Abstract:
An inverter transformer in which to overall structure and manufacturing process can be simplified despite its closed magnetic path structure, and a cost increase can be suppressed. Primary windings ( 24   a ,  24   b ,  24   c ) and secondary windings ( 25   a ,  25   b ,  25   c ) wound around a plurality of rod-like cores ( 23   a ,  23   b ,  23   c ) have leakage inductances. The primary windings ( 24   a ,  24   b ,  24   c ) axe wound around respective rod-like cores ( 23   a ,  23   b ,  23   c ) such that magnetic fluxes being induced in respective cores by the currents flowing through the primary windings ( 24   a ,  24   b ,  24   c ) are directed reversely to magnetic fluxes being induced in adjacent cores.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inverter transformer for use in an inverter circuit to light a discharge lamp, such as a cold cathode fluorescent lamp, as a light source of a lighting device for a liquid crystal display. 
     2. Description of the Related Art 
     Currently, a liquid crystal display (LCD) is increasingly used as a display unit for a personal computer, and the like. The LCD lacks a light emitting function, and therefore requires a lighting device, such as a back-light system or a front-light system, and a cold cathode fluorescent lamp (CCFL) is generally used as a light source for such a lighting device. In case of discharging and lighting a CCFL having a length, for example, about 500 mm, an inverter circuit is used which is adapted to generate a high-frequency voltage of 60 kHz, about 1600 V at the time of starting discharge. The inverter circuit controls a voltage applied to the CCFL such that after the CCFL is discharged, the voltage is lowered to about 1200 V which is a voltage required for keeping the discharge. Some inverter circuits include a closed magnetic path type inverter transformer and also a ballast capacitor, and the ballast capacitor additionally required prohibits reduction in dimension and cost. Further, even after discharging a CCFL, the voltage at the time of starting discharge must be maintained, which is disadvantageous in view of safety. 
     Recently, an open magnetic path type inverter transformer is employed which leverages the function of a leakage inductance serving as a ballast capacitance in place of a ballast capacitor. Some of such open magnetic path type inverter transformers may use a bar-shaped magnetic core (I-core), and others may use a combination of a bar-shaped magnetic core and a rectangular frame-shaped magnetic core (refer to Japanese Patent Application Laid-Open No. 2002-353044). 
       FIG. 19  is an equivalent circuit of an inverter transformer having a leakage inductance as described above. Referring to  FIG. 19 , the inverter transformer includes an ideal transformer  1  having no loss with a winding ratio of 1:n, leakage inductances L 1  and L 2 , and a mutual inductance Ls, and CCFLs  2 . In the inverter transformer, the leakage inductances L 1  and L 2  function as a ballast inductance, and the CCFLs  2  can be lighted normally without using a ballast capacitor. 
       FIG. 20  is a schematic view of a traditional inverter transformer  1 A of open magnetic path type. The inverter transformer  1 A includes a bar-shaped magnetic core (I-core)  3  indicated by a dashed line, a bobbin  4  defining a hollow  5  to house the bar-shaped magnetic core  3 , a primary winding  6  wound around the bobbin  4 , a secondary winding  7  wound around the bobbin  4 , a terminal block  9  provided with terminal pins  8  for the primary winding  6 , and a terminal block  11  provided with terminal pins  10  for the secondary winding  7 . Since a high voltage is induced at the secondary side, the secondary winding  7  is divided by partitions  12  formed at the bobbin  4  in order to prevent surface discharge. The inverter transformer  1 A in  FIG. 20 , which employs a bar-shaped magnetic core as described above, is simple in structure compared with an inverter transformer (not shown) which employs a magnetic core having a closed configuration, such as a rectangular core. However, magnetic flux leaks from the bar-shaped magnetic core, especially from the ends thereof 
       FIG. 21  is an exploded perspective view of another traditional inverter transformer  1 B. The inverter transformer  1 B includes a bar-shaped magnetic core  3 , a rectangular frame-shaped magnetic core  13 , a bobbin  14  having a hollow to house the bar-shaped core  3 , and primary and secondary windings  6  and  7  wound around the bobbin  14 . The end portions of the bar-shaped magnetic core  3  are engaged with respective recesses  15  of the rectangular frame-shaped magnetic core  13  such that gap sheets formed of a non-magnetic material are put between the bar-shaped magnetic core  3  and the rectangular frame-shaped magnetic core  13  so as to form gaps therebetween, thereby generating a prescribed amount of leakage inductance. In the inverter transformer  1 B thus structured, magnetic flux leaking from the bar-shaped core  3  passes through the rectangular frame-shaped magnetic core  13 , and leakage flux is small compared with an inverter transformer employing only a bar-shaped magnetic core (without a rectangular frame-shaped magnetic core). 
     In an inverter transformer involving leakage inductance, leakage flux may possibly influence neighboring components or wires, or emit noises, and the components and wires must be appropriately located in order to keep away from the leakage flux thus placing restrictions on arrangement of components and wires. This may result in increase of product dimension or deterioration of characteristics. Also, if a magnetic material is placed at the path of the leakage flux, the flux path may be influenced when the leakage flux passes through the magnetic material, which causes the leakage inductance to vary or fluctuate disturbing stability, further causing the inverter transformer to undergo variation in characteristic and consequently to undergo change in operation. 
     Thus, an inverter transformer including only a bar-shaped magnetic core is simple in structure but suffers increase in leakage flux distribution range, and also has difficulty in adjusting the amount of leakage inductance. On the other hand, an inverter transformer including a rectangular frame-shaped magnetic core together with a bar-shaped magnetic core has a smaller leakage flux distribution range than the inverter transformer including a bar-shaped magnetic core only, but incurs increase in number of components, and a molding or machining process is required for producing the rectangular frame-shaped magnetic core. Also, when engaging the bar-shaped magnetic core with the rectangular frame-shaped magnetic core, a complex and troublesome process of putting gap sheets therebetween is required for adjusting leakage inductance. 
     An inverter transformer incorporating only a bar-shaped magnetic core generates a wide distribution range of leakage flux as described above. Such an inverter transformer is magnetically shielded in order to prevent the inverter transformer from affecting neighboring components, and also to prevent the neighboring components from affecting the inverter transformer. This solution by magnetically shielding a product, however, requires a shielding case, and this leads to increase in product dimension and product cost. Also, processes of fixing the inverter transformer to the shielding case and taking out lead wires from the shielding case are additionally required, thus making cost reduction further difficult. And, a defective fixing of the inverter transformer to the shielding case may raise deterioration in reliability. On the other hand, an inverter transformer employing a rectangular frame-shaped magnetic core together with a rectangular frame-shaped magnetic core, while generating a reduced amount of leakage flux, has a complicated structure and requires additional troublesome manufacturing processes thus pushing up production cost. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in the light of the above problems, and it is an object of the present invention to provide an inverter transformer which has an open magnetic path structure but is simple in structure, and which has its production process simplified compared with a traditional open magnetic path structure including a rectangular frame-shaped magnetic core, thus preventing cost increase. 
