Patent Abstract:
A semiconductor assembly includes a stack with a semiconductor module and a cooler, wherein the semiconductor module is provided in contact with the cooler. A clamping assembly is adapted to exert a force on the two sides of the stack. The stack is provided with a through hole between the two sides thereof and a part of the clamping assembly including an electrically conductive part which extends through the through hole of the stack. Thereby, a compact mechanical arrangement is provided while obtaining improved electrical properties, such as lower inductance and more even current distribution.

Full Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to semiconductor assemblies and more particularly to a semiconductor assembly comprising a press pack module. 
       BACKGROUND ART 
       [0002]    Many different types of electrical converters make use of press pack modules for their easiness to bring them into series connection with the help of a mechanical stack, where more than one module is provided in series. Press pack modules are made to be mounted in between cooler or pressure plates and pressure is applied to that stack to ensure proper electrical and thermal contact between the individual press pack modules. For HVDC converters, up to 20 modules can be put in series connection into one stack and more than a hundred stacks can be needed for the complete converter. This means that a large number of heavy and sometime expensive mechanical parts are needed to create those stacks. 
         [0003]    An example of a prior art press pack stack is disclosed in  FIG. 1 . As seen in this figure, a standard stack comprises two or more rods, equally spread around the stack. Two yokes are provided, one at each end of the stack, to enclose the stack. Furthermore, a spring packet is provided on the top of the stack to provide a pressure thereon. In some press stack packs this spring packet is omitted and instead special yokes are provided which allow the use of their inherent mechanical elasticity as spring force. 
         [0004]    Another example of a press pack stack is disclosed in the U.S. patent application publication US2010/0133676 A1. 
       SUMMARY 
       [0005]    An object of the present invention is to provide a press pack stack with a simplified mechanical design. 
         [0006]    According to the invention, there is provided a semiconductor assembly comprising a stack comprising a semiconductor module and a cooler, wherein the semiconductor module is provided in contact with the cooler, the stack having a first side and a second side; a clamping assembly being adapted to exert a force on the stack; the semiconductor assembly being characterised in that the stack is provided with a through hole between the first side and the second side and that part of the clamping assembly extends through the through hole of the stack, wherein the part of the clamping assembly extending through the through hole of the stack comprises an electrically conductive part configured to conduct electricity. Thereby, a compact mechanical arrangement is provided while obtaining improved electrical properties, such as lower inductance and more even current distribution. 
         [0007]    In an embodiment, a single bus bar is provided. Thereby, the number of parts and thereby the weight are kept to a minimum. 
         [0008]    In an embodiment, an insulation element is provided to electrically insulate the electrically conductive bus bar from the semiconductor module. In this way, an end portion of the stack can be contacted by means of the bus bar. 
         [0009]    In an embodiment, the through hole is provided at the centre of the stack. This ensures even distribution of the forces exerted on the stack and reduces the mechanical parts for stacking, in turn reducing the complexity and the costs of the assembly. 
         [0010]    In an embodiment, the semiconductor module is generally flat and has a first planar side and a second, opposite, planar side. This enables a compact design with reliable electrical and thermal connection to adjacent coolers. This is particularly advantageous when the first and second planar sides function as module power connections. 
         [0011]    In an embodiment, the cooler is arranged such that substantially the entire area of a side of the semiconductor module ( 20   a ) is in contact with the cooler. Good electrical and thermal connections across the entire area of the semiconductor module sides are thereby ensured. 
         [0012]    In an embodiment, the cooler is electrically conductive, so that it can be used for connecting to the semiconductor module. 
         [0013]    In an embodiment, the semiconductor module and the cooler are circular. This provides a design with a small footprint and also improves the homogeneity of the electromagnetic coupling in the module. 
         [0014]    In an embodiment, the clamping assembly comprises a first clamping element adapted to exert a force on the first side of the stack and a second clamping element adapted to exert a force on the second side of the stack. In this way, the semiconductor assembly can be provided as a separate part for subsequent connection to a piece of electrical equipment. 
