Abstract:
A MOSFET die and a Schottky diode die are mounted on a common lead frame pad and their drain and cathode, respectively, are connected together at the pad. The pad has a plurality of pins extending from one side thereof. The lead frame has insulated pins on its opposite side which are connected to the FET source, the FET gate and the Schottky diode anode respectively by wire bonds. The lead frame and die are molded in an insulated housing and the lead frame pins are bent downwardly to define a surface-mount package.

Description:
[0001]    This application is a continuation Application Ser. No. 09/645,060, filed Aug. 24, 2000, which is a continuation of Application Ser. No. 09/161,790, filed Sep. 28, 1998, which is a continuation of Application Ser. No. 08/816,829, filed Mar. 18, 1997, now U.S. Pat. No. 5,814,884, and claims the priority of Provisional Application Serial No. 60/029,483 filed Oct. 4, 1996. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to semiconductor devices, and more specifically relates to a novel device in which a plurality of die, which may be of diverse size and of diverse junction pattern, are fixed to a common lead frame and within a common package or housing.  
         BACKGROUND OF THE INVENTION  
         [0003]    Numerous electrical circuits, for example, DC to DC converters, synchronous converters, and the like require a number of semiconductor components such as MOSFETs and Schottky diodes. These components are frequently used in portable electronics apparatus and are commonly separately housed and must be individually mounted on a support board. The separately housed parts take up board space. Further, each part generates heat and, if near other components, such as microprocessors, can interfere with the operation of the microprocessor.  
           [0004]    It would be desirable to reduce the board space required by plural semiconductor devices and to reduce part count and assembly costs in power converters and other power subsystems for high-density applications.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0005]    In accordance with the invention, two or more diverse semiconductor die are laterally spaced and mounted on a common lead frame with a first one of each of their power terminals electrically connected to the lead frame. The main lead frame body then has a first set of externally available pins which are used to make connection to the first one of the power terminals of each of the diverse die. The die are also provided with second power terminals at the tops of the die, and these are connected to respective external pins of the lead frame which are isolated from one another and from the first set of external pins. One or more of the die may also contain a control terminal, such as the gate electrode of a MOSFET die, and a further and isolated pin of the lead frame is connected to this gate terminal.  
           [0006]    The lead frame and die are then over-molded with a suitable insulation compound housing, with the various pins extending in-line and beyond the edge surfaces of the housing and available for external connection.  
           [0007]    The housing may take the form of a surface-mounted housing with a very small “footprint”. By way of example, a MOSFET die and a Schottky diode die may be contained within and may have their drain electrodes and cathode electrodes respectively soldered to a common conduction lead frame pad to be interconnected within the housing. The FET source and gate terminals on top of the die are wire bonded to insulated lead frame pins and the top Schottky diode anode is also connected to an isolated pin so that any desired external connection can be made to the package.  
           [0008]    While any package style can be used, the novel invention has been carried out with an SO-8 style small outline package.  
           [0009]    The novel package of the invention can improve efficiency of a DC to DC converter by reducing power drain on batteries, leading to a longer life. For desk top systems, the device reduces power dissipation and heat generation near temperature-sensitive parts such as microprocessors. The device also provides substantial savings in board space while reducing component count and assembly costs.  
           [0010]    For example, the use of a copackaged FET Type IRF7422D2 (a (−20) volt 90 mohm P channel FET) and a Schottky diode (30 volt, 1 ampere) in a buck converter circuit provided a 60% saving in board space and assembly cost.  
           [0011]    Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a circuit diagram of a known buck converter circuit using a P channel MOSFET.  
         [0013]    [0013]FIG. 2 is a circuit diagram of a buck converter circuit employing an N channel MOSFET and a parallel Schottky diode.  
         [0014]    [0014]FIG. 3 is a perspective diagram of an SO-8 style package which can be used to house both the MOSFET die and Schottky die of FIGS. 1 and 2 in accordance with an embodiment of the invention.  
         [0015]    [0015]FIG. 4 is a schematic top view of the package of FIG. 3 with the die of the circuit of FIG. 1 copackaged on a common lead frame.  
         [0016]    [0016]FIG. 5 shows a top view of the lead frame of the package of FIGS. 3 and 4 with the MOSFET die and Schottky diode die fastened to the lead frame.  
         [0017]    [0017]FIG. 6 is an enlarged view of the portion of FIG. 5 which is within the dashed line in FIG. 5.  
         [0018]    [0018]FIG. 7 is a schematic top view of an alternative embodiment of the package of FIG. 3 with the die of the circuit of FIG. 1 copackaged on a common lead frame.  
         [0019]    [0019]FIG. 8 shows a top view of the lead frame of the package of FIG. 7 with the MOSFET die and the Schottky diode die fastened to the lead frame.  
         [0020]    [0020]FIG. 9 is an enlarged view of the portion of FIG. 8 which is within the dashed line in FIG. 8.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring first to FIG. 1, there is shown a conventional buck converter circuit, sometimes known as a step down converter, which is commonly used to reduce the voltage to integrated circuits and processors on the circuit board of a portable electronic device or the like. For example, the circuit might be used to reduce an input voltage of 12 volts DC to 5 volts DC (or 3.3 volts DC in some cases) to drive an integrated circuit or other load (not shown).  
