CAPACITIVE FUNCTIONAL TEST OF VARISTORS OF A VEHICLE ON-BOARD ELECTRICAL SYSTEM HIGH-VOLTAGE PROTECTION DEVICE

Description

[0001] It is common practice to equip vehicles with electric drives powered by batteries. This involves not only DC-DC converters but also inverters and charging circuits, all connected to the drive system and battery within the vehicle's electrical system. Firstly, to achieve high performance, these components must be high-voltage, operating at a nominal voltage exceeding 60 V, for example, 400 or 800 V. This necessitates enhanced safety measures, such as high-voltage insulation and, in particular, a high-voltage electrical system that is galvanically isolated from the low-voltage sections. Secondly, these high-voltage components require monitoring and control, for which low-voltage signals are used.These low-voltage signals are exchanged via lines that extend from the high-voltage on-board branch into the low-voltage section.

[0002] JP 6 070470 B2 discloses an ignition device. The ignition device comprises an ignition coil with a primary coil and a secondary coil magnetically coupled to each other; a spark plug with a center electrode and a ground electrode; an ECU 20 for applying a secondary voltage to a gap to generate a spark discharge in a gap that is a space between the center electrode 10a and the ground electrode 10b; a diode 18 for limiting the secondary voltage V2 so that it does not exceed a predetermined value; and a switching element 19 connected in parallel to the spark plug 10 and electrically disconnecting the secondary coil 12b and the diode 18. The ECU 20 determines whether an anomaly occurs at the diode 18 when the secondary voltage V2 is applied to the gap and switches the switching element 19 to a non-connected state when an anomaly is detected at the diode 18.

[0003] WO 2010 / 079132 A2 discloses an overvoltage protection element comprising a housing and at least one suppressor diode arranged in the housing as an overvoltage-limiting component. Monitoring of the functionality and condition of the overvoltage protection element during operation is made possible by arranging the suppressor diode in the bridge branch of a diode bridge circuit and by connecting a capacitance meter to the suppressor diode, whereby the capacitance meter detects the capacitance or any change in capacitance of the suppressor diode.

[0004] It was recognized that this resulted in the task of providing ways to secure these low-voltage lines with regard to dangerous high-voltage potentials.

[0005] This problem is solved by the subject matter of the independent claims. Further properties, features, embodiments and advantages become apparent with the dependent claims, the description and the figure.

[0006] It is proposed to provide a vehicle electrical system high-voltage protection device that protects against high-voltage potentials by means of varistors or other voltage limiting elements. This can be achieved in two ways: firstly, by having the varistors (or other voltage limiting elements) themselves divert dangerous high-voltage potentials to a reference potential, and / or secondly, by having the varistors (or other voltage limiting elements) provide a current flow that is triggered by dangerous high-voltage potentials (on a low-voltage line), so that this current flow can be reliably detected by further measures, such as insulation monitors. In this case, the varistors (or other voltage limiting elements) serve as a safeguard by initiating conduction when a high-voltage potential appears at a point in the electrical system where it would not occur under normal operating conditions.

[0007] Varistors and other voltage limiting devices have the characteristic of conducting when the voltage applied across them exceeds a threshold voltage (or breakdown voltage), and otherwise not conducting. This threshold voltage is, for example, 20 V or 40 V. Furthermore, these devices exhibit different capacitive properties in a faulty state, such as after an overload or exceeding an aging limit, compared to a fault-free state (where, for example, the nominal capacitance value is present). These properties can be implemented using a varistor (as mentioned), or by a voltage limiting element such as a gas discharge tube, a spark gap, a protection diode, a thyristor circuit, a DIAC, a Zener diode, and / or a four-layer diode.The following describes properties, characteristics, and embodiments that apply to both the varistor mentioned therein and the other voltage limiting elements. In particular, the term "varistor" is used to represent: a varistor (component), a gas discharge tube, a spark gap, a protection diode, a thyristor circuit, a DIAC, a Zener diode, and / or a four-layer diode, or other components with the aforementioned properties.

