Abstract:
A cooling element includes a base plate and several spaced-apart cooling fins which are arranged on a flat side of the base plate. The end faces of the cooling fins together with the formed cooling channels form an inflow side and an outflow side for cooling air. The end face of the cooling fins on the inflow side and the outflow side are configured to produce a low flow resistance. This substantially reduces the counterpressure exerted on the cooling air flow and thereby obviates the need for high-power fans, without a reduction in the removed heat per unit time {dot over (Q)}.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
         [0001]    This application claims the priority of German Patent Application, Serial No. 101 53 512.0, filed Oct. 30, 2001, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a cooling element with a base plate and several spaced-apart cooling fins which are arranged on a flat side of the base plate. The end faces of the cooling fins together with the formed cooling channels form an inflow and outflow side for cooling air.  
           [0003]    A cooling element of this type is commercially available and illustrated in greater detail in FIG. 1. The cooling element includes a base plate  2  having a flat side  4  and cooling fins  6 , end faces  8  and  10  formed by the cooling fins as well as cooling channels  16  having an inflow side  12  and an outflow side  14 . Cooling air produced by a fan  18  is blown through the cooling channels  16 . The fan  18  is directly attached to the inflow side  12  of the cooling element. Each of the cooling channels  16  is formed by two adjacent cooling fins  16  and a section of the flat side  4  of the base plate  2 . The outflow side  14  is arranged in opposition to the inflow side  12  and is formed by the end faces  10  of the cooling fins  6  and the cooling channels  16 . FIG. 1 does not show the end faces  8  of cooling fins  6  which together with a cooling channels  16  form the inflow side  12 , since these are covered by the fan  18 . Several semiconductor components  22  are mounted with a heat-conducting mounting plate  24  on the flat side  20  of the base plate  2  that faces away from the cooling fins  6 .  
           [0004]    Modern high-efficiency semiconductor devices have a heat flux density of approximately 10 5  W/cm 2 . These devices can be cooled efficiently only by using a cooling element with a high fin ratio R V , which is defined as the quotient of the cooling fin width or thickness R B  to the fin spacing R A . The efficiency of the cooling element is limited by the achievable temperature increase ΔT of the cooling air. This temperature increase ΔT depends on the geometry of the cooling element and, more particularly, is proportional to the rib ratio R V . A cooling element with a rib ratio R V =1 has an upper limit value for the increase in the cooling air temperature of approximately 24K as determined by the flow conditions. This increase in cooling air temperature is capable to remove an average heat current {dot over (Q)}.  
           [0005]    The increase of the air temperature in the cooling element is a result of a momentum transfer between the cooling air and the cooled surface at the boundary between the cooling fin  6  and cooling channel  16 , whereby the momentum transfer depends on the degree of turbulence in the flow and increases with increasing degree of turbulence. The degree of turbulence, on the other hand, depends on the surface characteristic of the rib  6 , the physical properties of the cooling air as well as the inherent momentum of the cooling air (airspeed).  
           [0006]    For a given temperature increase and rib characteristic, such as geometry and roughness, the removable heat current {dot over (Q)} can only be increased by increasing the degree of turbulence. This can be achieved, for example, by increasing the airspeed. To achieve this, either the mass flow of the air has to be increased or the cross-section of the flow channel has to be reduced. Both measures necessitate an increase in the required fan power.  
           [0007]    With a constant air mass flow {dot over (m)}, the flow velocity increases with decreasing cross-sectional A of the flow channel, which is equal to the product of fin spacing R A  and height of the cooling fins R H . Increasing the flow velocity causes an increased counterpressure in the flow channel. The pressure drop of a flow channel can be expressed by the following equation using Bernoulli&#39;s law:  
         Δ                 P     =       ζ   *     ρ   2     *       (          s          t       )     2       =     ζ   *     ρ   2     *       (       m   .       ρ   *     R   H     *     R   A         )     2                               
 
           [0008]    As seen from this equation, decreasing the fin spacing R A  cause an increase ΔP in the pressure. The form factor ζ describes herein the flow resistance of the flow channel. This form factor ζ is essentially composed of three components, which are:  
           [0009]    a) the surface properties along the flow channel (roughness),  
           [0010]    b) the conditions at the inflow end of the flow channel (geometry), and  
           [0011]    c) the conditions at the outflow end of the flow channel (geometry).  
           [0012]    While for the momentum transfer the form factor ζ along the flow channel should be as large as possible, the components b) and c) listed above and also relating to the form factor ζ are undesirable and detrimental, since they tend to increase the system cost.  
