Abstract:
A feed through capacitor, includes an electrical conductor having a first end being in electrical communication with a second end, the first and second ends being couplable to respective means for conveying electrical energy, a groundable electrically conductive housing enclosing at least a portion of the conductor and being spaced apart from the conductor, and a capacitor bank having at least one capacitor element, the capacitor element being in electrical communication with both the conductor and the housing. A method of forming a feed through capacitor is further included.

Description:
RELATED APPLICATION 
   The present application claims the benefit of U.S. Provisional Application Ser. No. 60/796,469, filed May 1, 2006, and which is incorporated herein in its entirety by reference. 

   TECHNICAL FIELD 
   The present invention relates to the field of electromagnetic interference. More particularly, the present invention relates to a feed through capacitor (FTC) for filtering electromagnetic interference. 
   BACKGROUND OF THE INVENTION 
   Electronic equipment used in commercial and military applications must adhere to conducted emission requirements at the input power terminals of the electronic equipment. These requirements are levied on the electronic equipment manufacturers by agencies such as the FCC, European Union, and, in the case of the U.S. Military, the procuring branch of the service. The goal of these requirements is to minimize the negative interactions that may occur between various pieces of electronic equipment. The negative interactions occur because the noise currents, normally produced by switching action in one piece of electronic equipment, interfere with the proper operation of another piece of electronic equipment. Typically, the method of coupling the negative interactions is conduction through a common shared power bus (DC or AC). Additionally, noise current feeding on the power bus may set up electromagnetic interference that couples into the surrounding electronic equipment via electromagnetic radiation. Noise currents are typically AC and have frequencies that are much higher than the operating frequency of the power source. 
   In order to meet the conducted emission requirements, inductive components are used in conjunction with capacitors in electrical networks referred to as EMI filters. EMI filters are highly effective in attenuating noise currents that emanate from electronic equipment. Noise currents can be characterized by the path that they take during conduction. Two conduction paths exist: normal mode (sometimes referred to as differential mode) and common mode. See  FIGS. 1 and 2  respectively. Normal mode noise currents typically feed into one input power terminal of the electronic equipment and exit from the electronic equipment via the remaining input power terminal (as closed). The sum of all the normal mode noise currents entering and exiting a particular piece of electronic equipment input power terminals is zero. In contrast, the sum of common mode noise currents entering and leaving the electronic equipment input power terminals is not zero. These currents typically find alternate paths through the equipment chassis and the application earth ground structure. These alternate and less predictable paths can be highly disruptive. 
   In addition to the noise currents that feed into and out of equipment power terminals, power producing currents must also feed. The high frequency noise currents, both normal mode and common mode, essentially modulate the lower frequency power producing component. Power producing currents also feed in the normal mode. Capacitors, used in EMI filters, must efficiently pass the lower frequency power producing component and attenuate the high frequency noise currents. 
   Common mode noise currents typically occur when energy from the switching transition within the electronic equipment capacitively couples into the equipment ground structure or system ground structure. The magnitude of the common mode noise current is proportional to the parasitic capacitor magnitude and the time/rate of change of the voltage across the parasitic capacitance. Accordingly, a large voltage swing, and a fast switching transition results in a large common mode current magnitude. The duration of the common mode current pulse is proportional to the duration of the electronic equipment switching transitions. The frequency spectrum for the common mode noise currents is often discrete since the switching transitions are usually periodic. The fundamental component of the frequency spectrum is the same as the switching frequency of the electronic equipment and the harmonics are at multiples of the fundamental frequency. The magnitude of the common mode noise frequency spectrum envelope drops gently, starting at the fundamental frequency, to its first null, which in an ideal system is equivalent to the inverse of the switching transition time. This frequency spectrum, with its large energy content at the higher frequencies, can be quite disruptive if mitigation techniques, such as EMI filters, are not employed. 
