In recent years, so-called low-pollution vehicles such as fuel cell cars, electric cars, hybrid cars and the like have been developed. On the other hand, in contrast to roll out of vehicles with such equipment, there have been large increases in solar power generating capacity, wind power generating capacity, natural refrigerant heat pump water heater capacity and the like. In particular, hybrid cars have increased in Japan and abroad. In such a hybrid car or the like, the voltage of a battery is stepped down to the voltage for electrical components, and direct current is converted to high-frequency alternating current in a case in which a motor or the like is inverter-controlled, via a switching power source or the like.
In a circuit of the above switching power source, a reactor consisting of a core (magnetic core) and a coil wound around the core is arranged. As a property of the reactor, in addition to small size, low loss, and low noise, reliable inductance characteristics with a wide range of direct current is necessary, that is, superior direct current superposition characteristics. Therefore, as a core for reactor, a core in which iron loss is low and magnetic permeability is reliable from low magnetic fields to high magnetic fields, that is, a core having superior constant magnetic permeability, is desirable.
Generally, a core for a reactor is constructed from material such as silicon steel plates, amorphous ribbons, ferrite oxide or the like, and the core constructed from these materials is produced by stacking of flat plate materials, powder compacting forming, powder compacting sintering or the like. Furthermore, in order to improve direct current superposition characteristics, apparent magnetic permeability is controlled by forming an appropriate gap in a magnetic path of the core.
Accompanied by increase in power output of a motor, inverter, or the like, the core of a reactor or the like has been required to be used under large currents and stronger magnetic fields. In such a core for a reactor, it is desirable that inductance not decrease even in higher magnetic fields. However, in the core constructed from the above materials such as silicon steel plates, amorphous ribbons, ferrite oxide or the like, magnetic flux density is saturated at higher magnetic fields since they are highly magnetically permeable materials, and as a result, inductance may be decreased. In order that such a core in which inductance is greatly varied by superposed current may be used in a reactor, design is required so that gaps of the core are increased in thickness, the number of gaps is increased, or the like. However, such design of a core may result in leakage of magnetic flux, increase in loss, increase in noise, and increase in size of the reactor. This is undesirable for use installed in a vehicle or the like in which good fuel economy performance is required and installation space is limited. In addition, since the assembly processes increase, it is disadvantageous from the viewpoint of production cost.
As a core that is different in material organization structure, a powder magnetic core produced by a compression forming a soft magnetic metallic powder such as iron is known. Compared to a laminated magnetic core of silicon steel plates or the like, the powder magnetic core has good material yield during production, and therefore, material cost can be reduced. Furthermore, there is greater freedom of forming, and characteristics can be improved by appropriately designing the shape of the magnetic core. Furthermore, by improving electrical insulating characteristics among metallic powders by mixing electrically insulating material such as organic resin, inorganic powder, or the like into the metallic powder, or by coating the surface of the metallic powder with an electrically insulating coating, eddy-current loss in the magnetic core can be greatly reduced, and superior magnetic characteristics can be obtained, particularly in high frequency ranges. From these characteristics, attention has been drawn to the powder magnetic core as a core for a reactor.
Conventionally, as a raw material for a core for a reactor, material such as silicon steel plates in which 3 to 6.5% of Si is contained in Fe, has been used. However, the silicon steel plate is hard and has poor characteristics for forming into shape. Therefore, from the viewpoints of low cost and superior shaping characteristics, use of powder magnetic cores in which soft magnetic powder having an insulating coating on the surface thereof is compact-formed, has been increasingly common (See patent document 1, for example).
As a method of production of the powder magnetic core, a method is known in which an inorganic insulating coating is formed on a surface of the soft magnetic powder, thermosetting resin powder is mixed into the soft magnetic powder, the powder mixture is compressed and formed, and resin hardening treatment is performed on the resultant powder compact (See patent document 2, for example). Furthermore, since further lower iron loss in powder magnetic cores has been required in recent years, a method is known in which compression forming is performed to obtain a powder compact (powder magnetic core), heat treatment is performed to loosen distortion due to powder compacting forming and hysteresis loss is reduced (See patent document 3, for example).
Patent documents are as follows:    Patent document 1: Japanese Unexamined Patent Application Publication No. Hei 09 (1997)-102409    Patent document 2: Japanese Unexamined Patent Application Publication No. Hei 09 (1997)-320830    Patent document 3: Japanese Unexamined Patent Application Publication No. 2000-235925
In a powder magnetic core, more reliable superposition characteristics can be obtained compared to silicon steel plates or the like; however, it has thus far been impossible to construct a reactor without a magnetic gap, and reactance is adjusted by dividing a core used for the reactor and by filling gap material between the divided core. However, in this case, it is very complicated to assemble the reactor while arranging the gap materials between the divided core and aligning the divisions. Here, if the core used for the reactor has superior superposition characteristics, there may be no need to divide the core, and assembly of the reactor may be facilitated, enabling reduction of the gap material arranged between the divided core, and as a result, magnetic flux leakage can be controlled, loss can be reduced, noise can be reduced, and the size of the reactor can be reduced.
In view of the above circumstances, an object of the present invention is to provide a powder magnetic core in which reliable superposition characteristics are exhibited such that variation in inductance value is small even if superposition current is varied, and in which the number of cores used in the reactor can be reduced.
In order so solve the above subject, as a result of the inventors' research, it was found that by forming layered gaps inside the powder magnetic core, the powder magnetic core can exhibit superior superposition characteristics without dividing the core or arranging gap material, and the present invention was completed.