         [0015]    In an embodiment, the semiconductor assembly comprises a piece of electric equipment, preferably a capacitor, wherein the bus bar at a first end portion is electrically connected to the first side of the stack and at a second end portion is electrically and mechanically connected to a first pole of the piece of electric equipment. By integrating the clamping assembly into the piece of electrical equipment, a compact design is achieved and the lengths of the electrically conductive paths are kept to a minimum, improving the electrical properties of the assembly. 
         [0016]    In an embodiment, the stack comprises a plurality of semiconductor modules and a plurality of coolers, wherein each of the semiconductor modules is provided between two of the coolers. In this way, a large number of semiconductor modules can be included in one stack. 
         [0017]    In an embodiment, the stack comprises two semiconductor modules and three coolers, wherein a connection is preferably provided for electrical connection to a central, second cooler of the three coolers. The stack then lends itself to operating as a phase leg in an inverter, for example. In this case, the semiconductor modules comprise high voltage semiconductors, such as Isolated Gate Bipolar Transistors. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]    The invention is now described, by way of example, with reference to the accompanying drawings, in which: 
           [0019]      FIG. 1  shows a prior art press pack stack, 
           [0020]      FIG. 2  shows a sectional view of a semiconductor assembly in the form of a simple press pack stack according to the invention, 
           [0021]      FIG. 3  shows a cross-sectional top view of a semiconductor module comprised in the semiconductor assembly of  FIG. 2 , 
           [0022]      FIG. 4  shows a side view of a second embodiment of a semiconductor assembly in the form of a press pack stack according to the invention, 
           [0023]      FIG. 5  shows a side view, partially in section, of a third embodiment of a semiconductor assembly in the form of a press pack stack according to the invention, 
           [0024]      FIG. 6  shows a sectional view of a fourth embodiment of a semiconductor assembly in the form of a press pack stack according to the invention mounted to a piece of electrical equipment in the form of a capacitor, 
           [0025]      FIG. 7  shows an exploded perspective view of the capacitor of  FIG. 6 , and 
           [0026]      FIG. 8  shows the capacitor of  FIG. 7  after assembly. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    In the following, a detailed description of preferred embodiments of a semiconductor assembly according to the invention will be given. 
         [0028]      FIG. 1  has been discussed in the background art section and will not be further dealt with herein. 
         [0029]      FIG. 2  shows a sectional view of a semiconductor assembly according to the invention, generally designated  10 . This semiconductor assembly is a simple form of a press pack stack comprising just one semiconductor module  20   a  and a cooler  30   a  abutting each other, i.e., being in direct physical contact or engagement with each other. This semiconductor module  20   a  is generally flat and has a first planar side  22  and a second, opposite, planar side  24 . Referring to  FIG. 3 , showing a cross-sectional view of the circular semiconductor module  20   a,  a central through hole  26  is provided in the semiconductor module between the first and second sides  22 ,  24 . A plurality of semiconductors  28 , such as Isolated Gate Bipolar Transistors (IGBTs) are provided in the module and are electrically connected to the first and second planar sides of the semiconductor module  22 ,  24 , and these sides therefore function as module power connections. 
         [0030]    Again referring to  FIG. 2 , a cooler  30   a  with a first planar side  32  and a second planar side  34  opposite to the first planar and abutting or engaging the first side of the semiconductor module  20   a.  The cooler  30   a  is provided with a central through hole  36  between the opposite planar sides, which hole is aligned with the through hole  26  of the semiconductor module  20   a.  The cooler has good electrical and thermal conductivity and it is thus preferred that the cooler is made of a material with good electrical conductivity, such as copper or aluminium, and that channels for coolant (not shown), such as water, are provided inside the cooler. The cooler  30   a  is circular with a diameter equal to or exceeding the diameter of the semiconductor module  20   a,  in order to ensure electrical and thermal conduction between the semiconductor module  20   a  and the cooler across  30   a  the entire first side  22  of the semiconductor module  20   a.    
         [0031]    The semiconductor module  20   a  and the cooler  30   a  together form a stack, generally designated  38 . 
