Balance transformer

A small loop path 9A composed of a part of each of a first core 51 and a second core 52 is disposed with two windings 2A and 3A of a transformer portion 4A, and a small loop path 9B composed of another part of each of the first core 51 and the second core 52 is disposed with two windings 2B and 3B of a transformer portion 4B. A magnetic flux generated at the transformer portion 4A circulates along the small loop path 9A, and a magnetic flux generated at the transformer portion 4B circulates along the small loop path 9B with each flux circulating in a mutually reverse direction.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No. 2006-138841 filed on May 18, 2006 and Japanese Patent Application No. 2007-78867 filed on Mar. 26, 2007, which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a balance transformer used in a circuit for parallel-driving a plurality of discharge lamps to balance current shunted to the plurality of discharge lamps, and in particular, to a balance transformer suitable for use in a DC/AC inverter circuit for parallel-driving cold cathode fluorescent lamps (CCFL) for back light of various types of display panels used for Laptops, liquid crystal televisions, and the like.

2. Description of the Prior Art

Heretofore, as a parallel-drive circuit of CCFL, for example, those disclosed in International Patent Publication No. WO2005/038828, U.S. Pat. No. 6,781,325, and Japanese Unexamined Patent Publication No. 2003-31383 are known.

Further, Japanese Unexamined Patent Publication No. 2006-12781 proposes a balance transformer disposed with a plurality of transformer portions comprising a primary winding and a secondary winding disposed coaxially applicable to a parallel drive circuit of the CCFL of this type, particularly as a balance transformer applicable to the parallel drive circuit disclosed in the International Patent Publication No. WO2005/038828.

The balance transformer disclosed in the above described Japanese Unexamined Patent Publication No. 2006-12781 can be broadly classified into three types in its specific mode. A first type is disposed with a plurality of transformer portions in series, and at the same time, is disposed with a loop-shaped common core communicated to the interior of the winding of each transformer (see FIGS. 11, 12, and 14 of the above described Japanese Unexamined Patent Publication No. 2006-12781). A second type is disposed with a plurality of transformer portions in parallel, and at the same time, is disposed with a common core provided with a plurality of leg portions inserted into the interior of the winding of each transformer, respectively (see FIG. 13 of the above described Japanese Unexamined Patent Publication No. 2006-12781). Further, a third type is disposed with an individual core for each of the plurality of transformer portions disposed in parallel (see FIG. 15 of the above described Japanese Unexamined Patent Publication No. 2006-12781).

However, the balance transformers of the first and second type are liable to cause magnetic interference since a magnetic path for each magnetic flux generated at each transformer portion is mutually not isolated, and there is a fear that an accuracy of the operation for balancing the current to each CCFL is lowered.

On the other hand, though each magnetic path is mutually isolated by providing a separate core for each transformer, the balance transformer of the third type is hardly able to attempt at the miniaturization of the individual transformer portion because of the requirement of a core for each transformer portion, and at the same time, the number of component parts of each transformer portion is increased, thereby causing a problem of the increase in the cost of production.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above described circumstances, and an object of the invention is to provide a balance transformer in which the magnetic flux generated at each transformer portion hardly causes magnetic interference, and moreover, the number of component parts is reduced with the component parts miniaturized, and the low cost can be attempted.

The balance transformer according to the present invention is a balance transformer disposed in a circuit for driving a plurality of discharge lamps, and comprises:a first transformer portion having a first primary winding and a first secondary winding;a second transformer portion having a second primary winding and a second secondary winding; anda magnetic core comprising a loop-shaped outer frame portion and a short portion for shorting inside of the outer frame portion,wherein a first small loop path composed of a part of the outer frame portion and the short portion is disposed with only two windings of the first transformer portion from among all the windings, and a second small loop path composed of another part of the outer frame portion and the short portion is disposed with only two windings of the second transformer portion from among all the windings, andwherein a first magnetic flux generated at the first transformer portion circulates along the first small loop path, and a second magnetic flux generated at the second transformer portion circulates along the second small loop path in a direction reverse to the first magnetic flux.

Further, in the balance transformer of the present invention, the four windings are approximately coaxially disposed, and the magnetic core is composed of a combination of a bar-shaped first core disposing in each interior of the four windings and a second core coupled with the first core.

