Patent Publication Number: US-7911308-B2

Title: Low thermal impedance conduction cooled magnetics

Description:
BACKGROUND OF THE INVENTION 
     Power processing systems are used to provide electrical power to a broad variety of applications, from automobiles to zeppelins. In many if not all of these applications, the size and mass of the power processing system are among the first design considerations. For most power processing systems, overall size and mass are typically determined by magnetic components, such as transformers and inductors. If these magnetic components can be made smaller and lighter, then the overall systems in which they are included become smaller, lighter, and usually less expensive. 
     In turn, for both transformers and inductors, size and mass are generally based on thermal considerations. That is, as heat transfer is improved, size can be reduced, because winding current densities and core voltage can increase without excessively raising the temperature. Accordingly, substantial effort has been directed toward achieving efficient heat transfer between the winding and ambient and between the core and ambient. 
     In many conventional power processing systems, magnetic components are cooled by free-convection or forced-air cooling. In these systems, heat transfer is limited by the heat transfer coefficient of air, which is typically in the range of 0.4 to 0.8 mW/cm 2 /° C. for free convection and 1.0 to 3.0 mW/cm 2 /° C. for forced-air cooling. 
     In other conventional power processing systems that utilize liquid cooling (e.g., transformer oil), the heat transfer coefficient is typically improved by more than a factor of ten. While this enables the associated magnetic components to be significantly reduced in size, the inconvenience and economic cost of providing the liquid coolant flow frequently offsets this performance advantage. Furthermore, in cases where the coolant contacts only the outer surface of the winding, thermal resistance of the winding itself may become the limiting factor. 
     In power systems rated above about 50 kW, cooling the system frequently involves a cold-plate, which may be either forced-air or liquid-cooled. In such cases, a low thermal impedance path is desired between both the winding and the core to the cold plate. One of the key challenges in obtaining the low thermal impedance path is the relatively poor thermal conductivity of electrical insulation materials in the winding. Accordingly, any design which involves heat transfer to a base-plate should have relatively short heat flow paths through the electrical insulation. 
     In various systems including power processing systems, a potting material or other encapsulant is frequently used to encapsulate various types of components, as is well known to those skilled in the art of electrical and electronic packaging. One conventional method of potting includes the use of a potting cup, a mold, or some other vessel, into which the components to be protected are placed, and an encapsulant or potting compound such as an epoxy or resin is poured or injected into the vessel to cover the components. The potting compound is then cured and hardened. Such a potting compound can provide the internal components with varying degrees of protection from environmental contamination, electrical insulation, structural support, and a thermally conductive path from the component to ambient. 
     However, further improvement in the conduction of heat away from power processing systems and various other electrical and electronic components is desired. 
     SUMMARY OF THE INVENTION 
     The present invention provides for the efficient cooling of an element, for example, a magnetic element such as an inductor or transformer having windings and a core. Various aspects of the invention include reduced cost, reduced size, reduced mass, reduced heat load into surrounding air, and an improved capability to use high-flux density core materials, further reducing cost and size as compared to the prior art. 
     In one aspect, the invention provides a thermally conductive vessel, such as a potting cup, with a cavity that is adapted to conform to a surface of the element. The element is placed in the cavity, and due to the closely matching shape, a relatively small gap remains between the element and the thermally conductive vessel. A thermally conductive encapsulant, such as a resin or potting material fills the gap between the element and the vessel, further improving the cooling ability of the apparatus. 
     Another aspect of the invention provides a winding with a uniform surface, reducing the necessary size of the gap between the winding and the vessel. The winding with a uniform surface can be provided with an edge winding, fabricated by bending a rectangular metal bar around its short edge, or a machined winding, machined from an extruded metal tube. 
     These and other aspects of the invention are more fully comprehended upon review of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention. 
