Patent Grant
9435832
The invention refers to an electronic component, in particular a current sensor.
These types of current-sense resistors are known, for example from EP 0 605 800 A1 and EP 1 253 430 A1, and allow measurement of an electrical current according to the four-wire technology. In this case, the electrical current to be measured is passed via two contacts through the current-sense resistor while the electrical voltage is measured via two further contacts, which drops over the resistance element of the current-sense resistor. From the voltage measured it is then possible, by applying Ohm's law, to calculate the electrical current which flows through the current-sense resistor.
From EP 1 253 430 A1 it is furthermore known that a measurement value recording system in the form of an ASIC (ASIC: Application Specific Integrated Circuit) is directly arranged on a current-sense resistor so that the current-sense resistor forms a current sense module together with the ASIC. Current measurement using this type of current sense modules offers the advantage of very precise measurement and a low construction size; however, such current sense modules are expensive and are therefore only used for precision applications where a high precision of measurement is important.
In the industrial area there are numerous applications (for example current limit switches, electronic fuses, and others), for which it is only necessary to have a monitoring function in place, whereas the precision of measurement is less important. For some applications it is even desirable that the current measurement only reacts slowly to changes in the current to be measured in order to avoid excessively frequent switching off due to short-term current peaks. It can therefore be desirable that the current measurement reacts to a change in the current to be measured according to a time constant which is in the range of seconds. Furthermore, it should also be possible to measure both a direct current and an alternating current, such as a 50 Hertz alternating current, an alternating current strongly distorted by switching operations, or even a high-frequency alternating current.
Reference is also made to U.S. Pat. No. 3,026,363 concerning the prior art. This patent discloses a component for measurement of the temporal average of an electrical signal, such as a broadband signal. Here there is indeed a temperature measurement made by means of a thermoelement but the temperature difference between the resistance element and its connection parts is not measured.
Furthermore, a current-sense resistor is known from DE 10 2004 062 655 A1 for which the heating up of the current-sense resistor and the resultant change in the resistance value is taken into account in order to avoid temperature-dependent measurement errors. In this regard too, however, the temperature difference between the resistance element and its connection parts is not measured. This patent application therefore just discloses temperature compensation for a current-sense resistor.
Finally, regarding the general technical background, one is referred to US 2004/0227522 A1 and US 2010/0040120 A1.
The object of the invention is therefore to create another option for current measurement which is less costly to produce and also, preferably, avoids the above-mentioned disadvantages.
This object is achieved by a current sensor according to the invention.
The invention is based on the technical insight that a temperature difference arises when operating a current sensor between the resistance element made out of a resistance material on the one hand and the neighboring connection parts made out of a conductor material on the other hand. This temperature difference is due to the fact that the resistance material of the resistance element is generally of a higher impedance than the conductor material of the connection parts so that an appropriately larger thermal power loss occurs in the resistance element generating heat, which is thermally removed through the connection parts. This temperature difference between the resistance element on the one hand and the connection parts on the other hand, in turn depends on the size of the electrical current, which flows through the resistance element, so that this temperature difference constitutes a measure for the electrical current.
The invention therefore provides a temperature measuring device, which measures the temperature difference between the resistance element on the one hand and at least one of the two connection parts on the other hand in order to derive the electrical current to be measured from this. It is necessary to mention here that the temperature measuring device is meaningfully galvanically separated from the connection parts and the resistance element.
In a preferred exemplary embodiment of the invention, consisting of the temperature sensors and the evaluation circuit, the temperature measuring device has at least one thermoelement electrically insulated from a shunt with a hot contact point and a cold contact point, wherein the hot contact point thermally contacts the resistance element while the cold contact point thermally contacts one of the two connection parts. The thermoelement generates a thermoelectric voltage in an energized condition of the current sensor according to the Seebeck Effect, wherein the measured thermoelectric voltage constitutes the temperature difference and, therefore, a measure of the electrical current. Thus the thermoelement has elements, which are preferably aligned in the current sensor parallel to the direction of the current, so that the hot contact point lies on the resistance element while the cold contact point lies one of the connection parts.
