Abstract:
A power module employs at least one capacitor electrically coupled across the input terminals to reduce voltage overshoot. The capacitor may be surface mounted to a high side collector plating area and a low side emitter plating area. The power module may employ a lead frame and terminals accessible from an exterior of a module housing, for making electrical couplings to externally located power sources and/or loads.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This disclosure is generally related to electrical power systems, and more particularly to power module architectures suitable for rectifying, inverting and/or converting electrical power between power sources and loads. 
   2. Description of the Related Art 
   Power modules are typically self-contained units that transform and/or condition power from one or more power sources for supplying power to one or more loads. Power modules commonly referred to as “inverters” transform direct current (DC) to alternating current (AC), for use in supplying power to an AC load. Power modules commonly referred to as “rectifiers” transform AC to DC. Power modules commonly referred to as “DC/DC converters” step up or step down a DC voltage. An appropriately configured and operated power module may perform any one or more of these functions. The term “converter” is commonly applied generically to all power modules whether inverters, rectifiers and/or DC/DC converters. 
   Current flowing through various inductive paths within the module transiently stores energy which increases energy loss, reduces efficiency, and generates heat. When the flow of current changes, as in such a high frequency switching environment, large voltage overshoots often result, further decreasing efficiency. These large voltage overshoots typically reduce the power rating of the power module or require the use of circuitry devices with higher ratings than would otherwise be required, thus significantly increasing the cost of the power module. 
   To minimize the negative effects of current gradients, noise and voltage overshoots associated with the switching process of the module, large capacitors are generally placed in a parallel arrangement between the positive and negative DC connections or from each DC connection to a ground or chassis. These large capacitors are commonly referred to as “X” or “Y” capacitors. Relatively large external capacitors of about around 100 micro Farads are needed. By “external” it is meant that the element referred to is located outside of a power module. High frequency noise, and voltage overshoots that are initiated in the module by the switching process travel away from the source of the noise and voltage overshoots. A low impedance network may be used to provide a return path for the high frequency energy associated with noise and voltage overshoots. The further the energy travels, the more difficult it is to provide a low impedance network to return the energy. Therefore, capacitors attached between the positive and negative DC connections or from the DC connections to ground must be relatively large to minimize the impact of noise, and voltage overshoots. In addition, these external capacitors typically cause stray inductance, which renders the capacitor ineffective at frequencies higher than about 10 kHz. 
   These and other problems are avoided and numerous advantages are provided by the method and device described herein. 
   SUMMARY OF THE INVENTION 
   The disclosure is directed to an architecture for a power module that limits or dampens voltage overshoot, permitting the power module to handle larger loads, and/or allowing the use of circuitry with lower ratings than would otherwise be required and thus reducing cost. 
   In one aspect, a power module comprises: a lead frame forming at least a portion of a module housing; a first set of terminals accessible from an exterior of the lead frame; a second set of terminals accessible from the exterior of the lead frame; a positive DC bus received at least partially in the module housing; a negative DC bus received at least partially in the module housing; a number of high side switches received in the module housing and selectively electrically coupling a first one of the first set of terminals to respective ones of the second set of terminals; a number of low side switches received in the module housing and selectively electrically coupling a second one of the first set of terminals to respective ones of the second set of terminals; and at least one capacitor electrically coupled between the positive DC bus and the negative DC bus. 
   In another aspect, a power system comprises: a lead frame; a plurality of electrical terminals carried by the lead frame; a first bus bar coupled to the lead frame; a second bus bar coupled to the lead frame; a high side substrate coupled to the lead frame, the high side substrate comprising a number of electrically conductive high side collector areas and a number of electrically conductive high side emitter areas, the high side emitter areas electrically isolated from the high side collector areas; a low side substrate coupled to the lead frame, the low side substrate comprising a number of electrically conductive low side collector areas and a number of electrically conductive low side emitter areas, the low side emitter areas electrically isolated from the low side collector areas; a number of high side switches physically coupled to the high side substrate; a number of low side switches physically coupled to the low side substrate; and a number of capacitors, each of the capacitors electrically coupled between one of the high side collector areas and one of the low side emitter areas. 