[0008] The aforementioned function of the varistors is crucial for the safety measures, and therefore, according to the subject matter of the claim, the functionality of these varistors is verified. This is done by determining the capacitance of the varistors, as the capacitance indicates whether the varistor is in a faulty or fault-free state. The capacitance in a fault-free state is known, for example, from the varistor's datasheets and is thus a known property of the varistor. If a varistor is applied at a high voltage at which it conducts, it can be damaged, for example, by an excessively high current flow (i.e., higher than the maximum current specified in the design), which leads to its malfunction.This cannot be reliably verified with voltages below the varistor's threshold voltage, because at excessively low voltages (below the threshold) the varistor does not conduct, and in a faulty state the varistor also exhibits a high resistance (regardless of the applied voltage).

[0009] Testing using high voltages (above the threshold voltage) carries the risk of a dangerous touch voltage.

[0010] However, the functionality of the varistor can be determined using the capacitance test proposed here, without having to apply a high test voltage above the threshold value in order to check the line that then starts (in a fault-free state).

[0011] Therefore, a method for verifying the functionality of a vehicle electrical system high-voltage protection device (hereinafter referred to as a protection circuit) is described, which incorporates a varistor. The method is performed on a varistor (or voltage limiting element) through which the potential of a low-voltage line leading out of a high-voltage section is connected to a ground terminal of the vehicle's high-voltage protection circuit. The high-voltage section is separated by insulation from a low-voltage section in which the vehicle's high-voltage protection circuit is located. The ground terminal of the vehicle's high-voltage protection circuit is connected to a ground potential of the low-voltage section, and this ground potential is galvanically isolated from the high-voltage section.

[0012] Here, the capacitance of the varistor is determined, and this capacitance allows us to deduce whether the varistor is functioning or not, and whether it is in a faulty or fault-free state. The capacitive component of the varistor's AC impedance is thus measured. After this measurement, for example, is taken, it is evaluated and, in particular, compared with a nominal capacitance value. For instance, the difference between the measured capacitive component and the nominal capacitance value (which the varistor exhibits in a fault-free state) can be calculated, or the capacitive component can be compared with a capacitance interval that defines a range the varistor exhibits in a fault-free state (e.g., taking into account individual component variations). The nominal capacitance value represents the capacitance of a functioning varistor, i.e.,of a varistor in a fault-free state; a corresponding interval would indicate the capacitance or capacitance range of a functioning varistor.

[0013] The faulty condition of the varistor is detected when a comparison reveals that the capacitive component deviates from the nominal capacitance by more than a predetermined amount. This nominal value can, for example, be defined by the limits of a capacitance interval that characterizes the capacitance of a functioning varistor. In this case, the comparison with the limits corresponds to a comparison of the measured capacitive component with the interval defined by the limits. For simplicity, the following text refers to a comparison with a nominal capacitance value, which can refer to a comparison with a single value, a comparison with the limits of an interval representing a functioning varistor, or with values ​​within that interval.

[0014] The specified value indicates how much the varistor can deviate from its nominal capacitance value while still remaining in a fault-free state, or it represents the threshold beyond which a faulty varistor should be assumed. This value also accounts for detection or measurement errors, as well as variations between individual varistors, to prevent the erroneous assumption of a faulty varistor due to typical measurement or detection errors, or due to normal variations in the nominal capacitance value.

[0015] The measurement can be performed by an active process in which an AC signal is applied to the varistor, and the resulting AC voltage signal is measured. The capacitance, i.e., the capacitive component, is then determined by the ratio of the applied AC signal to the AC voltage signal in a known manner. A complementary measurement is also possible, in which an AC voltage signal is applied to the varistor, and the corresponding AC signal present at the varistor is measured. When an AC signal is applied, the AC voltage directly applied to the varistor can be determined, or a voltage that varies with or depends on the voltage applied to the varistor can be measured to derive the AC voltage signal.When an AC voltage signal is applied to the varistor, the AC current signal can be detected by measuring the current flowing through the varistor itself, or by determining the current using a shunt resistor for current measurement by measuring the voltage across the shunt, which corresponds to the AC current. Both the AC voltage signal and the AC current signal can be direct measurement signals, or they can be derived signals, particularly using amplifiers, voltage dividers, shunt resistors, or similar devices.