         [0022]    The circuit of FIG. 1 is well known and uses a P channel MOSFET  10  for the switching function under the control of a suitable control circuit  11  connected to the FET gate G. FET  10  may be a 20v, 90 m-ohm die available from the International Rectifier Corporation. A Schottky diode  12  which may be a 30 volt, 1 ampere die has its cathode connected to the drain D of FET  10  and is used to perform output current recirculation into inductor  13  and capacitor  14 . As will be later shown, and in accordance with the invention, FET  10  and Schottky diode  12  are provided in die form and are mounted on a common lead frame of a single package shown by dotted line block  15 . This novel combination produces a 60% space saving on the support board of the device and reduces assembly cost.  
         [0023]    It will be apparent that the invention can be employed in many other circuit configurations. For example, FIG. 2 shows a synchronous buck converter circuit using an N channel MOSFET  20  as the switching device, an N channel MOSFET  21 , and a Schottky diode  22  in parallel for synchronous rectification.  
         [0024]    In accordance with the invention, FET  21  and Schottky diode  22  may be die which are copackaged within a common housing, as shown by dotted block  23 . This circuit is useful to avoid losses found in the “lossy” forward voltage drop of the Schottky diode  12  of FIG. 1. It also eliminates the effects of the inherent body diode of the vertical conduction FET  21  from the circuit since the Schottky diode  22  handles the reverse current flow seen by the synchronous rectifier during the “wait” state of controller  24 .  
         [0025]    FET  21  of FIG. 2 may be a 30v, 35 m-ohm die available from the International Rectifier Corporation.  
         [0026]    Housings  15  and  23  may take the form of a known housing Type SO- 8 , shown in FIGS. 3 and 4. Thus, FIG. 3 shows an SO- 8  surface mount housing with eight in-line pins  1  to  8  (FIG. 4) which extend from a plastic insulation housing  30 . As seen in FIG. 4, the FET die  10  and Schottky diode  12  are internally mounted on a common lead frame, as will be later described and are interconnected to enable their external connection as in FIG. 1 or  2  (with an appropriate FET die  10  or  21 ) or in other circuit configurations.  
         [0027]    In FIG. 4, the drain of FET  10  and cathode of Schottky diode  12  are connected to one another and to pins  5  to  8  of a common lead frame section as will be later described. The source and gate of FET  10  are connected by wire bonds to isolated pins  3  and  4 , respectively, and the anode of Schottky diode  12  is connected by wire bonds to isolated pins  1  and  2 .  
         [0028]    [0028]FIGS. 5 and 6 show the lead frame and FET  10  and Schottky  12  die in more detail. Thus, a lead frame  40  is provided which contains a main pad body  41  from which pins  5  to  8  integrally extend. The main pad body  41  is larger than the main pad body of a conventional lead frame so that both the FET die  60  and the Schottky diode  12  may be mounted to it. According to a novel aspect of the invention, the walls of plastic insulation housing  30  are thinner than a conventional housing to accommodate the larger main pad body without significantly reducing resistance to moisture.  
         [0029]    The lead frame also contains pins  1  to  4  and respective bond pad extensions which are within molded housing  30 . These are originally integral with the lead frame body  40  (during molding), but are shown in their severed condition which isolates pins  1  to  4  from one another and from main pad  41 . Typically, pins  1  to  4  are coplanar with each other and with the main bond pad  41 .  
         [0030]    Lead frame  40  is a conductive frame and may have a conventional lead frame solder finish. The bottom cathode surface of diode  12  and the bottom drain surface of FET  10  are connected to pad  41  as by a conductive epoxy die attach compound and are thus connected to pins  5  to  8 . Alternatively, the cathode surface of diode  12  and the drain surface of FET  10  are soldered to pad  41  or are connected to the pad using a conductive glass containing silver particles.  
         [0031]    The top anode electrode of Schottky diode  12  is wire bonded by gold bonding wires  50  and  51  to pins  1  and  2 , respectively (before molding), while the source electrode and gate electrode of die  10  are bonded by gold wires  52  and  53  to the internal bonding extensions of pins  3  and  4 , respectively, also before molding the housing  30 . Alternatively, aluminum bonding wires are used. The internal bonding extension of the pins are typically silver or gold plated. The bonding wires are generally bonded to the die surface and to the internal bonding extensions using thermosonic ball bonding, as is known in the art, though other processes may be used.  
         [0032]    Thereafter, the molded housing, which may be a mold compound such as NITTO MP7400. It is formed in a conventional molding operation. However, other types of housings, such as a ceramic housing, a hermetic housing or an injection molded metal housing, may be used.  
         [0033]    It should be noted that other package styles could be used, but the copackaging in a surface-mount package conserves considerable board space. The resulting device can be soldered down to a printed circuit board using conventional mass production soldering techniques.  
         [0034]    [0034]FIGS. 7 and 8 shows an alternative embodiment of the invention in which the source of FET  10  is connected by wire bonds  151  and  152  to isolated pins  2  and  3 , the gate of FET  10  is connected by wire bonds  153  to isolated pin  4 , and the anode of Schottky diode  12  is connected by wire bonds  150  to isolated pin  1 . The drain of FET  10  and the cathode of Schottky diode  12  are connected to one another and to pins  5  to  8  of a common lead frame section in the manner described above.  
         [0035]    [0035]FIGS. 8 and 9 show the lead frame of this embodiment and the FET  10  and the Schottky diode  12  in greater detail. The lead frame  140  is similar to the lead frame  40  described above and includes a similar main pad body  141 . The bottom cathode surface of Schottky diode  12  and the bottom drain surface of FET  10  are connected to pad  141  in a similar manner to that described above, and the top anode electrode of Schottky diode  12  and the source and gate electrodes of FET die  10  are similarly bonded to the internal bonding extensions of the pins as described above. Similarly, the housing  130  is formed in the manner described above.  
         [0036]    Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not only by the specific disclosure herein, by only by the appended claims.