Abstract:
A high-power semiconductor module ( 10 ) has a number of flat semiconductor chips ( 14 ) which rest with their lower face flat on a base plate ( 11 ), establishing first electrical contacts, and have a cover plate ( 13 ), which is arranged parallel to the base plate ( 11 ), applied to their upper face with pressure, establishing second electrical contacts.  
     In a module such as this, simplified cooling is made possible in that those faces, or outer faces, of the base plate ( 11 ) and of the cover plate ( 13 ) which face away from the semiconductor chips ( 14 ) are each electrically isolated from the semiconductor chips ( 14 ).

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to the field of power electronics. It relates in particular to a high-power semiconductor module as claimed in the precharacterizing clause of claim 1.  
           [0002]    A high-power semiconductor module of this type is known, for example, from an article by H. Zeller, “High Power Components—From the State of the Art to Future Trends”, PCIM Nuremberg, May 1998 (FIG. 3).  
         PRIOR ART  
         [0003]    Widely different versions of high-power semiconductor modules are known from the prior art (see, for example, EP-A1-0 597 144, U.S. Pat. No. 5,978,220 or 6,087,682).  
           [0004]    High-power semiconductor modules are operated at voltages up to 10 kV per module. Voltages of well above 100 kV are reached by connecting them in series (for example for high-voltage direct-current transmission). Water or water/glycol is generally used to cool such modules efficiently. The coolers used for this purpose are—in a comparable manner to the configuration described in U.S. Pat. No. 4,574,877—connected in series with the modules in a stack to which pressure is applied, and are accordingly likewise at high potentials. The coolers must be operated using deionized water since, otherwise, current could flow to lower potentials via the cooling medium. The requirement to be able to connect such modules in series means that they are constructed in such a way that the one end surface corresponds to the emitter potential, and the opposite surface corresponds to the collector potential. A more recent version of such a module, in which a number of IGBT chips are connected in parallel within the module in a “press pack” configuration, is illustrated and described in the article by H. Zeller mentioned initially. The press pack configuration, in which the electric connection for the semiconductor chips arranged in the module is established by means of a pressure contact, has the advantage over making contact by means of bonding wires that, in the event of a fault, there is a stable short circuit in the module, so that this avoids any interruption in the current flow in a series circuit in this case.  
           [0005]    In other applications (for example for traction), the initial situation is somewhat different: here, lower voltages are used, so that there is no need to connect the modules directly in series. However, on the other hand, deionized water must not be used since this can lead to complications in this application. For this reason, modules such as these should be isolated from the cooling medium.  
           [0006]    This is typically achieved by using AlN substrates within the module and/or by introducing large AlN plates between the semiconductor module and the cooler. AlN coolers are even used in certain situations. Such isolated modules unfortunately frequently have restricted reliability (thermal expansion between the base plate and the substrates), while the large AlN plates and coolers are very expensive.  
         DESCRIPTION OF THE INVENTION  
         [0007]    The object of the invention is thus to provide a high-power semiconductor module which links the increased reliability of pressure-contact modules with the capability to use a cooler through which industrial water flows, and to specify an application for such a high-power semiconductor module.  
           [0008]    The object is achieved by the totality of features in claims 1 and 13. The essence of the invention is to design the module at the same time using a press pack configuration, and to configure the outer faces of the module such that they are electrically isolated from the semiconductor chips accommodated in the module. This results in DC isolation from an externally located cooling apparatus, while at the same time making reliable contact.  
           [0009]    One preferred refinement of the invention is distinguished in that a first electrically conductive, elastic connecting element, preferably in the form of a first contact spring, is in each case arranged between the upper face of the semiconductor chips and the cover plate, in that the base plate is an electrically insulating substrate which has a first metal coating on the inner face, in that the semiconductor chips are mounted by techniques such as bonding, soldering or welding, preferably by soldering, on the first metal coating, in that the first metal coating has the cover plate applied to it, with pressure, in an area located outside the semiconductor chips, establishing a third electrical contact, and in that the third electrical contact is established via a second electrically conductive, elastic connecting element, preferably in the form of a second contact spring.  
           [0010]    One preferred development of this refinement is characterized in that the cover plate is a first isolation plate, on whose inner face a first metallic contact plate is arranged, via which the second electrical contacts with the semiconductor chips are established, and in that a second metallic contact plate is arranged on the first metallic contact plate, and electrically isolated from it, via which the third electrical contact with the first metal coating on the base plate is established.  
           [0011]    In order to provide further protection for the module, an electrically insulating housing is arranged between the base plate and the cover plate, and encloses the semiconductor chips and the associated contact devices.  
           [0012]    Further embodiments can be found in the dependent claims. 
       
