Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and is a continuation-in-part of U.S. Provisional Patent Application Ser. No. 61/933,535, filed on Jan. 30, 2014 entitled LOW PROFILE, HIGHLY CONFIGURABLE POWER MODULE FOR EQUAL CURRENT SHARING OF MANY PARALLELED WIDE BAND GAP POWER DEVICES which is hereby incorporated by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under grant FA8650-10-C-2124 awarded by the United States Air Force and grant DE-EE0006429 awarded by the Department of Energy. The United States government has certain rights in the invention. 
    
    
     REFERENCE TO A MICROFICHE APPENDIX 
     Not Applicable. 
     RESERVATION OF RIGHTS 
     A portion of the disclosure of this patent document contains material which is subject to intellectual property rights such as but not limited to copyright, trademark, and/or trade dress protection. The owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent files or records but otherwise reserves all rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to improvements in wide band gap power modules. More particularly, the invention relates to improvements particularly suited for providing a configurable consistent power module design for multiple applications. In particular, the present invention relates specifically to a parallel path power module allowing for current sharing at high switching frequencies. 
     2. Description of the Known Art 
     As will be appreciated by those skilled in the art, power modules are known in various forms. Patents with information of interest to power modules include: U.S. Pat. No. 7,687,903, issued to Son, et al. on Mar. 30, 2010 entitled Power module and method of fabricating the same; U.S. Pat. No. 7,786,486 issued to Casey, et al. on Aug. 31, 2010 entitled Double-sided package for power module; U.S. Pat. No. 8,018,056 issued to Hauenstein on Sep. 13, 2011 entitled Package for high power density devices; U.S. Pat. No. 8,368,210 issued to Hauenstein on Feb. 5, 2013 entitled Wafer scale package for high power devices; U.S. Pat. No. 6,307,755 issued to Williams, et al. on Oct. 23, 2001 entitled Surface mount semiconductor package, die-leadframe combination and leadframe therefore and method of mounting leadframes to surfaces of semiconductor die. Additional articles include: R. K. Ulrich and W. D. Brown, “Advanced Electronic Packaging,” New Jersey: John Wiley &amp; Sons, Inc., 2006, p. 203; and Shengnan Li, “Packaging Design of IGBT Power Module Using Novel Switching Cells,” Ph.D. dissertation, University of Tennessee, 2011, http://trace.tennessee.edu/utk_graddiss/1205. Each of these patents and publications are hereby expressly incorporated by reference in their entirety. 
     Wide band gap power semiconductors, including Silicon Carbide, SiC, and Gallium Nitride, GaN, offer numerous advantages over conventional Silicon, Si, based power electronic devices, including: 
     1. Reduced intrinsic carriers allowing for higher temperature operation 
     2. Increased carrier mobility 
     3. Higher electrical breakdown strength 
     4. Reduced on-resistance 
     5. Faster switching speeds 
     6. Increased thermal conductivity 
     These benefits allow for designers to implement systems which are substantially smaller, more efficient, and more reliable that the current state-of-the-art systems. Higher temperature operation allows for the reduction of the cooling system required to remove waste heat. The potential also exists to switch from an active, i.e. forced air or liquid, cooling scheme to passive, natural convection, cooling, elimination of thermal shielding materials, and operation in extreme environments where traditional technology will fail. High frequency switching reduces switching losses and allows for a major reduction in the size of filtering elements in a power converter. 
     The promises of wide band gap power technology, however, are hindered by the power packaging necessarily to interconnect, protect, and integrate the devices into a power conversion system. Power packages for Si devices are generally designed to house one large device per switch position, often with a single antiparallel diode. Commercially available wide band gap devices, however, are not available as large, monolithic elements due to issues with wafer quality and yield. Accordingly, while the relative power density, per die area, for SiC is substantially higher than Si, in order to reach high currents, in the hundreds of amps, many SiC devices must be placed in parallel. 
     There is a fundamental issue with paralleling many devices in conventional packages which were not designed to effectively account for issues such as current sharing. This is particularly important due to the extremely high switching speeds of wide band gap devices, often hundreds of times faster than Si equivalents. Mismatches in inductances between the devices may cause uneven stresses and current overshoot during switching events. Additionally, the materials, attaches, and interfaces of established power module technology are not capable or reliable at the temperatures which wide band gap devices are operable. 
     From this, it may be seen that the prior art is very limited in its teaching and utilization, and an improved power module is needed to overcome these limitations. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved power module using parallel power devices. In accordance with one exemplary embodiment of the present invention, a power module is provided with low inductance equalized current paths to many paralleled devices, allowing for even current sharing and clean switching events. The power module is capable of running at junction temperatures ranging from 200 to 250° C., depending on devices, operating conditions, etc. and may carry very high currents, 100 s of amps and greater. Chiefly, these enhancements fall into three categories: 1 performance, 2 function, and, 3 usability. This technology is designed from the ground up to embrace the characteristics and challenges of wide band gap power devices. Features of the power module include the following highlights: 
     Matched footprint with industry standard 62 mm base plates. 
     Equalized power paths for effective paralleling of bare die power devices. 
     Large active area available for devices, 7.5 mm×71 mm per switch position. 
     Low module height, 10 mm. 
     Low inductance achieved with wide, low profile power contacts. 
     Short current path and large conductor cross section area for massive current carrying, &gt;500 A. 
     Internal gate &amp; source kelvin interconnection substrate with individual ballast resistors. 
     High reliability bolted connection of the internal gate &amp; source kelvin interconnection substrate. 
     Standardized, and configurable 1 mm, 2 mm, 0.1 in, and 0.05 in pitch gate drive connectors. 
     Gate drive connectors on either left or right size of the module. 
     Option for internal temperature sensing RTD and associated input/output connectors. 
     Reduced unique part count to reduce system cost. 
     Reduced unique part count to increase modularity. 
     Configurable as half bridge, full bridge, common source, and common drain topologies. 
     Voltage creepage extenders incorporated into the plastic housing. 
     Lightweight through the use of low density materials, ˜140 g in total. 
     Materials, attaches, and voltage blocking passivation capable of operating up to 250° C. 
     These and other objects and advantages of the present invention, along with features of novelty appurtenant thereto, will appear or become apparent by reviewing the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the following drawings, which form a part of the specification and which are to be construed in conjunction therewith, and in which like reference numerals have been employed throughout wherever possible to indicate like parts in the various views: 
         FIG. 1  shows a perspective view of the power module. 
         FIG. 2  shows an exploded view of the power module. 
         FIG. 3  shows a relative size to thickness comparison of the power module. 
         FIG. 4  shows the equalized current flow for multiple paralleled devices. 
         FIG. 5  shows the power contact design. 
         FIG. 6  shows the low profile power contact bending. 
         FIG. 7  shows the power module base plate. 
         FIG. 8  shows the gate and source kelvin secondary substrate, 
         FIG. 9  shows the gate &amp; source kelvin board half bridge arrangement. 
         FIG. 10  shows the gate &amp; source kelvin board common source arrangement. 
         FIG. 11  shows the gate &amp; source kelvin board common drain arrangement. 
         FIG. 12  shows the single layer modular gate and source kelvin example layout. 
         FIG. 13  shows the power substrate half bridge arrangement. 
         FIG. 14  shows the power substrate common source arrangement. 
         FIG. 15  shows the power substrate common drain arrangement. 
         FIG. 16  shows the high temperature plastic housing topside features. 
         FIG. 17  shows the high temperature plastic housing backside features 
         FIG. 18  shows the housing attach to power module assembly. 
         FIG. 19  shows the power contact guides. 
         FIG. 20  shows the half bridge, single channel common source or drain module. 
         FIG. 21  shows the full bridge, dual channel common source or drain module. 
         FIG. 22  shows the extended single housing side-by-side module configuration. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1  of the drawings, one exemplary embodiment of the present invention is generally shown as a power module  100 . The power module  100  is configurable in multiple useful power electronic topologies such as half bridge, full bridge, common source, and common drain and can be configured in up to two separate channels. It is uniquely suited to take advantage of all wide band gap technology has to offer, while being flexible enough to meet the demands of many customer systems through custom configurations. 
     The power module  100  consists of the primary elements outlined in  FIG. 2 . These include the base plate  200 , power substrate  300 , power contacts  400 , power devices  500 , gate &amp; source kelvin interconnection board  600 , gate drive connectors  700 , injection molded housing  800 , and fasteners  900 . 
     Specific focus was placed on using a footprint common in the power electronics industry, a 62 mm×107 mm base plate  200  with M6 mounting holes 48 mm×93 mm apart. Using a common footprint allows for customers with existing systems to evaluate these high performance modules  100  without investing in a complete system redesign. 
     While the length and width of the module  100  fits industry standards, the height of the module is 2× to 3× thinner than contemporaries. It is 10 mm thick in total. This dramatically reduces the module inductance and increases current carrying capability partially by utilizing lower path lengths. It may also provide a major source of system level volume savings in a power converter. 
     The comparison of top size to thickness dimensions of the power module  100  are presented in  FIG. 3  in the top and side view comparison. The module  100  measures 65 mm×110 mm×10 mm. The plastic housing  800  extends like a sheath over the base plate  200  for voltage isolation, which accounts for the extra 3 mm on each side over the base plate  200  dimensions. It has a volume of 71.5 cm3 and weighs approximately 140 g. 
     The power module  100  utilizes 57.5 mm×73 mm, 42 cm2, of the total footprint area for conduction. This is an impressive 60% utilization solely for current carrying. The remaining area is used for mounting, 5%, gate drive connections, 5%, and plastic features including minimum wall thickness, voltage creepage extenders, and strengthening ribs, 30%. 
     Power Loop 
     As noted by  FIG. 4 , the driving focus of the power module  100  power loop  110  is effectively paralleling large numbers of devices  500 . Shown are a first power device  501 , second power device  502 , third power device  503 , fourth power device  504 , fifth power device  505 , sixth power device  506 , seventh power device  507 , eight power device  508 , ninth power device  509 , tenth power device  510 , and eleventh power device  511 . The module  100  can either have two or four switch positions, depending on configuration, which is detailed later.  FIG. 4  shows the upper position  480  and the lower position  490 . There is a large amount of flexibility in the formation of each switch position, such that they are tailored to specific applications without costly module  100  modifications. For example, the positions may have an equal number of diodes to the power switches  500 , only a few diodes, or none at all.  FIG. 4  is a representation of the power loop  110 , depicting the even, shared current paths  120  for current traveling from the “V+” terminal  410  to the “Mid” terminal  420 , the V-terminal  430  is also shown that is used for devices  500  in the lower position  490 . An additional benefit of this layout is that the even spacing of each device  500  aids in the spreading of the heat sources across the module  100  instead of concentrating them in a few locations. 
     As displayed in  FIG. 4 , nearly the entire width of the power module  100  is utilized for the conduction of current. Many benefits would be lost if the module  100  was tall. In the worst case, the length the current would travel through the power contact  410 ,  420 ,  430  would be longer than the path it travels once it reaches the substrates  300 . Accordingly, the power contacts  400  were designed to have a low height such that they contribute a negligible amount to the resistance and inductance of the system. 
     The low height of the power contacts  400  was achieved by using a dual bending process. First, the power contacts  400  are formed through either a metal stamping operation or by etching followed by forming in a press brake. The 90° bend at the base  450  creates an “L” shaped connector with a vertical body  460 . The base  450  is soldered down to the power substrates  300 . The base  450  is relatively thin in comparison to the overall shape. This reduces the area consumed by this bond, allowing for more active device  500  area inside of the module  100 . To improve adhesion of this thin bond, staggered holes, called solder catches  454  are etched or formed along the bonding surface  452  on the bottom of the base  450 . Molten solder travels up the catches  454  through capillary action. Once solidified, the solder inside of the catches  454  substantially improves bond strength in many directions. An exemplary contact  400  with solder catches  454  is presented in  FIG. 5 . 
     Also shown in  FIG. 5  is how the “L” shaped contacts  400  are bent a second time at the end of the fabrication process to form a contact top  470 . Before bending, the vertical body  460  of the contact  400  allows for a single piece plastic housing  800  to be dropped into place, as there are no undercuts present. The radius of the second bend  472  is not as tight as the first bend  462 . This provides some tolerance in the process and is a smoother bending operation. The second radius  472  is facilitated through a pre-formed radius  810  in the plastic housing  800 , which, at this stage, is touching the leading edge  464  of the contacts  400 . Specifically designed rotating bending hardware presses flatly on the opposite surface  466 , folding the contacts  400  down over the captive fasteners  900 . An illustration of the bending of the “L” shaped contacts into “C” shaped forms is pictured in  FIG. 6 . 
     Underneath the folded contacts  400  are low profile threaded fasteners  900  shown as nuts  900 . These fasteners  900  are captured underneath the power contacts  400 . They are otherwise loose. The captive fasteners  900  serve an important purpose. When the module  100  is bolted to buss bars, the loose fasteners  900  and the contacts  400  are pulled upwards into the bussing, creating a quality electrical connection. If the fasteners  900  were affixed to the housing  800 , they would act to pull the bussing down into the module  100  and could create a poor connection due to the stiffness of the buss bars. 
     Base Plate 
     The base plate  200  is a critical element of the module, providing mechanical support, heat spreading, and a means to effectively bolt down to a heat sink or cold plate. The material properties of the base plate  200  become increasingly important as the temperature of operation elevates. An effective example is found in the coefficient of thermal expansion, CTE, where materials in the assembly expand at different rates due to heat and may create large stresses in their interfaces. 
     The power module  100  utilizes a Metal Matrix Composite, MMC, material, which is a composite of a high conductivity metal, copper, aluminum, etc., and either a low CTE metal such as moly, beryllium, tungsten, or a nonmetal such as silicon carbide, beryllium oxide, graphite. These composite materials combine the best features of each contributing element, allowing for a high thermal conductivity with a CTE which is matched with the power substrate  300  to which it is attached. 
       FIG. 7  shows how the power module  100  base plate  200  was designed to match an industry standard 62 mm geometry, which has a set diameter and location for the mounting holes  203  in the corners. The thickness of the plate  200  was fine-tuned through the use of parametric finite element analysis of the CAD model. This was achieved by sweeping the thickness between pre-defined practical limits and measuring the thermal and mechanical responses. The material and thickness combinations that achieved the best thermal performance with a minimal mechanical deflection were selected. Additional features of the power module  100  plate include machined or molded, depending on the MMC material, standoffs  210  with a threaded board hole  212 , and housing hole  290  each. The standoff  210  provides a planar surface with the power substrate  300  such that the internal gate &amp; source kelvin board  600  can be bolted down without bowing. 
     Gate Drive Loop 
     Independent electrical paths for each switch position are required to form gate and source kelvin connections, which are necessary for controlling the power switches. This becomes difficult with the number of devices  500  in parallel, as ideally the gate and source kelvin routing would not interfere with the wide, equalized power paths.  FIG. 8  shows how the power module  100  and its variations include a single piece secondary substrate  600  which is placed over the power substrate  300  and then bolted down to the base plate  200 . 
     As shown in  FIG. 8 through 11 , the gate &amp; source kelvin substrate  600  has two interconnection channels  602 ,  604  which may be located in one of four positions top or first  611 , upper middle or second  612 , lower middle or third  613 , and bottom or fourth  614  to define die apertures such as an external die aperture  603  or middle die aperture  605  to allow for a multitude of module  100  configurations. Essentially, the relative layout of each interconnection channel  602 ,  604  is the same; however, the location and direction are adapted to match the associated die aperture  603 ,  605  and die  500  placement and rotation to match each topology. This is illustrated in  FIG. 9 ,  FIG. 10 , and  FIG. 11  with the arrows indicating the gate direction for a half bridge, common source, and common drain topology, respectively. Each of these may consist of a single or dual channel arrangement, depending on the layout of the power substrate  300  and the format of the power contacts  400  and housing  800 . 
     As shown in  FIG. 12 , to aid in paralleling, individual ballast resistors  640  may be included on the interconnection board  600 . While there are many different layouts these boards can utilize such as parallel planes, clock tree distribution, etc., one of the more effective is a low cost single layer modular arrangement with many bonding locations  642 . As shown, a gate track  650  and source track  652  go across the length of the interconnection channels  654 . Source wire bonds are formed directly on the source track  652 . Each gate is bonded to individual gate pads  651  which are connected to the gate track through resistors  640 . The values of the resistors  640  are device and application dependent and will vary between module  100  configurations. 
     Power Substrate 
       FIG. 13  shows the power substrate  300  which is a metal-ceramic-metal layered structure designed to handle very high currents and voltages with the arrows again showing the gate direction to be matched with the boards  600 . Metals may be copper or aluminum at varying thicknesses, while the ceramic materials are typically alumina, Al203, aluminum nitride, AlN, or silicon nitride, Si3N4. The metal layers  302  are etched into topology specific patterns  330 ,  340 ,  350  as illustrated in  FIG. 13  for a half bridge substrate  330 ,  FIG. 14  for a common source substrate  340 , and  FIG. 15  for a common drain substrate  350  showing the upper and lower die  500  positions for each configuration. Also note that each of these layouts may be split into a dual channel arrangement by etching a line down the center of the substrates  300 . They may also be split into individual substrates per channel if desired. This may be useful for more harsh environments as the smaller substrates will experience less stress. 
     Housing 
     The housing  800  is formed in an injection molding process with reinforced high temperature plastic. The housing  800  serves many functions in addition to being a protective barrier to the sensitive semiconductors  500 . This includes voltage blocking, mechanical support for the captive fasteners  900 , guides for the power contact bending process, entry zones for gel passivation, vents for the gel passivation process, and self-strengthening internal ribs  812 . Many of these features are depicted in  FIG. 16  and  FIG. 17 . High aspect ratio trenches are placed around the periphery of the power contacts  400  to increase the surface distance between exposed metal contacts, increasing voltage blocking capability. 
       FIG. 16  shows the high temperature plastic housing topside features including the creepage extenders  802 , the passivation entries and vents  804 , the captive fasteners apertures  806 , the labeling area  808 , and the power contact pinch and radius  810 .  FIG. 17  shows the backside features including the strengthening ribs  812 , the thick bolt hole core sections  814 , the bolt head clearance recess  816 , the bottoms of the fastener insets  818 , the power contact entryways  820 , and the wire bond clearance apertures  822 . 
       FIG. 18  shows how the housing  800  slides over the electronic sub assembly to form the top of the module  100 , with the power contacts  400  routed through the narrow openings  820 . The housing  800  is bolted  830  at two points to threaded holes  290  on the base plate  200 . At this stage the gel passivation material is injected into the module  100  and fully cured. Multiple openings and vents  804  assist this assembly step. The slices  820  in the housing  800  for the power contacts  400  have drafted “guides” to aid this process, and a rounded fillet  810  on top to aid in the bending procedure. These are illustrated in  FIG. 19 . 
     Configurability 
     As discussed earlier in this document, the power module  100  is configurable in a variety of useful power electronic topologies. These include half bridge, common source, and common drain. Splitting the channels, through layout changes in the power substrate  300  and gate &amp; source kelvin board  600  and alterations to the power contacts  400  and housing  800 , allows three more configurations, including a full bridge, common source dual channel, and common drain dual channel. 
       FIG. 20  displays the first external configuration  150  for half bridge, single channel common source, and single channel common drain configurations. There are four locations  701 ,  702 ,  703 ,  704  for the gate driver connections  700 , two on each side. Either or both sides may be used for this purpose. For the dual channel arrangement  152 , shown in  FIG. 21 , the power contacts  400  are split and provide two fully isolated channels. Gate drive connectors  700  on both sides are now required. This arrangement is used for a full bridge, dual channel common source, and dual channel common drain topologies. 
     For higher currents and for customers who desire a single module, a larger side-by-side arrangement of a dual power module  200  may be fabricated from two modules built side by side into a single housing  800 . This is illustrated in  FIG. 22 . 
     Reference numerals used throughout the detailed description and the drawings correspond to the following elements:
         power module  100     power loop  110     shared current paths  120     first external configuration  150     dual channel arrangement  152     base plate  200     mounting holes  203     standoffs  210     threaded holes  212     threaded holes  290     power substrate  300     metal layers  302     first topology pattern half bridge substrate  330     second topology pattern common source substrate  340     third topology pattern common drain substrate  350     power contacts  400     first power contact  410     second power contact  420     third power contact  430     base  450     bonding surface  452     solder catches  454     vertical body  460     first bend  462     leading edge  464     opposite surface  466     contact top  470     second bend  472     upper position  480     lower position  490     power devices  500     first paralleled power device  501     second paralleled power device  502     third paralleled power device  503     fourth paralleled power device  504     fifth paralleled power device  505     sixth paralleled power device  506     seventh paralleled power device  507     eight paralleled power device  508     ninth paralleled power device  509     tenth paralleled power device  510     eleventh paralleled power device  511     gate &amp; source kelvin interconnection board  600     first interconnection channel  602     external die aperture  603     second interconnection channel  604     internal die aperture  605     first interconnection position  611     second interconnection position  612     third interconnection position  613     fourth interconnection position  614     individual ballast resistors  640     bonding locations  642     gate track  650     gate pads  651     source track  652     interconnection channels  654     gate drive connectors  700     first gate driver connection location  701     second gate driver connection location  702     third gate driver connection location  703     fourth gate driver connection location  704     housing  800     creepage extenders  802     passivation entries and vents  804     captive fasteners apertures  806     labeling area  808     power contact pinch and radius  810     strengthening ribs  812     bolt hole core sections  814     bolt head clearance recess  816     fastener insets  818     power contact entryway slices  820     wire bond clearance apertures  822     bolt  830     fasteners  900         

     From the foregoing, it will be seen that this invention well adapted to obtain all the ends and objects herein set forth, together with other advantages which are inherent to the structure. It will also be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. Many possible embodiments may be made of the invention without departing from the scope thereof. Therefore, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. 
     When interpreting the claims of this application, method claims may be recognized by the explicit use of the word ‘method’ in the preamble of the claims and the use of the ‘ing’ tense of the active word. Method claims should not be interpreted to have particular steps in a particular order unless the claim element specifically refers to a previous element, a previous action, or the result of a previous action. Apparatus claims may be recognized by the use of the word ‘apparatus’ in the preamble of the claim and should not be interpreted to have ‘means plus function language’ unless the word ‘means’ is specifically used in the claim element. The words ‘defining,’ ‘having,’ or ‘including’ should be interpreted as open ended claim language that allows additional elements or structures. Finally, where the claims recite “a” or “a first” element of the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements.

Technology Category: h