[0016] As mentioned, the capacitive component can be actively measured, i.e., by applying an excitation signal (an AC current or voltage signal) and detecting the resulting AC voltage or current signal, which represents the response to the excitation signal. With multiple varistors, a separate active detection device can be provided for each varistor, or the same detection device can be used by multiplexing, with this common device being connected alternately or sequentially to the different varistors to measure their capacitive component. The capacitive components of the varistors can thus be detected with individual devices (e.g., one per varistor). A separate signal source (current or voltage source for the applied AC voltage or current signal) can be provided for each varistor or for a subgroup thereof.Alternatively, or in combination with this, a separate measuring device can be provided for each varistor or for a subgroup thereof, which detects the resulting signal (AC voltage signal when an AC current is applied and AC current signal when an AC voltage is applied). Thus, each varistor or for a subgroup thereof has its own measuring device that detects the resulting signal present at the associated varistor.

[0017] The following describes a method for measuring the capacitive components of several varistors using signal multiplexing. Here, the same device measures the capacitive component of several or all varistors, particularly sequentially using signal multiplexing. It can be provided that the same signal source applies an AC signal to all varistors or to a subgroup thereof. This can be done via a multiplexing device, i.e., sequentially, or the AC signal (or the AC voltage signal) can be applied to all varistors or to a subgroup thereof simultaneously. Alternatively, or in combination with this, the same measuring device can measure the resulting signal at the varistors or at the subgroup thereof, i.e., at all varistors or the subgroup thereof, particularly sequentially.When sequentially acquiring the resulting signals from the varistors, signal multiplexing is preferably used during measurement. Furthermore, the same device can simultaneously acquire the resulting signal from all varistors or a subgroup thereof (without signal multiplexing), as this allows for the detection of a significantly increased capacitive component even with multiple varistors, thus determining that at least one of the varistors is faulty. In this case, signal multiplexing would not be necessary. Using signal multiplexing, particularly during measurement, offers the advantage that it not only detects that at least one varistor is faulty, but also identifies which varistor is faulty. The detection would therefore be varistor-specific, and not based on a subgroup or all varistors.

[0018] The method is performed on a varistor through which the potential of a low-voltage line is connected to the vehicle electrical system ground. This low-voltage line is routed away from the high-voltage section and, in the event of insulation faults within the high-voltage section, can carry an excessively high voltage (relative to ground), causing the varistor to trigger (i.e., become partially or fully conductive). This applies particularly to the varistor or varistors whose capacitive component is being monitored. Specifically, this applies to varistors whose monitored capacitive component is compared to a nominal capacitance value. In other words, the varistor whose state is being monitored is preferably connected to a reference potential such as the vehicle electrical system ground. This connection is preferably indirect, i.e., it involves at least one impedance, which is preferably inductive and / or has a resistive component.Thus, the method can be implemented on a varistor connected to the reference potential (e.g., vehicle electrical system ground) via an inductor and / or a resistor. In particular, if the varistor is connected via a circuit or impedance with an inductive component, a low-pass filter is formed between the varistor and the vehicle electrical system ground. Therefore, embodiments provide for connecting the varistor(s) to the reference potential via a low-pass filter or a high-blocking filter with attenuation that increases with frequency. This allows the (active) detection of the capacitive component of the varistor's AC impedance while isolating the reference potential from this detection, as the inductor blocks the AC components generated by the active detection from the reference potential.In other words, the capacitive component of the varistor's AC impedance is preferably (actively) detected by a circuit with a series-connected impedance between this circuit and the vehicle's electrical system ground. This impedance attenuates AC voltage or current signals to the vehicle's electrical system ground. For detecting the capacitive component, an AC signal is preferably used whose frequency is configured such that the impedance produces a frequency-dependent attenuation of at least 10, 30, or 50 dB at that frequency. The impedance is configured accordingly (with respect to the frequency of the AC signal used). The frequency can be greater than 1 kHz, 10 kHz, preferably at least 100 kHz, 1 MHz, or 5 MHz.

[0019] The following describes a vehicle high-voltage protection circuit particularly suitable for implementing the method described here. The vehicle high-voltage protection circuit (or simply "protection circuit") comprises at least one protective terminal configured for connection to a low-voltage line leading from a high-voltage area. The protection circuit also includes at least one varistor. This varistor connects the protective terminal to a ground terminal of the vehicle high-voltage protection circuit. The term "ground terminal" here generally refers to a reference potential associated with the vehicle chassis. This also applies to the vehicle's electrical system ground. The ground terminal mentioned here is specifically the vehicle's electrical system ground or, more generally, an electrical system-related reference potential.

