Vehicle electrical system with active bridge rectifier and overvoltage protection during load shedding, rectifier arrangement, associated operating procedure and means for its implementation

Abstract

A vehicle electrical system (4) with an active bridge rectifier (1) connected to a generator (2) via a number of phase terminals (u - y) and having DC-side terminals (B+, B-), and with means (3) configured to detect a load shedding at the active bridge rectifier (1) when a voltage applied to the DC-side terminals exceeds a first threshold value, and to pulse-short-circuit the phase terminals (u - y) upon detection of a load shedding, thereby injecting a pulsed current into the vehicle electrical system (4), characterized in that at least one electrical system capacitor (C1) is provided which is configured to smooth the pulsed current, and that the vehicle electrical system (4) has voltage limiting means configured to limit the voltage between the DC-side terminals (B+,B-) of the bridge rectifier (1) to clamp to a predetermined maximum voltage when the voltage applied to the DC-side terminals exceeds a second threshold, the second threshold being greater than the first threshold.

Description

[0001] The present invention relates to a vehicle electrical system with an active bridge rectifier and means for protection against overvoltages during load shedding, as well as an associated operating method and means for implementing this operating method. State of the art

[0002] Rectifiers of various designs can be used to supply DC systems from AC systems. The present application relates to active (controlled) bridge rectifiers that have active switching elements, for example in the form of known MOS field-effect transistors. In vehicle electrical systems, six-pulse bridge rectifiers are frequently used, corresponding to the three-phase generators commonly installed there. However, the invention is equally suitable for bridge rectifiers for other phase numbers, e.g., for five-phase generators, and in other application scenarios.

[0003] As explained, for example, in DE 10 2009 046 955 A1, the use of active bridge rectifiers in motor vehicles is desirable, among other reasons, because they have lower power losses compared to passive (uncontrolled) bridge rectifiers.

[0004] A critical fault condition, particularly with active bridge rectifiers, is load dumping. This occurs when, with a highly excited generator and a correspondingly high output current, the load on the generator or the associated bridge rectifier (e.g., by switching off loads) suddenly decreases and this is not absorbed by capacitive elements in the DC network (e.g., the battery in a vehicle's electrical system).

[0005] In extreme cases, the generator and its associated bridge rectifier can supply energy to the vehicle's electrical system for up to approximately 300 to 500 ms. This energy must be dissipated (quenched) in the bridge rectifier to protect downstream electrical components from overvoltage damage. With passive or uncontrolled bridge rectifiers, this protection is typically provided by the rectifier diodes themselves, as they can convert the energy loss into heat. However, currently available active switching elements, such as MOS field-effect transistors, would be destroyed due to the high power dissipation. Therefore, additional protection strategies are required.

[0006] During load shedding, for example, the generator phases can be at least temporarily short-circuited by simultaneously activating all switching elements of the upper or lower rectifier branch, as disclosed, for example, in DE 198 35 316 A1 and discussed in the aforementioned DE 10 2009 046 955 A1. A corresponding control signal can also be clocked in such a way that a minimum voltage level is not undershot and a maximum voltage level is not exceeded.

[0007] However, multiple clockings of the control signal within a half-cycle have disadvantages, including rapid current changes during the initiation and termination of corresponding phase short circuits. These, in combination with existing line inductances, can lead to voltage dips or spikes. The latter, in particular, can damage components.

[0008] Vehicle electrical systems and the handling of load drops are also known from US 2005 / 0 046 397 A1, US 6 359 421 B1, US 2012 / 0 091 973 A1, US 5 629 606 A, US 2011 / 0 075 460 A1 and DE 10 2006 047 213 A1.