In the preferred exemplary embodiment of the invention, the temperature measuring device has a thermopile with a number of thermoelements which are electrically connected in series. This offers the advantage that also relatively low temperature differences between the resistance element on the one hand and the connection parts on the other lead to a metrologically evaluatable thermoelectric voltage, since the thermoelectric voltages from the individual thermoelements add together within the thermopile.
In this connection, the thermopile is preferably arranged in such a way that the cold contact points of the thermoelement alternately contact the first connection part and the second connection part thermally. It is, however, possible as an alternative, that the cold contact points of the thermoelement of the thermopile continuously thermally contact only the first connection part or only the second connection part.
The concept of thermal contacting used within the framework of the invention requires that the respective contact points are attached to the resistance element or the connection part in such a way that there is a low heat transfer resistance. For example, this thermal contacting can occur in such a way that the hot or the cold contact points are firmly glued onto the resistance element or the connection parts using a heat conductive adhesive. One other option for thermal contacting is to firmly solder the hot or the cold contact points to the resistance element or the connection parts using an electrically insulating carrier. However, concerning the thermal contacting of the hot or the cold contact points, the invention is not restricted to the above examples but may also be implemented in another way.
In the preferred exemplary embodiment of the invention there are two thermopiles provided electrically insulated from the shunt, wherein the hot contact points on both thermopiles each thermally contact the resistance element. On the first thermopile the cold contact points thermally contact the first connection part, whereas on the second thermopile the cold contact points thermally contact the second connection part. Thus the first thermopile measures the temperature difference between the resistance element on the one hand and the first connection part on the other hand. The second thermopile, on the other hand, measures the temperature difference between the resistance element and the second connection part.
In one variant of this exemplary embodiment both of these thermopiles are connected together on the component to form a series connection and are connected with two common contacts on the component. Therefore, the thermoelectric voltage dropping over both contacts mirrors the sum of both temperature differences between the resistance element and both connection parts.
In another variant of this exemplary embodiment, both thermopiles are, on the other hand, each connected with two contacts electrically separated from each other on the component and also separate from each, in order to measure the thermoelectric voltages of both thermopiles separately from each other. This is then advantageous if a direct current is measured and the conductor material of the connection parts is not thermoelectrically adapted to the resistance material of the resistance element. In this case, an asymmetrical temperature distribution arises from the resistance element to both connection parts in the current sensor due to the Peltier Effect. The direction of the current can be derived from the difference of the thermoelectric voltages of both thermopiles, whereas the absolute value of the sum of both thermoelectric voltages constitutes a measure of the size of the electrical current to be measured.
It was already mentioned above that the hot contact points of the thermoelements thermally contact the resistance element since the resistance element in an energized condition is hotter than the neighboring connection parts due to its larger specific electrical resistance. In this case, the hot contact points of the thermoelement can lie next to each other in a row, wherein the row of hot contact points can run essentially transverse to the direction of the current in the electronic component.
The temperature inside the resistance element is, however, usually not spatially constant, but instead drops from a so-called “hot spot” in the middle of the resistance element in the direction of the neighboring connection parts. Furthermore, the temperature of the resistance element is also not constant in the lateral direction with reference to the direction of the current, but drops from the middle of the resistance element to the exposed edges. It can therefore be advantageous if the hot contact points are arranged in such a way that they are distributed spatially over the resistance element so that the thermopile measures an average value of the temperature of the resistance element, whereby the measurement can be improved.
Indirect measurement of a current by means of a temperature measurement usually is a relatively slow method of measurement, which can be advantageous for certain applications in order to avoid unnecessarily frequent switching off due to short-term current peaks. Therefore, the temperature measuring device reacts to a temporal change in the electrical current and, therefore, also the temperature difference according to a certain first time constant, wherein the first time constant is greater than 0.5 s, 1 s, 2 s, 5 s or even 10 s. However, with regard to the time constants for the temperature measuring device, the invention is not restricted to the above-stated example values but can be also realized with other time constants.
In order to improve the temporal dynamics of the current measurement there is also the possibility, as part of the invention, that the temporal change in the measured temperature difference is evaluated instead of or in addition to the absolute value of the measured temperature difference so that, for example, a statement can already be obtained about the current after just 0.3-0.5 seconds. This evaluation of the temporal change in the measured temperature difference is very suitable for monitoring for short circuits in an electrical circuit.
