Abstract:
A high voltage capacitor has a monolithic body made of layers of dielectric material and further has first and second external contacts located on the body. First and second nonoverlapping electrodes electrically connected to the respective first and second contacts are located on respective first and second layers of dielectric material within the body. A floating electrode not connected to either of the contacts is located on a different, third layer of dielectric material. The floating electrode overlaps the first and second electrodes and forms two serially connected capacitors therewith.

Description:
[0001]    This application is a continuation-in-part of U.S. application Ser. No. 10/136,789, filed May 3, 2002, which is hereby incorporated by reference in its entirety herein, which, in turn, is a continuation-in-part of U.S. application Ser. No. 09/865,816, filed May 25, 2001, now U.S. Pat. No. 6,545,854. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally concerns capacitors and more particularly, the organization of internal electrodes within a capacitor having a high voltage breakdown rating.  
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates to the placement of internal electrodes within a multi-layer capacitor made of a dielectric material such as a ceramic dielectric material. Capacitance between spaced-parallel plate regions is a function of their separation. Further, plate density cannot be particularly high in a multi-layer capacitor that relies on only a relatively thin ceramic layer to limit the breakdown voltage. Metal plate regions of alternating polarity are stacked in a parallel relationship and partially overlap each other. The metal plate regions are parallel and overlapping so as to create capacitance along the elementary model of two parallel plate electrodes. The formula for the capacitance of the conventional parallel-plate ceramic capacitor is:  
           [0004]    Cap=k A  
           [0005]    d  
           [0006]    where  
           [0007]    Cap is the capacitance in farads,  
           [0008]    k is the dielectric constant in farads per meter,  
           [0009]    A is the area of electrode overlap in square meters, and  
           [0010]    d is the distance of separation between plates in meters.  
           [0011]    Although d would desirably be minimized for greatest capacitance, in high voltage capacitors, d cannot be indefinitely small or else the capacitor will be subject to failure from voltage breakdown of the insulating ceramic dielectric. For example, referring to FIG. 4, a known capacitor  10  having a high voltage breakdown rating has a substantially monolithic thee-dimensional body  12  comprised of layers of dielectric material  14 . Conductive first electrodes  16  are placed on a first layer of dielectric material  15  and are connected to a conductive first contact  18  on an external portion of the body  12 . Conductive second electrodes  20  are also placed between the same layer of dielectric material  15  and are connected to a conductive second contact  22  on another external portion of the body  12 . A conductive third electrode  24  is placed on a different, second layer of dielectric material  26 . The third electrode  24  is not electrically connected to either of the contacts  18 ,  22  and overlaps with both the first and second electrodes  16 ,  20 . Referring to FIG. 4B, a first capacitor  28  is formed between the first and the third electrodes  16 ,  24 , and a second capacitor  30  in a series circuit with the first capacitor  28  is formed between the second and the third electrodes  20 ,  24 .  
           [0012]    A typical ceramic dielectric will have a voltage rating of 100 volts per mil (0.001 in.) thickness. For example, if the capacitor  10  is designed to have an operating voltage of about 2,000 volts, an axial plate separation, that is, the thickness t of the ceramic layer  15  must be about 10 mils.  
           [0013]    Another aspect of high voltage ceramic capacitor design relates to the distance dl of separation between electrodes  16 ,  20  of opposite polarity. The plate separation dl should be 50% greater than the layer thickness and hence the electrode separation t. This is because a voltage breakdown is more likely to occur along the unavoidable imperfections of the seams  32  between the layers  15 ,  17 . Thus, the distance d1 should be about 15 mils, that is, 1.5×10 mils.  
           [0014]    Capacitors so constructed use high voltages, commonly about 750 volts. When the electrodes of the capacitor are subjected to high voltages, for example, on the order of hundreds and, with safety margins, even thousands of volts, the seam  32  is subject to developing voltage breakdown paths between the electrodes  16 ,  20 .  