[0009] Therefore, there remains a need for improved protection strategies for active bridge rectifiers during load shedding. Disclosure of the invention

[0010] Against this background, the present invention proposes a vehicle electrical system with an active bridge rectifier and means for protection against overvoltages during load shedding, as well as an associated operating method and means for implementing this operating method with the features of the independent claims. Advantages of the invention

[0011] A key aspect of the present invention is the use of on-board network capacities as energy storage during load shedding operation of an active bridge rectifier. As explained, such load shedding operation can include connecting the generator phases to each other ("short-circuiting") in a pulsed manner by simultaneously activating (and thus simultaneously switching on) all active switching elements of a rectifier branch. Corresponding periods in which the active switching elements of a rectifier branch are simultaneously activated and thus switched on (and thus a short circuit of the generator phases is established) are referred to in this application as "short-circuit phases." In the terminology of this application, such short-circuit phases are "initiated" and "deactivated." This is done by activating the active switching elements.A "clocked" short circuit, in this context, refers to the alternating establishment and termination of a conductive connection according to a specific frequency or control pattern, whereby, for example, the duration of the short-circuit phases can be set or predetermined. This is discussed below in connection with the... Fig. Sections 1A to 1C are explained in more detail.

[0012] As is generally known, an active bridge rectifier has half-bridges, each with its own active switching elements that define an "upper" and a "lower" or "high-side" and "low-side" rectifier branch. The active switching elements in the upper or high-side rectifier branch allow one or more AC-side terminals to be connected to a positive DC-side terminal, and the active switching elements in the lower or low-side rectifier branch allow one or more AC-side terminals to be connected to a negative DC-side terminal. The active bridge rectifier is connected to a corresponding number of generator phases via the AC-side terminals, while the DC-side terminals supply a DC electrical system.The negative DC-side connection can be connected to ground. If two of the active switching elements of a rectifier branch are simultaneously switched to conduct, the correspondingly connected generator phases are short-circuited. If all (e.g., all three) generator phases are short-circuited, thus initiating a corresponding short-circuit phase, no current can flow into the DC electrical system. The generator phases can also be short-circuited by additional switching elements (i.e., not the active switching elements of the rectifier branches). Such additional switching elements can be found, for example, in the short-circuit circuit 6 described below. Fig. 4. They can also be arranged in the form of a bridge circuit, for example.

[0013] A corresponding rectifier operation includes, for example, detecting a load shedding by evaluating the voltage at a (usually positive) DC-side terminal of the bridge rectifier. If a load shedding is detected, a load shedding operation is initiated, which involves a pulsed short-circuiting of the generator phases. Thus, corresponding short circuits are pulsedly initiated and then interrupted, whereby the frequency and duration of the individual phases can be set or predefined.

[0014] If, for example, all active switching elements of the lower rectifier branch are controlled in a pulsed manner, the generator with the connected bridge rectifier behaves like a pulsed current source. This will also be explained below with reference to the Fig. 2 illustrated.

[0015] To maintain the voltage supply in the vehicle electrical system using a pulsed current source, an energy storage device in the form of a capacitor is required. This capacitor is referred to as the "vehicle electrical system capacitance" in this application. According to the present invention, the vehicle electrical system capacitance is connected to the bridge rectifier or a DC-side terminal of the bridge rectifier via a cable of a specific length. The entire arrangement functions as a buck converter. During short-circuit phases, the vehicle electrical system is powered by the energy stored in the vehicle electrical system capacitance. When the short circuit is deactivated, the current fed into the vehicle electrical system by the bridge rectifier is divided into a portion that flows directly into the vehicle electrical system and a portion that charges the vehicle electrical system capacitance (see Figure 1). Fig. 1B).

[0016] The cable connecting the vehicle's electrical system inevitably has a corresponding inductance. A rapid voltage increase at a DC-side connection of the bridge rectifier induces a back EMF, resulting in brief voltage spikes. Typical values ​​for such cables include a length of 1.5 m and an inductance of, for example, 1.5 to 2.5 µH. These voltage spikes must be limited, as otherwise the switching elements of the bridge rectifier can be destroyed, and damage to the connected regulators and the bridge rectifier's control electronics may occur.

[0017] The present invention therefore provides for a reduction of corresponding voltage spikes. This can be achieved, for example, by the measures described in detail below. These measures include, among others, the provision of internal clamping elements (i.e., clamping elements integrated into the half-bridges of the bridge rectifier), external clamping elements (i.e., clamping elements between the DC terminals of the bridge rectifier), and the controlled use of avalanche breakdown in the active switching elements of the bridge rectifier.

