Patent Publication Number: US-7220949-B2

Title: Capacitor of magnetron

Description:
This application claims the benefit of the Korean Patent Application No. 2005-028209, filed on Apr. 4, 2005, which is hereby incorporated by reference as if fully set forth herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a magnetron, and more particularly, to a capacitor of a magnetron, designed to have excellent withstand voltage and capacitance, thereby enhancing noise shielding efficiency, and allowing reduction in filling amount of insulating filler in the capacitor together with size reduction of the capacitor. 
   2. Discussion of the Related Art 
   Generally, a magnetron is applied to microwave ovens, plasma illuminating devices, driers, and other high frequency systems. In the magnetron, thermal electrons are emitted to a cathode by application of power, and generate microwaves by electromagnetic field. Then, the microwaves are output as a heat source to heat a target. 
   A conventional magnetron will be described with reference to  FIGS. 1 to 4 . 
   Referring to  FIG. 1 , the overall construction of the magnetron will be described. 
   The magnetron generally comprises a high frequency generator for generating microwaves by an applied voltage, an output portion for emitting the microwaves generated from the high frequency generator, and an input portion for applying a the voltage to the high frequency generator. 
   The high frequency generator of the magnetron comprises upper and lower plate-shaped yokes  11   a  and  11   b , an anode cylinder  12 , cooling fins  13 , upper and lower magnetic poles  14   a  and  14   b , an A-shaped seal member  15   a , an F-shaped seal member  15   b , a ceramic stem  16 , magnets  17   a  and  17   b , vanes  21 , and a cathode  22 . 
   The anode cylinder  12  is located in an inner space defined between the upper and lower yokes  11   a  and  11   b.    
   Each of the cooling fins  13  is connected at one end to the anode cylinder  12  and at the other end to the upper or lower yoke plate  11   a  or  11   b . The cooling fins  13  act to dissipate heat from the anode cylinder  12  to the upper and lower yokes  11   a  and  11   b.    
   The upper and lower magnetic poles  14   a  and  14   b  are disposed to upper and lower ends of the anode cylinder  12 , respectively. The A-shaped seal member  15   a  is equipped to surround an outer surface of the upper magnetic pole  14   a , and the F-shaped seal member  15   b  is equipped to surround an outer surface of the lower magnetic pole  14   a . The magnets  17   a  and  17   b  are equipped to the outer surfaces of the upper and lower magnetic poles. 
   The upper and lower magnetic poles  14   a  and  14   b , the A-shaped seal member  15   a  and the F-shaped seal member  15   b , and the magnets  17   a  and  17   b  are symmetrically equipped to the upper and lower ends of the anode cylinder  12 , respectively. 
   The lower end of the F-shaped seal member  15   b  is opened, and the ceramic stem  16  is equipped thereto. The ceramic stem  16  is penetrated with an outer connecting lead  25 , which is connected to a center lead  23  and a side lead  24 . 
   The anode cylinder  12 , the A-shaped seal member  15   a , the F-shaped seal member  15   b , and the ceramic stem  16  close a space from which the microwaves are generated. 
   The anode cylinder  12  has the vane  21  equipped therein, and is formed at the center of the vane  21  with a chamber  21   a  where the microwaves are generated. The chamber  21   a  of the vane is equipped with the cathode  22  to which the center lead  23  is inserted. At this time, the vane  21  acts as a positive electrode, and the cathode  22  acts as a negative electrode. The microwaves are generated by interaction of the vane and the cathode. 
   The output portion of the magnetron comprises an antenna feeder  31 , an A-shaped ceramic member  32 , and an antenna cap  33 . 
   The antenna feeder  31  is connected to the vane  21 , and the A-shaped ceramic member  32  is located between an upper end of the A-shaped seal member  15   a  and the antenna cap  33 . Thus, the microwaves generated from the chamber  21   a  of the vane  21  and the cathode  22  are guided by the antenna feeder  31 , and are then emitted to the outside through the A-shaped ceramic member  32 . 
   The input portion of the magnetron comprises a filter box  40 , a capacitor  50 , and a choke coil  60 . 
