Abstract:
A capacitor module for use with an IGBT including an epoxy housing having two holes completely therethrough. Within the housing are two wound capacitors which are connected in parallel. Additionally, there are two mounting inserts which at least partially line the holes of the housing. The wound capacitors are also connected to the first and second mounting inserts which serve as the terminals for the capacitor module. As a result, the capacitor module exhibits very low self-inductance as well as very low ESR.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to capacitors for use with an Insulated Gate Bipolar Transistor (IGBT) and, more particularly, it relates to a capacitor module designed for direct mounting to an IGBT and a method for making the same. 
     An IGBT is a device that combines the features of a power transistor and a MOSFET. In particular, an IGBT has the high input impedance and high-speed characteristics of a MOSFET with the conductivity characteristics (low saturation voltage) of a bipolar transistor. See Dorf, The Electrical Engineering Handbook, CRC Press (1993), ISBN 0-8493-0185-8. 
     IGBTs are frequently used for power switching applications. In such applications, snubber circuits are used to &#34;snub out&#34; transient spikes generated by the IGBT when it switches from one state to another. It is known to use leaded capacitors in snubber circuits; however, the leads, which connect the capacitor(s) to the IGBT terminals, generate inductance and equivalent series resistance (ESR). Both of which lead to degraded performance of the snubber circuit and, consequently, degraded switching characteristics for the IGBT. 
     An additional drawback of using leads to connect capacitor(s) directly to IGBT terminals is uncertain reliability. Because leads are typically fragile and sensitive to vibration, they present a &#34;weak link&#34; in power applications which use IGBTs such as electric vehicles, motor applications, controllers, high power inverters and power conditioning systems. 
     SUMMARY OF THE INVENTION 
     The present invention involves a capacitor module including a housing having at least two holes. Within the housing are at least two wound capacitors and, at least partially lining the holes, are first and second mounting inserts. The capacitors are connected in parallel by way of the first and second mounting inserts which serve as the terminals for the capacitor module. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with accompanying drawing, in which: 
     FIG. 1 shows an exemplary embodiment of the present invention being incorporated into an exemplary application; 
     FIG. 2A shows a cut-away top view of the exemplary embodiment of the present invention used in FIG. 1 without the capacitors; 
     FIG. 2B shows a cut-away side view of the exemplary embodiment of the present invention used in FIG. 1 without the capacitors; 
     FIG. 3A shows the cut-away top view of FIG. 2A with the capacitors; 
     FIG. 3B shows a cut-away side view of FIG. 2B with the capacitors; 
     FIG. 4 and 4B shows an exemplary embodiment of a mounting insert used in the embodiment shown in FIGS. 1, 2A, 2B, 3A and 3B; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an exemplary application of the present invention. In FIG. 1, a capacitor module 110 is bolted, via bolts 112, to bus bars 114 and an Insulated-Gate Bipolar Transistor (IGBT) 116. Both the bus bars 114 and IGBT 116 are well known in the art, for example, exemplary bus bars are manufactured by Rogers Corporation of and an exemplary IGBT is manufactured by International Rectifier. 
     Referring back to FIG. 1, an important purpose of capacitor module 110, in the exemplary application, is to facilitate the operation of IGBT 110 while minimizing inductance and equivalent series resistance (ESR). This purpose is desirably met while, at the same time, satisfying size constraints. Capacitor module 110 facilitates IGBT operation by providing the desired parameters (e.g., capacitance and voltage rating) for snubbing potential spikes during switching of IGBT 116 which, in turn, improves the service life of IGBT 116 and attenuates noise going back into the lines. 
     Furthermore, capacitor module 110 is able to minimize inductance and ESR by eliminating external leads. In fact, as measured by laboratory experiments, the inductance of capacitor module 110 is approximately 90% less than conventional leaded capacitors for the exemplary application. 
     Moreover, capacitor module 110 facilitates IGBT installation and assembly by forming a complete subassembly which mounts directly to IGBT 116, thereby reducing assembly time for power supply manufacturers. 
     In addition to the above advantages, housing the capacitors windings (see FIGS. 3A and 3B) in an epoxy housing, improves flame retardance as well as heat transfer characteristics. 
