Laminate type varistor

Abstract: A laminate type varistor has at least one pair of first and second inner electrodes; a varistor layer, the at least one pair of first and second electrodes and the varistor layer being laminated; and a first outer electrode and a second outer electrode electrically connected to the first inner electrode and a second inner electrode, respectively. In the laminate type varistor, the first inner electrode and the first inner electrode are separated by a predetermined distance from the outer electrode so that the first inner electrode has no electrode surface facing to an electrode surface of the second inner electrode.

Description: The present invention relates to a laminate type varistor adapted for being incorporated into a small-capacitance high-frequency circuit, or the like. Conventionally, as shown in Fig. 5, a laminate type varistor is configured in the following manner. That is, at least two inner electrodes 20a and 20b making a pair to each other and a varistor layer 21 are laminated. Ceramic layers 22 and 23 are provided as outermost layers for protecting the laminate. The inner electrodes 20a and 20b are electrically connected to outer electrodes 24 and 25 respectively. The varistor layer 21 has a dielectric constant. The inner electrodes 20a and 20b are formed to have surfaces W facing each other through the varistor layer 22 (Unexamined Japanese Patent Publication (kokai) Nos. Hei. 5-6806 and Hei. 5-6807). Similarly, conventionally, also in the case where pairs of inner electrodes 30a, 30b; 31a, 31b;... are provided in a laminate type varistor as shown in Fig. 6. The laminate type varistor is configured in the following manner. That is, the inner electrodes 30a, 30b; 31a, 31b;... are formed so that the inner electrodes have surfaces W facing each other through varistor layers 32a, 32b... respectively. Protection ceramic layers 33 and 34 are provided as outermost layers. The inner electrodes 30a, 30b; 31a, 31b;... are electrically connected to outer electrodes 35 and 36 respectively (Unexamined Japanese Patent Publication (kokai) Nos. Hei. 5-283208 and Hei. 8-55710). In the laminate type varistor configured as described above, the capacitance increases as the facing surfaces W of the inner electrodes 20a, 20b, 30a, 30b, 31a, 31b... increase in terms of areas. However, if the capacitance is large, a high-frequency signal may be passed through the varistor or the waveform of the signal may be distorted in the case where the varistor is used in a high-frequency circuit. To prevent this problem, it is necessary to set the capacitance to a value of about several tens of pF. In the aforementioned configuration, however, it is difficult to set the capacitance to a value of about several tens of Pf. US 5,324,986 describes a laminate type varistor, wherein a first and second inner electrode and at least one non-connected type inner electrode are embedded into a ceramics sintered body. The first and second inner electrodes are electrically connected to outer electrodes, and the non-connected type inner electrode is not electrically connected to the outer electrodes. The non-connected type inner electrode may be arranged so as to be overlapped with both the first and second inner electrodes while being separated by a ceramics layer. Alternatively, a plurality of non-connected type inner electrodes may be arranged so as to be respectively overlapped with the first and second inner electrodes. The reason for the arrangement of the first and second inner electrodes and the non-connected type inner electrodes is to effect a reduction of a generation of mesh-shaped holes. It is the object of the present invention to provide a laminate type varistor in which the capacitance can be set to a small value while a varistor voltage is kept in a value equivalent to that of a conventional varistor. This object is solved by the subject matters of the independent claims. Preferred embodiments are defined in the dependent claims. A laminate type varistor according to the present invention comprises at least one pair of first and second inner electrodes; a varistor layer, the at least one pair of first and second electrodes and the varistor layer being laminated; and a first outer electrode and a second outer electrode electrically connected to the first inner electrode and a second inner electrode, respectively, wherein the first inner electrode and the second inner electrode are separated by a predetermined distance from the outer electrode so that the first inner electrode has no electrode surface facing to an electrode surface of the second inner electrode. A laminate type varistor according to the present invention comprises: a ceramic sintered body comprising at least one pair of first and second inner electrodes; a varistor layer, the at least one pair of first and second electrodes and the varistor layer being laminated; and protection ceramic layers as outermost layers of the ceramic sintered body; and a first outer electrode and a second outer electrode electrically connected to the first inner electrode and a second inner electrode, respectively; wherein a width of the ceramic sintered body is equal to or longer than the sum of the lengths of the first and second inner electrodes. In the accompanying drawings: Fig. 1 is an explanatory view showing a laminate type varistor according to an embodiment of the present invention; Fig. 2 is an explanatory view showing a laminate type varistor according to another embodiment of the present invention; Fig. 3A is an explanatory view showing a laminate type varistor according to a further embodiment of the present invention; Fig. 3B is an explanatory view showing a laminate type varistor of a modified embodiment of the embodiment shown in Fig. 3A; Fig. 4 is a perspective view showing the structure of an inner electrode; Fig. 5 is an explanatory