AU2019100388A4

AU2019100388A4 - Protection circuit

Info

Publication number: AU2019100388A4
Application number: AU2019100388A
Authority: AU
Inventors: Thomas Jackson
Original assignee: JACKSON TRADEMARK HOLDINGS Pty Ltd
Current assignee: JACKSON TRADEMARK HOLDINGS Pty Ltd
Priority date: 2018-04-09
Filing date: 2019-04-09
Publication date: 2019-05-16
Anticipated expiration: 2027-04-09

Abstract

PROTECTION CIRCUIT A surge suppression circuit comprising, in each current path between any two connections, a weak printed circuit board track, a thermal cut-off fuse, and a metal oxide varistor, connected in series, wherein the thermal cut-off fuse is mounted adjacent a face of the metal oxide varistor, wherein the surge compression circuit further comprises a thermal bond between the thermal cut-off fuse and the metal oxide varistor. 201 220 210 220 210 220 230 230 230 Fig. 2B

Description

PROTECTION CIRCUIT

Related Application [0001] The present application claims priority from Australian Provisional Patent Application No. 2018901165 which is herein incorporated by reference in its entirety.

Technical Field [0002] The present invention relates generally to protection circuits. More particularly, the present invention relates to surge suppression circuits having thermal runaway and overcurrent protection.

Background [0003] Metal Oxide Varistors (MOVs) are commonly used in surge suppression circuits. However, MOVs are known to suffer from “thermal runaway”, a condition in which an increase in temperature changes the operational conditions of the MOV in a manner which causes a further increase in temperature. More particularly, the resistance of the MOV lowers as the MOV heats up. The trigger voltage lowers, and the MOV slides into catastrophic thermal runaway, possibly culminating in a small explosion or fire. Fuses can be used in combination with MOVs, but this arrangement results in a high component count, increasing cost and physical space required. There is a need for an alternative arrangement.

Summary [0004] It is an object of the present invention to meet this need or to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements.

[0005] According to a first aspect, the present disclosure provides a surge suppression circuit comprising, in each current path between any two connections, a weak printed circuit board track, a thermal cut-off fuse, and a metal oxide varistor, connected in series, wherein the thermal cut-off fuse is mounted adjacent a face of the metal oxide varistor, wherein the surge compression circuit further comprises a thermal bond between the thermal cut-off fuse and the metal oxide varistor.

22509117 (IRN: P300910AU)

2019100388 09 Apr 2019 [0006] In certain embodiments, the metal oxide varistor and thermal cut-off fuse are substantially encapsulated within heatsink tubing.

[0007] The weak printed circuit board track is in series with the metal oxide varistor and the thermal cut-off fuse, wherein the weak printed circuit board track has a reduced width compared to other printed circuit board tracks in the circuit. The weak printed circuit board track is intended to rupture open-circuit in the event of an overcurrent fault condition in the protective MOV circuit.

[0008] According to a second aspect there is provided a surge suppression circuit comprising: in a first current path between an earth connection and an active connection, a first weak printed circuit board track, a first thermal cut-off fuse, and a first metal oxide varistor, connected in series, wherein the first thermal cut-off fuse is mounted adjacent a face of the first metal oxide varistor;

in a second current path between an active connection and a neutral connection, a second weak printed circuit board track, a second thermal cut-off fuse, and a second metal oxide varistor, connected in series, wherein the second thermal cut-off fuse is mounted adjacent a face of the second metal oxide varistor; and in a third current path between a neutral connection and an earth connection, a third weak printed circuit board track, a third thermal cut-off fuse, and a third metal oxide varistor, connected in series, wherein the third thermal cut-off fuse is mounted adjacent a face of the third metal oxide varistor;

wherein a thermal bond is located between at least one of the first, second and third thermal cut-off fuse and the respective first, second and third metal oxide varistor.

[0009] In certain embodiments, the first thermally bonded metal oxide varistor and first thermal cut-off fuse are substantially encapsulated within first heatsink tubing; the second thermally bonded metal oxide varistor and second thermal cut-off fuse are substantially encapsulated within second heatsink tubing; and the third thermally bonded metal oxide varistor and third thermal cut-off fuse are substantially encapsulated within third heatsink tubing.

[0010] Other aspects and embodiments are also disclosed.

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Brief Description of the Drawings [0011] One or more embodiments of the present invention will now be described with reference to the drawings, in which:

[0012] Fig. 1A shows a schematic circuit diagram of a surge suppression circuit;

[0013] Fig. IB shows a printed circuit board layout of the circuit shown in Fig. 1A;

[0014] Fig. 2A shows a schematic circuit diagram of another surge suppression circuit; and [0015] Fig. 2B shows a printed circuit board layout of the circuit shown in Fig. 2A.

