Varistor

Abstract

The present disclosure specifies a varistor comprising a ceramic body, which comprises a functional ceramic, and electrodes arranged inside the ceramic body. The electrodes include non-floating electrodes, which are electrically connected to external contacts of the varistor, respectively. The electrodes include at least three floating electrodes, which are electrically isolated with respect to the external contacts. At least two floating electrodes are arranged in the same layer, and each of the floating electrodes overlaps with at least two further electrodes. At least two floating electrodes overlap with one of the non-floating electrodes, respectively. A distance (D1) is defined along a longitudinal axis of the ceramic body between two of the electrodes overlapping with a first floating electrodes, and a distance (D2) is defined perpendicular to the longitudinal axis between the first floating electrode and one of the overlapping electrodes. The distance (D1) is at least twice the distance (D2).

Claims

1-15. (canceled)

16. A varistor comprising: a ceramic body comprising a functional ceramic; electrodes arranged inside the ceramic body, the electrodes including non-floating electrodes that are electrically connected to external contacts of the varistor, respectively, the electrodes further including at least three floating electrodes that are electrically isolated with respect to the external contacts; and wherein at least two floating electrodes are arranged in the same layer, and wherein each floating electrode overlaps with at least two further electrodes, and wherein at least two floating electrodes overlap with one of the non-floating electrodes, and wherein a distance (D1) is defined along a longitudinal axis of the ceramic body between two of the electrodes overlapping with a first floating electrodes, and a distance (D2) is defined perpendicular to the longitudinal axis between the first floating electrode and one of the overlapping electrodes, and wherein the distance (D1) is at least twice the distance (D2).

17. The varistor according to claim 16, wherein the floating electrodes overlapping each other are arranged in two parallel layers along the longitudinal axis of the ceramic body.

18. The varistor according to claim 16, wherein the floating electrodes are arranged in multiple parallel layers along a longitudinal axis of the ceramic body.

19. The varistor according to claim 16, wherein the functional ceramic comprises metal oxide.

20. The varistor according to claim 19, wherein the functional ceramic comprises grains.

21. The varistor according to claim 15, wherein the grains have a diameter between 100 nm and 20 ?m.

22. The varistor according to claim 21, wherein at least two abutting grains are arranged in series in between adjacent electrodes.

23. The varistor according to claim 16, wherein the distance between adjacent electrodes is between 400 nm and 20 ?m.

24. The varistor according to claim 16, wherein the electrodes arranged inside the ceramic body are flat.

25. The varistor according to claim 15, wherein the electrodes have a thickness between 1 and 3 ?m.

26. The varistor according to claim 16, wherein the electrodes comprise silver, palladium, copper, another metal or a combination thereof.

27. The varistor according to claim 16, wherein the varistor is manufactured by multilayer technology.

28. The varistor according to claim 16, wherein the height of the ceramic body measured perpendicular to the longitudinal axis is 100 ?m or less.

29. The varistor according to claim 16, wherein the overlap between the at least one floating electrode and the two further electrodes is at least 5% and at most 45% of the extension of the floating electrode in direction of the longitudinal axis.

30. The varistor according to claim 16, wherein the external contacts are shaped as caps.

31. The varistor according to claim 16, wherein the functional ceramic comprises grains, wherein the grains have a diameter between 100 nm and 20 ?m.

32. The varistor according to claim 16, wherein the thickness of the ceramic body is 100 ?m or less, and the thickness of the electrodes is between 1 and 3 ?m, the number of electrode layers is less than the ceramic body thickness divided by the electrode thickness.

33. The varistor according to claim 16, further including metal oxide grains, the diameter of metal oxide grains is in the range of 5 ?m to 20 ?m.

34. The varistor according to claim 33, wherein the metal oxide grains are selected from zinc oxide, bismuth oxide, chrome oxide or manganese oxide that show highly non-linear electrical characteristics, and wherein a mixture of the metal oxide as well as doping of the metal oxide improves the performance of the varistor.

35. The varistor according to claim 16, wherein the ceramic body is rectangular in shape, the two non-floating electrodes protrude into the ceramic body on each of two opposing ends, both non-floating electrodes overlap with different floating electrodes, and both of the floating electrodes overlap again with two floating electrodes that are located in a middle region of the ceramic body and stacked upon each other.

36. The varistor according to claim 16, wherein nine non-floating electrodes protrude into the ceramic body from the first and second opposing ends, and wherein, inside the ceramic body, multiple parallel layers of floating electrodes are arranged along the longitudinal axis from the first end to the second end.

