Transformer with ferrite core and method for driving it

Abstract: Provided are an Mn-Zn ferrite in which the core loss is low and Bs is high at high temperatures; a transformer capable of being small-sized and suitable to use at high temperatures; and a method for efficiently driving the transformer. Also provided is a transformer capable of being small-sized and suitable to use in a broad temperature range including high temperatures. Further provided are a high-efficiency transformer capable of being small-sized, in which the core loss in the ferrite core is low and the saturation magnetic flux density is high therein at the temperature at which the transformer is driven; and a high-efficiency driving method for the transformer. Depending on its use, Mn-Zn ferrite to be the ferrite core for the transformers comprises, as the essential components, specific amounts of Fe 2 O 3 , ZnO and MnO, and, as the side components, specific amounts of SiO 2 , CaCO 3 , Nb 2 O 5 and ZrO 2 . The core of the transformers is made of the Mn-Zn ferrite.

Description: The invention relates to a transformer with an Mn-Zn ferrite core in which the core loss is low and the saturation magnetic flux density is high at its driving temperature falling between 50 and 70°C or so, and to a method for driving the transformer. Mn-Zn ferrites are much used as materials for transformers for various communication appliances, household and industrial appliances, etc. The temperature at which transformers in conventional switching power supplies are used falls between 60 and 100°C, and the switching frequency for the power supplies falls between 10 and 100 kHz. The minimum core loss in transformers is within the practical temperature range, and various studies of additive addition, element substitution, firing condition change and others have heretofore been made so as to lower the core loss in transformers within the practical temperature range (see Japanese Patent Application Laid-Open (JP-A) Hei-8-169756, etc.). With the recent tendency toward small-sized and lightweight switching power supplies, high frequency has become used for switching frequency. Low core loss materials for a frequency falling between 500 kHz and 1 MHz or so have been developed (see JP-A Hei-8-148323, etc.). However, since transformers generate heat by themselves and are used in high-temperature conditions, the temperature of the transformer core being actually driven is often high to fall between 80 and 110°C. Ferrites for power supplies of that type have been proposed in JP-A Hei-3-141612 and Hei-7-297020. The Mn-Zn ferrites proposed therein have an essential component of ZnO of being not smaller than 10 mol%, and contains an additive of Nb 2 O 5 in the former (Hei-3-141612) or Nb 2 O 5 combined with ZrO 2 in the latter (Hei-7-297020) and additionally SnO 2 and TiO 2 in an amount of not smaller than 300 ppm, to thereby lower the core loss in those ferrites. However, it is still desired that magnetic oxide materials shall satisfy not only low core loss but also high saturation magnetic flux density therein so as to realize small-sized and efficient transformers within the practical temperature range noted above. Mn-Zn ferrites having a ZnO content of not smaller than 10 mol% could realize reduced core loss therein, but could not realize increased saturation magnetic flux density within the practical temperature range noted above, especially at a temperature falling between 100 and 110°C. In this connection, the core loss reduction in Mn-Zn ferrites could be realized by element substitution of adding SnO 2 and TiO 2 to the ferrites. However, no magnetic oxide materials are known capable of realizing increased saturation magnetic flux density within the practical temperature range noted above. On the other hand, where the practical temperature range for magnetic oxide materials is broad, for example, it falls between 20 and 120°C, the materials are indispensably required to satisfy low core loss therein. However, if the saturation magnetic flux density in the materials varies, depending on the temperature at which the power supplies comprising the materials are used, and, as a result, if the permeability of the materials is thereby varied, the inductance of the materials shall vary, depending on the ambient temperature even though the magnetic field around the materials does not vary. For these reasons, therefore, when the materials are used for power smoothing, the ripple in the outputted power will vary, thereby causing the problem that the power-outputting characteristics of the power supplies comprising the materials vary. Mn-Zn ferrites