   The nature of the common mode frequency spectrum places performance constraints on an FTC that is used as an EMI filter. The FTC must provide a low impedance path to chassis ground for common mode noise current. The low impedance path must be across the entire common mode noise frequency spectrum. The impedance exhibited by the FTC to chassis ground must be lesser than the common mode noise source impedance and the impedance of the load circuit to chassis ground. These performance constraints translate into requirements for the electrical elements of the FTC including capacitance, equivalent series inductance (ESL) and equivalent series resistance (ESR). The FTC capacitance value must be large enough to shunt the low frequency fundamental common mode spectral component. Additionally, the FTC must exhibit a low ESL, enabling it to effectively shunt the higher frequency common mode spectral components. Finally, the ESR of the FTC must be low, enabling it to effectively shunt all common mode spectral components. 
   FTC&#39;s used in high current applications must pass the power source current without introducing power loses. Additionally, FTC&#39;s need to provide a low impedance thermal path to the surrounding environment (air or chassis), for power losses that do occur within the FTC. Failure to minimize power losses or provide a low impedance thermal path will result in excessive FTC operating temperature which will reduce the FTC reliability. Excessive power losses within the FTC can also impact the operation of the application circuit by reducing the available circuit input voltage or by contributing to a temperature rise in the application circuitry. These requirements drive the FTC design to maximize cross-sectional conductor area and minimize the length of the FTC conducting element. 
   FTC&#39;s used in high voltage applications must also support the high voltage without breaking down. This requirement places constraints on the insulating materials used in the construction of the capacitor. Additionally, when insulating material cannot be employed, the FTC design must rely on clearance and creepage distances to avoid voltage breakdown. Clearance is the minimum distance required to avoid breakdown (through air) for a given potential. Creepage is the minimum distance required to avoid breakdown (over an insulating surface) for a given potential and insulating material. Clearance and creepage distances become important design requirements at the FTC terminations and dictate the physical size of the capacitor design. 
   When these low impedance, high current requirements and high voltage requirements are impressed on prior art FTC&#39;s, a costly, bulky design results. In addition, the prior art FTC becomes difficult to produce which results in excessive lead time and integration risk. There is a need then in the industry for cost effective, low lead time FTC&#39;s exhibiting low impedance, high current capability and high voltage capability. 
   Commercial off-the-shelf FTC&#39;s available today are targeted for low current and low voltage applications. FTC&#39;s capable of exhibiting low impedances over a wide frequency range as would be the case with 0.3 microfarad ceramic capacitor, passing a source current of 200 amps, and of supporting voltages of 1000 volts are not readily available. Even custom FTC&#39;s, as distinct from commercial off-the-shelf FTC&#39;s, do not support the capacitance requirement or the 200 amp current unless three or more custom FTC&#39;s are configured in parallel. Further, current custom FTC&#39;s have a clearance and creepage distance of 3.2 mm. For the application noted above, clearance and creepage distances of 5 to 10 mm, depending on the pollution degree environment, are required. Further, present custom FTC&#39;s are both expensive and have long lead times for delivery, typically in the realm of 22 weeks or more. 
   SUMMARY OF THE INVENTION 
   The present invention substantially meets the aforementioned needs of the industry. The FTC of the present invention meets the following requirements: 
   1. Capacitance: 0.3 microfarad; 
   2. Capacitance tolerance: +/−10%; 
   3. Capacitor technology: ceramic, X7R, SMT; 
   4. ESL: 2 nH; 
   5. ESR: 0.1 ohms at the resonant frequency; 
   6. Minimum resident frequency: 7 MHz; 
   7. Current rating: 200 A with internal hot spot temperature rise less than 35° C.; 
   8. DC resistance: 0.000125 ohms; 
   9. Voltage rating: 1000 V; 
   10. Insulating resistance: 10 Mohm; 
   11. Dielectric withstand voltage: 2000 V; 
   12. Ambient operating temperature: −55° C. to 100° C.; 
   13. Volume: no greater than 36 mm×38 mm×38 mm; and 
   14. Lead time: no greater than 4 weeks. 
   The feed through capacitor of the present invention has been designed to meet the aforementioned goals. Testing of devices made in accordance with the present application has shown the feed through capacitor of the present invention to fully meet the aforementioned goals. 