[0018] Internal bracketing elements are described in detail in the Fig. Figure 6 illustrates this. Using such clamping elements, the gate of the active switching elements of the rectifier branch that is not short-circuited can, for example, be autonomously controlled above a breakdown voltage. For this purpose, the positive DC terminal of the bridge rectifier is connected to the gate terminals via a blocking Zener diode. Only when the voltage at the positive DC terminal of the bridge rectifier exceeds the blocking voltage of the Zener diode (which corresponds to the desired breakdown voltage) during a short-circuit phase, does a current flow to the gate terminals, thereby also switching the active switching elements of the previously non-short-circuited rectifier branch into conduction.These remain conductive until the voltage of the positive DC-side terminal of the bridge rectifier falls below the reverse voltage of the Zener diode, and therefore no current flows to the respective gate terminals.

[0019] As an external clamping element, a Zener diode, for example, can be provided directly between the DC-side terminals. Such a Zener diode is directly connected between the DC-side terminals and can, for example, be integrated into the bridge rectifier. Below its forward voltage, it blocks current flow from the positive terminal to the negative terminal. It is preferably located in close proximity to the bridge rectifier so that it can completely prevent voltage spikes caused by the line inductance. The Zener diode is selected such that its breakdown voltage is below the maximum voltage of the voltage spikes and within a range acceptable for the aforementioned components. Thus, a suitable Zener diode clamps the voltage drop between the DC-side terminals to its breakdown voltage.Voltage peaks can also be limited by using a varistor, which is integrated between the DC-side terminals of the rectifier according to the possibilities explained above.

[0020] It is also possible to provide external voltage clamping via a controllable switching element between the DC-side terminals directly at the bridge rectifier. Such a switching element can be controlled, for example, by a control unit that is also used to control the rectifier. For this purpose, voltage monitoring can be performed, or alternatively, a pulsed connection between the DC-side terminals can be established for a predetermined period (e.g., a few milliseconds) whenever a short circuit is cleared by the control unit. The activation duration or pulse frequency can be determined based on a measurement and / or modeling of the occurring or expected voltage peaks.

[0021] As mentioned, clamping can also be achieved by utilizing avalanche breakdown in the active switching elements of the bridge rectifier, specifically in the active switching elements of the rectifier branch that is not short-circuited. As those skilled in the art know, avalanche breakdown is one of the three breakdown types in semiconductor devices. Avalanche breakdown causes a steep increase in current above a certain reverse voltage. The trigger for avalanche breakdown is the avalanche effect (also called avalanche multiplication or carrier multiplication). The avalanche effect is reversible, provided the permissible total power dissipation of the device is not exceeded. For this purpose, the active switching elements of the rectifier branch that is not short-circuited are designed to be avalanche-resistant, thus achieving clamping without the need for additional switching elements.

[0022] The elements described (Zener diodes, varistors, and controllable switching elements, which may also be designed to be avalanche-proof) are collectively referred to here as "voltage-dependent elements." Such voltage-dependent elements are characterized by their design and / or control signal such that they transition from a high-resistance to a low-resistance state at a predetermined voltage. This also applies to active switching elements, which transition from a non-conductive to a conductive state according to a corresponding control signal.

[0023] The described elements (Zener diodes, varistors, and controllable switching elements) are preferably arranged "directly" at the bridge rectifier so that the conductor section between the bridge rectifier and the respective element is as short as possible, allowing a back EMF to build up in this section due to conductor inductance. As explained, the vehicle's electrical system may include a capacitor, for example, a known smoothing capacitor. This capacitor is connected to the bridge rectifier via a conductor with a length of [m] meters and / or a conductor inductance of [m] µH.In contrast, the line or line section through which these elements (Zener diodes, varistors, and controllable switching elements) arranged "directly" at the bridge rectifier are connected has a length of only n meters and / or a line inductance of m µH, where n is at most 0.5, 0.4, 0.3, 0.2, 0.1, 0.05, or 0.01 times m. This allows the effects of the line inductances to be almost completely eliminated.

[0024] Wiring the active switching elements of the bridge rectifier in the form of a clamp circuit to limit voltage spikes can also be advantageous. This allows for a reduction of voltage spikes without the need for additional components. For example, the aforementioned clamp circuit can be implemented in the upper rectifier branch and the short-circuit circuit in the lower rectifier branch.

[0025] A computing unit according to the invention, e.g. a control unit of a motor vehicle or a rectifier control, is, in particular in terms of programming, equipped to carry out a method according to the invention.

