of the LED for bundling or scattering the light and, in particular, the thermal budget of the LED, form a primary component in the form of efficient thermal management. The inorganic LED belongs technically to the product group of electroluminescent emitters, as well as the well-known fluorescent tubes, which need no additional cooling.
This impression often suggests to the LED user that even for the LED no further cooling is necessary. Only 35 percent of the applied electrical energy is converted into a light output according to the current state of LED technology. The larger remaining part is heat loss, which must also be dissipated as in conventional semiconductors, necessarily to extend the component life by effective thermal management to the environment.
Passive heat dissipation method
In many lighting applications, special LED heatsinks in a round shape are often used. Not only because of the passive and thus noiseless cooling method, but this type of execution is also in demand and use. The star-shaped aluminum heat sinks are produced in the so-called extrusion or extrusion process. Relatively low tooling costs can realize aluminum profiles in a round shape and also provide a real added value for the user regarding specific thermal conductivity, weight and mechanical strength about the heat dissipation.
Various variants have a massive aluminum core for LED recording. At this moment, the heat emitted by the LED is absorbed and distributed homogeneously into the individual cooling ribs with adapted rib geometry. The full aluminum core for LED recording also allows the user freedom of design; the LED with the associated holding system and reflector as well as the optics by an additional mechanical reworking installed directly in the milled solid core. The efficient, specially adapted to the LED cooling system heat sink concept is thus an important and integral part of the lighting design, the LED heat sink changes its appearance from an industrial part to a presentable design element.
The individual LED heatsinks in round shape are matched to the respective LED types and their technical parameters. Mechanical CNC reworking or modifications according to customer specifications to adapt the heat sinks exactly to the desired luminaire design are demanded by the users more than ever. If no matching LED heat sink can be found for the application, for example, due to the design requirements or technical parameters, it is possible to set up a custom profile, but also to customize its own LED heat sink.
Newly developed production technologies enable customizable LED heat sinks. Here, the admission to the LED, likewise as with the extruded profiles, consists of a massive aluminum full core. The solid core can be adapted to the respective size of the LED and its holder system. Circumferentially, the solid core contains a special groove geometry, which is introduced via special processing machines. In a further operation, individual aluminum sheets are now pressed, which in their entirety represent the rib geometry of the heat sink. In addition to the press-fitting process, an additional thermal adhesive ensures maximum contact between the aluminum sheets and the solid core. Also, the Wärmeleitkleber avoids air pockets and thus causes the smallest heat transfer resistance.
The design of the LED heat sink can be adapted relatively easily and flexibly to all LED modules and sizes regarding core diameter for LED recording, the number of ribs, thickness and geometry. The execution of the copper receiving core for heat spreading, various surface finishes and any mechanical processing are also feasible. Finally, it should be noted that individual LED heat sink solutions can be implemented without great effort and additional tooling costs, even for smaller quantities, especially according to customer-specific requirements and application-technical requirements.
Even so-called LED linear modules require efficient thermal management for larger power losses. Luminaires, which are made up of LED linear modules, are often used in industrial design, architecture, the furniture industry or in advertising, for example. The unilaterally open U-shaped heat sinks have integrated guide grooves for holding rigid or flexible LED-Line modules and retractable cover plates or Plexiglas panes, which may vary in material thickness. Linear blades with a single adhesive option can also be glued to the heat sink bottom provided for this purpose. The fixing of the Plexiglas panes and the LED linear modules takes place using frontal cover plates, which can also be attached directly to the cooling profile. The cover plates screwed to the front side must also be adapted to customer specifications, so that, for example, the entire module would have to be used as a table or ceiling lamp via an unwinding system. The heat dissipation of the absorbed heat to the environment is ensured in the different versions by an external, depending on the performance class adapted cooling fin structure. Various variants in different length and surface finishes, which are included in the standard range, complete the product portfolio. The heat dissipation of the absorbed heat to the environment is ensured in the different versions by an external, depending on the performance class adapted cooling fin structure. Various variants in different length and surface finishes, which are included in the standard range, complete the product portfolio. The heat dissipation of the absorbed heat to the environment is ensured in the different versions by an external, depending on the performance class adapted cooling fin structure. Various variants in different length and surface finishes, which are included in the standard range, complete the product portfolio.
Once the thermal management for LED cooling has been expertly selected and adapted, the connection of the LED to the heat sink must also be optimized. This plays a decisive role since in a poor heat transfer from the LED to the heat sink, the heat conduction or the heat transfer is reduced and consequently the LED temperature increases. As is known, an increase in temperature, in turn, has a direct influence on the function and light output of the LED. However, adapted thermal contact resistances require careful consideration of the tolerances created by the manufacturing processes in advance.
Unevenness and roughness as well as air inclusions must be leveled between contact point, LED and heat sink, as these significantly impair the heat transfer. For an excellent thermal functionality, an excellent thermal heat transfer between LED and heat sink is a significant and not negligible factor.
Low heat transfer resistance
So-called thermal conduction materials, also called thermal interface material, when used correctly, offer perfect solutions, to keep the thermal heat transfer resistances as low as possible. If the LED is fastened to the flat mounting surface using screws or with the aid of the associated holder system, unusually thin heat-conducting materials are recommended. Aluminum, Kapton, graphite or phase change heat transfer foils provide excellent solutions. Furthermore, it is also possible to bond the COB directly to the heat sink using double-sided adhesive heat-conducting foils based on polyimide or two-component epoxy resin adhesives.
Regardless of the choice of the TIM (Thermal Interface Material), however, it must be ensured that these materials are suitable for LEDs or ideal for the application and do not evaporate volatile organic compounds (VOCs). In the worst case, the outgassed components may precipitate on the lens surface of the LED and cause significant defects. To ensure a long-term and flawless functioning of the combination of LED, heat sink and thermal interface material, the available thermal interface materials should be considered and selected by their respective specifics.
In addition to good thermal conductivity and a material thickness adapted to the application, further decision criteria are the right choice of thermal conductivity, such as electrical insulation or conductivity, temperature range, dielectric strength (dielectric strength), chemical composition (environmental compatibility) and flexibility (Shore hardness). To consider or to consider. Likewise, the materials used must have easy handling, good aging resistance, and long service life.