Abstract:
A heat exchanger including a metal base formed of a plurality of interfitted elements and fins formed in part from recompressed expanded graphite, wherein the fins are securely connected to the base by placing a deformable edge portion of each fin between each pair of interfitting elements and thereafter compressing the interfitting elements toward one another to deformably retain the edge portions of each fin there between.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of manufacturing a heat exchanger designed to cool at least one power electronic component. The invention also relates to such a heat exchanger. 
     2. Brief Description of the Related Art 
     Heat exchangers are used conventionally to avoid detrimental overheating of power electronic components, such as Insulated Gate Bipolar Transistor (IGBT) circuits or equivalent equipments. As described in Document FR-A-2 777 986, such a heat exchanger can comprise a plate on which tubes are mounted through which a coolant fluid circulates. As envisaged in US-A-2004/0000391, such a heat exchanger can further comprise a metal base on which graphite fins are mounted, those fins being engaged in grooves that are machined in the base. Even when a friction fit is used, the mechanical fastening of those fins is not permanent, in particular insofar as the successive increases in the temperature of the base give rise to variations in the dimensions thereof that do not correspond to the changes in the fins. Therefore, the thermal contact between the fins and the base tends to deteriorate over time, which adversely affects the overall effectiveness of the heat exchanger. 
     When making a heat exchanger that comprises a metal base, and fins of high thermal conductivity (expressed in watts per meter degree Celsius (W/m° C.)), a problem arises concerning how to fasten together the various elements of different natures. The fins can be assembled mechanically onto the base, with the above-mentioned drawbacks, by brazing or by adhesive bonding. Fastening by brazing guarantees heat transfer that is of good quality, which is desirable in applications in which maximum cooling effectiveness is desired. However, that solution suffers from the drawback of requiring coatings to be applied to the parts that are to be assembled prior to assembly thereof, those coatings often being costly in view of the uses for which they are designed. In addition, the nature of certain fin materials does not make it possible to accommodate such treatments and/or to withstand the increase in temperature resulting from the brazing. Adhesive bonding is sometimes applied. In which case, the thermal improvement procured by the material of the fin is, for the most part, lost in the adhesive bond in which the thermal conductivity of the products used is very low. 
     SUMMARY OF THE INVENTION 
     More particularly, an object of the invention is to remedy those drawbacks by proposing a method of manufacturing a heat exchanger that makes it possible to fasten the fins to the base optimally, both in mechanical terms and in thermal terms. 
     To this end, the invention provides a method of manufacturing a heat exchanger designed to cool at least one power electronic component, the heat exchanger comprising a metal base and fins, each of said fins has a portion made of recompressed expanded graphite, said method being characterized in that it comprises a step consisting in: 
     a) embedding an edge of at least one fin by squeezing a deformable portion between two distinct component pieces of the base. 
     Thanks to the invention, the deformable portion is compressed between the two component pieces of the base, so that it holds the fin stationary mechanically and establishes optimized thermal contact between the base and the fin. 
     In advantageous but optional aspects of the invention, such a method can incorporate one or more of the following characteristics:
         It further comprises a step b) prior to step a) and in which the deformable portion is formed by making provision for the edge of the fin to have a thickness that is greater than the thickness of the portion of the fin that is designed to project relative to the base. At the end of step b), the thickness of the deformable edge is advantageously greater by in the range 20% to 40% than the thickness of the portion of the fin that is designed to project relative to the base.   It further comprises a step b′) prior to step a) and in which the deformable portion is formed as an independent part and then said part is disposed in the vicinity of the edge of the fin, between the pieces of the base. During step b′), the deformable portion is formed with a generally channel-section shape, suitable for capping the edge of the fin.   During step b) or b′), the deformable portion is provided with a height substantially equal to the height of embedding of the fin between the above-mentioned pieces of the base.       

     The invention also relates to a heat exchanger suitable for being manufactured by the above-mentioned method, and more specifically to a heat exchanger comprising a metal base and fins, each of said fins has a portion made of recompressed expanded graphite, said heat exchanger being characterized in that the base is made up of a plurality of distinct pieces between at least two of which an edge of a fin is embedded, a portion of the fin being deformed by being squeezed between the two above-mentioned pieces of the base, at the edge of the fin. 