         [0032]    A clamping assembly, generally designated  40 , is adapted to exert a force F on the stack  38 , in this case on the first side  32  of cooler  30   a  in the direction towards the semiconductor module  20   a,  whereby the cooler  30   a  acts as a yoke. Thus, the clamping assembly  40  extends through the through hole  26  of the semiconductor module  20   a  and the through hole  36  of the cooler  30   a.  The part extending through the through holes  26 ,  36  is an electrically conductive bus bar  42 , which has a first, threaded, end portion  42   a,  on which a clamping element in the form of a nut  44   a  is threaded, and a second end portion  42   b  opposite to the first end portion  42   a.  The electrically conductive bus bar may be an aluminium rod or alternatively a central steel rod provided with a jacket of aluminium. Although this embodiment comprises an electrically conductive part in the form of a bus bar, the electrically conductive part extending through the through hole may be embodied in many different forms, such as a bar or a strip, which conducts electricity within an inverter, a substation, a battery bank or any other electrical apparatus. 
         [0033]    It is assumed that the semiconductor module  20   a  rests with its second side  24  on a fixed plane, shown with dashed line in the figure. This fixed plane may be another cooler, an additional yoke, or the surface of a piece of electric equipment, as will be described below. Thus, when the nut  44   a  is tightened, a downward directed force F will be exerted on the yoke or cooler  30   a.  Since the yoke or cooler  30   a  is rigid, this force will be transmitted onto the semiconductor module  20   a.  Since the semiconductor module  20   a  cannot give way due to the fixed plane on which the second side  24  thereof abuts, the semiconductor module  20   a  and the cooler  30   a  will be pressed into tight contact with each other, ensuring good electric and thermal conductivity there between. 
         [0034]    A second embodiment of a semiconductor assembly will now be described with reference to  FIG. 4 . In this embodiment, a plurality, namely four semiconductor modules  20   a ,  20   b ,  20   c ,  20   d , are provided in a stack. Correspondingly, a plurality of coolers, namely five coolers  30   a ,  30   b ,  30   c ,  30   d ,  30   e,  are provided in such a way that each of the semiconductor modules  20   a - d  is provided between two of the coolers  30   a - e . This means that the semiconductor modules  20   a - d  and the coolers  30   a - e  are arranged alternately in a stack  38  having a first upper end as shown in the figure and a second, opposite lower end. 
         [0035]    A clamping assembly  40  comprises a bar  42  with a first upper threaded end portion  42   a  and a second lower threaded end portion  42   b.  A first clamping element in the form of a first nut  44   a  is threaded onto the first end portion  42   a  of the bar and a second clamping element in the form of a second nut  44   b  is threaded onto the second end portion  42   b  of the bar. 
         [0036]    A spring package  46  is arranged between the first nut  44   a  and the first, uppermost cooler  30   a,  as shown in the figure. The spring package is shown as a being built up from a plurality of cup springs but it will be realized that other forms of springs may be provided. 
         [0037]    A first yoke insulation element  48   a  is provided between the spring package  46  and the first cooler  30   a  so as to provide electrical insulation there between. Correspondingly, a second yoke insulation element is provided between the second nut  44   b  and the lowermost cooler  30   e  so as to provide electrical insulation there between. 
         [0038]    Connections (not shown in the figure) are provided for electrical connection to the semiconductor modules  20   a - d  and optionally to one or more of the coolers  30   a - e.    
         [0039]    The semiconductor assembly  10  is in this embodiment held together by means of the clamping assembly  40 , without relying on any fixed plane as in the embodiment shown in  FIG. 2 . This means that this semiconductor assembly may be releasably mounted to a piece of electrical equipment, such as a capacitor or a transformer. 