having a ZnO content of not smaller than 10 mol% could realize reduced core loss therein, but the temperature-dependent variation in the saturation magnetic flux density in them is large within the practical temperature range noted above. For the reasons mentioned above, it is desired to develop ferrites and transformers suitable to use at high temperatures and also in a broad temperature range including high temperatures. On the other hand, desired are transformers in which the core loss is low at their driving temperatures under the conditions under which they may generate a small quantity of heat by themselves. For such transformers in that situation, it is still desired to lower the high-frequency core loss in them so as to meet the requirement of small-sized and lightweight switching power supplies. The driving temperature for transformers generally falls between 50 and 70°C. However, with advance knowledge of the generation of heat in transformers, ferrites for transformers may be so designed that the core loss in the transformers comprising them could be the lowest within a temperature range falling between 80 and 100°C (see JP-A Hei-3-141612, Hei-7-297020, Hei-8-169756, etc.). Of those patent publications, JP-A Hei-8-169756 discloses a low core loss Mn-Zn ferrite core which consists essentially of from 25 to 40 mol% of MnO and from 6 to 25 mol% of ZnO with the balance of Fe 2 O 3 and contains, as side components, from 0.002 to 0.040 wt.% of SiO 2 and from 0.02 to 0.20 wt.% of CaO and in which the side component elements are segregated to be in the grain boundaries in such a manner that the half width of their concentration distribution is not larger than 10 nm. They say in JP-A Hei-8-169756 that the side components may further include Nb 2 O 5 , Ta 2 O 5 , ZrO 2 and V 2 O 5 . The composition of the samples actually demonstrated in their Examples was so designed that the core loss in them could be the lowest at 90°C. Briefly, the composition of the essential components in those samples is composed of 53.5 mol% of Fe 2 O 3 , 34.5 mol% of MnO and 12.0 mol% of ZnO. As in JP-A Hei-8-169756, the ferrite is so designed that the core loss therein could be the lowest at temperatures higher then its driving temperature. However, under the conditions under which transformers may generate a small quantity of heat by themselves, it is necessary that the core loss in the transformers is lower within a temperature range of from 50 to 70°C within which the transformers are driven. To meet the requirement, JP-A Hei-8-191011 discloses an Mn-Zn-Co ferrite in which the core loss is reduced within the driving temperature noted above. The additive oxides that may be in the ferrite include SiO 2 , CaO, ZrO 2 and Ta 2 O 5 . However, the Mn-Zn-Co ferrite in JP-A Hei-8-191011 is still problematic in that the core loss therein within the driving temperature range that falls between 50 and 70°C could not be reduced to a satisfactory level, and therefore requires further studies on it. Parent Application No EP-A-0 931 779 is directed to a ferrite in which the core loss is low and the saturation magnetic flux density is high at high temperatures. The present application seeks to provide a high-efficiency transformer capable of being small-sized, of which the core is made of a ferrite of such that the core loss therein is low and the saturation magnetic flux density therein is high within the driving temperature range (between 50 and 70°C) for the transformer. In accordance with the present invention there is provided: (1) A transformer of which the core is made of a ferrite that consists essentially of iron oxide in an amount of from 54 to 55 mol% calculated as Fe 2 O 3 , zinc oxide in an amount of from 6 to 11 mol% calculated as ZnO, and manganese oxide, and contains, as side components, silicon oxide in an amount of from 80 to 150 ppm calculated as SiO 2 , calcium oxide in an amount of from 600 to 1000 ppm calculated as CaCO 3 , niobium oxide in an amount of from 150 to 400 ppm calculated as Nb 2 O 5 , and zirconium oxide in an amount of from 40 to 300 ppm calculated as ZrO 2 , and in which the saturation magnetic flux density (direct current) in the ferrite at 60°C is not smaller than 480 mT, the minimum core loss in the ferrite is within a temperature range falling between 50 and 70°C and the minimum core loss (at a frequency of 100 kHz) is not larger than 260 kW/m 3 . (2) The transformer of (1), wherein the zinc oxide content of the ferrite falls between 6 and 9.5 mol% calculated as ZnO. (3) The transformer of (1) or (2), wherein the saturation magnetic flux density (direct current) in the ferrite