37. The varistor according to claim 16, wherein a charge carrier must travel at least fourteen times through the functional ceramic to get from a non-floating electrode at a first end to a non-floating electrode at a second end that opposes the first end.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In the following the invention is described based on embodiments with reference to the figures. The figures serve solely to illustrate the invention and are therefore only schematic and not drawn to scale. Some parts may be exaggerated or distorted in dimension. Therefore, neither absolute nor relative dimensions can be taken from the figures. Identical parts or parts with equivalent effect are referred to by the same reference number.

[0027] FIG. 1 shows a cross section of a first embodiment of a varistor according to the present invention;

[0028] FIG. 2 shows a cross section of a second embodiment of a varistor according to the present invention;

[0029] FIG. 3 shows a cross section of a third embodiment of a varistor according to the present invention;

DETAILED DESCRIPTION OF THE DRAWINGS

[0030] In FIG. 1 a first embodiment of a varistor 1 according to the present invention is shown. Two non-floating electrodes 3a protrude into the ceramic body 2 from the two end faces opposing each other along the longitudinal axis 5 of the ceramic body 2. Three floating electrodes 3b are provided inside of the ceramic body 2. The first floating electrode 3b overlaps with a first non-floating electrode 3a and a second floating electrode 3b, while the second floating electrode 3b overlaps with the first floating electrode 3b and the third floating electrode 3b, which in turn overlaps with the second non-floating electrode 3a. All of the electrodes 3, especially the floating electrodes 3b, are arranged in two parallel layers along the longitudinal axis 5 of the ceramic body 2, while two of the floating electrodes 3b are arranged in the same layer. This allows for a very thin design of the whole electronic component.

[0031] When a voltage applied to the varistor 1 exceeds the varistor voltage, the charge carriers move from the first non-floating electrode 3a to the first floating electrode 3b, afterwards from the first floating electrode 3b through the overlapping region to the second floating electrode 3b, then from the second floating electrode 3b to the third floating electrode 3b, and thereafter from the third floating electrode 3b to the second non-floating electrode 3a. Therefore the charge carriers have to pass four times the functional ceramic compared to one pass the charge carriers undergo in a conventional varistor 1 where the non-floating electrodes 3a overlap each other.

[0032] The distances between the electrodes 3 have been chosen such that a distance D1 between adjacent electrodes, i.e. electrodes 3 that are arranged in one and the same layer, is at least twice the distance D2 between overlapping electrodes 3. So the distances between the electrodes 3 obey the following expression: D1>2*D1; In this way, the charge carriers are forced to elongate their path compared to a varistor 1 that just has overlapping non-floating electrodes 3a without floating electrodes 3b. As the paths through the functional ceramic take similar effects in terms of resistance, the elongated charge carrier path shows the same behaviour as resistors connected in series. In the given arrangement, where the floating electrodes 3b are arranged in two parallel layers along one axis, the varistor voltage behaves according to the following expression: U=2*(n+1)*X, wherein n is the number of floating electrodes 3b and X is a given varistor voltage for just one path through the functional ceramic at a given distance. Hence, the varistor voltage can be comfortably adjusted to a desired voltage class by adding or removing floating electrodes 3b in between the non-floating electrodes 3a.

[0033] The functional ceramic of the ceramic body 2 comprises grains which are predominantly responsible for the resistance between two overlapping electrodes 3 as the resistance is generated by the grain boundaries. The resistance, and therefore the varistor voltage, can be tuned by adjusting the grains' properties such as the material or the diameter of the grains. The grains comprises a metal oxide such as zinc oxide, bismuth oxide, chrome oxide or manganese oxide which show highly non-linear electrical characteristics. Additionally a mixture of the metal oxide as well as doping of the metal oxide can improve the performance of the varistor 1.

[0034] Ceramic grains can be produced in a wide range of different diameters ranging from 100 nm to 20 ?m. As the varistor voltage and the resistance are caused by the grain boundaries, it is necessary to have at least two abutted grains in series to ensure a grain boundary between adjacent electrodes 3 and to avoid a short circuit between the electrodes 3. Therefore, the distance between two adjacent electrodes 3 has to be, dependent on the grain size in between the electrodes 3, in between 400 nm to 20 ?m. Hence, the varistor voltage can be increased not just by adding additional floating electrodes 3b in between floating electrodes 3b, but also by increasing the distance D2 between overlapping electrodes 3.