   In a further aspect, method of forming a power module comprises: providing a lead frame; coupling a substrate comprising a high side and a low side to the lead frame, the high side comprising a number of high side collector areas and a number of high side emitter areas electrically isolated from the high side collector areas, the low side comprising a number of low side collector areas and a number of low side emitter areas electrically isolated from the high side collector areas; mounting a number of high side switches to the high side of the substrate; mounting a number of low side switches to the low side of the substrate; surface mounting at least one capacitor to a low side emitter area; and surface mounting the at least one capacitor to a high side collector area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. 
       FIG. 1  is an isometric view of a power module comprising a housing, integrated cold plate, DC bus terminals, AC phase terminals, and power semiconductor devices. 
       FIG. 2A  is an isometric view of the power module of  FIG. 1  with a cover removed and some portions broken or removed to show the DC bus, the AC bus, and the power semiconductor devices carried by various regions on a substrate. 
       FIG. 2B  is a top plan view of the power module of  FIG. 2A  showing a representative sampling of wire bonds electrically connecting various power semiconductor components, buses, and layers in the substrate as an inverter. 
       FIG. 3  is a schematic cross sectional view of one embodiment of the DC bus comprising a pair of L-shaped vertical DC bus bars spaced by an electrical insulation. 
       FIG. 4  is a schematic cross sectional view of one embodiment of the DC bus comprising a pair of generally planar DC bus bars spaced by an electrical insulation. 
       FIG. 5A  is a partial isometric view of a portion of a low side of the power converter illustrating the surface mounting of snubber capacitors to a low side emitter area of the substrate. 
       FIG. 5B  is an isometric view of a portion of a high side of the substrate illustrating the surface mounting of the snubber capacitors of  FIG. 5B  to high side collector area of the substrate. 
       FIG. 6  is an electrical schematic of the switches, freewheeling diodes, and snubber capacitors according to an illustrated embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with power modules, power semiconductors and controllers have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention. 
   Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
     FIGS. 1 ,  2 A, and  2 B show a base power module  10 , generally comprising: a lead frame or housing  12 , an integrated cold plate  14  attached to the housing  12  via bushings  15 , a DC bus  16 , an AC bus  18 ; circuitry  20  electrically coupled between the DC bus  16  and AC bus  18 , forming a high side  20   a  and a low side  20   b  of the power module  10 . The base power module  10  may further include one or more gate drivers  22  for driving some of the power semiconductors  20 . 
   Two sets of DC bus terminals  24 ,  26  extend out of the housing  12 . In some applications one set of DC bus terminals  26  is electrically coupled to a positive voltage or high side of a power source or load and the other set of DC bus terminals is  24  is electrically coupled to a negative voltage or low side of the power source or load. In other applications, the DC bus terminals  24 ,  26  are electrically coupled to respective DC bus terminals  24 ,  26  on another power module. A set of AC phase terminals comprises three pairs of AC bus phase terminals  28   a ,  28   b ,  30   a ,  30   b ,  32   a ,  32   b , extending out of the housing  12 . In some applications, one pair of AC phase terminals coupled to a respective phase (A, B, C) of a three phase power source or load. In other applications, some of the AC phase terminals are interconnected across or between the pairs, and coupled to power sources or loads. 
     FIG. 3  shows a schematic cross-sectional view of the power module  10  taken along section line  3 — 3  of FIG.  2 A.  FIG. 3  is not an exact cross-sectional view, but has been modified to more accurately represent the electrical connections which would otherwise not be clearly represented in the FIG.  3 . 
