Motor vehicle electrical system switch, motor vehicle electrical system and method for operating a motor vehicle electrical system switch

Description

[0001] The subject matter relates to a motor vehicle electrical system switch, a motor vehicle electrical system with such a switch and a method for operating a motor vehicle electrical system switch.

[0002] The increasing automation and autonomy of automotive driving is leading to increased safety requirements for electrical consumers. Safety requirements are also playing an increasingly important role in the increasing electrification of the powertrain.

[0003] Safety-relevant consumers such as braking systems, steering systems, optical and / or radar-based environmental sensors (LIDAR), control units and the like must be permanently and stably supplied with electrical energy to enable safe automated and / or autonomous driving.

[0004] For this purpose, it is already proposed to provide at least two on-board power supplies. These on-board power supplies can redundantly supply electrical energy to safety-relevant consumers. One on-board power supply can be, for example, a conventional lead-acid battery, a lithium-ion battery, a brake fuel cell, or the like. Another on-board power supply can be a generator, which is used, for example, as an alternator in combustion engines.

[0005] Despite the redundant supply of safety-relevant consumers by two on-board power supplies, problems can arise from voltage drops in the on-board power supply connected to one of the on-board power supplies. A voltage drop in one of the on-board power supplies also leads to a voltage drop in the safety-relevant consumers, despite the connection to the second, redundant on-board power supply. It is required that a voltage drop below a certain limit, for example, 9V, may only occur for a certain time, for example, only up to 0.5ms. After that, the voltage must again exceed a certain limit, for example, the aforementioned 9V.

[0006] The publication WO 2016 / 045836 A1 relates to an on-board power supply system, in particular an electrical on-board power supply system, for a motor vehicle with a plurality of consumers and a method for supplying consumers in an on-board power supply system of a motor vehicle.

[0007] Based on this state of the art, the object of the invention was to provide a motor vehicle electrical system switch which ensures a stable connection of the on-board electrical system power supplies with safety-relevant consumers.

[0008] This object is achieved by a motor vehicle electrical system switch according to claim 1, a motor vehicle electrical system according to claim 14 and a method according to claim 15.

[0009] A motor vehicle electrical system switch comprises at least two inputs. The inputs can be formed as terminal lugs, crimp terminals, terminal studs, terminal lugs, solder lugs, or the like. The inputs can be routed into a common housing. The inputs can be made of aluminum or aluminum alloy, or copper or copper alloy.

[0010] In this case, one of two on-board power supplies can be connected to each of the inputs. The on-board power supplies can be configured as already explained in the introduction. The on-board power supplies are preferably connected in parallel to the inputs and to other consumers. It is possible for an on-board power supply to be exclusive to one or more safety-relevant consumers and to be connected to these via the vehicle on-board power supply switch in question. However, it is also possible for at least one of the on-board power supplies to supply other consumers, which are in particular not safety-relevant. Thus, an on-board power supply and / or an on-board power supply and other consumers can be electrically contacted exclusively at the inputs.

[0011] The on-board power supplies can be batteries, accumulators, supercapacitors, generators, fuel cells, or the like. Lead-acid batteries or lithium-ion batteries are particularly used. At least one of the on-board power supplies can be directly short-circuited to an input via an electrical cable. At least one of the on-board power supplies can be connected to one of the inputs via an AC / DC rectifier or a DC / DC converter.

[0012] The vehicle's on-board power supply switch can also provide an output for a consumer, particularly a safety-relevant consumer, of the vehicle. Mechanically, the output can be configured in accordance with the above description of the inputs. The output can also be routed into the common housing.

[0013] One or more loads, particularly safety-relevant loads, can be connected to the output. These loads can be, for example, the loads mentioned above. The safety-relevant loads require an input voltage above a certain threshold, e.g., greater than 9V, and are designed to withstand voltage drops below this threshold for a certain time, e.g., a maximum of 0.5ms. Extended voltage drops below the threshold may cause malfunctions in the safety-relevant loads. Such malfunctions are unacceptable and not permitted in automated / autonomous driving.