     In order to achieve the object described above, according to an aspect of the present invention, there is provided an inverter transformer which is used in an inverter circuit to invert DC into AC, transforms a voltage inputted at a primary side and outputs the transformed voltage at a secondary side, and which includes a plurality of winding units, each of the winding units including: a bar-shaped magnetic core; and a primary winding and a secondary winding which are wound around the bar-shaped magnetic core, and which have respective leakage inductances. In the inverter transformer described above, the primary windings are wound around respective bar-shaped magnetic cores in such a manner that a magnetic flux generated in one magnetic core by a current flowing through a primary winding provided around the one magnetic core is directed opposite to a magnetic flux generated in another magnetic core adjacent to the one magnetic core by a current flowing through a primary winding provided around the adjacent magnetic core. 
     In the aspect of the present invention, at least one portion of each winding unit may be covered with respect to the longitudinal direction by a magnetic resin formed of a resin containing a magnetic substance. 
     In the aspect of the present invention, the magnetic resin may cover the entire portion of each winding unit 
     In the aspect of the present invention, the magnetic resin may cover both end portions of each winding unit and/or a portion of each winding unit located at a boundary area between the primary and secondary windings. 
     In the aspect of the present invention, an external unit having a larger saturation magnetic flux density than the magnetic resin may be disposed so as to cover at least one portion of the circumference of a transformer body which includes the plurality of winding units and the magnetic resin. 
     In the aspect of the present invention, the external unit may have a smaller magnetic resistance than the magnetic resin. 
     In the aspect of the present invention, the external unit may have either a squared C configuration or a substantially circular configuration in cross section so as to cover the circumference of the transformer body. 
     In the aspect of the present invention, the external unit may include a plurality of members, and the members may be combined into a box configuration so as to cover the transformer body. 
     In the aspect of the present invention, the external unit may be a sintered compact. 
     In the aspect of the present invention, the magnetic resin may have a smaller relative magnetic permeability than the magnetic cores. 
     In the aspect of the present invention, the magnetic substance contained in the resin may be Mn—Zn ferrite, Ni—Zn ferrite, or iron powder. 
     Since the primary windings are wound in such a manner that a magnetic flux generated in one magnetic core by a current flowing through a primary winding provided around the one magnetic core is directed opposite to a magnetic flux generated in another magnetic core adjacent to the one magnetic core by a current flowing through a primary winding provided around the adjacent magnetic core, leakage flux spreading around the inverter transformer is reduced, thus having smaller influences on the components and wires arranged around the inverter transformer. This structure also contributes to making it harder for the characteristics of the inverter transformer to suffer the effects of metals present around the inverter transformer, thus enabling the leakage inductance of the inverter transformer to be stabilized. On the other hand, since the secondary windings are wound in such a manner that voltages induced in the secondary windings have the same polarity, there is no voltage difference between the secondary windings W 2  thus proving favorable in terms of withstand voltage and consequently improving safety, and as a result the number of components is reduced, the device can be downsized, and eventually the device can be produced inexpensively. 
     Also, since the magnetic cores are totally or partly covered by the magnetic resin, leakage flux spreading around the inverter transformer is reduced, thus having smaller influences on the components and wires arranged around the inverter transformer. This structure also keeps the characteristics of the inverter transformer from suffering the effects of metals present around the inverter transformer, thus enabling the leakage inductance of the inverter transformer to be stabilized. 
     Further, since the magnetic resin is disposed so as to perform magnetic shielding, a case for magnetic shielding is not required thus preventing cost increase. This eliminates a work process of fixing the inverter transformer to the case, or taking out lead wires from the case, and consequently the production process is simplified. And at the same time, since the inverter transformer is resin-molded, the inverter transformer has its mechanical strength increased thus enhancing the product reliability. 
     Still further, since the external unit, which has a larger saturation magnetic flux density than the magnetic resin, is disposed so as to cover at least one portion of the circumference of the inverter transformer body that comprises the plurality of winding units and the magnetic resin, most of magnetic fluxes leaking out from the magnetic cores so as to pass through the magnetic resin and then to leak out further from the magnetic resin are adapted to pass through the external unit. Consequently, the amount of the leakage fluxes can be reduced effectively compared when the magnetic fluxes is prevented from leaking out by the magnetic resin only without providing the external unit, and therefore the thickness of the magnetic resin can be reduced, which results in reduction of the entire cross section area of the inverter transformer thus downsizing the inverter transformer. 
     And, the number of turns and the leakage inductance on the winding can be adjusted to the optimum conditions of the circuit operation by adjusting the magnetic characteristics such as relative magnetic permeability of the magnetic resin and adjusting the coverage area and thickness of the magnetic resin. Consequently, the inductance value can be adjusted without changing the number of turns on the primary and secondary windings and the configuration and characteristics of the magnetic core, thus providing applicability to various inverter transformers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top plan view of an inverter transformer according to a first embodiment of the present invention; 
         FIG. 2  is an explanatory view of states of windings and directions of magnetic fluxes generated by respective windings in an inverter transformer according to the present invention; 
         FIGS. 3(   a ) and  3 ( b ) are explanatory views of winding methods for primary windings  1 W in inverter transformers according to the present invention; 
         FIG. 4  is an explanatory view of positions A and B for measuring a magnetic field on an inventive sample according to the present invention and a comparative sample of a conventional product; 
         FIG. 5  is a graph showing measurement results at several positions A shown in  FIG. 4  on the inventive and comparative samples; 
         FIG. 6  is a graph showing measurement results at several positions B shown in  FIG. 4  on the inventive and comparative samples; 
         FIGS. 7(   a ),  7 ( b ) and  7 ( c ) are respectively schematic top plan, front elevation, and partial cross-sectional views of an inverter transformer according to a second embodiment of the present invention, and  FIGS. 7(   d ) and  7 ( e ) are respectively schematic front elevation and partial cross-sectional views of an inverter transformer according to a third embodiment of the present invention; 
         FIGS. 8(   a ) and  8 ( b ) are respectively schematic top plan and front elevation views of an inverter transformer according to a fourth embodiment of the present invention, and  FIG. 8(   c ) is a front elevation view of an inverter transformer according to a fifth embodiment of the present invention; 
         FIGS. 9(   a ) and  9 ( c ) are respectively schematic top plan and front elevation views of an inverter transformer according to a sixth embodiment of the present invention, and  FIG. 9(   b ) is a perspective view of an external unit used in the inverter transformer according to the sixth embodiment; 
         FIGS. 10(   a ) and  10 ( c ) are respectively schematic top plan and front elevation views of an inverter transformer according to a seventh embodiment of the present invention, and  FIG. 10(   b ) is a perspective view of an external unit used in the inverter transformer according to the seventh embodiment; 
