Patent Publication Number: US-6713937-B2

Title: Minitab rectifier for alternators

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 09/909,454, filed Jul. 19, 2001 now U.S. Pat. No. 6,627,975, entitled MINITAB RECTIFIER FOR ALTERNATORS which claims priority to U.S. Provisional Application No. 60/274,991, filed Mar. 12, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to semiconductor devices and more specifically relates to a novel diode structure for application to automotive structures. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices are commonly used in automotive applications such as in three-phase bridge designs for automotive alternators. These are generally mounted in the hostile environment of “under-hood” locations. Despite this hostile environment (heat, vibration, shock forces, etc.), high reliability and thermal efficiency are key requirements. 
     Button diodes of a well-known type are commonly used for automotive alternators. Such button diodes have a cylindrical shape with a cylindrical conductive outer rim and flat top and bottom electrodes which are insulated from the rims, and define the cathode and anode electrodes of the device. These button diodes are commonly mounted on two separate heat sink sections which form the positive and negative d-c rails for the output of the three-phase bridge circuit. The cathode electrodes of the diodes are mounted on the negative bus and the three other button diodes are flipped over with their anodes mounted on the positive bus. 
     Conventional button diodes have been found to be unreliable for alternator outputs above about 2 kW, which is needed for many modern automotive alternator applications. Further, the upper-facing or free electrode of the button diode requires a separate clip connector for connection to the circuit a-c output leads and to terminate either the anode or cathode terminal of the button diode. 
     It would be desirable to provide a diode structure which is reliable for operation in an alternator application at output powers in excess of 2 kW, and which can be applied to existing alternator structures and heat sinks. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with the invention, a novel alternator diode is provided which can directly replace a button diode in an existing alternator structure, but can be reliably used for higher output power. 
     The novel diode of the invention consists of a diode package of a TO 220 type outline and which has an internal diode die having one electrode mounted to a first lead frame section and its other electrode wire bonded to an extending second lead frame tab which is insulated from the first section. The die and lead frame sections are over-molded with a conventional plastic housing, with the bottom of the first lead frame section exposed for surface mounting and with the second and tab section of the lead frame extending through the side wall of the housing. The end of the second section is preferably forked to define an easy screw or bolt connector connection to the common a-c connection. The exposed bottom surface of the first section and the extending tab are preferably metallized with a solderable finish. The die within the package can be mounted with the anode side up or down to define the diode to be connected to the positive or negative d-c bus respectively. 
     In one embodiment of the invention, the diode may be a Zener diode which, at 25° C. has a Zener voltage of 28/33 volts; a forward voltage drop of 1 volt at 100 amperes; an R junction-case of 0.6° C./W; a lead current rating greater than 75 amperes and an I AV  (180° Rect.) of 80 amperes at a case temperature of 125° C. This rating is suitable for many alternator designs with outputs available above the 3 kW level. Further, the novel structure provides lower assembly costs and, critically, more reliable operation at higher power in the hostile “under-hood” environment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a known three-phase bridge alternator circuit for automotive alternator operation. 
     FIG. 2 is a side view of a prior art button diode. 
     FIG. 3 is a top view of FIG.  2 . 
     FIG. 4 is a side view in partial cross-section, of the button diode of FIG. 2 mounted on a heat sink 
     FIG. 5 is a top view of FIG.  4 . 
     FIG. 6 is a top view of the novel diode of the invention. 
     FIG. 7 is a side view of FIG.  6 . 
     FIG. 8 is a partial cross-section of the device of FIG. 7 when mounted on a heat sink. 
     FIG. 9 is a cross-section of the diode of FIGS. 6 and 7. 
     FIG. 10 is the first and second lead frame sections used to manufacture a plurality of diodes of the kind shown in FIGS. 6,  7  and  9 . 
     FIG. 11 shows a side view of FIG.  10 . 
     FIG. 12 shows the lead frame sections of FIG. 10 after they are welded together. 
     FIG. 13 is a side view of FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a circuit diagram of a typical alternator circuit to which the diode of the invention may be applied. Thus, an automotive battery may have its positive and negative terminals connected to d-c heat sink rails  20  and  21  respectively. The cathode terminals of diodes  22 ,  23  and  24  are connected to the common heat sink rail  21  and the anode terminals of diodes  25 ,  26  and  27  are connected to the common heat sink rail  20 . The a-c nodes  28 ,  29  and  30  of the three-phase bridge are connected to the terminals of alternator motor  31  and the d-c terminals  20  and  21  are connected to the field control  32  of the field winding of the motor  31 . 