   The filter box  40  is fixed to a lower end of the high frequency generator. The capacitor  50  is fixed to the filter box  40  while being connected to the choke coil  60 , which is connected to the outer connecting lead  25  while being located inside the filter box  40 . 
   The filter box  40  is spaced a predetermined distance for insulation from the choke coil  60 , a coupled portion between the outer connecting lead  25  and the choke coil  60 , and the outer connecting lead  25 . Moreover, the filter box  40  is made of an electrically conductive material, such as a steel plate, so as to prevent the microwaves from being leaked to the outside. 
   The capacitor  50  will be described with reference to  FIG. 2 . 
   The capacitor  50  comprises an insulating case  51  fixedly inserted into the filter box  40 , an insulating base  52  equipped to one end of the insulating case  51 , two central conductors  53  inserted into the insulating base  52 , a dielectric material  54  surrounding the central conductors  53  within the insulating case  51 , insulating filler  55  filled in the insulating case  51 , and a ground plate  56  equipped to the one end of the insulating case  51  while being grounded to the filter box  40 . 
   After the central conductors  53  and the dielectric material  54  are fixed in the insulting case  51 , the insulating case is filled with the insulating filler  55 , and the insulating filler  55  is cured for a predetermined period of time (about 10 hours). The insulating filler  55  includes an epoxy resin. 
   The dielectric members constituting the capacitor will be described with reference to  FIGS. 3 and 4 . 
   The dielectric members  54  are disposed between the outer surfaces of the central conductors  52  and the insulating case  51  so as to face each other. The dielectric members  54  consist of barium titanate, BaTiO 3 . 
   Each of the dielectric members  54  is substantially formed in a semicircular shape, and is formed with inner and outer electrodes  54   a  and  54   b  on inner and outer surfaces thereof, respectively. Here, the inner and outer electrodes  54   a  and  54   b  are formed in semicircular shapes. 
   The inner and outer electrodes  54   a  and  54   b  are formed by plating a material having excellent electric conductivity, such as silver, on the surfaces of the electrodes. Here, the inner electrode  54   a  contacts the rod-shaped central conductor  52 , and the outer electrode  54   b  is connected to the ground plate  56 . The dielectric members  54  have predetermined withstand voltage and capacitance. 
   In order to produce a capacitor having a higher capacitance with a reduced size, it is advantageous to increase the withstand voltage and capacitance of the dielectric members  54 . Here, the withstand voltage and capacitance of the dielectric members  54  are proportional to the dielectric constant ε of the dielectric members, effective surface areas of the inner and outer electrodes  54   a  and  54   b , and wire diameters of the central conductors  53 , but inversely proportional to the distance between the inner electrode and the outer electrode. Here, the dielectric constant ε is determined by a dielectric material, the effective surface areas are defined by heights and widths of the respective electrodes, and the wire diameter of the central conductors is defined by the radius a of the inner electrode. 
   The capacitances of the dielectric members  54  are varied according to the shapes thereof. Moreover, when the dielectric members have a higher withstand voltage, a capacitor can be manufactured to have a large capacitance with a reduced size by reducing the distance between the inner electrode  54   a  and the outer electrode  54   b.    
   Meanwhile, the ground plate  56  extends to the outside of the insulating case  51 , and is grounded to the filter box  40 . As a result, the inner and outer electrodes  54   a  and  54   b , and the dielectric members  54  are grounded while repeating charge and discharge of electrons through the ground plate  56 . 
   Operation of the magnetron constructed as described above will now be described as follows. 
   When power is applied to the magnetron, a predetermined voltage is supplied to the central conductors  53  of the capacitor  50 . At this time, the dielectric members  54  have predetermined withstand voltage and capacitance. 
   The dielectric members  54  perform charge and discharge of electrons through the ground plate  56 , and stabilize overvoltage surges applied to the capacitor. The capacitor supplies the stabilized voltage to the leads  23  and  24  through the outer connecting lead  25 . Additionally, direct current is generated by interaction between the capacitor  50  and the choke coil  60 , thereby shielding noise. 
   Electrons are emitted from the cathode  22  to the vane  21 , so that microwaves are generated from the chamber of the vane. Then, the microwaves are guided to the outer portion by the antenna feeder  31  connected to the vane  21 , and radiated through the A-shaped ceramic member. 
