Patent Publication Number: US-8975993-B2

Title: Transformer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transformer. 
     2. Related Background Art 
     The balun transformer disclosed in Japanese Patent Application Laid-Open No, 10-326715 has conventionally been known as an example of transformers for use in small electronic devices and the like. This type of conventional transformer is constructed by joining a flat core to a drum-shaped core having a center flange and quadrangular end flanges at both ends. Two windings (primary and secondary windings) are wound, for example, one by one as lower and upper tiers, about each winding groove (winding core part) formed between the center flange and the end flanges. Electrodes are disposed on side faces of the flanges, while terminals of the windings are connected to their corresponding electrodes. 
     SUMMARY OF THE INVENTION 
     During thermocompression bonding of winding terminals to the electrodes in transformers constructed basically as mentioned above, the heat of thermocompression bonding may deteriorate the surroundings of the electrode connecting portions. Thus deteriorated parts may cause mounting failures when included in a surface to be mounted on a substrate. For preventing this from happening, the connecting portions may be placed on a surface opposing the plate-like core on the side opposite from the mounting surface. As a method for improving the DC superposition characteristic in thus constructed transformer, a gap may be provided between the plate-like core and the flange so as to suppress the magnetic saturation. In a simple flat structure such as that of the conventional plate-like core, however, a magnetic flux may pass between the plate-like core and the terminal electrode, thereby causing an eddy current, which produces an eddy-current loss. 
     For employing thus constructed transformer as a step-up transformer with a high transformer ratio, it is necessary for the turn ratio of the secondary winding to the primary winding to be as high as possible. For this purpose, winding the secondary winding about the winding core part of the drum core in a reciprocating manner into multiple tiers and then winding the primary winding about the outermost tier may be considered. However, this may increase the stray capacitance between the first and secondary windings depending on the positional relationship between the winding start portions of the primary and secondary windings. In particular, when the stray capacitance between the higher voltage side of the secondary winding (the winding start side of the secondary winding) and the primary winding increases, the LC resonance may be so high that the output voltage of the transformer becomes unstable, thereby generating ringing. Hence, how to wind the primary and secondary windings about the winding core part has become an important problem. 
     For solving such a problem, it is an object of the present invention to provide a transformer which can improve the DC superposition characteristic without incurring eddy-current losses. It is another object of the present invention to provide a step-up transformer which can reduce the stray capacitance and stabilize the output voltage. 
     The transformer in accordance with the present invention comprises a drum core, made of ferrite, having a winding core part and flanges disposed at both ends of the winding core part; a winding wound about the winding core part; a terminal electrode, disposed at the flange, for connecting with a terminal of the winding on a top face of the flange; and a plate-like core, made of ferrite, opposing the top face; wherein the plate-like core has, in a part opposing the top face, a first opposing portion opposing no terminal electrode and a second opposing portion opposing the terminal electrode; wherein a first gap is formed between the top face and the first opposing portion by a spacer; and wherein a second gap greater than the first gap is formed between the terminal electrode and the second opposing portion by a recess in the plate-like core provided so as to correspond to the second opposing portion. 
     In this transformer, in the part opposing the top face, the plate-like core has the first opposing portion opposing no terminal electrode and the second opposing portion opposing the terminal electrode. The first gap is formed between the top face and the first opposing portion by the spacer. The second gap greater than the first gap is formed between the terminal electrode and the second opposing portion by a recess in the plate-like core which is provided so as to correspond to the second opposing portion. As a consequence, in this transformer, magnetic fluxes pass between the top face and the first opposing portion where the first gap is formed, but are inhibited from passing between the terminal electrode and the second opposing portion where the second gap greater than the first gap is formed. This can restrain the terminal electrode from generating eddy currents, whereby the DC superposition characteristic can be improved without incurring eddy-current losses. 
     Preferably, the second gap is at least 3 times the first gap. This can reliably secure the gap between the terminal electrode and the plate-like core. Therefore, the transformer can further inhibit the terminal electrode from generating eddy currents, so that the DC superposition characteristic can be improved without incurring eddy-current losses. 
     Preferably, the plate-like core has a thickness of 0.25 mm or more at the recess. This can restrain the plate-like core from being deflected by heat during when the transformer is in use. 
     Preferably, the transformer is a step-up transformer, the terminal electrode includes input and output terminals; the winding includes a primary winding connected to the input terminal and a secondary winding connected to the output terminal; the primary winding has a diameter greater than that of the secondary winding; the secondary winding has a number of turns greater than that of the primary winding; the secondary winding is wound in a plurality of tiers about the winding core part, while a winding start portion thereof for the winding core part is covered with an upper tier of the secondary winding; and the primary winding is wound on the outside of the upper tier of the secondary winding. In this case, the secondary winding is wound in a plurality of tiers about the winding core part, the winding start portion of the secondary winding for the winding core part is covered with the upper tier of the secondary winding, and the primary winding is wound on the outside of the upper layer of the secondary winding. This interposes the upper tier of the secondary winding between the winding start portions of the primary and secondary windings and thus can prevent these winding start portions from coming into contact with each other. Therefore, this step-up transformer can lower the stray capacitance between the winding start portions of the primary and secondary windings, thereby stabilizing the output voltage. 