[0026] Implementing the process in software form is also advantageous, as this incurs particularly low costs, especially if the executing control unit is already used for other tasks and is therefore already present. Suitable data carriers for providing the computer program include floppy disks, hard drives, flash memory, EEPROMs, CD-ROMs, DVDs, etc. Downloading the program via computer networks (Internet, intranet, etc.) is also possible.

[0027] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.

[0028] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or on their own, without leaving the scope of the present invention.

[0029] The invention is schematically illustrated in the drawing using an exemplary embodiment and is described in detail below with reference to the drawing. Brief description of the drawings Fig. Figures 1A to 1C show arrangements with active bridge rectifiers and their function in schematic representation. Fig. Figure 2 shows a current waveform when controlling an active bridge rectifier during load shedding. Fig. Figure 3 shows a schematic representation of an arrangement for testing a load drop, which can be operated according to the invention. Fig. Figure 4 shows a schematic representation of an arrangement with an active bridge rectifier that can be operated according to the invention. Fig. Figure 5 shows current and voltage waveforms during the control of an active bridge rectifier during load shedding according to the prior art and according to an embodiment of the invention. Fig. Figure 6 shows a schematic representation of an arrangement with an active bridge rectifier that can be operated according to the invention.

[0030] The figures show identical or corresponding elements with the same reference symbols. Further explanation is omitted. embodiment(s) of the invention

[0031] The Fig. Figures 1A to 1C show schematic representations of arrangements with active bridge rectifiers and their function.

[0032] In Fig. Figure 1A schematically illustrates a conventional arrangement with a bridge rectifier 1 and a generator 2, using a three-phase system as an example. The bridge rectifier 1 is in Fig. Figure 1A is shown as a six-pulse bridge rectifier configured for rectifying the three-phase current from a three-phase generator 2. However, a four-, five-, six- or seven-phase generator 2 and a correspondingly adapted bridge rectifier 1 can also be used in the same way. Fig. 1B and Fig. Figure 1C shows, for example, arrangements with a five-phase generator 2 and corresponding bridge rectifiers 1.

[0033] The bridge rectifier 1 has three half-bridges U, V and W, which are connected via inputs u, v and w to corresponding outputs of the generator 2, and thus to the respective generator windings.

[0034] The half-bridges U, V, and W are connected on their output side, for example, to a positive battery terminal B+ and a negative battery terminal B- and / or corresponding supply lines B+ and B- of a DC power supply. Terminal B- can be connected to ground. The half-bridges U, V, and W each have active switching elements S1 to S6, which are each integrated into an upper branch H (highside) and a lower branch L (lowside) of the respective half-bridges U, V, and W.

[0035] An input u, v and w can be connected to B+ and / or B- according to a corresponding circuit configuration of the active switching elements S1 to S6, whereby simultaneous control of both switching elements of a half-bridge U, V and W (i.e. S1 / S4, S2 / S5 and S3 / S6) is to be avoided in normal operation in order to prevent "hot paths" between B+ and B-.

[0036] The active switching elements S can be driven via their respective gate terminals G by a control unit 3 via control lines (not shown) according to a control pattern. Normal operation of the generator involves controlling the active switching elements S such that a current signal present at a corresponding input u, v, and w of a connected generator winding of generator 2 is alternately routed to B+ and B-. Typically, this occurs such that when a positive half-wave is present at inputs u, v, and w, the respective signal is routed to B+, and when a negative half-wave is present, the signal is routed to B-. Setting an output voltage at B+ can also be achieved by appropriate clocking.

[0037] A load drop can occur in a Fig. The arrangement shown in 1A is detected based on a voltage applied to B+. If a defined threshold is exceeded, a load shedding can be detected.

[0038] Rectifier 1 can be controlled upon detection of a load shedding event by short-circuiting the phase windings of generator 2, each connected to the half-bridges of rectifier 1 via one of the inputs u, v, and w, for a defined period. As a result, the current fed into the vehicle electrical system drops to zero. A corresponding short circuit can be created by simultaneously activating and thus conducting the switching elements S1 to S3 or S4 to S4 of each rectifier branch H or L, respectively. When the short circuit is released, the current rises again. This process can be used to regulate the output voltage and / or to reduce generator overvoltage. As mentioned, however, problems can arise due to current or voltage spikes caused by line inductances. These voltage spikes generate high power losses in the half-bridges, which can lead to thermal damage.

