Magnetic core

A magnetic core includes a first core having a predetermined magnetic permeability and a second core formed of the same material as the first core. The second core forms a closed magnetic circuit together with the first core. The second core is configured to radiate heat through a heat radiating unit. At least one of the first core and the second core is configured to be wound with a coil. The magnetic core includes a third core that is arranged between the first core and the second core and has a lower magnetic permeability than the first core.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic core.

Conventionally, a reactor, which is a type of induction device, has a pair of cores formed of ferrite with high magnetic permeability and a non-magnetic membrane formed of plastic with low magnetic permeability arranged between the cores to obtain desirable DC superposition characteristics. See, for example, Japanese Laid-open Patent Publication No. 2001-102217.

It is known that a change in the electric current flowing in a coil of an induction device causes heat generation in not only the coil but also cores. However, in the induction device described in the aforementioned document, plastic arranged between the cores, which exhibits a low thermal conductivity, suppresses heat transfer from one of the cores (a first core) to the other one of the cores (a second core). Accordingly, when a cooler is arranged in the first core to radiate heat from the first core, for example, the plastic prevents heat transfer from the second core. The heat is thus easily accumulated in the second core. This problem also occurs in a case in which an air gap is formed between the cores instead of arranging the plastic between the cores.

To solve the problem, the plastic or the air gap may be omitted so that the cores formed of ferrite are allowed to contact each other to facilitate heat transfer from one core to the other. However, in this configuration, improved DC superposition characteristics cannot be obtained.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide a magnetic core that ensures improved DC superposition characteristics and enhances heat radiation performance.

To achieve the foregoing and in accordance with a first aspect of the present invention, a magnetic core that includes a first core and a second core is provided. The first core has a predetermined magnetic permeability. The second core is formed of the same material as the first core and forms a closed magnetic circuit together with the first core. The second core is configured to radiate heat through a heat radiating unit. At least one of the first core and the second core is configured to be wound with a coil. The magnetic core further includes a third core arranged between the first core and the second core, the third core having a lower magnetic permeability than the first core.

According to a second aspect of the present invention, an induction device having the magnetic core of the first mode and a coil wound around the magnetic core is provided.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A magnetic core according to one embodiment of the present invention will now be described with reference toFIGS. 1A to 1C.

As illustrated inFIGS. 1A to 1C, an I-shaped core12serving as a second core, which is shaped like a flat elongated rectangular plate as viewed from above, is adhered to a heat radiation board11serving as a heat radiating unit (a heat radiator) formed of aluminum. Specifically, the I-shaped core12is fixed to the heat radiation board11and held in tight contact with the heat radiation board11. The I-shaped core12is a ferrite core made of ferrite of, for example, a MnZn based material or a NiMn based material.

A dust core member13ais adhered to the surface of the I-shaped core12opposite to the adhesion surface adhered to the heat radiation board11. The dust core member13ais shaped identically to the I-shaped core12as viewed from above. Also, the dust core member13ais adhered to the I-shaped core12, while being stacked with each other at coinciding positions as viewed from above. In other words, the dust core member13ais fixed to the I-shaped core12and held in tight contact with the I-shaped core12. The dust core member13a, which is shaped like a flat plate, configures a dust core13serving as a third core.

The dust core13(the dust core member13a) is formed by subjecting, to compression molding, dust material, which is powder of, for example, Fe—Al—Si magnetic material having surfaces coated with insulating plastic. The dust core13exhibits lower magnetic permeability and higher saturation magnetic flux density than a ferrite core. The thermal conductivity of the dust core13is preferably set to 8 to 10 [W/mK], which is higher than the thermal conductivity of plastic such as PET (polyethylene terephthalate).

An E-shaped core15serving as a first core is arranged on the surface of the dust core13opposite to the adhesion surface adhered to the I-shaped core12. As a result, the E-shaped core15, the dust core13, the I-shaped core12, and the heat radiation board11are arranged sequentially in this order and held in tight contact. As viewed from above, the E-shaped core15is oriented in an E shape reversed clockwise at 90 degrees. The E-shaped core15is formed of the same material as the I-shaped core12. That is, the E-shaped core15is a ferrite core formed of, for example, a MnZn based material or a NiMn based material.

The E-shaped core15includes a substantially plate-like flat portion15a, a pair of pillar-like first leg portions15b, and a pillar-like second leg portion15c. The flat portion15ais shaped identically to each of the cores12,13as viewed from above. The two first leg portions15bextend from opposite ends of the flat portion15atoward the I-shaped core12. The second leg portion15cextends from the middle of the flat portion15atoward the I-shaped core12. When the E-shaped core15is assembled with the I-shaped core12and the dust core13, the distal surfaces of the first leg portions15band the second leg portion15care in contact, or, specifically, tight contact, with the dust core13. The I-shaped core12, the dust core13, and the flat portion15aof the E-shaped core15are arranged parallel to one another. The E-shaped core15is not fixed to the heat radiation board11unlike the I-shaped core12. The E-shaped core15does not contact the heat radiation board11, which is a heat radiating unit.