     Preferably, the winding start portions of the primary and secondary windings are located at respective positions different from each other in an axial direction of the winding core part. This can more reliably prevent the winding start portions of the primary and secondary windings from coming into contact with each other. Therefore, this step-up transformer can lower the stray capacitance between the winding start portions of the primary and secondary windings, thereby stabilizing the output voltage. 
     Preferably, the primary winding is wound sparsely such that turns thereof are in no contact with each other. This can reduce leakage fluxes, thereby further inhibiting the voltage waveform from ringing. 
     Preferably, the winding start portion of the secondary winding is located closer to a center of the winding core part, while a middle part of turns in the secondary winding is located between the winding start portion of the secondary winding and the flange. This prevents the winding start portion of the secondary winding from being arranged adjacent to the flange, so that the secondary winding does not interfere with the flange when covering the winding start portion of the secondary winding, whereby it becomes easier for the upper tier of the secondary winding to cover the winding start portion of the secondary winding. Therefore, this step-up transformer can lower the stray capacitance between the winding start portions of the primary and secondary windings, thereby stabilizing the output voltage. 
     The step-up transformer in accordance with the present invention comprises a drum core having a winding core part and flanges disposed at both ends of the winding core part; input and output terminals disposed at the flanges; a primary winding connected to the input terminal; and a secondary winding connected to the output terminal; wherein the primary winding has a diameter greater than that of the secondary winding; wherein the secondary winding has a number of turns greater than that of the primary winding; wherein the secondary winding is wound in a plurality of tiers about the winding core part, while a winding start portion thereof for the winding core part is covered with an upper tier of the secondary winding; and wherein the primary winding is wound on the outside of the upper tier of the secondary winding. 
     In this step-up transformer, the secondary winding is wound in a plurality of tiers about the winding core part, the winding start portion of the secondary winding for the winding core part is covered with the upper tier of the secondary winding, and the primary winding is wound on the outside of the upper layer of the secondary winding. This interposes the upper tier of the secondary winding between the winding start portions of the primary and secondary windings and thus can prevent these winding start portions from coming into contact with each other. Therefore, this step-up transformer can lower the stray capacitance between the winding start portions of the primary and secondary windings, thereby stabilizing the output voltage. 
     Preferably, the winding start portions of the primary and secondary windings are located at respective positions different from each other in an axial direction of the winding core part. This can more reliably prevent the winding start portions of the primary and secondary windings from coming into contact with each other. Therefore, this step-up transformer can lower the stray capacitance between the winding start portions of the primary and secondary windings, thereby stabilizing the output voltage. 
     Preferably, the primary winding is wound sparsely such that turns thereof are in no contact with each other. This can reduce leakage fluxes, thereby further inhibiting the voltage waveform from ringing. 
     Preferably, the winding start portion of the secondary winding is located closer to a center of the winding core part, while a middle part of turns in the secondary winding is located between the winding start portion of the secondary winding and the flange. This prevents the winding start portion of the secondary winding from being arranged adjacent to the flange, so that the secondary winding does not interfere with the flange when covering the winding start portion of the secondary winding, whereby it becomes easier for the upper tier of the secondary winding to cover the winding start portion of the secondary winding. Therefore, this step-up transformer can lower the stray capacitance between the winding start portions of the primary and secondary windings, thereby stabilizing the output voltage. 
     The present invention can provide a transformer which can improve the DC superposition characteristic without incurring eddy-current losses. The present invention can also provide a step-up transformer which can reduce the stray capacitance and stabilize the output voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating the transformer in accordance with a first embodiment of the present invention; 
         FIG. 2  is a front view of  FIG. 1 ; 
         FIG. 3  is a side view of  FIG. 1 ; 
         FIG. 4  is a top plan view of  FIG. 1 ; 
         FIG. 5  is a bottom plan view of  FIG. 1 ; 
         FIG. 6  is a front view of a plate-like core in the transformer illustrated in  FIG. 1 ; 
         FIG. 7  is a side view of  FIG. 6 ; 
         FIG. 8  is a top plan view of  FIG. 6 ; 
         FIG. 9  is a bottom plan view of  FIG. 6 ; 
         FIG. 10  is a perspective view illustrating the step-up transformer in accordance with a second embodiment of the present invention; 
         FIG. 11  is a front view of  FIG. 10 ; 
         FIG. 12  is a side view of  FIG. 10 ; 
         FIG. 13  is a top plan view of  FIG. 10 ; 
         FIG. 14  is a bottom plan view of  FIG. 10 ; 
         FIG. 15  is a front view illustrating the step-up transformer without the plate-like core; 
         FIG. 16  is a side view of  FIG. 15 ; 
         FIG. 17  is a top plan view of  FIG. 15 ; 
         FIG. 18  is a bottom plan view of  FIG. 15 ; 
         FIG. 19  is a circuit diagram of the drum core in accordance with an example; 
         FIG. 20  is a sectional view taken along a line XX-XX of  FIG. 17 ; 
         FIG. 21  is a sectional view illustrating how the primary and secondary windings are wound about the drum core in accordance with a comparative example; 
         FIG. 22  is a sectional view illustrating a main part of the step-up transformer in accordance with a third embodiment of the present invention; 
         FIG. 23  is a sectional view illustrating a main part of the step-up transformer in accordance with a fourth embodiment of the present invention; 
         FIG. 24  is a sectional view illustrating a main part of the step-up transformer in accordance with a fifth embodiment of the present invention; and 
         FIG. 25  is a sectional view illustrating a main part of the step-up transformer in accordance with a sixth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, preferred embodiments of the transformer in accordance with the present invention will be explained in detail with reference to the drawings. 
