ELECTRONIC SWITCH

Description

[0001] The invention relates to an electronic switch having a first and a second terminal, between which an electrically conductive connection can be established or interrupted, and having at least one bidirectional turn-off semiconductor switching element connected between the first and second terminals.

[0002] In the future, semiconductor-based circuit breakers, hereinafter referred to as electronic switches, could replace conventional miniature circuit breakers in both AC and DC networks. These switches interrupt the current path using semiconductor switches such as IGBTs. The electronic switch has at least one, but usually two, semiconductor switches connected in anti-serial or anti-parallel in the current path.

[0003] Such electronic switches offer advantages in terms of the speed with which fault currents can be interrupted. This is advantageous, for example, in DC networks in future industrial plants, which are intended to reduce losses, ensure direct energy exchange between converters, storage systems, and motors, and achieve increased robustness. The short cable lengths result in very small series inductances in the supply lines, which in turn cause fault currents to exhibit a very steep rise. The necessary rapid interruption is achieved better by semiconductor switches than by mechanical switches, as semiconductor switches can interrupt the fault within a few hundred nanoseconds after detection.

[0004] A problem here is the current that continues to flow after a shutdown due to the inductances of the lines and / or loads. Due to the lack of a freewheeling path, this leads to a high voltage across the electronic switch. If this voltage exceeds the dielectric strength of the semiconductor switch(es), it can be damaged.

[0005] Various solutions are known to address this problem. For example, a capacitor can be installed parallel to the semiconductor switches, slowing the voltage rise after shutdown to such an extent that the dissipation of the inductively stored energy is unproblematic. The electronic switch can also have a varistor arranged either in series with the capacitor or between the poles of the power grid. Such solutions are known, for example, from WO 2019 / 011642 A1 and WO 2020 / 099103 A1. A disadvantage of the known solutions is that the capacitors used are large and heavy components, which contribute significantly to the overall size of the electronic switch.

[0006] It is an object of the present invention to provide an improved electronic switch which, in particular, has a reduced size compared to known solutions.

[0007] This object is achieved by an electronic switch having the features of claim 1.

[0008] The electronic switch according to the invention has a first and a second terminal, between which an electrically conductive connection can be established or interrupted. Typically, the electronic switch with these terminals is serially integrated into a pole of an electrical network and can thus interrupt or establish the electrical connection in this pole.

[0009] The electronic switch further comprises a bidirectional, turn-off semiconductor switching element, which is arranged between the first and second terminals, thus connecting the first and second terminals. This bidirectional semiconductor switching element can establish or interrupt the connection between the first and second terminals. Since the semiconductor switching element is bidirectional, the direction of current flow is irrelevant; the semiconductor switching element can interrupt current in both directions and generate voltage in both directions.

[0010] Finally, the electronic switch includes a voltage limiting circuit connected in parallel with the semiconductor switching element. The voltage limiting circuit includes multiple bidirectional suppressor diodes. In the case of multiple suppressor diodes, these are arranged in a series circuit. It is understood that the bidirectional suppressor diodes can be integrated components or individual unidirectional components connected in reverse series.

[0011] For the invention, it was recognized that suppressor diodes advantageously enable the limitation of a build-up of voltage across the semiconductor switch. Since this limitation occurs via a non-destructive breakdown of the suppressor diodes and causes a corresponding current flow, such a voltage limitation results in power loss in the suppressor diode. It has been found that suppressor diodes are available that can achieve a peak power of several tens of kW over a turn-off period of approximately 200 µs, which makes their use in modern DC networks possible even at operating voltages of, for example, 800 V and with inductively stored energies in the range of 1 to 50 J.

[0012] The use of suppressor diodes eliminates the need for a capacitor, which is otherwise often used for voltage limitation. Since suppressor diodes are significantly smaller and lighter than a capacitor dimensioned for the same network, this results in significant weight and space savings, even when multiple suppressor diodes are used. The varistors used in conventional solutions can also be eliminated. Another advantage is that suppressor diodes have an exceptionally high response speed, allowing them to achieve voltage limitation even with low line inductances and a correspondingly rapid current rise.

