LED lighting systems get thermal management

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Guest columnist Tim Haas, product marketing manager for e-mech at Avnet Abacus says that improvements in LED lighting solutions are imposing secondary challenges in thermal management which are just as fundamental.Heat and light are often synonymous. Although LED lighting creates more light and less heat with the available energy, it still generates a proportion of heat – quite a lot in the case of the latest high brightness products.
The small and delicate semiconductor devices at the centre of an LED design have definite limits on their ability to tolerate heat. Good thermal management remains as important for efficient LED applications as it was with less efficient established sources. Thermal management of light sourcesWhile artificial light is normally generated to be projected, often the application is housed in an enclosure which can significantly impede heat dissipation without adequate ventilation. Sometimes this only requires vents in the enclosure and heat sink(s) on the power components within. Today there are pre-formed heat sinks available for most semiconductor packages from vendors such as Aavid Thermalloy which help channel the heat away from the transistors’ junction – the most heat-critical element of an integrated circuit.
These standard heat sinks are often formed, or ‘stamped’ from metals including copper and aluminium, in an array of shapes. Normally they are fitted directly to the package using pads or clips designed for the purpose, providing a good heat-conductive path from the semiconductor’s transistor junction, to the packaging, through the heat sink and out to the ambient environment. The effectiveness of this form of passive heat dissipation is dependent on the maximum allowable junction temperature, the design of the heat sink and the ambient temperature. Aavid Thermalloy heat sink designs may, for example, include elaborate extrusions or fins, which increase the surface area and produce more effective heat convection. Developments in this area include increasing the effectiveness of the material, by forming pipes that are then filled with a liquid which conducts the heat away from the device to the environment more efficiently than a solid material; Aavid Thermalloy HiContact Cold Plates use this approach.
  This form of quasi-passive cooling enables more effective cooling for a given mass, making smaller heat sink more effective. Other areas exploring the materials used include ceramic heat sinks, which can be formed in to almost any shape and give almost as good heat conduction as copper.  Ceramic heat sinks are also cost effective to produce, and can provide additional functionality in the form of component mounting, becoming both a socket and a heat sink for some components such as LEDs. In applications where the ambient temperature is too high for effective passive cooling, forced air is normally used. This involves the use of a fan fitted inside the enclosure, which forces the warmed air out through ventilation slots.More advanced fans now integrate temperature sensors, allowing them to remain off when the ambient temperature is below a predefined maximum and thereby further saving power. The most common types of fans in use today are axial fans and radial fans.
Axial fans are constructed such that the air flow is in a constant direction, passing through the fan in the same plane. Radial fans normally have an exhaust flow that is 90 degrees to the air intake. Fans where the exhaust is at an angle between these two extremes are called diagonal fans, and there are also fans which implement alternative mechanical formats, that feature rotating cylinders instead of rotating blades, which are termed ‘blowers’.Whichever fan is used it represents additional cost, particularly in extreme environments that must feature ingression protection against dust, humidity, salt, oil and other contaminants which could harm the fan or the sensitive components within the housing. Small is beautifulThe use of fans is commonplace within larger enclosures, but the trend within today’s electronics devices is size reduction; from the integrated devices that empower them, to the end-applications such as mobile phones. However, while the size of the components continues to reduce, the heat they generate doesn’t and it remains a growing issue.
As mentioned earlier, the materials used to create heat sinks is a constant area of development and could be the solution to the problem of dissipating heat in ever-smaller devices. Techniques such as piped heat sinks are being augmented by the introduction of another new method which employs a ‘heat sheet’ such as the Panasonic PGS Graphite Sheet. These are made from a graphite compound which is pliable and can be formed into various shapes, almost like a sheet of paper. It is not only relatively cheap to produce, and offers very attractive thermal characteristics.
It conducts heat in the X and Y planes far more effectively than standard materials such as copper and aluminium, while in the Z plane (thickness) it shows comparable effectiveness. These qualities mean the material can be formed around components and channel heat away from hotspots much more effectively than other forms of passive cooling and, in smaller applications like a mobile phone where active cooling is not an option, enable more reliable designs.

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