           [0015]    Thus, there is a need for an improved multilayer high voltage ceramic capacitor that has a substantially higher breakdown voltage rating.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention provides a multi-layer capacitor that has a significantly higher voltage breakdown threshold than known capacitors of comparable size. The multi-layer capacitor of the present invention is especially useful in applications where higher voltages may be expected and thus, can be used in a wider range of more rigorous applications than known comparable capacitors. The multi-layer capacitor of the present invention has a construction that substantially strengthens potential voltage breakdown paths within the capacitor and thus, provides capacitors having operating voltages ranging from about 1,000 volts to 10,000 volts and higher.  
           [0017]    According to the principles of the present invention and in accordance with one embodiment, the present invention provides a multilayer capacitor having a substantially monolithic body made of layers of dielectric material with first and second external contacts located on the body. A first electrode connected to the first contact is located on a first layer of dielectric material within the body, and a second electrode connected to the second contact is located on a second layer of dielectric material different from the first layer. The first and second electrodes are nonoverlapping with each other. A floating electrode not electrically connected either of the contacts is located on a third layer of dielectric material different from the first and second layers. The floating electrode overlaps the first and second electrodes and forms serially connected capacitors therewith. Locating the electrodes on different layers of dielectric material provides the multilayer capacitor with a higher voltage breakdown threshold than known capacitors of comparable size.  
           [0018]    In one aspect of this invention, additional floating electrodes are located on different layers of dielectric material and provide additional serially connected capacitors to increase the voltage breakdown threshold of the multilayer capacitor.  
           [0019]    These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1A is a centerline cross-sectional view of one embodiment of a multilayer capacitor having a high breakdown voltage threshold in accordance with the principles of the present invention. FIG. 1B is an electrical schematic diagram of the capacitor of FIG. 1A.  
         [0021]    [0021]FIG. 2A is a centerline cross-sectional view of another embodiment of a multilayer capacitor having a high breakdown voltage threshold in accordance with the principles of the present invention. FIG. 2B is an electrical schematic diagram of the capacitor of FIG. 2A.  
         [0022]    [0022]FIG. 3A is a centerline cross-sectional view of a further embodiment of a multilayer capacitor having a high breakdown voltage threshold in accordance with the principles of the present invention. FIG. 3B is an electrical schematic diagram of the capacitor of FIG. 3A.  
         [0023]    [0023]FIG. 4A is a centerline cross-sectional view of a known multilayer capacitor, and FIG. 4B is an electrical schematic diagram of the capacitor of FIG. 4A. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    As described earlier, known multilayer high voltage capacitors as illustrated in FIG. 4A have a limited voltage breakdown threshold because of the potential for conduction along a seam  32  between ends of the electrodes  16 ,  20 . To overcome that limitation, referring to FIG. 1A, a multilayer high voltage capacitor  130  has a plurality of metallized or conductive first electrodes  132  disposed on respective first layers of dielectric material  134 . A plurality of conductive second electrodes  136  are disposed on respective second layers of dielectric material  138  different from the first layers  134 . The first and second electrodes  132  and  136  are electrically connected to respective first and second external electrical contacts  140 ,  142 . A plurality of conductive floating electrodes  144  are disposed on respective third layers of dielectric material  146  that are different from the first and second layers  134 ,  138 , respectively. The floating electrodes  144  are not connected to either of the contacts  140 ,  142 ,  
         [0025]    The first electrodes  132  are non-overlapping with the second electrodes  136 ; however, each of the floating electrodes  144  overlaps at least one of the first electrodes  132  and at least one of the second electrodes  136 . As shown in FIG. 1B, a plurality of first capacitors  146  are formed between respective first electrodes  132  and respective floating electrodes  144 . In addition, a plurality of second capacitors  148  are formed between respective floating electrodes  144  and respective second electrodes  136 . The capacitors  146 ,  148  are connected in series between the external contacts  140 ,  142 .  
         [0026]    Placing the first and second electrodes  132 ,  136  on respectively different layers of dielectric material  134 ,  138 , reduces the potential for conduction between ends  150 ,  152  of the respective electrodes  132 ,  136 . Further, the voltage breakdown threshold between the ends  150 ,  152  can be controlled by varying the distance separating the ends  150 ,  152  and/or the thickness of the layer of dielectric material  134 . Depending on the application, the thickness of the layer of dielectric material  138  is in a range of about 4-10 times the thickness of the dielectric material  134 . If the voltage breakdown threshold between electrodes  132 ,  144  is about 1,500 volts and the voltage breakdown threshold between electrodes  136  and  144  is about 1,500 volts, then the voltage breakdown threshold rating for the high voltage capacitor  130  is about 3,000 volts.  
