Inductor devices – Coil or coil turn supports or spacers – Printed circuit-type coil
Reexamination Certificate
1999-04-01
2001-03-06
Donovan, Lincoln (Department: 2832)
Inductor devices
Coil or coil turn supports or spacers
Printed circuit-type coil
C336S223000, C336S232000
Reexamination Certificate
active
06198374
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to multi-layer transformers, more specifically, to multi-layer transformers with improved magnetic coupling and dielectric breakdown voltage between windings in the multi-layer transformers.
2. Description of Related Art
The use of multi-layer transformers is widely known. In general, a multi-layer transformer is constructed with the following process. A magnetic material, for example, ferrite, is cast into tape. The tape is then cut into sheets or layers, and vias are formed at the required locations in each of the tape layers to form conductive pathways. Conductive pastes are subsequently deposited on the surface of the tape layers to form the spiral windings which terminate at the vias. After that, a number of the tape layers with corresponding conductive windings are stacked up with vias in appropriate alignment to form a multi-turn transformer structure. The collated layers are joined together by heat and pressure. The structure is then transferred to a sintering oven to form a homogenous monolithic ferrite transformer. With the above process, many transformers can be made at the same time by forming an array of vias and conductive windings on the surface of the ferrite layers. The transformer may be singulated pre or post firing.
FIGS. 1-2
show an example of a traditional ferrite transformer formed by using the above process.
However, a transformer constructed in the above process has a uniform magnetic permeability throughout the multi-layer structure. Some of the magnetic flux lines generated by the conductive windings cut through the adjacent windings. For example, in a structure where primary windings and secondary windings are disposed in an interleaving relationship on different layers, not all flux lines generated by the primary windings cut through the secondary winding. This yields inefficient flux linkage between the primary windings and the secondary windings. The efficiency of the flux linkage between primary windings and secondary windings can be determined by a magnetic coupling factor. Generally, the magnetic coupling factor between primary and secondary windings is defined as &agr;=
L
pri
-
L
leak
L
pri
,
wherein L
pri
represents primary magnetizing inductance, and L
leak
represents the inductance measured across the primary winding with the secondary winding shorted. It has been determined empirically that coupling is a function of proximity between windings. A transformer (as shown in
FIGS. 1 and 2
) with a uniform permeability has a magnetic coupling factor of 0.83.
Though a closer spacing between the windings in adjacent layers can obtain a higher magnetic coupling factor, the ferrite layers must be made thick enough to withstand a minimum voltage where no dielectric breakdown occurs between the windings. For example, the thickness of a typical NiZn ferrite material requires more than 7 mils to withstand 2400 VAC.
In order to obtain a high magnetic coupling factor, another method has been suggested in U.S. Pat. No. 5,349,743. The '743 patent suggests forming apertures and sing two separate materials to limit the magnetic flux paths to a well defined core area to increase coupling. However, this method is very expensive and limits transformer miniaturization due to the need to make apertures and fill them with a different material than the tape.
Thus, there is a need in the art for an improved multi-layer transformer with a higher magnetic coupling between the windings. Also, there is a need for such an improved multi-layer transformer to be constructed in a lower cost and smaller size, and/or to be readily mass producable in an automated fashion, as well as to meet regulatory safety requirements.
SUMMARY OF THE INVENTION
To overcome the limitations in the art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention provides a method and apparatus of providing a multi-layer transformer with an improved magnetic coupling without affecting its electrical isolation characteristics.
The present invention provides a layer of low permeability dielectric material, thinner than but mechanically and chemically compatible with the higher permeability tape. The thin layer can be disposed on top of, on bottom of, or in between the conductive windings. It is understood that the thin layer may be screen-printed or pasted onto the tapes. The thin layers create areas of different permeability within the structure. The dielectric material in the thin layer also chemically interacts with the ferrite tape during sintering to selectively lower the ferrite permeability in the screened areas. The low permeability dielectric material forms high reluctance paths for the magnetic flux between the windings, thus encouraging the magnetic flux formation in the desired magnetic core volume rather than taking short cuts between windings. Thus, more flux linkages are formed between all primary and secondary windings thereby significantly improving the magnetic coupling factor.
In one embodiment of the present invention, a transformer having a multi-layer tape structure comprises a plurality of tapes being stacked one over the other having a magnetic core area proximate a center of the tapes of the transformer, a primary winding disposed on at least one of the tapes, a secondary winding disposed on at least one of the tapes, a first plurality of interconnecting vias connecting the primary winding between the tapes, a second plurality of interconnecting vias connecting the secondary winding between the tapes, and a layer being disposed proximate at least one of the primary and secondary windings between the tapes, wherein the layer is made of a lower permeability dielectric material in comparison to that of the tapes to form high reluctance paths for magnetic flux between the windings such that the magnetic flux flow is maximized in the magnetic core area.
Further in one embodiment of the present invention, the primary winding and the secondary winding may be disposed in an interleaved relationship on the tapes.
Still in one embodiment of the present invention, the primary winding and the secondary winding may be disposed on adjacent tapes.
Still in one embodiment of the present invention, the primary winding and the secondary winding may be disposed on the same tape.
Yet in one embodiment of the present invention, the layer is mechanically and chemically compatible with the tapes.
Further in one embodiment of the present invention, the layer is screen-printed onto the primary and secondary windings.
Further in one embodiment of the present invention, the layer is pasted onto the primary and secondary windings.
Still in one embodiment of the present invention, the layer is in a tape format.
One of the advantages of the present invention is that the magnetic coupling between the primary winding and the secondary winding is significantly improved. The magnetic coupling factor in the present invention can reach approximately 0.95.
In the present invention, the low permeability dielectric material (i.e. the thin layer) is formulated to have a higher dielectric volt/mil ratio than the traditional ferrite material (e.g. NiZn ferrite material) used to form the tape layers. Thus, another advantage of the present invention is that it allows an overall reduction in tape thickness required to meet dielectric test voltages, thereby using less overall material for each transformer.
A third advantage of the present invention is the lower cost of manufacture. A screen-printing process is much faster than a process of forming apertures in volume. Screens are also generally much lower cost than tooling to make apertures. In addition, tooling size and speed limit how small apertures can practically be in tape layers, whereas screens can be made inexpensively with fine details. Thinner ferrite tape layers also reduce the overall transformer height and/or weight.
The present invention also provides a method for c
Donovan Lincoln
Mai Anh
Merchant & Gould P.C.
Midcom, Inc.
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