Thermally protected metal oxide varistor (TMOV) devices
The TMOV device with a silica sand, melamine cyanurate, and silicone heat-absorbing layer, combined with a lower-melting-point solder connection, addresses premature TCO activation, ensuring enhanced thermal endurance and arc extinction for circuit protection.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- DONGGUAN LITTELFUSE ELECTRONICS CO LTD
- Filing Date
- 2025-12-12
- Publication Date
- 2026-06-30
AI Technical Summary
Conventional thermally protected metal oxide varistors (TMOVs) are prone to early thermal cutoff (TCO) activation due to prolonged operation or high-temperature environments, leading to potential failure and damage from electrical arcs.
A thermally protected metal oxide varistor (TMOV) device with a heat-absorbing layer made of silica sand, melamine cyanurate, and silicone, and a thermal cutoff element connected with a lower-melting-point solder, designed to prevent premature TCO activation, and includes a heat-absorbing paste to manage heat and extinguish arcs.
Enhances thermal endurance and prevents premature TCO activation, effectively extinguishing electrical arcs to protect connected components from damage.
Smart Images
Figure 2026108578000001_ABST
Abstract
Description
Technical Field
[0001] The present disclosure generally relates to the field of circuit protection devices. More specifically, the present disclosure relates to a thermally protected metal oxide varistor with enhanced thermal durability.
Background Art
[0002] A metal oxide varistor (MOV) is a non-linear device that provides voltage-dependent transient voltage suppression in an electronic circuit. The MOV has a high electrical resistance when a low voltage is applied and a low electrical resistance when a high voltage is applied. When the MOV is connected in parallel with a circuit component to be protected, it can clamp the voltage to a safe level when a high transient voltage occurs in the circuit. Thus, the MOV absorbs energy that would otherwise damage the component to be protected.
[0003] A drawback associated with conventional MOVs is their susceptibility to electrical punch-through when high local currents flow, which can lead to excessive heating and subsequent ignition. For example, under abnormal overvoltage conditions, the MOV may overheat and potentially cause thermal runaway and / or dielectric breakdown. This can lead to the ejection of hot gases causing electrodes on the outer surface of the MOV to rupture. To address these problems, thermally protected MOV (TMOV) devices have been developed. Referring to Figures 1A and 1B, front and rear perspective views of a prior art TMOV device 10 are shown. The TMOV device 10 includes an MOV chip 12, which is formed of a material having voltage-dependent nonlinear resistance characteristics as described above. The TMOV device 10 further includes first and second conductive electrodes 14a, 14b positioned on both sides of the MOV chip 12, and conductive first and second lead wires 15, 16 to facilitate the electrical connection of the TMOV device 10 in a circuit. The first lead wire 15 is directly connected to the first electrode 14a attached to the first side of the MOV chip 12, and the second lead wire 16 is connected to a dielectric barrier 17 located on the second side of the MOV chip 12. The dielectric barrier 17 prevents a direct electrical connection between the second lead wire 16 and the second electrode 14b. The TMOV device 10 further includes a thermal cutoff (TCO) element 19, which has a first end electrically connected (e.g., by soldering) to the second lead wire 16 attached to the dielectric barrier 17, and a second end electrically connected to the second electrode 14b. The TCO element 19 is made of a conductive material and is adapted to melt and break apart when a predetermined temperature (e.g., 140°C to 240°C) is reached.
[0004] Under normal operation, the TMOV device 10 behaves like a conventional MOV device, clamping the voltage to a safe level if a high transient voltage occurs in the connected circuit. However, if the TMOV device 10 overheats, the TCO element 19 melts and breaks, thereby blocking the current flowing through the TMOV device 10 and preventing thermal runaway and dielectric breakdown. This reduces the risk of catastrophic failure (e.g., fire).
[0005] In some cases, the TCO element of a TMOV device may open earlier than usual (i.e., not under actual failure conditions). For example, this can occur when the TMOV device is operating for extended periods and / or in high-temperature environments. Therefore, it is desirable to provide a TMOV device with enhanced thermal endurance, in which the TCO element of such a TMOV device is less prone to opening earlier than usual. This improvement may be helpful in addressing these and other considerations. [Overview of the project]
[0006] This summary of the invention is provided to introduce, in a simplified form, selected from the concepts further described below, in modes for carrying out the invention. This summary is not intended to identify the main or essential features of the claimed subject matter, nor is it intended to assist in determining the scope of the claimed subject matter.