   The integrated cold plate  14  comprises a metal base plate  39 , a direct copper bonded (DCB) substrate  40  which is attached to the metal base plate by a solder layer  41 . A cooling header  42  including a number of cooling structures such as fins  42   a , one or more fluid channels  42   b , a fluid inlet  42   c  and a fluid outlet  42   d  for providing fluid connection flow to and from the fluid channels  42   b , respectively. 
   The DCB substrate  40  typically comprise a first copper layer  40   a , a ceramic layer  40   b  and a second copper layer  40   c  which are fused together. The second copper layer  40   c  may be etched or otherwise processed to form electrically isolated patterns or structures, as is commonly known in the art. For example, the second copper layer  40   c  may be etched to form regions of emitter plating  43   a  (i.e., emitter plating areas or emitter areas) and collector plating  44   a  (i.e., collector plating areas or collector areas) on a low side of the power module  10  (i.e., side connected to DC bus bar  34 ). Also for example, the second copper layer  40   c  may be etched to form regions of emitter plating  43   b  and collector plating  44   b  on the high side of the power module  10  (i.e., the side connected to DC bus bar  36 ). 
   A conductive strip  45  or wire bonds may extend between the collector plating  44   a  of the low side and the emitter plating  43   b  of the high side, passing through respective passages  46  formed under the DC bus bars  34 ,  36 . As illustrated, the conductive strip  45  has be exaggerated in length on the low side of the power module  10  to better illustrate the electrical connection with the collector plating  44   a.    
   Power semiconductor devices  20  are attached to the various structures formed in the second copper layer  40   c  via a solder  47 . The power semiconductor devices  20  may include one or more switches for example, transistors  48  such as integrated bipolar gate transistors (IGBTs) or metal oxide semiconductor field effect transistors (MOSFETS). The power semiconductor devices  20  may also include one or more diodes  50 . The power semiconductor devices  20  may have one or more terminals directly electrically coupled by the solder  47  to the structure on which the specific circuit element is attached. For example, the collectors of IGBTs  48  may be electrically coupled directly to the collector plating  44   a ,  44   b  by solder  47 . Similarly, the cathodes of diodes  50  may be electrically coupled directly to the collector plating  44   a ,  44   b  by solder  47 . 
   The DC bus  16  comprises a pair of L-shaped or vertical DC bus bars  34   a ,  36   a . The upper legs of the L-shaped DC bus bars  34   a ,  36   a  are parallel and spaced from one another by the bus bar insulation  38 . The lower legs of the L-shaped DC bus bars  34 ,  36  are parallel with respect to the substrate  40  to permit wire bonding to appropriate portions of the substrate. For example, the negative DC bus bar  34   a  may be wire bonded to the emitter plating  43   a  of the low side, while the positive DC bus bar  36   a  may be wire bonded to the collector plating  44   b  of the high side. The emitters of the IGBTs  48  and anodes of the diodes  50  may be wire bonded to the respective emitter plating  43   a ,  43   b . Wire bonding in combination with the rigid structure of the DC bus  16  and housing  12  may also eliminate the need for a hard potting compound typically used to provide rigidity to protect solder interfaces. For low cost, the copper layers  40   a  and  40   c  may be nickel finished or aluminum clad, although gold or palladium may be employed at the risk of incurring higher manufacturing costs. 
     FIG. 4  shows another embodiment of the DC bus  16  for use in the power module  10 , the DC bus  16  comprising a pair of generally planar DC bus bars  34   b ,  36   b  parallel and spaced from one another by a bus bar insulation  38 . The DC bus bars  34   b ,  36   b  are horizontal with respect to a substrate  40  (FIGS.  1  and  2 ), with exposed portions to permit wire bonding to the various portions of the substrate  40 . 
