Patent Application
20100085141
The invention relates to a fuse for interrupting a voltage and/or current-carrying conductor in case of a thermal fault and a method for producing the fuse according to the class of the independent claims.
Especially devices with a very high current load often do not provide the possibility for separating the corresponding control and/or power electronics from the power source in case of a thermal fault, i.e. for example when very high ambient temperatures, which are significantly above 100 EC, occur as a result of middle- or low-resistance short circuits. Appropriate temperature fuses for preventing thermal damages are, however, necessary in particular in motor vehicles.
It is, for example, known from the American patent U.S. Pat. No. 6,737,770 B2 how to separate the coil of a brushless motor from the power source by means of a fuse. In so doing, an end of the fuse is soldered on; so that when a certain limit temperature is exceeded, the mechanically biased part of the fuse leads to a separation of the soldered joint.
In the European patent EP 1 120 888 A1, a heat-resisting mechanism is disclosed, which is thermally coupled to a heat sink of a circuit breaker and separates the power source of a brushless motor from the coil. As is the case in the American patent U.S. Pat. No. 6,737,770 B2, an end of the fuse is also soldered on here. When a certain limit temperature is exceeded, the mechanically biased part of the fuse thus leads to a separation of the soldered joint. A corresponding fuse is furthermore known from the patent WO 00/08665.
The German patent DE 39 09 302 A1 reveals a fuse, in which a new alloy with a high electrical resistance arises from the melting of two highly electrically conductive alloys. Said new alloy prevents a further flow of high currents.
A disadvantage of the aforementioned fuses is, for example, the limited service life as a result of a permanently mechanically biased, soldered joint. Furthermore, insufficiently high tolerances can arise due to a simultaneous influence of temperature and current. A satisfactory and safe usage, in particular for the automotive field, is therefore basically not provided.
The invention relates to a fuse for interrupting a voltage and/or current-carrying conductor in case of a thermal fault, having a conductor bar ensuring an electrically conductive connection of the voltage and/or current-carrying conductor during correct operation. In an advantageous manner, the conductor bar melts upon an increase in temperature above the melting point so that the electrically conductive connection of the voltage and/or current-carrying conductor is interrupted due to the surface tension of the conductor bar. The melting point of the conductor bar is selected thereby in such a way that on the one hand a melting of the conductor bar can be ruled out during correct operation, while on the other hand the melting is ensured in case of a thermal fault. In particular for electric motors with or without electronics, a safe and reliable de-activating path is consequently ensured, which essentially depends on the temperature and not on the current, when inadmissibly high temperatures occur, for example, due to breakdowns of components or short circuits resulting from external impacts or malfunctions of insulating materials. In this way, an activation of the fuse is also possible for disturbances, which only lead to small currents beneath the admissible maximum currents. Moreover, a mechanical bias of the fuse can be avoided so that said fuse is not exposed to any additional stress. This fact leads to a significantly longer service life with respect to the fuses according to the state of the art.
The invention furthermore relates to a method for producing the fuse with a retaining element and a conductor bar for interrupting a voltage and/or current-carrying conductor in case of a thermal fault. The retaining element has a first and a second part, the second part serving to connect said retaining element to the voltage and/or current-carrying conductor and the conductor bar being affixed, respectively inserted, in a force-fitting or positive-locking manner on or into the first part of said retaining element. The fuse can consequently be advantageously produced independent from the later application.
Additional advantages of the invention are apparent from the characteristics indicated in the independent claims as well as from the drawing and the following description.
According to the invention, at least one end of the conductor bar is held by a retaining element of the fuse. Said retaining element has a first part for holding the conductor bar and a second part for connecting the retaining element to a stamped grid, a printed circuit board or the like. In this way the fuse can very easily be integrated into varying applications.
The first part of the retaining element is configured in an advantageous manner as a hollow body having one open side. The conductor bar is fixed inside the hollow body by a soldering metal, the melting point of the solder lying below that of the conductor bar and above the maximally admissible temperature for correct operation.