view showing a laminate type varistor as a conventional example; and Fig. 6 is an explanatory view showing a laminate type varistor as another conventional example. The present invention will be described in detail below with reference to the accompanying drawings. Each of the laminate type varistors shown in the drawings is configured in the following manner. That is, ceramic green sheets are formed from a ceramic material containing ZnO as a main component. Electric-conductive paste of Pd, Ni, Ag-Pd, or the like, is printed on each of the ceramic green sheets to form inner electrodes. The ceramic green sheets are laminated and baked to obtain a ceramic sintered body having protection ceramic layers as outermost layers. Then, Ag or Cu baked layers are plated with Ni, Sn, solder, or the like, to thereby provide outer electrodes on outer surfaces of the ceramic sintered body so that the outer electrodes are electrically connected to the inner electrodes. A laminate type varistor 61 shown in Fig. 1 is configured in the following manner. That is, two inner electrodes, a first inner electrode 1a and a second inner electrode 1b, make a pair to each other. A varistor layer 2 are laminated and baked to thereby obtain a ceramic sintered body 62 having protection ceramic layers 3 and 4 as outermost layers. The first and second inner electrodes 1a and 1b are electrically connected to first and second outer electrodes 5 and 6, respectively. A laminate type varistor 71 shown in Fig. 2 has a plurality of pairs of first and second inner electrodes 10a, 10b; 11a, 11b;... This laminate type varistor 72 is configured in the following manner. That is, a plurality of varistor layers 12a, 12b... and protection ceramic layers 13 and 14 as outermost layers are laminated and baked to thereby obtain a ceramic sintered body 72. First and second outer electrodes 15 and 16 are provided so as to be electrically connected to the pairs of the first and second inner electrodes 10a, 10b; 11a, 11b;... In the case where about six layers are to be laminated as the varistor layers in the multilayer-structure laminate type varistor 71, each layer may be formed to have a thickness of about 60 µm. In the laminate type varistors 61, 71 shown in Figs. 1 and 2, the first and second inner electrodes 1a, 1b; 10a, 10b; 11a, 11b;... each making a pair to each other are separated by a predetermined distance L 1 from each other so that the respective pairs of the first and second inner electrodes 1a, 1b; 10a, 10b; 11a, 11b;... are formed respectively on the same planes which are the varistor layers 2; 12a; 12b;... so that the pairs of the first and second inner electrodes have no surfaces facing each other. For example, as shown in Fig. 4, the inner electrode 1a(1b) has an electrode surface 51 and a tip end surface 52. In this case, the electrode surface 51 of the first inner electrode 1a does not face to that of the second inner electrode 1b. In the laminate type varistors 61, 71, the varistor voltage and capacitance are affected by the distance L 1 by which the pairs of the first and second inner electrodes 1a, 1b; 10a, 10b; 11a, 11b;... are separated from each other. For example, when the varistor voltage is 12 V, the separation distance L 1 may be set to about 66 µm. For example, when the varistor voltage is 27 V, the separation distance L 1 may be set to about 120 µm. As shown in Fig. 1, a width L 3 of the ceramic sintered body 62 is longer than the sum of a length L 4 of the first inner electrode 1a and a length L 5 of the second inner electrode. Preferably, the width L 3 is from more than 0 to not more than 800µm. In the present invention, the distance L 1 is preferably not more than half of the width L 3 . These relationships among the lengths, width and distance is also applied to the ceramic sintered body 72 as shown in Fig. 2. Besides the case where the pairs of first and second inner electrodes 1a, 1b; 10a, 10b; 11a, 11b;... are formed respectively on the same planes which are the varistor layers 2, 12a, 12b... For example, the inner electrodes 1a and 1b making a pair to each other may be separated from each other by a predetermined separation distance L 2 in the thickness direction as seen in the laminate type varistor 81 shown in Fig. 3A so that the first and second inner electrodes 1a and 1b are disposed in different planes separated by the varistor layer 2, but they are formed as inner electrodes having no surfaces facing each other. In this case, the separation distance L 2 by which the first and second inner electrodes 1a and 1b are separated from each other, can be secured by the distance between the inner ends where the first and second inner electrodes 1a and 1b do not face each other and the thickness of the varistor layer 2 interposed between the first and second inner electrodes 1a and 1b. Further, in addition to the embodiment shown in Fig. 3A, it is possible to form the distance L 1 between the first and second inner electrodes 1a and 1b, in the varistor as shown in Fig. 3B. The lengths, thickness and distance relationships of Figs. 1 and 3A can also be applied to a ceramic sintered body 92 of a varistor 91 as shown in Fig. 3B. In case of Figs. 3A and 3B, a thickness L 6 of the ceramic sintered body is preferably from not less than 0 to not more than 800µm. The distance L 2 is less than the thickness L 6 . Further, in this case, the width L 3 is equal to or longer than the sum of the length L 4 of the first inner electrode 1a and the length L 5 of the second inner electrode 1b. In comparison with the characteristic of conventional laminate type varistors having 1 varistor layer and 6 varistor layers, and the present invention's laminate type varistors having 1 varistor layer, 6 varistor layers and 27 varistor layers respectively were produced on the basis of the configurations of the laminate type varistors shown in Figs. 1 