Still further, in the balance transformer of the present invention, the second core is mutually and integrally formed with a base portion extending in parallel to the first core; with outer leg portions protruding toward the first core at both end portions of the base portion, respectively; and with a middle leg portion protruding toward the first core at a central portion of the base portion.

Moreover, in the balance transformer of the present invention, the first secondary winding and the second secondary winding are wound by being split into a plurality of wound sections, respectively, and each width of the plurality of wound sections are set to become larger than the width of each wound region of the first primary winding and the second primary winding.

In addition, in the balance transformer of the present invention, a bobbin wound with all windings is disposed with a primary side terminal connected with the first primary winding and the second primary winding and a secondary side terminal connected with the first secondary winding and the second secondary winding at mutually different side surfaces of the bobbin, respectively.

Still further, in the balance transformer of the present invention, between the first primary winding and the first secondary winding in the first transformer portion, and between the second primary winding and the second secondary winding in the second transformer portion, an insulating wall is disposed, respectively, and the insulating wall is provided with a groove portion.

Further, in accordance with the balance transformer of the present invention, the first transformer portion and the second transformer portion are mutually coupled through a coupling portion, and the coupling portion is disposed with an opening portion for exposing a part of the magnetic core.

Moreover, in the balance transformer of the present invention, two of the short portions are provided, and the first small loop path and the second small loop path are configured to pass through a separate short portion, respectively.

In addition, in accordance with the balance transformer of the present invention, another side surface of the bobbin is provided with a wall portion for positioning the magnetic core.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of a balance transformer according to the present invention will be described below in detail with reference to each drawing accompanied herewith.FIGS. 1 to 4show the embodiment of the balance transformer according to the present invention. Incidentally, three dimensional orthogonal coordinate system shown in each Figure shows a corresponding relationship of the aspects between the Figures, and according to the following description, the direction of an axis X of the three dimensional orthogonal coordinate system can be referred to as the front to back (an arrow direction is front), the direction of an axis Y as the left to right (an arrow direction is the right), and the direction of an axis Z as the upward to downward (direction of the arrow is upward).

First, usingFIGS. 1 to 3, a configuration of the balance transformer according to one embodiment of the present invention will be described.FIG. 1is a top plan view showing the entire configuration of the balance transformer according to one embodiment of the present invention,FIG. 2is an oblique view from a front side of a bobbin shown inFIG. 1, andFIG. 3is an exploded view of a magnetic core shown inFIG. 1.

A balance transformer1of the present embodiment, for example, is used for balancing the current shunted to a plurality of CCFLs in a DC/AC inverter circuit which discharges and lights a cold cathode fluorescent lamp (CCFL) for use of back light of various types of display panels used for Laptops, liquid crystal televisions, and the like, and as shown inFIG. 1, is provided with a first transformer portion4A comprising a first primary winding2A and a first secondary winding3A, a second transformer portion4B comprising a second primary winding2B and a second secondary winding3B, and a magnetic core5.

The four windings2A,3A,2B and3B are wound around a bobbin6formed by an insulating material such as a plastic resin. This bobbin6, as shown inFIG. 2, is formed by mutually integrating a first winding axis portion61A disposed with the first transformer portion4A, a second winding axis portion61B disposed with the second transformer portion4B, and a coupling portion62disposed between the first and second winding axis portions61A and61B. The first and second winding axis portions61A and61B are formed with a core insertion hole63extending in the left to right direction (direction Y in the Figure), respectively, and the coupling portion62is formed with an opening portion62ato allow a part of the magnetic core5inserted into the core insertion hole63(for further detail, a first core51to be described later) to be exposed.

Incidentally, the opening portion62ais for performing the coupling with middle leg portions52d,52e, and52fof a first core51and a second core52to be described later. By providing such opening portion62a, a creepage distance from the first transformer portion4A and the second transformer portion4B to the middle leg portions52d,52e, and52fof the second core52becomes long, and insulation properties can be sufficiently secured.

To be more in detail, the first winding axis portion61A comprises a first primary side winding portion65A wound with the first primary winding2A, a first secondary side winding portion66A wound with the first secondary winding3A, and a first insulating wall portion67A disposed between these first primary side winding portion65A and first secondary side winding portion66A. The first secondary side winding portion66A is split into three winding sections by an end flange68and two partition flanges69, and each winding section is configured to be wound with approximately one third of the first secondary winding3A. Further, each partition flange69is formed with a notch portion69afor delivering the first secondary winding3A to adjacent winding sections.