Description of Embodiments [0016] Described herein are embodiments of a surge suppression circuit having thermal protection. The surge suppression circuit comprises a thermal cut-off fuse mounted electrically in series with a MOV, and adjacent one face of the MOV. The surge suppression circuit also comprises a weak printed circuit board track that is also provided in series with the MOV to provide a second form of protection. The described surge suppression circuit is configured to be installed in power distribution accessories (e.g. power boards, double adapters or the like), process control systems, communication systems, and the like, for the purpose of protecting against electrical surges and spikes, including those caused by electrical faults.

[0017] Fig. 1A shows a schematic circuit diagram of an example surge suppression circuit 100. Fig. IB shows an example printed circuit board layout 101 of the circuit 100 shown in Fig. 1A. The circuit 100 has a MOV 110, a thermal cut-off fuse 120, and a weak printed circuit board track 130 connected in series between an active phase connection 141 and a neutral connection 142. Earth connection 143 is not connected.

[0018] As is apparent from Fig. IB, the thermal cut-off fuse 120 is mounted adjacent one face of the MOV 110. The weak printed circuit board track 130 provides overcurrent protection. Furthermore, the thermal cut-off fuse 120 is thermally bonded, via a thermal bond to the MOV 110. In some forms, the thermal bond could be achieved using a thermal conductive paste or grease which can be located between and in contact with the thermal cut-off fuse 120 and the

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MOV 110 in order to achieve thermal heat transfer. In one implementation, the bond can be provided in the form of a thermal glue compound which is used between the thermal cut-off fuse 120 and the MOV 110, the thermal glue compound setting hard and having predefined thermal transfer properties. In another implementation a thermal transfer compound is placed between the thermal cut-off fuse 120 and the MOV 110, and that combination is then encapsulated within heatsink tubing.

[0019] The operating point of the thermal cut-off fuse 120 is at a temperature below what is required to put the MOV 110 in thermal runaway. When the MOV 110 heats up, due to an increased (fault condition) current through the MOV 110, to a temperature above the operating point of thermal cut-off fuse 120, it causes thermal cut-off fuse 120 to rupture open circuit which opens the electrical circuit between the neutral connection 142 and the active phase connection 141.

[0020] An overcurrent fault condition causes the weak printed circuit board track 130 to rupture which opens the electrical circuit between the neutral connection 142 and the active phase connection 141. In a specific example, for a double adapter having a supply voltage of 230 240VAC at 50 Hz , a maximum rated current of 10A and a maximum rated input power of2400W, the weak printed circuit board track 130 of surge suppression circuit 100 has a width of 1.5 mm. However, depending upon the specific operating and overcurrent parameters, various characteristics of the weak print circuit board track (including an alternate track width) can be utilised as defined by Onderdonk’s equation,) and Preece’s equation as known by those skilled in the art.

[0021] Fig. 2A shows a schematic circuit diagram of another surge suppression circuit 200, and Fig. 2B shows a printed circuit board layout 201 of the circuit 200 shown in Fig. 2A. Circuit 200 is similar to the circuit 100 shown in Fig. 1A, with the difference being that the circuit 200 has a MOV 210, a thermal cut-off fuse 220 and a weak printed circuit board track 230 connected in series between each of the active phase connection 141, neutral connection 142, and earth connection 143. More specifically, the surge suppression circuit 200 comprises, in a first current path between an earth connection 143 and an active connection 141, a first weak printed circuit board track 230, a first thermal cut-off fuse 220, and a first metal oxide varistor 210, connected in series, wherein the first thermal cut-off fuse 220 is mounted adjacent a face of the first metal oxide varistor 210. Furthermore, the surge suppression circuit 200 comprises, in a second current

22509117 (IRN: P300910AU)

2019100388 09 Apr 2019 path between an active connection 141 and a neutral connection 142, a second weak printed circuit board track 230, a second thermal cut-off fuse 220, and a second metal oxide varistor 210, connected in series, wherein the second thermal cut-off fuse 220 is mounted adjacent a face of the second metal oxide varistor 210. Furthermore, the surge suppression circuit 200 comprises, in a third current path between a neutral connection 142 and an earth connection 143, a third weak printed circuit board track 230, a third thermal cut-off fuse 220, and a third metal oxide varistor 210, connected in series, wherein the third thermal cut-off fuse 220 is mounted adjacent a face of the third metal oxide varistor 210.

