Abstract:
A power module includes at least one semiconductor die holding structure. Each die holding structure has a substantially cylindrical outer profile and a central axis. Each die holding structure is disposed within a common cylindrical EMI shield. A plurality of semiconductor devices are mounted to each die holding structure to form a substantially symmetric die mounting pattern respect to the central axis of the die holding structure.

Description:
BACKGROUND 
       [0001]    The subject matter of this disclosure relates generally to high speed semiconductor switches, and more particularly a coaxial power module that reduces electromagnetic interference (EMI) caused by high speed semiconductor switching. 
         [0002]    More high power wide bandgap semiconductors, such as the silicon carbide (SiC) metal oxide semiconductor field-effect transistor (MOSFET) are becoming commercially available to the power conversion industry. These devices have high switching speeds and high temperature capabilities making them ideal for usage in high frequency and high-end applications such as military and avionics. Such applications are very sensitive to radiated and conducted electrical noise. Since these new semiconductors are switched with such high speeds, the interconnections and the packaging implementations of these functional blocks could be a source for both radiated and conducted electrical energy that can disrupt sensitive control and communications equipment. 
         [0003]    Commercial power semiconductor modules historically have been designed to replicate an industry standard package so as to be able to replace existing field-installed devices. The foregoing approach is no longer a viable solution with the onset of new faster switching and high temperature technology. Existing industry standard packages have inherent geometries that are not conducive to implementing tight power interconnection. Many approaches have been implemented in attempts to reduce inductance using local planar interconnecting bus work within a module. None of these approaches have carried the concept down to the die level interconnections. Recent attempts at reducing inductance within a module have resulted in large conduction loops formed by the wire-bonds and associated routing strategies. Further, modules have been attached to DC bus bars using rectangular power contacts separated by large spaces and having to be bolted on to the bus bars themselves, creating additional interconnection inductances, and thus negate whatever gains are made by internal planar interconnections inside the power module. Modules bolted to bus bars disadvantageously take up large spaces making it difficult to design very tightly connected high-density converter/inverter modules for high-end applications such as avionics or defense related designs. 
         [0004]    In view of the foregoing, there is a need for a technique to reduce EMI caused by high power, high speed semiconductor switches to a level that will not cause interference with sensitive control and communications equipment that either employs high power, high speed semiconductor switches or that is in the immediate vicinity of such high power, high speed semiconductor switches. The technique would be particularly advantageous if it could be applied to provide a structure having a cylindrical geometry that is conducive to the mounting of power semiconductor modules, for example, in motor rotor or stator or missile cases. 
       BRIEF DESCRIPTION 
       [0005]    An exemplary power module embodiment comprises: 
         [0006]    a cylindrical EMI shield; 
         [0007]    at least one semiconductor die holding structure comprising a substantially cylindrical outer profile and a central axis, wherein the die holding structure is disposed within the cylindrical EMI shield; and 
         [0008]    a plurality of semiconductor devices mounted to each die holding structure to form a substantially symmetric die mounting pattern with respect to the central axis of the die holding structure. 
         [0009]    According to another embodiment, a power module comprises: 
         [0010]    a cylindrical EMI shield; 
         [0011]    at least one circumferential wrapped semiconductor die holding structure comprising a substantially cylindrical outer profile and a central axis, wherein the die holding structure is disposed within the cylindrical EMI shield; and 
         [0012]    a plurality of semiconductor devices mounted to each die holding structure to form a substantially symmetric die mounting pattern with respect to the central axis of the die holding structure. 
         [0013]    According to yet another embodiment, a power module comprises: 
         [0014]    a cylindrical EMI shield; 
         [0015]    at least one multi-stack radial semiconductor die holding structure comprising a substantially cylindrical outer profile and a central axis, wherein the die holding structure is disposed within the cylindrical EMI shield; and 
         [0016]    a plurality of semiconductor devices mounted to each die holding structure to form a substantially symmetric die mounting pattern with respect to the central axis of the die holding structure. 
     
    
     
       DRAWINGS 
         [0017]    The foregoing and other features, aspects and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0018]      FIG. 1  is a perspective diagram illustrating a coaxial power module according to one embodiment; 
           [0019]      FIG. 2  is an end view illustrating a BNC gate connector and cooling fluid piping coupled at one end of the coaxial power module depicted in  FIG. 1 ; 
           [0020]      FIG. 3  is a perspective diagram illustrating a coaxial power module according to another embodiment; 
           [0021]      FIG. 4  is an end view illustrating a BNC gate connector and cooling fluid piping coupled at one end of the coaxial power module depicted in  FIG. 3 ; 
           [0022]      FIG. 5  illustrates a circumferential power overlay (POL) assembly according to one embodiment, and that is suitable for use with the coaxial power modules depicted in  FIGS. 1 and 3 ; 
           [0023]      FIG. 6  is a cross-sectional view illustrating cooling fluid inlet and outlet pipes coupled to the circumferential POL assembly depicted in  FIG. 5 ; 
           [0024]      FIG. 7  illustrates a radial wire-bond sub-assembly stack according to one embodiment, and that is suitable for use with the coaxial power modules depicted in  FIGS. 1 and 3 ; 
           [0025]      FIG. 8  illustrates a single radial wire-bond sub-assembly for the radial wire-bond sub-assembly stack depicted in  FIG. 7 ; and 
           [0026]      FIG. 9  is an end view is the sub-assembly depicted in  FIG. 8 . 
       