[0039] In the Fig. 1B and Fig. Figure 1C shows arrangements with a five-phase generator 2 and a rectifier 1 with five half-bridges. The inputs are labeled u to y, and the half-bridges accordingly U to Y. The half-bridges U to Y each have active switching elements S1 to S10. The switching elements S1 to S10 are illustrated as switches with a parallel-connected diode, but in reality, they are implemented, for example, as MOS field-effect transistors as shown in Fig. 1A trained. Furthermore, the regulations comply with the Fig. 2B and Fig. 2C those of Fig. 1A.

[0040] In a downstream electrical system 4, a system capacitor C1 and a resistive load R1 are arranged. The resistive load corresponds, for example, to a consumer in the electrical system 4. The lines in the electrical system 4 result in a line inductance L1, as explained in more detail below.

[0041] In Fig. Figure 1B depicts a momentary state of normal operation of the corresponding arrangement. It is assumed here that at this moment, current flows from generator 2 into rectifier 1 at phases u and v, while current flows into generator 2 at phases w, x, and y. Switching elements S1 and S2 establish a conductive connection between inputs u and v and the positive DC output U+ through appropriate control. Switching elements S8, S9, and S10 establish a conductive connection between inputs w, x, and y and the negative DC output U-. For this moment, a positive voltage potential is present at inputs u and v, while a negative voltage potential is present at inputs w, x, and y.Through inputs u and v, a current flows from generator 2 into rectifier 1, as illustrated by the corresponding arrows; through inputs w, x, and y, a current flows from rectifier 1 into generator 2. The current fed in by the generator becomes too high during load shedding and in this case is, for example, 150 A.

[0042] Since a vehicle electrical system capacitor C1 is provided in this case, part of the current, for example 130 A, flows into it and charges it. The remaining portion, here 20 A, flows into the actual vehicle electrical system 4 or the resistive load R1.

[0043] To avoid excessive current flows during load shedding, a switching state can be periodically set, as described in Fig. Figure 1C shows that all switching elements S6 to S10 of the lower rectifier branch L are connected. Alternatively, the conductive connection can also be established via switches S1-S5. Inputs u to y, and thus the corresponding generator phases, are thereby connected (short-circuited). No current flows from generator 2 to rectifier 1. In this switching state, the vehicle electrical system 4 is supplied exclusively from the vehicle electrical system capacitor C1. The switching state of the Fig. 1C is set until the voltage between the DC-side voltage terminals B+ and B- falls below a permissible value. Then the switching state of the Fig. 1B is set until the voltage between the DC-side voltage terminals B+ and B- exceeds the permissible value again, etc. When switching between the switching states of the Fig. 1B and Fig. As explained, voltage dips and spikes occur at 1C.

[0044] In Fig. Figure 2 shows a generator current curve for such control of an active bridge rectifier 1 during load shedding according to the prior art. In the diagram shown, the generator current i is plotted as current I in A on the y-axis against time t in ms on the x-axis.

[0045] As already mentioned in connection with the Fig. As explained in section 1A, in the context of conventional control during load shedding, the switching elements S1 to S3 or S4 to S6 (or the switching elements S1 to S5 or S6 to S10) are activated according to the Fig. 1B and Fig. 1C) Each rectifier branch H or L is simultaneously activated during specific time periods 21, thus short-circuiting the generator phases. No current is supplied to the vehicle electrical system. During time periods 22, the short circuit is broken and current is supplied to the vehicle electrical system. As shown in Fig. As can be seen in Figure 2, the current increases abruptly from 0 A to, for example, 100 A and then decreases just as abruptly. This, in conjunction with line inductances in the vehicle electrical system, as explained, can cause voltage spikes with corresponding negative effects.

[0046] Fig. Figure 3 shows a schematic representation of an arrangement 30 for testing or simulating a load shedding. The arrangement 30 comprises a generator 2 with a bridge rectifier, as described in the Fig. 1A to 1C are explained. A voltage U1 is applied to this.