         FIGS. 1A-1C  illustrate a magnetic component and potting cup according to an exemplary embodiment of the present invention; 
         FIG. 2  illustrates an embodiment including a flat surface for making forced contact with a cold plate or heat sink; 
         FIG. 3  illustrates an embodiment including external fins; 
         FIG. 4  illustrates an embodiment including a finned interior cavity; 
         FIG. 5  illustrates an embodiment including a magnetic element having a laminated E-I core and an edge winding; 
         FIG. 6  illustrates an extruded metal tube; 
         FIG. 7  illustrates a machined winding including terminals and coils; 
         FIG. 8  illustrates an exemplary embodiment including a U-I core and two machined windings; 
         FIG. 9  illustrates three axially adjacent edge windings; 
         FIG. 10  illustrates a two-element double layer concentric edge winding; 
         FIG. 11  illustrates a four-element double layer edge winding with transposed wiring; and 
         FIG. 12  illustrates an embodiment including a U-I core and two edge windings. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Like reference numerals designate like elements throughout the specification. 
     The present invention relates generally to a thermally conductive vessel that houses a component, where the thermally conductive vessel has an interior cavity that accurately conforms to the shape of corresponding surfaces of the component.  FIGS. 1A-C  illustrate an exemplary embodiment in which the component is an inductor  100  including a winding  102  and a core  104 , and the thermally conductive vessel is a potting cup  106 .  FIG. 1A  illustrates the inductor  100  including the winding  102  and the core  104  prior to insertion into the potting cup  106 .  FIG. 1B  illustrates the potting cup  106  prior to the insertion of the inductor  100 .  FIG. 1C  illustrates an assembled unit  108  including the inductor  100  inserted into the potting cup  106 . 
     Those skilled in the art will comprehend that the invention is not limited to any particular element, and according to various embodiments the element is essentially any object that would benefit from a rapid transfer of thermal energy. For example, in some embodiments the element is a magnetic component such as a standard inductor  100 , as illustrated in  FIG. 1 , a common-mode inductor, or a saturating reactor. In other embodiments, the magnetic component is a standard transformer, a fly-back transformer, or a voltage-regulating transformer. In the embodiments described below, for brevity the element will be described as including a magnetic core  104  and one or more windings  102 . Furthermore, the thermally conductive vessel is not limited to a potting cup  106 , or any other particular structure or material. 
     As illustrated in the embodiment of  FIGS. 1A-C , for example, the thermally conductive vessel includes an interior cavity  110  that has a shape that is generally the inverse of the outer shape of the element to be inserted into the cavity  110 . With this shape, after insertion of the element into the thermally conductive vessel, a small space or gap  112  remains between the interior surface of the vessel and the adjacent surfaces of the winding and the core. In general, to promote the conductance of heat away from the element, it is desirable that this gap is as small as possible. That is, limited in part by differences in thermal expansion of the element and the vessel, in general, the more accurately the shape of the cavity  110  in the thermally conductive vessel matches the shape of the corresponding surface of the element, the better. 
     The gap  112  between the element and the thermally conductive vessel is generally filled with a thermally conductive material such as a resin or other potting material. This resin provides a high thermal conductivity path from the element, such as the core  104  and the winding  102 , to the thermally conductive vessel or potting cup  106 . For example, some embodiments include a resin with a thermal conductivity that exceeds 0.5 W/(m·K). 
     Another aspect of the resin is the improvement in the strength and other mechanical properties of the structure. However, due to potential differences in thermal expansion of the element and the vessel, a resin that is very rigid might result in a structural failure. Thus, some embodiments of the invention include a resin that can be strained by at least 5% without yielding. 
     Accordingly, in the embodiment illustrated in  FIG. 1 , low thermal impedance paths are established between the winding  102  and the potting cup  106 , and between the core  104  and the potting cup  106 . The potting cup  106  serves to provide heat transfer, mechanical support for the magnetic components, and also serves to protect both the winding  102  and core  104  from environmental damage such as shock, vibration, and moisture. 
     Several further embodiments enable efficient heat removal from the vessel. A first embodiment, illustrated in  FIG. 2  includes a flat surface  200 , such as a bottom surface of the vessel  202  such that efficient heat transfer can be achieved when this surface  200  is brought into forced contact with the surface of a cold-plate or heat sink  204 . 