[0014] Two switches can be provided within the vehicle electrical system switch, for example, enclosed in a common housing, in particular encapsulated in a common housing, in particular as an integrated circuit. The switches serve to switch a respective electrical connection between an input and a common node of the vehicle electrical system switch.

[0015] A first switch may be arranged between the first input and the common node. In addition to the first switch, a second switch may be arranged between the second of the inputs and the common node.

[0016] For example, at least one of the switches is a normally open switch, which is closed when a corresponding control voltage is applied. However, at least one of the switches can also be a normally closed switch, which is opened when a voltage is applied.

[0017] The switches establish an electrical connection between the respective input and the common node. The common node is connected to the output. The potential of the common node can be tapped at the output.

[0018] During normal operation, electrical energy flows via the first and / or second switch from a respective on-board power supply to the node, and from there via the output to the at least one consumer. Typically, the electrical energy flows from the on-board power supply with the higher electrical potential relative to ground to the at least one consumer.

[0019] The switches being closed during normal operation can cause compensating currents between the on-board power supplies. These compensating currents are particularly critical in the event of a fault on one of the on-board power supplies and must be prevented, as described below.

[0020] For particularly fast response time, compact design and to ensure the necessary energy in the event of a fault, it is proposed that the switches be designed as semiconductor switches and that their body diodes are each connected in the forward direction towards the common node. This means that in a consumer-counter arrow system, electrical energy can flow from a respective on-board electrical system power supply via the body diodes to the common node. In particular, a current flow direction from the input towards the common node is possible. Since both body diodes are each connected in the forward direction towards the common node, current flows in the consumer-counter arrow system from the node via the body diode to the input are impossible. The body diodes thus isolate the two on-board electrical system power supplies from each other when the switches are open.When the switch is closed, the current flows directly through the switch, and only a minor fraction flows through the body diodes. When the switch is open, a current may flow in the direction of current flow from the input to the common node via the body diodes, where a voltage drop of, for example, 0.7V is possible.

[0021] To protect safety-relevant consumers, it is now proposed that a monitoring circuit be used to monitor a first voltage at the first input and a second voltage at the second input and / or a voltage at the common node. The monitoring circuit measures, in particular, the potential relative to ground at the first and / or second input and / or the common node. If reference is made below to a voltage measurement, this can be measured at at least one of the mentioned points.

[0022] The vehicle electrical system switch in question is connected to the positive battery terminal, i.e., the high side, so that a positive potential relative to ground can be detected by the monitoring circuit. As previously explained, a voltage above a certain threshold, for example, 9V, is almost essential for safety-relevant consumers. A voltage drop below this threshold for an excessively long period of time will lead to malfunctions of these consumers.

[0023] The two on-board power supplies supply at least one consumer with an on-board power supply voltage, for example, 12V, 24V, or 48V, via the vehicle's on-board power supply switch, although other voltage paths are also possible. The voltages of the two on-board power supplies are, in particular, virtually identical.

[0024] During normal operation, the switches are closed. Current can flow from the inputs to the output, from the output to the inputs, and between the inputs via the closed switches. The switches essentially represent short circuits between the inputs and the output. This means that if the voltage of one of the two on-board power supplies is too low, a compensating current flows from the on-board power supply with the higher voltage to the on-board power supply with the lower voltage. Such compensating currents flow in addition to the currents from the on-board power supply with the higher voltage to the load. This is not critical as long as the potential of the on-board power supply with the lower voltage is high enough.

[0025] In the event of a critical voltage drop in one of the on-board power supplies or the connected on-board power supply, for example, below the limit value, e.g., 9V, compensating currents flow through the closed switches, which can lead to a corresponding voltage drop in the on-board power supply and the on-board power supply, where no fault originally occurred. The compensating currents therefore lead to a voltage drop at the output, even though one of the on-board power supplies is functioning correctly.

[0026] To prevent this, it is now proposed that, depending on the magnitude of at least one of the monitored voltages, an opening signal be generated to simultaneously open both switches. This means that in the event of a fault, if a voltage drop occurs on one of the two inputs, both switches are opened immediately and simultaneously.