[0013] Advantageous embodiments of the device according to the invention emerge from the claims dependent on claim 1. In this case, the embodiment according to claim 1 can be combined with the features of one of the subclaims or preferably also with those of several subclaims. Accordingly, the following additional features can be provided for the electronic switch: According to the invention, the electronic switch comprises a capacitor which is arranged in parallel with one or more of the suppressor diodes. Measurements have shown that despite the theoretically extremely fast reaction of the suppressor diodes to an overvoltage in a real circuit, the voltage initially significantly exceeds the summed breakdown voltage of the suppressor diodes and only drops into the range of the summed breakdown voltage after a time period in the range of 1 µs has elapsed.In the following, the term "summed breakdown voltage" is used, even if only one suppressor diode is present, where in this case the summed breakdown voltage is the breakdown voltage of the single suppressor diode. Depending on the configuration of the electronic switch—that is, depending on the relationship between the operating voltage of a connected network, the dielectric strength of the semiconductor switching element, and the summed breakdown voltage of the suppressor diodes—this initial overvoltage may be unproblematic or may require additional measures.

[0014] Figure 1 shows a measured curve of the current 101 across an exemplary electronic switch as well as the curve of the voltage 102 across the electronic switch.

[0015] Here, the electronic switch has three suppressor diodes connected in series, but not the capacitor. The electronic switch is switched on at time 103, with a short circuit occurring directly on the load side of the switch. The current then rises and is switched off by the switch immediately after the fault is detected, resulting in a peak current of 470 A due to the inductance used. At this time 104, the voltage 102 shows a clear peak with a height of just over 1600 V.

[0016] Since this initial voltage pulse can also be problematic for the semiconductor switching element, the capacitor is advantageously arranged in parallel with at least one of the suppressor diodes. This mitigates the initial voltage spike, as the affected suppressor diode(s) is initially bypassed until the capacitor is charged. As a result, only a reduced total breakdown voltage is present initially, and the initial voltage spike, which exceeds the breakdown voltages, is also correspondingly reduced.

[0017] After the capacitor is charged, the bridging is no longer present and the suppressor diode(s) connected in parallel to the capacitor contribute with their breakdown voltage to the total counter voltage that is built up across the electronic switch.

[0018] Because the capacitor used in the electronic switch does not have to dampen the voltage buildup across the electronic switch for the entire turn-off period, but only for a short initial period of less than 10 µs, its capacitance can be significantly smaller than in conventional solutions. Therefore, its size and weight are significantly smaller.

[0019] In certain embodiments, the capacitor can be arranged in parallel to exactly one of the suppressor diodes.

[0020] According to the invention, several suppressor diodes are present, so that the capacitor is arranged in parallel with fewer than all of the suppressor diodes. It is advantageous if, in addition to the bridged suppressor diode(s), further suppressor diodes are arranged in series, since then, even at the moment of shutdown, a countervoltage is already built up against the inductively driven current flow, thus preventing the short-circuit current from building up further. This prevents a further buildup of inductively stored energy, which then has to be dissipated.

[0021] It is expedient if the sum of the breakdown voltages of all suppressor diodes is less than the dielectric strength of the semiconductor switching element. This limits the voltage across the semiconductor switching element to a value that prevents damage to the semiconductor switching element. At the same time, it is advantageous if the sum of the breakdown voltages of all suppressor diodes is greater than 60%, in particular greater than 80%, and in a special embodiment, greater than 90% of the dielectric strength of the semiconductor switch. A summed breakdown voltage that is as high as possible allows the inductively driven current to dissipate more quickly after the electronic switch is turned off.

[0022] The capacitor can advantageously have a capacitance of less than 10 µF, in particular less than 5 µF. In special embodiments, the capacitor can have a capacitance of less than 2 µF. It is thus significantly smaller than a typical capacitor used in known voltage limiting solutions, which has a capacitance of, for example, 50 µF.