It should also be mentioned that the temperature measuring device is constructionally integrated in the preferred exemplary embodiment into the electronic component. To do this, the temperature measuring device can, for example, have a printed circuit board, which is fastened to the connection part and/or to the resistance element, wherein the printed circuit board carries the thermopiles. Alternatively, however, there is also the possibility that the thermopiles are attached directly onto the surface of the component, wherein the thermopiles are electrically separated from the component.
Furthermore, within the framework of the invention, there is also the possibility that the temperature measuring device also has an evaluation unit in addition to the thermopile, which determines the electrical current, which flows through the electronic component based on the measured temperature difference. This evaluation unit can, for example, be realized as an ASIC or in some other way.
In one exemplary embodiment of the invention the electronic component not only has a temperature measuring device but also a controllable switching element, wherein the switching element is connected with the temperature measuring device and switches depending on the measured temperature difference. In the preferred exemplary embodiment of the invention the switching element disconnects an electrical circuit if the measured temperature difference shows that the electrical current flowing through the current sensor has exceeded a prescribed maximum value. The switching closes the electrical circuit again if the temperature difference measured by the temperature measuring device goes below a prescribed minimum value. In this case, a switching hysteresis can be provided in such a way that the maximum value is greater than the minimum value. Resetting of the switching element can also take place by hand, however.
It was already mentioned above that the electrical current flowing through the current sensor can also be measured by means of a voltage measurement according to the known four-wire technology. In a preferred exemplary embodiment of the invention there is also provision for a voltage measurement unit, which measures the electrical voltage, which drops over the resistance element in order to allow for derivation of the electrical current. In this way, the current measurement takes place using two different measuring principles, namely, on the one hand, measurement of the temperature difference between the resistance element and the neighboring connection parts and, on the other hand, by means of a voltage measurement according to the known four-wire technology. This offers the advantage of a redundancy for the current measurement.
The voltage measurement unit for measurement of the voltage, which drops over the resistance element is preferably constructionally integrated into the electronic component so that the component together with the voltage measurement unit and/or the temperature measuring device forms a current sense module. To this effect, the voltage measurement unit can have a printed circuit board, which is fastened to the connection part and/or to the resistance element, wherein the printed circuit board can, for example, have an ASIC as described in patent application EP 1 253 430 A1 or in EP 1 363 131 A1 mentioned above. Furthermore, the voltage measurement unit has two voltage tapping points, which are connected electrically with both connection parts in order to measure the voltage, which drops over the resistance element.
In the preferred exemplary embodiment of the invention, the voltage measurement unit on the one side and the temperature measuring device on the other side are arranged on opposing sides of the electronic component. In this connection the temperature measuring device with the thermopile is preferably arranged on the underside of the current sensor, that is on the assembly side, on which the current sensor can be attached, according to the surface mounting technology (SMD: Surface Mounted Device), to a printed circuit board. The voltage measurement unit, on the other hand, is preferably arranged on the upper side of the current sensor, that is on the side opposite the assembly side.
It should also be mentioned that the current measurement is temporally significantly more dynamic according to the four-wire technology than the current measurement through measurement of the temperature difference between the resistance element and the neighboring connection parts. Thus, the voltage measurement unit reacts according to a certain second time constant to a temporal change of the electrical current flowing through the electronic component, wherein the second time constant is smaller than the first time constant of the temperature measuring device. For example, the second time constant of the voltage measurement unit can be less than 100 ms, 50 ms, 20 ms or 10 ms.
In an advantageous development of the invention the thermoelectric voltage generated by the thermopile is not only evaluated as a measurement parameter but also used for power supply. In another development of the invention there is a power supply unit provided, which supplies the temperature measuring device, the voltage measurement unit, the evaluation unit and/or the controllable switching element with the electrical power required for operation, wherein the power supply unit is fed by the thermopile so that no external power supply is needed. In order to achieve an adequately high supply voltage the thermopile can have more than 20, 50, 100, 200, 500 or even more than 1000 thermoelements and make an output voltage of more than 50 mV, 100 mV or even more than 200 mV available.