     A second embodiment, illustrated in  FIG. 3 , includes external fins  300  on the vessel  302 , such that either free-convection or forced-air cooling is enhanced. Variations of this embodiment include any number from one or more fins  300 . Furthermore, the term “fins” is not intended to limit the shape or the structure in this embodiment, and generally refers to any shape or structure adapted to increase the surface area of the thermally conductive vessel. 
     A third embodiment, illustrated in  FIG. 4 , includes an internal cavity  400  in the thermally conductive vessel  402  for cooling with a circulating liquid coolant. In this embodiment, the liquid coolant is injected into the internal cavity  400  through an inlet and evacuated from the internal cavity  400  via an outlet (not illustrated). In a further embodiment, the internal cavity surfaces include fins  404  for further improving heat transfer. 
     When conventional windings are utilized in the embodiments illustrated in  FIGS. 1-4 , the efficiency of heat transfer out of the magnetic component (e.g., the inductor  100 ) may be less than desired. Further improvements to the overall heat transfer can be achieved in a magnetic component by configuring all elements of the winding for improved heat transfer to the winding surface. 
     A winding is a coil of conductive material, such as a metal, generally shaped as a circular helix. The helical shape functions to concentrate a magnetic field, generated by a current in the winding, through the center of the winding, and further, to increase the inductance of the winding material. Most conventional windings are formed of round or rectangular wire wound in multiple layers. 
     First, with a conventional winding, the topography of the outer surface of the winding is not smooth or accurately defined. Small variations in wire tension during a winding process and variations in insulation between layers can cause appreciable variations in the winding outer surface. Thus, for production designs utilizing a conventional winding, the gap between the winding surface and the corresponding surface of the potting cup must be made relatively large. This in turn reduces the heat transfer between the winding and the potting cup. Second, electrical insulation between layers of the winding further reduces heat transfer. In particular, the innermost layers of the winding utilize a heat flow path including all of the insulating layers external to those innermost layers. Third, achieving the winding termination is generally difficult, especially where the conductor cross section is large. 
     Thus, some embodiments of the present invention utilize an edge winding  500 , as illustrated in  FIG. 5 . Generally, an edge winding  500  is a rectangular conductor wound on its narrow edge. In one embodiment, an edge winding  500  includes a conductor having a generally uniform rectangular cross-section where the thickness dimension is less than the width dimension, and where the axis of bending is parallel to the thickness dimension. Such edge windings  500  can be made utilizing conventional coated copper, coated aluminum strip materials, or other suitable materials using appropriate winding machines. An edge winding  500  may provide an outer surface  502  that is a circular cylinder having generally uniform and precise dimensions. This enables efficient heat transfer when applied to a magnetic component having an edge winding  500  and a magnetic core  506 , incorporated into a vessel  504  having internal surfaces that accurately conform to the edge winding surfaces. 
     Still other embodiments utilize a different winding structure, which brings benefits to a system in which a relatively large conductor cross-section and a relatively small number of turns are desirable. According to these embodiments, an extruded tube  600  made of aluminum, copper, or another suitable material as illustrated in  FIG. 6  is machined to create both the machined winding  700  and the associated terminals  704 . For example, as detailed in Davis, U.S. Pat. No. 3,731,243, four cuts on an extruded tube  600  using properly angled, ganged, circular saws provides the required spiral slitting  702 , while conventional milling removes material adjacent the terminals  704 . The machined winding element  700  can be anodized or coated to provide needed electrical insulation. 
     A machined winding  700  as illustrated in  FIG. 7  enables a higher ratio of the width to the thickness of the conductor than possible with an edge winding  500 . Further, dimensional tolerances of the machined winding  700  can be made very precise. Manufacturing is made more flexible because windings  700  of various turn numbers and lengths can be made using the same tube stock. And the end terminals  704  of the machined winding  700  are generally of superior strength and rigidity. That is, because the termination  704  is an integrated part of the machined winding  700 , there is no need for separate terminals to be attached to the machined winding  700 . This adds reliability, while reducing manufacturing costs. 
