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Thermal Engineering of Lights

Different light sources produce different amounts of heat. With candles and other historical lighting sources, flame was used as a source of light, and this presented a number of safety issues in the home and workplace. When incandescent bulbs were invented by Edison, he discovered the glow from a hot tungsten wire could be used in bulbs for electrically powered lights. While Edison’s light source was far safer than candles or gaslights, his bulbs still were based on a heat generated source of light, namely a glowing hot filament of wire.

The problem with glowing filaments in incandescent bulbs is that they are not very energy efficient. 90% of the electricity creates heat and non-visible radiation (infrared light), and only 8% of the energy produces visible light. LEDs create light electronically without the use of a glowing filament. For this reason, LEDs are far more efficient. LEDs convert 15 – 25% of their power to light with plans for increasing this percentage as the technology matures. However the remaining 85 – 75% of the power is converted to heat. This can pose a problem for the LED itself.

LEDs and Heat: Too much heat can kill an LED

Within an LED is a semiconductor chip which emits light when electrical current is passed through it. The point in the chip where electricity converts from positive to negative charge is known as the junction and the temperature at that location is known as junction temperature. (Symbol Tj) In the lab, LEDs are tested and rated at a Tj of 25°C (ambient). However, in the real world the Tj can typically be 60°C or higher even when engineered properly.

The higher the Tj, the less light is emitted from the LED. Therefore it is typical for an LED to emit about 10% less light than is stated on the manufacturer’s ratings when used in a real world application. This is expected from lighting engineers.

The problem arises when the LED light fixture is not properly engineered, causing the Tj to increase well beyond its optimum level, 100°C or higher. A high Tj can decrease the light output of the LED by 25% and reduce its life dramatically. A LED expected to last 50,000 hours can actually burn up in a day or two if not properly engineered.

Even in major retail outlets, one can see cut-rate lighting products that skip on the thermal design and engineering entirely, and have no cooling system whatsoever, yet still claim in their literature to have 100,000 hour LED life. The U.S. Department of Energy (DOE) warns consumers to be vigilant to the numerous false claims about lighting products in the marketplace today.

What causes a high Junction Temperature?

Three things greatly affect the Junction Temperature:

  1. Ambient Temperature: This is the temperature surrounding the light fixture. A light in a freezer would be operated below freezing. A streetlight in Miami could run at very high heats.
  2. The amount of current being driven through the LED. More current equals more light and more heat. To produce more light out of an LED and thus use less LEDs (saving cost) the current to the LED can be increased. If this current is too high, the LEDs are said to be "over-driven" and excess heat will result, shortening the life of the LED.
  3. The thermal path of the heat. Heat will naturally dissipate if allowed to do so and the LED fixture can be engineered to provide a path for the heat to dissipate. Various methods are utilized including fans, metal heat sinks and vents. The science of what to use and how to engineer the fixture is called Thermal Engineering.

What is Thermal Engineering?

Thermal Engineering is the science of heat. Continental Thermal Engineers study how much heat is created by a device, and design cooling systems to reduce the heat created by the LED and thus maintain proper junction temperature.

The Thermal Engineer will first import the CAD design into a CAE (Computer Aided Engineering) System. He will build a virtual model of the entire fixture and test the thermal path of the system. Utilizing the CAE tools he can spot hot spots and problems with the fixture. He will work with the system until he has a thermal design that will effectively manage the heat from the LED and keep the junction temperature at an acceptable range.

Later this design will be prototyped and the junction temperature will be tested.

Heat Sinks:

Thermal Engineers typically utilize heat sinks to provide a good thermal path. They are called heat sinks, since they operate much like a sink in a kitchen that lets the dish water drain away. This is a passive device, in that it contains no moving parts and cannot break or wear out. The fins in the metal aid the dissipation of the heat.

Types of Heat Sinks:

Sometime the heat sink can be incorporated into the circuit board, especially for low power LED applications. There are many heat sink designs for LEDs that are attached to the circuit board. These can be "off-the-shelf" or custom designed. The shape and numerous fins are designed to effectively dissipate the heat. Sometimes the fixture itself becomes the heat sink. For high powered lighting products, a fan may be added, especially if the fixture is compact and located in a space with little air flow. However, since fans can break and will not last as long as the LEDs, lighting engineers prefer a passive solution if at all possible.

 

Summary:

Proper Thermal Engineering is a critical part of the LED lighting product design process, since only with effective heat management can the long life and significant cost savings benefits of LED lighting be fully realized. Continental has Thermal Engineers who have the capability of using advanced engineering software to model the heat generated by different LED designs, and to model the effectiveness of different heat sink designs to verify and validate the efficiency of a temperature management strategy for the entire lighting system.

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