         [0027]    Higher voltage breakdown thresholds can be obtained by adding successive capacitors in series as shown in FIGS. 2A and 2B. Referring to FIG. 2A, a high voltage capacitor  149  has an external contact  140  connected to a plurality of conductive first electrodes  132  that are disposed on respective first layers of dielectric material  150 . A second external contact  142  is connected to conductive second electrodes  136  that are disposed on respective layers of dielectric material  152  different from the layers  150 . The first electrodes  132  are non-overlapping with the second electrodes  136 . A plurality of first floating electrodes  144  are disposed on respective layers of dielectric material  154  that are different from the first layers  150  and second layers  152 . A plurality of second floating electrodes  156  are disposed on respective layers of dielectric material  158  that are different from the first layers  150 , second layers  152  and third layers  154 . The first and second floating electrodes  144  and  156  are not connected to either of the external contacts  140 ,  142 . However, the first floating electrode  144  overlaps both the first electrode  132  and the second floating electrode  156 . Further, the second floating electrode  156  also overlaps the second electrode  136 . As shown in FIG. 2B, first capacitors  160  are formed by respective first electrodes  132  and respective first floating electrodes  144 . Second capacitors  162  are formed by respective first floating electrodes  144  and respective second floating electrodes  156 . Third capacitors  164  are formed by respective second floating electrodes  156  and respective second electrodes  136 . Respective ones of the capacitors  160 ,  162  and  164  are in a series circuit between the external contacts  140 ,  142 . If each of the capacitors  160 ,  162 ,  164  has a voltage breakdown threshold of about 1,500 volts, then the high voltage capacitor  149  has a voltage breakdown threshold of about 4,500 volts.  
         [0028]    The voltage breakdown threshold can be increased by adding further floating electrodes, for example, as shown in FIGS. 3A and 3B, a high voltage capacitor  122  has third floating electrodes  166 . Thus, a plurality of capacitors  168  are formed by respective second floating electrodes  156  and respective third floating electrodes  166 . In addition, a fourth plurality of capacitors  170  are formed by respective third floating electrodes  166  and respective second electrodes  136 . Respective ones of the capacitors  160 ,  162 ,  168 ,  170  are respective series circuits between the external contacts  140 ,  142 . If each of the capacitors  160 ,  162 ,  168  and  170  has a voltage breakdown threshold of 1,500 volts, then the high voltage multilayer capacitor  122  has a voltage breakdown threshold of about 6,000 volts.  
         [0029]    Thus, by placing electrodes on different layers of dielectric material within each of the capacitors  130 ,  149 ,  122 , potential voltage breakdown paths within the capacitors are substantially strengthened, thereby providing capacitors with significantly higher voltage breakdown thresholds than known capacitors of comparable size. As will be appreciated, there is no limit to the number of series capacitors that can that can be formed within one chip; and capacitors having operating voltages up to about 10,000 volts and higher can be made. Further, the substantially higher breakdown voltage threshold substantially increases the range of applications in which the capacitors can reliably be used.  
         [0030]    While the invention has been illustrated by the description of one embodiment and while the embodiment has been described in considerable detail, there is no intention to restrict nor in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those who are skilled in the art, for example, in the described embodiment, the multi-layer capacitors are made of a ceramic dielectric. As will be appreciated, in an alternative embodiment, the multi-layer capacitor may be made of a plastic dielectric, for example, a MYLAR or PET plastic film. With a plastic film dielectric, the electrodes are often made of aluminum or silver; and the structure is glued together and not sintered. Further, as will be appreciated, the shape of the multi-layer capacitor can vary depending on a particular application.  
         [0031]    Therefore, the invention in its broadest aspects is not limited to the specific detail shown and described. Consequently, departures may be made from the details described herein without departing from the spirit and scope of the claims which follow.