    
    
       [0027]    While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0028]      FIG. 1  is a perspective diagram illustrating a coaxial power module  10  according to one embodiment. According to particular embodiments, power module  10  comprises a cylindrical main body  12  that surrounds a circumferential POL assembly or a multi-stack radial wire bond assembly, both described in more detail herein. The cylindrical main body  12  comprises a metal housing that forms a Faraday shield/cage surrounding the POL or wire bond assemblies to capture EMI and prevent airborne electrical noise that may otherwise be caused by the power module  10 . 
         [0029]    Coaxial power module  10  further comprises a cylindrical power connector  14  coupled to one end of the cylindrical main body  12 . According to one embodiment, the cylindrical power connector  14  is a coaxial type multi-pole power connector that comprises a center/inner conductor  16  and a cylindrical outer conductor  18  surrounding the inner conductor  16 . According to some embodiments, the inner conductor  16  may be configured as a drain or collector conductor, while the cylindrical outer conductor may be configured as a source or emitter conductor. Although cylindrical power connector  14  depicts a single switch structure, other embodiments can be easily implemented to provide a multi-switch structure. 
         [0030]      FIG. 2  is an end view illustrating a BNC gate connector  24  and cooling fluid conduit fluid inlet and outlet ports  20 ,  21  at one end of the coaxial power module  10  depicted in  FIG. 1 . BNC as used herein is an abbreviation for bayonet Neil-Concelman type connector that is a small device for connecting coaxial type cables. According to one embodiment, the end of the cylindrical main body  12  opposite the end coupled to the cylindrical power connector  14  comprises fluidic interface connections  20 ,  21  for receiving and expelling a suitable cooling fluid, a shield connection  22 , and a BNC type gate connector  24 . In summary explanation, a coaxial power module  10  comprises a cylindrical geometry having power interconnections implemented with multi-pole cylindrical connectors  14  placed on one end of the cylinder  12  and cooling fluid ports  20 ,  21  and a gate driver control signal connector  24  placed on the opposite end of the main body cylinder  12 . 
         [0031]    All power interconnections are preferably implemented such that conduction path loops are minimized and all current paths have conductor pairs, whose currents oppose each other, thus causing flux cancellation in the power interconnections within the module  10  and its associated power interface(s). Die interconnections within the module  10  are also preferably implemented to reduce the current loops and to have the same flux cancellation strategy to reduce the overall inductance of the module  10 . 
         [0032]      FIG. 3  is a perspective diagram illustrating a coaxial power module  30  according to another embodiment. Coaxial power module  30  is identical to coaxial power module  10  except coaxial power module  30  comprises a multilayer blade type power connector  32  having very thin high voltage insulation  34  between power contacts  36 ,  38  that may be, for example, a source or emitter contact  36 , and a drain or collector contact  38 . Although multilayer blade type power connector  32  depicts a single switch structure, other embodiments can be implemented using the principles described herein to provide a multi-switch structure. 
         [0033]      FIG. 4  is an end view illustrating a shield connection  22 , a BNC type gate connector  24  and cooling fluid conduit inlet and outlet ports  20 ,  21  placed at one end of the cylindrical main body shield  12  depicted in  FIG. 3 . 
         [0034]    According to particular embodiments, the coaxial power modules  10 ,  30  further each comprise a die holding structure that may be implemented using a multi-stack radial structure or a circumferential wrapped structure. The multi-stack radial structure and circumferential wrapped structure may be implemented using either wire bonding or POL technology described in further detail herein. 
         [0035]      FIG. 5  illustrates a circumferential power overlay (POL) assembly  50  according to one embodiment, and that is suitable for use with coaxial power modules  10 ,  30 . POL assembly  50  comprises a plurality of semiconductor dies disposed circumferentially inside POL  50 . The semiconductor dies may comprise, for example, diode dies  52  and/or MOSFET/insulated gate bipolar transistor (IGBT) dies  53 . POL assembly  50  further comprises an outer layer of copper conductor  54  that ultimately makes contact with the outer cylindrical conductor  18  of a corresponding cylindrical power connector  18  to form an external emitter/source connector. Copper conductors  56  are configured to provide an electrical pathway between the collector/drain portion of each semiconductor die  52 ,  53  and the inner conductor  16  of a corresponding cylindrical power connector  18  to form an external collector/drain connector. Other embodiments may just as easily employ power connectors having geometries other than cylindrical, such as the blade type power connector  32  depicted in  FIG. 3 , depending upon the desired application. Ceramic spacers  58  are employed at various locations to provide the requisite electrical isolation between the outer layer of copper conductor  54  and a corresponding cylindrical Faraday shield  12  in contact with the spacers  58  to complete the assembly, such as depicted in  FIGS. 1 and 3 . The spacers  58  allow a metal/Faraday shield  12  to slide over the entire module, thus creating the Faraday shield that encloses EMI, and thus blocking airborne electrical noise that may otherwise enter or exit the POL assembly  50 . The shield  12  preferably comprises a location to mount a screw or similar hardware allowing attachment of a wire and to capacitively or hard wire decouple the shield  12  to a reference power connection so as to provide a low conduction path for high frequencies back to their place of origin. 