         FIGS. 11(   a ) and  11 ( b ) are respectively schematic top plan and front elevation views of an inverter transformer according to an eighth embodiment of the present invention,  FIG. 11(   c ) is a perspective view of an external unit used in the inverter transformer according to the eighth embodiment, and  FIG. 11(   d ) is a front elevation view of another inverter transformer according to the eleventh embodiment including a different type transformer body; 
         FIGS. 12(   a ) and  12 ( b ) are respectively schematic top plan and front elevation views of an inverter transformer according to a ninth embodiment of the present invention,  FIG. 12(   c ) is a perspective view of an external unit used in the inverter transformer according to the ninth embodiment,  FIG. 12(   d ) is a front elevation view of another inverter transformer according to the ninth embodiment of the present invention, and  FIG. 12(   e ) is s perspective view of an external unit used in an inverter transformer according to a tenth embodiment; 
         FIGS. 13(   a ) and  13 ( b ) are respectively schematic top plan (partly sectioned) and cross-sectional views (taken along line A-A) of an inverter transformer according to an eleventh embodiment of the present invention,  FIG. 13(   c ) is a cross-sectional view of an inverter transformer according to a twelfth embodiment of the present invention, and  FIG. 13(   d ) is a perspective view of an external unit and a plate member used in an inverter transformer according to a thirteenth embodiment of the present invention; 
         FIGS. 14(   a ) and  14 ( b ) are respectively schematic top plan and front elevation views of an inverter transformer according to a fourteenth embodiment of the present invention; 
         FIGS. 15(   a ) and  15 ( b ) are respectively schematic top plan and front elevation views of an inverter transformer according to a fifteenth embodiment of the present invention, and  FIG. 15(   c ) is a front elevation view of another inverter transformer according to the fifteenth embodiment of the present invention; 
         FIGS. 16(   a ) and  16 ( b ) are respectively schematic top plan and front elevation views of an inverter transformer according to a sixteenth embodiment of the present invention, and  FIG. 16(   c ) is a front elevation view of an inverter transformer according to a seventeenth embodiment of the present invention; 
         FIGS. 17(   a ) and  17 ( c ) are respectively schematic top plan and front elevation views of an inverter transformer according to an eighteenth embodiment of the present invention,  FIG. 17(   b ) is a perspective view of an external unit used in the inverter transformer according to the eighteenth embodiment, and  FIG. 17(   d ) is a front elevation view of an inverter transformer according to a nineteenth embodiment of the present embodiment; 
         FIGS. 18(   a ) and  18 ( b ) are respectively schematic top plan and front elevation views of an inverter transformer according to a twentieth embodiment of the present invention, and  FIG. 18(   c ) is a front elevation view of an inverter transformer according to a twenty first embodiment of the present invention; 
         FIG. 19  is an equivalent circuit of an inverter transformer having a leakage inductance; 
         FIG. 20  is a schematic top plan view of a traditional inverter transformer including a bar-shaped magnetic core; and 
         FIG. 21  is an exploded perspective view of another traditional inverter transformer including a bar-shaped magnetic core. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will hereinafter be described with the accompanying drawings. 
     A first embodiment of the present invention will be described with  FIG. 1 . An inverter transformer  10  according to the first embodiment is for lighting three CCFLs concurrently. The number of CCFLs to be lighted is not limited to three but may alternatively be other than three, as long as primary windings are wound around respective bar-shaped magnetic ores in such a manner that a magnetic flux generated in one magnetic core by a current flowing through a primary winding provided around the one magnetic core is directed opposite to a magnetic flux generated in another magnetic core adjacent to the one magnetic core by a current flowing through a primary winding provided around the adjacent magnetic core as described later. In such a case, the magnetic cores are provided in a number equal to the number of the CCFLs. In the following description, for the purpose of simplification as appropriate, primary windings  24  ( 24   a ,  24   b  and  24   c ) are reference-marked as W 1 , secondary windings  25  ( 25   a ,  25   b  and  25   c ) are reference-marked as W 2 , rectangular tubular bobbins  26  ( 26   a ,  26   b  and  26   c ) are referred to simply as bobbins  26 , and bar-shaped magnetic cores  23  ( 23   a ,  23   b  and  23   c ) are referred to simply as cores  23 . 
     The inverter transformer  10  shown in  FIG. 1  is for lighting three CCFLs as mentioned above. Three bobbins  26  are shaped identical with one another. Three cores  23  are inserted through respective bobbins  26 , which are engagingly fitted to each other. The cores  23  are formed of a soft magnetic material, for example, Mn—Zn ferrite, and have a relative magnetic permeability of, for example, 2000. The inverter transformer  10  generally includes the three cores  23 , the three bobbins  26  having respective primary windings W 1  and secondary windings W 2  wound therearound, primary winding terminal blocks  38   a  each engagingly attached to one end of each bobbin  26 , and secondary winding terminal blocks  39   a  each engagingly attached to the other end of each bobbin  26 . The primary and secondary winding terminal blocks  38   a  and  39   a  are formed of an insulating material and are disposed apart from each other so as to sandwich the bobbins  26 . Terminal pins  40   a  are fixedly attached to the primary winding terminal blocks  38   a , and terminal pins  41   a  are fixedly attached to the secondary winding terminal blocks  39   a.    
     The primary winding terminal blocks  38   a  are each provided with a hole or groove (not shown) for accommodating lead wires (not shown) of the primary winding W 1 , which are connected to the primary winding terminal pins  40   a . The secondary winding terminal blocks  39   a  are each provided with a hole or groove (not shown) for accommodating lead wires (not shown) of the secondary winding W 2 , which are connected to the secondary winding terminal pins  41   a . Those lead wires, each coated with an insulating material, are inserted through the hole or put in the groove so as to secure sufficient surface distance and insulation. 
     The bobbins  26  are each provided with a partition  57   a  which separates the primary winding W 1  and the secondary winding W 2 . Specifically, the primary winding W 1  is wound around the bobbin  26  between the primary winding terminal block  38   a  and the partition  57   a , and the secondary winding W 2  is wound around the bobbin  26  between the secondary winding terminal block  39   a  and the partition  57   a . Since a high voltage is generated at the secondary winding W 2 , the secondary winding W 2  is split into several sections by means of insulating partitions  4   b  so that a sufficient surface distance is secured to prevent creeping discharge. The insulating partitions  4   b  are each provided with a notch for connecting adjacent sections of the secondary winding W 2 . 
     The operation of the inverter transformer  10  described above will hereinafter be explained. Magnetic flux generated in the core  23  leaks out from the core  23  so as to provide leakage inductance. That is to say, the magnetic path formed by the core  23  is not a closed magnetic path, and the inverter transformer  10  virtually has an open magnetic path structure having a leakage inductance. Accordingly, there is generated not only a magnetic flux that passes entirely through the core  23  so as to interlink the primary winding W 1  and the secondary winding W 2 , but also a leakage flux that interlinks either with the primary winding W 1  only or with the secondary winding W 2  only thus failing to contribute to providing electromagnetic coupling between the primary winding W 1  and the secondary winding W 2 , whereby leakage inductance is generated. The leakage inductance acts as ballast inductance so as to duly discharge and light the CCFLs connected to the secondary windings W 2 . 
