Abstract:
A power converter includes a housing, heat generating circuitry in the housing, and a heat conducting metal sheath on the outside of the housing. The housing has an elongated-box shape including two relatively smaller ends and four relatively larger sides and a heat-conducting metal sheath on the outside of the housing. The heat conducting sheath covers substantially all of the four relatively larger sides.

Description:
BACKGROUND 
     This invention relates to power conversion packaging. 
     U.S. Pat. No. 6,031,726, issued Feb. 29, 2000, and incorporated by reference, shows a packaging arrangement for a small, high-power-density power converter in which a primary-side converter piece and a secondary-side converter piece mate to form the converter. The two pieces house populated circuit boards in an encapsulant. Each of the circuit boards supports one winding of a main power conversion transformer, and a hole in the encapsulant passes through the center of the winding. After the two pieces are mated, two u-shaped core pieces are inserted through the holes to complete the transformer. At the bottom of the converter, lead frames carry signals and power from the encapsulated circuit boards&#39; terminals at the ends of the converter. A heat-sink sleeve slips over the assembly to complete the converter. The finished converter is mounted on an external circuit board with the terminals soldered to conductive runs on the surface of the board and a portion of the body lying in a rectangular hole in the circuit board for efficient use of space. 
     SUMMARY 
     In general, in one aspect, the invention features a power converter that includes a housing, heat generating circuitry in the housing, and a heat conducting metal sheath on the outside of the housing. The housing has an elongated-box shape including two relatively smaller ends and four relatively larger sides, and a heat-conducting metal sheath on the outside of the housing, The heat conducting sheath covers substantially all of the four relatively larger sides. 
     Implementations of the invention may include one or more of the following features. The metal sheath may be a longitudinally extruded piece that includes fins that run along the length of the box shape. The metal sheath may include two pieces, one of the pieces covering substantially all of three of the relatively larger sides, the other of the pieces covering substantially all of the fourth of the relatively larger sides. At least a portion of the sheath may include a solderable metal surface. Terminals may project from at least one of the smaller ends and be configured to be soldered to conductive runs on a surface of a printed circuit board. The heat conducting metal sheath may have a solderable surface configured and oriented to be soldered to the surface of the printed circuit board when the terminals are soldered to the conductive runs. The other of the pieces may form one of the four relatively larger sides of the housing. The one piece may include three separate sub-pieces that are respectively mounted on each of three of the larger sides. 
     In general, in another aspect, the invention features an assembly that includes a circuit board having an aperture and a power converter in the shape of an elongated box. The power converter is mounted on the circuit board with a portion of the power converter lying in the aperture. The power converter has a heat sink that surrounds four longer sides of the elongated box and terminals that project from ends of the power converter and are soldered to conductive runs on the surface of the circuit board. There are heat conductive connections between portions of the heat sink and the circuit board. 
     In general, in another aspect of the invention, a series of power converters are made on a manufacturing line, each of the power converters including a heat sink selected from among a set of different kinds of heat sinks and a core converter. The manufacturing line is configured to enable lot-of-one mixing of power converters in which each power converter in the series may have a different one of the available heat sinks. 
     Implementations of the invention may include one or more of the following features. The different kinds of heat sinks may differ at least in their capacities for sinking heat, or in their sizes. The core converters of all of the power converters may be the same. The core converters of at least some of the power converters may be selected from among different available kinds of converters. 
     In general, in another aspect the invention features a power converter having a box-shaped housing having two ends and four sides and heat-generating electronic circuitry held in the housing. Heat-conducting metal shields span substantially all of two opposing sides of the housing. A mounting mechanism attaches the converter to a circuit board in a position in which one of the heat conducting metal shields lies to one side of the circuit board and the other metal shield lies to the other side of the circuit board. 
     In general, in another aspect, the invention features a method comprising providing an external heat sinking surface on a power converter, and connecting the heat sinking surface to a substrate external to the power converter. 