       FIGS. 1 to 5  are perspective, front, side, top plan, and bottom plan views of the transformer in accordance with the first embodiment, respectively.  FIGS. 6 to 9  are front, side, top plan, and bottom plan views of a plate-like core in the transformer in accordance with the first embodiment, respectively. 
     The transformer  101  in accordance with this embodiment is used for voltage transformation in a small device such as a camera. As illustrated in  FIGS. 1 to 5 , the transformer  101  comprises a drum core  102 , a plate-like core  103 , a primary winding  104 , a secondary winding  105 , input terminals (terminal electrodes)  106 ,  106 , and output terminals (terminal electrodes)  107 ,  107 . Here, the transformer  101  has a length (in the vertical direction in  FIG. 4 ) of about 3.2 mm, a width (in the horizontal direction in  FIG. 4 ) of about 2.5 mm, and a height (in the vertical direction in  FIG. 2 ) of about 1.2 to 2.4 mm. 
     The drum core  102  is made of ferrite and has a winding core part  121  and flanges  122 U,  122 L. The winding core part  121  is shaped like a substantially quadrangular prism, for example. The flanges  122 U,  122 L, each shaped like a substantially rectangular parallelepiped having a cross-sectional area greater than that of the winding core part  121 , are disposed at both ends of the winding core part  121 . 
     Each of the primary and secondary windings  104 ,  105  is wound about the winding core part  121  clockwise (in a right-hand turn) as seen from the flange  122 U side. The secondary winding  105  is initially wound about the winding core part  121 , and then the primary winding  104  is wound about the outer periphery of the secondary winding  105 . The primary winding  104  has a diameter which is about 2 to 5 times that of the secondary winding  105 . Here, the diameter of the primary winding  104  is about 50 to 100 μm, while the diameter of the secondary winding  105  is about 10 to 40 μm. 
     One input terminal  106  is disposed on the upper face (hereinafter referred to as “top face”)  123 U, side face  124 U, and bottom face  125 U of the flange  122 U so as to exhibit a substantially U-shaped form. The other input terminal  106  is similarly disposed on the flange  122 L. The output terminals  107 ,  107  are disposed on the flanges  122 U,  122 L, respectively, as with the input terminals  106 ,  106 . The input terminals  106 ,  106  are located closer to one side of the flanges  122 U,  122 L (the right side in  FIG. 2 ), while the output terminals  107 ,  107  are located closer to the other side of the flanges  122 U,  122 L. Each of the input and output terminals  106 ,  107  has a width (in the horizontal direction in  FIG. 7 ) which is about ⅕ that of each of the flanges  122 U,  122 L. At the top faces  123 U,  123 L, of the flanges  122 U,  122 L, the terminals of the primary winding  104  are connected to the input terminals  106 ,  106 , while the terminals of the secondary winding  105  are connected to the output terminals  107 ,  107 . 
     The input and output terminals  106 ,  107  are formed by transferring a conductive paste mainly composed of Ag, for example, to the top faces  123 U,  123 L, side faces  124 U,  124 L, and bottom faces  125 U,  125 L, of the flanges  122 U,  122 L, burning the paste at a predetermined temperature (e.g., about 700° C.) thereafter, and further plating it with a metal. For example, Sn can be used for metal plating. The input and output terminals  106 ,  107  may be constituted by a plate material made of a metal, so as to be attached to their corresponding positions of the flanges  122 U,  122 L. For example, phosphor bronze plated with metals (Ni and Sn) can be used for the metal plate material. 
     The plate-like core  103 , which is a substantially rectangular member made of ferrite, is used for lowering magnetic resistance, so as to improve the inductance of the transformer  101 . The plate-like care  103  has such a size as to cover the drum core  102 , e.g., a width (in the horizontal direction in  FIG. 4 ) of about 2.5 mm, a length (in the vertical direction in  FIG. 4 ) of about 3.2 mm, and a thickness of about 0.45 mm in this embodiment. As illustrated in  FIG. 3 , the plate-like core  103  is arranged such that the vicinities of both longitudinal ends thereof oppose the top faces  123 U,  123 L, of the flanges  122 U,  122 L. 
