Abstract:
A power electronics convertor is disclosed which includes a hermetically sealed housing, a dielectric cooling fluid contained in the housing, and a bank of capacitive elements disposed within the housing and submerged in the cooling fluid, wherein each capacitive element is wound about a hollow aluminum core forming an open interior region within the center of the capacitive element that is dimensioned and configured to optimize heat transfer between the capacitive element and the cooling fluid and reduce thermal stress in the capacitor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The subject invention relates to bulk capacitors, and more particularly, to an immersion cooled bulk film capacitor for use in aerospace power electronics converter equipment. 
         [0003]    2. Description of Related Art 
         [0004]    Power conversion equipment requires the rapid switching of large currents by power electronic semiconductor devices. Typically, in power convertor circuits, reactive current is provided by a bank of bulk capacitors, as disclosed for example in U.S. Pat. No. 5,729,450 to Dimino et al., the disclosure of which is herein incorporated by reference in its entirety. Bulk film capacitors are an important aspect in any aerospace power electronic converter. 
         [0005]    Often these capacitors are sized by their current rating and thus become thermally limited due to excessive heating and internal hot spots. Unless, properly designed and controlled, high rates of heat generation can result in high operating temperatures for electronic equipment. This will jeopardize its safety and reliability. Therefore, thermal control has become increasingly important in the design and operation of power conversion equipment. 
         [0006]    Several cooling techniques are commonly used in electronic equipment. These include conduction cooling, natural convection and radiation cooling, forced-air cooling, liquid cooling and immersion cooling. The immersion cooling technique is the most reliable, since it does not involve any penetration into the electronic enclosure and the component resides in a completely sealed liquid environment. The subject application, employs the technique of immersion cooling to optimize heat transfer and reduce thermal stress in a uniquely constructed bulk film capacitor. 
       SUMMARY OF THE INVENTION 
       [0007]    The subject invention is directed to a power electronics converter and more particularly to a new and useful immersion cooled capacitor for use in power electronics converters. The power electronics convertor includes a housing, a cooling liquid contained within the housing, and at least one capacitive element disposed within the housing and submerged in the cooling liquid. 
         [0008]    Preferably, there is a bank of capacitive elements disposed within the housing and submerged in the cooling liquid. Each capacitive element in the bank of capacitive elements has an open interior region configured to optimize heat transfer between the capacitive element and the cooling liquid and reduce thermal stress in the capacitor. 
         [0009]    The housing of the power electronics convertor is hermetically sealed and the cooling liquid is a non-conductive liquid, such as for example, a fluorocarbon fluid. Each capacitive element is wound about a thermally conductive hollow core forming the open interior region of the capacitive element. In one aspect of the invention, the hollow core of the capacitive element has a smooth interior surface finish. Alternatively, the hollow core of the capacitive element has a roughened interior surface finish to help initiate nucleated boiling of the cooling fluid. 
         [0010]    The capacitive element may be an inductively wound element or a non-inductively wound element. In either case, the capacitive element is configured with two alternating layers of metallized foil and two alternating layers of insulative film, wherein one foil layer provides a positive terminal for the capacitor and the other foil layer provides a negative terminal for the capacitor. Those skilled in the art should readily appreciate that although the two terminals are referred to herein as positive and negative, bulk film capacitors are typically not polarized. 
         [0011]    In the case of the non-inductively wound element, end portions of one foil layer extend from one end of the capacitive element and end portions of the other foil layer extend from an opposed end of the capacitive element. Preferably, the end portions of the respective foil layers are joined to one another by an end connection, and a terminal tab is provided at each end connection, whereby one end terminal defines a positive terminal for the capacitor and the other terminal defines a negative terminal for the capacitor. 
         [0012]    These and other features of the immersion cooled capacitor of the subject invention and the manner in which it is employed within a power electronics convertor will become more readily apparent to those having ordinary skill in the art from the following enabling description of the preferred embodiments of the subject invention taken in conjunction with the several drawings described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    So that those skilled in the art to which the subject invention appertains will readily understand how to make and use the immersion cooled capacitor of the subject invention without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein: 
           [0014]      FIG. 1  is a perspective view of a power electronics convertor constructed in accordance with a preferred embodiment of the subject invention, with the wall of the hermetically sealed housing cut away to show a bank of immersion cooled capacitors submerged in a dielectric cooling liquid contained within the housing; 
           [0015]      FIG. 2  is a perspective view of an immersion cooled capacitor constructed in accordance with a preferred embodiment of the subject invention, which includes an open interior region that allows the dielectric cooling liquid to flow therethrough; 
           [0016]      FIG. 3  is a cross-sectional view of the immersion cooled capacitor disposed within the housing of the power electronics convertor, taken along line  3 - 3  of  FIG. 1 ; 
           [0017]      FIG. 4  is an illustration of the alternating foil and film layers of an inductively wound capacitor constructed in accordance with a preferred embodiment of the subject invention; and 
           [0018]      FIG. 5  is an illustration of the alternating foil and film layers of a non-inductively wound capacitor constructed in accordance with a preferred embodiment of the subject invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    Referring now to the drawings, wherein like reference numerals identify similar structural features or aspects of the subject invention, there is illustrated in  FIG. 1  a power electronics converter  100  containing, among other things, a bank of immersion cooled bulk film capacitors constructed in accordance with a preferred embodiment of the subject invention and each one designated generally by reference numeral  10 . Those skilled in the art will readily appreciate that the power electronics convertor  100  will also include a number of switches, magnets and other components, all of which generate a substantial amount of heat, affecting the performance reliability of the convertor. 
