Abstract:
A novel and apparatus for cooling, supporting, and packaging power electronics. An elongated heat exchanger having a rectangular cross-section may carry and cool plural electronic devices. Plural heat exchangers may be arranged to form a chassis for two-sided cooling. Electronic devices on the heat exchangers may be interconnected and configured using circuitry bonded to a power pack cover. Circuitry on the cover, e.g., for reducing internal inductance, CTE mismatch and preventing voltage overshoot at turn-off, may be cooled through physical connections with electronic devices carried by the heat exchangers.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to power packs and, more specifically, to complex power packs with integrated cooling. 
     This application is related to application Ser. No. 09/040,112 filed Mar. 18, 1998 for “Semiconductor Power Pack” by Azotea et. al., the contents of which is incorporated herein in its entirety by reference. 
     Power pack manufacturers of today have not yet solved the built in problems of power pack geometries. These problems may include: suitable thermal management, “turn-off” inductance, and complicated device configuration. 
     In the power industry, issues concerning thermal interface, case temperature, and the cost of thermal management are generally not the responsibility of most manufacturers. A common practice in the power industry is to achieve maximum power levels by increasing the thermal gradient between the junction temperature of the silicon die and a “cold plate”. Increasing the thermal gradient does not solve the underlying problem of an inadequate case-to-heat sink interface but adds cost and volume to the system design. 
     The forward blocking voltage (BVf) of power packs must be de-rated by the total internal inductance of the power pack multiplied by the rate of change of the peak turn-off current. The inductance values in power packs vary with geometry and are mostly a function of wiring length and mutual inductance. 
     Power packs having only one power device (e.g., one semiconductor power switch) may be referred to as power modules. A power pack may include plural power modules. 
     One known power module design includes a base plate, a lid, and a power device sandwiched between the lid and base plate. Such power modules may be single or double sided cooled but may not provide an optimum mounting and cooling area as compared to the total volume of the module. 
     For more complex power packs (i.e., two or more power devices in a power pack), the problems of providing a low volume package with suitable cooling (e.q., heat exchangers) and providing simultaneous operation of multiple power devices within a power pack having inductance remain. 
     One known technique for cooling involves attaching a heat exchanger to each individual power device in a power pack. The size and complexity of the power pack increases as the number of devices to be cooled increases, since each heat exchanger must have access to a coolant. 
     As for simultaneous operation of several power devices or even several power modules in one power pack, the driver circuitry may be located a distance away from the power device which increases the inductance of the power pack because of the extra lead length necessary to reach the power pack from the driver circuit. In parallel power device applications, the increased inductance may cause voltage over-shoot on turn-off which may damage power pack components. 
     Accordingly, it is an object of the present invention to provide a novel power pack. 
     It is another object of the present invention to provide a novel method of cooling a power device. 
     It is yet another object of the present invention to provide a novel heat exchanger. 
     It is still another object of the present invention to provide a novel method of cooling electronic devices. 
     It is a further object of the present invention to provide a novel method of providing structural support and cooling for heat generating electronic devices. 
     It is yet a further object of the present invention to provide a novel power pack having a heat exchanger as a chassis. 
     It is still a further object of the present invention to provide a novel power pack having increased power dissipation, decreased forward break down voltage derating factor, low inductance, and multiple power device configurations from the use of internal or external circuits. 
     It is yet still a further object of the present invention to provide a novel power pack in which a snubber capacitor may be implemented to limit the voltage over-shoot and to manage stray inductance. By locating the snubber cap on the cover and making the connection from the device to the snubber capacitor very short, coefficient of thermal expansion (CTE) problems are reduced, and less inductance between the power device and the capacitor is created making the snubber more effective. Additionally, the impedance may be matched. 
     It is yet still another object of the present invention to provide a novel power pack in which the cost is significantly reduced by mass producing the power device and effecting the configuration thereof by changing the external circuit. 
     These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of one embodiment of a heat exchanger of the present invention. 
     FIG. 2 is a pictorial view of a second embodiment of the heat exchanger of the present invention illustrating variations in the size of the fluid passageways. 
     FIG. 3 is a pictorial view of a third embodiment of the heat exchanger of the present invention illustrating variations in the size of the fluid passageways. 
     FIG. 4 is a pictorial view of a fourth embodiment of the heat exchanger of the present invention illustrating generally orthogonal fluid passageways. 
     FIG. 5 is a pictorial view of a fifth embodiment of the heat exchanger of the present invention illustrating a device mounted on the heat exchanger. 
     FIG. 6 is a pictorial view of a sixth embodiment of the heat exchanger of the present invention illustrating slot mounting of the HTP assemblies. 
     FIG. 7 is a top plan view of a seventh embodiment of the heat exchanger of the present invention illustrating an octagon construction with devices mounted on each of the vertical surfaces and vertical fluid passageways. 
     FIGS. 8 a - 8   c  are a pictorial view of a one embodiment of a heat exchanging chassis of the present invention. 
     FIG. 9 is a pictorial view of a one embodiment of a heat exchanging power pack of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIG. 1, heat exchanger  10  may be a rigid elongated member having at least one fluid coolant channel  12 , a plurality of flat surfaces  14 , and thermal expansion holes  16 . Electrical devices such as power modules may be mounted on the flat surfaces  14 . The heat exchanger  10  may serve as a chassis for mounting power semiconductor devices, electronic parts, printed circuit boards, and passive power components such as thick film resistors. 