     The generated leakage flux, however, not only provides leakage inductance but also have an adverse effect on devices arranged near the inverter transformer  10 , and therefore should be prevented from spreading out from the inverter transformer  10 . In the present invention, the primary windings W 1  are arranged around respective cores  23  such that magnetic fluxes generated by currents flowing through the primary windings W 1  are directed opposite to each other in any adjacent cores  23 , thereby preventing the leakage flux from spreading out from the inverter transformer  10 . 
     The operation of the primary windings W 1  of the inverter transformer  10  arranged as described above will be described with reference to  FIG. 2 . Magnetic fluxes Φ 1  and Φ 3 , which are generated respectively in the cores  23   a  and  23   c  (first group core) by respective currents flowing through the primary windings W 1  wound around two non-adjacent cores  23   a  and  23   c  of the three cores  23 , are directed identical with each other. A magnetic flux Φ 2 , which is generated in the core  23   b  (second group core) disposed between the two first group cores, is directed opposite to the magnetic fluxes Φ 1  and Φ 3 . 
     There are two kinds of methods as shown in  FIGS. 3(   a ) and  3 ( b ), in which the primary windings W 1  are arranged so as to generate the magnetic fluxes Φ 1 , Φ 2  and Φ 3  as described above. Shown in  FIG. 3(   a ) is one method, in which all of the primary windings W 1  around the first and second group cores are wound in the same direction, and the polarity of a voltage e applied to the primary windings W 1  around the first group cores is opposite to the polarity of a voltage e applied to the primary winding W 1  around the second group core. Shown in  FIG. 3(   b ) is the other method, in which the primary windings W 1  around the first group cores are wound in the opposite direction to the primary winding W 1  around the second group core, and the electrodes of a voltage e applied to all the primary windings W 1  around the first and second group cores have the same polarity. In whichever methods, the magnetic fluxes Φ 1  and Φ 3  generated in the cores  23   a  and  23   c  (first group cores) are directed opposite to the magnetic flux Φ 2  generated in the core  23   b  (second group core) disposed adjacent to the cores  23   a  and  23   c  (first group cores) 
     When all of the magnetic fluxes Φ, Φ 1  and Φ 2  are directed identical with one another, magnetic fluxes leaking out from the ends of the cores  23  repel one another, and most of them do not go through adjacent cores and spread out in the air around thus increasing leakage flux. On the other hand, in the inverter transformer  10  according to the first embodiment, the magnetic fluxes Φ 1  and Φ 3  generated in the first group cores  23   a  and  23   c  are directed opposite to the magnetic flux Φ 2  generated in the second group core  23   b  disposed between the first group cores  23   a  and  23   c  as described above, and therefore magnetic fluxes leaking out from the ends of two adjacent cores, specifically, the cores  23   a  and  23   b , and the cores  23   b  and  23   c , do not repel each other, which causes an increased portion of the magnetic flux to go through adjacent cores. This reduces the amount of leakage flux that spreads out in the air around the inverter transformer. Consequently, influences on components and wirings disposed around the inverter transformer are reduced. The inverter transformer according to the present embodiment includes three cores, but the present invention is not limited to this structure and the inverter transformer may include any other plural number of cores insofar as magnetic fluxes going through adjacent cores are directed opposite to each other as described above. 
     The secondary windings W 2  are arranged such that the electrodes of voltages induced in the secondary windings W 2  around the first and second group cores  23  have the same polarity. For example, referring to each of  FIGS. 3(   a ) and  3 ( b ), since the primary windings W 1  are wound around the cores  23  such that the magnetic flux generated in the middle core is directed opposite to the magnetic fluxes generated in the adjacent cores, the secondary winding W 2  around the middle core is wound in the opposite direction to the secondary windings W 2  wound around the adjacent cores so that the electrodes of voltages induced in all the secondary windings W 2  have the same polarity. 
     As mentioned above, a high-frequency voltage of about 1600 V are generated in the secondary windings of the inverter transformer  10  for lighting CCFLs, and a voltage of about 1200 V for keeping the CCFLs discharging. However, since the voltages induced in the secondary windings W 2  have the same polarity as described above, there is no voltage difference between the secondary windings W 2  thus proving favorable in terms of withstand voltage and consequently enhancing safety. 
     The characteristics of the inverter transformer  10  according to the first embodiment will be described with reference to  FIGS. 4 ,  5  and  6 . As for  FIGS. 5 and 6 , the primary windings W 1  and the secondary windings W 2  were arranged as shown in  FIG. 3(   a ), specifically such that all the primary windings W 1  were wound around the cores  23  in the same direction while the secondary winding W 2  around the core  23   b  was wound in the opposite direction to the secondary windings W 2  around the cores  23   a  and  23   c . And, the electrode of a voltage to the primary winding W 1  around the core  23   b  had a polarity opposite to that of the primary windings W 1  around the cores  23   a  and  23   c . Accordingly, the magnetic flux generated in the core  23   b  was directed opposite to the magnetic fluxes generated in the cores  23   a  and  23   c . Referring to  FIG. 4 , the measurement of magnetic field was performed at positions (measurement points A) with respective distances d 1  above from the middle part of the winding top surface in the vertical direction dY, and at positions (measurement points B) with respective distances d 2  away from the middle part of the winding side surface in the horizontal direction dX orthogonal to the core length. 
     The measurement was performed on an inventive sample structured according to the present embodiment, and a comparative sample traditionally structured such that magnetic fluxes generated in the cores by currents flowing through the primary windings are directed identical with one another. The measurement results at the measurement points A are shown in  FIG. 5 , and the measurement results at the measurement points B are shown in  FIG. 6 . The magnetic filed due to leakage flux decreases with increase of the distances d (d 1  and d 2 ), more specifically, is inversely proportional approximately to the square of the distances d (d 1  and d 2 ). The measurement results show that the inventive sample has a smaller magnetic field than the comparative samples at both the measurement points A and B as shown in  FIGS. 5 and 6 , respectively, and substantially smaller especially at the measurement points A. 
     Specifically, for example, the inventive sample has magnetic fields of 6.9 A/m and 36 A/m respectively at the measurement point A with the distance d 1  of 2 cm and the measurement point B with the distance d 2  of 2 cm, while the comparative sample has magnetic fields of 91 A/m and 62 A/m, respectively. Thus, the present invention is effective in reducing the magnetic field attributable to leakage flux from the inverter transformer, especially effective with respect to the vertical direction dY above the top surface of the winding. The effect is rather small with respect to the horizontal direction dX orthogonal to the core length, because the magnetic fluxes which leak laterally from the cores  23   a  and  23   c  located at both sides spread in the air around. 
     Second and third embodiments of the present invention, which further enhance the effect achieved by the first embodiment, will be described with reference to  FIGS. 7(   a ),  7 ( b ) and  7 ( c ), and  FIGS. 7(   d ) and  7 ( e ), respectively. In explaining the second and third embodiments in  FIGS. 7(   a ) to  7 ( e ), any component parts corresponding to those in  FIG. 1  are denoted by the same reference numerals, and a detailed description thereof will be omitted below. 