     The alternator diodes  22  to  27  in the prior art have been button diodes, having the structures shown in FIGS. 2 and 3 for the case of diode  22 . Thus, diode  22  has a cylindrical rim  40  with top and bottom electrodes  41  and  42  which are cathode and anode electrodes respectively, and are symmetric with respect to one another. An insulation bead, such as bead  43  insulates rim  40  from electrode  41 . The anode electrode  42  is similarly insulated. A conventional silicon diode die, not shown, is contained within the rim  40  and its top and bottom electrodes are connected to electrodes  41  and  42  respectively. 
     FIGS. 4 and 5 show the manner in which diode  22  is connected to negative heat sink rail  21 . Thus, cathode  41  is soldered, or otherwise affixed to heat sink  21 . A thin conductive spring clip  50  having an extending tab  51  is forceably clipped onto the top of diode  22  and contacts anode electrode  42  and provides the terminal for node  28  in FIG.  1 . 
     Each of diodes  23  and  24  are similarly connected to heat sinks  21  and are arranged to have their clips connected to nodes  29  and  30  respectively. 
     Diodes  25 ,  26  and  27  are similarly constructed, but these are flipped over so that their anodes  42  are fixed to heat sink  20  and their cathodes are connected to spring clips for connection to nodes  28 ,  29  and  30  respectively. 
     In accordance with the present invention, diodes  22  to  27  of FIG. 1 have a modified TO  220  type structure as shown in FIGS. 6 and 7 for diode  125  (which is connected at the location of diode  25  in FIG.  1 ). 
     Diode  125  has a surface mounted anode electrode  142  (FIGS.  7  and  9 ), an extending cathode tab-type electrode  141 , and an insulation housing  160  which maybe conventionally transfer molded. The extending cathode tab  141  is forked, as shown in FIG. 6 for easy connection at node  28  in FIG.  1 . 
     The internal structure of diode  125  is best shown in FIG.  9 . Thus, a silicon die  161  has conventional aluminum cathodes and anode electrodes  162  and  163  on its opposite surfaces. Anode electrode is soldered by solder mass  164  to the relatively thick first lead frame section  147 . Solder mass  147  may be enclosed by insulation epoxy bead  165 . The top anode electrode  163  is wire bonded by wire bond  166  to cathode terminal  141 . 
     The bottom of anode section  147  may be metallized to be easily solderable, and the portion of cathode terminal  141  which protrudes through the side wall of insulation housing  160  is also metallized to be solderable. The anode section  147  may then be easily soldered to the top of heat sink  20  as shown in FIG.  8 . 
     Each of the diodes in the locations of diodes  26  and  27  will have the same structure as shown in FIGS. 6,  7  and  9  and are also soldered to the common heat sink  20 . 
     The diodes in the positions of diodes  22 ,  23  and  24  are identical to the diode of FIGS. 6,  7  and  9 , except that the polarity is reversed. Thus, these diodes have the die  161  reversed within housing  160  in FIG. 9 so that the lead frame section  147  is a cathode and section  141  is an anode. 
     FIGS. 10 to  13  show the novel lead frame which can be used to make the device of FIGS. 6 to  9 . Thus, in FIGS. 10 and 11, two lead frame sections  170  and  171  are shown. Lead frame section  170  is a lead frame which is used for a type D-2 pak lead frame and provides a relatively thick lead frame section for the first lead frame sections  147 . Such D-2 pak housings are made by International Rectifier Corporation of El Segundo, Calif. The second lead frame section  171  is etched from a thin copper strip and defines the terminal elements  141  of FIGS. 6 to  9 . These two strips  170  and  171  are laser welded together as shown in FIGS. 12 and 13. Silicon die  161  are then die bonded to the lead frame sections  147  and their tops are wire bonded to sections  141 . The die are oriented to have cathode electrodes  163  either down or up, as needed for the diodes in locations  22 ,  23 ,  24  or  25 ,  26 ,  27  respectively. The lead frames are then over-molded with insulation plastic housing  160  and the individual segments are separated to define the individual die. 
     In the above devices, both in a forward and reverse configuration, various metallization may be used. Thus, the wire bond to leads may employ aluminum metallizing. The solder die attach to the heat sink may employ a Ti/Ni/Ag metallization. There are also two silicon wafer types which can be used; one with an aluminum anode and a Ti/Ni/Ag cathode; or a Ti/Ni/Ag anode and a Ti/Al cathode. 
     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 by the specific disclosure herein.