         [0036]    The cylindrical module structure advantageously provides the ability to cluster this type of geometry and pack it very tightly on a bus bar; whereas known module structures are interconnected to their bus bars on a face bolt-on approach taking up a large area on the bus bars, and making it necessary to access both sides for replacement purposes without first disassembling the power converter. 
         [0037]      FIG. 6  is a cross-sectional view illustrating ceramic cooling fluid inlet and outlet pipes  20 ,  21  coupled to the circumferential POL assembly  50  depicted in  FIG. 5 . According to one aspect, a cooling fluid enters the innermost portion of fluid pipe  20  and exits through the outermost portion of fluid pipe  21  subsequent to passing through one or more heat exchangers in close proximity to the POL assembly  50  to provide cooling of the semiconductor dies  52 ,  53 . 
         [0038]      FIG. 7  illustrates a radial wire-bond sub-assembly stack  60  disposed within a cylindrical Faraday shield  12  according to one embodiment, and that is suitable for use with coaxial power modules  10 ,  30 . The wire-bond sub-assembly stack  60  comprises a semiconductor device gate wire  62  and gate return wire  64  that are connected to a BNC connector  24  placed on one end of the cylindrical Faraday shield  12 . The radial wire-bond sub-assembly stack  60  is attached to a cylindrical multi-pole connector  14  at an end of the cylindrical Faraday shield  12  opposite the end of the Faraday shield  12  with the BNC connector  24 . 
         [0039]      FIG. 8  illustrates a single radial wire-bond sub-assembly portion  70  of the radial wire-bond sub-assembly stack  60  depicted in  FIG. 7 . According to one embodiment, sub-assembly portion  70  comprises at least one MOSFET/IGBT die  53  and at least one diode die  52 . According to one embodiment, each sub-assembly portion  70  of the radial wire-bond sub-assembly stack  60  further comprises an integral heat exchanger  76  that functions to cool the corresponding dies  52 ,  53 . Various insulators  78  are configured to provide the requisite insulation between the heat exchanger  76 , wire-bonds  79 , and power terminals. 
         [0040]      FIG. 9  is an end view of the coaxial power module depicted in  FIG. 7 . A cooling pipe slot  78  in the cylindrical Faraday shield  12  depicted in  FIG. 7  accommodates cooling fluid inlet and outlet piping/conduit for the integral heat exchanger(s)  76  also depicted in  FIGS. 7 and 8 . Another opening  80  in the cylindrical Faraday shield  12  accommodates mounting of a BNC connector such as the BNC connector  24  depicted in  FIGS. 1 and 2 . 
         [0041]    In summary explanation, cylindrical power module embodiments implement power interconnections that form coaxial or planar structures down to the die interconnecting level, providing extremely narrow conduction loops and flux cancellation techniques. Integral heat exchanger manifolds cool the dies and can be decoupled via capacitors to provide a path for conducted electrical noise back to the source. An overall shield prevents airborne electrical noise by providing a Faraday cage surrounding the cylindrical module. The Faraday cage can also be decoupled using hardwire or capacitor interconnections to a ground system. The cylindrical power module power connectors are cylindrical or planar blade types which provide for extremely small conduction loops to the main bus bar connections. This type of power interconnection advantageously allows for very small footprints and enables very dense packing of the power module(s). 
         [0042]    The embodiments described herein advantageously enable new device technologies to take advantage of faster switching speeds without presenting negative impacts on the overall electrical performance of the system. These embodiments reduce power module inductance and interconnecting resistance voltage overshoots to avoid ringing, while voltage drops due to interconnection impedances are reduced thus reducing power loss. These embodiments may also incorporate many small SiC devices to produce a large current carrying module making it possible to commercially offer high current all SiC device modules, a feature that is difficult at best using present module technologies due to difficulties with interconnecting a large number of small SiC devices using wire-bonding with present module geometries. Further, the embodied cylindrical structures described herein advantageously provide a geometry that is much more conducive to the mounting of power semiconductor modules without limitation, in motor rotor or stator or missile cases. 
         [0043]    In further summary explanation, a coaxial power module implemented according to the principles described herein provides a packaging approach focused on reducing electro-magnetic interference by reducing interconnection parasitic, implementing magnetic flux cancellation, and by containing EMI within a Faraday shield. The embodied coaxial power modules can be implemented having current ratings from low current to hundreds of amps and a voltage range up to kilovolts. A preferred method of cooling is liquid cooling for particular embodiments due to large power handling capabilities. According to particular embodiments, a coaxial power module is implemented with quick disconnects allowing for quick and easy replacement. Further, embodiments can advantageously be employed, without limitation, as an integral part of a power conversion system such as a single power switch, a phase configuration having multiple power semiconductor switches, and/or a multiple phase configuration having multiple power semiconductor switches. 
         [0044]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.