Thermal management for LED bulbs: Technology is evolving

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Position for thermal interface materials (Courtesy: https://consumer.dow.com)

The quality and the reliability of the light of an LED are both temperature-dependent. This makes thermal management critical to maximise an LED’s output. Since LEDs are semiconductors, they emit a substantial amount of heat in a very small area. That’s why the most appropriate thermal management solutions should be considered to dissipate the highly concentrated waste heat.

By Deepshikha Shukla

Heat adversely affects an LED by reducing both its efficiency and its life expectancy. By reducing the junction temperature by just a few degrees, the lifetime of an LED can be increased by thousands of hours. Moreover, reducing the temperature of the LED’s surrounding environment will also lower the junction temperature and hence extend the useful life of the LED.

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LED lighting is a greener lighting solution, with advantages that include lower power consumption, longer lifetime, compactness and no use of mercury. As the LED industry shifts to higher brightness LEDs and higher density LED arrays, more effective thermal management solutions are required to ensure the operating temperature of the LED junction is low. The shift in the LED light’s wavelength with a rise in temperature is an important concern for spectrum sensitive products.

High-intensity applications such as sports venues, street lamps, industrial and office lighting will lead to increasingly compact LED designs that deliver more lumens, draw more power and, consequently, have to manage heat more efficiently than ever before.

The role of thermal management in improving LED efficiency
Thermal management materials are designed to dissipate heat away from critical areas. They can be used beneath LEDs as an interface between the PCB and the outer casing, conducting the heat away, both above and below the PCB surface. The method chosen will depend upon the design of the assembly. Thermal management materials can also be used to encapsulate associated electronic circuits such as LED drivers, improving the efficiency of the LED unit as a whole. This can help distribute the heat in LED arrays to reduce the temperature at the LED junctions.

Products range from thermally conductive encapsulation resins, offering both heat dissipation and environmental protection, to thermal interface materials used to improve the efficiency of heat conduction at the LED junction. Such compounds are designed to fill the gap between the device and the heat-sink, and thus reduce the thermal resistance at the boundary between the two. Conductive materials such as copper, as well as more effective thermal adhesives and grease, eliminate air pockets that might otherwise impair heat transfer between a heat-sink and a semiconductor material.

Heat pipes transfer heat from the LED to a remote sink
(Courtesy: https://www.1-act.com)

The design of an LED array will also dictate which thermal interface material is chosen. For example, if it is a large array, across which temperature fluctuations are likely, it may be beneficial to use a material that is not subject to movement during thermal cycling. A surface curing thermal paste or one of the latest phase change materials would be ideal choices for these larger LED assemblies.

An encapsulated PCB is also protected from adverse environmental influences such as mechanical shock or high humidity. A thermally conductive ‘room temperature vulcanising’ material (RTV) would be the appropriate choice for both adhesive strength and thermal management. Encapsulation resins provide high levels of protection against adverse environmental factors as well as total heat dissipation. They also provide structural stability and flame retardancy. These heat-sinks are better able to dissipate excess heat when they include fins, which increase the surface area.

Heat-sink technologies
Attaching a heat-sink directly to an LED light casing is ineffective because the casing is not the primary cause of excess heat build-up. Attaching the heat-sink directly to a circuit board also removes some, but not all of the excess heat. Attaching the heat-sink directly to the LED is the best option.

LED lighting includes a circuit board and the LED, with the latter being encased in a lamp. The plastic for LED housings must have the capability to dissipate the heat generated by the LED. It must also provide high mechanical integrity as well as comply with the anti-flammability requirements stipulated for IEC and UL safety standards.

Heat pipe embedded metal core PCBs efficiently spread heat from LED devices, providing a reduction in thermal resistance as compared to pure aluminium PCBs. Integrating heat pipes, high conductivity plates, or vapour chambers with the heat-sink can reduce the LED temperature, and allow heat-sinks to handle higher heat fluxes. The number, shape, size and location of heat pipes would be specific to the design.

The remote sink solution has two heat pipes, one in direct contact with the LED device at one end, which serves as the evaporator. At the other end, the heat pipe is connected to the heat-sink, the condenser. These heat pipes are in direct contact with both the LED at the bottom and heat dissipating fins at the top. A wall or other enclosure can be placed in between the LED and the heat-sink to separate the two.

Heat pipes have an effective thermal conductivity range of 10,000 to 100,000W/m K, compared to aluminium’s thermal conductivity of about 180W/m K. So, the embedded heat pipes can be used to improve the performance of aluminium heat-sinks. The heat pipes can spread the heat throughout the heat-sink due to their highly effective conductivity. This reduces the thermal gradient and subsequently reduces the maximum temperature at the LED source.

High conductivity heat-sinks can also improve the size, weight and power of the LED fixture compared to standard heat-sinks. Embedding heat pipes in a high conductivity sink can increase the thermal conductivity, providing an opportunity to reduce the heat-sink plate thickness and fin area. This can be implemented in a variety of LED applications including large arrays and outdoor lighting.

The longer the heat-sink, the higher the heat dissipation (Courtesy: https://www.1-act.com)

Testing and analysis have confirmed that the performance can be further improved with longer heat pipes. The improvement in thermal resistance with heat pipes increases approximately linearly with an increase in the length of the heat-sink. The benefit will be more noticeable in natural convection heat-sinks as the fan operation plays a major role in forced convection performance.

Technologies developed for thermal management in LEDs
Flexible graphite is being used extensively to remove heat from LED circuit boards, drivers, and other ‘smart lighting’ components, and will play a leading role in the thermal management of the next generation of LED luminaires.

Flexible graphite’s key properties include high in-plane thermal conductivity, low thermal contact resistance, being lightweight, flexible, compressibility, cost-effectiveness, infinite duty life, and the sustainable cradle with a unique structure and properties. Flexible graphite can be used as a heat spreader, heat shield or thermal interface material.

Fischer Elektronik’s new FSF 15P and FSF 20P phase change materials are available as sheets or rolls, and can be customised to flow at a phase change temperature between 52°C and 48°C, respectively.

The MechaTronix CoolStar series of designer LED coolers covers the whole range of cooling capacities from 1000 lumens all the way up to 6000 lumens, which positions them in the spotlight, downlight and luminaires space.

The longer the heat-sink, the higher the heat dissipation (Courtesy: https://www.1-act.com)

Electrical isolation can be achieved using materials that are thermally insulating, such as with FR4 boards. Vapour chambers are used to spread the high input heat flux over the entire surface of the vapour chamber.

GlacialTech has released its new 250W heat-sink for outdoor floodlights and for single COB or multi-chip LEDs. The Igloo SS250 features an efficient heat-sink with heat pipe designs for single COB and multi-chip LEDs to speed up heat dissipation.

The future
Recent trends in the LED industry include high power LED packages, novel architectural form factors, as well as smart and connected lighting. All these trends have one thing in common—more electrical power being packaged within increasingly slim and lightweight form factors. The resulting increase in power density and the inclusion of temperature-sensitive electronics drive the need for innovative thermal management solutions to ensure maximum performance and long life.

Systems have been designed to effectively remove heat and improve the longevity of LED system components, as well as to reduce maintenance and warranty costs. Materials and processes are evolving to meet the thermal management challenges of the next generation of LED luminaires.

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