[0023] According to the invention, the semiconductor switching element comprises two semiconductor switches connected in anti-serial or anti-parallel configurations. Such circuits comprising two semiconductor switches, for example, two IGBTs, are available as integrated modules, but they can also be present as individual, separate components. In the case of anti-serial switches, these preferably comprise the known freewheeling diodes arranged in series. The semiconductor switches preferably have a dielectric strength of at least 600 V, in particular at least 1200 V.

[0024] The electronic switch can have a pre-charging circuit. This comprises a series circuit with a switching device, for example another bidirectional semiconductor switching element, and a current-limiting resistor. The pre-charging circuit is connected in parallel to the semiconductor switching element, i.e., is also arranged between the first and second terminals. The pre-charging circuit can be used to charge capacitances present on the load side with a limited current flow before the electronic switch is switched on, i.e., to reduce the voltage difference between the two terminals before the actual switching on. This avoids the almost unlimited current flow that would otherwise occur when the electronic switch is switched on, provided there is a significant load-side capacitance.

[0025] The electronic switch can advantageously be used in an electrical network, in particular a DC network. One such electronic switch or a plurality of electronic switches can be used in the network. The electronic switch is preferably connected with its first and second terminals to one pole of the network, so that the current path of the load current leads through the semiconductor switching element. It is advantageous if the summed breakdown voltage of the suppressor diodes is greater than the operating voltage of the electrical network, so that upon shutdown, a sufficient countervoltage is generated to completely dissipate the current. The electrical network preferably has an operating voltage of at least 400 V, in particular at least 800 V.

[0026] The electronic switch can comprise a control device that controls the semiconductor switching element. The control device can be configured to measure the current flowing through the semiconductor switching element. The current measured by the control device thus corresponds to the current flowing in the network, i.e., the load current. The control device can further be configured to cause the semiconductor switching element to shut down if the current exceeds a threshold value. The threshold value is selected to be characteristic of a fault current. In this case, the electronic switch is therefore operated as a circuit breaker.

[0027] Further advantages and features can be found in the following description of exemplary embodiments with reference to the figures. In the figures, the same reference numerals denote the same components and functions.

[0028] They show: Figure 1a diagram showing current and voltage curves for a shutdown process, Figure 2 a DC voltage system with a first electronic switch not belonging to the invention, Figure 3 a second electronic switch which is an example of the invention, Figure 4 another diagram showing current and voltage curves for a shutdown process.

[0029] The Figure 2 shows a DC voltage system 10 with a load 8 fed from a DC network 7. The DC voltage of the DC network 7 is applied between the first potential 41 and the second potential 42. An electronic switch 1 is arranged between the DC network 7 and the load 8. In addition to this electronic switch 1, there is also a contactor or isolator 80 with which the load 8 can be galvanically isolated from the DC network 7. However, the contactor or isolator 80 is not absolutely necessary for the function of the electronic switch 1.

[0030] The inductances present in the supply lines are represented by element 25. These can be parasitic inductances of lines or also inductive components such as chokes, coils or transformers. The electronic switch 1 has a series connection of two turn-off semiconductor switches 2 between its first connection 11 and its second connection 12. These are arranged anti-serially to one another. This means that the switching element of the two turn-off semiconductor switches 2 can each carry a current with a different direction and thus switch. To increase the blocking capacity, i.e. the voltage to be switched, the number of turn-off semiconductor switches 2 in the series connection 5 can be further increased.

[0031] A diode 15, 16 is arranged antiparallel to the respective switching element of the turn-off semiconductor switch 2. Thus, a current can flow through the diode 15, 16 in the opposite direction to the current flowing through the corresponding switching element of the turn-off semiconductor switch 2. The antiserial arrangement of the two turn-off semiconductor switches 2 results in the current flowing between the first terminal 11 and the second terminal 12 through a diode of one turn-off semiconductor switch 2 and through a switching element of the other turn-off semiconductor switch 2.

