The best known high voltage capacitor of this type to date is a two-ganged type high voltage capacitor as disclosed in, for instance, Japanese Utility Model Examined Publication No. 19388/1989 or 48112/1985. This high voltage capacitor comprises a through type capacitor having two spaced-apart through holes, independent electrodes formed on one of opposite surfaces, where the through holes open, and a common electrode provided on the other surface shared by the independent electrodes, the common electrode being bonded by means of soldering or the like to a raised portion of a grounding member. Conductors clad with insulating tubes pass through the respective through holes of the through type capacitor and a through hole of the grounding member, and they are soldered by electrode connectors to the independent electrodes of the through type capacitor. The grounding member has a central raised portion formed on one side. An insulating case is fitted on the outer circumference of the raised portion of the grounding member on one side thereof so as to surround the through type capacitor, and an insulating cover is fitted on the other side so as to surround the conductors. The insulating case is usually made of a thermoplastic resin such as polybuthylene telephthalate (PBT) to achieve cost reduction. A thermosetting insulating resin such as an epoxy resin is provided on the inner and outer sides of the through type capacitor, which is surrounded by the insulating case and cover, thus ensuring moisture resistance and electric insulation. The conductors have terminal sections, such as tab connectors formed on the side of the insulating case for connection to the outside.
Since this high voltage capacitor comprises a thermosetting resin such as an epoxy resin provided on the inner side of the through type capacitor, it is necessary to reduce thermal stress generated in withstand voltage tests or heat shock tests or in use or shrinkage stress generated at the time of hardening. Heretofore, this has been achieved by covering the conductors with insulating tubes of silicone rubber or the like. The silicone rubber insulating resin tubes are elastic and can thus prevent interface separation between a porcelain element constituting the through type capacitor and the epoxy resin.
However, since the prior art high voltage capacitor comprises a through type capacitor with insulating resin provided on the inner and outer sides of the capacitor, the contact interface between the through type capacitor and the insulating resin extends broadly along the inner and outer peripheries of the through type capacitor. Therefore, the likelihood of interface separation occurring is high, and voltage breakdown failure is liable to occur. In addition, since the capacitor uses a two-ganged through type capacitor with an insulating resin provided therearound, size reduction is limited. Furthermore, the use of the two-ganged through type capacitor leads to high cost. The above problems are also present in a magnetron which uses the before-mentioned high voltage capacitor. In order to solve the problems described above, independent capacitor type high voltage capacitors using two independent through type capacitors, which had been commonly used before the two-ganged type, were reconsidered. However, the independent capacitor type high voltage capacitor comprises independent through type capacitors, which results in insufficient mechanical strength when they are bonded to a grounding member. Furthermore, conductors are mounted in the respective through type capacitors, and external connectors are fitted on and removed from tab-type terminal sections of the conductors. Therefore, insufficient mechanical strength leads to looseness in the conductors to cause interface separation of the conductors, the dielectric body and the grounding member from the insulating resin. In such a case, the withstand voltage characteristic is greatly deteriorated.
Furthermore, even with a structure in which an insulating case is provided for each through type capacitor with an insulating resin provided around the through type capacitor in the insulating case, it is difficult to ensure sufficient mechanical strength to withstand external forces exerted when connecting the external connectors. Besides, the insulating resin has to be provided independently for each insulating case, thus leading to an increase in the number of insulating resin pouring steps and increasing the cost. Furthermore, when water drops or the like collect on the surface of the insulating case, creeping discharge may be produced along the insulating case surface to result in withstand voltage failures.
Study was further conducted of a structure in which two through type capacitors are covered by a single insulating case and an insulating resin is provided to fill the inner space of the case. In this case, excess insulating resin is provided, thus increasing thermal stress generated in heat cycle tests to increase the likelihood of withstand voltage failure or the like. Besides, the overall size and cost are increased.