     An inverter transformer  40  according to the second/third embodiment includes cores  23 , bobbins  26 , primary windings W 1 , secondary windings W 2 , primary winding terminal blocks  38   a , and secondary winding terminal blocks  39   a , and these components are partly (the second embodiment) or totally (the third embodiment) covered by a magnetic resin  6 . The primary windings W 1  are arranged around the cores  23  in the same way as the first embodiment, so that magnetic fluxes generated in the cores  23  by currents flowing through the primary windings W 1  are directed opposite to each other on adjacent core basis. 
     Referring to  FIGS. 7(   a ),  7 ( b ) and  7 ( c ) showing the second embodiment, a core  23   a , a bobbin  26   a , a primary winding  24   a , a secondary winding  25   a , and an insulation resin  50  to enclose the above-mentioned members constitute a first winding unit  51   a ; a core  23   b , a bobbin  26   b , a primary winding  24   b , a secondary winding  25   b , and an insulation resin  50  to enclose the above-mentioned members constitute a second winding unit  51   b ; and a core  23   c , a bobbin  26   c , a primary winding  24   c , a secondary-winding  25   c , and an insulation resin  50  to enclose the above-mentioned members constitute a third winding unit  51   c . The first, second and third winding units  51   a ,  51   b  and  51   c  thus constituted make up a winding assembly  51 . The winding assembly  51  is circumferentially covered by the aforementioned magnetic resin  6  except the bottom face as shown in  FIG. 7(   b ) (this resin coverage structure refers to “a transformer body  55 B” as described later), and with interspaces between the winding units  51   a ,  51   b  and  51   c  being filled up. The magnetic resin  6  may alternatively be arranged to cover the top face only of the circumference of them winding assembly, or the side faces or the bottom face only. The magnetic resin  6  covers the winding assembly  51  longitudinally from one ends of the cores  23   a ,  23   b  and  23   c  to the other ends thereof, and portions of the primary and secondary winding terminal blocks  38   a  and  39   a.    
     Referring to  FIGS. 7(   d ) and  7 ( e ) showing the third embodiment, the winding assembly  51  is circumferentially covered by the magnetic resin  6  including the bottom face as shown in  FIG. 7(   d ) (this resin coverage structure refers to “a transformer body  55 A” as described later). 
     The magnetic resin  6  is formed of a mixture produced by mixing a magnetic substance of powder gained by pulverizing sintered Mn—Zn ferrite, and, for example, a thermosetting epoxy resin, where the Mn—Zn ferrite powder accounts for 80% in terms of volume ratio. In case of the inverter transformer  40 , the mixture thus produced is applied to the winding assembly  51  (the first, second and third winding units  51   a ,  51   b  and  51   c  constituted respectively by the cores  23   a ,  23   b  and  23   c , the bobbins  26   a ,  26   b  and  26   c , the primary windings  24   a ,  24   b  and  24   c , the secondary windings  25   a ,  25   b  and  25   c , and the insulation resins  50 ) by molding, spreading, or the like, and is heated and cured by a temperature of, for example, 150 degrees C., whereby the mixture applied turns into the magnetic resin  6 . The magnetic substance for the magnetic resin  6  is not limited to Mn—Zn ferrite, but may be Ni—Zn ferrite or ion powder, and the resin material may alternatively be nylon, and the like, which achieves a similar effect. The relative magnetic permeability of the magnetic resin  6  is determined so as to effectively shield against leakage flux coming out from the cores  23  and at the same time to duly constitute an open magnetic path structure. In the present embodiments, the relative magnetic permeability of the magnetic resin  6  can be controlled by changing the property of the magnetic substance, or changing the mixing ratio of the magnetic substance to the resin. For example, Mn—Zn ferrite or Ni—Zn ferrite achieves a relative magnetic permeability of several tens, and iron power achieves a relative magnetic permeability of several hundreds. 
     In the inverter transformer  40  shown in  FIGS. 7(   a ),  7 ( b ) and  7 ( c ) according to the second embodiment, the magnetic resin  6  is arranged so as to cover the top and side faces only of the winding assembly  51  (including the first, second and third winding units  51   a ,  51   b  and  51   c ). In the inverter transformer  40  shown in  FIGS. 7(   d ) and  7 ( e ) according to the third embodiment, the magnetic resin  6  is arranged so as to cover the top, sides, and bottom faces, that is to say the entire circumferential faces, of the winding assembly  51 , where the interspaces between the first, second and the third winding units  51   a ,  51   b  and  51   c  are filled up with the magnetic resin  6  in the same way as the second embodiment. In the transformers  40  according to the second and third embodiments, the magnetic resin  6  covers the winding assembly  51  longitudinally from the one ends of the cores  23   a ,  23   b  and  23   c  to the other ends thereof, and portions of the primary and secondary winding terminal blocks  38   a  and  39   a , as described above. In this connection, all of the cores  23   a ,  23   b  and  23   c  (the winding assembly) are covered together by the magnetic resin  6  composed of one piece in the embodiments described above, but the present invention is not limited to this structure and the cores  23   a ,  23   b  and  23   c  (the first, second and third winding units  51   a ,  51   b  and  51   c ) may be covered individually by three separate pieces of magnetic resins. 
     The operation of the inverter transformers  40  according to the second and third embodiments will hereinafter be described. 
     Since the magnetic resin  6  has a significantly smaller relative magnetic permeability than the cores  23 , all of magnetic fluxes generated at the cores  23  are not adapted to pass through the magnetic resin  6 , but some parts of the magnetic fluxes are allowed to leak beyond the magnetic resin  6  due to the difference of their magnetic resistances, and thus leakage inductance is provided. That is to say, the magnetic path generated by the cores  23  and the magnetic resin  6  is not a closed magnetic path, and therefore the inverter transformer  40  substantially has an open magnetic path structure having leakage inductance. Accordingly, there are generated not only magnetic fluxes that pass entirely through the cores  23  so as to interlink the primary windings W 1  and the secondary windings W 2 , but also leakage fluxes that interlink either with the primary windings W 1  only or with the secondary windings W 2  only thus failing to contribute to providing electromagnetic coupling between the primary windings W 1  and the secondary windings W 2 , whereby leakage inductance is generated. The inverter transformer  40  operates in the same way as an inverter transformer structured with an open magnetic path and not covered by the magnetic resin  6 , and the generated leakage inductance acts as ballast inductance so as to duly discharge and light the CCFLs connected to the secondary windings W 2 . 
     Unlike a traditional inverter transformer, in the inverter transformer  40  according to the second/third embodiment, the winding assembly  51  is surrounded by the magnetic resin  6  thereby causing the leakage inductance to act as ballast inductance, and at the same time most of the magnetic fluxes leaking from the cores  23  are adapted to pass through the magnetic resin  6  thus reducing the amount of magnetic fluxes leaking beyond the magnetic resin  6 . Consequently, the range of leakage flux spreading out from the inverter transformer  40  is limited. Thus, the inverter transformer  40  is further effective in reducing leakage flux, because of the magnetic resin  6  reducing leakage flux as described above in combination with the leakage flux reducing effect achieved by the primary windings W 1  arranged around the cores  23  in the same way as the first embodiment, especially in the direction dX as shown in  FIG. 4 . 
