MINITAB RECTIFYING DIODE PACKAGE
WITH TWO DIFFERENT TYPES OF DIODES
This application claims the benefit of 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 three 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 are needed for many modern automotive alternator applications. Further, the upward-facing or free electrode (anode or cathode) of the button diode requires a separate clip connector for connecting one of the circuit a-c output leads to said free electrode of the button diode.
It would be desirable to provide a diode structure which is reliable for operation in an alternator application at an output power 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
5 75 amperes and an 1AV (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
10 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 20 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 25 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.
30 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.
35 DETAILED DESCRIPTION OF THE
FIG. 1 is a circuit diagram of a typical alternator circuit to which the diode of the invention may be applied. Thus, an
4q 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
45 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.
50 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 symmet
55 ric 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
60 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 force
65 ably clipped onto the top of diode 22 and contacts anode electrode 42 and provides the terminal for node 28 in FIG. 1.