DESCRIPTION OF RELATED ART Electronic equipment normally generates heat when used. Said heat increases the temperature of said equipment as well as the surroundings. In general, the lifetime of electronic equipment decreases progressively as the temperature rises.

In addition, soldered joints in such equipment start to soften at a certain temperature. The current trends in high packing density mean smaller size but higher thermal density, additionally increasing heat output. It is therefore desirable to cool electronic equipment efficiently.

A common way of cooling electronic equipment is to use heat sinks and cooling fans. A heat sink absorbs heat generated by the electronic equipment, and in many applications transports the heat to a cooling fan where it is radiated to the surroundings. Natural convection of the heat sink is however a desired alternative for cost-effective design.

This generally assumes that the surroundings are cooler than the heat sink-otherwise heat will go the other way, i. e. the heat sink will absorb heat.

Indoors it is often easy to control the environment surrounding electronic equipment. Outdoors, however, it is usually more difficult, for instance because of direct solar

radiation. One way of solving this problem is to use sunscreens to protect the heat sinks, and in some cases also the cooling fans.

The use of a sunscreen may, however, be insufficient. In strong sunlight, a sunscreen may reach 90° C, while the heat sink it protects preferably holds at most 65° C. This means that heat will be transferred from the sunscreen to the heat sink and possibly, ultimately to the electronic equipment.

In general, the amount of heat transferred from a surface depends on several factors. This radiative heat transfer is governed by Stefan-Bolzmann's law, which states that P=A£6T4 where P is the emanated power, A is the surface area, s is the emittance (s=1 for black body and s=0 for an ideally shiny surface), 6 is a constant, and T is the temperature.

From this, a formula for heat transfer between two objects, for example a sunscreen and a heat sink, can be obtained: m4 rp4<BR> T sunscreen-Theatsink P = As 1/sunscreen'-L/heatsink"-*- A positive value of P indicates a net radiative heat transfer from the sunscreen to the heat sink, while a negative value indicates the opposite.

There are, however, a number of solutions to the problem of keeping down the temperature of the heat sink, a number of which are described below.

Reflecting the solar radiation reaching the sunscreen prevents a substantial increase of its temperature. Painting the sunscreen in a reflective colour, e. g. white or metallic, is a simple way of achieving this. However, for various reasons, e. g. aesthetical, the sunscreens often have to conform to certain colour requirements. In this case the solution is seldom viable.

Reflection of the IR thermal radiation (3-50 pm) reaching the heat sink can be achieved by coating the heat sink with some kind of IR reflecting material, e. g. chrome. However, the surface must then be coated in order to prevent corrosion of said material, which then increases the thermal emissions.

Controlling the climate, i. e. the temperature, of the environment between sunscreen and heat sink could be an efficient way of solving the problem. A decreased surrounding temperature decreases proportionally the temperature of the electronic equipment. Air conditioners and the like are however costly and space consuming, and this solution is therefore not always viable.

For the reasons stated above it is desired to find a more efficient heat protection for electronic equipment in housings exposed to strong infrared radiation, particularly solar radiation. Said heat protection should preferably have low weight and not add volume.

SUMMARY OF THE INVENTION The present invention aims to solve the problem of how to protect electronic equipment against radiative heat, when

the housing of said equipment is exposed to strong electromagnetic radiation, particularly solar radiation.

One object of the present invention is to provide an arrangement for facilitating heat transfer from electronic equipment protected by a sunscreen, particularly where the sunscreen is positioned in strong solar radiation.

According to the present invention, there is provided an arrangement for reducing the radiative heat transfer from a sunscreen to a heat sink connected to electronic equipment, by decreasing the emittance inwards of the sunscreen and thereby reducing the emanated heat power from the sunscreen towards the heat sink.

The arrangement according to the invention is defined in claim 1.

Preferred embodiments of the arrangement according to the invention are defined in claims 2-5.