     The heat exchanger  10  may be manufactured with “high k” (thermally conductive) materials having electrically conductive or dielectric properties such as Copper (Cu) and Beryllium oxide (BeO). Other materials include silicon carbide (SiC), aluminum nitride (AlN) and alumina (Al 2 O 3 ). The heat exchanger may preferably be manufactured from a volume of metalized ceramic having undergone a DBC profile. Silicon may be mounted directly on a metalized ceramic heat exchanger which provides a reduction in thermal resistance, increases power, and provides an appropriate CTE match between silicon and heat exchanger. For other substrate materials such as copper, thermal expansion holes  16  may be formed in the metal substrate for modifying the composite coefficient of thermal expansion to approximate silicon. Methods of determining the composite CTE of objects are generally known in the art. 
     Power modules, power devices, or other circuitry may be bonded by brazing or soldering to the heat exchanger  10 . The bond provides a low thermal resistance interface between heat generating devices and the heat exchanger  10 . 
     Cross-sections of the heat exchanger  10  may preferably be rectangular but any cross-sectional shape may be used as long as mounting surfaces exist. For example, the heat exchanger may be a volume of honey comb material surrounded by a copper foil skin. 
     The “multi-sided” heat exchanger  10  is designed to increase dissipation area, provide low thermal resistance, provide a low thermal resistance bonding interface, and allow a direct composite coefficient of thermal expansion match between heat exchanger and silicon power die. To manufacture such a heat exchanger, a copper block may be drilled or extruded to form a chassis and covered with a metal foil or metalized to provide attachment surfaces, which can be patterned. Green ceramic heat exchangers may also be manufactured by extrusion. 
     Thermal performance of the heat exchanger  10  may be maximized by optimizing relations of power equation 
      P=[(K)(A)(T j −T a )/L]  (1) 
     where A is the total surface area, 
     L is the length of the semiconductor junction to coolant interface, 
     T j  is the junction temperature, 
     T a  is the coolant temperature, and 
     K is the thermal conductivity of the interface. 
     In operation, devices (not shown) mounted on the plurality of surfaces  14  generate heat. Heat generated may be transferred to the heat exchanger  10  through a low thermal resistance bond with the heat exchanger  10 . The generated heat may be transferred away from the heat exchanger by passing a fluid coolant through the channel  12 . Plural channels may be used to increase the surface contact area of the coolant to the heat exchanger. Heat may also be transferred away from the heat exchanger without using a channel  12 . For example, planar surfaces of the heat exchanger  10  may be placed in contact with a coolant. 
     As shown in FIG. 2, there may be a number of passageways through the heat exchanger, all the same size or variable in size to modify the rate of heat flow. In FIG. 3, the passageways nearest the skin are larger to accommodate greater fluid flow nearest the devices to be bonded to the flat surfaces. In FIG. 4, the larger diameter passageways are nearest the center of the heat exchanger. 
     As shown in FIGS. 4 and 5 the passageways may extend through the heat exchanger in directions normal to each other, and in FIG. 6 the passageways may be located at one end of the heat exchanger so that HPT assemblies may be slotted into the heat exchanger from the other end. 
     The heat exchangers of the present invention may be any convenient shape so long as they possess the surfaces to which to bond the devices to be cooled. As shown in FIG. 7, the heat exchanger may be an octagon in cross section and may have vertical passageways extending therethrough. 
     With reference to FIG. 8, the heat exchanging chassis  20  may include two spaced apart heat exchangers  28  and  30 . Together the heat exchanging chassis  20  and electronic devices  24  and  26  may form a power pack sub-assembly. Additional electronic devices may be bonded to sides of each heat exchanger  28  and  30 . For example, heat exchanger  28  may be rotated to a vertical position in order to use all four areas of the heat exchanger for power devices and other electronic devices  24  and  26  which may be attached by low thermal resistance methods. 
     The structure of the chassis  20  significantly increases the mounting and cooling area of a power pack using a simple rectangular geometry. Other heat exchanger geometry&#39;s such as hexagons or milled slots may increase area and power dissipation but increase cost and complexity. Top or double sided power device cooling may be provided by attaching the top side of an electronic device such as a power module  26  to second heat exchanger  30 . Power device mounting and location on the chassis  20  may be determined according to heat dissipation requirements. 
     With reference to FIG. 9, a power pack  60  may include a heat exchanging chassis  40 , power modules  42 , and a frame which includes a power pack cover  44 , a chassis support plate  46  and the circuity  47  carried by the cover  44 . The heat exchanging chassis may include plural heat exchangers spaced apart from each other. 
     The power modules  42  may be bonded to the chassis  40  to provide support and cooling for the power modules  42 . Other devices may also be bonded to the chassis for support and cooling. The chassis support plate  46  may be a metal or ceramic substrate that may also serve as heat exchanger. The cover  44  may be a ceramic substrate having metalized surfaces patterned for bonding with circuitry  47 . The circuity  46  may include a snubber, power device driver circuitry, and/or power pack configuration circuitry. Holes  48  provide electrical conduction path to the interior of the power pack for connection with the power modules  42 . 
     Some thermal management may be provided for the circuitry  47  on the cover  44  through the physical connections with the power modules  42  such as by connecting power module striplines  50  to the cover  44  and cooling the circuitry  47  on the cover  44  with the heat exchanging chassis  40  through the power modules  42 . Short striplined electrodes from the power modules  42  may be connected to “sliding female” or “Pem nut” interfaces (not shown). A “sliding male” or screw interface allows an electrical and thermal connection from the cover  44  to the chassis  40 . High “K” materials such as ceramics and back filled epoxy may be used to enhance thermal management. 
     The power pack  60  may include only one heat exchanger  51  which may be configured with circuitry  47  as a half-bridge, a full-bridge, AC switch or other special circuit. An example of circuitry  47  may be a circuit integrating a non-inductive current resistor, a damping resistor, and a discharge capacitor. Also film resistors may be used. 
     The power pack  60  provides a novel approach in device configuration, package inductance, snubbing, current sensing, CTE matching and component cooling. 
     While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.