[0032] The series circuit of the turn-off semiconductor switches 2 is connected to the first potential 41 of the DC network 7. The electronic switch 1 also has two suppressor diodes 3, which form a series circuit 5. The series circuit 5 is connected in parallel to the two semiconductor switches 2, i.e., like the two semiconductor switches, between the first and second terminals 11, 12. The present DC voltage system 10 is a network with an operating voltage of 400 V. In this example, semiconductor switches 2 with a dielectric strength of 650 V are to be used.

[0033] In this example, as in the following examples, the specific number of suppressor diodes 3 results from a consideration of the existing parameters of the DC voltage system 10. If these are known, the number and type of suppressor diodes 3 selected can be adapted accordingly, as is the case in this example. If the parameters, for example the inductances connected to the electronic switch 1, are unknown, a configuration of the electronic switch 1 can be selected that functions in every case within the framework of minimum and maximum parameters, for example a minimum and maximum line inductance. In the first exemplary embodiment, it is assumed that a very high line inductance or possibly also a load inductance of 200 µH is connected to the electronic switch on the load side. This ensures that the current rise in the event of a fault is relatively slow.The time required to detect a fault plays only a minor role in this case, and the fault can be cleared with a relatively low current. The voltage peak at the beginning of the shutdown process, as shown in . Figure 1 - with other parameters - is therefore small.

[0034] Therefore, in this example, two suppressor diodes 3 of the 30KPA216CA type with a breakdown voltage of approximately 265 V can be selected. Others with slightly higher breakdown voltages can also be selected, as long as the dielectric strength of the semiconductor switches 2 is not exceeded. In other design variants, for example, six suppressor diodes 3 with a breakdown voltage of 95 V can also be selected. The number can be determined by considering, for example, the energy that the suppressor diodes 3 must absorb during shutdown. If the current during shutdown in this example is 200 A, then the peak power of the suppressor diodes 3 is 200 A * 265 V, or slightly more than 50 kW. This peak power can be absorbed by the suppressor diodes 3 as long as the width of the current peak, which corresponds approximately to half the duration of the shutdown, is not greater than approximately 300 µs.If more than two suppressor diodes 3 are used, the power converted in each of the suppressor diodes 3 decreases according to the lower voltage and more time is available for the duration of the shutdown - at least with regard to the suppressor diodes 3.

[0035] If a current now flows at the first potential 41 from the DC network 7 to the load 8 through the electronic switch 1, then the diode 15 is conductive and the diode 16 is blocked. The current flows through the switching element of the turn-off semiconductor switch 2, which is arranged in parallel with the diode 16. If the electronic switch 1 now switches off by blocking the turn-off semiconductor switches 2, the current is driven further by the inductance of the element 25, for example the line inductances. The further driven current now leads to a rapid voltage build-up across the electronic switch 1, i.e. between the first and second terminals 11, 12. Once the sum of the breakdown voltages of the suppressor diodes 3 is reached, they begin to conduct current and thus limit the voltage drop across the series circuit 5 to the sum of the breakdown voltages.The voltage drop across the two turn-off semiconductor switches 2 largely corresponds to this voltage. This protects the turn-off semiconductor switches 2 from a further increase in voltage and possible damage.

[0036] In measurements such as those of the Figure 1It can be seen that the voltage across suppressor diodes 3 initially, i.e. directly after switch-off, rises to a value that is higher than the summed breakdown voltage of suppressor diodes 3. The summed maximum clamping voltage or even more may be reached here. After switch-off, the voltage drops to the value of the summed breakdown voltage within a period of approximately 1 µs. The initial overvoltage reached depends on the switched-off current. In the first exemplary embodiment, a high inductance and therefore a relatively low fault current were assumed. The initial overvoltage therefore does not pose a significant problem and does not require any countermeasures.