A coil16is wound around the second leg portion15cof the E-shaped core15. The coil16is a planar coil formed by punching a copper plate in a rectangular frame-like shape. The coil16is wound around the second leg portion15cto be parallel to the I-shaped core12and the dust core13. The coil16is fixed to a surface, which is a main surface, of a non-illustrated circuit board, for example.

In this manner, the I-shaped core12, the dust core13, and the E-shaped core15configure a magnetic core10. On the other hand, the I-shaped core12, the dust core13, the E-shaped core15, and the coil16configure a reactor20serving as an induction device.

As indicated by arrows Y1inFIG. 1A, the reactor20has a closed magnetic circuit in which a magnetic flux flows from the second leg portion15cto the flat portion15a, the first leg portions15b, the dust core13, the I-shaped core12, the dust core13, and the second leg portion15cor in the opposite direction as the coil16receives electric power. Accordingly, each of the leg portions15b,15cfunctions as a magnetic path forming portion, which is a magnetic leg, for forming a magnetic path by which a magnetic flux proceeds in the direction toward the I-shaped core12or the opposite direction, which is the direction away from the I-shaped core12. The dust core13is arranged between the I-shaped core12and the E-shaped core15. More specifically, the dust core13, which is formed by the single dust core member13a, extends between each of the leg portions15b,15cand the I-shaped core12.

A method for forming, or manufacturing, the magnetic core10and the reactor20will hereafter be described.

First, the I-shaped core12and the dust core13are adhered and fixed to each other. Then, the I-shaped core12, to which the dust core13has been adhered, is adhered and fixed to the heat radiation board11. Subsequently, the coil16is arranged with respect to the I-shaped core12and the dust core13.

Then, by joining the E-shaped core15to the I-shaped core12, the dust core13, and the coil16, the magnetic core10and the reactor20are completed. Specifically, to assemble the E-shaped core15, the second leg portion15cis passed through the coil16while adjusting the positions of the first leg portions15band the second leg portion15cto prevent the first and second leg portions15b,15cfrom contacting the coil16.

Operation of the magnetic core10and that of the reactor20will now be described.

When the electric current flowing in the coil16changes, the magnetic flux in the I-shaped core12and the E-shaped core15changes, thus causing heat generation in the I-shaped core12and the E-shaped core15. The heat produced by the I-shaped core12is transferred from the I-shaped core12to the heat radiation board11, which is in tight contact with the I-shaped core12, and radiated. In other words, the I-shaped core12and the heat radiation board11are thermally connected to each other.

In contrast, the E-shaped core15does not contact a heat radiating unit such as the heat radiation board11. This prevents the heat generated by the E-shaped core15from being transferred directly to the heat radiating unit, which is the heat radiation board11, to be radiated, unlike the heat produced by the I-shaped core12. However, the dust core13, which is arranged between the I-shaped core12and the E-shaped core15, allows the heat produced by the E-shaped core15to transfer to the I-shaped core12through the dust core13and then to the heat radiation board11, as indicated by arrows Y2inFIG. 1A. The heat generated by the E-shaped core15is thus easily radiated. In other words, the E-shaped core15(the leg portions15b,15c) and the I-shaped core12are thermally connected to each other through the dust core13.

The illustrated embodiment has the advantages described below.

(1) The dust core13having lower magnetic permeability than ferrite is arranged between the I-shaped core12and the E-shaped core15both formed of ferrite. This configuration ensures improved DC superposition characteristics. When the electric current in the coil16changes and causes the E-shaped core15to generate heat, the heat is transferred to the I-shaped core12through the dust core13and radiated through the heat radiation board11. This enhances heat radiation performance, in addition to the improved DC superposition characteristics.

(2) The dust core13, which is formed by the single flat plate-like dust core member13a, is arranged between each of the leg portions15b,15cand the I-shaped core12. This configuration decreases the number of the components compared to a configuration in which independent dust cores13are arranged for the respective leg portions15b,15c. The magnetic core10is thus easily manufactured.

(3) The flat plate-like I-shaped core12is fixed to the heat radiation board11and the E-shaped core15includes the leg portions15b,15c. This facilitates fixation of the core to the heat radiation board11compared to a case where the I-shaped core12is replaced by an E-shaped core or an L-shaped core having a pillar-like magnetic path forming portion extending toward the E-shaped core15.

(4) Particularly, in the illustrated embodiment, the I-shaped core12fixed to the heat radiation board11has a flat plate-like shape. This prevents the position of the coil16from being restricted to a specific position due to the core fixed to the heat radiation board11, unlike a case employing an E-shaped core, for example, instead of the I-shaped core12. The coil16is thus easily installed. Further, the E-shaped core15, which includes the leg portions15b,15c, is installed after the coil16is installed. This facilitates assembly of the E-shaped core15without causing contact between the coil16and the E-shaped core15.