and 2. The results of comparison about the characteristic are shown in the following Table 1. The capacitance (pF) was reduced extremely in comparison with that of the conventional laminate type varistor. Also the withstand electrostatic voltage resistance measured in terms of the rate of the change of the varistor voltage after 100 times repetition of a pulse of 30 KV was substantially equivalent to or better than that of the conventional laminate type varistor. Conventional Present invention 1 layer 6 layers 1 layer 6 layers 27 layers Sintering Temperature (°C) 1165 1134 1200 1150 1320 1225 1280 1210 1250 1170 Varistor Voltage (V) 12 27 12 27 12 27 12 27 12 27 Capacitance (pF) 205 110 1050 420 70 20 90 40 95 50 Electrostatic Voltage Resistance (%) -6 -4 0 0 -9 -7 -5 -3 0 0 Further, as seen from the above Table 1, the varistor voltage is determined by the separation distance by which the inner electrodes are separated from each other. Accordingly, a laminate type varistor having a target characteristic can be obtained easily if the separation distance and the total number of varistor layers are adjusted in accordance with the required value of capacitance. Incidentally, the baking temperature in the present invention is set to be more or less higher than that in the conventional case. This is because the number of varistor layers is increased in number by space for separating the inner electrodes from each other are interposed. As described above, in the laminate type varistor according to the present invention, inner electrodes making a pair to each other are separated from each other so that the inner electrodes are formed to have no electrode surfaces facing each other. Accordingly, the capacitance can be set to a small value while the varistor voltage is kept in a value equivalent to that of the conventional laminate type varistor. Even in the case where the varistor according to the present invention is used in a high-frequency circuit, the high-frequency signal can be prevented from passing through the varistor or the waveform of the signal can be prevented from being distorted. Further, because the varistor voltage can be determined by the separation distance by which the inner electrodes are separated from each other, a laminate type varistor having a target characteristic can be obtained easily if the separation distance and the total number of varistor layers are adjusted in accordance with the required value of capacitance.

Claims: A laminate type varistor (61,71,81,91) comprising: at least one pair of first and second inner electrodes (1a, 1b; 10a, 10b; 11a, 11b; ... ); a varistor layer (2), said at least one pair of first and second inner electrodes and said varistor layer being laminated; and a first outer electrode (5, 15) and a second outer electrode (6, 16) electrically connected to said first inner electrode and a second inner electrode, respectively; wherein said first inner electrode and said second inner electrode are separated by a predetermined distance (L1) from each other so that said first inner electrode has no electrode surface facing to an electrode surface of said second inner electrode; characterized in that each inner electrode of the laminate type varistor is electrically connected to either said first outer electrode or said second outer electrode. The laminate type varistor according to claim 1, wherein said first and second inner electrodes of said pair are formed on different planes, separated through said varistor layer. The laminate type varistor according to claim 1, wherein said pair of first and second inner electrodes is formed on one and the same plane of said varistor layer. The laminate type varistor according to one of claims 1 to 3, wherein a plurality of said pair of first and second inner electrodes are provided. A laminate type varistor (61,71,81,91) consisting in : a ceramic sintered body (62, 72, 82, 92) comprising at least one pair of first and second inner electrodes (1a, 1b; 10a, 10b; 11a, 11b; ... ); a varistor layer (2), said at least one pair of first and second inner electrodes and said varistor layer being laminated; and protection ceramic layers (3, 4, 13, 14) as outermost layers of said ceramic sintered body; and a first outer electrode (5, 15) and a second outer electrode (6, 16) electrically connected to said first inner electrode and said second inner electrode, respectively; wherein a width (L3) of said ceramic sintered body is equal to or longer than the sum of the lengths of said first (L4) and second (L5) inner electrodes; characterized in that each inner electrode of the laminate type varistor is electrically connected to either said first outer electrode or said second outer electrode. The laminate type varistor according to claim 5, wherein the width of said ceramic sintered body is longer than the sum of the lengths of said first and second inner electrodes. The laminate type varistor according to claim 5 or 6, wherein a distance between said first and second inner electrodes is not more than half of the width of said ceramic sintered body. The laminate type varistor according to claim 6 or 7, wherein said pair of first and second inner electrodes is formed on one and the same plane of said varistor layer. The laminate type varistor according to one of claims 5 to 7, wherein said first and second inner electrodes of said pair are formed on different planes, separated through said varistor layer. The laminate type varistor according to one of claims 5 to 7 or 9, wherein a thickness of said ceramic sintered body is longer than a distance (L2) in a thickness direction between said first and second inner electrodes.

Inventors: OGASAWARA TADASHI, TAKEHANA MIKIKAZU, TANAKA RYUICHI, OGASAWARA, TADASHI, TAKEHANA, MIKIKAZU, TANAKA, RYUICHI
Applicants: TDK CORP, TDK CORPORATION
Application year: 1998
Main classification: H01C7
IPC codes: H01C7