In order to achieve a still better fixing of the conductor bar to the retaining element, the hollow body has at least one raised portion on its outer circumference, which constitutes a point of force application for a mechanical deformation of the hollow body for holding the conductor bar. The first part can, however, also alternatively be configured as an obtuse contact surface.
The first part and the second part of the retaining element are advantageously implemented as one piece. It is also, however, possible for both parts to be welded or riveted together. In order to allow for a good connection and one made as easy as possible to the stamped grid or to the printed circuit board, the second part of the retaining element is of bar-, wire- or strip-like form. It is furthermore possible in this connection for the second part to be angled with respect to the preferred orientation of the conductor bar for the purpose of strain relief. The retaining element can also additionally be an integral component part of the stamped grid.
In a particularly advantageous manner, the conductor bar consists of metal or a highly electrically conductive alloy, in particular a soft solder alloy such as Sn, SnAG, SnAgCu or the like. A sufficiently sound thermal connection to the environment as well as a sufficiently low specific resistance of the conductor bar is furthermore ensured by a sufficiently large cross-section. In so doing, said bar warms up only slightly with respect to the environment even when the current is at a maximally admissible level. Furthermore, an improved, i.e. more reliable, melting behavior in connection with the surface tension is achieved if the conductor bar has a flux core. It is also advantageous if the core of the conductor bar comprises an activator-medium, which consists of carboxylic acid or a salt of the carboxylic acid, contains carboxylic acid or a salt of the carboxylic acid or contains a mixture of carboxylic acid and a resin or a salt of the carboxylic acid and a resin. As a result, a significant increase in the activation temperature for such a fuse is possible with respect to a fuse on the basis of media containing rosin as a flux. By using the activator-media as a flux instead of using rosin, the thermal application range of such a fuse can be greatly expanded in this way.
As an alternative to a flux core, the conductor bar can also have a flux coating, which contains a carboxylic acid or a salt of a carboxylic acid. In particular the flux coating can be embodied by a coat of lacquer. This provides the advantage of being able to apply the coat of flux to the fuse from the outside after soldering the conductor bar to the retaining element. On the one hand, such a procedure can be easily implemented during manufacturing, and on the other hand it does not require transient liquid phase soldering, whereby the flux potentially runs when soldering the conductor bar into the retaining element. The fuse can thereby prematurely activate.
Provision is made in an advantageous manner for the following steps with regard to the method according to the invention for producing the fuse:
It is additionally advantageous if the hollow body is mechanically deformed before or after being heated. The heating can also first take place after inserting the conductor bar into the hollow body. Moreover, it is possible in an advantageous way to attain heating by a thermal pulse, which is impressed on the second part of the retaining element, on the raised portion of the hollow body or on the conductor bar. The thermal pulse can also alternatively be impressed in a non-contact manner by laser or infrared light. The duration of the thermal pulse must thereby be selected in such a way that the conductor bar definitely melts only in the interior of the hollow body, in particular in the region of a base or of the raised portions of said body. A melting outside of the hollow body as a result of a thermal pulse lasting too long is on the other hand worth avoiding. In this context, splashing the conductor bar with a coolant outside of the hollow body of the retaining element, immersing the conductor bar in the coolant or mechanically clamping it to a thermal ground can be advantageous, the jaws of a holding tool serving as a thermal ground. If the second part of the retaining element is of strip-like form, the additional strip material can also serve as a thermal ground, provided the impressing of the thermal pulse occurs before the second part is punched out.
Corresponding to the previous embodiments of the conductor bar with a flux coating, provision can also be made in the method for a step, which applies a flux or activator to the conductor bar, the flux in this form of embodiment develops, for example a lacquer layer around the conductor bar. In particular when a conductor bar without an internal flux core is used, said aforementioned step provides the advantage that a significantly simpler and more reliable manufacturing method can be employed for the production of the fuse.