Further, a width (a length in the direction to Y in Figure) W2of each winding section of the first secondary side winding portion66A is formed to be larger than a width W1of a winding area of the first primary side winding portion65A. As a result, as against the number of windings (for example, about 10 T) of the first primary winding2A wound around the first primary side winding portion65A, the number of windings of the first secondary winding3A wound around the first secondary side winding portion66A (for example, about 300 T for each winding section, and a total of about 900 T) can be increased to a large extent. By producing difference in the number of windings between the first primary winding2A and the first secondary winding3A, potential difference between both ends of the first primary winding2A can be suppressed low.

However, when the difference is thus produced in the number of windings, potential difference between the first primary winding2A and the first secondary winding3A becomes large, and therefore, a sufficient attention must be paid to ensure insulation between these windings. The present embodiment is configured such that the width (the length in the direction to Y in Figure) of the first insulating wall portion67A is sufficiently secured, and at the same time, a groove portion67ais formed in its peripheral surface, thereby making the creepage distance between the first primary side winding portion65A and the first secondary side winding portion66A long so that a sufficient insulation can be obtained.

On the other hand, the second winding axis portion61B comprises a second primary side winding portion65B wound with the second primary winding2B, a second secondary side winding portion66B wound with the second secondary winding3B, and a second insulating wall portion67B disposed between these second primary side winding portion65B and second secondary side winding portion66B. The configurations of these second primary side winding portion65B, second secondary side winding portion66B, and second insulating wall portion67B are the same as the configurations of the first primary side winding portion65A, first secondary side winding portion66A, and first insulating wall portion67A in the above described first winding axis portion61A, and therefore, the detailed description thereof will be omitted.

Further, the bobbin6is integrally formed with five terminal supports71to75. As shown inFIG. 1, the terminal support73holds two primary side terminals7protruding in front (downward in the Figure), and the terminal supports72and74hold one each of a primary side terminal7protruding in front and a secondary side terminal8protruding at the back (upward in the Figure). The terminal supports71and75hold secondary side terminal8projecting backward, one for each. Each end portion of the first secondary winding3A is configured to be connected to an entwining portion8aof each secondary side terminal8of the terminal supports71and72, and each end portion of the second secondary winding3B is configured to be connected to an entwining portion8aof each secondary side terminal8of the terminal supports74and75. Further, each end portion of the first primary winding2A and each end portion of the second primary winding2B are configured to be connected to the primary side terminal7of any of the terminal supports72to74.

Thus, the primary side terminal7and the secondary side terminal8are disposed at mutually different side surfaces of the bobbin6, particularly desirably disposed at mutually opposing side surfaces, so that the insulating properties with the primary windings2A and2B, and the secondary windings3A and3B can be sufficiently secured. Further, the parts layout and routing of the wirings on a circuit board can be simplified. Incidentally, from among each of the secondary side terminals8, particularly those connected to the high voltage sides of the secondary windings3A and3B may be disposed at the side surfaces mutually different from the side surfaces disposed with the primary side terminal7, and those connected to the low voltage sides are not necessarily disposed in the same manner.

Further, as shown inFIG. 2, the upper surface of the terminal support71is provided with a wall portion71aextending along the left edge portion, and the terminal supports72to74are provided with wall portions72ato74aextending along the front edge portion, respectively.

By providing the wall portions71a, and72ato74ain this manner, the positioning of the core can be made, so that the fluctuation of the characteristic by displacement of the core can be suppressed.

On the other hand, as shown inFIG. 3, the magnetic core5, for example, is configured by mutually combining the first core51and the second core52formed respectively by a ferrite of soft magnetic material (in addition, materials such as permalloy, sendust, and iron carbonyl, and dust core which compression-moulds these fine particles can be used).

The first core51formed in the shape of a bar is configured to be inserted into the core insertion hole63from the right end side of the second secondary side winding axis portion61B shown inFIG. 2, and held inside the bobbin6in a state in which its left end portion contacts with the wall portion71aof the terminal support71(seeFIG. 1).

