1.  Field of the Invention
The present invention relates to a wound core, and more particularly, to a wound core for a toroidal transformer. 
2.  Description of the Related Art
Recently, toroidal transformers employing toroidal windings, which are thin, light, and low in leakage flux, have often been used in audio equipment and in monitors of computer systems.  It is required to provide wound cores that permit an improvement in the productivity of the toroidal transformers and allow the toroidal transformers to provide full performance. 
A wound core is made by winding several hundred turns of a grain-oriented silicon steel strip, about 0.2 to 0.3 mm thick, around a cylindrical jig.  In the prior art, the wound core has a rectangular section with long longitudinal sides and an annular plan shape.  Further, the surface of the wound core is covered with an insulating film around which a copper wire such as an enameled wire is wound according to a toroidal winding technique, to form a transformer (toroidal transformer). 
By the way, the ideal sectional shape of a core of a winding is circular.  Many patent and utility model applications have disclosed wound cores having a circular section.  For example, there are Japanese Patent Publication (Kokoku) Nos.  60-28375, 61-22851, and 5-29289 (corresponding to U.S. Pat.  Nos.  5,115,703 and 5,188,305).  Note that, these disclosures employ a cylindrical coil bobbin for winding a wire as nearly circular as possible. 
The core for a toroidal transformer must be longitudinally elongated in section, to reduce the area occupied by the transformer.  The diameter of the core must be short to reduce its weight.  A core having a rectangular section produces a transformer which has a high ratio of iron and copper to its volume. 
As described above, in the prior art, the section of the wound core for the toroidal transformer is rectangular, and copper wire is wound around the wound core according to a toroidal winding technique, to form the toroidal coil.  Therefore, spaces are formed between the wound core and the toroidal coil. 
The spaces between the wound core and the coil (toroidal coil) increase the size of the coil and increase the length of the copper wire, per turn, around the wound core, to increase the resistance of the coil.  In addition, the spaces may cause noise and vibration. 
An object of the present invention is to provide a wound core for a toroidal transformer that allows full performance and improves the productivity of toroidal transformers. 
According to the present invention, there is provided a wound core for a toroidal transformer, having circular or elliptic longitudinal ends in section.  Further, according to the present invention, there is provided a wound core for a toroidal transformer, having polygonal longitudinal ends in section.  Further, according to the present invention, there is also provided a wound core for a toroidal transformer, having circular or elliptic starting part and ending part in section.  In addition, according to the present invention, there is provided a wound core for a toroidal transformer, having polygonal starting part and ending part in section. 
According to the present invention, there is provided a wound core for a toroidal transformer, having an elliptic section.  Further, according to the present invention, there is provided a wound core for a toroidal transformer, having curved corners in section.  Further, according to the present invention, there is also provided a wound core for a toroidal transformer, having beveled corners in section. 
According to the present invention, there is provided a toroidal transformer comprising a wound core and a toroidal winding, wherein the wound core has circular or elliptic longitudinal ends in section.  Further, according to the present invention, there is provided a toroidal transformer comprising a wound core and a toroidal winding, wherein the wound core has polygonal longitudinal ends in section.  Further, according to the present invention, there is also provided a toroidal transformer comprising a wound core and a toroidal winding, wherein the wound core has circular or elliptic starting part and ending part in section.  In addition, according to the present invention, there is provided a toroidal transformer comprising a wound core and a toroidal winding, wherein the wound core has polygonal starting part and ending part in section. 
According to the present invention, there is provided a toroidal transformer comprising a wound core and a toroidal winding, wherein the wound core has an elliptic section.  Further, according to the present invention, there is provided a toroidal transformer comprising a wound core and a toroidal winding, wherein the wound core has curved corners in section.  Further, according to the present invention, there is also provided a toroidal transformer comprising a wound core and a toroidal winding, wherein the wound core has beveled corners in section. 
The wound core may be manufactured by continuously cutting a material strip into shaped bands and by winding a unit length of the shaped bands around a jig.  The jig may be cylindrical and be turned around its axis to wind a unit length of the shaped bands around the jig, to form the wound core. 
A longitudinal center line of each of the shaped bands may be aligned with the widthwise center of the jig, and a unit length of the shaped bands may be wound around the jig.  The material strip may be cut into the shaped bands with the maximum and minimum widths of a first band of the shaped bands being substantially adjacent to the minimum and maximum widths of a second band of the shaped bands. 
The material strip may be cut into the shaped bands with a linear side-edge of the material strip serving, as it is, as a linear side-edge of one or two of the shaped bands.  The material strip may be cut into two shaped bands with each linear side-edge of the material strip serving, as it is, as a linear side-edge of each of the shaped bands, and with narrowed and widened parts of a first band of the shaped bands being substantially adjacent to widened and narrowed parts of a second band of the shaped bands, so that a remainder of the material strip between the first and second shaped bands may be discarded.