In order to check for the correct production of the fuse, provision can also be made in the method for a step to check the connection between the first part of the retaining element and the conductor bar affixed or inserted in a force-fitting or positive-locking manner. In so doing, the check can take place automatically or optically in an advantageous way. A probe, which is disposed as to be freely displaceable, can therefore also be used to cover a region to be checked in the first part of the retaining element. In so doing, the opportunity is provided to also assure the correct manufacturing and with it the accurate functionality of the manufactured fuse by the continued use of present devices for the control of the manufacturing of a printed circuit board, respectively its assembly. This is done without an extensive technical outlay with additional expenses. An operating result for confirming a flawless soldering can be specially supplied during the checking step if a solder meniscus is detected when the first part of the retaining element is connected to the conductor bar, which is inserted in a force-fitting and/or positive-locking manner. Such a function check can be simply and cost effectively implemented by the proposed use of the probe and the evaluation of the reflection pattern of the solder joint.
The invention is paradigmatically described below with the aid of the
An example of embodiment of the fuse 10 according to the invention for interrupting a voltage and/or current-carrying conductor 12 in case of a thermal fault is depicted in
The conductor bar 14 is made from metal or a highly electrically conductive alloy, in particular a soft solder alloy like tin (Sn), tin-silver (SnAg), tin-silver-copper (SnAgCu) or the like. Its cross-section, its thermal connection to the environment as well as its specific resistance is selected in such a way that the conductor bar 14 warms up only marginally with respect to the environment when a maximally admissible current is present. This requirement is met, for example, by a conductor bar 14 of bar-like form with a very small specific resistance. The melting point of the conductor bar 14 is furthermore selected in such a way that on the one hand melting can be assuredly ruled out during correct operation, while on the other hand said melting is ensured in case of a thermal fault, i.e. when temperature increases occur as a result of operational disturbances, such as, for example: breakdowns of electronic components, malfunctions of the insulating materials, middle- or low-resistance short circuits due to external impacts or the like, in connection with the surface tension of the conductor bar 14. Said melting thus interrupts the current path between the two retaining elements. An assured melting of the conductor bar 14 can furthermore be achieved as a result of said bar 14 additionally having a flux core 18, whereby the flux is known to the specialist and need not be specified here. A suitable flux is, however, especially characterized in that it is non-corrosive during correct operation and furthermore does not age or ages only to a small extent.
Each retaining element 16 consists of a first part 20 for holding the conductor bar 14 and a second part 22 for connecting the retaining element 16 to the voltage and/or current-carrying conductor 12, which, for example, can be configured as a stamped track of a stamped grid, as a conductor path of a printed circuit board, as a cable or the like. The first part 20 is configured in the example of embodiment according to
The second part 22 of the retaining element 16 is of bar-, wire-, or strip-like form for connecting to the voltage and/or current-carrying conductor 12, depending on whether said voltage and/or current-carrying conductor 12 relates to a stamped track, a cable or a conductor path. In an advantageous manner, the first part 20 and the second part 22 of the retaining element 16 are embodied as one piece. It is, however, also conceivable that the two parts 20 and 22 are welded or riveted together. In order to ensure an improved strain relief of the fuse, particularly the second part 22, which is of wire-like form, can also be angled. This is, however, not shown in the figures.
A third and a fourth example of embodiment of the retaining element 16 can be seen in
The manufacture of the fuse 10 according to the invention takes place now in such a way that the conductor bar 14 is affixed, respectively inserted, on or into the first part 20 of the retaining element 16 in a form-fitting and/or positive-locking manner. Provision can additionally be made for the solder 28 to initially be applied in or on the first part 20 of the retaining element 16. In so doing, the contact surface 38, respectively the base 30 and/or an interior wall 32, respectively interior surface 34, of the first part 20 are covered with solder 28, which melts at a lower temperature in comparison to the conductor bar 14. By means of a suitable device, the retaining element 16 and/or the conductor bar 14 are then heated to a temperature value between the melting point of the solder 28 and the melting point of the conductor bar 14. While the solder 28 is fluid, the conductor bar 14 is inserted, respectively affixed, in such a way into or onto the first part 20 of the retaining element 16 so that the conductor bar 14 comes into contact with the solder 28. Finally the cooling down of the fuse 10 occurs and with it the connection of the conductor bar 14 with the retaining element 16 in a positive-locking manner, for example by splashing the conductor bar 14 outside the first part 20 with a coolant. The conductor bar 14 can also alternatively be immersed in the coolant or mechanically clamped to a thermal mass, for example to the jaws of a holding tool. If the second part 22 of the retaining element 16 is of strip-like form, the additional strip material can also serve as a thermal ground.