   However, the capacitor for the conventional magnetron has problems as follows. 
   Firstly, although the dielectric members are formed to have the semicircular shapes in order to increase the effective surface areas of the dielectric members, the outer electrode is formed to have an undesirably enlarged surface area compared to that of the inner electrode. That is, the outer electrode has the undesirably enlarged surface area compared to an effective surface area thereof. Thus, the size, in particular, a width W, of the capacitor is increased, and the amount of epoxy resin required to fill the insulating case is undesirably increased, thereby increasing the time for curing the epoxy resin. As a result, there are problems of increasing a time for manufacturing the products, a price of the products, and the size of the capacitor. 
   Secondly, the wire diameter of the central conductors is also increased in order to increase the withstand voltage and capacitance of the capacitor. However, in order to increase the wire diameter of the central conductors, the diameter of the central conductors must be greatly increased. In this case, costs for manufacturing the central conductors are increased, so that the sizes of the central conductors and the capacitor are increased together with an increase of a filling amount of the epoxy resin. 
   Thirdly, since the dielectric members have the semicircular shapes, the outer diameter of the dielectric members is remarkably increased when increasing a distance b-c between the inner electrode and the outer electrode. As a result, as the size of the dielectric members is remarkably increased, the size of the capacitor and the filling amount of the epoxy resin are increased. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to a magnetron that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   An object of the present invention is to provide a capacitor of a magnetron, designed to have excellent withstand voltage and capacitance, and to have a reduced size and a filling amount of epoxy resin, thereby reducing the time for manufacturing products employing the magnetron. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a capacitor of a magnetron comprises: two central conductors disposed inside a ground plate and connected to a choke coil; and two dielectric members disposed at the outside of the central conductors so as to face each other, respectively, each dielectric member including inner and outer electrodes disposed on inner and outer surfaces thereof such that the inner electrode is connected to an associated central conductor and the outer electrode is connected to the ground plate, wherein a converging angle of less than 180° is defined between lines extending from both sides of the dielectric member, each side being formed between corresponding ends of the inner and outer electrodes. 
   Preferably, the converging angle is 65˜80°. 
   The inner electrode of the dielectric member may have either a round shape or a flat shape. Moreover, the outer electrode of the dielectric member may have either a round shape or a flat shape. 
   Preferably, each of the central conductors corresponds to the inner electrode of the dielectric member, and has an enlarged portion larger than the inner electrode. For example, the enlarged portion may have either a round shape or a flat shape. 
   In another aspect of the present invention, a capacitor of a magnetron comprises: two central conductors disposed inside a ground plate and connected to a choke coil, each of the central conductors having an enlarged portion formed to have a larger diameter than that of the central conductor at a predetermined portion thereof; and two dielectric members disposed at the outside of the central conductors so as to face each other, respectively, each dielectric member including inner and outer electrodes disposed on inner and outer surfaces thereof such that the inner electrode is connected to the enlarged portion of an associated central conductor and the outer electrode is connected to the ground plate. 
   The enlarged portion may have either a round shape or a flat shape. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
       FIG. 1  is a constructional view illustrating a conventional magnetron; 
       FIG. 2  is a cross-sectional view illustrating a capacitor of  FIG. 1 ; 
       FIG. 3  is a perspective view illustrating the capacitor of  FIG. 1 ; 
       FIG. 4  is a perspective view illustrating dielectric members of the capacitor of  FIG. 1 ; 
       FIG. 5  is a perspective view illustrating one embodiment of a capacitor according to the present invention; 
       FIG. 6  is a top view illustrating the capacitor of  FIG. 5 ; 
       FIG. 7  is a perspective view illustrating one example of dielectric members of  FIG. 5 ; 
       FIG. 8  is a perspective view illustrating an alternative example of the dielectric members of  FIG. 5 ; and 
       FIG. 9  is a perspective view illustrating a central conductor of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   The preferred embodiments of the invention will now be described with reference to  FIGS. 1 to 5 . 
   Referring to  FIGS. 5 and 6 , a capacitor  100  of a magnetron of the invention will be described. In  FIG. 5 , an insulating case and insulating filler are not illustrated since they have the same constructions as those of the conventional magnetron. 