[0027] Detecting a voltage dip is quick and easy. For example, it is only necessary to measure the potential at the common node. A simple voltage measurement can be achieved in a very short time. The fast response time, i.e., the rapid measurement of the voltage dip, can cause the breakers to open quickly.

[0028] In particular, the reaction time between detecting the voltage drop and opening the switches is below the critical limit for the loads, during which the voltage may fall below the threshold. This time is, for example, less than 0.5 ms. This means that with the circuit in question, a voltage drop below the threshold can be limited to a duration shorter than the maximum permissible duration for the loads.

[0029] In conventional circuits, opening both switches would also interrupt the power supply to the load. This is prevented by the fact that the opposing body diodes in the forward direction continue to allow power to be supplied to the common node. A current continues to flow from the on-board power supply and the fault-free on-board network from the input via the body diode to the common node and the output or load. However, compensating currents are prevented by the other body diode.

[0030] The circuit in question ensures that the moment a fault occurs, the two on-board power supplies are separated from each other and compensating currents are prevented, while at the same time ensuring that the consumers are supplied with energy with an essentially stable voltage at the output.

[0031] By ensuring a power supply with a substantially stable voltage via the body diodes, it is then possible to determine which of the two on-board systems the fault actually occurred in. This check may take longer than simply measuring the voltage level. However, since the consumer remains supplied with power with a substantially stable voltage, a longer period of time can be used for this purpose.

[0032] As soon as it has been determined which of the two on-board electrical systems or which of the two on-board electrical system power supplies caused the voltage drop, the other switch or the other connection to the on-board electrical system power supply can be closed again and the consumer is supplied with energy with a stable voltage via the intact on-board electrical system power supply.

[0033] Closing the switch relieves the load on the body diode. The body diode is designed to carry a high operating current only for a certain period of time. A voltage drop of 0.7V occurs across the body diode, and the power loss causes the switch to heat up. Therefore, the switch must be closed to prevent damage to the switch or the body diode.

[0034] According to one embodiment, it is proposed that the monitoring circuit triggers the opening signal when at least one of the voltages falls below a threshold. As already explained, the motor vehicle electrical system switch in question is preferably connected to the high side of the battery, so that a fault is caused by a voltage drop. This voltage drop below a threshold, for example, 9V, can be detected, for example, directly at the node. Once the voltage drop is detected, the opening signal is output, so that compensating currents are immediately prevented.

[0035] According to one embodiment, it is proposed that the monitoring circuit triggers the opening signal in the event of a short circuit at at least one of the inputs. One possible cause of a voltage drop can be a short circuit. In the event of a short circuit, the voltage drops very rapidly with a steep edge. Both the current of the power supply connected to the vehicle electrical system at which the short circuit occurs and the current of the vehicle electrical system power supply connected to the intact vehicle electrical system on the other side of the vehicle electrical system switch flow across the short circuit, with this current flowing across the two switches in the manner of a compensating current. The short circuit also short-circuits the consumer, so that it is no longer supplied with power and / or a substantially stable voltage.

[0036] By simultaneously opening the switches immediately after the short circuit is detected, this compensating current is prevented, and the consumer no longer "sees" the short circuit. The on-board power supply of the intact on-board power system also no longer "sees" the short circuit, since the body diode is reverse-biased toward the short circuit.

[0037] According to one embodiment, it is proposed that the monitoring circuit compares at least one of the voltages with a reference potential, in particular a ground potential. The monitoring circuit measures the voltage relative to the ground potential. If a voltage drop occurs at one of the inputs, this voltage drop also occurs at the other input and the node, since the two inputs are short-circuited via the closed switches and the node. To prevent compensating currents, the switches are then immediately opened.

[0038] According to one embodiment, it is proposed that the monitoring circuit, immediately after the opening signal is generated, checks a respective state of at least part of the vehicle electrical system at the first input and at least part of the vehicle electrical system at the second input and, depending on the check, generates a closing signal for exactly one switch, preferably only the first or only the second switch. The closing signal is generated for only exactly one of the two switches. The closing signal is generated for the switch connected to the input at which the vehicle electrical system is faulty. The switch connected to the input at which the vehicle electrical system is faulty remains open.