[0020] The protection circuit also includes a detection device. This device is connected to the varistor via a signal transmission connection. This connection can be a direct line connection, or connections via filters, capacitive couplings, and / or voltage dividers or amplifiers. The detection device is configured to detect the capacitive component of the varistor's AC impedance, primarily due to the signal transmission connection. Furthermore, the protection circuit includes an evaluation device. This device is connected downstream of the detection device. The evaluation device is configured to compare the capacitive component detected by the detection device with the nominal capacitance of the varistor. Additionally, this evaluation device is configured to output an error signal at a signal output if the deviation exceeds a predefined value.This indicates that the varistor is faulty. The detection device is therefore configured to perform the step described here of detecting the capacitive component. In general, the evaluation device is configured to compare the capacitive component detected by the detection device as described.

[0021] In this context, the sensor need not necessarily be downstream of the detection device, but can also be part of this detection device and, for example, be downstream of an entity configured to detect the capacitive component of the varistor's AC impedance, such as a measuring device. According to one embodiment, the detection device comprises at least one measuring device, an AC signal source, and a comparator that performs the comparison step and can accordingly output the signal indicating a faulty condition.

[0022] An AC current source or an AC voltage source can also be used in the protection circuit, as described above in the procedure. If the protection circuit includes an AC current source, it is connected to the at least one varistor (for signal transmission) and configured to impress an AC current across the varistor. The AC current thus serves as the excitation. The detection device or a measuring device thereof is connected to the varistor for signal transmission and is configured to detect the voltage resulting from the impressed AC current across the varistor (as a reaction). The AC current source can be part of the detection device or be an AC current source outside the detection device (which includes a measuring device and, if necessary, a comparator).

[0023] Alternatively, an AC voltage source can be used for excitation, i.e., for active measurement. This source is connected to the at least one varistor via signal transmission. Furthermore, the AC voltage source is configured to apply an AC voltage to the varistor, which is connected to the source directly or indirectly via capacitive coupling, a resistor network, and / or an amplifier. The sensing device is also connected to the varistor (via signal transmission). The sensing device is configured to detect the current resulting from the applied AC voltage across the varistor. The AC voltage source can be part of the sensing device (which may also include a measuring device and / or a comparator). Alternatively, the AC voltage source can be located externally to the sensing device.

[0024] In specific embodiments, the (active) detection of the varistor's capacitive component involves stimulating it with an alternating signal originating from a communication unit connected to the low-voltage line to which the varistor is also connected. In this case, a communication signal (sensor signal, control signal, etc.) serves to stimulate the varistor. This communication signal is transmitted during the execution of a sensor evaluation process, a control or regulation process, or a process for supplying low-voltage components with low voltage. The signal source for the alternating signal (i.e., the alternating current source or the alternating voltage source) would then not be a dedicated signal source, but rather a control unit, sensor unit, or similar device that performs an additional function and is connected to the low-voltage line to which the varistor is also connected.Therefore, the terms AC source and AC power source are intended to refer to both dedicated AC signal sources and AC signal sources that have an additional function and generate an AC signal as part of that function, such as a data signal, communication signal, control signal, sensor signal, or similar. A low-voltage power supply circuit, which transmits power via the low-voltage line, can also provide this AC signal for measuring the capacitive component, particularly if this AC signal is used in the high-voltage section for power supply, for example, to a low-voltage component.

[0025] In this case, therefore, to implement the protective circuit or to carry out the procedure described here, it would only be necessary to add a passive detection device to an existing device, that is, a detection device that does not have its own signal source, but which has a measuring unit and / or an evaluation unit.

[0026] The protection circuit can include multiple varistors. A detection device can be provided for each varistor, or for a subgroup thereof. This device is connected to the respective varistor via signal transmission. This connection can also be provided for only one measuring device of the detection circuit, or for a measuring device that is assigned to the detection circuit. In this case, there is a fixed assignment between the varistor and the detection device. Alternatively, a selector switch can be provided, via which the detection circuit (or its measuring device) is connected to all varistors or to a subgroup thereof. In this case, the selector switch is configured to connect the detection circuit to a selected varistor or to a selected subgroup of varistors.The selection switch is thus configured to select one element (varistor or subgroup) from among several varistors or subgroups in order to connect this element to the detection device. The detection device is then able to successively determine the respective capacitive components of the different varistors or subgroups of varistors. Signal multiplexing is performed by means of the selection switch. In particular, the selection switch is configured to connect the AC current or AC voltage signal source successively to different varistors or different subgroups. Alternatively, a common signal source can be provided that applies the AC signal (AC current or AC voltage) to all varistors or all subgroups, while only the detection device can select the individual varistors or subgroups via the selection switch.