   The present invention is a feed through capacitor, including an electrical conductor having a first end being in electrical communication with a second end, the first and second ends being couplable to respective means for conveying electrical energy, a groundable electrically conductive housing enclosing at least a portion of the conductor and being spaced apart from the conductor, and a capacitor bank having at least one capacitor element, the capacitor element being in electrical communication with both the conductor and the housing. The present invention is further a method of forming a feed through capacitor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic depiction of electronic equipment with normal mode AC noise currents; 
       FIG. 2  is a schematic depiction of electronic equipment with common mode AC noise currents; 
       FIG. 3  is an exploded view of the feed through capacitor of the present invention; 
       FIG. 3A  is an end elevational view of the assembled FTC taken from the end that includes the internal nut. 
       FIG. 4  is a sectional view of the FTC taken along the section line A-A of  FIG. 3A . 
       FIG. 5  is an end elevational view of the housing; 
       FIG. 6  is a sectional view of the housing taken along section line B-B of  FIG. 5 ; 
       FIG. 7  is an end elevational view of the conductor; 
       FIG. 8  is a sectional view of the conductor taken along section line C-C of  FIG. 7 ; 
       FIG. 9  is an elevational view of the flange spacer; 
       FIG. 10  is a sectional view of the flange spacer taken along section line D-D of  FIG. 9 ; 
       FIG. 11  is an elevational view of the flanged washer; 
       FIG. 12  is a sectional view of the flanged washer taken along the section E-E of  FIG. 11 ; 
       FIG. 13  is an elevational view of the flat washer; 
       FIG. 14  is a sectional view of flat washer taken along the section line G-G of  FIG. 13 ; 
       FIG. 15  is a side elevational view of the internal nut; 
       FIG. 16  is an end elevational view of the internal nut; 
       FIG. 17  is an end elevational view of the external nut; 
       FIG. 18  is an elevational view of the external nut taken along section line F-F of  FIG. 17 ; 
       FIG. 19  is a sectional view of a capacitor element taken along the section line H-H of  FIG. 20 ; 
       FIG. 20  is a perspective view of a capacitor element; 
       FIG. 21  is a perspective view of a partially assembled FTC with a taped, threaded contact centered within the housing and a capacitor element of the capacitor bank disposed between the threaded contact and the housing; 
       FIG. 22  is a partially assembled feed through capacitor including the threaded contact centered within the housing; 
       FIG. 23  is a side perspective view of a partially assembled feed through capacitor; 
       FIG. 24  is a perspective view of a partially assembled feed through capacitor; and 
       FIG. 25  is a perspective view of a pair of FTC&#39;s integrated into an electronic assembly. 
       FIG. 26  is a perspective view of a capacitor bank including three capacitor elements. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The feed through capacitor of the present invention is shown generally at  10  in the figures. The feed through capacitor (FTC)  10  includes eight components; housing  12 , threaded contact  14 , flanged spacer  16 , flanged washer  18 , flat washer  20 , internal nut  22 , external nut  24 , and capacitor bank  26 . 
   Generally, the threaded contact  14  acts as a conductor that provides a path for the power source current. The insulating flanged spacer  16  typically disposed on a first end of the threaded contact  14  centers the threaded contact  14  in the housing  12 . The flanged spacer  16  establishes and maintains the required clearance and creepage distances at the first end of the FTC  10 . On the opposed second end of the FTC  10 , the insulating flanged washer  18  centers the threaded contact  14  in the housing  12  and establishes and maintains the required clearance and creepage distances in cooperation with the flanged spacer  16 . Such distances are noted above. The insulating washer  20  provides a flat insulating surface for the capacitor bank  26  (known as an EESEAL). The capacitor bank  26  may include a single EESEAL or a plurality of individual EESEALs. Each EESEAL(s) of the capacitor bank  26  has a bore defined therein, the bore being slid over the threaded contact  14 . The EESEAL(s) occupies the space within the housing  12  defined between the spaced apart flanged washer  18  and the flat washer  20 . Preferably, one to three EESEALs may occupy this space resulting in capacitance of 0.1 μFd (one EESEAL being employed) to 0.3 μFd (three EESEALs being employed) depending upon the number of EESEALs used. Internal nut  22  is used to compress the EESEAL(s) against the flat washer  20  and against the internal wall of the housing  12 . The internal nut  22  also acts as a mounting surface for external electrical terminal lug connections. The flanged surface (described in greater detail below) of the threaded contact  14  acts as a mounting surface for external electrical terminal connections as well. The external nut  24  is used to secure the FTC  10  into the EMI filter assembly, as depicted in  FIG. 25 . 