[0035] The electrodes 3 are shaped as thin films and are flat in a direction of the height 6 of the varistor 1. The thickness of the electrodes 3 can be in between 1 and 3 ?m. By using a flat design of the electrodes 3, the height 6 of the whole varistor 1 can be kept small. Various different techniques, such as vacuum coating, surface coating, lamination, plating, or printing can be processes that are suitable for manufacturing the electrodes 3. Metals such as silver, palladium, copper, alloys or a mixture of different metals are suitable as material for the electrodes 3. As metals have a high electrical conductivity, they can be shaped in very thin layers without increasing their resistance. The height 6 of a varistor 1 according to the first embodiment can be just 100 ?m or less.

[0036] In FIG. 2 a second embodiment of a varistor 1 according to the present invention is shown. The ceramic body 2 is rectangular in shape. Two non-floating electrodes 3a protrude into the ceramic body 2 on each of two opposing front ends. Both non-floating electrodes 3a overlap with different floating electrodes 3b, and both of this floating electrodes 3b overlap again with two floating electrodes 3b which are located in the middle of the ceramic body 2 stacked upon each other. Therefore, a charge carrier has to move four times through the functional ceramic to get from a non-floating electrode 3a from one front end to another non-floating electrode 3a on the front end opposing the first front end. Consequently, a varistor 1 according to the second embodiment has a higher varistor voltage than a varistor 1 according to the first embodiment.

[0037] In the embodiment shown in FIG. 2 the overlap between two electrodes 3, in particular in between two floating electrodes 3b, is much higher than in the first embodiment shown in FIG. 1. In FIG. 1 the overlap between floating electrodes 3b is about 10% of the extension of a floating electrode 3b, whereas there is about 30% overlap in the second embodiment shown in FIG. 2. As a capacitance is generated by the overlap of the electrodes 3, the capacitance of the second embodiment, which has an overlap that is three times the overlap of the first embodiment, is much higher than the capacitance in the first embodiment shown in FIG. 1. Accordingly, the capacitance of the varistor 1 can be adjusted nearly independent from the varistor voltage as the capacitance is dominantly influenced by the overlap and the varistor voltage is dominantly influenced by the distance D1 of the overlapping electrodes 3, the grain size of the functional ceramic, the material of the functional ceramic and especially by the number of floating electrodes 3b that are arranged between the non-floating electrodes 3a.

[0038] By employing a symmetrical arrangement of the electrodes 3, as in FIG. 2, thermoplastic tensions inside the ceramic body 2 which can occur during thermal manufacture processes as sintering are reduced. Additionally, on both front ends of the ceramic body 2, where the non-floating electrodes 3a protrude into the ceramic body 2, the external contacts 4 connected to the external contacts 4 are shaped as caps 4. The cap 4 arranged at a front end is electrically connected to all non-floating electrodes 3a protruding into the ceramic body 2 from this side. A varistor 1 with caps 4 can be integrated and mounted comfortably in an application as it is a surface-mounted device. Hence, a varistor 1 with caps 4 can also handily be processed by a pick and place automat.

[0039] In FIG. 3 a third embodiment of a varistor 1 according to the present invention is shown. In this embodiment the non-floating electrodes 3a also protrude into the ceramic body 2 from two front ends opposing each other, whereby in this embodiment nine non-floating electrodes 3a protrude from each side. Inside the ceramic body 2 multiple parallel layers of floating electrodes 3b are arranged, reaching from one frontend side to the other frontend side along the longitudinal axis 5.

[0040] In this embodiment a charge carrier has to travel at least fourteen times through the functional ceramic to get from a non-floating electrode 3a at a first front end to a non-floating electrode 3a at a second frond end opposing the first. Therefore, the varistor voltage of a varistor 1 according to the third embodiment is much higher compared to the other two embodiments. Furthermore, the current surge capability of the varistor 1 is increased because multiple parallel layers of floating electrodes 3b are arranged in the ceramic body 2. By adding additional layers of electrodes 3 the current surge capability can be even increased further. Hence, a varistor 1 according to the third embodiment is not just suitable for applications where a high varistor voltage is needed, but also for applications where high currents occur.

LIST OF USED REFERENCE SYMBOLS

[0041] 1 varistor [0042] 2 ceramic body [0043] 3 electrodes [0044] 3a non-floating electrodes [0045] 3b floating electrode [0046] 4 external contact/cap [0047] 5 longitudinal axis [0048] 6 height [0049] D1 distance between two further electrodes [0050] D2 distance between two overlapping electrodes