In general terms, the invention also encompasses the general principle of having redundant current measurement using a measurement value recording system, wherein the measurement value recording system measures the electrical current flowing through the electronic component according to different physical measuring principles such as a four wire measurement of current and voltage, on the one hand, and through measurement of the temperature difference between the resistance element and the neighboring connection parts on the other hand. This general principle of having redundant current measurement can, however be realized using other measuring principles.
In the preferred exemplary embodiment of the invention the connection parts and/or the resistance element are plate-shaped, in particular in a planar or bent form. Alternatively, however, there is also the possibility that the component is formed using a convex or round material or one of another shape or is formed from pieces of pipe. Furthermore, the connection parts are preferably welded to the resistance element, in particular by means of electron beam welding. The individual current-sense resistors can therefore be cut out of a so-called Tri-Band, wherein the Tri-Band can consist of two outer lying copper strips, which are electron beam welded with Manganin® strips lying in the middle. This type of manufacture of the current sensor from a composite material is very inexpensive, as is explained in detail in EP 0 605 800 A1.
It should also be mentioned that the conductor material of the connection parts has a smaller specific electric resistance than the resistance material of the resistance element. For example, the conductor material of the connection parts can have a specific electric resistance of less than 5·10−7 Ωm, 2·10−7 Ωm, 1·10−7 Ωm, 5·10−8 Ωm or even 2·10−8 Ωm. On the other hand, the resistance material of the resistance element is low-ohmic, in particular with a specific electrical resistance of less than 50·10−7 Ωm, 20·10−7 Ωm, 10·10−7 Ωm or even 5·10−7 Ωm. The resistance material does, however, have a higher impedance than the conductor material with a specific electrical resistance of more than 10·10−8 Ωm, 20·10−8 Ωm, 50·10−8 Ωm, 10·10−8 Ωm, 10·10−7 Ωm or 20·10−7 Ωm.
The resistance material of the resistance element is preferably a resistance alloy, in particular copper-manganese-nickel, in particular Cu84Ni4Mn12 (Manganin®), nickel-chromium, nickel-chromium-aluminum-silicone, copper-nickel, nickel-iron, copper-nickel-manganese or copper-nickel.
It should also be mentioned that the resistance material of the resistance element preferably has a high temperature constancy concerning its specific electrical resistance. For example, the linear temperature coefficient of the resistance material can be less than 1·10−3K−1, 0.5·10−3K−1, 0.2·10−3K−1, 0.1·10−3K−1, 0.05·10−3K−1 or 0.03·103K−1.
Furthermore, it should also be mentioned that the resistance material of the resistance element has a different thermoelectric power in the thermoelectric potential series than the conductor material of the connection parts so that the measurement using thermoelements generates a respective thermoelectric voltage.
Furthermore, the resistance material of the resistance element usually has a lower thermal conductivity than the conductor material of the connection parts, so that the temperature in the connection parts drops more slowly outwards than in the resistance element itself. For example the thermal conductivity of the resistance material of the resistance element can lie in the range of 20 Wm−1K−1 up to 500 Wm−1K−1.
Other advantageous developments of the invention are explained in more detail below together with the description of the preferred exemplary embodiments of the invention on the basis of the figures. The figures show as follows:
The current sense module 1 has a current-sense resistor 2, which consists of two plate-shaped connection parts 3, 4 made out of copper or a copper alloy and a plate-shaped resistance element 5 made out of Manganin®, wherein the current-sense resistor 2 is cut out of a composite material strip (“Tri-Band”), which consists of two copper strips, which are electron beam welded with Manganin® strips lying in the middle, which is already known from the patent application EP 0 605 800 A1 mentioned above.
A voltage measurement unit 6 is arranged on the upper side of the current sense module 1, which voltage measurement unit essentially consists of a printed circuit board 7 and electronic components (for example an ASIC) of the printed circuit board 7, wherein the voltage measurement unit 6 is connected electrically with both connection parts 3, 4 and measures the voltage, which drops over the resistance element 5, in order, by applying Ohm's law, to calculate the electrical current, which flows through the current-sense resistor 2. The voltage measurement unit 6 can have a conventional form here, as described in the above-mentioned patent application EP 1 253 430 A1.