   More particularly, with respect to the structural features of the various components of the FTC  10  as noted above, the housing  12  of the FTC  10  is generally cylindrical in shape having an interior bore  30 , as depicted in  FIGS. 3-6 . The interior bore  30  preferably has a smooth finish. The interior bore  30  is formed as a housing body  31 . At a first end of the housing body  31 , a beveled edge  32  extends to an outer margin  34 . The outer margin  34  is preferably threaded with threads  36 . 
   A flange  38  is formed integral to the housing body  31  at the opposite end of the housing body  31  at which the beveled edge  32  is formed. The flange  38  has a greater diameter than the diameter of the outer margin  34 . The flange  38  extends radially inward, partially closing the interior bore  30 . A center bore  40  with a diameter that is somewhat less than the diameter of the interior bore  30  is formed in the flange  38 , the center bore  40  extending into the interior bore  30 . 
   The second component of the FTC  10 , as depicted in  FIGS. 3-4 ,  7  and  8 , is the threaded contact  14 . The threaded contact  14  has an elongate cylindrical stud body  44 . Integral flange  46 , having a diameter that is greater than the diameter of the cylindrical stud body  44 , is formed on the stud body  44 . The integral flange  46  is formed proximate the first end  48  of the stud body  44 . Threads  50  are defined on the outer margin of the stud body  44  between the flange  46  and the first end  48 . At the opposed second end  52  of the stud body  44 , threads  54  are formed on the exterior margin of the stud body  44  extending approximately ⅓ the length of the stud body  44 . 
   The third component of the FTC  10  is the flanged spacer  16 . See  FIGS. 3-4 ,  9  and  10 . The flanged spacer  16  has a hub  58  with a smoothly finished contact bore  60  defined therein. A step  62  connects the hub  58  to a flange  64 . The flange  64  has significantly greater diameter circumferential margin  68  than the diameter exterior margin of the hub  58 . The circular recess  66  is defined in the flange  64 . A circular recess  66  is preferably concentric with the contract bore  60 . Together, the circular recess  66  and the contact bore  60  define a passageway that extends axially through the flanged spacer  16 . In a preferred embodiment, the flanged spacer  16  is made of a non-metallic, non-electrically conductive material. Most preferably, the flanged spacer  16  is made of the material commonly known as DELRIN (Polyoxymethylene). 
   The fourth component of the FTC  10  is the flanged washer  18 , as depicted in  FIGS. 3-4 ,  11 , and  12 . The flanged washer  18  has a washer body  70 . The washer body  70  presents a circumferential exterior margin  72 . A step  76  leads to a flange  74 , the flange  74  presenting a circumferential margin  75  that is lightly greater in diameter than the diameter of the circumferential exterior margin  72 . A circular recess  78  is defined in the side of the flanged washer  18  presenting the flange  74 . The recess  78  has an inside circumferential margin  80 . A contact bore  82  extends through the washer  18  and into the recess  78 . Margin  80  has a considerably greater diameter than the diameter of the contact bore  82 . In a preferred embodiment, the flanged washer  18  is made of a non-metallic, non-electrically conductive material. Most preferably, the flanged washer  18  is made of the material commonly known as DELRIN (Polyoxymethylene). 
   The fifth component of the FTC  10  is the flat washer  20 . See  FIGS. 3-4 ,  13 , and  14 . Like the flanged washer  18 , the flat washer  20  is preferably made of a non-metallic, non-electrically conductive material and is also most preferably made of the plastic material DELRIN (Polyoxymethylene). The flat washer  20  has a washer body  83 . The body  83  defines a circumferential exterior margin  84 . The flat washer  20  has a centrally disposed, smooth finished contact bore  86  defined therein. 