     Moreover, unlike an edge winding  500 , because a machined winding  700  is constructed from an extruded tube  600 , the inner and outer surfaces are not constrained to be round, or any other shape. As illustrated in  FIG. 8 , this enables the profile of the machined winding  700  to conform to the shape of a magnetic core  104  having essentially any shape, thus reducing or eliminating dead space between the winding  700  and core  104  when non-circular cores are used, such as the rectangular-cross-section core  104 . 
     In some embodiments of the invention, a plurality of individual edge windings  500  or machined windings  700  are generally aligned in an axial direction, as illustrated in  FIG. 9 . This way, transformers with multiple primary and secondary windings can be structured using a rectangular conductor of uniform width and varying thicknesses. Some variations of these embodiments include transformers with at least partially interleaved primary and secondary windings, reducing the leakage inductance between any two windings. 
     In embodiments already discussed such as that illustrated in  FIG. 5 , where the edge winding  500  is a single layer, inter-layer insulation may be eliminated. Utilization of single-layer edge windings  500  may save cost, improve the packing factor, and eliminate the associated component of thermal resistance. The same principle applies to embodiments having a single-layer machined winding  700 . 
     However, other design considerations may result in the favorability of embodiments including concentric windings  1000 ,  1100  as illustrated in  FIGS. 10-11 . In these embodiments, a small space or gap  1006  remains between the inner winding  1002 ,  1102  and the outer winding  1004 ,  1104 . When the gap  1006  between respective windings is relatively small and is uniformly filled with a thermally conductive resin, good heat transfer can be achieved for all elements of the winding. Some embodiments further include spacers  1008  between the windings, for facilitating concentric alignment of the individual windings. 
     As already discussed, various embodiments of the invention include a magnetic core  104 , as conventionally utilized in a variety of applications known to those skilled in the art. Returning to  FIG. 5 , the magnetic core  104  may include stacked laminations  510 , which are electrically insulated from one another to reduce or prevent eddy currents, and may be made of steel, iron, or another suitable material. Alternatively, the core may be structured from a ferromagnetic powder, or a sintered ceramic material. 
     Embodiments of the invention can include E-E or E-I core elements  512   a ,  512   b , as illustrated in  FIG. 5 , both known to those skilled in the art. In these cases, windings  500  can be placed on just one prong  514 , or on two or three of the prongs (not illustrated). In particular, for three-phase applications, it often makes sense to use E cores where all three prongs  514  have the same dimensions and where windings  500  are applied to each prong  514 . All of these schemes can be structured with conventional windings, machined windings  700 , or edge windings  500  as illustrated in  FIG. 5 , where the potting cup  106  conforms to the winding  102  and core  104  surfaces. 
     Embodiments of the invention can include U-U or U-I cores, known to those skilled in the art, as illustrated in  FIG. 12 . These embodiments may include two winding elements  102 , such as the edge windings  500  illustrated. Such embodiments have a thermal advantage over an E-E or E-I embodiment as illustrated in  FIG. 5  because the effective heat flow section is proportionately larger than with the single winding design. Accordingly, the embodiment in  FIG. 12  can usually achieve higher power densities than the embodiment in  FIG. 5 . 
     In embodiments such as those illustrated in  FIG. 5  or  12 , or other embodiments, one or more gaps  508  (e.g., air gaps) may be provided in the core  506  to increase the reluctance of the magnetic circuit. Other embodiments utilize a core  506  made from a material having a relatively low permeability, likewise to increase the reluctance of the magnetic circuit. These reductions in reluctance can reduce the corresponding inductance of the magnetic device, resulting in an appreciable magnetizing current, desirable in certain applications as known to those skilled in the art. 
     As illustrated in  FIG. 11 , embodiments utilizing two or more sets of two or more concentric windings  1100  can enable improved electric coupling between axially adjacent windings and reduced proximity losses in high frequency applications provided appropriate winding transpositions are made. With the interconnections  1108  illustrated in  FIG. 11 , the flow of circulating currents between parallel connected branches is reduced or minimized. 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.