[0037] However, if electronic switch 1 is connected to a line with only a low inductance, a fault current rises very rapidly. Current rise rates of 200 A / µs can be reached. In this case, the considerable dead time that elapses before electronic switch 1 detects a fault current and switches off causes the current to assume higher values, such as 470 A, before the switch-off occurs.

[0038] In this case, the initial overvoltage can become problematic. Such a case is Figure 1 shown. Figure 3shows an electronic switch 1 according to a second example. In the second example, it is assumed that the operating voltage of a second, not shown, DC voltage system is 800 V. Instead of the suppressor diodes 3 used in the first example, those of the type 30KPA280CA are used here, whose breakdown voltage is 334 V. In this case, three suppressor diodes 3 connected in series are necessary, since only with three suppressor diodes 3 is the summed breakdown voltage higher than the operating voltage of 800 V. The semiconductor switches 2 used in the second example are intended to have a dielectric strength of 1200 V. To avoid repetition, reference is made to the description of Figure 2 and to the reference symbols introduced there.

[0039] A real electronic switch 1, which is constructed according to the invention, in particular according to the embodiment of Figure 3typically also includes a pre-charging circuit 30 as shown in Figure 3 This comprises a bidirectional semiconductor switch 31 in series with a current limiting resistor 32. This pre-charging circuit 30 serves to charge the capacitances present on the load side before switching on the semiconductor switches 2 in order to avoid an extremely high inrush current. If, in an embodiment not belonging to the invention, the capacitor 6 is connected in such a way that it bridges all existing suppressor diodes 3, then the pre-charging circuit is also advantageously used to discharge the capacitor 6 before switching on the semiconductor switches 2. If, on the other hand, the capacitor is as in Figure 3 shown connected in such a way that one or more suppressor diodes 3 remain in series with it, then at least for the capacitor 6 no discharge is necessary during a switch-on process.

[0040] The sum of the breakdown voltages of the suppressor diodes 3 in this example is 3 x 344 V = 1032 V. If, after a switch-off process of the electronic switch 1, this voltage arises across the suppressor diodes 3 and thus also across the two switchable semiconductor switches 2 and if switchable semiconductor switches 2 with a dielectric strength of 1200 V are still used, then the maximum overvoltage thus achieved does not pose a problem.

[0041] In practice, however, the already described initial value of 1032 V is significantly exceeded. Measurements show an initial voltage value of between 1600 V and 1700 V, i.e., slightly more than three times the maximum terminal voltage, which is approximately 1400 V. Such an overvoltage would disadvantageously require semiconductor switches 2 with a higher dielectric strength, i.e., at least a dielectric strength of 1600 V. These are significantly more expensive than those with a dielectric strength of 1200 V and therefore unattractive.

[0042] To address this problem, the second embodiment includes a capacitor 6 connected in parallel with a first of the three suppressor diodes 3. The capacitance of capacitor 6 in this embodiment is 1 µF, more than an order of magnitude less than typical capacitors in known suppression networks such as those mentioned in the introduction.

[0043] The capacitor 6 ensures that at the first moment of commutation of the load current from the turn-off semiconductor switches 2 to the suppressor diodes 3, one of the suppressor diodes 3 is bypassed by the capacitor 6. At the moment of switch-off, the capacitor 6 is uncharged because, in the switched-on state, almost no voltage is dropped across the electronic switch 1. By bypassing the first suppressor diode 3, the breakdown voltage of the series circuit of suppressor diodes 3 is reduced by the breakdown voltage of such a suppressor diode 3, in this case by approximately 344 V to 688 V. The initially occurring overvoltage is thus also reduced to a value of approximately 1100 V, whereby semiconductor switches 2 with a dielectric strength of 1200 V can be used.