     Implementations of the invention may include one or more of the following features. The substrate may include a printed circuit board. The heat sinking surface may be connected to the substrate by soldering or by thermally conductive adhesive. The heat sink may be plated with nickel. The substrate may include a printed circuit board having a conductive pattern on its surface and the heat sinking surface may be soldered to the conductive pattern. 
     Other advantages and features of the invention will become apparent from the following description and from the claims. 
    
    
     DESCRIPTION 
     FIG. 1 is a perspective view of a converter mounted in a partially broken-away circuit board. 
     FIG. 2 is an exploded view of the converter of FIG.  1 . 
     FIG. 3 is an exploded view of a converter. 
     FIG. 4 is a perspective view of a converter mounted in a partially broken-away circuit board. 
     FIG. 5 shows cross-sectional views of heatsinks. 
     FIGS. 6A and 6B show a converter being mounted into an aperture in a circuit board. 
    
    
     As shown in FIGS. 1 and 2, in an improvement of the converter shown in U.S. Pat. No. 6,031,726, the heat sink  10  is a long extruded piece of aluminum that is plated with nickel after extrusion. The heat sink has two sides  12 ,  14  and a bridge  16  that meet to form a unshaped extrusion. Each side  12 ,  14  has two fins  18 ,  20 , and  22 ,  24  that run along the length of the extrusion. The bridge  16  has two fins  26 ,  28 , each of which has two smaller fins that project upwardly. Together the fins increase the dissipation of heat from the heat sink into the ambient air. 
     The bottom of the converter is an extruded aluminum lid  30  that extends the length of the converter. The lid has two side flaps  32 ,  34 , that meet the lower edges  36 ,  38  of the sides of the heat sink. Thus, substantially all of the four relatively longer side walls  40 ,  41 ,  42 ,  44  of the converter are sheathed in metal that sinks heat dissipated by the internal components of the converter. The heat sink is attached to the converter by soldering the inner surface of the nickel plated heatsink directly to metallized heat generating regions which are exposed at the surfaces of the side walls  42 ,  44  of the converter. For example, the primary-side piece  52  of the converter includes a power semiconductor (e.g., a MOSFET) which is mounted to one side of a ceramic substrate. The other side of the substrate is clad with copper and exposed (shown as region  49 ) on side wall  44 . The inner surface of the heat sink is soldered to region  49  as well as to other exposed, metallized, surfaces on the opposite side wall  42 . The lid is attached to the converter by means of thermally conductive epoxy which enhances the conduction of heat to the metal sheath formed by the heat sink and the lid. 
     On each end of the converter, electrical terminals  43  are held in two insert-molded terminal mounting blocks  46 ,  48 . The terminals project from the ends of the converter and have two folds  45 ,  47  terminating in connection surfaces  50 . The connection surfaces  50  are configured to be soldered to circuit runs  57  on the printed circuit board  90  on which the converter is mounted. The lower edges  36 ,  38  of the sides of the heat sink lie at the same level as the connection surfaces. Because the heat sink is plated with nickel, the lower edges of the heat sink can be soldered to conductive metal regions on the surface of the board. This enables heat to be more easily conducted from the heat sink to the circuit board, enhancing the overall flow of heat away from the converter. 
     The opposite ends of the terminals  51 ,  53  from the connection surfaces extend beneath the mated primary-side piece  52  and the secondary-side piece  54  of the converter and are configured to make contact with (and be soldered to) the connection pads  55 ,  56  that lie on projecting portions of the encapsulated circuit boards that are housed in the two mated pieces  52 ,  54 . Each of the two mating pieces has a hole  156 ,  158  which passes through windings on the printed circuit boards that are encapsulated in the pieces. Two u-shaped core pieces  60 ,  62  are inserted through the holes to form the main transformer of the converter. 
     One of the molded terminal blocks has a key  70  between two of the terminals. The key mates with a slot  72  in the rectangular hole of a circuit board through which the converter is mounted to prevent inadvertent mounting in the wrong direction. 