[0027] The at least one varistor is preferably connected to the ground terminal via a high-pass filter. This prevents the alternating signal (AC current or AC voltage) intended for active measurement from being routed to the ground terminal or from penetrating other components of the vehicle's electrical system via the ground terminal. In one embodiment, the at least one varistor is connected to the ground terminal via an inductor and / or a resistor. The use of a resistor also provides attenuation of AC current or AC voltage components during transmission to the ground terminal.

[0028] As mentioned, the method and devices described here serve to protect at least one low-voltage line leading out of a high-voltage area. The high-voltage area is housed in an enclosed enclosure. Low-voltage lines lead out of this enclosure, for example, for control, censoring, or supplying (low voltage) components within the high-voltage area.

[0029] In particular, the at least one low-voltage conductor penetrates the housing wall that spatially encloses the high-voltage area. In other words, the at least one low-voltage conductor penetrates the boundary by which the high-voltage area is spatially limited, especially in relation to the low-voltage area or the surroundings of the high-voltage area. In one embodiment, the at least one low-voltage conductor penetrates insulation that separates the high-voltage section from the low-voltage section.

[0030] This will be described in more detail below using the example of a high-voltage vehicle electrical system.

[0031] A high-voltage vehicle electrical system with a spatially defined high-voltage area is provided, from which at least one low-voltage line leads. This at least one low-voltage line penetrates the spatial boundary of the high-voltage area. This boundary separates the high-voltage area from its surroundings, in particular from a low-voltage area. The low-voltage line can lead from the high-voltage area to the low-voltage area. If a high-voltage protection circuit, as described herein, is provided, then the at least one protective terminal of this circuit is connected to the at least one low-voltage line. If several low-voltage lines are provided, then the protection circuit preferably has at least the same number of protective terminals, so that an individual protective terminal is provided for each low-voltage line.

[0032] The high-voltage section is separated from the low-voltage section by insulation. In other words, at least one low-voltage line passes through this insulation and out of the high-voltage section. This insulation can be achieved by physically confining the high-voltage section or by housing or enclosure it. The aforementioned protective circuit is located in the low-voltage section. The low-voltage section has a ground potential, for example, the potential of the chassis, and the ground connection of the protective circuit is connected to this potential. This ground potential is separate from the high-voltage section.

[0033] The protective connection or low-voltage line is galvanically connected to ground potential. Therefore, the varistor can detect insulation faults or inadequate galvanic isolation between the high-voltage and low-voltage sections on the one hand, and between the low-voltage section, ground potential, or ground connection on the other.

[0034] The low-voltage line can be used as a sensor line. In particular, the low-voltage line can lead to a sensor within the high-voltage section. Such components are, for example, temperature sensors, voltage sensors, current sensors, pressure sensors, or similar devices. The low-voltage line can also be a data line, especially a data line for communication with components within the high-voltage section. For example, the data line can be a control line to operate a component within the high-voltage section from the low-voltage section. Conversely, the data line can also be used by a component within the high-voltage section to operate a component within the low-voltage section, or generally to communicate with it, for example, to transmit sensor data.Examples of this include communication between a battery management system within the high-voltage range and a component in the low-voltage range.

[0035] Finally, a low-voltage line can be a low-voltage supply line that runs from a component in the high-voltage area to the low-voltage area, from which it receives a low-voltage supply. Examples of this are low-voltage power sources that are connected to a sensor or communication component within the high-voltage area via a low-voltage supply line and are configured to supply this component within the high-voltage area. These lines have in common that, under fault-free operation—that is, with fault-free insulation or galvanic isolation from ground potential or the ground connection—they exhibit a voltage of less than 60 V (specifically less than 24 V). For example, data lines can carry voltages that vary between 0 and 5 V or between 0 and 12 V. This also applies to the supply line.Embodiments provide for several low-voltage lines, of which one or more are sensor lines, one or more are data lines and / or one or more are low-voltage supply lines.