     In advantageous but optional aspects of the invention, such a heat exchanger can incorporate one or more of the following characteristics:
         The deformed portion is constituted by the edge of the fin, which edge is integral with the portion of the fin that projects relative to the base.   The deformed portion is a part that is interposed between the edge of the fin and at least one of the pieces of the base between which the edge is embedded.   The zones in which two adjacent pieces of the base meet have profiles with crenellations suitable for interfitting with one another, the deformed portion being disposed between two end zones of said profiles.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood and other advantages of the invention appear more clearly from the following description of two embodiments of a heat exchanger that complies with the principle and with the manufacturing method of the invention, the description being given merely by way of example and with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a heat exchanger of the invention equipping two electronic components; 
         FIG. 2  is a diagrammatic side view of the heat exchanger of  FIG. 1  while it is being manufactured; 
         FIG. 3  is a view on a larger scale of the detail III of  FIG. 2 ; 
         FIG. 4  is a fragmentary side view of the heat exchanger of  FIG. 1 ; 
         FIG. 5  is a view analogous to the  FIG. 4  view for a second embodiment of the heat exchanger of the invention; and 
         FIG. 6  is a fragmentary diagrammatic side view of the heat exchanger of  FIG. 5  while it is being manufactured. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The heat exchanger  1  shown in  FIGS. 1 to 4  comprises a base  11  that is generally rectangular block shaped and on which fins  12  are mounted that enable heat to be dissipated into the ambient atmosphere, which heat is transmitted to the base  11  by two electronic components  2  and  2 ′ that are mounted on that face  111  of the base  11  that is opposite from the face  112  from which the fins  12  extend. 
     The base  11  is made by assembling together extruded aluminum elements  113  that are identical, except for the end elements  113 A and  113 B. In a variant, the end elements can be identical to the other elements  113 , in which case, the small end faces  116  and  117  of the base  11  are ribbed. 
     The elements  113 ,  113 A, and  113 B of the base  11  are distinct parts that must be assembled together to form said base. 
     Each of the sides of the elements  113  has a profile with crenellations formed of a succession of grooves  114  and of ribs  115  that extend parallel to the end faces  116  and  117  of the base  11 , which faces are defined by respective ones of the elements  113 A and  113 B. The grooves  114  and ribs  115  of the various elements  113  are disposed in such a manner that they can interfit with and be wedged in the grooves and ribs of an adjacent element  113  when the base  11  is assembled. In other words, the crenellated profiles provided on the two sides of the same element  113  are complementary with each other over most of the height H 113  of each element  113  that extends between the faces  111  and  112 . 
     In the vicinity of the face  112 , the crenellated profiles are not exactly complementary, so that there remains, after assembly of the base  11  and between two adjacent elements  113 , a recess  118  for receiving a root  121  of a fin  12 , which root constitutes the edge of the fin that is held stationary by being embedded between facing faces  119  and  119 ′ of two elements  113  whose crenellated profiles are co-operating with each other. In other words, the surfaces  119  and  119 ′ co-operate to define a recess  118  between them. 
     With the exception of the elements  113 A and  113 B, each element  113  is provided both with a surface  119  and with a surface  119 ′, which surfaces are distributed on either side of a midplane P 113  of said element. 
     As can be seen more particularly in  FIG. 3 , before the fin  12  is mounted onto the base  11 , its root  121  has a thickness e 121  that is greater than the thickness e 122  of the portion  122  of the fin  12  that is designed to project beyond the face  112  of the base  11  once the heat exchanger  1  is assembled. The root  121  is integral with the portion  122 . In practice, the thickness e 121  is greater than the thickness e 122  by in the range 20% to 40%. For example, the thickness e 121  can be about 2 millimeters (mm) while the thickness e 122  is about 1.6 mm. 
     While a fin  12  is being manufactured, the root or deformable edge  121  is shaped to have the thickness e 121  over a height h 121  that is substantially equal to the depth P 118  of the recess  118  into which it is designed to be inserted, said depth being equal to the height of the surface  119 ′ as measured perpendicularly to the faces  111  and  112 . 
     Each fin  12  is formed of a core  125  made from a plate of recompressed expanded graphite also commonly known as “flexible graphite”, said core being clad with cladding  123  made up of two metal foil strips of aluminum  123 A and  123 B disposed against the side faces of the fin  12 , which foil strips impart to the fin both good strength and also a certain amount of flexibility, enabling said fin to deform. The core  125  and the foil strips  123 A and  123 B can be fastened together by mechanical anchoring and co-lamination. 
     When manufacturing the heat exchanger  1 , the root  125  of a fin  12  is inserted between the facing faces  119  and  119 ′ of each pair of elements  113 . Then the base  11  is assembled by exerting on the faces  116  and  117  a clamping force represented by the arrows F 1  in  FIG. 2 , until the ribs  115  are fully embedded in the corresponding grooves  114 , the recesses  118  thus being formed. The base  11  is then held in this configuration by means of the tight mutual interfitting of the crenellations of the elements  113 . 
