In most cases, spot lights and flash lights with long-life LED modules in various output cate­gories score highly. Their high lumi­nance enables a targeted light control with overall low power consump­tion. As with all semi­con­duc­tors, however, the waste heat has to be removed effi­ciently, other­wise, it leads to hazardous high temper­a­tures despite high effi­ciency of the tiny LED chip area. Thanks to special LED cooling modules replacing passive heat sinks, modern cooling solu­tions with active air move­ment allow for targeted heat dissi­pa­tion, along­side with size reduc­tion and mate­rial gains. Also, completely new design possi­bil­i­ties can be real­ized, mini­mizing main­te­nance of even very complex lighting systems, such as in museums, theaters, and places of worship, storage facil­i­ties, street lighting or stadiums (Fig. 1). Active cooling opens new hori­zons in effi­cient LED lighting.

ebm-papst has devel­oped a line of active compact cooling systems specif­i­cally for the market-oriented designs of the new high-perfor­mance LEDs.

Also, completely new design possi­bil­i­ties can be real­ized, mini­mizing main­te­nance of even very complex lighting systems, such as in museums, theaters, and places of worship, storage facil­i­ties, street lighting or stadiums (Fig. 1). Active cooling opens new hori­zons in effi­cient LED lighting.

Selecting the right light source

The correct amount of light deter­mines how we perceive the world. This is why there are lighting spec­i­fiers, who can make us see things “in the right light”. However, it is often diffi­cult even for experts to select the right light source. Ideally, a light source should have a universal appli­ca­tion, requiring little space or power. CoB lights (Chip on Board) meet a whole range of industry require­ments. However, its semi­con­ductor chip must have targeted cooling in order to main­tain its life­time and color fidelity. To meet the CoB require­ments, ebm-papst has devel­oped a line of active compact cooling systems specif­i­cally for the market-oriented designs of the new high-perfor­mance LEDs. It saves space and allows completely new lighting possi­bil­i­ties.

LED bulbs – compact and effi­cient?

Figure 2: The life expectancy of the LED is essen­tially depen­dent on the temper­a­ture, which is why targeted heat removal is espe­cially impor­tant.

When you look at the CoB in detail, you will quickly notice several prob­lems (Fig. 2). As a semi­con­ductor, the LED chip can only operate up to a prede­ter­mined junc­tion temper­a­ture. Should the temper­a­ture rise, the LED quickly develops chal­lenges. These include a reduc­tion in CRI (color rendering index), effi­cacy and most impor­tantly, a reduc­tion in life­time.

But even at lower temper­a­tures, the mate­rial ages rapidly, lumi­nance and effi­ciency decrease, the color range reduces- in short, its useful life is down. Despite their high effi­ciency, the waste heat of LED surfaces and the high power density of the LED light sources can be formi­dable. This amount of waste heat must be dissi­pated in a targeted manner, either by means of conven­tional (often over­sized) passive cooling or via targeted active heat dissi­pa­tion (see text on the box).

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

In prin­ciple, the following must be consid­ered: Energy (heat) always flows from hot to cold. For cooling solu­tions, the total heat resis­tance, i.e. the sum of indi­vidual paths of thermal resis­tance, must be taken into account. Here, a signif­i­cant differ­ence between passive and active cooling concept already emerges: The “cooling pathway” LED Chip – substrate – heat sink – air is always the same, but the mate­rial part of the same cooling perfor­mance varies greatly. The more mate­rial is used, the larger the heat sink-is required.

Smaller LEDs with the same output and passive cooling are not yet capable of producing smaller fixture designs; because they require large heat sinks as the thermal dissipation/heat transfer to the air becomes a limiting factor for the heat transfer. Passively cooled LEDs there­fore require a high use of mate­rial and are usually neither compact nor envi­ron­men­tally friendly. At this point, active cooling concepts offer several distin­guishing advan­tages (Fig. 3).

Future-proof active cooling

Figure 4: Heat sinks and fans can be combined into a compact module for common LED cooling solu­tions, which facil­i­tates assembly.

Since the heat dissi­pa­tion from the heat sink to the air is the main resis­tance in the energy discharge, the largest cooling reserves can also be released. A key feature of active cooling is the targeted air supply to the heat sink. Forced convec­tion, or more specif­i­cally turbu­lent flow is gener­ated towards the heat sink, which consid­er­ably improves heat transfer from the thermal mass of the heat sink itself into the neigh­boring reser­voir of air which surrounds the light fixture.