Another aspect of this type of high voltage capacitor is that it has an important application as a filter of a magnetron in a microwave oven and is, therefore, frequently used in environments of high relative humidity or with much dust. Therefore, it is required to a high degree of withstand voltage under humid conditions. The prior art high voltage capacitor, however, has an insulating case comprising a thermoplastic resin such as PBT, which is fitted on the outer circumference of a raised portion of the grounding member. This means that most of the path extending from the conductors to the grounding member is constituted of the surface of the insulating case made of a thermoplastic insulating resin. Thermoplastic resin such as PBT is inferior in tracking resistance characteristics to thermosetting resins such as an epoxy resin or an unsaturated polyester resin. Therefore, it has been difficult to obtain a high voltage capacitor having satisfactory tracking resistance characteristics. The tracking resistance characteristics may be improved by using a thermosetting resin such as an epoxy resin or an unsaturated polyester resin for the insulating case. In this case, however, the cost is increased.
As a further problem with a prior art structure, in which an epoxy type thermosetting insulating resin is provided on the inner and outer sides of a through type capacitor to ensure moisture resistance and electric insulation, the bonding strength and adhesion between the dielectric porcelain element of the capacitor and the thermosetting insulating resin are insufficient. Therefore, in high temperature loading tests or moisture resistance loading tests, separation or cracks may occur in the contact interface between the dielectric porcelain element and the insulating resin, thus leading to electric breakdown.
As a still further problem, a gap or crack may be generated due to separation in the contact interface between the through type capacitor and a thermosetting resin such as an epoxy resin by thermal stress generated in withstand voltage tests, heat shock tests, in use or stress generated due to shrinkage at the time of hardening. In consequence, the withstand voltage characteristics deteriorate. The thermal stress or hardening shrinkage stress in a thermosetting insulating resin such as an epoxy resin may be reduced by covering a portion of the conductor that extends in the through hole capacitor with an insulating tube of silicone rubber or the like. In this case, however, since it is necessary to cover the conductor with an insulating tube of silicone rubber or the like, the number of components is increased, thus increasing the number of assembling steps because it is necessary to fit the insulating tube.
A yet further problem is posed by the use for the insulating case and insulating cover of an epoxy or like synthetic resin which has adhesion to the insulating resin. In this case, the adhesion of the insulating case and insulating cover to the insulating resin is greater than the adhesion of the through type capacitor to the insulating resin. Therefore, thermal stress generated due to temperature variations in heat shock tests, heat cycle tests or in use causes repeated shrinkage and expansion of the insulating resin originating at the insulating case and insulating cover. Consequently, separation, gaps or cracks may occur in the contact interface between the dielectric porcelain element and the insulating resin, causing electric field concentration in the separated part of the resin, gap or crack formed therein, thus resulting in creeping discharge breakdown or the like.
A further problem is posed by the use of the insulating case and insulating cover as molds for charging the insulating resin by having the case and cover fitted at one end thereof on or in the grounding member. In this step, if the contact of fit of the insulating case and insulating cover to the grounding member is insufficient, the charged insulating resin may flow out through the insufficient contact portions to the outside, thus resulting in defective products.
To prevent production of defective products due to the flow-out of the insulating resin, it is necessary to fit the insulating case and insulating cover to the grounding member in perfect contact therewith. However, failure of fitting or defective fitting of the insulating case and insulating cover to the grounding member is liable to result in the production of defective products. Besides, since the insulating case and insulating cover are necessary, the number of components and number of assembling steps are increased, thereby increasing costs.
In this type of high voltage capacitor it is very important to increase the bonding strength between the dielectric porcelain element of the through type capacitor and the insulating resin provided therearound, in order to ensure reliability. With the prior art high voltage capacitor, however, the bonding strength is about 20 to 40 kgf/cm.sup.2 in a measurement temperature range of 80.degree. to 140.degree. C. Therefore, it has been impossible to prevent separation, gaps or cracks from occurring in the contact interface between the dielectric porcelain element and the insulating resin due to thermal stress generated due to temperature change in heat shock tests, heat cycle tests or in use. Therefore, there has been a problem that moisture intrudes into the separated portions, gaps or cracks in the contact interface between the dielectric porcelain element and the insulating resin in reliability tests such as high temperature loading tests, moisture resistance loading tests, in use under high temperature or high relative humidity conditions. In addition, electric field concentration in the separated areas, gaps or cracks is prone to cause voltage breakdown.