Are there heat sinks for LED lights

Active cooling for compact high-performance LEDs

In most cases, lamps and luminaires with long-life LED modules in different performance classes therefore score points. Their high luminance allows targeted light guidance with an overall low power consumption. As with any semiconductor, however, the waste heat must be efficiently dissipated, because despite the high level of efficiency on the small LED chip area, this quickly leads to harmful high temperatures. Modern cooling solutions with active ventilation through special LED cooling modules as a replacement for passive heat sinks now allow targeted heat dissipation with smaller dimensions and less material use than before.

ebm-papst has developed a series of active compact cooling specifically for the customary designs of the new, powerful LEDs.

On the one hand, completely new design variants can be implemented and, on the other hand, the maintenance of complex lighting systems, e.g. B. in museums, theaters, places of worship, warehouses, street lamps or stadiums, can be reduced to a minimum (header image). Active cooling opens up new horizons for efficient LED lighting technology.

Choosing the right light source

The right light largely determines how we perceive the world. It is not for nothing that there is the profession of lighting, who "puts things in the right light". But when it comes to choosing the right light source, it is often difficult, even for experts. Ideally, a light source can be used universally, requiring little space and electricity. CoB lights (Chip on Board) meet a whole range of relevant requirements. However, your semiconductor chip must be specifically cooled in order to maintain its service life and color fidelity. For this reason, ebm-papst has developed a series of active compact cooling specifically for the customary designs of the new, high-performance LEDs that meet the requirements of the CoBs. It saves space and allows completely new lighting options.

LED light sources - compact and efficient?

Image 2: The service life of the LED is largely dependent on the temperature, which is why the targeted dissipation of heat is particularly important.

If you go into detail about CoB lamps, problems quickly arise (Fig. 2). As a semiconductor, an LED chip may only be operated up to a specified junction temperature. If the temperature rises above this, LED causes problems in a very short time. This includes, for example, the deterioration in the CRI (color rendering index), the efficiency and, above all, a shortening of the service life.

But even at lower temperatures, the material ages quickly, the luminance and efficiency decrease, the color spectrum shifts - in short, the useful life is reduced. Despite the high degree of efficiency, the waste heat from the LED surfaces and the high power density of the LED light source are considerable. This amount of heat must be dissipated in a targeted manner, either through conventional (often oversized) passive cooling or via targeted active heat dissipation (see box).

Image 3: Active cooling solutions also impress with their compact design.

Basically, the following must be taken into account: Energy (heat) always flows from hot to cold. With cooling solutions, the total thermal resistance counts, i.e. the sum of the individual heat transfers. This shows a significant difference between passive and active cooling concept: The "cooling path" LED chip - carrier - heat sink - air is always the same, but the use of materials for the same heat dissipation varies greatly. Because the more material is required, the larger the required heat sink.

Smaller LEDs with the same power and passive cooling do not result in smaller luminaires because they require large heat sinks, since the heat transfer to the air is the limiting factor for heat transfer. Passively cooled LEDs therefore require a large amount of material and are therefore usually neither compact nor environmentally friendly. Active cooling concepts can offer several advantages here (Fig. 3).

Future-proof active cooling

Image 4: Heat sink and fan can be combined to form a compact module for common LED cooling solutions, which makes assembly easier.

Since the heat transfer from the heat sink to air is the main resistance in the dissipation of energy, the greatest cooling reserves can also be released here. The essential feature of active cooling is the targeted air supply to the heat sink. For this purpose, a forced convection or, more precisely, a turbulent flow is generated on the heat sink, which considerably improves the heat transfer from the thermal mass of the heat sink itself to the air reservoir that surrounds the luminous element.

A system usually works as follows: The small, highly stressed LED is attached to a heat sink with thermal paste. This is four to six times smaller because of the significantly lower thermal resistance, which enables greater heat transfer from the LED to the luminous element, and carries the fan that provides cold fresh air. The electronics cooling specialists from St. Georgen have now combined heat sinks and fans into a compact module for common LED cooling solutions, which makes assembly easier (Fig. 4). The smaller design saves not only material but also weight, and the targeted air flow also ensures that deposits such as dust, which can impair heat transfer, do not even adhere.

Reliable, inaudible, durable

If the requirements that modern lighting technology demands of LED lights are implemented and the concept found is optimized using simulation programs with material-specific, aerodynamic and drive-specific details, efficient, reliable cooling modules can be built even in the smallest of spaces. The six times smaller dimensions compared to passive cooling speak for themselves. Important prerequisites for using active cooling are low operating noise and a long service life.

Most people can only perceive noises from around 12 dB (A), the fans used reach values ​​between 7 and 19 dB (A), while comparable fans are only available on the market from 18 dB (A) and up. For comparison: the noise level in an office is around 35 dB (A), so the modules cannot be heard even in museums or theaters. The power consumption of the fan is between 0.18 and 1.1 W at 12 VDC. This means that the modules can safely dissipate between 38 and 200 W of waste heat.

Important prerequisites for using active cooling are low operating noise and a long service life.

Depending on the performance class, the round and square axial compact fans have a diameter or a side length of 40, 50, 60, 80 or 92 or 119 mm with an overall height between 10 and 25 mm. For radial fans with an air deflection of 90 °, the dimensions are 51, 76 or 97 mm with a height of 15 to 33 mm. Compared to passive cooling solutions with comparable cooling performance, 50 to 100% higher luminous intensities are possible with the same size. A very positive benefit of targeted active cooling is the color fidelity of the LED series associated with the low temperature. In museums in particular, a high color rendering index (CRI) is essential in order to see the illuminated objects in the right light.

Since the cooling modules were developed for worldwide maintenance-free use, their service life is matched to that of the CoB light sources. At 40 ° C the value is between 87,500 and 97,500 h, i.e. around 10 years; At an ambient temperature of 20 ° C, the service life is doubled and can therefore outperform the LED itself. The GreenTech technology from ebm-papst also takes into account an environmentally friendly and comprehensive service life concept for development, production, operation and disposal.

Thanks to their small dimensions, modern compact modules for active LED cooling allow completely new lighting concepts, the chip-specific modules / designs drastically shorten the time-to-market for customers and improve the environmental balance of the lighting concepts used due to the low maintenance requirements.

Active cooling basics

The heat transfer coefficient, which is important for heat dissipation, describes the ability of the air to dissipate energy from the surface of a heat sink. It depends, among other things, on the air density and the coefficient of thermal conductivity of the heat-dissipating material and the air. The coefficient for heat conduction is usually calculated using the temperature difference between the media involved. In contrast to thermal conductivity, the heat transfer coefficient is not a material constant, but is strongly dependent on the flow velocity or the type of flow (laminar or turbulent) as well as the geometric relationships and the surface properties. This uses the active cooling for more efficient heat dissipation.

With laminar flow, the air moves approximately in parallel layers. The heat is transported between the layers only through the very slow heat conduction. In the case of turbulent flow, on the other hand, there is intensive swirling and redeployment. The result is an almost perfect mixing of the air flow. The heat transport in such a turbulent flow is therefore considerably more efficient than in the laminar flow, as is the case with passive cooling (Fig. 5). As an everyday example, e.g. a small hair dryer can deliver around 1.0-1.5 kW through a turbulent blower flow. An electric convection heater, on the other hand, with 1.5 kW requires a considerably larger design due to the largely laminar flow with the same output.

Image 5: The picture shows how the LED heats the heat sink (red, 55 ° C) and the fan blows the cool ambient air (blue, 25 ° C) through the heat sink and the maximum temperature on the LED to approx. 60 ° C limited.