     The inverter transformer  40  shown in  FIGS. 7(   a ),  7 ( b ) and  7 ( c ) according to the second embodiment, in which the bottom face of the winding assembly  51  is not covered by the magnetic resin  6 , is desirable and suitable when mounted on a substrate or chassis made of a non-magnetic material. Specifically, when the inverter transformer  40  according to the second embodiment is mounted on a non-magnetic substrate or chassis, the magnetic paths of magnetic fluxes leaking from the cores  23  in the bottom direction are not influenced by anything thus reducing variation or change in the property. On the other hand, since the other faces than the bottom face, that is to say, the top and side faces, are covered by the magnetic resin  6 , the range of leakage flux spreading out from the inverter transformer  40  is limited. Consequently, leakage inductance is duly achieved without having influence on neighboring components, and at the same time the height of the inverter transformer  40  can be reduced due to its bottom face not covered by the magnetic resin  6 . 
     The inverter transformer  40  shown in  FIGS. 7(   d ) and  7 ( e ) according to the third embodiment, in which the top, side, and bottom faces of the winding assembly  51  are covered by the magnetic resin  6  longitudinally from one ends of the cores  23  to the other ends thereof, is desirable and suitable when mounted on a substrate or chassis made of a magnetic material. Specifically, since the bottom face of the inverter transformer  40  according to the third embodiment is also covered by the magnetic resin  6 , magnetic fluxes leaking from the cores  23  are not subject to the influence of the magnetic substrate or chassis disposed under the bottom face due to the magnetic shielding function of the magnetic resin  6 , and therefore the magnetic paths of the magnetic fluxes are not changed thus reducing variation in the property. 
     For optimizing the operation of an inverter transformer, the numbers of turns on primary and secondary windings and leakage inductance must be adjusted, but the characteristic of leakage inductance is caused to vary with a change in the magnetic property of the magnetic path of leakage flux. On the other hand, in the inverter transformer  40  of the present invention, leakage inductance is adjusted according to the optimal conditions for the circuit operation by adjusting the magnetic properties (such as relative permeability), thickness, and area range of the magnetic resin  6 . As a result, the operation of the inverter transformer  40  can be flexibly optimized for application to various kinds of inverter transformers simply by adjusting the value of leakage inductance without changing the numbers of turns on the primary windings W 1  and the secondary windings W 2  and also the configuration and property of the cores  23 . 
     In the inverter transformers  40  according to the second and third embodiments, the magnetic resin  6  is disposed so as to cover the bar-shaped cores  23  entirely from one end to the other, but insofar as leakage inductance is duly provided, the magnetic resin  6  does not necessarily have to entirely cover the cores  23  and may alternatively be disposed so as to partly cover the cores  23 . Such a partial coverage structure is employed in fourth and fifth embodiments of the present invention described below. 
     The fourth and fifth embodiments mentioned above will be described with reference to  FIGS. 8(   a ),  8 ( b ) and  8 ( c ). In explaining the examples shown in  FIGS. 8(   a ),  8 ( b ) and  8 ( c ), any component parts corresponding to those in  FIGS. 1 and 7(   a ) to  7 ( e ) are denoted by the same reference numerals, and a detailed description thereof will be omitted below. 
     Referring to  FIGS. 8(   a ),  8 ( b ) and  8 ( c ), in inverter transformers  20  according to the fourth and fifth embodiments, both end portions of cores  23  including portions of bobbins  26  and primary and secondary winding terminal blocks  38   a  and  39   a , i.e. end portions  511  of a winding assembly  51  are individually covered by two separate magnetic resins  6 , respectively, while the middle portions of the cores  23  are not covered thereby. In the inverter transformer  20  of the fourth embodiment, the two separate magnetic resins  6  are disposed so as to cover the top and side faces only of the end portions  511  as shown in  FIG. 8(   b ), which is common to the second embodiment (refer to  FIG. 7(   b )), and which generates similar effects. On the other hand, in the inverter transformer  20  of the fifth embodiment, the two separate magnetic resins  6  are disposed so as to cover the top, side, and bottom faces of the end portions  511  as shown in  FIG. 8(   c ), which is common to the third embodiment (refer to  FIG. 7(   d )), and effects similar to those in the third embodiment are achieved. 
     In the inverter transformers  20  according to the fourth and fifth embodiments, since both end portions of the cores  23  (the winding assembly  51 ) are covered totally or partly by respective magnetic resins  6 ,  6 , most of leakage fluxes ΦR coming out from the end portion of the cores  23  are adapted to pass through the magnetic resins  6  functioning as a shield, and consequently the amounts of leakage fluxes ΦS spreading out in the open air around are reduced. Since the inverter transformers  20  according to the fourth and fifth embodiments are of an open magnetic path structure like the inverter transformer  40  according to the second and third embodiments, leakage inductance is generated at primary windings W 1  and secondary windings W 2  and functions as ballast inductance so as to duly light CCFLs. 
     In the fourth and fifth embodiments described above, the end portions of the cores  23  ( 23   a ,  23   b  and  23   c ) are covered together by the one piece magnetic resin  6 , but the present invention is not limited to this structure and may alternatively be structured such that the end portions of the cores  23  are covered individually by three separate piece magnetic resins, respectively. In the inverter transformers  20  according to the fourth and fifth embodiments, leakage inductance is adjusted according to the optimal conditions for the circuit operation by adjusting the magnetic properties (such as relative permeability), thickness, and area range of the magnetic resin  6 . 
     In the fourth and fifth embodiments, since the leakage fluxes ΦS coming from the end portions of the cores  23  and spreading out in the open air around are reduced as described above, components arranged close to the end portions of the cores  23   a  are kept magnetically uninfluenced, and at the same time, the inverter transformer  20  is prevented from getting influenced by magnetic fluxes coming from the components thus reducing variation and change in characteristics. Also, influences can be eliminated that may possibly arise when components including a magnetic substance are arranged close to the end portions of the cores  23 . 
     Also, in the fourth and fifth embodiments, a partition portion  52  of the winding assembly  51  (composed of the first, second and third winding units  51   a ,  51   b  and  51   c ) provided with partitions  57   a  to separate the primary windings W 1  from the secondary windings W 2  may be covered by an additional magnetic resin. The partition portion  52  is an area where leakage flux is generated abundantly, and covering the partition portion  52  by a magnetic resin is very effective in further reducing the amount of magnetic flux exiting out from the inverter transformer  40  in the open space around. This measure of covering the partition portion  52  by a magnetic resin may be effectively implemented not only in the inverter transformer  20  according to the fourth or fifth embodiment but also in a traditional inverter transformer. 
     A sixth embodiment of the present invention will be described with reference to  FIGS. 9(   a ),  9 ( b ) and  9 ( c ). In explaining the example shown in  FIGS. 9(   a ),  9 ( b ) and  9 ( c ), any component parts corresponding to those in  FIGS. 1 ,  7 ( a ) to  7 ( e ), and  8 ( a ) to  8 ( c ) are denoted by the same reference numerals, and a detailed description thereof will be omitted below. 