           [0013]    The effect of the form factor ζ in particular of the components b) and c) above, is typically neglected in conventional embodiments of cooling elements. If the heat flow {dot over (Q)} to be removed by the cooling element is to be increased, the pressure drop increases quadratically with the flow velocity and linearly with the form factor ζ. This is a reason why commercially available cooling elements require heavy-duty fans to produce the mechanical power to move the cooling air. However, such fans are large and expensive.  
           [0014]    It would therefore be desirable and advantageous to provide an improved cooling element which obviates prior art shortcomings and which is so configure that a fan requiring less power can be used.  
         SUMMARY OF THE INVENTION  
         [0015]    According to one aspect of the present invention, a cooling element cooled with cooling air having a flow direction includes a base plate and a plurality of spaced-apart cooling fins spaced apart transversely to the flow direction and arranged on a flat side of the base plate so as to form cooling channels. The cooling fins have end faces which cooperate to form an inflow side and an outflow side for the cooling air. The end faces of the cooling fins on the inflow side and the outflow side are configured so as to provide a low flow resistance. As a result, the fan power can be reduced due to the reduced pressure drop. This advantageous effect increases with decreasing spacing between the cooling fins of a cooling element. The fin density is considered to be high when the fin spacing is approximately equal to the fin width.  
           [0016]    According to an advantageous embodiment of the cooling element of the invention, the spaced-apart cooling fins are offset in the flow direction of the cooling air in such a way that the inflow and outflow sides have a wave-like shape. This measure further decreases the pressure drop and simultaneously reduces the fan power requirement.  
           [0017]    According to another advantageous embodiment of the cooling element of the invention, each end face of each cooling fin on the inflow side is formed convex and each end face of each cooling fin on the outflow side is wedge-shaped. These different configurations of the end faces of each cooling fin gives the cooling fin the shape of an elongated drop that is oriented opposite the flow direction. This produces an ideal form of the cooling fins by reducing the counterpressure, and thereby also the mechanical power requirements of the fan. However, this disadvantageously makes the fabrication process for the cooling element quite complex.  
           [0018]    According to another embodiment of the cooling element of the invention, each end face of each cooling fin on the inflow and outflow side is inclined and/or the spaced-apart cooling fins are mutually offset in the flow direction of the cooling air in such a way that the inflow and outflow side each form a zigzag pattern. The two zigzag surfaces in this embodiment are in phase. This arrangement produces a particularly economical solution for a cooling element according to the invention. By inclining each fin end with the aforedescribed arrangement, a mini-region (each fin) and a macro-region (arrangement of the fins) results, each of which contributes to the reduction in the counterpressure.  
           [0019]    According to yet another advantageous embodiment, each cooling fin can include transverse ribs oriented in the flow direction of the cooling air. In addition, a second base plate can be arranged so that its flat side contacts the narrow sides of the free ends of the cooling fins. At least one the base plate can include spaced-apart grooves oriented in the flow direction of the cooling air, into which grooves a cooling rib can be pressed. For improved heat conduction, the base plate(s) and the cooling fins can be made of extruded aluminum or another metal with a high thermal conductivity. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0020]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:  
         [0021]    [0021]FIG. 1 is a perspective view of a conventional cooling element;  
         [0022]    [0022]FIG. 2 shows a flow pattern on the cooling fins of the cooling element according to FIG. 1  
         [0023]    [0023]FIG. 3 shows a flow pattern on the cooling fins of a first embodiment of a cooling element according to the present invention;  
         [0024]    [0024]FIG. 4 shows a flow pattern on the cooling fins of a second embodiment of a cooling element according to the present invention; and  
         [0025]    [0025]FIG. 5 is a perspective view of a third embodiment of a cooling element according to the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0026]    Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals.  
         [0027]    Turning now to the drawing, and in particular to FIG. 2, there is shown a flow pattern of cooling air on cooling fins of a commercially available cooling element according to FIG. 1. The cooling air produced by the fan  18  is depicted in FIG. 2 by arrows A. Also seen in FIG. 1 are on the inflow side  12  turbulent zones B which generate the undesirable counterpressure. This effect increases with increasing width R B  of the fins. This effect is particularly pronounced in extruded cooling fins. Turbulent zones C also form on the outflow side  14  of the flow channels  16  due to the movement of the cooling air at the edges of the fin ends. The turbulent zones C can even experience a flow reversal. The flow distribution is a result of the geometry at the inflow and outflow sides of the flow channels  16 . These two components substantially affect the form factor ζ of the cooling element which is responsible for the generation and the magnitude of the pressure drop on the cooling element.  
