EP2301053B1 - Thermal fuse - Google Patents

Description

Technical area

The invention relates to a thermal fuse for interrupting a current flow in modules, in particular for use in the automotive sector

State of the art

In order to protect electrical modules against overheating, irreversible thermal fuses are required, which interrupt a current-carrying conductor if the ambient temperature is too high, ie trigger the fuse. The thermal fuses are designed so that the trip temperature is not reached due to a high current flow, so that it is ensured that they can not be triggered by a high current but only by an excessively high ambient temperature. A thermal fuse of the above type thus serves to provide an independent Abschaltpfad for electrical modules available, with inadmissible high temperatures in the module, for example, due to failures of components, short circuits, for example, by external influence, malfunction of insulation materials and the like Current flow is safely interrupted.

Conventional thermal fuses are usually based on the concept of a fixed spring, such as a spring. a soldered leaf spring, which opens a contact by a spring force when triggered. In this case, even in the unreleased case, a mechanical force is permanently exerted on the joint, which leads to quality problems, especially in long periods of use, such as. long operating times in the automotive sector. In particular, a disruption of the solder joint may occur after some time.

An alternative embodiment of a thermal fuse uses a conductive fusing element of a fusible material that begins to melt at a triggering temperature and thereby breaks an electrical connection. Such a melting element is usually arranged between two connection regions, at which the molten material of the melting element collects after melting due to the surface tension. A separation was successful when a deposit or a drop of fused material has formed on one or both terminal areas, without leaving a conductive bridge of fusible material between the terminal areas.

In experiments with connection areas, the front side, on which the fusible element completely rests, just as large as the cross-section of the contact surface of the fusible element or cup-shaped connection areas, which completely surrounds the fusible element at its ends, it was observed that they do not always reliably trigger since a conductive bridge of fusible material remains between the coverings or between the drops at the connection areas.

document US 5,631,621 For example, discloses a thermal fuse according to the preamble of claim 1.

It is therefore an object of the present invention to provide a thermal fuse of the type mentioned above, which triggers reliable.

Disclosure of the invention

This object is achieved by the thermal fuse according to claim 1.

Further advantageous embodiments of the invention are specified in the dependent claims.

According to one aspect, a thermal fuse for interrupting a current flow in modules is provided, in particular for use in the automotive sector. The thermal fuse comprises a terminal having a terminal portion and a fuse of fused material attached at one end to the terminal portion to provide an electrically conductive connection between the fuse and the terminal. The connecting element has a propagation region for receiving molten melting material. The propagation region has a spreading surface on which a melt material part of the molten melting material spreads during melting of the melting element, wherein the propagation surface corresponds to an inner surface of a funnel-shaped structure.

One idea, in the above thermal fuse, is to form the propagation areas for receiving the molten melt material such that when triggered, the separation of the current path through the fuse element is assisted by utilizing the surface tension of the molten melt material. In particular, the connection elements in the propagation region have a surface structure in which, in particular, an energy barrier with respect to the propagation of the molten melting material is avoided. This can be achieved in particular by providing the propagation surface only with curvatures of 0 or positive curvatures, and in particular avoiding negative curvatures in order to reduce the surface energy. It can thereby minimize the risk that bridges of fusible material remain between the connection areas of the thermal fuse.

Furthermore, the thermal fuse may have two connection elements, between which the fusible element is received, so that ends of the fusible element are attached to the corresponding connection elements.

According to a further embodiment, the propagation surface may correspond to an inner surface of a funnel-shaped structure. In particular, the tip of the funnel-shaped structure may be flattened with a surface which is equal to or smaller than the cross-sectional area of the end of the fusible element.

Brief description of the drawings

• Figuren 1 a und 1 b eine herkömmliche Thermosicherung im nicht ausgelösten bzw. ausgelösten Zustand;
• Figur 2 eine herkömmliche Thermosicherung mit becherförmigen Anschlussbereichen;
• Figuren 3a und 3b eine Thermosicherung mit erweiterten Anschlussbereichen in einem nicht ausgelösten bzw. ausgelösten Zustand;
• Figuren 4a und 4b eine weitere Thermosicherung in einem nicht ausgelösten bzw. ausgelösten Zustand; und
• Figuren 5a und 5b eine erfindungsgemäβe Ansführungsform einer Thermosicherung in einem nicht ausgelösten bzw.
• ausgelösten Zustand.