at 60°C is not smaller than 500 mT. (4) A method for driving a transformer of any one of (1) to (3) at a temperature falling between 50 and 70°C, at a frequency falling between 20 and 500 kHz and in an exciting magnetic flux density falling between 125 and 500 mT. In JP-A Hei-8-169756, proposed is an Mn-Zn ferrite having a ZnO content of from 6 to 25 mol%, in which the core loss is low at temperatures around 90°C. In this, however, the samples concretely demonstrated in Examples have a ZnO content of 12.0 mol% and an Fe 2 O 3 content of 53.5 mol%. Therefore, the invention of JP-A Hei-8-169756 shall be obviously differentiated from the present invention. In JP-A Hei-8-148323, shown are magnetic materials having a reduced degree of core loss within a range of medium frequency to high frequency bands. In this, the comparative samples referred to in Examples have a ZnO content of 8.0 mol%. However, nothing that relates to addition of Nb 2 O 5 to ferrites is referred to in JP-A Hei-8-148323. Therefore, the technical idea for the invention of JP-A Hei-8-148323 is basically different from that for the present invention. In JP-A Hei-8-191011, shown are Mn-Zn-Co ferrites for transformers in which the core loss at the driving temperature for the transformers is reduced. However, nothing that relates to addition of Nb 2 O 5 to the ferrites is referred to in JP-A Hei-8-191011. Therefore, the technical idea for the invention of JP-A Hei-8-191011 is also basically different from that for the present invention. The present invention is described in detail hereinunder. The ferrite core material for the transformers of the invention comprises a ferrite having the composition mentioned below, in which the saturation magnetic flux density Bs (direct current) at 60°C is not smaller than 480 mT, the core loss is the lowest within a temperature range falling between 50 and 70°C and the minimum core loss (at a frequency of 100 kHz) is not larger than 260 kW/m 3 . Essential Components: Iron oxide in an amount of from 54 to 55 mol%, preferably from 54.5 to 55 mol% (calculated as Fe 2 O 3 ), Zinc oxide in an amount of from 6 to 11 mol%, preferably from 6 to 9.5 mol%, even more preferably from 7.0 to 9.0 mol% (calculated as ZnO), and Manganese oxide as the balance (calculated as MnO). Side Components: Silicon oxide in an amount of from 80 to 150 ppm (calculated as SiO 2 ), Calcium oxide in an amount of from 600 to 1000 ppm (calculated as CaCO 3 ), Niobium oxide in an amount of from 150 to 400 ppm (calculated as Nb 2 O 5 ), and Zirconium oxide in an amount of from 40 to 300 ppm (calculated as ZrO 2 ). The ferrite composition is characterized by high Bs at temperatures falling within the driving temperature range for transformers of from 50 to 70°C or so and by reduced core loss in the temperature range, as discussed in Parent EP-A-0 931 779. Therefore, using the core material of the ferrite of this type in producing transformer cores realizes high-efficiency transformers. More concretely, the ferrite cores in the transformers can be small-sized, and therefore the transformers themselves can be small-sized. In addition, since the core loss in the transformers is low, the power to be consumed by the transformers and also the heat to be generated by them can be reduced. Bs (direct current) at 60°C in the ferrite having the composition mentioned above is not smaller than 480 mT, but preferably not smaller than 500 mT, more preferably not smaller than 505 mT. Though not specifically defined, the uppermost limit of Bs may be 600 mT or so. The temperature range within which the core loss in the ferrite is the lowest, and also the minimum core loss in that temperature range are measured at a frequency falling between 20 and 500 kHz (preferably between 75 and 150 kHz) and in an exciting magnetic flux density falling between 125 and 500 mT (preferably between 200 and 350 mT). The core loss in the ferrite is the lowest within a temperature range falling between 50 and 70°C, and the minimum core loss (at 100 kHz and in 200 mT) is at most 260 kW/m 3 , preferably at most 250 kW/m 3 . Though not specifically defined, the lowermost limit of the core loss may be 100 kW/m 3 or so. Bs in the others of which the composition of the essential components falls outside the ranges noted above could not be high, and the core loss in those will be high. The reason why the Fe 2 O 3 content of the ferrite is defined to fall within the range noted above is as follows. In order that transformers could be most efficiently driven in power supplies and the like within the driving temperature range for the transformers, the core loss in the