An advantage with the present solution to the problem is that the thermal radiation transfer from sunscreen to heat sink is reduced. This will keep the heat sink at a lower temperature.

Other advantages with the present solution to the problem are that the arrangement has low weight, does not require extra volume and is invisible to the naked eye.

Yet another advantage with the present solution to the problem is that the solution uses tested materials and fabrication techniques.

The invention will now be described in more detail with the aid of the description of the embodiment and with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a simplified view of electronic equipment in a cabinet.

Figure 2 shows the equipment and cabinet from figure 1 exposed to strong solar radiation.

Figure 3 shows the equipment and cabinet exposed to strong solar radiation, with the arrangement according to one embodiment of the invention.

Figure 4 shows the equipment and cabinet exposed to strong solar radiation, with the arrangement according to another embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Figure 1 shows a simplified view of electronic equipment in a cabinet. A cabinet 12, comprising a sunscreen 13, houses electronic equipment 15 and a heat sink 14 thermally connected to each other. The dominating part of the heat emanated from the electronic equipment should be absorbed by the heat sink 14. The heat sink 14 should then emit the heat, often through openings (not shown) in the sunscreen 13.17 indicates the radiative heat transfer, which in this case is directed from the heat sink 14, as the heat sink 14 is warmer than the surroundings (see theory above).

Figure 2 shows the same equipment and cabinet as in figure 1 in strong solar radiation. A difference between the figures is that solar radiation 11 from the sun 10 reaches the cabinet 12 in figure 2.

The dominating part of the heat generated by the electronic equipment 15 should, as before, be emitted by the heat sink 14 via convection. When, however, solar radiation 11 impinges on the sunscreen 13, the sunscreen 13 temperature increases as it absorbs the energy of the solar radiation 11. The maximum absorption by the sunscreen 13 is in the wavelength interval 0.3-2.5 pm-i. e. roughly the visible and the near infrared part of the electromagnetic spectrum- is 1120 W/m2.

Additionally, most of the colours required by customers and regulations absorb significantly in this range. In a 24 hour cycle, the temperature of a sunscreen can in this case reach 90° C, while a preferred temperature of a heat sink is around 65° C. The net heat transfer 17 for a typical area of m2 can then be 13 W (see above for formulae). The heat transfer 17 is then directed to, instead of from, the heat sink 14, as indicated by the arrows in figure 2. At the same time, it is common for the electronic equipment 15 to generate 20 W of heat. This heat should be absorbed and radiated by the heat sink 14. The 13 W of heat transferred 17 to the heat sink 14 from the sunscreen 13 is then of the same magnitude as the 20 W of heat generated by the electronic equipment 15. It is therefore desired to reduce the heat transfer 17 from sunscreen 13 to heat sink 14 indicated in figure 2.

It is not sufficient to coat the heat sink 14 with an IR reflective material. In order to prevent corrosion of said material, the surface must be additionally coated. This

coating then increases the thermal emissions, owing to the fact that the coating has a high IR emission value, s (see above).

The window glass industry, on the other hand, has the problem of how to keep heat inside a room. A solution to this problem is to coat the outside of the innermost sheet of glass with a layer of low IR emission material, which to a certain extent prevents heat from emanating from said sheet to the next. There are many standard ways of coating glass panes for this purpose: evaporation, sputtering, CVD (Chemical Vapour Deposition), spraying a metal foil and so on. There are also many materials, many which are suitable for harsh climatic conditions: for example stannous oxide, SnO, and stannic oxide, Sn02.

The solution from the window glass industry can, after modification, be applied to the problem of reducing the heat transfer 17 from sunscreen 13 to heat sink 14. Figure 3 shows roughly the same equipment and cabinet in strong solar radiation, protected by the arrangement according to the invention. The main differences between this figure and figure 2 are: the heat sink 14 is missing and the figure shows the arrangement according to the invention: a coating layer 18 that reduces the radiated heat transfer 17 from the sunscreen 13 inwards. For reasons of clarity, the layer 18 is much thicker in the figure than in reality. See below under figure 4 for a further discussion.