In contrast to this, the second core52, as shown inFIG. 3, is configured by being mutually and integrally formed with a base portion52aextending in parallel with the first core51, outer leg portions52band52cprotruding toward the first core51at both ends of the base portion52a, respectively, and the middle leg portions52dand52eprotruding toward the first core51in the center portion of the base portion52a, respectively. This second core52, as shown inFIG. 1, is disposed such that the two middle leg portions52dand52econtact with the front surface of the first core51between the first transformer portion4A and the second transformer portion4B so that the two outer leg portions52band52ccontact with the front surface of the first core51at the left end side and the right end side of the first core51, respectively.

Incidentally, in the present embodiment, a loop-shaped outer frame portion is composed of the base portion52aof the second core52, two outer legs portions52band52c, and the first core51, and a short portion for shorting the interior of the outer frame portion is composed of the two middle leg portions52dand52eof the second core52.

Further, the first small loop path9A is composed of approximately the left half of the first core51disposed as shown inFIG. 1, approximately the left half of the base portion52aof the second core52, and the outer leg portion52bof the left side and the middle leg portion52dof the left side, and a second small loop path9B is composed of approximately right half of the first core51, approximately right half of the base portion52aof the second core52, and the outer leg portion52cof the right side and the middle let portion52eof the right side.

As shown inFIG. 1, the first small loop path9A is disposed with two windings2A and3A only of the first transformer portion4A from among the four windings2A,3A,2B and3B, and the second small loop path9B is disposed with two windings2B and3B only of the second transformer portion4B from among the four windings2A,3A,2B, and3B. A first magnetic flux generated at the first transformer portion4A circulates along the first small loop path9A, and a second magnetic flux generated at the second transformer portion4B circulates along the second small loop path9B in a direction reverse to the first magnetic flux.

As a result, in the balance transformer1of the present embodiment, a magnetic path by the first magnetic flux generated at the first transformer portion4A and a magnetic path by the second magnetic flux generated at the second transformer portion4B can be mutually isolated. Consequently, the magnetic interference by the two magnetic fluxes can be prevented, and a balancing accuracy of the current toward each CCFL can be improved.

Incidentally, though the second core52is provided with two middle leg portions52dand52ecomprising the short portion, the short portion may be composed of one middle leg portion52fsimilarly to the second core52A of the modified example shown inFIG. 4. In this case, the first small loop path9A and the second small loop path9B are common in the middle leg portion52f. While the magnetic flux has properties to pass through preferably the shortest possible magnetic path, a magnetic interference is slightly generated in the middle leg portion52fin which the two small loop paths9A and9B are common. However, no significant trouble arises in the characteristics of the circuit. On the other hand, the advantage is afforded to be able to reduce the cost of production owing to the simplification of the core shape.

Further, though the four windings2A,3A,2B, and3B are approximately coaxially disposed, these disposing positions can be suitably changed if the two windings2A and3A only of the first transformer portion4A are disposed on the first small loop path9A, and the two windings2B and3B only of the second transformer portion4B are disposed on the second small loop path9B. For example, in the first transformer portion4A, the first primary winding2A can be disposed at the first core51side, and the first secondary winding3A can be disposed at the second core52side, and these windings can be also disposed at the outer leg portion52bor the middle leg portion52dof the second core52(the same disposition can be made also in the second transformer portion4B).

Further, the terminal arrangements may be appropriately changed from those in the embodiment.

Further, though the above described embodiment shows an embodiment comprising two transformer portions of the first transformer portion4A and the second transformer portion4B, the number of the transformer portions is not limited to two, but a third transformer portion and a fourth transformer portion may be appropriately added.

The balance transformer of the present invention is configured such that the first magnetic flux generated at the first transformer portion circulates along the first small loop path composed of a part of the magnetic core, and the second magnetic flux generated at the second transformer portion circulates along the second small loop path composed of another part of the magnetic core in a direction reverse to the first magnetic flux. Consequently, the magnetic path of the first magnetic flux and the magnetic path of the second magnetic flux can be mutually isolated, so that the magnetic interference by the two magnetic fluxes can be prevented, and a balancing accuracy of the current toward each discharge lamp can be improved similarly to the case where a separate core is provided for each transformer portion.

Further, the magnetic core is configured to be composed of the loop-shaped outer frame portion and the short portion, so that the first and second transformer portions can use a common magnetic core. Thus, comparing with the conventional art provided with a separate core for each transformer portion, the number of component parts can be made small, thereby the reduction in size and cost can be attempted.