[0039] According to one embodiment, it is proposed that the monitoring circuit checks a network impedance and / or a voltage and / or an impulse response at the part of the vehicle electrical system at the first input and the part of the vehicle electrical system at the second input. In this case, for example, voltage can be measured at the first input and / or at the second input and / or the node. In addition, the current and the direction of current flow at the first input and / or at the second input and / or the node can be measured. The measured values can be digitized by an AD converter and processed in a processor. Depending on the processing, the monitoring circuit can output the closing signal.

[0040] To avoid misunderstandings, it should be noted that the part of the vehicle electrical system at the first input and the part of the vehicle electrical system at the second input are each to be understood as including at least the connection of the input to the respective vehicle electrical system power supply. Furthermore, a connection to at least one, preferably several, additional consumers connected to the respective vehicle electrical system power supply may be meant. A part of a vehicle electrical system can thus be formed from the vehicle electrical system power supply and / or the consumers connected to it.

[0041] According to one embodiment, it is proposed that the semiconductor switches are dimensioned such that a voltage drop across a respective body diode after the switch is opened is less than 1V, preferably less than 0.7V. When the switch is open, a current continues to flow via the body diode in the forward direction from the intact on-board power supply to the load. This current causes a voltage drop of preferably 1V, in particular 0.7V or less, across the body diode. However, this voltage drop is not critical for the safety-relevant load, since the voltage of the on-board power supply minus the voltage drop across the body diode is greater than the minimum voltage for the load, in particular the safety-relevant load.

[0042] According to one embodiment, it is proposed that the monitoring circuit generates the closing signal within 100 ms, preferably 40 ms, of the opening signal. The monitoring circuit thus has between 40 and 100 ms to check both parts of the two vehicle electrical systems beyond the two inputs and determine on which side the fault, in particular the short circuit, occurred that led to the voltage drop. If the correct side where the fault occurred is detected, the other side is connected to the load by closing the switch.

[0043] According to one embodiment, it is proposed that after the switches are opened, the output is conductively connected to the first and second inputs via the respective body diodes, and the body diodes enable a current flow from the respective input to the output. As already explained, a current flow is enabled in the consumer meter arrow system from the on-board power supply to the output. However, the body diodes block a current flow from the higher potential to the lower potential in the consumer meter arrow system. This prevents a compensating current between the on-board power supply that is intact and the on-board power supply that is faulty and caused the voltage drop. Since at the time of the voltage drop it is not yet clear which of the two on-board power supplies caused the voltage drop, both switches are opened.However, the power supply to the consumer at the output with an essentially stable voltage is ensured via the body diodes.

[0044] According to one embodiment, after the switches are opened, the output is conductively connected to the first and second inputs via the respective body diodes, but the body diodes block a current flow from the output to the respective input. Thus, a current flow from the common node to the input is also blocked by the body diodes.

[0045] According to one embodiment, it is proposed that at least one of the semiconductor switches be connected with a variable switching characteristic, in particular that the switching characteristic is dependent on a load at the output. The switching behavior of the semiconductor switch, in particular a dependence on the current and / or voltage, can be adjusted via software. A dynamic load, which, for example, has a high starting current during startup and requires a lower current during normal operation, can also be variably protected by flexibly defining the switching characteristic, e.g., how much current flows for how long (dI / dt).

[0046] As already explained at the beginning, according to one embodiment, the aforementioned safety-relevant load can be connected to the output. A continuous power supply in a redundant manner must be ensured, especially for safety-relevant loads.

[0047] According to one embodiment, it is proposed that the output be short-circuited to the common node. Preferably, no further component is provided in the on-board power supply switch between the common node and the output.

[0048] A further aspect is a motor vehicle electrical system with a motor vehicle electrical system switch as previously described, with a first on-board electrical system energy supply connected to the first input, with a second on-board electrical system energy supply connected to the second input and with a safety-relevant consumer connected to the output.

[0049] Another aspect is a method for operating a motor vehicle electrical system switch as described above. At least one voltage is monitored. This particularly also means monitoring a voltage at a common node. When the switches are closed, the voltages at the inputs and the common node are virtually identical and differ only in the voltage drop across the respective switches, which is in the range of a few 100 mV. Depending on the magnitude of at least one of the voltages, both switches are opened simultaneously.