   Because the DC bus bars  34 ,  36  are parallel, counter flow of current is permitted, thereby canceling the magnetic fields and their associated inductances. In addition the parallel DC bus bars  34 ,  36  and bus bar insulation  38  construct a distributed capacitance. As will be understood by one of ordinary skill in the art, capacitance dampens voltage overshoots that are caused by the switching process. Thus, the DC bus bars  34 ,  36  of the embodiments of  FIGS. 3 and 4  create a magnetic field cancellation as a result of the counter flow of current, and capacitance dampening as a result of also establishing a functional capacitance between them and the bus bar insulation  38 . 
   As best illustrated in  FIGS. 5A ,  5 B and  6 , the circuitry  20  includes a number of snubber capacitors  53  that are electrically coupled between the DC bus bars  34 ,  36  to clamp voltage overshoot. For example, some of the snubber capacitors  53  are electrically coupled directly (i.e., surface mounted) to the emitter plating  43   a  on the low side  20   b  of the power module  10  and are electrically coupled directly (i.e., surface mounted) to the collector plating  44   b  on the high side  20   a  of the power module  10 . While the Figures show two snubber capacitors for each switching pair combination, the power module  10  may include fewer or a greater number of snubber capacitors as suits the particular application. Significant savings may be realized by effective clamping of voltage overshoot. For example, if switching is maintained below approximately 900V, a transformer may be eliminated. The snubber capacitors  53  can be soldered in the same operation as the soldering of the substrate  40  to the cold plate  14 , or the soldering of other elements of the circuitry  20  to the substrate  40 , simplifying manufacturing and reducing costs. 
   As best illustrated in  FIGS. 2A and 2B , the circuitry  20  also includes a number of decoupling capacitors  55  which are electrically coupled between the DC bus bars  34  or  36  and ground to reduce EMI. In contrast to prior designs, the decoupling capacitors  55  are located on the substrate  40  inside the housing  12 . For example, some of the decoupling capacitors  55  are electrically coupled directly to the emitter plating  43   a  on the low side  20   b  of the power module  10  and some of the decoupling capacitors  55  are electrically coupled directly to the collector plating  44   b  on the high side  20   a  of the power module  10 . The decoupling capacitors  55  can be soldered in the same operation as the soldering of IGBTs  48  and  50  to the substrate  40 . 
   As best illustrated in  FIGS. 1 and 2A , the DC bus bars  34 ,  36  each include three terminals  24 ,  26 , spaced along the longitudinal axis, to make electrical connections, for example, to a DC power source. Without being restricted to theory, Applicants believe that the spacing of the terminals  24 ,  26  along the DC bus bars  34 ,  36  provides lower inductance paths within the DC bus bars  34 ,  36  and to the external DC voltage storage bank. 
   In contrast to typical power modules, the DC bus bars  34 ,  36  are internal to the housing  12 . This approach results in better utilization of the bus voltage, reducing inductance and consequently permitting higher bus voltages while maintaining the same margin between the bus voltage and the voltage rating of the various devices. The lower inductance reduces voltage overshoot, and problems associated with voltage overshoot such as device breakdown. The increase in bus voltage permits lower currents, hence the use of less costly devices. The bus bar insulation  38  between the DC bus bars  34 ,  36  may be integrally molded as part of the housing  12 , to reduce cost and increase structural rigidity. The DC bus bars  34 ,  36  may be integrally molded in the housing  12 , or alternatively, the DC bus bars  34 ,  36  and bus bar insulation  38  may be integrally formed as a single unit and attached to the housing  12  after molding, for example, via post assembly. 
   The power semiconductors  20  are directly mounted on the substrate  40  which is directly attached to the cold plate  14  via solder layer  41 , the resulting structure serving as a base plate. The use of a cold plate  14  as the base plate, and the direct mounting of the power semiconductors  20  thereto, enhances the cooling for the power semiconductors  20  over other designs, producing a number of benefits such as prolonging the life of capacitors  55 . 