If the first part 20 of the retaining element 16 is configured as a hollow body 24, a force-fit connection between the retaining element 16 and the conductor bar 14 can additionally be achieved prior to or after heating by a mechanical deformation serving as a stamping process by means of the raised portions 36.
The heating occurs by a thermal pulse, which is impressed on the second part 22 of the retaining element 16, on the raised portion 36 of the hollow body 24 or on the conductor bar 14. A contactless heating by a laser, infrared light or the like is also possible. In so doing, the duration of the thermal pulse must be selected in such a way that the conductor bar 14 definitely melts only in the interior 26 of the hollow body 24, in particular in the region of the base 30 or the raised portions 36 of the hollow body 24. Melting of the conductor bar 14 outside of the hollow body 24 as a result of the thermal pulse lasting too long is worth preventing with the aid of the cooling procedure already described. As a rule, said procedure can, however, be dispensed with because the thermal pulse can be applied very precisely. Finally it should be mentioned that the heating can alternatively take place right after inserting the conductor bar 14 into the hollow body 24.
Furthermore, the quality of the fuse produced, respectively terminated, should also be examined. For a terminated fuse, the soldering between the fuse and the termination, i.e. the retaining element, is essential for its operation and reliability. The open geometry of the termination (flat or U-shaped) introduced here allows for an AOI (AOI=automated optical inspection). Said AO I is employed here in the same fashion as it can also be employed in the case of printed circuit board assembly. In the method proposed here, the soldering meniscus is analyzed, which only forms when the soldering is done correctly. In
Thermal fuses with an internal flux core were described. Known thermal fuses on the basis of molten bridges are characterized in contrast by a flux, which has been coated on the molten bridge. The flux used in such a fuse is thereby based on rosin, which becomes liquid at approximately 100 EC and produces a high vapor pressure at 140 EC, which leads to a rapid evaporation. For this reason, the customary molten bridges are always enclosed by a ceramic sleeve, which is intended to prevent the loss and aging of the flux. This ceramic sleeve, however, enlarges the structural shape, increases the self-heating and the heating output (on account of the long connections) and increases the manufacturing costs. It has become apparent in tests that a flux core containing rosin leads to a mechanical deformation of the molten bridge through its vapor pressure already from temperatures starting at approximately 120 EC.
Promising candidates are found in the class of organic carboxylic acids (or their salts), which have melting temperatures in the range of up to >170 EC. For this reason, such materials permit the construction of fuses, which first activate at an ambient temperature of 170 EC. This represents a significantly higher activation temperature for fuses with respect to the known fuses. These organic carboxylic acids by themselves or mixed with resins can be used as an alternative to rosin based fluxes. In their pure form, carboxylic acids are therefore not designated as a flux but as an “activator”. For the aforementioned application as a flux, respectively its replacement, pure carboxylic acid or a synthetic flux consisting of an activator and resin can be used. In the latter case, the resin used should also have the characteristics which were previously stated.
As an alternative to the fuse with the previously described internal flux core, the molten bridge could also be externally coated with a refractory flux lacquer, respectively activator lacquer. For this purpose, the active substance, for example a carboxylic acid, is mixed with a bonding agent to form a lacquer, which is to be externally applied.
Vis-à-vis an internal flux core, the necessity for a transient soldering process as is depicted in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 014 332.1 | Mar 2007 | DE | national |
| 10 2008 003 659.5 | Jan 2008 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP08/51769 | 2/14/2008 | WO | 00 | 9/24/2009 |