Preferred embodiments will be explained in more detail with reference to the accompanying drawings. Show it

• FIGS. 1 a and 1b a conventional thermal fuse in the untripped or tripped state;
• FIG. 2 a conventional thermal fuse with cup-shaped connection areas;
• FIGS. 3a and 3b a thermal fuse with extended connection areas in a non-tripped or tripped state;
• FIGS. 4a and 4b another thermal fuse in an untripped or tripped state; and
• FIGS. 5a and 5b An inventive embodiment of a thermal fuse in an untripped or
• triggered condition.

Description of embodiments

In the following description, like reference numerals designate elements of like or comparable function.

In the FIGS. 1 a and 1 b, a conventional thermal fuse 1 is shown, which has two connection elements 2, between which a conductive fusible element 3 is arranged. The fusible element 3 is fastened with its two ends to a respective connection region 4 of the connection lines 2, for example soldered. The cross section of the connection elements 2 at the contact point to the fusible element 3 and the cross section at the ends of the fusible element 3 are substantially the same, so that the connection elements 2 substantially flush with respect to their surfaces in the fusible element 3.

If the ambient temperature of the thermal fuse 1 exceeds a threshold value, the fusible material of the fusible element 3 melts. The fusible material of the fusible element is preferably a low-melting metal or an alloy, such as an aluminum alloy. Lot, which in the molten state, a high surface energy, i. Surface tension. On the surface or in the interior of the fusible element 3, a flux 5 may be provided, that breaks through the oxide skin during melting and increases the wetting or the surface tension.

Due to the surface tension of the molten melting material, the liquid melting material creeps beyond the connection region 4 of the connection elements 2 beyond a propagation surface 6 of the connection elements 2, in which the connection region 4 is located. As a result, additional fusible material is distributed on the previously exposed surface of the connecting element 2, so that the liquid melting material from the center of the fusible element 3, which has previously made the conductive connection between the connecting elements 2, is withdrawn. This takes place until the melting element 3 is completely divided into two pieces of molten material at the connection elements 2, which collect as drops or coating on the respective propagation surface 6 of the connection element 2. As a result, the conductive connection between the connection elements 2 should be interrupted.

In experiments with such thermal fuses, it has been found that they do not always dissolve reliably because, despite the melting of the melt material, conductive bridges of melt material remain between the connection elements 2. One reason for this is evidently that the melting material has too low a surface tension, that is to say the affinity of the molten melting material, on the provided propagation surface 6 of the connection element 2, ie in a propagation region, is not sufficient to completely separate the melt material between the connection elements 2.

wobei E der gesamten Oberflächenenergie, E₀ einem Anteil der konstanten materialabhängigen Oberflächenenergie, E₁/r_k einem Anteil der krümmungsabhängigen Oberflächenenergie (mit r_k = Krümmungsradius) entspricht.For an understanding of the phenomenon, an analysis of the surface free energy is useful. The surface tension corresponds to the difference between the surface free energies of the surface of the connection element 2 and the surface of the liquid melt material. The surface energy is composed, according to the general Gibbs-Thomsen relation, of a constant, material-dependent proportion and a proportion which depends on the curvature of the surface on which the molten material is to be distributed: $$\mathrm{e}\mathrm{=}{\mathrm{e}}_{\mathrm{0}}\mathrm{+}{\mathrm{<figure-callout\; id="1"\; label="e"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">e</figure-callout>}}_{\mathrm{1}}\mathrm{/}{\mathrm{r}}_{\mathrm{k}}\mathrm{.}$$

where E is the total surface energy, E ₀ a proportion of the constant material-dependent surface energy, E ₁ / r _k a proportion of the curvature-dependent surface energy (with r _k = radius of curvature).

As in the formation of the thermal fuse according to the FIGS. 1 a and 1 b is the radius of curvature r _k = ∞, the curvature-dependent surface energy E ₁ / r _k = 0.

In the thermal fuse 1 of Fig. 2 with a cup-shaped connection region 4 of the connection element 2, which completely surrounds the melting element 3 at its ends, the curvature-dependent component of the surface free energy is even positive, since the edge of the cup-shaped connection region 4 is positive curvature. These positive curvatures represent energy barriers which counteract spreading of the melted melt material over the propagation region 6 on the connection elements 2 and thus increase the risk that too little material is drawn onto the propagation surface 6 or surface of the connection elements 2 and thereby leaving a conductive bridge of fusible material between the terminal elements 2.

By suitably choosing the geometry of the propagation region 6 of one or more of the connection elements 2, it can now be ensured that the curvature-dependent fraction of the surface energy during and after the triggering is negative and thus the contraction of the melted melt material parts 7 at the connection regions is supported.