     Referring to  FIG. 9(   a ), in an inverter transformer  40  according to the sixth embodiment, a winding assembly  51  is entirely covered by a magnetic resin  6 , including interspaces between first, second and third winding units  51   a ,  51   b  and  51   c , in the same way as the third embodiment (refer to  FIG. 7(   d )), wherein the winding assembly  51  and the magnetic resin  6  constitute a transformer body  55 . As mentioned previously, a transformer body  55 , in which a winding assembly  51  is entirely covered, that is to say, has its top, side and bottom faces covered by a magnetic resin  6 , is designated as “a transformer body  55 A” (refer to  FIG. 7(   d )), while a transformer body  55 , in which a winding assembly  51  has its top and side faces only covered by a magnetic resin  6 , is designated as “a transformer body  55 B” (refer to  FIG. 7(   d )). 
     Referring to  FIGS. 9(   a ) to  9 ( c ), in the inverter transformer  40  according to the sixth embodiment, the transformer body  55 A is enclosed by an external unit  56  with primary and secondary winding terminal blocks  38   a  and  39   a  sticking out. The external unit  56  is composed of sintered compacts formed of, for example, Mn—Zn ferrite, or Ni—Zn ferrite, and has a larger saturation magnetic flux density and a smaller magnetic resistance than the magnetic resin  6 . Referring to  FIG. 9(   b ), the external unit  56  includes a first section  56   a  having a hollow  56   h  to receive the transformer body  55 A, and a second section  56   b  disposed on the first section  56   a  so as to cover up the transformer body  55 A. 
     Referring to  FIGS. 9(   b ) and  9 ( c ), the first section  56   a  includes a bottom  58 , side walls  59  vertically disposed at the both sides of the bottom  58 , a front end wall  60  vertically disposed at the front end (lower in  FIG. 9(   a )) of the bottom  58 , and a rear end wall  61  (not seen in the figures) vertically disposed at the rear end (upper in  FIG. 9(   a )) of the bottom  58 . A cutout  62  is formed at each of the front end wall  60  and the rear end wall  61 , and some portions of the primary and secondary winding terminal blocks  38   a  and  39   a  protrude through respective cutouts  62 . That is to say, the external unit  56  is adapted to enclose the transformer body  55 A with the terminal blocks  7  and  8  sticking out. 
     In the inverter transformer  40  according to the sixth embodiment, since the external unit  56  (sintered compact) having a larger saturation magnetic flux density than the magnetic resin  6  is provided so as to enclose the transformer body  55 A, most of magnetic fluxes leaking from the cores  23   a ,  23   b  and  23   c  so as to pass through the magnetic resin  6  and then to leak beyond the magnetic resin  6  are now adapted to pass through the external unit  56 . Thus, with provision of the external unit  56 , magnetic flux can be prevented from leaking out from the inverter transformer  40  more effectively than when the external unit  56  is not provided. Consequently, the cross section area of the structure according to the sixth embodiment can be reduced compared with the structure in which magnetic flux is prevented from leaking out by means of the magnetic resin  6  only, and the inverter transformer  40  can be downsized. 
     Since the external unit  56  has a smaller magnetic resistance than the magnetic resin  6 , magnetic flux leaking out beyond the magnetic resin  6  passes through the external unit  56  more effectively. Consequently, magnetic flux can be further prevented from leaking out from the inverter transformer  40 , which enables further downsizing of the inverter transformer  40 . 
     The inverter transformer  40  according to the sixth embodiment is produced as follows. The winding assembly  51  is put in the hollow  56   h  of the first section  56   a  of the external unit  56  with the primary and secondary winding terminal blocks  38   a  and  39   a  fitted in the respective cutouts  62 , and a resin material (the magnetic resin  6 ) is filled in the hollow  56   h  so as to mold the winding assembly  51 . The magnetic resin  6  is heated at, for example, about 150 degrees C. for curing, and the transformer body  55 A, which is composed of the winding assembly  51  and the magnetic resin  6  filled around the winding assembly  51 , is obtained in the hollow  56   h . Then, the second section  56   b  of the external unit  56  is put on the first section  56   a  so as to lid the hollow  56   h  having the transformer body  55 A therein, thus the first section  56   a  and the second section  56   b , in combination, enclose the transformer body  55 A, and the inverter transformer  40  is obtained. Since the winding assembly  51  is molded by filling the magnetic resin  6  in the hollow  56   h , the production is eased enhancing the productivity. In this connection, the second section  56   b  of the external unit  56  may be omitted so that the external unit  56  is constituted by the first section  56   a  only. 
     In the sixth embodiment, the external unit  56  is structured so as to cover the top, side, bottom, and front end and rear end (except the primary and secondary winding terminal blocks  38   a  and  39   a ) faces of the transformer body  55 A, but the present invention is not limited to this structure and arrangement. For example, an inverter transformer may include a transformer body  55 B in place of the transformer body  55 A, and also may alternatively be structured in combination with any one of various external units as described below. 
     Referring to  FIGS. 10(   a ),  10 ( b ) and  10 ( c ), an inverter transformer  40  according to a seventh embodiment includes an external unit  56 A which is shaped into a rectangular tube so as to cover the top, side, and bottom faces of a transformer body  55 A. The external unit  56 A has a larger saturation magnetic flux density and a smaller magnetic resistance than a magnetic resin  6 . 
     In the seventh embodiment, the external unit  56 A does not cover the front end and rear end faces of the transformer body  55 A but still covers most area of the outer surface thereof, and magnetic flux leaking out from the inverter transformer  40  can be duly reduced, and also the inverter transformer  40  can be downsized. And, since the external unit  56 A has a smaller magnetic resistance than the magnetic resin  6 , magnetic flux can be further prevented from leaking out from the inverter transformer  40 , which enables further downsizing of the inverter transformer  40 . 
     Referring to  FIGS. 11(   a ),  11 ( b ) and  11 ( c ), an inverter transformer  40  according to an eighth embodiment includes an external unit  56 B which is composed of a roof  63  and two side walls  64  vertically disposed at the both sides of the roof  63  so as to have a squared C shape in cross section, and which covers the top and side faces of a transformer body  55 B. The external unit  56 B has a larger saturation magnetic flux density and a smaller magnetic resistance than a magnetic resin  6 . 
     In the eighth embodiment, the external unit  56 B does not cover the bottom face of the transformer body  55 B compared with the external unit  56 A in the seventh embodiment described above but still covers a substantial area of the outer surface thereof, and magnetic flux leaking out from the inverter transformer  40  can be duly reduced, and also the inverter transformer  40  can be downsized. And, since the external unit  56 B has a smaller magnetic resistance than the magnetic resin  6 , magnetic flux can be further prevented from leaking out from the inverter transformer  40 , which enables further downsizing of the inverter transformer  40 . 