     The plate-like core  103  has, in the parts opposing the top faces  123 U,  123 L, first opposing portions  131 ,  131  opposing none of the input and output terminals  106 ,  107  and second opposing portions  132 ,  132  opposing the input and, output terminals  106 ,  107 . The plate-like core  103  also has a third opposing portion  133  in the part opposing the winding core part  121 . As illustrated in  FIGS. 6 to 9 , each first opposing portion  131  is a substantially rectangular part having a width T 1  of about 1.44 mm and a length T 2  of about 0.7 mm, for example. All of the side faces  131   a ,  131   b ,  131   c  of the first opposing portion  131  are tilted. Bach of the second opposing portions  132 ,  132 , which are substantially rectangular parts (parts surrounded by dash-single-dot lines in  FIG. 9 ) holding the first opposing portion  131  therebetween in the substantially U-shaped part covering the first opposing portion  131 , has a width T 3  of about 0.53 mm and a length T 4  of about 01 mm, for example. The third opposing portion  133  is a substantially rectangular part having a width. T 5  of about 2.5 mm and a length T 6  of about 1.4 mm. Both side faces  133   a ,  133   a  of the third opposing portion  133  are tilted. 
     The first opposing portion  131  is provided with spacers  134 ,  134  which are separated from each other in the width direction of the plate-like core  103  and form a first gap d 1  between the plate-like core  103  and the flanges  122 U,  122 L (see  FIG. 2 ). As illustrated in  FIG. 7 , each of the spacers  134 ,  134  is a spherical protrusion projecting from the plate-like core  103  and having, for example, a diameter of about 0.2 mm and a height da of about 0.03 to 0.1 mm, which is about 0.05 mm here. The height da corresponds to the first gap d 1 . That is, the first gap d 1  is 0.05 mm. The thickness dt of the plate-like core  103  at the first opposing portion  131  is about 0.35 to 0.8 mm, which is 0.45 mm here. 
     Each second opposing portion  132  serves as a recess depressed from the first opposing portion  131 , while a second gap d 2  greater than the first gap d 1  is formed by the recess between the second opposing portion  132  and its corresponding one of the input and output terminals  106 ,  107  (see  FIG. 2 ). The amount of depression of the second opposing portion  132  is about 0.1 mm, for example, while the sum dc of the amount of depression and the height da of the spacer  134  corresponds to the second gap d 2 . Hence, the second gap d 2  is 0.15 mm, which is 3 times the first gap d 1 . Preferably, the second gap d 2  is at least 3 times the first gap d 1 . This can increase the magnetic resistance between the flanges  122 U,  122 L and their corresponding second opposing portions  132 ,  132 , inhibit magnetic fluxes from passing between the flanges  122 U,  122 L and the second opposing portions  132 ,  132 , and allow the magnetic fluxes to pass only between the flanges  122 U,  122 L and their corresponding first opposing portions  131 ,  131 . The thickness dm of the plate-like core  103  at the second opposing portion  132  serving as a recess, which is preferably at least 0.25 mm, is 0.35 mm here. 
     In order to identify the polarity of the transformer  101  when mounting it to a substrate, the upper and bottom faces of the plate-like core  103  are provided with orientation identification marks  135 ,  135 . The plate-like core  103  is secured to the upper faces of the flanges  122 U,  122 L by an adhesive applied between the spacers  134 ,  134 . 
     In thus constructed transformer  101 , the plate-like core  103  has, in the parts opposing the top faces  123 U,  123 L, the first opposing portions  131 ,  131  opposing none of the input and output terminals  106 ,  107  and the second opposing portions  132 ,  132  opposing the input and output terminals  106 ,  107 . The spacers  134 ,  134  form the first gap d 1  between the top faces  12313 ,  123 L and their corresponding first opposing portions  131 ,  131 . Between the input and output terminals  106 ,  107  and their corresponding second opposing portions  132 ,  132 , the second gap d 2  greater than the first gap d 1  is formed by the recesses in the plate-like core  103  provided so as to correspond to the second opposing parts  132 ,  132 . Therefore, in the transformer  101 , magnetic fluxes pass between the top faces  123 U,  123 L and their corresponding first opposing portions  131 ,  131  where the first gap d 1  is formed, but are inhibited from passing between the plate-like core  103  and the input and output terminals  106 ,  107  where the second gap d 2  greater than the first gap d 1  is formed. This restrains the input and output terminals  106 ,  107  from generating eddy currents, whereby the DC superposition characteristic can be improved without incurring eddy-current losses. 
     Since the second gap d 2  is at least 3 times the first gap d 1 , the gap between the plate-like core  103  and the input and output terminals  106 ,  107  can be secured reliably. This can also increase the magnetic resistance between the flanges  122 U,  122 L and their corresponding second opposing portions  132 ,  132 , inhibit magnetic fluxes from passing between them, and allow the magnetic fluxes to pass only between the flanges  122 U,  122 L and their corresponding first opposing portions  131 ,  131 . Therefore, this transformer  101  can further restrain the input and output terminals  106 ,  107  from generating eddy currents, whereby the DC superposition characteristic can be improved without incurring eddy-current losses. 
     The plate-like core  103  has a thickness of 0.25 mm or more at the second opposing portion  131  serving as a recess and thus can be restrained from being deflected by heat during when the transformer  101  is in use. 