   The capacitor  100  comprises two central conductors  120  disposed inside a ground plate  110  and connected to a choke coil, and two dielectric members  130  disposed at the outside of the central conductors  120  so as to face each other, respectively. Each dielectric member  130  includes inner and outer electrodes  131  and  132  disposed on inner and outer surfaces thereof such that the inner electrode  131  is connected to an associated central conductor  120  and the outer electrode  132  is connected to the ground plate  110 . A converging angle θ of less than 180° is defined between lines extending from both sides of the dielectric member  130 , in which each side of the dielectric material  130  is formed between corresponding ends of the inner and outer electrodes  131  and  132 . 
   The ground plate  110  is equipped to one end of the insulting case  51  (see  FIG. 2 ) while being grounded to the filter box  40  (see  FIG. 1 ). The ground plate  110  has a substantially rectangular shape opened at both sides thereof, and has a flange  111  extending perpendicular to the ground plate  110  toward the outside. The flange  111  is formed with fastening holes  112  for fastening the ground plate  112  to the filter box. 
   The insulating case is filled with the insulating filler, which fills a space between the dielectric members  130  and an upper space of the case. The insulating filler is the same as that of the conventional magnetron. 
   Each of the dielectric members  130  is formed with the inner and outer electrodes  131  and  132  on inner and outer surfaces thereof, respectively. The inner and outer electrodes  131  and  132  are formed by plating a material having excellent electric conductivity, such as silver, on their surface. 
   A first embodiment of the dielectric members will be described with reference to  FIG. 7 . 
   The inner and outer electrodes  131  and  132  of each dielectric member  130  preferably have a round shape. Alternatively, the inner and outer electrodes  131  and  132  may have a circular or elliptical shape. As such, when the inner and outer electrodes  131  and  132  of the dielectric member  130  are formed to have the round shapes, the inner and outer electrodes have larger effective surface areas than when they have flat shapes. In particular, when the inner and outer electrodes  131  and  132  are formed to have the elliptical shapes, the effective surface areas of the electrodes can be further increased in comparison to the circular shape. 
   More preferably, the converging angle θ defined between the lines extending from both sides of the dielectric member  130  is about 65˜80°, in which each side of the dielectric material  130  is formed between the corresponding ends of the inner and outer electrodes  131  and  132 . This converging angle can remarkably reduce the width of the dielectric members  130  in comparison to the conventional construction while securing the effective surface areas of the inner and outer electrodes  131  and  132 , thereby permitting desired capacitance and withstand voltage. 
   Moreover, although the distance between the inner and outer electrodes  131  and  132  of the dielectric members  130  is increased, the size of the dielectric members  130  is only slightly increased, and thus the size, in particular, the width, of the capacitor  100  is not significantly increased. As a result, the amount of the insulating filler is not significantly increased. 
   A second embodiment of the dielectric members will be described with reference to  FIG. 8 . 
   Inner and outer electrodes  231  and  232  of each dielectric member  230  preferably have a flat shape. As a result, the inner and outer electrodes  231  and  232  cannot but have reduced effective surface areas in comparison to the electrodes having the round shape as shown in  FIG. 7 . On the contrary, the dielectric members  230  are advantageous in terms of enhanced quality thereof and reduced frequency of defective products since they allow stable formation and treatment of the electrodes. 
   Although not shown in the drawings, alternatives of the dielectric member will be described. 
   The inner and outer electrodes of each dielectric member may have a round shape and a flat shape, respectively. In this manner, the inner electrode can have a greater effective surface area than that of the outer electrode. 
   The inner and outer electrodes of each dielectric member may have a flat shape and a round shape, respectively. In this manner, the outer electrode can have a greater effective surface area than that of the inner electrode, and the width of the dielectric members can be reduced. 
   The construction of the central conductors will be described with reference to  FIG. 9 . 
   Each of the central conductors  120  contacts the inner electrode  131  of the dielectric member  130 , and has the enlarged portion  121  having a larger diameter than that of the central conductor. With the enlarged portion  121 , the wire diameter of the central conductor  120  is increased without increasing the diameter of the central conductor  120 , thereby allowing the capacitance of the capacitor  100  to be increased. Preferably, the enlarged portion  121  has a slightly larger area than that of the inner electrode  131 . 