         [0040]    A third embodiment of a semiconductor assembly will now be described with reference to  FIG. 5 . In this embodiment, a plurality, namely two semiconductor modules  20   a,    20   b,  are provided in a stack  38 . Correspondingly, a plurality of coolers, namely three coolers  30   a,    30   b,    30   c,  are provided in such a way, that each of the semiconductor modules  20   a,    20   b  is provided between two of the coolers  30   a - c . This means that also in this embodiment the semiconductor modules  20   a ,  20   b,  and the coolers  30   a - c  are arranged alternately in a stack  38  having a first upper end as shown in the figure and a second, opposite lower end. 
         [0041]    A clamping assembly  40  comprises a bar  42  with an upper threaded end portion  42   a.  The lower end portion  42   b  of the bar  42  is attached to a piece of electrical equipment, such as a capacitor. A clamping element in the form of a nut  44   a  is threaded onto the upper end portion  42   a  of the bar. 
         [0042]    A spring package  46 , similar to the one shown in  FIG. 4 , is arranged between the first nut  44   a  and the first, uppermost cooler  30   a,  as shown in the figure. The spring package  46  exerts a force directly on the first, uppermost cooler  30   a,  which in this embodiment functions as a yoke. The yoke insulating elements shown in  FIG. 4  are in this embodiment omitted. 
         [0043]    In this embodiment, where the stack of coolers and semiconductor modules is shown in section, an insulation element  50  is provided to electrically insulate the central bar  42  from the semiconductor modules  20   a,    20   b.  The insulation element  50  also electrically insulates a negative DC connection or terminal “−DC” and a positive DC connection or terminal “+DC” from each other. However, the insulation element  50  does not extend all the way up to the spring package  46  which means that the first, uppermost cooler  30   a  is in electrical connection with the bar  42 . This means that since the bar  42 , in this case acting as a bus bar and being in electrical connection with the negative DC connection, is in direct electrical connection with the first, uppermost cooler  30   a,  also this first cooler will take the same electrical potential, i.e., “−DC”. 
         [0044]    An electrically insulating means  62  is provided between the negative DC connection or terminal “−DC” and the positive DC connection or terminal “+DC” to electrically separate these from each other. 
         [0045]    Connections in the form of the uppermost and lowermost coolers  30   a ,  30   c  are provided for electrical connection to the semiconductor modules  20   a ,  20   b.    
         [0046]    A phase connection is provided for electrical connection to the central, second cooler  30   b,  which, being electrically insulated from the bus bar  42 , takes the electrical potential “−DC” or “DC”, in dependence of the operation of the semiconductor modules  20   a,    20   b.  In this embodiment, the semiconductor assembly  10  is therefore suitable for providing the phase voltage of a converter, such as an HVDC converter. 
         [0047]    A fourth embodiment of a semiconductor assembly will now be described with reference to  FIGS. 6-8 . In this embodiment, a plurality, namely two semiconductor modules  20   a,    20   b  are provided in a stack  38 . Correspondingly, a plurality of coolers, namely three coolers  30   a,    30   b,    30   c  are provided in such a way, that each of the semiconductor modules  20   a,    20   b  is provided between two of the coolers  30   a - c . This means that also in this embodiment the semiconductor modules  20   a,    20   b  and the coolers  30   a - c  are arranged alternately in a stack  38  having a first upper end as shown in the figure and a second, opposite lower end. 
         [0048]    A clamping assembly  40  comprises a bar  42  with an upper threaded end portion  42   a.  A clamping element in the form of a nut  44   a  is threaded onto the upper end portion  42   a  of the bar  42 , acting as a bus bar. The lower end portion  42   b  of the bar  42  is attached to a piece of electrical equipment in the form of a capacitor  60 , preferably by means of a screw joint, wherein the capacitor is provided with a screw thread electrically connected to one of the electrical connections. 
         [0049]    A spring package  46  in the form of a cup spring is arranged between the nut  44   a  and the first, uppermost cooler  30   a,  as shown in the figure. The spring package  46  exerts a force on the first, uppermost cooler  30   a  through a dedicated yoke  49 , i.e., a yoke not functioning as a cooler, and a yoke insulation element  48   a.    
         [0050]    The bus bar  42  is provided with a bus bar insulation  50  to electrically insulate the bus bar  42  from the semiconductor modules  20   a,    20   b.    