     FIGS. 6A and 6B show a converter  10  being mounted into an aperture  11  in a circuit board  90 . The tolerance in the positioning of the terminations  43  is no more than few thousandths of an inch relative to their nominal positions. Pads  57  on the circuit board can be located relative to the aperture  11  with similar accuracy. If the length and width of the aperture (the width of the aperture is marked X in FIG. 6A) are controlled to closely conform to the length and width of the body of the converter (the width of the body is marked Y in FIG.  6 A), the terminations  43  will be properly aligned relative to the pads  57  when the converter is installed into the aperture. 
     The heat sink need not be made from a single extrusion but can be formed of two or three pieces that are fastened to the outer walls of the converter, as shown in FIG.  3 . In the figure, the top piece  66  and two side pieces  64 ,  66  cover three sides of the converter and the lid  30  covers the fourth. One advantage of this arrangement is that the width of the gaps between the inner walls of the metal sides  86 ,  88  and the outer walls of the converter  40 ,  42  in the converter of FIG. 3 are more easily controlled than those in the converter of FIGS. 1 and 2, in which a single unshaped extrusion is used. Like the embodiment of FIG. 1, substantially all of the four long walls of the converter of FIG. 3 are sheathed in metal. 
     The number and configuration of the fins on the heat sink may be different than the ones shown in FIG.  1 . For example, FIG. 5 shows cross-sectional views of five heat sink designs  10   a - 10   e  with differing fin heights. In the Figure, dimension H is 0.35″, and dimensions A, B, C, D and E are, respectively, 1.075″, 0.875″, 0.725″, 0.600″ and 0.500″. Each heat sink has progressively shorter fins with the last example, in FIG. 10 e,  having no fins at all. Likewise, the thickness and pitch of the fins can be modified to provide desired thermal performance. In general, longer fins and increased fin density result in reduced thermal resistance. 
     The heat sink configuration, and hence the overall volume occupied by the converter, in a particular application depends upon the temperature and flow rate of the cooling airstream, the amount of heat dissipated by the converter and the converter maximum operating temperature. For example, for a given set of operating and environmental conditions, the fin height required in a 1200 LFM air stream, and thus the overall volume occupied by the converter, will be lower than that required in a 300 LFM air stream. By providing a variety of heat sinks  10  (e.g., heat sinks  10   a - 10   e,  FIG. 5) for use with a common core converter design configuration (e.g., the combination of primary and secondary-side pieces  52 ,  54 , mounting blocks  46 ,  48  and cores  60 ,  62 ) a customer can specify the smallest heat sink that provides the required thermal performance in a particular application, thereby minimizing wasted space. By providing supplies of different heat sinks on a manufacturing line, many identical (or even different) core converters can be mated in real time with different heat sinks, in varying size lots down to a “lot-of-one”, to satisfy a variety of different customer applications. 
     The heat sink need not cover all four long walls of the converter. For example, in FIG. 4, the lid  30  and the top plate  76  are made of aluminum and the side panels  80 ,  82  are made of non-metallic material (e.g., the material used to encapsulate the primary and secondary printed circuit boards or a plastic cover). Providing metal heat spreaders e.g. lid  30 , top plate  76 , in the air streams which flow over both the top and the bottom surfaces of the printed circuit board  90  is a more effective way of removing heat than just having a heat spreader in the air stream on one side of the printed circuit board. 
     Other embodiments are within the scope of the following claims. 
     The heat sink and lid need not be made from extrusions. For example, the heat sink could be manufactured by connecting metal fins to a metal base by means of soldering, brazing or use of adhesive (e.g., thermally conductive epoxy). Materials other than aluminum, such as copper or copper alloys, may be used to fabricate the heat sinks and lid and the heat sink, or the parts used to manufacture the heat sink, may be plated with materials other than nickel. Adhesive, epoxy or silicone material may be used to assemble the heat sink to the converter. The lid may be soldered to the heat sink.