     Thus, by forming the base  11 , by assembling together the elements  113 ,  113 A and  113 B and by clamping them together more tightly, the root  121  of each fin  112  is held stationary by it being compressed in a recess  118 . The force squeezing the root  121  is exerted by the elements  113 ,  113 A, or  113 B when they are clamped. 
     Due to the tighter clamping obtained by the force F 1 , each root  121  is deformed by being squeezed while being embedded in the corresponding recess  118 . The depth of the grooves  114  and the height of the ribs  115  are chosen in such a manner that the distance d 118  between the facing faces  119  and  119 ′ of a recess  118  is substantially equal to the thickness e 122 . 
     In view of the capacity of the flexible graphite core  125  to be compressed without breaking, the result of the fins  12  and the base  11  being assembled together mechanically by means of embedding is that the various fins  12  are held mechanically in effective manner, and that thermal contact is established in very satisfactory manner between the base  11  and the fins  12 . 
     In practice, in an aspect of the invention that is not shown, the thicknesses of the ribs  115  and of the grooves  114  guarantee that, after the elements  113  have been assembled together and by means of the force F 1 , the ribs  115  are fully embedded in the grooves  114 , thereby imparting to the resulting base a mechanical structure that is very rigid and non-separable. 
     The elements  113  can be obtained by extrusion, with manufacturing tolerances that are taken up during assembly of the base  11 , by the embedding of the fins  12 . No brazing is necessary in order to hold the fins  12  permanently on the base  11 , which is very advantageous in view of the nature of the core  125  of each fin  12 . 
     By way of example, the thickness e 121  can be about 2 mm, while the distance d 118  lies approximately in the range 1.6 mm to 1.9 mm. In which case, when the root  121  is embedded, its thickness is reduced by an amount lying in the range 5% to 20%. 
     In the second embodiment of the invention shown in  FIGS. 5 and 6 , elements that are analogous to the elements of the first embodiment bear identical references. In the second embodiment, the base  11  is formed by assembling together aluminum shaped-section elements  113 . Each of the fins  12  is formed of a core  125  and of metal cladding  123 . They have the same thickness over their entire height. Deformable bushes  124  are provided as parts that are structurally independent of the pieces  11  and  12  of the heat exchanger  1  prior to assembly thereof. These parts  124  are in the form of generally U or channel-section troughs, enabling each of them to cap the edge  121  of a respective fin  12 , the parts  124  being inserted into recesses  118  defined, as in the first embodiment, in the vicinity of one face  112  of the base  11 , between two facing faces  119  and  119 ′ belonging to respective ones of two different elements  113 . 
     In the second embodiment, the parts  124  play the same part as the deformable roots  121  of the first embodiment in that they are designed to deform by being squeezed under the effect of the clamping force exerted for assembling the base  11 . These parts  124  both hold the fins  12  stationary mechanically relative to the base  11 , and also establish excellent thermal contact between the fins  12  and the base  11 . 
     The parts  124  can be made of any material suitable for their function, in particular of metal foil strips, e.g. made of copper or of aluminum. They are preferably made of recompressed expanded graphite, also known as “flexible graphite”, like the cores  125  of the fins  12 . 
     The total thickness of a part  124  before it is inserted between two elements  113  is referenced e 124 . This thickness is reduced while the heat exchanger is being manufactured, due to the compression obtained by the clamping force F 1  exerted during assembly of the base  11 . In this situation, the distance d 118  between the facing faces  119  and  119 ′ of a recess  118  is not equal to the thickness e 122  of a fin  12  in its portion  122  that projects beyond the base  11 , given that the flanges of the channel section formed by each part  124  continue to have some thickness. 
     The height h 124  of each part  124  is at least equal to the depth P 118  of the recesses  118 . 
     Thus, when forming the base  11  by assembling together the pieces  113  and equivalent pieces, the fins  12  are held stationary in the recesses  118  by compressing the parts  124 , by means of the elements  113  and equivalent elements, to the extent that said parts deform. 
     In a variant of the invention that is not shown, a part forming a deformable portion can be disposed on one side only of a fin  12  in the corresponding recess  118 , namely either on the same side as the surface  119 , or on the same side as the surface  119 ′ of the corresponding recess  118 . In another variant, a part can be disposed on either side of the portion of the fin  112  that is embedded in a recess  118 , without the two parts being connected together by a piece corresponding to the web of the U or channel section shown in  FIG. 6 . 
     Regardless of the embodiment, the foil-strip cladding can be omitted, the fins then being made of graphite only. 
     In another variant, the cores  125  or the entire fins  12 , when cladding is omitted, can be made of a mixture comprising graphite and another material, such as a binder.