         [0028]    [0028]FIG. 3 shows the flow pattern on the cooling fins of a cooling element according to a first embodiment of the invention. The sake of clarity, only a few individual cooling fins  6  are shown. The cooling air produced by the fan  18  is also depicted by arrows A. In this first embodiment of the cooling element, the end faces  8  of the cooling fins  6  are convex and the end faces  10  are formed as wedges. The convex form of the end faces  8  of the cooling fins  6  of a cooling element prevent the formation of turbulent zones B at the inflow end of the flow channels  16 , since the cooling air A no longer impinges on a rebounding surface. The cooling ribs  6  are offset in the flow direction of the cooling air A at the inflow side  12  in a wavy pattern. As a result, the convex end faces  8  divert the cooling air A in the region between the end faces  8  into adjacent flow channels  16 . The end faces  8  of the different cooling fins  6  of the cooling element at the inflow side  12  are located on a concave curvature with respect to the flow in cooling air A. The curvature of the concave end faces  8  required for efficiently preventing turbulence depends on the airspeed of the supplied cooling air A. The air mass flow in each flow channel  16  of the fin arrangement of the cooling element can be increased by suitably shaping the end faces  8  on the inflow side  12 . As a result, the airspeed in the flow channels  16  of the cooling element also increases.  
         [0029]    The end faces  10  of the cooling fins on the outflow side  14  of the fin arrangement are wedge-shaped. It should be noted that the wedge-like shape of the end faces  10  should optimally be free of any edges. Advantageously, if the inclined surfaces of the wedge-shaped end faces  10  should be concave. In this way, the cooling air A can exit from the flow channels  16  without experiencing turbulence in spite of the increased airspeed.  
         [0030]    The two end faces  8  and  10  of each cooling fin  6  of the cooling element are formed in the shape of a drop oriented opposite the flow direction. More particularly, the wedge-like shape of the end faces  10  on the outflow side  14  of each fin  6  of the fin arrangement significantly reduced the counterpressure.  
         [0031]    [0031]FIG. 4 shows a flow pattern on the cooling fins  6  of another cooling element according to the invention. The cooling air produced by the fan  18  is also indicated by the arrows A. In this particularly advantageous embodiment, the cooling fins  6  have inclined end faces  8  and  10 . The inclined end faces  8  and  10  of each cooling fin  6  are beveled so as to extend in parallel in space. Moreover, the cooling fins  6  in this embodiment are mutually offset in the flow direction of the cooling air A on the flat side  10  of the base plate  2  so that (the envelopes of) both the inflow side  12  and the outflow side  14  form a zigzag pattern. Since each cooling fin has an identical angle of inclination, the zigzag-shaped inflow and outflow sides  12  and  14  have the same phase. The base plate  2  is made substantially longer (overhang) than that of the cooling element depicted in FIG. 1 so that the flow channels  16  in the inflow and outflow region of the fin arrangement are not entirely open. The angle of inclination (bevel angle) of the inclined end faces  8  and  10  determines the overhang of the base plate  2 . The length of the base plate  2  of the cooling element increases with increasing bevel angle of the end faces  8  and  10  of each cooling fin  6 .  
         [0032]    The aforedescribed advantageous shape of the end faces  8  and  10  of the cooling fins  6  of a fin arrangement of a cooling element produces an embodiment with a substantially reduced counterpressure. Each cooling fin  6  thereby forms a mini-region in the region of the inflow and outflow side  12  and  14 , whereby the arrangement of the fins according to the invention forms a macro-region, which separately contribute to a reduction in the counterpressure.  
         [0033]    [0033]FIG. 5 shows in a perspective view an advantageous cooling element according to the invention. This advantageous cooling element has cooling fins  6  which according to FIG. 4 are arranged on the base plate  2  and have inclined end faces  8  and  10 . The cooling fins  6  are pressed into grooves  26  provided in the base plate  2 . In addition, this cooling element has a second base plate  28 , which is placed with a flat surface on the narrow side of the free ends of the cooling fins  6 . The flat side of the second base plate  28  also has grooves into which the cooling fins  6  can be pressed. The flow channels  16  are closed off by the second base plate  28 , except for the inflow and outflow side  12  and  14 . In addition, high-power semiconductors can be releaseably secured on the flat surfaces  20  and  30  of the two base plates  2  and  28  opposite the grooved surfaces. To increase the surface area of the cooling fin  6 , the cooling fins are provided with transverse ribs extending in the flow direction of the cooling air A.  
         [0034]    By forming the cooling fins  6  in the inflow and outflow region of a fin arrangement of the cooling body according to the invention, the components b) and c) of the form factor ζ can be significantly reduced or even eliminated. This reduces substantially the counterpressure exerted on the cooling air flow, obviating the need for high-power fans  18 , while removing the same heat per unit time {dot over (Q)}. This not only reduces the system cost, but also maintenance expenses.  
         [0035]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.  
         [0036]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and their equivalents: