Abstract:
A dual-side thermal interface and cooling design of an integrated power module is disclosed which effectively reduces the equivalent thermal impedance on the power module by 20%. This in turn reduces the temperature rise of the junction temperature of the power devices inside the power module by 20% with an equivalent load current. As a consequence the weight and volume associated with the conventional cooling mechanism not employing a dual thermal interface is reduced, thus increasing the ambient operating temperature limit of a power converter in the module.

Description:
BACKGROUND OF THE INVENTION 
   This present invention generally relates to high-power-density power modules and, more specifically; to an electrically-powered, high-power-density power module useful as a power converter or inverter that can operate in an elevated temperature environment, with reduced weight and size, and increased reliability, which are critical for aircraft, space, military as well as many industrial applications. 
   In a completely sealed vehicle compartment, such as found on electrical aircraft and spacecraft subsystems, the ambient temperature of the power converters can be over 90 degrees centigrade, which is often dictated by use of hydraulic fluid as the coolant. However, a conventional power converter design is unable to achieve this operating temperature due to the built in thermal impedance of the power device or module and limited power device junction temperature. 
   IGBTs (Isolated Gate Bipolar Transistors) are popular power devices in use as pulse-width modulated power converters/inverters. However, their junction temperature is rated for operation at or below 125 degrees centigrade in accordance with FIG.  5 . 
   The devices must be derated to zero operating power at 150 degrees centigrade. The temperature effects on power semiconductor device parameters also include increased on-resistance or on-stage forward voltage drop, increased leakage current, reduced break down voltage and reduced switching speed. These effects significantly increase total power loss, thus increasing thermal stress and cooling requirements for the power devices and decreasing the converter efficiency. 
   As can be seen, there is a need for a dual-side thermal interface and cooling design which effectively reduces the equivalent thermal impedance on the power module by 20% and which also reduces the temperature rise of the junction temperature of the power devices inside the power module by 20% with an equivalent load current. Such a dual-side thermal interface and cooling design reduces the weight and volume associated with conventional cooling mechanisms not employing a dual thermal interface and increases the ambient operating temperature limit of the power converter. 
   SUMMARY OF THE INVENTION 
   In one aspect of the present invention, an integrated power module includes a power module in a fully-integrated electrical-and-thermal package having a top and a bottom side, a number of pairs of semiconductor or other types of power devices embedded in the package, a thermal interface on the bottom and top sides of the package for conducting heat generated by the internal power devices, and a heat remover in thermal communication with each of the thermal interfaces for dissipating the thermal interfaces. An electronic interface is also provided between the top thermal interface and its associated heat remover for supplying power to and receiving power from the semiconductor devices. 
   In another aspect of the invention there is disclosed a power module having a top and bottom side, a number of pairs of semiconductor or other types of power devices embedded in the module, a thermal interface on the bottom and top sides of the module, and a heat remover in thermal communication with each of the thermal interfaces. 
   In another aspect of the invention there is disclosed a power module having dual-sided thermal interfaces for improved cooling. 
   In yet another aspect of the invention there is disclosed a method for cooling an integrated power module containing a number of semiconductor power devices, and which includes dissipating heat and cooling the module by providing thermal interfaces on the opposed sides or the top and bottom surfaces of the module. The thermal impedance of a power semiconductor circuit and module is lowered by employing a new embedded thermal interface on the top side of a power module package, in addition to a compact bottom-side thermal interface provided via a base plate for increased cooling of the device, enabling elevated temperature operation. 
   Other aspects of the invention include having the top-side or front face thermal interface, for example a heat remover or exhaust fan, directly cool the internal power semiconductor dies, gate area of the switching devices, wire joints and bus joints, which are subject to heat flux concentration. A heat remover is employed adjacent the bottom face thermal interface to provide dual face cooling effective in reducing the inner thermal impedance between the power semiconductor devices and the external thermal interfaces. This feature in turn reduces the junction temperature rise of the power devices from their base plate/heat-sink temperature, thus allowing safer operation in elevated ambient temperatures and enabling a more compact package design and/or one of lower weight. 
   These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description, and claims 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a cross-sectional view of a generic embodiment of a power module in accordance with the present invention; 
       FIG. 2  is a perspective view of the power module of the invention having dual thermal interfaces used in the integrated module; 
       FIG. 3  is a perspective view of an experimental prototype of the integrated power module of the present invention; 
       FIG. 4  illustrates the temperature distribution for an integrated power module of the present invention; and 
       FIG. 5  is a chart illustrating junction temperature for operation of isolated Gate Bipolar Transistors according to the prior art. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
   The present invention generally provides unique, dual-side, thermal interfaces on an integrated power module used on a commercially available IGBT power converter. The converter results in improved cooling of the module, which finds use, for example, in aircraft cooling systems and actuation systems. This is unlike the prior art in that dual-sided cooling of the module occurs with increased efficiency. 
   Referring now to the accompanying drawings (in which like reference numbers indicate like parts throughout several views), and more particularly, to  FIG. 1 , there is shown a cross-sectional view of a generic embodiment of a power module in accordance with the present invention. The integrated power module  10  may include a PCB (Printed Circuit Board) electronic interface  12 , which may be a standard PCB provided with pins for establishing an electrical connection between electronic control circuits and the power devices. 
   A front face thermal interface  14 , which may be comprised of heat conducting materials such as copper, copper tungsten alloy or AlN (aluminum nitrite), may be encapsulated or embedded in a suitable epoxy material  26  (including for example any of several loaded epoxies, such as one sold under the trademark Stycast, which are engineered for high thermal conductivity and high voltage breakdown resistance) with pairs of power device dies  16 , comprised of IGBTs and diodes. The IGBTs may be replaced by other silicon power devices including MOSFETs (Metal Oxide Semiconductor Field Effect Transistor), or silicon carbide (SIC) and silicon-on-insulator (SOI) dies or chips, connected to the PCB electronic interface  12 . In fact, a three-phase bridge module may have six pairs of switching power devices and diode devices for a three-phase converter circuit. The typical cross section of the front face thermal interface  14  is further illustrated in  FIG. 2 , which is a perspective view of the power module of the invention. 
   One example of the materials used in the interface and their properties is set out in the following Table 1: 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Material 
               Thickness 
               Density 
               Specific Heat 
               Thermal Cold 
             
