Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2000-02-29
2001-09-11
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S307000, C361S303000
Reexamination Certificate
active
06288886
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high voltage capacitor and magnetron having a filter constituted of the high voltage capacitor.
2. Discussion of the Background
Well known examples of high voltage capacitors of this type in the prior art include those disclosed in Japanese Unexamined Patent Publication No. 316099/1996 and Japanese Unexamined Utility Model Publication No. 40524/1992. They generally adopt a structure achieved in which two through holes are formed over a distance from each other in a dielectric ceramic material constituting the capacitor. Individual electrodes that are independent of each other and a common electrode shared by the individual electrodes are provided at the two surfaces of the dielectric ceramic material at which the through holes open. The common electrode is secured onto a raised portion of a grounding metal by a means such as soldering. Conductors are inserted in the through holes at the capacitor and holes at the grounding metal. The conductors are soldered to the individual electrodes at the capacitor using electrode connectors or the like. An insulating case is fitted at the external circumference of the raised portion of the grounding metal, so as to enclose the capacitor. An insulating cover is fitted at the other surface of the grounding metal, so as to enclose the conductors. The insulating cover is mounted so that it comes into close contact with the internal circumferential surface of the raised portion of the grounding metal. Then, the insulating case and the spaces inside and outside of the capacitor enclosed by the insulating case are filled with a thermosetting insulating resin such as an epoxy resin to assure moisture resistance and good insulation.
However, the insulating cover is simply fitted so that it comes into close contact with the internal circumferential surface of the raised portion of the grounding metal and the insulating cover and the grounding metal are not bonded. As a result, the stress, which repeatedly occurs while the insulating resin is becoming hardened and contracting and during operation of the capacitor causes a gap to form at the boundary of the insulating cover and the internal circumferential surface of the raised portion of the grounding metal and also induces formation of a gap between the insulating resin and the grounding metal and between the insulating resin and the dielectric ceramic material. Thus, high voltage capacitors of this type in the prior art pose a risk of early defects occurring due to deterioration of characteristics and shorting occurring between electrodes.
Main causes of the gap formed between the insulating cover and the grounding metal include the stress occurring while the thermosetting insulating resin such as an epoxy resin is becoming hardened and contracting and the stress attributable to the electrostrictive phenomenon occurring at the dielectric ceramic material constituting the capacitor.
The main constituent of the dielectric ceramic material constituting the capacitor is barium titanate. Such a dielectric ceramic material is a ferroelectric material that belongs to the piezoelectric crystal class. A ferroelectric material belonging to the piezoelectric crystal class imparts a reverse piezoelectric effect. As a result, when a high AC voltage is applied, mechanical energy is generated inside the dielectric ceramic material constituting the capacitor. For instance, if this type of high voltage capacitor is employed in a filter of a magnetron in a microwave oven, a high AC voltage for oscillating the magnetron is applied to the capacitor. When such a high AC voltage is applied to the dielectric ceramic material the reverse piezoelectric effect mentioned earlier converts the electrical energy to mechanical energy. This results in the dielectric ceramic material expanding while the voltage is being applied and contracting to regain its original state when the voltage is not applied. A voltage of approximately 4 kV
O-P
having a commercial frequency or a frequency in the range of 20 kHz to 40 kHz is applied to oscillate the magnetron in a microwave oven. In addition, a transient voltage of 0 to 40 kV
P-P
is applied immediately before the magnetron starts to oscillate. In response to these AC voltages, the dielectric ceramic material constituting the capacitor repeats a process of expanding and contracting. This is referred to as an electrostrictive phenomenon of a dielectric ceramic material.
However, since the insulating cover is merely fitted in close contact with the internal circumferential surface of the raised portion of the grounding metal, the insulating cover and the internal circumferential surface are not bonded. This causes a gap to form at the boundary of the insulating cover and the internal circumferential surface of the raised portion of the grounding metal while the dielectric ceramic material constituting the capacitor repeats expanding and contracting in response to the AC voltage and, furthermore, induces separation at the interface of the insulating resin and the grounding metal.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a high voltage capacitor achieving a high degree of reliability in which deterioration of the characteristics and electrical shorting are avoided by increasing the bonding force at the interface of the grounding metal and the insulating resin filling the inside of the grounding metal and thus preventing any separation from occurring at the interface between them.
In order to achieve the object described above, the high voltage capacitor according to the present invention comprises at least one grounding metal, at least one capacitor, at least one conductor, at least one insulating tube, at least one insulating cover and an insulating resin.
The grounding metal is provided with a raised portion at one surface thereof, with the raised portion having a hole from one surface to another surface and an internal space that is continuous with the hole.
The capacitor, which includes a dielectric ceramic material having a through hole, assumes a structure provided with an electrode at each of the two surfaces at which the through hole in the dielectric ceramic material opens, with one of the electrodes secured onto the one surface of the grounding metal.
The conductor, which passes through the capacitor and the grounding metal, is connected to the other electrode achieving electrical continuity. The insulating tube covers the conductor.
The insulating cover, which is provided at the other surface of the grounding metal, has one end thereof inserted in the internal space at the raised portion. The insulating resin fills the space around the capacitor at one surface of the grounding metal and also fills the space around the capacitor inside the insulating cover.
When the high voltage capacitor assuming the structure described above is employed in a magnetron in a microwave oven, a high voltage capacitor that absorbs noise traveling through the conductors is achieved. The filtering function of the capacitor is achieved by using the conductors as power supply terminals and connecting the capacitor between the conductors and the grounding metal.
In addition, since the grounding metal is provided with a hole and the capacitor, too, is provided with a through hole passing through the dielectric ceramic material, the through terminal which achieves a potential higher than that of the ground can be mounted between the grounding metal achieving the ground potential and one of the electrodes at the capacitor while assuring a sufficient degree of electrical insulation achieved by the through holes.
Furthermore, since the insulating resin fills the spaces around the capacitor, a higher degree of reliability is achieved in reliability tests such as high temperature load tests and moisture resistance load tests or when the capacitor is utilized in a hot and highly humid environment.
The present invention is characterized in that one end of the insulating cover faces opposi
Fujiwara Isao
Itoh Yoshiyuki
Sasaki Setsuo
Sato Kazuo
Satoh Tsukasa
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Reichard Dean A.
TDK Corporation
Thomas Eric
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