[0047] Capacitors 31 and 32 and load resistors 33 and 34 of arrangement 30 represent capacitances and resistances, respectively, of a real vehicle electrical system. A voltage drop U2 occurs across these components. They are connected to generator 2 and bridge rectifier 1, respectively, via line 37 and can be switched using switches 35 and 36. Line 37 simulates the inductance of the vehicle electrical system, which results in a voltage difference between U1 and U2 in the event of a load shedding.

[0048] At the start of a load shedding test, both switches 35 and 36 are closed. The generator 2, or rather the bridge rectifier 1, supplies a current to the vehicle electrical system, which results from the voltage U2 and the load resistances 33 and 34.

[0049] A load shedding can be simulated by opening one of the switches 35 or 36. Opening switch 35 corresponds to a load drop to 0%, as would occur in reality, for example, when the battery terminal or the connecting cable to the generator is disconnected. Opening switch 36, on the other hand, simulates a partial load drop, as caused by disconnecting a larger resistive load in the vehicle electrical system. The magnitude of the "shed" load current can be adjusted by the resistance value of load resistor 34, and the magnitude of the remaining vehicle electrical system current by the resistance value of load resistor 33.

[0050] It should be noted that due to the pulsed current output of generator 2 or the downstream rectifier (see Fig. 2) An energy storage device, for example in the form of the aforementioned vehicle electrical system capacity, is required for the continuous voltage supply of the DC network. This is always the case in conventional DC networks, such as vehicle electrical systems. However, when the phase short circuits described above are interrupted (removed), a voltage increase occurs due to the line inductances. This is prevented according to the invention.

[0051] The measures according to the invention are based on the Fig. 4 further explained, in which an arrangement with a bridge rectifier 1 and a generator 2 according to the Fig. 1A is shown. The arrangement has a correspondingly designed control unit 3. The essential elements have already been described with reference to the Fig. 1A explained.

[0052] A current fed into a vehicle electrical system via the DC-side terminals B+ and B- of bridge rectifier 1 exhibits, during load shedding operation (i.e., during pulsed activation of all switching elements S1 to S3 or S4 to S6 of a half-bridge H or L), a waveform as shown in the Fig. Figure 2 shows the following. The on-board network 4 contains the on-board network capacitor C1 and the resistive load R1. The resistive load corresponds, for example, to a consumer in the on-board network 4.

[0053] The on-board power supply capacitance C1 can be used to smooth a current or voltage. The on-board power supply capacitance C1 acts like a buck converter, as explained previously. During short-circuit phases (periods 21 of the Fig. 2) No current is fed into the vehicle electrical system 4. During periods 21 and 22, the vehicle electrical system 4 is powered by the stored energy in the vehicle electrical system capacity C1. Fig. 2, i.e., when a corresponding short circuit is deactivated, the supplied currents are divided into partial currents that flow directly into the vehicle electrical system 4 and partial currents that charge the vehicle electrical system capacity C1 (see Fig. 1B and related explanations). When the short circuits are deactivated, i.e., when transitioning from periods 21 to periods 22 of the Fig. 2. This results in a sudden increase in current at the DC-side terminals B+ and B- of the bridge rectifier 1. This is due to the presence of a conductor section of a specific length, and therefore a specific conductor inductance L1, between the DC-side terminal B+ of the bridge rectifier 1 and, for example, a connection point 41 in the vehicle electrical system 4. Such a conductor section might, for example, have a length of 1.5 m and an inductance of 1.5 to 2.5 µH.

[0054] One way to prevent such voltage spikes is to provide a voltage-dependent element 5, which here is designed as an external clamping element. A "voltage-dependent element" is understood here to be an element that transitions from a high-resistance state to a low-resistance state when a certain voltage is applied between the DC-side terminals B+ and B- of the bridge rectifier 1. For example, as mentioned, a Zener diode or a varistor can be used as the voltage-dependent element 5 within the scope of the present invention. However, the voltage-dependent element 5 can also be designed as a switching element that is appropriately controlled when a short-circuit phase is deactivated, for example by means of the control device 3. For further possibilities, please refer to the explanations above.In particular, the invention can also be implemented without such an external clamping element by using internal clamping elements, as in . Fig. 6, or the active switching elements S1 to S6 are designed to be avalanche-proof. In the latter case, the switching elements S1 to S6 of the respective non-short-circuited rectifier branch (typically the upper rectifier branch H) reversibly transition to a low-resistance state from the corresponding blocking voltage, as explained above. The voltage-dependent element 5 is preferably selected or controlled such that the voltage at which element 5 transitions to the low-resistance state (hereinafter also referred to as the "clamp voltage") is significantly above the normal voltage or control voltage. The clamp voltage at the DC-side terminal B+ must be significantly (typically up to 10 V) above the short-circuit activation for the switching elements S1 to S6, which is used for load shedding operation, but significantly below the breakdown voltage of the switching elements S1 to S6.