             
               Layer 
               (mm) 
               (gm/cm 3 ) 
               (J/kg*K) 
               (W/m*K) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Silicon 
               0.4 
               2.34 
               712 
               148 
             
             
               Solder 
               0.1 
               8.42 
               176 
               50 
             
             
               Cu (1) 
               0.3 
               8.9 
               385.1 
               400 
             
             
               Al 2 O 3   
               0.635 
               3.8 
               795.35 
               21 
             
             
               Cu (2) 
               0.15 
               8.9 
               385.1 
               400 
             
             
               Al 
               — 
               2707 
               896 
               200 
             
             
               Stycast 
               — 
               — 
               — 
               1.5 
             
             
               Air 
               — 
               1.17 
               1005.7 
               0.028 
             
             
               Plastic 
               — 
               — 
               — 
               0.23 
             
             
                 
             
           
        
       
     
   
   In the embodiment shown in  FIG. 1 , the epoxy material  26  may replace the usual low thermal conductivity backfill material used for environmental protection, but must be capable of performing the same function. 
   A heat remover  20 , e.g., an exhaust fan or a heat pipe extending to an external fin system, may be provided on or in the PCB electronic interface  12  to remove heat from or directly cool the front face thermal interface  14 , the power device dies  16 , and the gate area of the switching devices, wire bonds and power bus joints (not shown), which have heat flux concentrations. In the embodiment illustrated in  FIGS. 1 and 2 , an internal fin system  18  may be created adjacent the front face thermal interface  14 . The internal fin system  18 , which may be bonded by epoxy to the front face thermal interface  14 , may contain an entry hole  21  and a mounting platform (not shown) for the heat remover  20  to provide forced air cooling. The cooling air driven by the heat remover  20  may travel in both directions over the top surface of the epoxy material  26  above the power device dies  16  and through a copper fin system  28 , removing heat from all the exposed surfaces as it exits the integrated power module  10 . 
   Still referring to  FIGS. 1 and 2 , the integrated power module  10  may include a second, bottom face thermal interface  22 , in the baseplate, to provide both heat removal, voltage insulation, and an acceptable thermal expansion match with the IGBTs and diodes. The bottom face thermal interface  22  may be comprised of a DCB (Direct Copper Bounding) copper/alumina/copper structure bonded to a copper base plate, or any similar structure that accomplishes the same purposes. The bottom face thermal interface  22 , connected to the opposite side of the power device dies  16  may also be connected using a high thermal conductive grease or may be directly hard bonded, such as by a high thermal conductivity epoxy, to one or more housed heat removers  24 , for cooling or conducting heat from the integrated power module  10  and power device dies  16 . 
   In the embodiment illustrated in  FIGS. 1 and 2 , dual exhaust fans  30  (one is shown) may drive cooling air through internal fin system  18  and copper fin system  28 . For low profile, these fans  30  may be mounted inside and in the center of the heat remover  20 , with air ducts connecting to the numerous internal fin system  18  channels that run parallel to the bottom face thermal inteface  22 . The size and spacing of said channels may be chosen to maximize heat removal, by striking the correct balance between large channel surface areas and the need to maintain enough copper in the through thickness direction to conduct heat through the block to channels more distant from the base plate. Any of several designs, including additional fin arrangements, external fan mounted systems, or hear pipe based systems, may accomplish the same goal, although not necessarily with the same high efficiency. 
   The dual-side cooling may be effective in reducing the inner thermal impedance between power device dies  16 , the front face thermal interface  14 , and the bottom face thermal interface  22 . This may reduce the junction temperature rise of the power device dies  16  from their baseplate/heat-sink temperatures, thus allowing safer operation in elevated ambient temperatures and enable a more compact, low weight module package design. 
   The integrated power module  10  has been tested and successfully operated at a full voltage of 480 RMS (650 VDC) and at a full power for a 3 horsepower AC machine at elevated temperatures that surpass 90 degrees centigrade. The power converter has been tested at full load in a sustained test routine. The new design reduces the equivalent thermal impedance of the power module over a conventional device by approximately 20%, as indicated in the following Table 2: 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
               Thermal 
               Thermal 
               Equivalent 
                 
                 
             
             
                 
               dispassion 
               dissipation 
               thermal 
             
             
                 
               front side 
               back side 
               impedance 
               Tj rise 
               Tj rise 
             
             
               Technologies 
               (%) 
               (%) 
               (pu) 
               (degree C.) 
               (pu) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Conventional 
               1.12 
               98.88 
               1 
               49 
               1 
             
             
               1 side 
             
             
               interface 
             
             
               Proposed 
               20.11 
               79.90 
               0.8 
               39 
               0.8 
             
             
               2 side 
             
             
               interface 
             
             
                 
             
           
        
       
     
   
   This may effectively reduce the temperature rise of the junction temperature of the power devices inside the module by about 20% with an equivalent load current.  FIG. 3  illustrates a perspective view of the experimental prototype of the integrated power module of the present invention used to obtain the above cited results, and  FIG. 4  illustrates the temperature distribution for an integrated power module having two-side cooling. 
   It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications made be made without departing from the spirit and scope of the invention as set forth in the following claims.