     The present invention is not limited to the above-mentioned embodiment. For example, while the spacer  134  is a protrusion projecting from the plate-like core  103  in the above-mentioned embodiment, a member independent from the plate-like core  103  may be used as a spacer instead. 
     The step-up transformer in accordance with the second embodiment of the present invention will now be explained. 
       FIGS. 10 to 14  are perspective, front, side, top plan, and bottom plan views of the step-up transformer in accordance with the second embodiment, respectively, while  FIGS. 15 to 18  are front, side, top plan, and bottom plan views illustrating the step-up transformer without its plate-like core.  FIG. 19  is a circuit diagram of the drum core in accordance with an example. 
     The step-up transformer  201 A in accordance with this embodiment is used for stepping up the voltage of a strobe light source for a camera, for example, and comprises a drum core  202 , a primary winding  203 , a secondary winding  204 , input terminals  205 U,  205 L, output terminals  206 U,  206 L, and a plate-like core  207  as illustrated in  FIGS. 10 to 14 . Here, the step-up transformer  201 A has a length (in the horizontal direction in  FIG. 13 ) of about 3.2 mm, a width (in the vertical direction in  FIG. 13 ) of about 2.5 mm, and a height (in the vertical direction in  FIG. 11 ) of about 1.2 to 2.4 mm. 
     The drum core  202  is made of ferrite and has a winding core part  221  and flanges  222 U,  222 L. The winding core part  221  is shaped like a substantially quadrangular prism, for example. The flanges  222 U,  222 L, each shaped like a substantially rectangular parallelepiped having a cross-sectional area greater than that of the winding core part  221 , are disposed at both ends of the winding core part  221 . 
     As illustrated in  FIGS. 16 and 17 , the input terminals  205 U,  205 L are disposed on the upper, side, and bottom faces of the respective flanges  222 U,  222 L. As with the input terminals  205 U,  205 L, the output terminals  206 U,  206 L are disposed on the flanges  222 U,  222 L, respectively. The input terminals  205 U,  205 L and output terminals  206 U,  206 L are formed by transferring a conductive paste mainly composed of Ag, for example, to the upper, side, and bottom faces of the flanges  222 U,  222 L, burning the paste at a predetermined temperature (e.g., about 700° C.) thereafter, and further plating it with a metal. For example, Sn can be used for metal plating. The input terminals  205 U,  205 L and output terminals  206 U,  206 L may be constituted by a plate material made of a metal, so as to be attached to their corresponding positions of the flanges  222 U,  222 L. For example, phosphor bronze plated with metals (Ni and Sn) can be used for the metal plate material. 
     The plate-like core  207 , which is a substantially rectangular member made of ferrite, is used for lowering magnetic resistance, so as to improve the inductance of the step-up transformer  201 A. The plate-like core  207  has such a size as to cover the drum core  202  and is arranged such that the vicinities of both longitudinal ends thereof oppose the upper faces of the flanges  222 U,  222 L. The plate-like core  207  is provided with recesses  271 ,  271  in the respective parts opposing the input terminals  205 U,  205 L and output terminals  206 U,  206 L. Protrusions  272 ,  272  for providing a gap between the plate-like core  207  and the flange  222 U are arranged on the plate-like core  207  in the part opposing the upper face of the flange  222 U and located between recesses  271 ,  271 . Similarly, the plate-like core  207  is provided with protrusions  272 ,  272  at the part opposing the upper face of the flange  222 L. The plate-like core  207  is secured to the upper faces of the flanges  222 U,  222 L by an adhesive applied between the protrusions  272 ,  272 . In order to identify the polarity of the step-up transformer  201 A when mounting it to a substrate, the upper face of the plate-like core  207  is provided with an orientation identification mark  273 . 
     Each of the primary and secondary windings  203 ,  204  is wound about the winding core part  221  clockwise (in a right-hand turn) as seen from the flange  222 U side. The primary winding  203  is connected to the input terminals  205 U,  205 L, while the secondary winding  204  is connected to the output terminals  206 U,  206 L. As illustrated in  FIG. 19 , the output terminal  206 L is grounded. The primary winding  203  has a diameter which is about 2 to 5 times that of the secondary winding  204 . Here, the diameter of the primary winding  203  is about 50 to 100 μm, while the diameter of the secondary winding  204  is about 10 to 40 μm. The number of turns of the secondary winding  204  is greater than that of the primary winding  203 , e.g., they are about 153 and 15, respectively, in this embodiment, whereby a primary voltage of 33 V can be raised to a secondary voltage of 330 V. The primary and secondary windings  203 ,  204  are electrically insulated from each other. For example, an insulation-coated copper wire can be used for the primary and secondary windings  203 ,  204 . The numbers of turns of the primary and secondary windings  203 ,  204  and the primary and secondary voltages can be changed as appropriate without being restricted to those mentioned above. 
     How the primary and secondary windings  203 ,  204  are wound about the winding core part  221  will now be explained in detail.  FIG. 20  is a sectional view taken along a line XX-XX of  FIG. 17 , and illustrating how the primary and secondary windings are wound about the drum core in accordance with the example. As illustrated in  FIG. 20 , the secondary winding  204  is initially wound about the winding core part  221 , and then the primary winding  203  is wound about the outer periphery of the secondary winding  204 . 
