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LED lighting, because of its energy-saving features, has become popular in the lighting industry. It features high efficiency, a long lifespan, and is cost-effective. LEDs consume far less energy than any other lighting solution on the market, making them an economically and environmentally sound choice.

The problem of heat dissipation of high-power LED luminaires such as large flood lights or large high bay lights limits the development of the high-power LED lighting industry. Even though LEDs emit much less energy as heat compared to all other types of lighting, it can still be an issue if not properly addressed in the design and manufacturing phases of LED fixtures. If heat is not properly dissipated within an LED fixture, it can lead to a decrease in its light efficacy and lifespan.

Unlike Edison light bulbs, where heat waste is radiated, the heat generated by LEDs is conducted. This heat must then be moved away from the device to preserve its performance. Fortunately, there have been many advancements in LED lighting technology that address the heat issue.

Understanding the key components of an LED light fixture

There are four main components of a complete LED light package:

Die – This is a semiconductor that emits blue light when an electric current runs through it.
Phosphor – This is applied to give a white or yellowish light. It is applied directly onto the die or mixed in the lens.
Substrate – Multiple dies are mounted on the substrate. It is made of aluminum or ceramic material.
Lens – The light emitted is extracted and directed by the lens.

An LED light package is typically mounted on a printed circuit board (PCB). A heat sink is then attached to the PCB. A heat sink is a heat exchanger that moves the heat produced by an electronic device to a fluid medium. This fluid medium is typically air or a liquid coolant. The heat is dissipated away from the device, thus allowing for the regulation of the device’s temperature. The heat sink shifts the heat of the chip to the heat sink through exact contact with the chip surface. It dissipates heat away from the LED package in three ways:

Convection – Heat is transferred from a solid to a moving fluid. For most LED applications, this will be air. Convection shifts heat utilizing fluid flow.

Radiation – Heat is transferred from two bodies of different surface temperatures through thermal radiation. Radiation doesn’t require any kind of medium. The heating object distributes heat directly to the adjacent space.

Conduction – Heat is transferred from one solid to another. The heat between objects in direct contact is transferred from one with a higher temperature to one with a lower temperature.

Junction temperature is the temperature at the junction between the LED die and the substrate it is mounted on. This junction usually has the highest temperature in the LED package which makes it an appropriate value to signify heat dissipation performance. Normally, junction temperatures can go up to 100°C or more.

Thermal designs used to reduce heat in LED packages

An LED fixture needs to be designed to keep LEDs cool by decreasing heat resistance from the LED. This is done by enhancing the three modes of heat dissipation in the fixture design. Thermal designs used to reduce heat include:

Positioning and Arrangement of LEDs

Designers often want to reduce the spacing of LEDs on a PCB to create condensed LED fixture designs. The challenge is that reduced spacing means an increase in thermal power density and, therefore, a rise in the temperature of the LEDs.

LED manufacturers often give suggested spacing requirements for LEDs and specify the rise in temperature that you can expect to see when spacing is reduced by a certain amount. A consistent and balanced chip arrangement delivers a similar heat load whether it is rectangular, hexagonal, or circular.

Types of LED modules

Direct In-Line Package (DIP) LEDs have a bullet-shaped design. They are used mainly for signage and display on household electronics.

Surface Mounted Chips LEDs are square-shaped diodes that are able to produce light in a full Red Green Blue spectrum. They are mounted on the PCB surface. Rather than have wire connections, they are soldered directly onto the PCB.

Chips on Board (COB) LEDs contain nine or more chips that share the same substrate. Because of this shared substrate, COB LEDs are usually affixed directly to a heat sink by utilizing a screw-type connector. They are more energy efficient and compact than Surface Mounted Chips LEDs. COB LEDs are typically mounted to a heat sink using a connector that attaches to the heat sink with screws.

  • A 10x10mm square array of COB LEDs can hold 38 times more LEDs than DIP LEDs.
  • MCOB LEDs merge several COB LEDs onto one aluminum plate. They can generate more than 130 lumen/Watt.
  • COB LED flip-chips are configured so that the die is constructed face down on a sub-mount. The sub-mount is usually made of ceramic or silicon. The sub-mount performs as a supporting substrate and a heat spreader. COB LED flip-chips have a 70% better heat transmission rate and 30% higher lumen output when compared to SMD chips.

Core Material of the Printed Circuit Board (PCB)

A PCB’s core material is used to redirect the heat away from the LED components. PCBs are one of the most sensitive elements of thermal design because of their prime location in the heat pat. There are three main types of PCB core materials:

FR-4 is a glass-reinforced exposed laminate. It is the most common type of the three materials.
Ceramic is mostly used for PCBs that will encounter harsh environments. It is not as cost-effective as FR-4 type PCBs, but they are the better choice in terms of the conductivity of the material used.
Metal Core Printed Circuit Boards (MCPCBs) have copper, aluminum, or steel alloy-based core material. Aluminum and copper deliver exceptional thermal conductivity and are many times more thermally conductive than FR4. MCPCBs are the least common PCB core material.

Contact Surfaces and Thermal Interface

Heat is transferred from one component to another through surface contact. This means that these points of contact must be reflected in the overall thermal design process.

Decreasing the points of physical contact will reduce the heat conduction across the interface. There will be gaps filled with air, which has low thermal conductivity. These gaps create thermal contact resistance. Because of this, it is important to produce contact surfaces as two-dimensional, smooth, and clean as possible.
There are two stages that are implemented to lessen thermal contact resistance:

Remove air by introducing a fluidic material into the gaps created by uneven surfaces. These materials can include adhesives or thermal grease.
If a smaller heat resistance is required, you can use a thermal interface material (TIM) which contains fillers that enhance the conduction process.

Heat Sink Design

Heat sinks are designed to conduct heat away from the LED and the PCB. They convect and radiate heat to the surrounding environment. A heat sink must transfer the heat away from the PCB. This prevents thermal buildup within the LED packages.

The heat sink material used to maximize the heat transfer between the PCB and the exposed convective surface must be highly conductive. Aluminum alloy is an example of this type of material.

Active Cooling

The previously listed thermal designs are known as passive cooling methods. This means that no artificially produced forces that enhance air circulation are used. They rely on natural convection for heat transfer.

For high-powered LED fixtures, heat transfer must be increased by using liquids or fans.

Liquid cooling systems are made up of a driving pump, a cold plate, and a fan-cooled radiator.

The heat that is produced by a high-power LED will transfer to liquids through a cold plate. From there, pump-driven liquids circulate in the system which absorbs the heat. Finally, a fan-cooled radiator chills the warmed fluids for the next circulation. The temperature of the LED is managed by the circulation of liquids.

Conclusion

The optimal performance of LED lighting systems depends on their operating temperature. Therefore, a thermal management system is essential. The heat dissipation performance of LED fixtures depends on several factors. These include materials, interfaces, geometries, and environmental conditions. Considering these factors is necessary to maintain LED temperature under working conditions.

Keeping the LEDs cool will extend their lifespan and help them to provide peak performance.

Interested in how LED lights are made? Read our blog post to learn more.

About the Author

Neil Peterson is Chief Operating Officer at LED Lighting Supply. He has been active in the LED industry for over 10 years and is responsible for product planning and management as well as revenue and operations at LED Lighting Supply. Much of Neil’s time is focused on customer engagement for large commercial and industrial lighting requirements. When not working, he enjoys family time, camping, fishing, and sports..

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