Inductor devices – With outer casing or housing – Potted type
Reexamination Certificate
1999-04-20
2001-02-20
Gellner, Michael L. (Department: 2832)
Inductor devices
With outer casing or housing
Potted type
C336S198000
Reexamination Certificate
active
06191675
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a small sized high voltage transformer comprising a primary coil, a secondary coil, and a magnetic core, and to an ignition transformer using the high voltage transformer.
2. Description of the Related Art
Transformers used in automobiles for ignition, and flyback transformers for driving cathode ray tubes are required to generate a high pulse voltage output of 10 kV to 35 kV. These transformers have firstly been assembled from a primary coil, a secondary coil and a magnetic core, and then a casting resin is injected into the coil part, and subsequently cured to complete the constitution of a transformer. In this type of transformers a high voltage is produced in the secondary coil by raising a special pulse voltage fed into the primary coil.
Described below is a detailed example of known manufacturing method for an ignition transformer of direct ignition type used for automobiles as illustrated in
FIG. 1. A
secondary coil
3
wound on a secondary bobbin
2
is arranged around an inner magnetic core
1
-
1
. After a primary coil
5
wound on a primary bobbin
4
is arranged and an inner magnetic core
1
-
2
is attached to the both ends of the coil, the parts are housed in a case
6
. A casting resin
7
is poured into the case
6
to fill the clearance in a transformer
8
and the void in coil
9
, and subsequently heat cured. Putting an exterior magnetic core
1
-
3
around the case completes the constitution of the transformer.
An example of conventional method widely used for manufacturing a flyback transformer for driving cathode ray tubes as shown in
FIG. 2
is also described below. A secondary coil
3
wound on an intermediate layer
10
coated on the secondary bobbin
2
is arranged around a primary coil
5
wound on a primary bobbin
4
. The parts are housed in a case
6
. A casting resin
7
is injected into the case
6
to fill the clearance in the transformer
8
and the narrow void in coil
9
, and then heat cured. Fitting of the magnetic core
1
completes the constitution of the transformer.
These transformers are required to function properly for a long period of use life under a high temperature in a cramped space in the transformer. Therefore, long term durability under heat and moisture has been a very important requirement. In the manufacture of such high voltage transformers, choice of combination of the casting resin and the material used for the bobbin is very important. The reason is that the lack of adhesion between the two materials may cause separation of the two materials. Difference in heat expansion coefficients between the two materials may case thermal stress, resulting in cracking in the casted resin. Thereby the dielectric breakdown in the coil may occur due to the electric discharge. In addition, withstanding voltage properties of the bobbin material and the casting resin are also required.
To avoid dielectric breakdown, attempts have been made to select a combination of a casting resin and a bobbin material which will give good adhesion between the two materials. For this reason, epoxy resins having a heat distortion temperature ranging from 90° C. to 120° C. have been widely used as the casting resins in combination with a bobbin material such as a blend of polyphenylene oxide and polystyrene (ex. Noryl, Trademark of GE Company) having heat distortion temperature of approximately 120° C. The reason why the above combination has been selected lies in the belief that surface of Noryl resin partially swells when contacted with a liquid epoxy resin thereby providing a good adhesion layer as the epoxy resin undergoes curing.
However, heat distortion temperatures of epoxy resins and bobbin materials conventionally used are not high enough. Therefore, these materials tend to soften when transformers are subjected to a temperature higher than 120° C. This has been a cause of mechanical deformation and dielectric breakdown of the materials employed in transformers.
In the conventional distributor system, one transformer is connected to multiple number of engines. On the other hand, recently, in order to improve power controllability of automobiles, a direct ignition system has been adopted, wherein plural transformers are connected directly to the same number of engines.
As a flyback transformer for driving cathode ray tubes, weight reductions of display is becoming major requirement in the market as well as the requirement for cost reduction. In these types of transformers, reduction in weight, size and cost of transformers are important issues.
However, in the conventional transformer, since the combination of materials were limited, and could not satisfy the severe requirement for use and its size reduction. When an epoxy resin of higher heat distortion temperature is used to improve heat resistance in combination with a conventional bobbin material, matching of heat expansion coefficients of the two materials becomes a problem resulting in poor adhesion between the two materials.
SUMMARY OF THE INVENTION
An object of the invention is to provide a small-sized heat resistant high voltage transformer and an ignition transformer by solving the problems.
Another object of the invention is to provide a heat resistant high voltage transformer which is capable of producing output voltage of 15-35 kV , using a casting resin and a coil bobbin both having a heat distortion temperature of 130° C. or above.
The heat distortion temperature herein referred is the temperature at which deformation of a casting resin composition or a molded bobbin starts to occur when exposed to that temperature. Generally, these values can be replaced with the values obtained at the loading of 1.82 MP a according to ASTM D648.
As for casting resins, epoxy resins containing 30-55 wt % of inorganic filler may be used.
The inorganic fillers used in the casting resins are silica, silica glass or a mixture of silica and silica glass. Alumina, hydrated alumina, calcium carbonate and other type of inorganic fillers may be added to modify the characteristics of the fillers as required.
Inorganic fillers contained in the mold composition of coil bobbins may be glass fiber, talc, or a mixture of glass fiber and talc, and can be modified by adding glass beads, mica, silica, alumina, calcium carbonate, or other inorganic fillers as required.
The coil bobbins are made from mold compositions containing 25-70 wt. % of an inorganic filler and a resin such as, polyphenylene sulfide, polyether sulfone, polyether imide, polyether ketone, and liquid crystal polymer. When an epoxy resin is precoated on the bobbin which swells with a solvent, 10-70 wt. % of an inorganic filler may be incorporated.
Pretreatments on the surface of coil bobbins, such as sandblast treatment and precoating with a solid epoxy resin are found to be very effective.
In order to maintain the heat distortion temperature of the casting resin at a temperature of at least 130° C., bis-phenol A di-glycidyl ether or bis-phenol F di-glycidyl ether can be used as a main ingredient for the epoxy resin. Addition of alicyclic epoxy compounds is particularly effective in keeping high heat distortion temperature. Alicyclic epoxy compounds are relatively low in viscosity before curing and are effective in raising heat distortion temperature of epoxy curing compositions. Examples of suitable alicyclic epoxy compositions are cyclohexene oxide, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, and etc.
Useful as curing agents for epoxy resins are methyl-tetrahydrophthalic anhydride, methyl-hexahydrophthalic anhydride, and hexahydrophthalic anhydride. Particularly useful in raising heat distortion temperature are methyl-hexahydrophthalic anhydride and hexahydrophthalic anhydride.
Imidazoles are found to be useful as catalysts for curing epoxy resins. Particularly effective are 2-ethyl-4-methyimidazole, its adduct with acrylonitrile, and 1-methyl-2-ethylimidazole.
A wide variety of inorganic fillers may be used for casting resins. Considering electric in
Iida Makoto
Kobayashi Kazutoshi
Sudo Ryoichi
Tajima Tetsuo
Antonelli Terry Stout & Kraus LLP
Gellner Michael L.
Hitachi , Ltd.
Nguyen Tuyen
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