On the assembly side of the current sense module 1, which is the underside, there is, on the other hand, a temperature measuring device 8 attached, which has a thermopile 9, wherein the thermopile 9 is arranged on a printed circuit board 10 and has two contacts 11, 12 at which a thermoelectric voltage can be measured, which reflects the temperature difference between the resistance element 5 on the one hand and the connection parts 3, 4 on the other hand. To this effect, the thermopile 9 has hot contact points 13 and cold contact points 14, 15, wherein the hot contact points 13 thermally contact the resistance element 5 through the printed circuit board 10, whereas the cold contact points 14, 15 alternately thermally contact the connection part 3 and the connection part 4 through the printed circuit board 10. Therefore, the thermoelectric voltage dropping over the contacts 11, 12 mirrors the temperature difference between the resistance element 5, on the one hand, and the connection parts 3, 4, on the other, wherein this temperature difference constitutes a measure of the electrical current, which flows through the current-sense resistor 2.
Thus,
One advantageous aspect of the current sense module 1 is the fact that the current measurement takes place redundantly, namely, on the one hand, by the voltage measurement unit 6 and, on the other hand, by the temperature measuring device 8.
A particularity of this exemplary embodiment is that the temperature measuring device 8 has two thermopiles 9.1, 9.2, which are galvanically separated from each other. The thermopile 9.1 measures the temperature difference between the resistance element 5 and the connection part 3, while the thermopile 9.2 measures the temperature difference between the resistance element 5 and the connection part 4.
The sum of both of the thermoelectric voltages, which are measured by both thermopiles 9.1, 9.2, constitutes here a measure of the size of the electrical current which flows through the current-sense resistor 2.
Furthermore, the Peltier effect causes an asymmetrical temperature distribution in the current-sense resistor 2 as can also be seen to some extent from
In contrast to the exemplary embodiment according to
Finally,
In this circuit diagram, it can also be seen that a differentiator 19 is provided, which differentiator forms the time derivation Δ{dot over (T)} of the temperature difference ΔT measured by the temperature measuring device 8.
Furthermore, there is an evaluation unit 20 provided, which determines the electrical current I, which flows through the current-sense resistor 2. Here, the evaluation unit 20 takes account, on the one hand, of the temperature difference ΔT and the time derivation Δ{dot over (T)} of the temperature difference ΔT and, on the other hand, the electrical voltage U, which drops over the resistance element 5 and which is measured by the voltage measurement unit 6.
The measurement value of the electrical current I measured in this way is passed on to a threshold element 21, wherein the threshold element 21 actuates a relay 22 or some other kind of switching element. If the measured electrical current I exceeds a prescribed maximum value IMAX, then the threshold element 21 actuates the relay 22 in such a way that the electrical circuit is disconnected in order to avoid any further increase in the current. If the electrical current I then goes below a prescribed minimum value IMIN, the threshold element 21 actuates the relay 22 in such a way that the electrical circuit is closed again. The threshold element 21, in this case, has a switching hysteresis in order to prevent excessively frequent switching.
Finally, it should also be mentioned that the current sense module 1 has a power supply unit 23, which is fed by the thermopile 9 and supplies the temperature measuring device 8, the differentiator 19, the evaluation unit 20, the threshold element 21, the relay 22 and the voltage measurement unit 6 with the electrical power required for operation, so that the current sense module 1 does not need any external power supply.
It should furthermore also be mentioned that there is a galvanic separation 24 provided between the evaluation unit 20 and the voltage measurement unit 6.
The invention is not limited to the preferred exemplary embodiments described above. Instead, a plurality of variants and modifications are possible, which likewise make use of the concept of the invention. Furthermore, the invention also claims protection for the features of the sub-claims independently of the features of the preceding claims to which they refer so that, as part of the invention, any number of combinations of the features mentioned in the claims or in the description are possible.
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| Number | Date | Country | |
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| Parent | 13582188 | US | |
| Child | 15016969 | US |