[0050] Alternatively or in addition to the voltage magnitude, the rate of change of the voltage may also be relevant and / or a voltage differential may be taken into account. For example, if the differential drops below a negative limit, a rapid voltage drop can be inferred, which is caused by a fault and could lead to the opening of the breakers.

[0051] After the switches are opened simultaneously, power can be supplied via the body diodes, as they connect a respective potential of the on-board power supply to the output in the forward direction. The on-board power supply with the higher potential causes a current to flow through the body diode to the consumer. The on-board power supply where the voltage drop occurred has the lower potential, and no current flows from it via the body diode in the forward direction to the common node and the consumer. Because the body diode is operated in the reverse direction, the intact on-board power supply does not "see" the defective on-board power supply, and no compensating currents flow.

[0052] After the switches are opened simultaneously, the electrical status of both on-board power supplies is checked.

[0053] Depending on the test results, the on-board electrical system and the on-board electrical system power supply in which no fault has occurred are connected to the common node and the output via the switch. The on-board electrical system power supply and the on-board electrical system in which the fault has occurred remain disconnected from the consumer and the other on-board electrical system power supply via an open switch.

[0054] The subject matter is explained in more detail below using a drawing showing exemplary embodiments. The drawing shows: Fig. 1 shows a block diagram according to an embodiment; Fig. 2a, b shows voltage curves at the common node; Fig. 3a-f shows switching states of a motor vehicle electrical system switch according to embodiments.

[0055] Fig. 1 shows a motor vehicle electrical system switch 2 in a motor vehicle electrical system. The motor vehicle electrical system switch 2 has two switches 4, 6. The first switch 4 is connected to a first on-board electrical system power supply 16 via an input contact 10. The second switch 6 is connected to a second on-board electrical system power supply 20 via an input contact 8. The switches 4, 6 are short-circuited together within the motor vehicle electrical system switch 2 via a common node 12. The node 12 is connected to an output contact 14.

[0056] The input contacts 8, 10 and the output contact 14 can be formed as connection lugs, connection tabs, plug contacts, crimp contacts, connection bolts, welding tabs, soldering lugs or the like.

[0057] The first on-board power supply 16 is, for example, a DC / DC converter which is connected to the input contact 10 on its high-side potential 16a.

[0058] The second on-board power supply 20 is, for example, a battery which is connected with its B+ potential 20a to the input contact 8 and with its B-contact 20b to ground 22.

[0059] The second on-board power supply 20 supplies, for example, an on-board power supply 24. The first on-board power supply 16 can also supply an on-board power supply, but this is not shown.

[0060] A consumer 25 is connected to output contact 14. Consumer 25 is, for example, a safety-relevant consumer.

[0061] The switches 4, 6 in the motor vehicle electrical system switch 2 each comprise a switching element 4b, 6b and a body diode 4a, 6a. The switches 4, 6 are, for example, semiconductor switches, in particular high-power semiconductor switches, for example MOSFETs, IGBTs, or the like.

[0062] In the Fig. 1 It can be seen that the body diodes 4a, 6a are each connected in the forward direction from an input contact 8, 10 in the direction of the common node 12.

[0063] The switching elements 4b, 6b can be controlled via a switching signal (not shown). Such control can be implemented, for example, via a monitoring circuit 26. The switches 4a, 4b can, for example, be normally open (NO), and an opening signal can be a decrease in the switching signal level. The switching elements 4b, 6b can also be normally closed (NC), and an opening signal can be a rise in the switching signal level.

[0064] The monitoring circuit 26 is connected to the common node 12 via a sensing line 26a and can, for example, measure the potential of the common node 12 relative to ground 22. Furthermore, the monitoring circuit 26 is connected to the respective on-board electrical systems at the on-board power supplies 16, 20 via sensing lines 26b, c. A state of the respective on-board electrical system can be determined via the sensing lines 26b, c, for example, an impedance, an impulse response, a resistance relative to ground, or the like.

[0065] With the aid of the vehicle electrical system switch 2 shown, it is possible to connect the safety-relevant consumer 25 redundantly and permanently to at least one of the vehicle electrical system power supplies 16, 20, even in the event of a fault.