Figure 4 shows an embodiment of the arrangement according to the invention. This figure is figure 3 with an added heat sink 14. This figure will be used as a basis for the discussion of the arrangement according to the invention.

As in figure 2, the solar radiation 11 increases the temperature of the sunscreen 13. With the layer 18 applied to the inside of the sunscreen 13, however, the radiated heat transfer 17 from the sunscreen 13 to the heat sink 14 is reduced. With less heat transferred 17 to the heat sink 14, it is easier for the heat sink 14 to radiate its heat, which in turn means that it is easier for heat to be transported from the electronic equipment 15 to the heat sink 14.

The layer 18 could be of practically any of the materials used in the window glass industry described above, for example stannous oxide, SnO, and stannic oxide, Sn02. The layer 18 does not necessarily have to cover the entire inside of the sunscreen 13. There are many different possibilities for coating the inside of the sunscreen 13.

For instance, only the parts of the sunscreen 13 deemed to be most harmful, i. e. radiate the most heat 17 to for example the heat sink 14, could be coated with a layer 18.

While this solution may seem to be near at hand, it is not obvious, however, as there are several unique aspects of the invention: -the object: protecting electronic equipment against heat as opposed to saving energy, -the area of application: sunscreens as opposed to windows, -the temperature: around 90° C as opposed to room temperature.

Advantages with this solution to the problem are: -a reduction in radiated heat transfer 17 from sunscreen 13 to heat sink 14, especially in strong solar radiation 11, -the use of already tested methods and materials,

-there is no increase in volume, -a very low weight, and -invisibility to the naked eye.

The reduction in radiated heat transfer can be calculated.

As earlier, the formula for heat transfer between sunscreen 13 and heat sink 14 is: rp4 m4<BR> T sunscreen-Theatsink p = A6 1/Ssunscreen'1/Eheatsink"1 The core of the invention can be described as trying to decrease P as much as possible by increasing the value of 1/esunscreen (by decreasing Esunscreen). Previously known sunscreens have approximate s-values of 0.90-0.95. As low- emittance materials even after ageing can have s-values below 0.3 (often around 0.2), the reduction of the heat transfer can be sizeable. Continuing the example above, on a warm day when the sunscreen is at its warmest, the heat transfer could be reduced from 13 W to 3 W-a reduction of more that 75%.

The arrangement according to the invention significantly reduces the heat transfer 17 from sunscreen 13 to heat sink 14 when the sunscreen 13 is warmer than the heat sink 14. It is unfortunately also true that the reverse heat transfer 17 -from heat sink 14 to sunscreen 13-is reduced when the heat sink 14 is warmer than the sunscreen 13. This results in a slight increase in the average temperature of the electronic equipment 15 over a 24-hour cycle, while the temperature peaks are cut. In environments where the cabinet 12 is expected to be exposed to much solar radiation 11, it is usually advantageous to accept a slight increase in average temperature as long as the temperature peaks are cut

significantly. The reason for this is that the temperature peaks shorten the life of electronic components more than a slight increase in average temperature does. It is impossible to give a general rule for when the arrangement according to the invention is beneficial. As a rule of thumb, however, an 6-value of the sunscreen 13 of less than 0.3 is usually required for the arrangement to give a reasonable improvement.

Where it reads heat sink 14 in the description, it should not be too narrowly interpreted. For purposes of this description the heat sink 14 could comprise cooling fans and other kinds of cooling equipment. It could also comprise parts of, or the entire housing of the electronic equipment 15, but it could also consist of just the housing.

Analogously, sunscreen 13 should not be too narrowly interpreted. It should be interpreted as more or less that or those parts of the cabinet, or whatever structure it belongs to, that could receive direct sunlight 11 or other strong radiation, particularly in the solar spectrum part of the electromagnetic spectrum.