[0044] In the second exemplary embodiment, it is assumed that the electronic switch 1 is designed for an operating current of 44 A and switches off at a five-fold overcurrent, i.e., at 220 A. The resulting power loss in one of the suppressor diodes 3 is therefore 220 A * 344 V, or approximately 75 kW. According to the data sheet of the suppressor diode 3 used here, such a peak power loss is possible for a current peak width of approximately 180 µs. Tests with an inductance of 230 µH, which is significantly oversized for typical line inductances, have shown that the resulting current peaks have a width in the range of a few µs, thus still leaving considerable margin. The described operating mode therefore poses no problem for the suppressor diodes 3 used, even if, for example, the maximum clamping voltage is used to consider the peak power.Since the second embodiment assumes a low line inductance, the entire switch-off process takes place within a few µs, so that the maximum power values ​​are unproblematic.

[0045] As in the first embodiment, the number of suppressor diodes 3 and their precise selection is a matter of weighing up various factors. In the second embodiment, more than three suppressor diodes 3, which have a lower breakdown voltage, can also be used. A higher counter voltage and thus faster shutdown could be achieved, for example, with six suppressor diodes 3 with a breakdown voltage of 190 V, i.e., a summed breakdown voltage of 1140 V. However, a higher initial overvoltage must also be expected. This can be counteracted by connecting the capacitor 6 in parallel with two or even three of the suppressor diodes 3. Thus, a finer adjustment of the voltages is achieved at the expense of an increased number of suppressor diodes 3. Reference symbol

[0046] 1 electronic switch 2 semiconductor switch 3 suppressor diodes 5 series circuit 6 capacitor 7 DC network 8 load 10 DC voltage system 11 first connection 12 second connection 15, 16 diodes 20 turn-off bidirectional semiconductor switching element 22 voltage limiting circuit 25 inductors 30 pre-charging circuit 31 bidirectional semiconductor switch 32 current limiting resistor 41 first potential 42 second potential 80 isolator 101 current waveform 102 voltage waveform 103, 104 points in time

Claims

1. Electronic switch (1) having - a first and a second terminal (11, 12) between which an electrically conductive connection is able to be established or interrupted, - at least one bidirectional turn-off semiconductor switching element (20) which comprises two semiconductor switches (2) connected in antiseries or in antiparallel and which is arranged between the first and second terminals (11, 12) and by means of which the connection between the first and second terminals is able to be established or interrupted, and - a voltage-limiting circuit (22) which is connected in parallel with the semiconductor switching element (20), wherein the voltage-limiting circuit (22) comprises a plurality of bidirectional suppressor diodes (3) in a series connection (5) and wherein a capacitor (6) is arranged in parallel with one or more, but less than all, of the suppressor diodes (3).

2. Electronic switch (1) according to Claim 1, in which the capacitor (6) is arranged in parallel with exactly one of the suppressor diodes (3).

3. Electronic switch (1) according to either of the preceding claims, in which the sum of the breakdown voltages of all the suppressor diodes (3) is less than the dielectric strength of the semiconductor switching element (20).

4. Electronic switch (1) according to Claim 3, in which the sum of the breakdown voltages of all the suppressor diodes (3) is greater than 60 %, in particular greater than 80 %, of the dielectric strength of the semiconductor switching element (20).

5. Electronic switch (1) according to one of the preceding claims, in which the capacitor (6) has a capacitance of less than 20 µF, in particular less than 5 µF.

6. Electronic switch (1) according to one of the preceding claims, in which the semiconductor switching element (20) has a dielectric strength of at least 1200 V.

7. Electronic switch (1) according to one of the preceding claims, having a precharging circuit (30) comprising a series connection comprising a switching device (31) and a current-limiting resistor (32), wherein the precharging circuit (30) is connected in parallel with the semiconductor switching element (20) .

8. Electrical network, in particular DC network (7) having at least one electronic switch (1) according to one of the preceding claims, in which the summed breakdown voltage of the suppressor diodes (3) is greater than the operating voltage of the electrical network.

9. Electrical network according to Claim 8, in which the operating voltage is at least 400 V, in particular at least 800 V.