Abstract:
A cooling arrangement for cooling a heat generating electrical component is disclosed, which arrangement comprises a heat spreading element ( 2 ) having a mounting surface ( 3 ) adapted to be thermally connected to the heat generating electrical component ( 1 ), and a heat rejection surface ( 4 ). The arrangement further comprises an enclosure ( 5 ) arranged to cover the heat rejection surface ( 4 ) and form an essentially closed compartment; an opening ( 13 ) leading into the compartment; an annular member ( 11 ) coaxially aligned with the opening; and an actuator connected to the annular member ( 11 ), and arranged to move the annular member ( 11 ) reciprocating away from/toward the opening ( 13 ). The annular member ( 11 ) hence generates a jet directed through the opening ( 13 ) toward the outside of the enclosure ( 5 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a cooling arrangement for cooling a heat generating electrical component. The present invention further relates to an electrical device comprising such a cooling device. 
       BACKGROUND OF THE INVENTION 
       [0002]    The need for active cooling arrangements for cooling heat generating electrical components, such as LEDs or ICs, is increasing as these components become more widely used in various applications. 
         [0003]    Examples of active cooling arrangements comprise fans, propellers or synthetic jets, that all enhance the heat transfer by forced convection. A synthetic jet is disclosed in U.S. Pat. No. 6,123,145, wherein a diaphragm in the wall moves to simultaneously move the volume in a container with the result that vortices are ejected from the chamber through the orifice. The synthetic jet that is hence generated impinges on a heated surface to cool it. 
         [0004]    Conventional active cooling arrangements are commonly subjected to fouling and limited lifespan, they make audible noise, are costly, heavy and require extra space and power, which is an issue when it comes to for example lighting, which is particularly demanding regarding these issues. 
       SUMMARY OF THE INVENTION 
       [0005]    In view of the above, an object of the present invention is to provide an improved cooling arrangement. 
         [0006]    According to an aspect of the present invention, there is provided a cooling arrangement for cooling a heat generating electrical component, comprising a heat spreading element that has a mounting surface adapted to be thermally connected to the heat generating electrical component, and a heat rejection surface; an enclosure arranged to cover the heat rejection surface and form an essentially closed compartment; an opening leading into the compartment; an annular member coaxially aligned with the opening; and an actuator connected to the annular member, and arranged to move the annular member reciprocating away from/toward the opening, so that the annular member generates a jet directed through the opening toward the outside of the enclosure. 
         [0007]    The jet causes forced heat convection from the heat rejection surface, thus enhancing the convective cooling of the heat spreading element. The arrangement enables zero net mass transfer, yet generating nonzero momentum transfer for forced convection in a narrow enclosure. An advantage with an outward jet is that it puffs out hot air while sucking in fresh air from elsewhere. Vortex shedding is generated by air displacement and occurs as the annular member moves reciprocating away from/toward the opening actuated by the actuator, with the cross section of the annular member in a plane normal to the direction of the motion. 
         [0008]    A “heat spreading device” is here a device that spreads the heat which is generated by a heat generating electrical component. Such a device may be a heat spreader and/or a heat sink, or a circuit board, such as a metal core printed circuit board, MCPCB. An “actuator” is here a device that is connected to the annular member and actuates the reciprocating movement of it. 
         [0009]    An active cooling device comprising an actuated tubular member is described in the non-published European Patent Application 07122623.7. However, this tubular member is adapted to primarily generate inward synthetic jets toward the object, and is further based on the realization that the object to be cooled is arranged inside an enclosure. Due to the enclosure such an arrangement is relatively big in size and accordingly requires a relatively big space when mounted. For example LEDs or ICs do not have an enclosure, and therefore put additional demands on a cooling arrangement. The present invention is based on the realization that the outward jet is generated even if the distance between the annular member and the heat spreader or enclosure wall is small. Although the arrangement is kept small in size, which is preferred or even required in many applications, such as general lighting applications, the outward jet(s) enhance convection. Furthermore, the surface of the annular member acts to pump the air, and since this surface may be relatively big compared to the orifice, the pumping action becomes more powerful. 
         [0010]    The opening may be arranged in the wall of the enclosure or alternatively in the heat spreading element, depending on the application. 
         [0011]    Moreover, the enclosure may comprise at least one additional opening, to increase the inward flow into the enclosure, hence boosting the flow inside the enclosure and intensifying the cooling of the heat rejection surface. Each opening affects the air displacement and hence the cooling effect of the cooling arrangement. 
         [0012]    In an embodiment the actuator may be integrated in the enclosure, which makes the cooling arrangement more compact. In this case the opening may be formed in the actuator, which makes the arrangement even more compact. 