[0066] Such an error case is in the Fig. 2a shown. Fig. 2a shows, by way of example and purely schematically, a voltage curve 28 at the common node 12. A standard voltage 32 is normally present at the common node 12. Normally, the switching elements 4b, 6b are closed. The standard voltage 32 results from the higher potential of one of the two

[0067] On-board power supplies 16, 20.

[0068] In the event of a fault, edge 30 in voltage 28 drops steeply. This can occur, for example, at time 36. Due to the steeply falling edge 30, voltage 28 falls below a lower limit 34. In conventional circuits, it takes until time 38 for the faulty on-board power supply 16, 20 to be disconnected from the common node. Time 38 can, for example, be 40 to 100 ms after time 36. The interval between times 36, 38 is longer than the maximum permissible duration of a voltage drop below limit 34 for a safety-relevant consumer, e.g., 0.5 ms.

[0069] If the lower limit value 34 is undershot for more than 0.5 ms, a fault in the safety-relevant consumer 25 can no longer be ruled out.

[0070] This is remedied by a vehicle electrical system switch 2 in which a voltage curve 40 according to Fig. 2b can be realized. The Fig. 2b is related to the Fig. 3a bis f described.

[0071] In the Fig. 3a It can be seen that the switching elements 4b, 6b are closed. This is the normal state. The standard voltage 32 flows from the on-board power supplies 16, 20 via the switching elements 4b, 6b and the common node 12 to the consumer 25.

[0072] In the Fig. 3b It is shown that a fault, for example a short circuit 42, 44 on the side of one of the two on-board power supplies 16, 20, can occur starting from the input contacts 8, 10. In such a case, the edge 30 of the voltage 40 drops steeply. This steep edge 30 can be detected via the monitoring circuit 26. This detection can be completed very quickly after the time 36, at the time 37. In this case, both switching elements 4b, 6b are immediately opened by the monitoring circuit 26. In particular, the time 37 is a maximum of 0.5 ms after the time 36. Since it is not known which of the short circuits 42, 44 has occurred, both switching elements 4a, 6a are opened first.

[0073] Fig. 3c shows an example in which a short circuit 42 has occurred on the side of the vehicle electrical system power supply 20. After the two switching elements 4b, 6b, as in Fig. 3c shown, have been opened by the monitoring circuit 26, a current flow 46 can still flow from the positive terminal 16a of the vehicle electrical system power supply 16 via the body diode 4a to the consumer 25. This is shown in the Fig. 2b There, it can be seen that after the falling edge 30 at time 37, the voltage 40 stabilizes again. The voltage 40 is 0.7 to 1 V below the standard voltage 32, which is due to the voltage drop across the body diode 4a.

[0074] In the period between times 36, 38, the monitoring circuit 26 can measure and check where the fault is located via the sensing lines 26b, c in the respective on-board networks of the on-board network power supplies 16, 20.

[0075] In the Fig. 3d It is shown that the fault was sensed on the part of the on-board power supply 20. In this case, the switching element 4b is closed. As shown in the Fig. 3d As can be seen, the current flow 48 occurs via the switching element 4b.

[0076] In the Fig. 2b It can be seen that at time 38, when the fault location has been determined, this leads to an increase in voltage 40 to standard voltage 32. In the Fig. 2b It can be seen that at no time did the voltage fall below the limit value 34.

[0077] Fig. 3e shows a case in which the short circuit 44 has occurred on the side of the vehicle electrical system power supply 16. This short circuit 44 also leads to a falling edge 30 and an immediate opening of the switching elements 4b, 6b. Since the vehicle electrical system power supply 20 continues to operate without errors, this leads, as shown in the Fig. 3e shown, to a current flow 46 through the body diode 6a. The voltage curve is according to the Fig. 2b .

[0078] How to Fig. 3c As described, the monitoring circuit 26 is used to measure the two on-board networks beyond the input contacts 8, 10 and to check on which side the error is located.