[0036] The Figure 1 shows a high-voltage vehicle electrical system with a protective circuit according to the invention to further explain embodiments of the items described here.

[0037] The high-voltage vehicle electrical system FB of the Figure 1The system has a spatially limited high-voltage area (HV) that borders a low-voltage area via insulation (IN). Low-voltage lines (NL) extend from the high-voltage area, penetrate the insulation (IN), and also pass through a housing containing the high-voltage area (HV), before entering the low-voltage area (NV). For clarity, the components connected by these low-voltage lines (NL) are not shown. However, it can generally be assumed that the depicted low-voltage lines connect at least one component of the low-voltage area (NV) to at least one component of the high-voltage area (HV).

[0038] The low-voltage lines NL are shown as examples of signal line SL, data line DL and supply line VL, whereby the lowest line of the lines NL can also be a communication line.

[0039] The high-voltage vehicle electrical system FB of the Figure 1 It is also equipped with a vehicle high-voltage protection circuit (protection circuit for short). This circuit has several protective terminals SA, SA' for connection to the low-voltage lines NL. The protective terminals serve to connect to the individual low-voltage lines and also to connect varistors V of the protection circuit. The varistors of the Figure 1Each varistor is assigned to a specific low-voltage line NL or a specific protective terminal SA. On the side facing away from the protective terminals, the varistors V are connected to a ground terminal MA of the protection circuit, specifically via the inductor L shown. This inductor prevents AC voltage or current components used in the measurement from reaching the ground terminal MA or the ground potential M of the low-voltage range during active measurement. The ground terminal MA of the protection circuit is, as shown, connected to ground or the ground potential M of the low-voltage range NV. The inductor generally acts as a low-pass filter, thus preventing the transition of AC components from the protection circuit to other potentials within the low-voltage range NV.

[0040] The protection circuit can be designed with a detection device EE', which includes an AC power source Q and a measuring resistor Rs. The AC power source is connected to the varistors via the measuring resistor Rs, specifically to the side of the varistors V facing away from the protection terminals. A measuring device ME is connected in parallel with the measuring resistor Rs. In the example shown, this can be a voltage measuring device. It measures the voltage that arises when the AC power source Q applies an AC signal to the varistors V via the measuring resistor Rs. The measuring device ME outputs a signal that indicates the magnitude of the capacitive component of the selected (measured) varistor V. This value is passed to the comparator K, which is connected downstream of the measuring device ME.This device compares the value corresponding to the capacitive component with a predefined value to determine whether the capacitive component, or the measured capacitance, of the varistor in question is within a nominal range or deviates from a nominal value by no more than a specified amount. The result is output to output A of the comparator, with the signal at output A indicating whether the varistor V in question should be classified as faulty due to an excessively deviating capacitive component.

[0041] A selector switch MX is provided to select the varistor V to be measured. This switch is connected (via a resistor RI) to the power source Q and can optionally connect both the power source Q and one of the protective terminals SA'. For clarity, the protective terminals SA' are shown separately from the protective terminals SA. The protective terminals SA connect the varistors to the low-voltage lines NL, while the protective terminals SA' connect the detection device EE' (or its AC power source Q) to the individual lines NL. Furthermore, the protective terminals SA' connect the measuring unit ME and its associated measuring resistor Rs to the low-voltage lines NL (via the selector switch MX). The protective terminals SA' thus serve to actively detect and measure the capacitive component of the varistors V, which in turn serve to dissipate current when the voltage of the low-voltage lines NL is too high (relative to ground).It is evident that the protective terminals SA' and SA are connected to the same potentials, so that in a specific embodiment within the protection circuit, the protective terminals SA' and SA are connected to each other to form a common protective terminal for the low-voltage lines NL. This means that each low-voltage line NL only needs to be contacted once.