   The sixth component of the FTC  10  is the internal nut  22 , depicted in  FIGS. 3-4 ,  15  and  16 . The internal nut  22  is comprised of a nut body  87 . A portion of the exterior margin is defined by a circumferential exterior margin  88 . A pair of opposed flats  90  intersect the circumference defining the circumferential exterior margin  88 . A threaded contact bore  92  is centrally, axially defined in the internal nut  22 . The width of the internal nut  22  is such that when snugged up against the flanged washer  18 , the internal nut  22  resides within the circular recess  78 . 
   The seventh component of the FTC  10  is the external nut  24 , depicted in  FIGS. 3-4 ,  17  and  18 . The external nut  24  has an interior threaded bore  94 . At a first edge of the interior threaded bore  94 , a beveled edge  96  is defined. The exterior margin  97  of the external nut  24  is preferably smooth finished and includes a pair of opposed circumferential exterior margins  98  that are joined by a pair of opposed flats  100 . 
   All of the components noted above with the exception of those specifically indicated to be most preferably made of the material DELRIN (Polyoxymethylene) are made of an electrically conductive metal. Preferably, the metal is a brass alloy. Most preferably, the brass alloy is an alloy noted as No. 464, (naval) leadfree. 
   The final component of the FTC  10  is the capacitor bank  26 . Preferably, the capacitor bank  26  is an EMI filter insert being a capacitor element(s)  104  known as an EESEAL™, manufactured by Quell Corporation of Albuquerque, N. Mex. 87109. The EESEAL is described in U.S. Pat. Nos. 5,686,697 and 6,613,979, incorporated herein by reference. In general, each EESEAL of the capacitor bank  26  utilizes three 0.033 μFd, 2000 V ceramic surface mount technology capacitors encapsulated within a silicon material, although fewer than three such capacitors are envisioned. The three capacitors are so arranged that they extend outward radially from a center bore defined in the EESEAL. The three capacitors are separated by 120°. When an EESEAL is inserted into the housing  12  (See  FIGS. 4 and 21 ), each of the EESEAL capacitors makes electrical contact with both the housing  12  and the threaded contact  14 . 
   The capacitor bank  26  is best viewed in  FIGS. 4 , and  19 - 21 . As noted above, the capacitor bank  26  preferably is comprised of three capacitor elements (preferably an EESEAL)  104 . Each of the capacitor elements  104  includes a silicon elastomer body  105 . A conductor bore  106  is centrally defined within the silicon elastomer body  105 . The body  105  presents a circumferential exterior margin  108 . 
   At least one capacitor  110  ( FIGS. 19 and 20 ) is included within the body  105 . The capacitor  110  is electrically coupled to conductor contact(s)  112 . When the capacitor element  104  is assembled within the housing  12  of the FTC  10 , the conductor contact(s)  112  is electrical communication with the threaded contact  14  residing within the conductor bore  106 . The capacitor  110  is electrically coupled to the circumferential exterior margin  108  by means of the housing contact  114 . When the capacitor element  104  is assembled within the housing  12 , the housing contact  114  is electrical communication with the housing  12 . Accordingly, there is an electrical path between the threaded contact  14  and the housing  12 . 
   As noted above and shown in  FIG. 26 , preferably there are three capacitors  110  in each capacitor element  104 . The capacitors  110  are preferably spaced equiangularly by the angles A, depicted in  FIG. 20 . Each of the angles A is 120°. Further, there are preferably three capacitor elements  104  in the capacitor bank  26 . When assembled within the housing  12 , each of the three capacitor elements  104  is shifted angularly by 40° with respect to the adjacent capacitor element(s)  104 . Thus, when the capacitor bank  26  is assembled using three capacitor element(s)  104 , there is a capacitor  110  at every successive 40° mark for the full 360° of revolution. 
   The above disclosure is not intended as limiting. Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the restrictions of the appended claims.