transformers shall be lowered within the driving temperature range. For this purpose, since the core loss in transformers varies depending on the temperature change therein, it is necessary that the core loss is the lowest within the temperature range within which the transformers are driven or around that temperature range (that is, the driving temperature range for transformers), and that the core loss is low within the driving temperature range. For this, it is well known to control the Fe 2 O 3 content of the ferrite for transformer cores. If the Fe 2 O 3 content is larger than 56 mol%, the core loss in the ferrite cores will be the lowest at temperatures lower than 50°C for the intended temperature range, or that is, the core loss therein will be high within the intended temperature range. Depending on the ratio of Fe/Mn in the ferrite having an Fe 2 O 3 content of from 54 to 55 mol%, the core loss in the ferrite could be the lowest at any temperature falling between 50 and 110°C. Therefore, in accordance with the intended temperature range for transformers, it is possible to select any desired composition of the ferrite in which the core loss could be lower within the temperature range. However, if the Fe 2 O 3 content of the ferrite is larger than 55 mol%, the minimum core loss in the ferrite may appear at lower temperatures, but when it is not larger than 56 mol%, the core loss in the ferrite will be reduced within the intended temperature range. On the other hand, however, if the Fe 2 O 3 content is smaller than 54 mol%, the minimum core loss in the ferrite will appear at higher temperatures, thereby resulting in that the core loss will be high in the intended temperature range. The reason why the ZnO content of the ferrite is defined to fall within the above-mentioned range is because, if it is larger than 11 mol%, Bs in the ferrite will lower, and if smaller than 6 mol%, the core loss therein will increase. The compositional range of the side components in the ferrite is defined essentially for the purpose of realizing the reduction in the core loss in the ferrite. If the amounts of the side components constituting the ferrite are outside the defined ranges, the core loss in the ferrite will increase. SiO 2 and CaO form an intergranular phase in the ferrite, while contributing to the increase in the resistance of the ferrite. Accordingly, if their amounts added to the ferrite are too small, their contribution to the resistance increase will be small; but if too large, they will cause abnormal grain growth in the ferrite whereby the core loss in the ferrite will increase. For these reasons, therefore, the amount of SiO 2 to be in the ferrite is defined to fall between 80 and 150 ppm, and that of and CaCO 3 is defined to fall between 600 and 1000 ppm. Nb 2 O 5 and ZrO 2 added to the ferrite along with SiO 2 and CaCO 3 act to prevent abnormal grain growth in the ferrite, while contributing to forming a thin and uniform high-resistance phase in the grain boundaries in the ferrite. In that manner, Nb 2 O 5 and ZrO 2 act to further lower the core loss in the ferrite. However, if their amounts are larger than the defined ranges, the side components will cause abnormal grain growth in the ferrite, thereby increasing the core loss therein. If, on the contrary, their amounts are smaller than the defined ranges, the side components will be ineffective and could not contribute to the reduction in the core loss in the ferrite. Using Ta 2 O 5 in place of Nb 2 O 5 will increase the core loss in the ferrite. The mean grain size of the ferrite may generally fall between 10 and 30 µm or so. In order to realize the reduction in the core loss in the ferrite, if desired, additional side components of tin oxide (its amount is calculated as SnO 2 ) and/or titanium oxide (its amount is calculated as TiO 2 ) may be added to the ferrite in an amount of not larger than 5000 ppm. However, the tetra-valent non-magnetic cations Sn and Ti may form solutes in the grains constituting the ferrite, thereby reducing Bs in the ferrite by a few %. Cobalt oxide may also be added to the ferrite. In practical use, the cobalt oxide content of the ferrite could fall between 0 and 3000 ppm or so, calculated as CO 3 O 4 . The effect of the present invention characterized in that the core loss in the ferrite is low and Bs therein is high within the predetermined temperature range noted above is further augmented when the amounts of the components constituting the ferrite fall within the preferred ranges noted above. The transformer of the invention has a core which is made of the ferrite having the composition and