[0055] However, the invention is not limited to the use of a voltage-dependent element 5, as described in the Fig. The limitations shown in Figure 4 are limited. It may also be possible to limit the voltage rise by appropriately connecting the switching elements S1 to S3 of the upper rectifier branch H in the half-bridges U, V, and W. For example, if switching elements S4 to S6 of the lower rectifier branch L are switched to conduct to initiate the phase short circuit, then, when the phase short circuit is removed, a corresponding clamping can briefly occur in a switching element S1 to S3 of the upper rectifier branch after the short circuit at switching elements S4 to S6 of the lower rectifier branch L has been removed. The same applies in reverse. Thus, in the event of a short circuit in the upper rectifier branch H, a switching element in the lower rectifier branch L can be connected for clamping.

[0056] A corresponding short circuit of the generator phases or corresponding phase connections u, v and w can also be produced in a separate short-circuit circuit 6, whereby clamping can be carried out in the lower rectifier branch L or the upper rectifier branch H or in an external voltage protection, for example the voltage-dependent element 5, e.g. a Zener diode or a varistor.

[0057] The effects resulting from the measures according to the invention are in Fig. 5 illustrates. Fig. 5 includes diagrams 501, 502 and 503.

[0058] Diagram 501 shows a current I in A on the ordinate versus a time t in ms on the abscissa. The current waveform essentially corresponds to the current waveform as also shown in Fig. 2 is shown. The corresponding phases are marked with the same reference symbols as in the Fig. 2. In phase 21, there is a short circuit, as explained previously; in phase 22, this short circuit is eliminated. As can be seen, a sudden increase in current occurs during the transition from phase 21 to phase 22.

[0059] Diagram 502 shows the resulting voltage waveforms in a circuit according to the state of the art, i.e., without preventing voltage spikes. In Diagram 502, as well as in the subsequently explained Diagram 503, voltages U in V are shown on the ordinate versus time t in ms on the abscissa. As can be seen, the voltages reach values ​​of up to several V during the transition from phase 21 to phase 22. The achievable voltage peak û is calculated as û = L × (di / dt) with i = I Gen (the generator current) according to Fig. 2. Conversely, the same applies to the transition from phases 22 to phases 21. Here, voltage dips occur.

[0060] In contrast, in diagram 503, the corresponding voltage peaks and voltage dips are significantly reduced due to the measures according to the invention.

[0061] Fig. Figure 6 shows the use of internal bracket elements. The ones in the Fig. The arrangement shown in section 6 largely corresponds to the Fig. 1A or the Fig.4. Only some of the elements are labeled with reference symbols. In parallel to the active switching elements S1 to S3 of the upper rectifier branch H, a Zener diode Z and a diode D are connected in the respective forward and reverse directions shown. The Zener diode Z ensures that a control line S1' to S3' is supplied with current from the positive DC voltage terminal B+ whenever the voltage applied there exceeds the breakdown voltage of the Zener diode Z. This also briefly short-circuits the corresponding active switching elements S1 to S3 (in addition to the active switching elements S4 to S6, which are controlled to short-circuit inputs u to w). The diode D ensures that current from a control signal does not flow into the vehicle's electrical system during normal operation.