     FIGS. 2A and 2B show a cut-away top view and a cut-away side view of capacitor module 110 without the capacitors therein. As shown in FIGS. 2A and 2B, capacitor module 110 includes a housing 210 which houses two capacitors (see FIGS. 3A and 3B) and which has two counter-sunk holes 214 bored all the way through. In the exemplary embodiment, holes 214 are shown in FIG. 2A as off-center in capacitor module 110 due to the spacing requirements of IGBT 116 terminals. 
     Each hole 214 bored through capacitor module 110 is, at least partially, lined with a conductive terminal mounting insert 216. In an exemplary embodiment of the present invention, as shown in FIG. 4A and 4B, insert 216 includes a wide portion 404 and a narrow portion 408. Lip 410 which transitions mounting insert 216 from wide portion 404 to narrow portion 408 is for the head of bolt 112 to press against in securing capacitor module 110 to bus bars 114 and IGBT 116. Mounting insert 216 is preferably constructed of tin-plated, nickel-coated copper or brass. 
     Referring back to FIGS. 2A and 2B, mounting inserts 216 are sufficiently spaced and extend beyond a face of housing 210 to provide terminals for connection to IGBT 116. In particular, the center-to-center spacing of inserts 216 substantially match the center-to-center spacing of the terminals for IGBT 116 and each mounting insert 216 protrudes beyond the bottom of housing 210 to form respective terminals: a positive terminal 218 and a negative terminal 220, respectively. 
     In the preferred embodiment, insert 216 extends approximately 0.25 inches beyond the bottom of housing 210. The two terminals are provided so that bolts 112 can be used to attach capacitor module 110 to IGBT 116. Bolts 112 extend through holes 212 in capacitor module 110, through corresponding holes in bus bars 114 and mate with the terminals of IGBT 116 thus providing a capacitor module subassembly for direct mounting to IGBT 116. Tightening bolts 112 physically secures, as well as electrically connects, capacitor module 110 to bus bars 114 and IGBT 116. 
     FIGS. 3A and 35 show top and side cut-away views with capacitor windings 310 therein. As shown in FIG. 3A, in the exemplary embodiment, each capacitor winding 310 is positioned completely on one side of a plane intersecting mounting inserts 216. In the exemplary embodiment, leads 312 are wrapped around respective mounting inserts 216 and connected to each capacitor winding 310 capacity windings can be connected in with series or parallel. In the exemplary embodiment, leads 312 are constructed of solid copper and soldered to the respective ends of capacitor windings 310. An alternate embodiment of leads 312 comprises a flat copper tab approximately 1/4 inch wide and 15-20 mils thick. 
     As leads 312 are the primary source of inductance, the length of leads 312 are minimized in order to minimize the inductance. Minimizing lead length by extending directly from insert 216 to capacitor windings 310 is particularly important since an additional 1/4 inch of lead length could approximately double the inductance. In so doing, each end of capacitor windings 310 are connected to either positive terminal 218 or negative terminal 220. 
     Wound capacitors for use in capacitor module 110 are well known in the art. An example of a wound capacitor 310 suitable for use with the present invention is found in U.S. Pat. No. 4,470,097 issued to Lavene on Sep. 4, 1984 which is herein incorporated by reference. Additionally, in the exemplary embodiment, capacitors 310 are at least partially covered with an insulating wrap or tape 320 in order to prevent direct contact between the body of a wound capacitor 310a and a portion 216a of mounting insert 216. Once capacitor windings 310 are connected to mounting inserts 216 via leads 312, they are positioned within a hard-casing mold and the mold is filled with an epoxy resin. Then, the epoxy is given time to cure, thereby forming a hard, solid housing for capacitors windings 310 and leads 312. 
     In the exemplary embodiment of the present invention, the epoxy contains a high alumina content such as substantially pure aluminum oxide. The high alumina content 1) reduces curing time, 2) improves flame retardance and 3) improves heat transfer characteristics. Improved heat transfer characteristics are important because capacitor windings 310 have heat rise which can adversely affect the rms current. In the exemplary embodiment of the present invention, the alumina content is approximately 30%. 
     The following table lists characteristics for embodiments of the capacitor module manufactured by Electronic Concepts, Inc. in Eatontown, N.J. It should be noted that TS represents the center-to-center distance between mounting inserts 216 and TD represents the wide portion diameter of mounting inserts 216. 