[0007] A thermally protected metal oxide varistor (TMOV) device according to one embodiment of the present disclosure may include a MOV chip, a first electrode located on a first side of the MOV chip, a second electrode located on a second side opposite to the first side of the MOV chip, a conductive first lead wire connected to the first electrode, a thermal absorption layer bonded to the second electrode with a thermal absorption paste, the thermal absorption layer being formed of a mixture of silica sand, melamine cyanurate, and silicone, the thermal absorption paste being formed of a mixture of melamine cyanurate and silicone, a thermal cutoff (TCO) element having a first end that penetrates the holes in the thermal absorption layer and is electrically connected to the second electrode with a certain amount of solder, the melting point of the solder being lower than the melting point of the TCO element, and a conductive second lead wire electrically connected to the second end of the TCO element.
[0008] A thermally protected metal oxide varistor (TMOV) device according to another embodiment of the present disclosure may include a MOV chip, a conductive first electrode located on a first side of the MOV chip, a conductive second electrode located on a second side opposite to the first side of the MOV chip, a conductive first lead wire connected to the first electrode, a thermal absorption layer bonded to the second electrode with a thermal absorption paste, the thermal absorption layer being formed of a mixture of silica sand, melamine cyanurate, and silicone, the thermal absorption paste being formed of a mixture of melamine cyanurate and silicone, a thermal cutoff (TCO) element having a first end that penetrates the holes in the thermal absorption layer and is electrically connected to the second electrode with a certain amount of solder, the melting point of the solder being lower than the melting point of the TCO element, and a conductive second lead wire electrically connected to the second end of the TCO element. [Brief explanation of the drawing]
[0009] [Figure 1A] These are a front perspective view and a rear perspective view showing a conventional thermally protected metal oxide varistor device. [Figure 1B] These are a front perspective view and a rear perspective view showing a conventional thermally protected metal oxide varistor device.
[0010] [Figure 2A] These are a front perspective view, a rear perspective view, and an exploded view showing a thermally protected metal oxide varistor device according to one embodiment of the present disclosure. [Figure 2B] These are a front perspective view, a rear perspective view, and an exploded view showing a thermally protected metal oxide varistor device according to one embodiment of the present disclosure. [Figure 2C] These are a front perspective view, a rear perspective view, and an exploded view showing a thermally protected metal oxide varistor device according to one embodiment of the present disclosure.
[0011] [Figure 3A] Figures 2A to 2C show a series of perspective views illustrating the assembly process of a thermally protected metal oxide varistor device. [Figure 3B] Figures 2A to 2C show a series of perspective views illustrating the assembly process of a thermally protected metal oxide varistor device. [Figure 3C] Figures 2A to 2C show a series of perspective views illustrating the assembly process of a thermally protected metal oxide varistor device. [Figure 3D] Figures 2A to 2C show a series of perspective views illustrating the assembly process of a thermally protected metal oxide varistor device. [Figure 3E] Figures 2A to 2C show a series of perspective views illustrating the assembly process of a thermally protected metal oxide varistor device. [Modes for carrying out the invention]
[0012] Herein, exemplary embodiments of thermally protected metal oxide varistor (TMOV) devices according to the present disclosure will be described in more detail below with reference to the accompanying drawings. However, TMOV devices may be embodied in many different forms and should not be construed as being limited to the embodiments described herein. Rather, these embodiments are provided so that specific exemplary forms of TMOV devices are conveyed by the present disclosure to those skilled in the art.
[0013] Referring to Figures 2A to 2C, a front view, rear view, and exploded view of an exemplary embodiment of a thermally protected metal oxide varistor device 100 (hereinafter, "TMOV device 100") according to the present disclosure are shown. The TMOV device 100 may include an MOV chip 112, which has conductive first and second electrodes 114a, 114b arranged on its opposing first and second sides. The MOV chip 112 may be formed of any MOV composition known in the art, such as zinc oxide granules embedded in ceramic. The present disclosure is not limited in this respect. The first and second electrodes 114a, 114b of the TMOV device 100 may be formed of a metal having good conductivity, such as, but not limited to, aluminum, copper, silver, and tin. The present disclosure is not limited in this respect.
[0014] The TMOV device 100 may further include a heat-absorbing layer 113, which is positioned on the second side of the MOV chip 112 in flat contact with the second electrode 114b. The heat-absorbing layer 113 may be formed from a mixture of silica sand, melamine cyanurate, and silicone, which may be poured into a mold, pressurized, cured (e.g., under high temperature and pressure in a temperature range of room temperature to 170°C), and then punched or cut into a desired shape (e.g., a disc as shown). In various embodiments, the aforementioned mixture may contain silica sand, melamine cyanurate, and silicone in a ratio of 8:1:1. The disclosure is not limited in this respect. The heat-absorbing layer 113 may be fixed to the second electrode 114b with a heat-absorbing paste, which will be further described below.