         [0013]    The actuator may preferably be a loudspeaker, which loudspeaker comprises a magnet, a coil and a membrane. The annular member may be connected to the coil of the loudspeaker, and the coil may be suspended and guided by the membrane. It is possible to tune the resonance frequency of a loudspeaker, just by adding moving mass to it. Hence, tuning of the resonance frequency of the loudspeaker is allowed by adjustment of the mass of the annular member. The additional mass of the annular member allows that the resonance frequency of the loudpeaker may be subsonic or low. Operation at a low frequency implies low audible noise as well as longer life time. Thus, sophisticated and expensive noise reduction is not required. Another advantage with loudspeakers is that vortex shedding (and synthetic jet formation) may be boosted by pumping of loudspeaker (cones) with modest excursion. Moreover, guidance of the annular member is integrated in a loudspeaker, which guidance is important to reduce play, and hence leakage. 
         [0014]    Other types of actuators include for examples a crank-connecting rod mechanism, a plunger or a membrane pump. 
         [0015]    The opening may be formed in the loudspeaker magnet, which make the arrangement compact, and no major construction modification of the loudspeaker is thus required. 
         [0016]    Furthermore, the cooling arrangement may comprise a tubular member that extends through the opening, which tubular member has a first open end arranged inside the enclosure connected to the annular member, and a second open end arranged outside the enclosure. The annular member may hence form a flange on the tubular member. Accordingly, tuning of the resonance frequency of the loudspeaker is allowed by adjustment of the mass of the annular and tubular member. Further, the moving air mass is approximately proportional to the length of the tubular member and the thickness of the flange. The surface of the flange of the tubular member and of the membrane pump the air, and since this surface is relatively big the pumping action becomes more powerful. 
         [0017]    The loudspeaker coil and the loudspeaker membrane may be arranged inside the compartment whereby the membrane divides the inside of the enclosure into two sub compartments, each provided with at least one opening. At least one of the openings leading from the first sub compartment may further be connected to at least one of the openings leading from the second sub compartment. This may be advantageous since the cooling arrangements becomes compact and more effective taking usage of the double action pump effects of the arrangement. Moreover, at least one of the openings leading from the first sub compartment may be connected to at least one of the openings leading from a second sub compartment via a λ-pipe, where λ is the wave length generated by the actuator. The pipe hence connects the backside of the loudspeaker membrane to the opposite side of the membrane. In case the loudspeaker backside must be closed, the pipe will act as the only additional opening. 
         [0018]    The distance between the annular member and an opposing surface may be adapted to allow a jet to develop toward the inside of the enclosure. Such an internal jet is advantageous for directing a jet toward the hot spot of the heat generating electrical component, such as a LED and/or the heat rejection surface of the heat spreader. The arrangement hence enables efficient cooling by synthetic jet impingement. In one embodiment, the distance between the annular member and the opposing surface may be at least 2 times the opening diameter of the annular member to allow the inner jet to develop, but preferably at maximum 10 times the opening diameter. 
         [0019]    The cooling arrangement according to the present invention may, furthermore, advantageously be comprised in an electrical device including electrical components. Other objectives, features and advantages will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In the following, embodiments of the present invention will be described in detail, with reference to the accompanying, exemplifying drawings, where the same reference numerals will be used for similar elements. All figures are schematic and not to scale. 
           [0021]      FIG. 1  is an exploded perspective view of a cooling arrangement according to an embodiment of the present invention. 
           [0022]      FIG. 2  is a cross-section view of an exemplary cooling arrangement according to a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    The invention will be described with reference to the exemplary cooling arrangement  10  in  FIG. 1 , where a LED  1  is thermally connected to the mounting surface  3  side of the heat spreading element  2  to be cooled by so called forced convection. An enclosure  5  surrounds the arrangement on the heat rejection surface  4  side of the heat spreading element  2 , thus forming an essentially enclosed compartment. An actuator, here a loudspeaker, is arranged inside the enclosure  5 , being integrated in the wall of the enclosure  5 , which loudspeaker comprises a coil  6 , a membrane  7  and a magnet  8 . In the illustrated example the loudspeaker is arranged opposite the heat rejection surface  4  side of the heat spreader  2 . Further, an opening  13  leads into the compartment and in the illustrated example it is formed in the magnet  8  of the loudspeaker. An annular member  11  is coaxially aligned with the opening  13 , and connected to the actuator, here the loudspeaker coil  6 . The loudspeaker coil  6  is further suspended by the loudspeaker membrane  7 . In this example the annular member  11  is moreover equipped with a tube  9 , which is arranged to extend through the opening  13  in the loudspeaker magnet  8  and the enclosure  5 . The first open end of the tube is arranged inside the enclosure  5  and the second open end is arranged outside the enclosure  5 . The annular member  11  hence forms a possibly cone-shaped flange on the tube  9 . 