Normally, the system works as follows: A small heavy-duty LED surface is attached to the heat sink with a thermal inter­face mate­rial. This provides a much lower thermal resis­tance enabling a greater transfer of heat from the LED into the heat sink, with between four- and six-fold decrease, the fan creating cold fresh air flow. The elec­tronic cooling experts from ebm-papst St. Georgen have now combined heat sinks and fans into a compact module for common LED cooling solu­tion designs, which makes instal­la­tion easier (Fig. 4). Its smaller design also saves not only in mate­rial but also in weight, and the targeted airflow also ensures that heat transfer impairing deposits such as dust do not adhere at all.

Noise-free, reli­able, long-lasting

The ever-changing require­ments of modern lighting LED tech­nology demand opti­mizing new concepts in simu­la­tion programs with mate­rial-specific, aero­dy­namic and drive-specific details, where effi­cient, reli­able cooling modules can be built in the smallest avail­able space. A six-fold decrease in dimen­sions compared to passive cooling speaks for itself. Other impor­tant require­ments for the use of active cooling are low oper­ating noise and a longer service life.

Most people can perceive noise starting from about 12 dB (A), the fans above reach values between 7 and 19 dB (A), whereas compa­rable fans avail­able on the market start from 18 dB (A) upwards. For compar­ison, the noise level in an office is about 35 dB (A), so the modules are inaudible even in museums or theaters. Power consump­tion of the fan is between 0.18 and 1.1 W at 12 VDC. This allows the modules to dissi­pate waste heat reli­ably at between 38 and 200 Watts.

Impor­tant require­ments for the use of active cooling are low oper­ating noise and a longer service life.

Depending on the output cate­gory, round and square axial compact modules have diam­e­ters and side lengths which include 40, 50, 60, 80 or 92 or 119 mm with an overall height of 10 to 25 mm. In the radial version with air deflec­tion at 90 °, the dimen­sions are 51, 76 or 97 mm with the height of 15 to 33 mm. Thus, compared to passive cooling solu­tions compa­rable to the cooling capacity, 50 to 100 % higher lumi­nance is possible with the same size. Another posi­tive benefit of the targeted active cooling is the low-temper­a­ture color fidelity of the LED arrays. Espe­cially in museums, a high CRI is essen­tial to see the illu­mi­nated objects in the right light.

As the cooling modules were devel­oped for world­wide main­te­nance-free use, their service life is compa­rable to the CoB light sources. At 40 °C, the value is 87,500 to 97,500 h, i.e. around 10 years; at an ambient temper­a­ture of 20 °C, the service life is doubled and can often far exceeds that of the LED itself. The green tech­nology from ebm-papst also takes into account an envi­ron­men­tally compat­ible overall service life concept for devel­op­ment, produc­tion, oper­a­tion and disposal.

As a result of their reduced size, modern compact modules for active LED cooling enable completely new lighting concepts, dras­ti­cally shorten the time-to-market for the chip-specific designs and improve the envi­ron­mental balance of the lighting concepts due to its low main­te­nance.

Active cooling prin­ci­ples

The heat discharge coef­fi­cient, which is impor­tant for heat dissi­pa­tion, describes the ability of the air to dissi­pate energy from the surface of a cooler. Among other things, it depends on the air density and the thermal conduc­tivity coef­fi­cients of the heat dissi­pating mate­rial as well as the air. The thermal conduc­tivity coef­fi­cient is usually calcu­lated using the temper­a­ture differ­ence of the parts 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 condi­tions and the surface texture. Active cooling employs more effi­cient heat dissi­pa­tion.

In the case of laminar flow, the air moves in approx­i­mately parallel layers. The heat is trans­ferred between the layers only by very slow heat conduc­tion. Conversely, in case of turbu­lent flow, inten­sive swirling and shifting occurs. This results in an almost perfect mixing of air flows. Heat transfer in turbu­lent flow is there­fore a lot more effi­cient as in the case of laminar flow, which is used in passive cooling (Fig. 5). To use an example from everyday life, a small hair dryer uses 1.0-1.5 kWatts per a blast of turbu­lent air. On the other hand, an elec­tric convec­tion heater with 1.5 kWatts builds up a lot more with a largely laminar inflow with the same output.

Figure 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 thereby limits the maximum temper­a­ture at the LED to approx. 60 °C.