     In the eighth embodiment described above, the roof  63  of the external unit  56 B is defined flat in accordance with the configuration of the transformer body  55 B but may alternatively be, for example, arced when the transformer body  55 B has an arced configuration. Also, a transformer body  55 A may be used in the eighth embodiment in place of the transformer body  55 B as shown in  FIG. 11(   d ). 
     Referring to  FIGS. 12(   a ),  12 ( b ) and  12 ( c ), an inverter transformer  40  according to a ninth embodiment includes an external unit  56 C which is composed of a roof  63  and two side walls  64 . The roof  63  is divided into a bridge portion  65  sandwiched between two openings and adapted to cover a partition portion  52 A (including a partition portion  52  of a winding assembly  51 ) of a transformer body  55 B provided with a partition  57   a , two end frame portions  66  adapted to cover both end portions  67  of the transformer body  55 B, and two side frame portions (not reference-numbered) perpendicularly adjacent to the side walls  64 . The external unit  56 C has a larger saturation magnetic flux density than a magnetic rein  6 . In the ninth embodiment, a transfer body  55 A may be used in place of the transformer body  55 B as shown in  FIG. 12(   d ). 
     Leakage flux is generated abundantly at the partition portion  52  of the winding assembly  51  as described above, but since the partition portion  52 A including the partition portion  52  is covered by the bridge portion  65  of the external unit  56 C and other portions thereof adjacent to the bridge portion  65 , most of magnetic flux leaking out via the partition portion  52 A is adapted to pass through the external unit  56 C, and therefore leakage flux from the inverter transformer  40  can be well reduced. Also, since the end frame portions  66  of the roof  63  cover respective end portions  67  of the transformer body  55 A, leakage flux from the inverter transformer  40  can be further reduced. 
     Referring to  FIG. 12(   e ), in a tenth embodiment shown in, an external unit  56 D is used, which differs from the external unit  56 C of the ninth embodiment in that the bridge portion  65  is eliminated so as to form one opening in a roof  63 . 
     Referring to  FIGS. 13(   a ) and  13 ( b ), in an inverter transformer  40  according to an eleventh embodiment, a transformer body  55 C′, in which a magnetic resin  6  covers the top and side faces of a partition portion  52  of a winding assembly  51 , is used in combination with an external unit  56 D (refer to  FIG. 12(   e )). Also, referring to  FIG. 13(   c ), in a twelfth embodiment, a transformer body  55 D′, in which a magnetic resin  6  covers the top, side and bottom faces of a partition portion  52  of a winding assembly  51 , is used. 
     Referring to  FIG. 13(   d ), in a thirteenth embodiment, a plate member  65   a  is separately attached after an external unit  56 D as shown in  FIG. 12(   e ) is attached to a winding assembly  51 . The plate member  65   a  is formed of a material equivalent to that of the external unit  56 D or a magnetic resin  6 . 
     Referring to  FIGS. 14(   a ) and  14 ( b ), an inverter transformer  40  according to a fourteenth embodiment includes an external unit  56 E which is composed of a plate having a rectangular configuration in plan view. The external unit  56 E is disposed under a transformer body  55 B so as to cover the bottom face of the transformer body  55 B. The external unit  56 E has a larger saturation magnetic flux density than a magnetic resin  6 . In the fourteenth embodiment, a transformer body  55 A may be used in place of the transformer body  55 B. 
     Referring to  FIGS. 15(   a ) and  15 ( b ), an inverter transformer  40  according to a fifteenth embodiment includes an external unit  56 F which is composed of first and second rectangular plates  56   c  and  56   d . The first and second plates  56   c  and  56   d  are disposed respectively at both sides of a transformer body  55 B so as to cover the side faces of the transformer body  55 B. The external unit  56 F has a larger saturation magnetic flux density than a magnetic resin  6 . In the fifteenth embodiment, a transformer body  55 A may be used in place of the transformer  55 B as shown in  FIG. 15(   c ). 
     Referring to  FIGS. 16(   a ) and  16 ( b ), an inverter transformer  40  according to a sixteenth embodiment includes an external unit  56 G which is composed of first and second members  56   e  and  56   f  each formed in a structure having a squared C shape in cross section. The first and second members  56   e  and  56   f  are disposed respectively at both end portions  67  of a transformer body  55 B so as to cover the top and side faces of respective end portions  67 . The external unit  56 G has a larger saturation magnetic flux density than a magnetic resin  6 . In the sixteenth embodiment, a transformer body  55 A may be used in place of the transformer body  55 B. 
     Referring to  FIG. 16(   c ), an external unit  56 H in a seventeenth embodiment is composed of first and second members  56   g  and  56   h  each formed in a structure constituting a rectangular frame configuration in cross section. The first and second members  56   g  and  56   h  are disposed respectively at both end portions  67  of a transformer body  55 A so as to cover the top, side, and bottom faces of respective end portions  67 . The external unit  56 H has a larger saturation magnetic flux density than a magnetic resin  6 . In the seventeenth embodiment, a transformer body  55 B may be used in place of the transformer body  55 A. 
     In the second to tenth embodiments shown in  FIGS. 7(   a ) through  12 ( e ), and in the fourteenth to seventeenth embodiments shown in  FIGS. 14(   a ) through  16 ( c ), an inverter transformer includes either a transformer body  55 A (where a magnetic resin  6  covers all circumferential faces of a winding assembly  51 ) or a transformer body  55 B (where a magnetic resin  6  covers the top and side faces only of a winding assembly  51 ). Also, in the eleventh to thirteenth embodiments shown in  FIGS. 13(   a ) to  13 ( d ), an inverter transformer includes either a transformer body  55 C′ or a transformer body  55 D′. The present invention, however, is not limited to this transformer body arrangement and any different type transformer bodies may be used in combination with an external unit  56  or any one of its modification. 
     For example, referring to  FIGS. 17(   a ),  17 ( b ) and  17 ( c ), a transformer body  55 C, in which a magnetic resin  6  is composed of three pieces adapted to cover respectively both end portions  511 ,  511  and a partition portion  52  of a winding assembly  51  at the top and side faces thereof, is used in combination with an external unit  56 B (an eighteenth embodiment). Also, referring to  FIG. 17(   d ), a transformer body  55 D, in which a magnetic resin  6  is composed of three pieces adapted to cover respectively both end portions  511 ,  511  and a partition portion  52  of a winding assembly  51  at the top, side and bottom faces, is used in combination with an external unit  56 B (a nineteenth embodiment). 
     And, referring to  FIGS. 18(   a ) and  18 ( b ), an external unit  56 F composed of first and second rectangular plates  56   c  and  56   d  is used in combination with a transformer body  55 C (a twentieth embodiment). Also, referring to  FIG. 18(   c ), an external unit  56 F composed of first and second rectangular plates  56   c  and  56   d  is used in combination with a transformer body  55 D (a twenty first embodiment). 
     INDUSTRIAL APPLICABILITY 
     An inverter transformer with an open magnetic path structure can be provided, whose entire structure and production process are simplified thus preventing cost increase.