the characteristics mentioned above, and is therefore well driven at a temperature falling between 50 and 70°C, at a frequency falling between 20 and 500 kHz (preferably between 75 and 150 kHz) and in an exciting magnetic flux density falling between 125 and 500 mT (preferably between 200 and 350 mT). To produce the ferrite cores for transformers of the invention, first prepared is a mixture of predetermined amounts of an iron oxide component, a manganese oxide component and a zinc oxide component. For the other embodiment of the ferrite cores for transformers of the invention, oxide cobalt may be added to the mixture in this stage. Compounds for the side components mentioned above are added to the mixture of the essential components. Their amounts shall be so controlled that the final mixture may have the predetermined compositional ratio. After the essential components and the side components have been mixed in that manner, a small amount of a suitable binder, for example, polyvinyl alcohol (PVA) is added thereto, and the resulting mixture is granulated, using a spray drier or the like, into grains having a grain size falling between 80 and 200 µm or so. Next, the grains are molded, and the resulting moldings are fired in an atmosphere having a controlled oxygen concentration, at a predetermined temperature falling between 1250 and 1400°C. The transformer of the invention may be produced by coiling a predetermined wire around the ferrite core having been molded in the manner mentioned above to have a predetermined shape. The shape and the size of the core may be varied, depending on the object and the use of the transformer to be produced. For example, the core may have any shape of toroidal forms, E-shaped forms, RM-shaped forms, ET-shaped forms, UU-shaped forms, FT-shaped forms, PQ-shaped forms, etc. The transformers of the invention can be small-sized. For example, for those suitable to use at high temperatures, the size of the core may be from 5 to 40 % smaller than that of conventional cores. The coiling count for the core may be varied, depending on the object and the use of the transformer to be produced. The transformer of the present invention can be used in switching power supplies for much small-sized and power-saving electronic appliances. In addition, the transformer is also usable in power supplies to be mounted on electric cars and hybrid cars. The invention is described in more detail hereinunder with reference to the following Examples, which, however, are not intended to restrict the scope of the invention. Example 1: Essential components of Fe 2 O 3 , MnO and ZnO were weighed each in a predetermined amount as in Tables 1 and 2, wet-blended, and then calcined in air at 850°C for 2 hours. Depending on the amounts of the essential components, predetermined amounts of side components of SiO 2 , CaCO 3 , Nb 2 O 5 and ZrO 2 were added to the resulting mixture, and wet-milled to obtain a ferrite powder. 0.8 % by weight, calculated as its solid content, of a binder of PVA was added to the powder, and granulated. The resulting grains were molded under a pressure of 1 ton/cm 2 into toroidal moldings (outer diameter: 24 mm, inner diameter: 12 mm, thickness: 5.5 mm). The moldings were fired in a mixed gas atmosphere of N 2 -O 2 having a controlled oxygen partial pressure, at a firing temperature of 1300° C for 5 hours. Thus were obtained toroidal cores having an outer diameter of 20 mm, an inner diameter of 10 mm and a thickness of 5 mm. Using Ta 2 O 5 in place of Nb 2 O 5 , a different core sample was produced in the same manner as herein. Through its fluorescent X-ray spectrometry, each final ferrite composition was verified to correspond to the composition of the starting components. Each core sample obtained herein was tested in an alternating current B-H analyzer (IWATSU-SYS232) for the core loss therein at an exciting magnetic flux density of 200 mT and at a frequency of 100 kHz. In addition, it was tested in a direct current B-H analyzer (YEW4192) for the saturation magnetic flux density Bs therein in a magnetic field of 15 Oe and at a temperature of 60°C. Tables 1 and 2 show the data of Bs and the minimum core loss in each sample, and the temperature at which the core loss was minimized. From the data in Tables 1 and 2, it is known that Bs in the samples of the invention is high within a temperature range (50 to 70°C) within which transformers may be driven and that the core loss therein is low within the driving temperature range. As opposed to those, the core loss in the sample having a ZnO content of smaller than 6 mol% is high (Sample No. 409), and Bs in the