         [0020]    Referring to  FIG. 1 , the power electronics convertor  100  includes a hermetically sealed housing  112  containing a cooling liquid  114  and having external cooling fins  116 . The cooling liquid  114  is a non-conductive liquid, such as for example, a fluorocarbon fluid. A preferred dielectric cooling fluid is 3M&#39;s Fluorinert™ FC-72, which is thermally and chemically stable, compatible with sensitive materials and non-flammable. Key properties of Fluorinert™ FC-72 include: a typical boiling point of 56° C.; a dielectric strength of 38 KV (0.01 inch gap); and a dielectric constant (1 KHz) of 1.76. 
         [0021]    Referring to  FIG. 2 , each of the immersion cooled capacitors  10  within the bank of capacitors define a single capacitive cell with positive and negative terminals  12 ,  14 . Those skilled in the art should readily appreciate that although the two terminals  12 ,  14  are referred to herein as positive and negative, bulk film capacitors are typically not polarized. 
         [0022]    As best seen in  FIG. 3 , each capacitor  10  includes a capacitive element  16  having an open interior region  15  that is dimensioned and configured to optimize heat transfer between the capacitive element  16  and the cooling liquid  114 . The open interior region  15  allows the dielectric cooling liquid  114  to move through the capacitive element  16  and initiate nucleated boiling at the center of the component within the sealed housing  112 , where a significant amount of heat is generated. 
         [0023]    The capacitive element  16  is cooled through nucleated boiling of the cooling liquid  114  and will have a heat transfer coefficient of about 15,000 W/m 2 K, as compared to 5 W/m 2 K with natural convection air. In operation, heat dissipated into the cooling liquid  114  from the capacitive element  16  is transferred to the walls of the housing  112 , and then to the external fins  116 . That heat can then be readily removed by external cooling means, such as, for example, by a cooling fan. 
         [0024]    With continuing reference to  FIGS. 2 and 3 , each capacitive element  16  consists of alternating layers of metallized foil and insulative film that are spirally wound about a thermally conductive hollow tubular core  18 , as described in more detail below with reference to  FIGS. 4 and 5 . The hollow tubular core  18  forms the open interior region of the capacitive element  16 . Preferably, the hollow core  18  is made from aluminum, although any thermally conductive material can be used. Those skilled in the art will readily appreciate that the core of a wound capacitor is typically formed by a solid post, as disclosed for example in U.S. Pat. No. 6,620,366 to Sagal. 
         [0025]    The high heat transfer coefficient of 3M&#39;s Fluorinert™ FC-72 and its ability to move through very small passages leads to the need for only a small diameter passage through the hollow core  18  of the capacitor  16 . This small core passage of core member  18  effectively enables the removal of heat from the capacitor element  16  and maintains the volumetric density thereof. The size of the bore hole of core member  18  can vary depending upon the application, but it is typically small in diameter relative to the overall diameter of the capacitor  16 . 
         [0026]    In one embodiment of the subject invention, the hollow core  18  has a smooth interior surface finish. In another embodiment of the subject invention, the hollow core  18  has a roughened interior surface finish to help initiate nucleated boiling of the cooling liquid  114  as it passes through the central passage of the hollow core  18 . 
         [0027]    The capacitive element  16  can be formed as an inductively wound element as illustrated in  FIG. 4 , or a non-inductively wound element as illustrated in  FIG. 5 . In either case, one of the foil layers provides a positive terminal for the capacitor  10  and the other foil layer provides a negative terminal for the capacitor  10 . 
         [0028]    In the case of the inductively wound element shown in  FIG. 4 , the alternating layers include two metallic foil layers  22 ,  24  and two insulating film layers  26 ,  28 . Flying leads or tabs (not shown) are inserted between the film/foil layers to create the positive and negative terminals of the capacitor  10 . 
         [0029]    In the case of the non-inductively wound element shown in  FIG. 4 , end portions of foil layer  22  extend from one end of the capacitive element  16  and end portion of the other foil layer  24  extend from the opposed end of the capacitive element  16 . The end portions of the respective foil layers  22 ,  24  are joined to one another by end connections  34 ,  36 . 
         [0030]    Preferably, the end connections  34 ,  36  is formed by applying a conductive spray. For this application the hollow core  18  is first covered and then the spray is applied. After the spray has been applied, the coolant channel of the core  18  is uncovered. Thereafter, terminal tabs (now shown) are inserted into the end connections  34 ,  36  on either side of the capacitive element  16 . 
         [0031]    Those skilled in the art will readily appreciate that there is no need to isolate the hollow aluminum core  18 , as typically the first layer in the stack is an insulative film layer. Nevertheless, if the first layer in the stack was a foil layer, it would not affect the capacitor since the hollow aluminum core  18  would only make contact with a single foil layer. 
         [0032]    While the immersion cooled capacitor  10  of the subject invention has been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that various changes and/or modifications may be made thereto without departing from the spirit and scope of the subject invention as defined by the appended claims.