[0042] The selector switch MX is used to select one of the varistors V. In the example shown, the varistor V connected to the signal line SL is selected. Selector switch MX applies an alternating current to this varistor, while the same switch MX also selects the voltage across the varistor V to be measured by the measuring unit ME. If the source Q is an AC voltage source, the process is the same: The voltage is applied to the varistor connected to the signal line SL via the measuring resistor Rs and via selector switch MX. The corresponding current is measured by detecting the voltage drop across the resistor Rs. Thus, the measuring unit is used to detect the current through the resistor Rs.Alternatively, the measuring unit can also directly detect the voltage drop across the varistor V (as represented by the detection device EE). Regardless of the specific design and connection of the measuring unit ME, it either detects the current flowing through the varistor using the measuring resistor Rs, which serves as a measuring resistor, or measures a voltage from which the voltage drop across the varistor V can be derived.

[0043] Instead of using a single sensing device EE' for all varistors V by means of a selector switch, sensing devices, as indicated by the reference symbol E, can also be provided. On the right, a first sensing device EE is shown as a dashed line and is connected in parallel to the rightmost varistor V. Another sensing device EE, shown as a dotted line, is connected to a different low-voltage line NL, namely line VL. In other words, the various sensing devices EE shown are connected to different varistors to measure the voltage drop across them. Each sensing device EE has a measuring unit ME' with which the voltage drop across varistor V is detected. Furthermore, each sensing device EE has a source Q' by means of which an AC voltage signal can be applied to the connected varistor V.If the source Q is an AC current source, then the measuring unit ME' detects the voltage drop across the connected varistor V. If the source Q' is an AC voltage source, then the measuring device ME' detects the current flowing between the source and the connected varistor. Each detection unit EE is configured to calculate the capacitive component of the connected varistor, in particular by determining the ratio of the measured or impressed current to the voltage drop or applied voltage. This result is passed on to the respective comparators K' as shown, with their respective outputs A' providing a signal indicating whether the measured capacitive component deviates from a nominal value by more than a specified amount. If the deviation is greater than the specified amount, then a faulty varistor is assumed. The outputs A' can be combined, for example, by means of a logic OR circuit.This detects whether at least one output A' emits a signal indicating a faulty varistor. The OR gate can have a combined output that indicates whether at least one of the individual outputs A' indicates a faulty varistor, thus generally identifying or outputting a fault in the protection circuit or a fault in the group of varistors.

Claims

1. Method for checking the functionality of a high-voltage protective device for an on-board vehicle electrical system, which high-voltage protective device has a varistor or a different voltage limiting element, having the steps of: - detecting the capacitive component of the alternating current impedance of the varistor or the voltage limiting element; - comparing the capacitive component to a rated capacitance value, wherein the rated capacitance value is characteristic of the capacitance of a functioning varistor or a functioning voltage limiting element; - determining a defective state of the varistor or the voltage limiting element if the comparison determines that the capacitive component deviates from the rated capacitance value by more than a predetermined magnitude, wherein the method is carried out on the varistor (V) or voltage limiting element, via which the potential of a low-voltage line (NV), which is led out of a high-voltage zone (HV), is connected to an earth connector (MA) of the high-voltage protection circuit for a vehicle, and the high-voltage zone is isolated by way of an insulation from a low-voltage zone (NV) in which the high-voltage protection circuit for a vehicle is located, and the earth connector (MA) of the high-voltage protection circuit for a vehicle is connected to an earth potential (M) of the low-voltage zone (NV) and the earth potential (M) is galvanically isolated from the high-voltage zone (HV) .

2. Method according to Claim 1, wherein the detection is carried out (a) by applying an alternating current signal to the varistor (V) or to the voltage limiting element and by detecting the associated AC voltage signal that is applied to the varistor (V) or to the voltage limiting element, or (b) by applying an AC voltage signal to the varistor (V) or to the voltage limiting element and by detecting the associated alternating current signal that is applied to the varistor or to the voltage limiting element.

3. Method according to Claim 1 or 2, wherein, for a plurality of varistors (V) or voltage limiting elements, the respective capacitive component of the alternating current impedance is detected and wherein - the capacitive components of the varistors (V) or the voltage limiting elements are detected using individual devices (EE) and, for each varistor (V) or for each voltage limiting element or a subgroup thereof, its own signal source (Q') applies an alternating current signal and / or, for each varistor (V) or voltage limiting element or a subgroup thereof, its own measuring device (ME') detects the resulting signal at the associated varistor (V) or at the voltage limiting element, or - the capacitive components of the plurality of varistors (V) or voltage limiting elements are detected by signal multiplexing (SA') using the same device (EE') and the same signal source (Q) applies an alternating current signal to all varistors (V) or voltage limiting elements or to a subgroup thereof and / or the same measuring device (ME) detects the resulting signal at the varistors (V) or at the voltage limiting elements for all varistors (V) or voltage limiting elements or for a subgroup thereof.

