Heat Management of LED`s

LED lighting is an indispensable part of our everyday life. It has largely replaced fluorescent lamps and energy saving lamps in many areas.

Especially for lighting systems in large event rooms (theaters, concert halls), street lighting or industrial buildings, only LEDs are used today. Not only the cost efficiency is a reason for the use of LED lighting, but also the service cost, which is significantly lower by the achievable lighting time of 50,000 hours and more (about 11 years at 12-hour operation).

For LEDs to reach a projected lifetime of 50,000 hours, the maximum temperature at the semiconductor (location of light generation) must be kept below a critical temperature. For in modern high-power LED currents flow up to 3 A, whereas only about 30% of the electrical power is converted into light. The resulting heat must be dissipated as efficiently as possible to the environment. For this purpose, a sufficiently large cooling is necessary.

A current rule of thumb is that the life of an electronic component is approximately doubled when the operating temperature is lowered by 10 ° C. In addition to the lifetime the long-term lighting intensity is also improved, which decreases too much at too high temperatures on the semiconductor chip.

In short, the cooler the barrier layer of an LED, the better and more durable the LED light.

Heat transfer

There are three mechanisms that transport heat from a heat source to a heat sink:

• Heat conduction
By direct physical contact without transport of moving media, e. g. power component to heat sink
• Convection
Combination of heat conduction and heat transfer through a moving medium. The heat from the hot region is e. g. transported by air or liquids to cooler areas (e. g., ambient air, heat pipe)
• Heat radiation
Heat transport by electromagnetic (infrared) radiation. There is no medium for heat transport needed. Black and matt surfaces emit heat better than shiny, silver ones.


(Housing) Internal heat management

As "internal thermal management" one can designate the heat transfer from the power component to the outer, surrounding housing. While doing so, the best possible heat-conducting materials should be used. Copper or aluminum have a very low specific thermal resistance (copper: 0.0025 m * K / W). Important in this heat path is that no air is trapped. Air is with 38.5 m * K / W a much worse heat conductor. Even electrical insulation films that are thermally conductive achieve significantly better values with 1.25 m * K / W. Due to the low thickness (Kapton® MT + with, for example, 0.025 mm thickness) they are only a low resistance within the heat path, but allow a very voltage-resistant galvanic isolation.

To thermally connect power LEDs to heatsinks, e. g. thermally conductive, double-sided adhesive pads (e. g., Nitto TR-5320F) are used. The self-adhesive, heat conductive punched parts are easy to apply, are an excellent installation aid and displace air by wetting the surface.

Typical thermal conductivities of commonly used materials

 Material

 Spec. thermal conductivity

 λ [W / mK]

 Copper

 ~ 400

 Aluminum

 234

 Silicon

 148

 Tin

 67

 Air

 0.0261


External heat management

The "external thermal management" refers to the release of heat to the environment. This is usually done via heat sink. Convection and heat radiation play a major role here. The convection mainly depends on the speed at which the ambient air flows past the heat sink. In most cases, the area around it is increased by cooling ribs or cooling fins.

The heat radiation contributes to the heat transport only at larger temperature differences. Black surfaces are recommended for this type of cooling, as they allow better heat radiation (dark emitter).

Emission coefficients ε of surfaces

Material

Emission coefficient ε

(Factor is between 0 ... .1)

Aluminum, polished

0.038

Aluminum, anodised, matt

0.8

Copper, polished

0.04

Ceramic, gray

0.9

Matt black lacquer surface

0.97

Roughly and only as a guideline you can set a required area of 25 cm² per 1 watt power loss. Influence parameters are e. g. the flow rate and the temperature difference of the air to the heat sink.

As a general rule, it can be stated that heat conduction is the most efficient instrument of heat dissipation within a housing. In the exterior of the housing, however, it is mainly the convection of heat sink or heat exchanger in liquid-based systems. Air pockets in the heat path are generally to be avoided!

But constructive measures around the heat path are also crucial for its efficiency. Hermetically sealed housings allow heat to accumulate more like those with ventilation holes. Small, horizontal surfaces are less effective as convection cooling than vertical, larger surfaces where the air can flow better due to heating (chimney effect). Additional external heating by e. g. solar radiation can be reduced by reflecting surfaces.

Analogy electrical and thermal currents

To calculate the heat flow, an analogy with the electric current is permitted. The same principles of the series circuit and parallel circuit can be applied as in the electrical circuit.

Thermal size

Electric size

Absolute thermal resistance Rth (K / W)

Electrical resistance R (Ω)

Heat flow Ф (W)

Electric current I (A)

Thermal conductivity λ (W / mK)

Electrical conductivity σ (s / m)

Temperature difference Δt (K)

Electrical voltage U (V)

 

Thermal management products by CMC

In line with the different requirements of the individual applications, CMC Klebetechnik offers a wide range of thermally conductive insulating adhesive tapes, silicone films and gap fillers.

More about heat conduction and heat management can be found HERE.