    
    
     BRIEF EXPLANATION OF THE FIGURES  
       [0013]    The invention will be explained in more detail in the following text with reference to exemplary embodiments and in conjunction with the drawing, in which:  
         [0014]    [0014]FIG. 1 shows a plan view, from above, of the base plate with the semiconductor chips and contact springs, as well as the surrounding housing of one preferred exemplary embodiment of the invention;  
         [0015]    [0015]FIG. 2 shows a longitudinal section of the cover plate associated with the module shown in FIG. 1; and  
         [0016]    [0016]FIG. 3 shows a longitudinal section through a completely assembled module as shown in FIGS. 1 and 2, and its use in a stack with a cooling apparatus. 
     
    
     APPROACHES TO IMPLEMENTATION OF THE INVENTION  
       [0017]    [0017]FIG. 3 shows a longitudinal section through a high-power semiconductor module  10  according to one preferred exemplary embodiment of the invention. The module has a base plate  11 , on whose upper face (inner face) as shown in FIG. 1 a number of semiconductor chips  14  (a total of 12 in the example) are fitted in a regular arrangement of four rows of three chips each. An electrically insulating housing  12  (preferably made of plastic) is provided at the edges around the semiconductor chips  14 , and this housing  12  can be compressed in the vertical direction by means of suitable measures (for example transverse corrugation). The high-power semiconductor module  10  is closed at the top by a cover plate  13  (see also FIG. 2). First contact springs  15  are arranged between the cover plate  13  and the free upper faces of the semiconductor chips  14 , via which first contact springs  15  (with appropriate pressure) an electrical contact is made between the cover plate  13  and the semiconductor chips  14 . Second contact springs  16 , which are placed between the two groups of six semiconductor chips  14 , provide a corresponding pressure contact between the cover plate  13  and the base plate  11 .  
         [0018]    An electrically insulating substrate  17  (for example AlN or some other ceramic) is used as the base plate  11 , and is provided on both the upper face and lower face with a respective metal coating  19  and  18 . The semiconductor chips  14  are soldered to the upper metal coating  19  on the base plate  11 , with space being left free for the second contact springs  16 . If the semiconductor chips  14  are, for example, IGBTs, these are soldered on the collector side. The emitter metalization of each semiconductor chip  14  is now fitted with a sprung contact (first contact spring  15 ). The contact springs  15 , which are in the form of pots in the illustrated example, may assume widely different shapes, provided the spring pressure is sufficiently great and the contact surface areas are sufficient for the specified high currents. The free surface areas of the upper metal coating  19  are provided with comparable second contact springs  16 . Thus, in the illustrated example, the central three contact springs  16  are at the collector potential, and the outer twelve contact springs  15  (of the semiconductor chips  14 ) are at the emitter potential. These two potentials or voltages can now be tapped off from above by means of the cover plate  13 .  
         [0019]    The cover plate  13  has a sandwich structure (FIG. 2): a narrow collector contact plate  23  (collector busbar) is bonded—isolated by an isolation plate  22 —onto a large-area emitter contact plate  21  (emitter busbar) The emitter contact plate  21  is itself isolated from the exterior by means of a further isolation plate  20 . The collector contact plate  23  presses against the central contact springs  16 , and thus makes contact via the upper metal coating with the collector side of the semiconductor chips  14 . The emitter contact plate  21  presses against the contact springs  15  which are arranged on the semiconductor chips  14 , and thus makes contact with the emitter side of the semiconductor chips  14 . Appropriately shaped contact lugs may be passed out of the housing  12  at the side from both contact plates  21  and  23 , and may be used as electrical connections for the module.  
         [0020]    The illustrated high-power semiconductor module  10  may now, as shown in FIG. 3, be stacked in a pressing apparatus, in which case only the base plate  11  need be cooled. A cooling apparatus  24  (represented by dashed lines in FIG. 3) is used for this purpose, and its cold surface rests against the lower metal coating  18  on the base plate  11 . Pressure is exerted on both sides of the stack (arrows in FIG. 3) to ensure both the electrical pressure contacts in the interior of the module and the thermal contact between the base plate  11  and the cooling apparatus  24 . A cooling box, which is known per se and through which water flows, may be used, for example, as coling apparatus  24 .