   It is desirable that the enlarged portion  121  be equipped to come to tight contact with the inner electrode  131  of the dielectric member  130 . For example, when the inner electrode  131  has the round shape as shown in  FIG. 7 , it is desirable that the enlarged portion  121  also have a round shape as shown in  FIG. 9 . On the other hand, when the inner electrode  231  has the flat shape as shown in  FIG. 8 , it is desirable that the enlarged portion  121  also has a flat shape. 
   Operation of the capacitor constructed as described above according to the invention will be described. 
   In order to generate microwaves having a predetermined frequency from the magnetron, a predetermined voltage must be supplied to the magnetron. Generally, a voltage of 20 kV is supplied to the magnetron. 
   At this time, the maximum electric field E applied to the dielectric members  130  can be determined as E=V/ln(b/a), and a capacitance C of the capacitor  100  can be determined as C=2π ∈ L/ln(b/a), in which a indicates the distance from the center of the dielectric member to the inner electrode  131 , b indicates the distance from the center of the dielectric member to the outer electrode  132 , and L indicates the height of the dielectric member. 
   At this time, since the maximum electric field E overcomes the insulative capacity, a lower maximum electric field E and a higher capacitance C are advantageous to manufacture the capacitor  100  with the reduced size and large capacitance. 
   Tests were performed by supplying a voltage of 20 kV to the magnetron, and results of the maximum electric field E, the withstand voltage, and the capacitance were obtained as follows. Here, the dielectric members  130  of the invention had a converging angle of 72° defined between the lines extending from both sides of each dielectric member  130 , in which each side is formed between corresponding ends of the inner and outer electrodes  131  and  132 . 
   Referring to  FIG. 4 , each conventional dielectric member  54  has a maximum electric field E of 9.0 kV/mm when a=1.45 mm, b=6.5 mm, L=5.0 mm, and V=20 kV. 
   Referring to  FIG. 7 , each dielectric member  130  of the invention has a maximum electric field E of 6.5 kV/mm when a=4.7 mm, b=9.0 mm, L=5.5 mm, and V=20 kV. 
   As such, according to the invention, it can be appreciated that, since the maximum electric field E serving as the insulation destructing pressure is lowered, the withstand voltage of the invention is enhanced by 2.5 kV/mm from 9.0 kV/mm to 6.5 kV/mm, resulting in an increase of the capacitance. 
   Additionally, the conventional dielectric member  54  has a distance (a-b) of 5.50 mm between the inner and outer electrodes  54   a  and  54   b , whereas the dielectric member  130  of the invention has a distance (a-b) of 4.3 mm between the inner and outer electrodes  131  and  132 . As a result, according to the invention, the inner and outer electrodes  131  and  132  are reduced in size, whereby the size of the capacitor  100  can be reduced. Moreover, since the dielectric members  130  of the invention are significantly reduced in width, the size of the capacitor  100  can be further reduced. 
   A high voltage capacitor  100  for a typical magnetron requires a capacitance of about 300˜500 pF. To achieve this capacitance, the dielectric members  54  have a volume of 630 mm 3 , whereas the dielectric members  130  of the invention have a volume of 500 mm 3 , which is reduced about 21% of that the conventional dielectric member  54 . 
   As apparent from the above description, the present invention has effects as follows. 
   Firstly, according to the invention, as the width of the dielectric member is remarkably reduced by reducing a substantial surface area of an outer electrode, the size and width of a capacitor can be reduced while maintaining the same capacitance. Moreover, even if the distance between the inner and outer electrodes is increased, the size of the dielectric member is not significantly increased. 
   Secondly, a withstand voltage and a capacitance are enhanced, thereby allowing a capacitor having a reduced size and a large capacitance to be manufactured. 
   Thirdly, as the size of the capacitor is reduced, the amount of insulating filler is reduced. Moreover, the curing time of the insulating filler is shortened, thereby reducing the manufacturing time. 
   Fourthly, since each central conductor has an enlarged portion formed at a predetermined portion thereof and an enlarged wire diameter, the wire diameter of the central conductor contacting the inner electrode can be increased without increasing the diameter of the central conductor. Thus, the capacitance can be further enhanced. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.