     
                                           TABLE I__________________________________________________________________________Cap W   L   H   TS  TD  ESR 25 C                                50 C                                    85 C                                        .sub.- I peak                                             DVDT                                                 ESL F resVDC  (uF)    (mm)        (mm)            (mm)                (mm)                    (mm)                        (ohms)                            (Arms)                                (Arms)                                    (Arms)                                        .sup.7 (A)                                             (V/us)                                                 (nH)                                                     (kHz)__________________________________________________________________________400  5.0 38  51  24  23  10  .005                            19  15  9   409  82  23  49110.0    38  67  32  23  10  .003                            28  22  14  817  82  23  34715.0    58  72  24  28  14  .005                            23  19  12  709  47  31  24220.0    58  89  25  28  14  .004                            29  23  14  945  47  31  21025.0    58  95  24  28  14  .003                            33  27  17  1181 47  31  18830.0    58  70  45  28  14  .003                            37  30  19  1417 47  31  17150.0    63  89  51  28  14  .003                            45  36  23  2207 44  32  130500  4.0 38  51  24  23  10  .005                            19  15  9   381  95  23  5498.0 38  67  32  23  10  .003                            28  22  14  763  95  23  38812.0    58  72  24  28  14  .005                            23  19  12  661  55  31  27116.0    58  89  25  28  14  .004                            29  23  14  882  55  31  23520.0    58  95  24  28  14  .004                            29  23  14  1102 55  31  21025.0    58  70  45  28  14  .003                            37  30  19  1378 55  31  18840.0    63  89  51  28  14  .003                            45  36  23  2060 51  32  145600  3.0 38  51  24  23  10  .006                            17  14  9   327  109 23  6346.0 38  67  32  23  10  .004                            24  19  12  654  109 23  44810.0    58  72  24  28  14  .005                            23  19  12  630  63  31  29712.0    58  89  25  28  14  .005                            26  21  13  756  63  31  27115.0    58  95  24  28  14  .004                            29  23  14  945  63  31  24220.0    58  70  45  28  14  .003                            37  30  19  1260 63  31  21030.0    63  89  51  28  14  .003                            45  36  23  1766 59  32  168800  2.0 38  51  24  23  10  .007                            16  13  8   272  136 23  7774.0 38  67  32  23  10  .004                            24  19  12  545  136 23  5496.0 58  72  24  28  14  .006                            21  17  11  472  79  31  3838.0 58  89  25  28  14  .005                            26  21  13  630  79  31  33210.0    58  95  24  28  14  .004                            29  23  14  787  79  31  29712.0    58  70  45  28  14  .004                            32  26  16  945  79  31  27120.0    63  89  51  28  14  .003                            45  36  23  1471 74  32  2051200 .68 38  51  24  23  10  .005                            19  15  9   352  517 23  13891.5 38  67  32  23  10  .003                            28  22  14  776  517 23  9352.2 58  72  24  28  14  .005                            23  19  12  596  271 31  6473.0 58  89  25  28  14  .004                            29  23  14  813  271 31  5543.5 58  95  24  28  14  .004                            29  23  14  948  271 31  5134.0 58  70  45  28  14  .004                            32  26  16  1083 271 31  4806.8 63  89  51  28  14  .003                            45  36  23  1682 247 32  3581500 .56 38  51  24  23  10  .006                            17  14  9   326  582 23  15301.2 38  67  32  23  10  .003                            28  22  14  698  582 23  10452.0 58  72  24  28  14  .005                            23  19  12  609  305 31  6792.5 58  89  25  28  14  .004                            29  23  14  762  305 31  6073.0 58  95  24  28  14  .004                            29  23  14  914  305 31  5543.5 58  70  45  28  14  .004                            32  26  16  1067 305 31  5135.6 63  89  51  28  14  .003                            45  36  23  1558 278 32  394__________________________________________________________________________ 
    
     Furthermore, in the preferred embodiment, the hard-casing mold is constructed of DAP and the epoxy is a two-part epoxy product by Ciba Geigy. 
     Although the invention is illustrated and described herein as embodied in a capacitor module with an epoxy case and designed for terminal mounting, the invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.