     The secondary winding  204  is wound regularly about the winding core part  221 , while its winding start portion (hereinafter referred to as “start wire”) S 2  for the winding core part  221  is located closer to the center of the winding core part  221 , more specifically between the flange  222 U on one side (left side in  FIG. 20 ) and the center portion of the winding core part  221 . The secondary winding  204  is wound as the first tier from the start wire  204  toward the flange  222 L on the other side and turned back toward the flange  222 U, before reaching the flange  222 L, so as to be wound as the second tier. While on the way to the flange  222 U, the second tier of the secondary winding  204  covers the start wire S 2 . As a consequence, a middle part of turns in the secondary winding  204  is located between the start wire S 2  and the flange  222 U. Preferably, the number of turns in the middle part of the secondary winding  204  located between the start wire S 2  and the flange  222 U is 1 to 10. The secondary winding  204  is turned back toward the flange  222 L at a position adjacent to the flange  222 U so as to be wound as the third tier, while the winding start portion S 3  of the second tier of the secondary winding  204  is covered with the third tier of the secondary winding  204 . The winding end portion of the secondary winding  204  is directly wound about the winding core part  221  at a position adjacent to the flange  222 L. The number of tiers by which the secondary winding  204  is wound may be any plural number without being restricted to 3. 
     The primary winding  203  is wound regularly on the outside of the upper tier of the secondary winding  204 , while its winding start portion S 1  is located at a position adjacent to the flange  222 U on one side. The primary winding  203  is wound tightly from the winding start portion S 1  to the position adjacent to the flange  222 L. 
     Operations and effects of the step-up transformer  201 A will now be explained. 
       FIG. 21  is a sectional view illustrating how the primary and secondary windings are wound about the drum core in accordance with a comparative example. In the step-up transformer  250  in accordance with the comparative example, the start wire S 2  is arranged at a position adjacent to a flange  252 U. Thus arranging the start wire S 2  at a position adjacent to the flange  252 U may cause a secondary winding  254  to interfere with the flange  252 U when winding the secondary winding  254  on the start wire S 2 , thereby making it harder to wind and leaving an unwound region  255  in which, as illustrated in  FIG. 21 , the secondary winding  254  is not wound on the start wire S 2 . When the winding start portion S 1  of the primary winding  253  is located in the unwound region  255 , so that the start wire S 2  and the winding start portion S 1  of the primary winding  253  come into contact with each other, the stray capacitance between the primary and secondary windings  253 ,  254  increases. When the stray capacitance increases, the LC resonance may be so high that the output voltage of the transformer becomes unstable, thereby generating ringing. 
     In the step-up transformer  201 A, by contrast, the secondary winding  204  is wound in a plurality of tiers about the winding core part  221 , while the start wire S 2 , which is the winding start portion thereof for the winding core part  221 , is covered with the upper tier of the secondary winding  204 , and the primary winding  203  is wound on the outside of the upper tier of the secondary winding  204 . This interposes the upper tier of the secondary winding  204  between the start wire S 2  and the winding start portion S 1  of the primary winding  203  and thus can prevent the start wire S 2  and the winding start portion S 1  of the primary winding  203  from coming into contact with each other. Therefore, the step-up transformer  201 A can lower the stray capacitance between the start wire S 2  and the winding start portion S 1  of the primary winding  203 , thereby stabilizing the output. 
     In the step-up transformer  201 A, the winding start portion S 3  of the secondary winding  204  is covered with the third tier of the secondary winding  204 . This can prevent the winding start portion S 3  of the second tier of the secondary winding  204  and the winding end portion of the primary winding  203  from coming into contact with each other, lower the stray capacitance, and stabilize the output. 
     In the step-up transformer  201 A, the start wire S 2 , which is the winding start portion of the secondary winding  204 , is located closer to the center of the winding core part  221 , while a middle part of turns in the secondary winding  204  is located between the start wire S 2  and the flange  222 U. This prevents the start wire S 2  from being arranged adjacent to the flange  222 U, so that the secondary winding  204  does not interfere with the flange  222 U when covering the start wire S 2 , whereby it becomes easier for the upper tier of the secondary winding  204  to cover the start wire S 2 . Therefore, this step-up transformer  201 A can lower the stray capacitance between the start wire S 2  and the winding start portion S 1  of the primary winding  203 , thereby stabilizing the output voltage. 
     The step-up transformer in accordance with the third embodiment of the present invention will now be explained. 
       FIG. 22  is a sectional view illustrating a main part of the step-up transformer in accordance with the third embodiment of the present invention. This step-up transformer  201 B is one in which the primary and secondary windings  203 ,  204  are wound differently from those in the step-up transformer  201 A in accordance with the second embodiment illustrated in  FIG. 20 . 