         [0051]    The design of the capacitor  60  will now be discussed. The front side  60   a  of the capacitor casing exhibits a plurality of terminals or poles, a first set labelled “−DC” and referring to a first pole of the capacitor, and a second set labelled “+DC” and referring to a second pole of the capacitor. The first set of terminals is usually at the same electrical potential as the casing of the capacitor  60 . 
         [0052]    An insulating sheet in the form of a lamination  62 , see particularly  FIGS. 7 and 8 , is provided on the front side  60   a  of the capacitor  60  and has a size which almost covers this front side  60   a.  First openings  62   a  are provided in the lamination  62  for the second set of terminals “+DC” and a second, central opening  62   b  is provided for the bus bar  42 . The lamination  62  is made of an electrically insulating material. 
         [0053]    In front of, i.e., outside of the lamination  62 , there is provided a conductive sheet  64  of electrically conductive material, such as sheet metal. First openings  64   a  are provided in the conductive sheet  64  for the second set of terminals “+DC” and a second, central opening  64   b  is provided for the bus bar  42 . However, the diameter of the central opening  64   b  of the conductive sheet is substantially larger than the diameter of the bus bar  42  to ensure electrical insulation there between. The conductive sheet  64  is provided with a circular area  64   c  around the central opening  64   b  which is adapted for tight contact with the innermost cooler  30   c.    
         [0054]    During assembly, the lamination  62  is fitted onto the second set of terminals “+DC” and the conductive sheet  64  is then also fitted onto this second set of terminals. Electrical and mechanical connection between the second set of terminals “+DC” and the conductive sheet  64  is provided by means of soldering, brazing or bolting etc. The bus bar  42  is then screwed or otherwise attached to the capacitor  60 , providing the potential “−DC” to the bus bar  42 . The different parts shown in  FIG. 4  are then threaded onto the bus bar  42  in the order innermost to outermost as shown in  FIG. 4 , ending with the nut  44   a,  which is screwed onto the threaded outer end portion  42   a  of the bus bar  42  until a desired force is exerted on the stack of coolers and semiconductor modules or by applying a force, such as a hydraulic force, and turning the nut to lock the spring. 
         [0055]    Connections are provided for electrical connection between a control unit  70  and the semiconductor modules  20   a,    20   b.    
         [0056]    A phase connection is provided for electrical connection to the central, second cooler  30   b,  which, being electrically insulated from the bus bar  42 , takes the electrical potential “−DC” or “+DC”, in dependence of the operation of the semiconductor modules  20   a,    20   b.  In this embodiment, the semiconductor assembly  10  is therefore suitable for providing the phase voltage of a converter, such as an HVDC converter. 
         [0057]    Preferred embodiments of a semiconductor assembly have been described. It will be appreciated that these made be modified within the scope defined by the appended claims without departing from the inventive idea. 
         [0058]    A piece of electrical equipment, such as a capacitor, has been described with an insulating sheet in the form of a lamination. It will be appreciated that this idea is applicable not only to the inventive semiconductor assembly described herein, but also to other assemblies, such as an assembly comprising the prior art press pack stack shown in  FIG. 1 . 
         [0059]    Although specific polarities have been set out in the drawings, it will be realized that any voltages and currents can be applied to the semiconductor assembly without departing from the inventive idea. 
         [0060]    In the described preferred embodiments, the semiconductor modules and the coolers are circular. It will be appreciated that the shape of these parts may vary from circular, such as square or hexagonal, without departing from the inventive idea. 
         [0061]    The lamination has been described and shown as being fitted onto the second set of terminals. In other embodiments, the lamination may be fitted inside or outside of the housing of the piece of electrical equipment. 
         [0062]    The central opening provided the conductive sheet for the bus bar has been described as being is substantially larger than the diameter of the bus bar itself in order to achieve electrical insulation. This insulation can also be achieved by other means, such as by sealing, for example with electrically insulating glue.

Technology Classification (CPC): 7