(5) The coil16is wound around the E-shaped core15(the second leg portion15c), which is formed of ferrite having high magnetic permeability, not dust material. This decreases the number of winding by which the coil16is wound compared to a case in which the core around which the coil16is arranged is formed of the dust material. The magnetic core10and the reactor20are thus effectively prevented from being enlarged in size.

The present invention is not restricted to the illustrated embodiment but may be embodied in the forms described below.

As illustrated inFIG. 2, the dust core13may be configured by a dust core member13aarranged between the second leg portion15cand one of the first leg portions15b(the left leg portion15b) and the I-shaped core12. In other words, the dust core13may be formed by a single member arranged between two or more of the leg portions15b,15cand the I-shaped core12. In this case, an additional dust core member13bmay be deployed between the other one of the first leg portions15b(the right leg portion15b) and the I-shaped core12. This configuration decreases the number of the components compared to a configuration in which dust core members are employed separately for the respective leg portions15b,15c. As a result, the magnetic core10is easily manufactured.

The dust core13may be configured by a plurality of dust core members that are arranged separately for the respective leg portions15b,15c. In other words, independent dust cores13are deployed for the respective leg portions15b,15c. Each of the dust cores13is arranged between the corresponding one of the leg portions15b,15cand the I-shaped core12.

The I-shaped core12and the dust core13may be shaped or sized differently from the E-shaped core15as viewed from above. For example, as viewed from above inFIG. 3, the dust core13may be larger in size than the E-shaped core15. In this case, the I-shaped core12is larger in size than the dust core13. This configuration prevents any portion of the distal surface of each leg portion15b,15cfrom becoming spaced from the dust core13when the E-shaped core15is installed with its position adjusted with respect to the coil16. Alternatively, the I-shaped core12and the dust core13may be larger in size than the E-shaped core15when the I-shaped core12and the dust core13are sized and shaped identically to each other, as viewed from above.

The present invention may be used in an electronic device having a plurality of reactors20mounted on a heat radiation board11. For example, to form a specific number of (multiple) reactors20with respect to the heat radiation board11, the specific number of I-shaped cores12each having a dust core13adhered to the I-shaped core12are adhered to the heat radiation board11. Then, a single circuit board having at least the specific number of coils16is arranged such that the coils16correspond to the associated I-shaped cores12(the associated dust cores13). Subsequently, the E-shaped cores15are mounted sequentially for the respective coils16to complete the reactors20. In this configuration, compared to a configuration in which an E-shaped core is fixed to the heat radiation board11instead of the I-shaped core12, the coils16formed on the single circuit board are easily arranged such that the multiple reactors20are efficiently formed. Alternatively, some or all of the reactors20may each be configured as a transformer having a plurality of coils16.

The E-shaped core15may be modified to a U-shaped core by removing the second leg portion15c. In this case, a coil16is wound around each first leg portion15b.

The heat radiation board11and the I-shaped core12, as well as the I-shaped core12and the dust core13, may be fixed together by any suitable method other than adhesion. For example, the E-shaped core15may be fixed using a holder that urges the E-shaped core15toward the heat radiation board11.

Instead of the I-shaped core12, an E-shaped core having three pillar-like magnetic path forming portions that extend toward the E-shaped core15, a U-shaped core having two magnetic path forming portions, or an L-shaped core having one magnetic path forming portion may be fixed to the heat radiation board11. In these cases, a flat plate-like I-shaped core without a leg portion or an L-shaped core having one leg portion may be employed instead of the E-shaped core15. However, to facilitate manufacture of the magnetic core, the configuration of the illustrated embodiment is preferable.

The E-shaped core15may be adhered to the heat radiation board11. In this case, the dust core13and the I-shaped core12are joined to the E-shaped core15sequentially in this order. In other words, the E-shaped core15may be caused to radiate heat through the heat radiation board11.

The coil16may be wound around each of the first leg portions15bor the flat portion15aof the E-shaped core15. Alternatively, the coil16may be arranged around the I-shaped core12instead of or in addition to the E-shaped core15.

The I-shaped core12may radiate heat through a heat radiating unit other than the heat radiation board11. For example, by holding the I-shaped core12in tight contact with a case accommodating the magnetic core10and the reactor20, the case is allowed to function as the heat radiating unit. Alternatively, refrigerant may be blasted onto the I-shaped core12.

The I-shaped core12and the E-shaped core15may be formed by a metal ribbon such as a Si steel plate instead of ferrite. Specifically, a core formed of a metal ribbon exhibits high magnetic permeability than the dust core13.

The dust core13(the dust core member13a) may be formed by subjecting powder of metal glass having surfaces coated with insulating plastic to compression molding.

The magnetic core10may be used in a transformer having a plurality of coils16, which serves as an induction device.