[0079] If the fault is on the side of the on-board power supply 16, as in Fig. 3f As shown, the switching element 6b must be closed again. This is time 38. A current 48 flows from the on-board power supply 20 via the switching element 6b to the consumer 25. Here, too, it is ensured that the voltage never falls below the limit value 34.

[0080] Using the arrangement shown, it is possible to design a redundant on-board power supply in such a way that a voltage drop below a critical limit for safety-relevant consumers is avoided.

Claims

1. Motor vehicle on-board electrical network switch (2) with - at least two inputs for respectively one of at least two on-board power supplies (16, 20), - at least one output for a load (25) of the motor vehicle, - at least two switches (4, 6), wherein - a first switch (4) is disposed between a first of the inputs and a common node (12), and - a second switch (6) disposed between a second of the inputs and the common node (12), and - the output is electrically connected to the common node (12), wherein - the switches (4, 6) are formed as semiconductor switches and are respectively connected with their body diodes (4a, 6a) in forward direction towards the common node (12), characterized in that - a monitoring circuit (26) monitors a first voltage at the first input and / or a second voltage at the second input and / or a voltage at the common node (12), and in that the monitoring circuit (26) triggers an opening signal for simultaneous opening of both switches (4, 6) as a function of an absolute value of at least one of the monitored voltages.

2. Motor vehicle on-board electrical network switch (2) according to claim 1, characterized in that - the monitoring circuit (26) triggers the opening signal when at least one of the voltages falls below a limit value (34).

3. Motor vehicle on-board electrical network switch (2) according to claim 1 or 2, characterized in that - the monitoring circuit (26) triggers the opening signal in the event of a short circuit (44) at at least one of the inputs.

4. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in that - the monitoring circuit (26) compares at least one of the voltages with a comparison potential, in particular a ground potential.

5. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in that - the monitoring circuit (26) evaluates a respective state of a part of the vehicle electrical network at the first input and of a part of the vehicle electrical network at the second input immediately after the opening signal is triggered and, depending on the evaluation, triggers a closing signal for one, preferably only the first or only the second switch (4, 6).

6. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in - the monitoring circuit (26) evaluates a mains impedance and / or a voltage and / or an impulse response at the part of the vehicle electrical network at the first input and the part of the vehicle electrical network at the second input.

7. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in that - the semiconductor switches are dimensioned in such a way that a voltage drop at a respective body diode (4a, 6a) after the switch is opened is less than 1V, preferably less than 0.7V.

8. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in that - the monitoring circuit (26) triggers the closing signal within 100ms, preferably 40ms, after the opening signal.

9. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in that - after opening of the switches (4, 6), the output is conductively connected to the first and second inputs via the respective body diodes (4a, 6a), and the body diodes (4a, 6a) enable a current flow from the respective input to the output.

10. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in that - after opening of the switches (4, 6), the output is conductively connected to the first and the second input via the respective body diodes (4a, 6a) and the body diodes (4a, 6a) block a current flow from the output to the respective input.

11. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in that - at least one of the switches (4, 6) is connected with a variable switching characteristic, in particular in that the switching characteristic is dependent on a load at the output.

12. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in that - a safety-relevant load (25) is connected to the output.

13. Motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, characterized in that - the output is short-circuited to the common node (12).

14. Motor vehicle electrical network with - a motor vehicle on-board electrical network switch (2) according to any one of the preceding claims, - a first on-board electrical power supply (16) connected to the first input, - a second on-board electrical power supply (20) connected to the second input and - a safety-relevant load (25) connected to the output.

15. Method of operating a motor vehicle on-board electrical network switch (2) according to one of the preceding claims 1-13, wherein - a first voltage at the first input and / or a second voltage at the second input and / or a voltage (40) at the common node (12) is monitored and, depending on an absolute value of at least one of the monitored voltages, both switches (4, 6) are opened simultaneously, - wherein, after the simultaneous opening of the two switches (4, 6), the body diodes (4a, 6a) of the two switches (4, 6) connect a respective potential of the on-board power supplies (16, 20) to the output in the forward direction, - wherein, after the simultaneous opening of the two switches (4, 6), the electrical state of the two on-board power supplies (16, 20) is evaluated and, depending on the evaluation, one of the two switches (4, 6) is closed.