   The power semiconductors  20  are operable to transform and/or condition electrical power. As discussed above, the power semiconductors  20  may include switches  48  and/or diodes  50 . The power semiconductors  20  may also include other electrical and electronic components, for example, capacitors  55  and inductors, either discrete or formed by the physical layout. The power module  10  and power semiconductors  20  may be configured and operated as an inverter (DC→AC), rectifier (AC→DC), and/or converter (DC→DC; AC→AC). For example, the power module  10  and/or power semiconductors  20  may be configured as full three phase bridges, half bridges, and/or H-bridges, as suits the particular application. 
   In at least one described embodiment, the power module  10  comprises three half bridges combined into a single three-phase switching module, or single half bridge modules that may be linked together to form a three phase inverter. As would be understood by one of ordinary skill in the art, the same DC to AC conversion may be accomplished with using any number of half bridges, which correspond to a phase, and each switching pair may contain any number of switching devices. For simplicity and clarity, many of the examples herein use a common three phase/three switching pair configuration, although this should not be considered limiting. 
   In at least one described embodiment, current flows from the power source through the positive DC bus bar  36  to the collector plating  44   b  on the high side of the power module  10 . Current is then permitted to flow through one or more of the switching devices  48  and/or diodes  50  on the high side to the emitter layer  43   b . The current passes to the collector layer  44   a  on the low side via the conductive strip  45  passing under the DC bus bars  34 ,  36 . A phase terminal allows current to flow from the collector layer  44   a  on the low side to a load such as a three phase AC motor. Similarly, the negative DC bus bar  34  couples the load to the switching devices  48  and/or diodes  50  on the low side via the emitter layer  43   a.    
   The overall design of the standard power module  10 , including the position and structure of the DC and AC buses  16 ,  18 , topology and modularity of substrates  40  and the inclusion of six phase terminals  28   a ,  28   b ,  30   a ,  30   b ,  32   a ,  32   b  in the AC bus  16  provides great flexibility, allowing the standard power module  10  to be customized to a variety of applications with only minor changes and thus relatively small associated costs. A number of these applications are discussed below. 
   Although specific embodiments of and examples for the power module and method of the invention are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the invention, as will be recognized by those skilled in the relevant art. The teachings provided herein of the invention can be applied to power module and power converters, rectifiers and/or inverters not necessarily the exemplary power module and systems generally described above. 
   While elements may be describe herein and in the claims as “positive” or “negative” such denomination is relative and not absolute. Thus, an element described as “positive” is shaped, positioned and/or electrically coupled to be at a higher relative potential than elements described as “negative” when the power module  10  is coupled to a power source. “Positive” elements are typically intended to be coupled to a positive terminal of a power source, while “negative” elements are intended to be coupled to a negative terminal or ground of the power source. Generally, “positive” elements are located or coupled to the high side of the power module  10  and “negative” elements are located or coupled to the low side of the power module  10 . 
   The power modules described above may employ various methods and regimes for operating the power modules  10  and for operating the switches (e.g., IGBTs  48 ). The particular method or regime may be based on the particular application and/or configuration. Basic methods and regimes will be apparent to one skilled in the art, and do not form the basis of the inventions described herein so will not be discussed in detail for the sake of brevity and clarity. 
   The various embodiments described above can be combined to provide further embodiments. All of the above U.S. patents, patent applications and publications referred to in this specification, including but not limited to: Ser. Nos. 60/233,992; 60/233,993; 60/233,994; 60/233,995 and 60/233,996 each filed Sep. 20, 2000; Ser. No. 09/710,145 filed Nov. 10, 2000; Ser. Nos. 09/882,708 and 09/957,047 both filed Jun. 15, 2001; Ser. Nos. 09/957,568 and 09/957,001 both filed Sep. 20, 2001; Ser. No. 10/109,555 filed Mar. 27, 2002; and Ser. No. 60/471,387 filed May 16, 2003, are incorporated herein by reference, in their entirety, as are the sections which follow this description. Aspects of the invention can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments of the invention. 
   These and other changes can be made to the invention in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all power modules, rectifiers, inverters and/or converters that operate or embody the limitations of the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.