[0015] The MOV chip 112, the first and second electrodes 114a, 114b, and the heat absorption layer 113 are illustrated in a circular shape, but this is not essential. One or more of the MOV chip 112, the first and second electrodes 114a, 114b, and the heat absorption layer 113 may have different shapes, for example, rectangular, triangular, or irregular shapes, without departing from the scope of the disclosure. The heat absorption layer 113 is shown as covering the entire second electrode 114b and the entire second side of the MOV chip 112. The disclosure is not limited in this respect. In various non-limiting embodiments, the heat absorption layer 113 may cover at least 80% of the second side of the MOV chip.
[0016] The TMOV device 100 may further include conductive first and second leads 115, 116 to facilitate the electrical connection of the TMOV device 100 within the circuit. The first lead 115 may be directly connected to a first electrode 114a on the first side of the MOV chip 112 by soldering, welding, conductive adhesive, etc. The second lead 116 may be connected to the first end of the thermal cutoff (TCO) element 119 by solder 121, etc. The TCO element 119 may penetrate an opening 123 formed in the heat absorption layer 113 (e.g., by drilling), or it may be attached to the second end of the second electrode 114b by solder, etc. This will be explained further below. The TCO element 119 may be made of a conductive material and may be adapted to melt and break apart when a predetermined temperature (e.g., 140°C to 240°C) is reached. In various embodiments, the TCO element 119 may be formed of solder having a melting point higher than the solder used to connect the TCO element 119 to the second lead wire 116 and the second electrode 114b.
[0017] Referring to Figures 3A to 3E, a series of diagrams illustrating the process of assembling the TMOV device 100 are shown. In Figure 3A, the first lead wire 115 may be electrically connected to the first electrode 114a, and the TCO element 119 may be electrically connected to the second electrode 114b. In various embodiments, the TCO element 119 may be electrically connected to the second electrode 114b with a certain amount of solder 127 having a melting point lower than the melting point of the solder forming the TCO element 119, as described above. Therefore, the solder 127 can be heated and reflowed to bond the TCO element 119 to the second electrode 114b without melting the TCO element 119.
[0018] In Figure 3B, the heat-absorbing paste 130 may be applied to the second electrode 114b, and may substantially cover the solder 127 at the junction between the TCO element 119 and the second electrode 114b. The heat-absorbing paste 130 may be formed from a mixture of melamine cyanurate and silicone. In various embodiments, the aforementioned mixture may contain melamine cyanurate and silicone in a 1:1 ratio. The disclosure is not limited in this respect.
[0019] In Figure 3C, the second lead wire 116 may be passed through the opening 123 of the heat absorption layer 113, and the heat absorption layer 113 may move or slide across the second lead wire 116 and the TCO element 119, rotate and / or tilt to engage flat with the second electrode 114b, and cover the second side of the MOV tip 112 as shown in Figure 3D. The heat absorption paste 130 may then be cured (for example, at 170°C for 2 to 4 minutes), after which the heat absorption layer 113 may be bonded to the second electrode 114b.
[0020] In FIG. 3E, a protective dielectric coating 132 may be applied over a portion of the MOV chip 112, the first and second electrodes 114a, 114b, the heat absorption layer 113, the TCO element 119, and the first and second lead wires 115, 116. In various embodiments, the dielectric coating 132 may be formed of epoxy and may be applied by dipping, jetting, or a similar technique. In various embodiments, the dielectric coating 132 may be completely omitted. The present disclosure is not limited in this regard.
[0021] During normal operation of the TMOV device 100, the heat absorption layer 113 and the heat absorption paste 130 can absorb heat and prevent the TCO element 119 from melting / openning earlier than normal. Such a situation may normally occur due to heat being stored in the TMOV device 100 during long-term operation and / or operation in a high-temperature environment. The heat absorption layer 113 and the heat absorption paste 130 can similarly prevent the solder 127 connecting the TCO element 119 to the second electrode and / or the solder 121 connecting the TCO element 119 to the second lead wire 116 from melting / opening earlier than normal.
[0022] When an actual fault condition (e.g., an extreme overvoltage condition) occurs in the TMOV device 100, the heat absorption capacity of the heat absorption layer 113 and the heat absorption paste 130 may exceed its limit, causing the TCO element 119 to melt. This blocks the current flowing through the TMOV device 100 and can prevent further overheating that could otherwise cause the TMOV device 100 to fire and damage surrounding components. When the TCO element 119 is in an open state, an electric arc may propagate across the gap left between the severed TCO element 119 and the second electrode 114a. The heat from the electric arc may cause the silicone within the heat absorption layer 113 and / or the heat absorption paste 130 to burn and decompose. When the silicone decomposes, the melamine cyanurate within the heat absorption layer 113 and / or the heat absorption paste 130 may be exposed and may also burn due to the heat from the electric arc. When melamine cyanurate burns and decomposes, it may undergo an endothermic chemical reaction that absorbs heat. This rapidly cools the electric arc. Additionally, certain by-products of the endothermic chemical reaction can become non-conductive gases (e.g., ammonia) that can impede the sustaining ability of the electric arc. Furthermore, other by-products of the endothermic chemical reaction may produce water, further cooling the electric arc. Thus, when a fault condition occurs in the TMOV device 100, the heat absorption layer 113 and / or the heat absorption paste 130 can absorb heat, release gases unfavorable for the sustained existence of the electric arc, and generate water that can further cool the electric arc. All these factors can contribute to rapid arc extinction. This protects the components connected to and / or located near the TMOV device 100 from damage that might otherwise occur if the electric arc could persist.
