Abstract:
A circuit board is provided with a plurality of MOSFETs (metal oxide semiconductor field effect transistors) each of which include a field effect transistor and an associated control circuit. The control circuits are mounted in a control section of the board remote from a power section mounting the MOSFETs. The present invention reduces the assembly steps required in the prior art wherein the MOSFET and its control circuit were mounted as preassembled units to the board. Moreover, the number of MOSFETs per unit area of board is increased by the present invention.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a circuit board having a plurality of power semiconductors, such as metal oxide semiconductor field effect transistors (MOSFETs) for handling electrical power. As is known, each MOSFET is associated with a control circuit and resistor elements. The present invention separates these three components into at least two sections on the circuit board. Thus, the MOSFETs are mounted as separate units directly to the board in a power section, with the control circuits mounted in a control section. 
     In the prior art, circuit boards for handling power typically include a plurality of MOSFETs. These MOSFETs have historically been mounted to a board substrate as a number of preassembled units. Each of the preassembled units includes the control circuit, as well as the MOSFET. Further, the subassemblies have typically been provided with separate, ceramic layers carrying the MOSFET and the control circuit. Resistors are sometimes mounted between the two ceramic layers. An outwardly extending mount structure attaches all of these components as a preassembled unit to the board substrate. 
     The prior art circuit boards had undesirably high assembly time and cost due to the various steps required to form the subassembled unit. Further, preassembled units require more space than would be desirable, and thus reduced the available “density” of MOSFETs per given area of circuit board. 
     SUMMARY OF THE INVENTION 
     In a disclosed embodiment of this invention, at least the control circuits for the power semiconductors, and the power semiconductors themselves, are mounted in separate sections. The control circuits are preferably associated with individual ones of the power semiconductors through circuit traces. By separating the two components, the two components can be mounted directly to the board, without any detailed or complex intermediate assembly steps. Moreover, the control for each power semiconductor is preferably also mounted in the same section as controls for the whole board. A thermal barrier is preferably mounted between a control section (carrying the controls) and a power section (carrying the power semiconductors, such as MOSFETs). 
     The present invention reduces the number of steps required to form a circuit board including a plurality of power semiconductors, and further allows for a greater density of power semiconductors per unit area of circuit board. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan schematic view of the prior art circuit board. 
         FIG. 2  is a cross-sectional view through the  FIG. 1  circuit board. 
         FIG. 3  is a plan view of the inventive circuit board. 
         FIG. 4  is a schematic view of the inventive circuit board. 
         FIG. 5  is a cross-sectional view through the inventive circuit board. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A circuit board  20 , as known in the prior art, is illustrated in  FIG. 1 . The circuit board is of a type utilized in power distribution networks, and in particular for aerospace applications. A number of power MOSFETs are associated with a power section  22  on the circuit board  20 . A connector  24  communicates through the circuit board  20  between a power supply and components to be powered. A communication section  26  includes control circuits, but control circuits generally for the entire circuit board  20 , rather than a particular control circuit for each of the MOSFETs in the power section  22 . While power MOSFETs are specifically disclosed, it should be understood that this invention would extend to circuit boards utilizing other types of power semiconductors. As an example, IGBTs, thyristors, SCRs, diodes, etc. could be utilized under the teachings of this invention. 
     As shown in  FIG. 2 , the board substrate  30  receives a number of preassembled units  23 , known in the art as “hybrid” units. The hybrid units are generally preassembled units  23  incorporating the MOSFET  38  mounted to one ceramic layer  19 , a surface mount technology (SMT) control circuit  29  mounted to another ceramic layer  27 , and resistors  36  sandwiched between the two ceramic layers  19  and  27 . Wire bond  25  electrically connects the ceramic layer  27  to layer  19 , and is encapsulated at  126 . Potting material  32  is formed within a cap  31 , and an outer mount structure with pins  40  holds the entire preassembled unit  23  to board substrate  30 . The potting materials environmentally protect the components and the boards. Control circuit  28  is mounted in communication section  26 , and is a control for the circuit board  20 , rather than any individual one of the preassembled units  23 . 
     The prior art required a good number of separate assembly steps to form the preassembled units  23 . Each of the individual components was assembled together to form the preassembled units  23 . Moreover, as can be appreciated from  FIG. 2 , a good deal of additional space is required outwardly of the MOSFET  38  to house all of the various components and the pins  40 . Thus, the prior art did not utilize the space on board substrate  30  as effectively as may be desired. 
       FIG. 3  is a plan view of an inventive circuit board  50 . Circuit board  50  incorporates a power section  54  receiving a number of separate MOSFETs. Connector  52  is provided as in the prior art. A separate control and communication section  56  is associated with the circuit board  50  and a thermal barrier  58  is provided between the control and communication section  56  and the power section  54 . As can be appreciated, the power section  54  carries a number of MOSFETs, and would be hot during operation. 
     As shown schematically in  FIG. 4 , the control and communication section  56  carries the control circuits (SMTs, as an example) for each of the individual MOSFETs, and the resistors. Further, as in the prior art, control circuits for controlling the overall board function are placed in the control and communication section  56 . The thermal barrier  58  may be formed by perforating a board substrate  90 , or by limiting the amount of copper traces along the thermal barrier  58  to limit the heat coupling between the power section  54 , and control and communication section  56 . By limiting the amount of copper, a material forming the board substrate  90  (e.g., FR4) provides a good thermal break and insulator. 
     As can be appreciated from  FIG. 5 , the control and communication section  56  carries the control circuit  60  for each MOSFET, and the thermal barrier  58  is formed between the control and communication section  56  and power section  54 . The MOSFETs  62 / 66  are each mounted as a naked die directly to the underlying board substrate  90 , and a wire bond  68  may connect the MOSFETs  62 / 66  to the board substrate  90 . Potting material  64  may also protect the MOSFET and the wire bond  68 . 
     As can be appreciated, and as shown in dotted line in  FIG. 5 , each MOSFET  62 / 66  is electrically communicated with its associated control circuit  60  by a circuit trace. Thus, the present invention requires a good deal fewer assembly steps in that the MOSFETs and associated controls are not assembled as a unit, but rather are mounted to the board as separate parts that are later communicated. 
     The present invention also better utilizes space and ensures that the density of power semiconductors per unit area of board is increased. As can be appreciated, at least some of the MOSFETs are positioned closer to each other in the power section than they are to their respective control circuits which are located in the control section. Similarly, the respective control circuits would be positioned closer to each other than they are to their respective MOSFETs. While the control circuit for each individual MOSFET may be maintained separately within the control section, it is also within the scope of this invention that the several control circuits be combined as part of a single control circuit. In such a case, the individual control functions for each MOSFET would still be accomplished by a control mounted within the control section, and such an embodiment would still come within the scope of an “individual control circuit” for each of the power semi conductors or MOSFETs. 
     Again, while MOSFETs are specifically disclosed, a circuit board incorporating other types of power semiconductors like IGBTs, thyristors, SCRs, diodes, etc. will also benefit from this invention. 
     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.