sample having a ZnO content of larger than 11 mol% is low (Sample No. 410). In the sample having an Fe 2 O 3 content of larger than 56 mol%, the minimum core loss appears at a low temperature, and the core loss at temperatures around the driving temperature range is high (Sample No. 411). In the samples in which the amounts of the side components all overstep the ranges defined herein, the core loss at temperatures around the driving temperature range is high (Samples Nos. 413 to 419). In the sample containing Ta 2 O 5 in place of Nb 2 O 5 , the core loss is high (Sample No. 420). In ferrite cores having an Fe 2 O 3 content of smaller than 54 mol%, the minimum core loss appears within a temperature range of from 100 to 110°C; and the core loss in those cores within the driving temperature range is high (for example, 431 kW/m 3 at 60°C). Therefore, the ferrite cores having such a small Fe 2 O 3 content are not suitable to use in transformers to be driven within the temperature range of from 50 to 70°C. Example 2: To the composition of Sample 402 in Example 1 (Table 1), cobalt oxide was added in an amount of 2000 ppm calculated as Co 3 O 4 to prepare Sample No. 402A. In this, cobalt oxide was mixed with the essential components, Fe 2 O 3 , MnO and ZnO prior to being calcined. The sample was evaluated for its characteristics in the same manner as in Example 1. The data obtained are shown in Table 3. Sample No. Bs (60°C) (mT) Minimum Core Loss (kW/m 3 ) Temperature of Minimum Core Loss (°C) 2A (cobalt oxide added) 520 260 60 The data in Table 3 verify the practical level of this sample. Example 3: Using the ferrite core of Sample No. 401 in Example 1 of the invention, produced was a transformer A. Briefly, the ferrite core was PQ-shaped, and its size was 59 mm x 42 mm x 27 mm (metatarsal diameter: 24 mm). For the coiling count for the core, the primary coil was in one turn, and the secondary coil was in 4 turns. The transformer A was found to be well driven at a temperature of 60°C, at a frequency of 100 kHz and in an exciting magnetic flux density of 200 mT. According to the present invention, obtained are ferrites in which the core loss is low and Bs is high at high temperatures. Also obtained are transformers capable of being small-sized and suitable to use at high temperatures. The transformers can be efficiently driven even at high temperatures. In addition, the transformers are suitable to use in a broad temperature range including high temperatures, and can be small-sized. Also according to the invention, obtained are high-efficiency transformers capable of being small-sized, in which the core loss in the ferrite core is low and the saturation magnetic flux density is high therein at the temperature at which the transformers are driven. The transformers of the invention can be efficiently driven according to the driving method for them of the invention. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof.

Claims: A transformer of which the core is made of a ferrite that consists essentially of iron oxide in an amount of from 54 to 55 mol% calculated as Fe 2 O 3 , zinc oxide in an amount of from 6 to 11 mol% calculated as ZnO, and manganese oxide, and contains, as side components, silicon oxide in an amount of from 80 to 150 ppm calculated as SiO 2 , calcium oxide in an amount of from 600 to 1000 ppm calculated as CaCO 3 , niobium oxide in an amount of from 150 to 400 ppm calculated as Nb 2 O 5 , and zirconium oxide in an amount of from 40 to 300 ppm calculated as ZrO 2 , and in which the saturation magnetic flux density (direct current) in the ferrite at 60°C is not smaller than 480 mT, the minimum core loss in the ferrite is within a temperature range falling between 50 and 70°C and the minimum core loss (at a frequency of 100 kHz) is not larger than 260 kW/m 3 . A transformer as claimed in Claim 1 wherein the zinc oxide content of the ferrite falls between 6 and 9.5 mol% calculated as ZnO A transformer as claimed in claim 1 or Claim 2, wherein the saturation magnetic flux density (direct current) in the ferrite at 60°C is not smaller than 500 mT. A method for driving a transformer as claimed in any one of claims 1 to 3 comprising driving the transformer at a temperature falling between 50 and 70°C, at a frequency falling between 20 and 500 kHz and in an exciting magnetic flux density falling between 125 and 500 mT.

Inventors: KURODA TOMOFUMI, OKAMOTO SHIGEO, SAITA HITOSHI, SATO NAOYOSHI, KURODA, TOMOFUMI, OKAMOTO, SHIGEO, SAITA, HITOSHI, SATO, NAOYOSHI
Applicants: TDK CORP, TDK CORPORATION
Application year: 1999
Main classification: C04B35
IPC codes: H01F1, H01F27, C04B35