[0022] Similarly to circuit 100, for the surge suppression circuit 200 at least one of the first thermal cut-off fuse 220 is thermally bonded to the first metal oxide varistor 210 via a first thermal bond, the second thermal cut-off fuse 220 is thermally bonded to the second metal oxide varistor 210 via a second thermal bond, and the third thermal cut-off fuse 220 is thermally bonded to the third metal oxide varistor 210 via a third thermal bond. In some forms, the thermal bond could be achieved using a thermal conductive paste or grease which can be located between each pair of thermal cut-off fuses 220 and the respective MOV 210 in order to achieve thermal heat transfer. In one implementation, the bond can be provided in the form of a thermal glue compound which is used between the thermal cut-off fuse 120 and the MOV 110, the thermal glue compound setting hard and having predefined thermal transfer properties.

[0023] In one form of the surge suppression circuit 200, the thermally bonded first metal oxide varistor 210 and first thermal cut-off fuse 220 are substantially encapsulated within a first heatsink tubing. Furthermore, the thermally bonded second metal oxide varistor 210 and second thermal cut-off fuse 220 are substantially encapsulated within a second heatsink tubing. Furthermore, the thermally bonded third metal oxide varistor 210 and third thermal cut-off fuse 220 are substantially encapsulated within a third heatsink tubing.

[0024] The operating point of the first, second or third thermal cut-off fuses 220 are at a temperature below what is required to put the adjacent first, second or third MOV 210 in thermal runaway. When the first, second or third MOV 210 heats up, due to an increased (fault condition) current through the first, second or third MOV 210, to a temperature above the operating point of the first, second or third thermal cut-off fuse 220 , it causes the first, second or third thermal cutoff fuse 220 to rupture open circuit which opens the electrical circuit between:

- earth connection 143 and active connection 141; or

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- active connection 141 and a neutral connection 142; or

- neutral connection 142 and earth connection 143.

[0025] The first, second and third weak printed circuit board tracks 230 of the surge suppression circuit 200 have a reduced width compared to another printed circuit board tracks of the surge suppression circuit 200, wherein the first, second and third weak printed circuit board tracks 230 are configured to rupture open circuit during an overcurrent fault condition. In a specific example, for a power board having a supply voltage of 240VAC at 50 Hz , a maximum current rating of 10A and a maximum input power rating of 2400W the first, second and third weak printed circuit board tracks 230 of the surge suppression circuit 200 can have a width of 1 mm. However, depending upon the specific operating and overcurrent parameters, various characteristics of the weak print circuit board track(s) (including an alternate track width) can be implemented as defined by Onderdonk’s equation and Preece’s equation as known by those skilled in the art.

[0026] Referring to Fig. 2B, each MOV 210 has mounted adjacent to a face thereof a respective thermal cut-off fuse 220.

[0027] Thus, not only do the circuits 100, 200 disclosed herein provide both thermal and overcurrent protection, circuits 100 and 200 also provide reduced component counts.

[0028] The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.

Claims

CLAIMS:

1. A surge suppression circuit comprising, in each current path between any two connections, a weak printed circuit board track, a thermal cut-off fuse, and a metal oxide varistor, connected in series, wherein the thermal cut-off fuse is mounted adjacent a face of the metal oxide varistor, wherein the surge compression circuit further comprises a thermal bond between the thermal cut-off fuse and the metal oxide varistor.
2. A surge suppression circuit according to claim 1, further comprising heatsink tubing which substantially encapsulates the thermally bonded metal oxide varistor and thermal cut off fuse.
3. A surge suppression circuit comprising:

in a first current path between an earth connection and an active connection, a first weak printed circuit board track, a first thermal cut-off fuse, and a first metal oxide varistor, connected in series, wherein the first thermal cut-off fuse is mounted adjacent a face of the first metal oxide varistor;

in a second current path between an active connection and a neutral connection, a second weak printed circuit board track, a second thermal cut-off fuse, and a second metal oxide varistor, connected in series, wherein the second thermal cut-off fuse is mounted adjacent a face of the second metal oxide varistor;

in a third current path between a neutral connection and an earth connection, a third weak printed circuit board track, a third thermal cut-off fuse, and a third metal oxide varistor, connected in series, wherein the third thermal cut-off fuse is mounted adjacent a face of the third metal oxide varistor; and wherein a thermal bond is located between at least one of the first, second and third thermal cut-off fuse and the respective first, second and third metal oxide varistor.
4. The surge suppression circuit according to claim 3, wherein:

the first thermally bonded metal oxide varistor and first thermal cut-off fuse are substantially encapsulated within first heatsink tubing;

the second thermally bonded metal oxide varistor and second thermal cut-off fuse are substantially encapsulated within second heatsink tubing; and

22509117 (IRN: P300910AU) the third thermally bonded metal oxide varistor and third thermal cut-off fuse are substantially encapsulated within third heatsink tubing.