Claims

[1] Vehicle electrical system (4) with an active bridge rectifier (1) which is connected to a generator (2) via a number of phase terminals (u - y) and has DC-side terminals (B+, B-), and with means (3) which are designed to detect a load shedding at the active bridge rectifier (1) when a voltage applied to the DC-side terminals exceeds a first threshold value and, in the event of a detected load shedding, to pulse the phase terminals (u - y) by short-circuiting them, thereby injecting a pulsed current into the vehicle electrical system (4), characterized by, that at least one on-board power supply capacitor (C1) is provided which is designed to smooth the pulsed current, and that the vehicle on-board power supply (4) has voltage limiting means which are designed to clamp a voltage between the DC-side terminals (B+, B-) of the bridge rectifier (1) to a predetermined maximum voltage when the voltage applied to the DC-side terminals exceeds a second threshold, wherein the second threshold is greater than the first threshold. [2] Motor vehicle electrical system (4) according to claim 1, wherein the bridge rectifier (1) has a number of active switching elements (S1 - S10) in a number of half-bridges (U - Y) corresponding to the number of phase connections (u - y), wherein a first group of the active switching elements (S1 - S10) can be controlled in groups for the pulsed short-circuiting of the phase connections (u - y). [3] Motor vehicle electrical system (4) according to claim 2, wherein the voltage limiting means have at least one voltage-dependent element (5, Z) configured to establish a conductive connection between the DC-side terminals (B+, B-) of the bridge rectifier (1) above the maximum voltage. [4] Motor vehicle electrical system (4) according to claim 3, wherein the at least one voltage-dependent element (5, Z) comprises at least one Zener diode (Z) and / or at least one varistor, the breakdown voltage or threshold voltage of which corresponds to the maximum voltage. [5] Motor vehicle electrical system (4) according to claim 3, wherein the at least one voltage-dependent element (5, Z) comprises at least one controllable switching element (5) which is configured to switch from a non-conductive to a conductive state above the maximum voltage. [6] Motor vehicle electrical system (4) according to one of claims 3 to 5, wherein the at least one voltage-dependent element (5, Z) is integrated between the DC-side terminals (B+, B-). [7] Motor vehicle electrical system (4) according to one of claims 3 to 5, in which a number of voltage-dependent elements (Z) are provided, each of which is connected between one of the DC-side terminals (B+, B-) of the bridge rectifier (1) and a gate terminal of at least one part of the active switching elements (S1 - S10). [8] Motor vehicle electrical system (4) according to one of claims 3 to 5, wherein the active switching elements (S1 - S10) are at least partially designed as voltage-dependent elements (5) in the form of avalanche-proof active switching elements (S1 - S10). [9] Motor vehicle electrical system (4) according to claim 2, wherein the voltage limiting means are configured to limit the voltage between the DC-side terminals (B+, B-) to the maximum voltage by controlling at least one active switching element which is not part of the first group of active switching elements (S1 - S10). [10] Method for operating a motor vehicle electrical system (4) according to one of the preceding claims, in which, upon detection of a load shedding at the active bridge rectifier (1), when a voltage applied to the DC-side terminals exceeds a first threshold value, the phase terminals (u - y) are pulsedly short-circuited, thereby injecting a pulsed current into the motor vehicle electrical system (4), characterized by, that the pulsed current is smoothed by means of the at least one on-board network capacitor (C1) and that by means of the voltage limiting means a voltage applied between the DC-side terminals (B+, B-) of the bridge rectifier (1) is clamped to a predetermined maximum voltage when the voltage applied at the DC-side terminals exceeds a second threshold, wherein the second threshold is greater than the first threshold. [11] Method according to claim 10, wherein voltage limiting means with a voltage-dependent element (5) is used, comprising at least one controllable switching element, wherein the voltage applied between the DC-side terminals (B+, B-) of the bridge rectifier (1) is clamped to the predetermined maximum voltage by controlling the at least one controllable switching element. [12] Method according to claim 10 or 11, in which a first group of active switching elements (S1 - S10) of the bridge rectifier (1) in a number of half-bridges (U - Y) corresponding to the number of phase terminals (u - y) is pulsed to pulse short-circuit the phase terminals (u - y) in groups. [13] Method according to claim 12, wherein the voltage between the DC-side terminals (B+, B-) is clamped to the maximum voltage by controlling at least one active switching element which is not part of the first group of active switching elements (S1 - S10). [14] Control device (3) for a motor vehicle electrical system (4) according to one of claims 1 to 9, which is configured to carry out a method according to one of claims 10 to 13. [15] Computer program comprising program code means which cause a computing unit, in particular a control device (3) according to claim 14, to carry out a method according to one of claims 10 to 13 when executed on the computing unit.

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