     In the step-up transformer  201 B, the start wire S 2  is located at a position adjacent to the flange  222 U, each tier of the secondary winding  204  is wound from the flange  222 U on one side to the flange  222 L on the other side, and the winding end portion of the secondary winding  204  is not directly wound about the winding core part  221 . 
     In the step-up transformer  201 B, the number of turns of the primary winding  203  is smaller than that in the step-up transformer  201 A in accordance with the second embodiment, while the winding start portion S 1  of the primary winding  203  is located closer to the center of the winding core part  221 , more specifically between the flange  222 U and the center portion of the winding core part  221 . As a consequence, the winding start portion S 1  of the primary winding  203  and the start wire S 2 , which is the winding start portion of the secondary winding  204 , are located at respective positions different from each other in the axial direction of the winding core part  221 . The primary winding  203  is wound about only a part of the upper tier of the secondary winding  204 , more specifically only about ⅔ of the upper tier. Preferably, the gap between the winding start portion S 1  of the primary winding and the start wire S 2  is at least one turn of the secondary winding. 
     In thus constructed step-up transformer  201 B, as in the step-up transformer  201 A in accordance with the second embodiment, the start wire S 2 , which is the winding start portion of the secondary winding  204  for the winding core part  221 , is covered with the upper tier of the secondary winding  204 , while the primary winding  203  is wound on the outside of the upper tier of the secondary winding  204 . Therefore, as with the step-up transformer  201 A in accordance with the second embodiment, the step-up transformer  201 B can reduce the stray capacitance between the start wire S 2  and the winding start portion S 1  of the primary winding  203 , thereby stabilizing the output voltage. 
     In the step-up transformer  201 B, as in the step-up transformer  201 A in accordance with the second embodiment, the winding start portion S 3  of the second tier of the secondary winding  204  is covered with the third tier of the secondary winding  204 . Therefore, as with the step-up transformer  201 A in accordance with the second embodiment, the step-up transformer  201 B can reduce the stray capacitance and stabilize the output voltage. 
     In the step-up transformer  201 B, the winding start portion S 1  of the primary winding  203  and the start wire S 2 , which is the winding start portion of the secondary winding  204 , are located at respective positions different from each other in the axial direction of the winding core part  221 . This can reliably prevent the winding start portion S 1  of the primary winding  203  and the start wire S 2  from coming into contact with each other. Therefore, the step-up transformer  201 B can reduce the stray capacitance between the start wire S 2  and the winding start portion S 1  of the primary winding  203 , thereby stabilizing the output voltage. 
     By winding the primary winding  203  about only a part of the upper tier of the secondary winding  204 , the step-up transformer  201 B can easily adjust the position of the winding start portion S 1  of the primary winding  203 , thereby simply regulating the gap between the winding start portion S 1  of the primary winding  203  and the start wire S 2 . Therefore, the step-up transformer  201 B can further reduce the stray capacitance between the start wire S 2  and the winding start portion S 1  of the primary winding  203 , thereby stabilizing the output voltage. 
     The step-up transformer in accordance with the fourth embodiment of the present invention will now be explained. 
       FIG. 23  is a sectional view illustrating a main part of the step-up transformer in accordance with the fourth embodiment of the present invention. This step-up transformer  201 C is one in which the primary winding  203  is wound differently from that in the step-up transformer  201 B in accordance with the third embodiment illustrated in  FIG. 22 . That is, in the step-up transformer  201 C, the winding start portion S 1  of the primary winding  203  is located at a position adjacent to the flange  222 U on one side, while the primary winding  203  is wound at substantially uniform intervals such that its turns are in no contact with each other. 
     In thus constructed step-up transformer  201 C, as in the step-up transformer  201 A in accordance with the second embodiment, the start wire S 2 , which is the winding start portion of the secondary winding  204  for the winding core part  221 , is covered with the upper tier of the secondary winding  204 , while the primary winding  203  is wound on the outside of the upper tier of the secondary winding  204 . Therefore, as with the step-up transformer  201 A in accordance with the second embodiment, the step-up transformer  201 C can reduce the stray capacitance between the start wire S 2  and the winding start portion S 1  of the primary winding  203 , thereby stabilizing the output voltage. 
     In the step-up transformer  201 C, as in the step-up transformer  201 A in accordance with the second embodiment, the winding start portion S 3  of the second tier of the secondary winding  204  is covered with the third tier of the secondary winding  204 . Therefore, as with the step-up transformer  201 A in accordance with the second embodiment, the step-up transformer  201 C can reduce the stray capacitance and stabilize the output voltage. 
     Since the primary winding  203  covers the secondary winding  204  as a whole, the step-up transformer  201 C can reduce leakage magnetic fluxes, thereby further inhibiting the voltage waveform from ringing. 
     The step-up transformer in accordance with the fifth embodiment of the present invention will now be explained. 
       FIG. 24  is a sectional view illustrating a main part of the step-up transformer in accordance with the fifth embodiment of the present invention. This step-up transformer  201 D is one in which the secondary winding  204  is wound differently from that in the step-up transformer  201 C in accordance with the fourth embodiment illustrated in  FIG. 23 . That is, in the step-up transformer  201 D, the start wire S 2  is located closer to the center of the winding core part  221 , more specifically between the flange  222 U on one side and the center portion of the winding core part  221 , while a middle part of turns in the secondary winding  204  is located between the start wire S 2  and the flange  222 U. 