         [0024]    The circumference of the enclosure  5  may further comprise one or several additional openings, and in the illustrated example it comprises three equidistant openings  12   a - c  along the circumference of the enclosure  5 . 
         [0025]    The tube  9  is here made of a temperature resistant material with low mass and lower or equal thermal expansion than the surroundings, e.g. Alsint ceramics, thin aluminum, or filled heat resistant polymers, the filling being present to reduce the coefficient of thermal expansion (CTE). 
         [0026]    A typical example of a loudspeaker which can be arranged to meet and exceed the jet formation criterion is PHILIPS/NXP 2403-254-22002. The jet formation criterion for round apertures can be found in “Formation Criterion for synthetic jets”, Ryan Holman et al., AIAA Journal, 43, 2110-2116, 2005. 
         [0027]    Each opening may be tapered toward the interior of the enclosure in order to boost the internal jet. Further, the edges of each aperture are preferably sharp to promote vortex shedding. By providing the surface of each aperture with grooves shaped as a helix or by having an aperture in the form of an orifice protruding into the enclosure the turbulence of the jet may be further increased or the shedding of vortices promoted. 
         [0028]    The flange of the tube  9  is arranged to be moved by the loudspeaker coil  6 . Hence, the opening  13 , here defined by the bore of the tube  9  and annular member  11 , forms an actuated opening  13 . During operation the reciprocating movement of the loudspeaker results in a translational motion of the flange  11  of the tube  9  which results in an air displacement within the enclosure  5 . Hence, the heat is dissipated from the heat spreading element  2  by actively leading the heated air through the opening  13  in the form of a jet toward the outside of the enclosure  5 , i.e. by means of forced convection. Additional flow may be generated via the additional openings  12   a - c  in the enclosure. The surface of the flange of the tube  9  as well as the membrane  7  pumps the air that forms this flow as well as the jet through the actuated opening  13 . 
         [0029]    In  FIG. 2  another example of a cooling arrangement  20  which functions as a double action pump is schematically shown. The loudspeaker membrane  7  divides the compartment of the arrangement so that a sub compartment  21 ,  22  is formed on each side of the membrane  7 . The openings  12   a - c  in the circumference of the enclosure  5  lead to the first sub compartment  21  on one side of the loudspeaker membrane  7 , whereas a second sub compartment  22  is formed on the opposite side of the loudspeaker membrane  7 , at the backside of the loudspeaker. 
         [0030]    The arrangement can work as a double action pump, on condition that there are openings that lead from both sub compartments  21 ,  22 . In the illustrated example an opening  23  is therefore arranged in the wall of the enclosure  5  and leads to the second sub compartment  22 . The opening  23  from the second sub compartment  22  is further attached to a nλ-pipe  24 , with length nλ, where n is a natural number and λ is the wave length of the waves generated by the loudspeaker. The second end of the nλ-pipe  24  is attached to the first sub compartment  21  via one of the additional openings  12   c  in the circumference of the enclosure. Using the double action pump properties, an additional pulsating flow in and out the enclosure  5  is generated, through the openings  12   a - c  in the enclosure  5 . This flow may cool the heat rejection surface  4  of the heat spreading element  2  more. Alternatively, the pipe may be a (n+½)λ-pipe (the length is not to scale). For a (n+½)λ-pipe, the air flow arrow through the opening  12   c  in pipe  24  changes direction. 
         [0031]    The person skilled in the art realizes that the present invention is not limited to the preferred embodiments. For example other gases (fluids) than air may be pumped. The openings may have any shape, such as round, square or oblique, and the number of additional openings is flexible. Furthermore, the actuator may be arranged outside the enclosure, but still be in connection with the annular member to achieve the reciprocating movement of it, which may promote lower operating temperatures of the actuator without the need to perforate the magnet. The apertures may for example be arranged in the circumference of the heat sink instead of in the enclosure, or in a tube attached to the loudspeaker coil, or even in the loudspeaker membrane. The opening may be arranged parallel to the heat spreader, which may allow more length available for an internal jet to develop. Or, the arrangement may comprise more than one annular member, actuated by the same actuator. 
         [0032]    Such and other obvious modifications must be considered to be within the scope of the present invention, as it is defined by the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word “comprising” does not exclude the presence of other elements or steps than those listed in the claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, a single unit may perform the functions of several means recited in the claims.