The FIGS. 3a and 3b, 4a and 4b such as 5a and 5b show embodiments for thermal fuses 1, which provide the molten melt material parts 7 within the corresponding propagation region 6 no or a negative curvature even after melting. Thus, the surface free energy is reduced, thereby promoting spreading of the molten melt material. This reduces the risk that bridges of molten material remain between the connection elements 2.

In the embodiment of the FIGS. 3a and 3b is a propagation region 6 formed as a surface portion of the connection element 2. The propagation region 6 comprises a flat surface which contains the connection region 4 of the connection element 2. In other words, the area of the terminal portion 4 is widened so that the surface of the terminal portion 4 where the fuse element 3 abuts before reflowing and the spreading surface 6 on which the molten fuse material propagates lie in a flat surface.

Preferably, the spreading surface 6 is formed with a size sufficient to receive so much molten melting material that it is ensured that no conductive bridge between the connecting elements 2 remains. The total area depends, among other things, on the surface tension of the enamel material (material properties) and the volume of the enamel element 3 off. Preferably, however, the surface is chosen so large that a drop of half the amount of the melt material of the fusible element 3 on the flat propagation surface 6 of the connection element 2 finds room. This can be found, for example, empirically.

In the embodiment of the FIGS. 4a and 4b the propagation surfaces 6 cup-shaped with a cup rim 8 and a cup bottom 9 are formed. The cup bottom 9 is preferably flat and has a larger area than corresponds to the connection region 4 of the melting element 3. The cup edges 8 of the propagation surface 6 are perpendicular or obliquely inwards or outwards from the cup bottom 9 in the direction of the opposite connection element 2 or in the direction of the melting element 3. The angle between the cup bottom 9 and the cup rim 8 forms a negative curvature which promotes the spreading of the molten melt material over the cup-shaped spreading surface 6.

Preferably, the volume of the cup-shaped spreading surface 6, that is to say the volume defined by the edge of the cup rim 8 opposite the cup bottom 9, has a size to accommodate the volume of the molten melting material part 7 which is at least half the volume of the melting material of the melting element 3 corresponds. A distribution of the molten melting material on the cup-shaped spreading surface 6 is in FIG. 4b shown.

In the FIGS. 5a and 5b a further thermal fuse 1 is shown, in which the propagation surface 6 is funnel-shaped. The thermal fuse 1 of FIG. 5 has for this purpose a connection funnel 10 as a propagation region, which is arranged around the surface 11 of the connection region 4 of the connection element 2. Between the surface 11 and the funnel-shaped propagation region 10 is also a negative curvature, which is the distribution and spreading of the molten melt material within the connection funnel 10th supported. As in the embodiment of the FIGS. 4a and 4b it is provided that the volume formed by the connection funnel 10 corresponds to at least half the volume of the melt material of the fusible element 3.

The melting material may in all embodiments be formed from low-melting solder, which is preferably provided with a flux, such as e.g. a flux soul in the interior of the fusible element 3 or as a surface covering of the fusible element 3 is formed.

Other embodiments, such as e.g. a hollow ball which is open to the opposite connection region and whose inner surface likewise has a negative curvature can be provided. The size of the area in which the molten melting material spreads, depends on the volume of the fusible element and the melt material part, which attaches to the respective connection element 3. The boundary of the propagation region should not have to be exceeded by the molten melting material during the melting of the fusible element 3 in order to completely separate the thermal fuse 1.

In the embodiments shown, the opposing connection elements 2 are identical. These can also be designed differently, which in particular can shift the distribution of the melting material parts accumulating during the melting process.

Claims (3)

1. Thermal fuse (1) for interrupting a power flew in modules, in particular for use in the automotive sector, comprising:

   - a connecting element (2) with a connecting region

   - a fusible element (3) of fusible material, which is attached by one end to the connecting region, in order to create an electrically conducting connection between the fusible element (3) and the connecting element (2);

   the connecting element (2) having an expansion region for accommodating melted fusible material,
   wherein
   the expansion region has an expansian area (6), on which part of the fusible material from the melted fusible material spreads out during the melting of the fusible element, characterized in that the expansion area (6) corresponds to an inside area of a funnel-shaped structure.
2. Thermal fuse (1) according to Claim 1, characterized in that two connecting elements (2) are provided, between which the fusible element (3) is accommodated, so that ends of the fusible element (3) are attached to the corresponding connecting elements (2).
3. Thermal fuse (1) according to Claim 1 or 2, characterized in that the tip of the funnel-shaped structure is flattened with an area which is equal to or smaller than the cross-sectional area of the end of the fusible element (3).

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