     In thus constructed step-up transformer  201 D, as in the step-up transformer  201 A in accordance with the second embodiment, the start wire S 2 , which is the winding start portion of the secondary winding  204  for the winding core part  221 , is covered with the upper tier of the secondary winding  204 , while the primary winding  203  is wound on the outside of the upper tier of the secondary winding  204 . Therefore, as with the step-up transformer  201 A in accordance with the second embodiment, the step-up transformer  201 D can reduce the stray capacitance between the start wire S 2  and the winding start portion S 1  of the primary winding  203 , thereby stabilizing the output voltage. 
     In the step-up transformer  201 D, as in the step-up transformer  201 A in accordance with the second embodiment, the winding start portion S 3  of the second tier of the secondary winding  204  is covered with the third tier of the secondary winding  204 . Therefore, as with the step-up transformer  201 A in accordance with the second embodiment, the step-up transformer  201 D can reduce the stray capacitance and stabilize the output voltage. 
     Since the primary winding  203  covers the secondary winding  204  as a whole, the step-up transformer  201 D can reduce leakage magnetic fluxes, thereby further inhibiting the voltage waveform from ringing as with the step-up transformer  201 C accordance with the fourth embodiment. 
     In the step-up transformer  201 D, as in the step-up transformer  201 A in accordance with the second embodiment, the secondary winding  204  does not interfere with the flange  222 U when covering the start wire S 2 , whereby it becomes easier for the upper tier of the secondary winding  204  to cover the start wire S 2 . Therefore, this step-up transformer  201 D can lower the stray capacitance between the start wire S 2  and the winding start portion S 1  of the primary winding  203 , thereby stabilizing the output voltage. 
     The step-up transformer in accordance with the sixth embodiment of the present invention will now be explained. 
       FIG. 25  is a sectional view illustrating a main part of the step-up transformer in accordance with the sixth embodiment of the present invention. This step-up transformer  201 E is one in which the primary and secondary windings  203 ,  204  are wound differently from those in the step-up transformer  201 A in accordance with the second embodiment illustrated in  FIG. 20 . 
     In the step-up transformer  201 E, the start wire S 2  is located at a position adjacent to the flange  222 U, each tier of the secondary winding  204  is wound from the flange  222 U on one side to the flange  222 L on the other side, and the winding end portion of the secondary winding  204  is not directly wound about the winding core part  221 . The secondary winding  204  is wound sparsely at its winding end portion. The winding end portion of the secondary winding  204  (the third tier of the secondary winding  204 ) is arranged closer to the flange  222 L than is the winding start portion S 3  of the second tier of the secondary winding  204  thereunder. 
     In the step-up transformer  201 E, the number of turns of the primary winding  203  is smaller than that in the step-up transformer  201 A in accordance with the second embodiment, while its winding end portion is located between the center part of the winding core part  221  and the flange  222 L. 
     In thus constructed step-up transformer  201 E, as in the step-up transformer  201 A in accordance with the second embodiment, the start wire S 2 , which is the winding start portion of the secondary winding  204  for the winding core part  221 , is covered with the upper tier of the secondary winding  204 , while the primary winding  203  is wound on the outside of the upper tier of the secondary winding  204 . Therefore, as with, the step-up transformer  201 A in accordance with the second embodiment, the step-up transformer  201 E can reduce the stray capacitance between the start wire S 2  and the winding start portion S 1  of the primary winding  203 , thereby stabilizing the output voltage. 
     In the step-up transformer  201 E, the winding end portion of the secondary winding  204  (the third tier of the secondary winding  204 ) is arranged on the winding start portion S 3  of the second tier of the secondary winding  204 . This can prevent the winding start portion S 3  of the second tier of the secondary winding  204  and the winding end portion of the primary winding  203  from coining into contact with each other, lower the stray capacitance, and stabilize the output voltage. 
     In the step-up transformer  201 B, the winding end portion of the primary winding  203  is located between the center portion of the winding core part  221  and the flange  222 L, so that the winding start portion S 3  of the second tier of the secondary winding  204  and the winding end portion of the primary winding  203  are located at respective positions different from each other in the axial direction of the winding core part  221 . This can reliably prevent the winding start portion S 3  of the second tier of the secondary winding  204  and the winding end portion of the primary winding  203  from coming into contact with each other, lower the stray capacitance, and stabilize the output voltage. 
     In the step-up transformer  201 E, the secondary winding  204  is wound sparsely at its winding end portion. Therefore, when the secondary winding  204  cannot be wound tightly in its third tier up to the flange  222 L, the winding end portion of the secondary winding  204  (the third tier of the secondary winding  204 ) can reliably be arranged on the winding start portion S 3  of the second tier of the secondary winding  204 . This can prevent the winding start portion S 3  of the second tier of the secondary winding  204  and the winding end portion of the primary winding  203  from coming into contact with each other, lower the stray capacitance, and stabilize the output voltage.