[0023] In this specification, elements or steps described in the singular and beginning with the word "a" or "an" should be understood not to exclude multiple elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “certain embodiments” in this disclosure are not intended to be construed as excluding the existence of further embodiments, including the features described therein.
[0024] While this disclosure refers to specific embodiments, numerous modifications, changes, and alterations are possible to the embodiments described without departing from the scope and scope of this disclosure as defined in the attached claims. Therefore, this disclosure is not limited to the embodiments described and is intended to encompass the entire scope defined by the following claims and equivalents.
Claims
1. MOV chip; A conductive first electrode positioned on the first side of the MOV chip; A conductive second electrode positioned on the second side of the MOV chip opposite to the first side; A conductive first lead wire connected to the first electrode; A heat-absorbing layer disposed on the second electrode; A thermal cutoff (TCO) element having a first end that penetrates the heat absorption layer and is electrically connected to the second electrode; and A conductive second lead wire electrically connected to the second end of the TCO element. A thermally protected metal oxide varistor (TMOV) device equipped with [specific feature].
2. The TMOV device according to claim 1, wherein the TCO element is formed of a conductive material and is adapted to melt and break apart when it reaches a predetermined temperature.
3. The TMOV device according to claim 1, wherein the TCO element is formed of solder.
4. The TMOV device according to claim 1, wherein the heat-absorbing layer is formed of a mixture containing melamine cyanurate.
5. The TMOV device according to claim 4, wherein the heat-absorbing layer is formed of a mixture containing silica sand, melamine cyanurate, and silicone.
6. The TMOV device according to claim 5, wherein the heat-absorbing layer is formed of a mixture of silica sand, melamine cyanurate, and silicone, in a ratio of 8:1:
1.
7. The TMOV device according to claim 1, wherein the heat absorption layer covers at least 80% of the second side of the MOV chip.
8. The TMOV device according to claim 1, wherein the heat-absorbing layer is bonded to the second electrode with a heat-absorbing paste.
9. The TMOV device according to claim 8, wherein the heat-absorbing paste is formed of a mixture of melamine cyanurate and silicone.
10. The TMOV device according to claim 9, wherein the heat-absorbing paste is formed from a mixture of melamine cyanurate and silicone in a ratio of 1:
1.
11. The TMOV device according to claim 8, wherein the heat-absorbing paste covers the junction between the TCO element and the second electrode.
12. The TMOV device according to claim 1, wherein the TCO element is connected to the second electrode with a fixed amount of solder having a melting point lower than the melting point of the TCO element.
13. The TMOV device according to claim 1, wherein the MOV chip, the first and second electrodes, the heat absorption layer, the TCO element, and a portion of the first and second lead wires are covered with a dielectric coating.
14. The TMOV device according to claim 13, wherein the dielectric coating is formed of epoxy.
15. MOV chip; A conductive first electrode positioned on the first side of the MOV chip; A conductive second electrode positioned on the second side of the MOV chip opposite to the first side; A conductive first lead wire connected to the first electrode; A heat-absorbing layer is bonded to the second electrode with a heat-absorbing paste, the heat-absorbing layer is formed of a mixture of silica sand, melamine cyanurate, and silicone, and the heat-absorbing paste is formed of a mixture of melamine cyanurate and silicone; A thermal cutoff (TCO) element having a first end that penetrates the holes in the heat absorption layer and is electrically connected to the second electrode with a certain amount of solder, the melting point of the solder being lower than the melting point of the TCO element; and A conductive second lead wire electrically connected to the second end of the TCO element. A thermally protected metal oxide varistor (TMOV) device equipped with [specific feature].
16. The TMOV device according to claim 15, wherein the heat-absorbing layer is formed of a mixture of silica sand, melamine cyanurate, and silicone, in a ratio of 8:1:
1.
17. The TMOV device according to claim 15, wherein the heat absorption layer covers at least 80% of the second side of the MOV chip.
18. The TMOV device according to claim 15, wherein the heat-absorbing paste is formed from a mixture of melamine cyanurate and silicone in a ratio of 1:
1.
19. The TMOV device according to claim 15, wherein the heat-absorbing paste covers the junction between the TCO element and the second electrode.