4. Method according to any one of the preceding claims, wherein the varistor (V) or the voltage limiting element is connected to the earth connector (MA) of the high-voltage protection circuit for a vehicle via an inductor (L) and / or a resistor.

5. High-voltage protection circuit for a vehicle having - at least one protective connector (SA) that is set up for connection to a low-voltage line (NL) which is led out of a high-voltage zone (HV), - at least one varistor (V) or at least one voltage limiting element, via which the protective connector (SA) is connected to an earth connector (MA) of the high-voltage protection circuit for a vehicle, wherein the earth connector (MA) of the high-voltage protection circuit for a vehicle is connected to an earth potential (M) of the low-voltage zone (NV) and the earth potential (M) is galvanically isolated from the high-voltage zone (HV), and - a detection device (EE; GE) which is connected to the varistor (V) or voltage limiting element in a signal-transmitting manner and is set up to detect the capacitive component of the alternating current impedance of the varistor (V) or the voltage limiting element, and - an evaluating device (K), wherein the evaluating device (K) is set up to compare the capacitive component, which is detected by the detection device (EE; GE), to a rated capacitive value of the varistor (V) and to output a fault signal at a signal output of the evaluating device (K) in the event of a deviation which exceeds a predetermined magnitude, which fault signal identifies the varistor (V) or the voltage limiting element as defective.

6. High-voltage protection circuit for a vehicle according to Claim 5, having (a) an alternating current source (Q) which is connected to the at least one varistor (V) or voltage limiting element and is set up to impress an alternating current into the varistor (V) or into the voltage limiting element, wherein the detection device (ME; EE) is connected to the varistor (V) or the voltage limiting element and is set up to detect the voltage that results due to the impressed alternating current at the varistor (V) or at the voltage limiting element, or (b) an AC voltage source which is connected to the at least one varistor (V) or voltage limiting element and is set up to apply an AC voltage to the varistor (V) or voltage limiting element, wherein the detection device (EE; GE) is connected to the varistor (V) or voltage limiting element and is set up to detect the current that results due to the applied AC voltage at the varistor (V) or at the voltage limiting element.

7. High-voltage protection circuit for a vehicle according to Claim 5 or 6, having a plurality of varistors or voltage limiting elements, wherein one detection device (EE') is provided for each varistor (V) or each voltage limiting element or a subgroup thereof, which is connected to the respective varistor (V) or voltage limiting element in a signal-transmitting manner, or a selection switch (MX) is provided, by means of which the detection device (EE') is connected to all varistors (V) or voltage limiting elements or a subgroup thereof and the selection switch (MX) is set up to connect the detection device (EE') to a selected varistor (V) or voltage limiting element or to a selected subgroup of the varistors (V) or voltage limiting elements.

8. High-voltage protection circuit for a vehicle according to Claim 5, 6 or 7, wherein the at least one varistor (V) or the at least one voltage limiting element is connected via an inductance (L) and / or a resistor to the earth connector (MA).

9. High-voltage on-board vehicle electrical system having a spatially delimited high-voltage zone (HV), out of which at least one low-voltage line (NL) is led, wherein the high-voltage on-board vehicle electrical system comprises a high-voltage protection circuit for a vehicle according to one of Claims 5-8, wherein the protective connector (SA) of the high-voltage protection circuit for a vehicle is connected to the at least one low-voltage line (NL), the high-voltage zone (HV) is isolated by means of an insulation (IN) from a low-voltage zone (NV) in which the high-voltage protection circuit for a vehicle is located, and the earth connector (MA) of the high-voltage protection circuit for a vehicle is connected to an earth potential (M) of the low-voltage zone (NV), wherein the earth potential (M) is galvanically isolated from the high-voltage zone (HV).

10. High-voltage on-board vehicle electrical system according to Claim 9, wherein the at least one low-voltage line (NL) is designed as a sensor line (SL), data line (DL) or low-voltage supply line (VL).