Vehicle electrical system and method for operating a vehicle electrical system

A vehicle electrical system includes a first system branch with a first nominal voltage U1, a second system branch with a second nominal voltage U2, at least one DC/DC converter configured to transmit energy between the first and second system branches, a first actuating unit to actuate the DC/DC converter(s), a first detection unit to detect an instantaneous voltage Uact,1 of the first system branch, and a comparison unit to compare the detected instantaneous voltage Uact,1 to a first upper voltage threshold value Uo,1 and to a first lower voltage threshold value Uu,1, wherein Uu,1<U1<Uo,1. The first actuating unit actuates the DC/DC converter(s) such that energy is transmitted from the first system branch to the second system branch if Uact,1>Uo,1, and such that energy is transmitted from the second system branch to the first system branch if Uact,1<Uu,1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/EP2012/072481 filed Nov. 13, 2012, which designates the United States of America, and claims priority to DE Application No. 10 2011 086 829.1 filed Nov. 22, 2011, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

This application relates to a vehicle electrical system, a vehicle having a vehicle electrical system and a method for operating a vehicle electrical system.

BACKGROUND

DE 10 2005 057 306 A1 discloses a method for stabilizing a direct voltage vehicle electrical system, in particular in a motor vehicle, having a direct voltage converter which is arranged in the vehicle electrical system and which is fed on an input side by at least one electrical energy source and which has on an output side a plurality of voltage-stabilizing electrical outputs which are each connected to a part of the vehicle electrical system with electrical loads connected thereto. Sudden changes in electrical load which effect the input side of the direct voltage converter on at least one part of the vehicle electrical system which is sensitive to voltage fluctuations can be at least partially compensated by respective voltage stabilization at the respective outputs using control means by varying the voltage and/or the electrical load on at least one part of the vehicle electrical system which is insensitive to voltage fluctuations.

SUMMARY

A vehicle electrical system, comprising a first vehicle electrical system branch with a first nominal voltage U1, a second vehicle electrical system branch with a second nominal voltage U2, at least one DC/DC converter which is designed to transfer energy between the first vehicle electrical system branch and the second vehicle electrical system branch, a first actuating unit which is designed to actuate the at least one DC/DC converter, a first detection unit which is designed to detect an instantaneous voltage Uact,1of the first vehicle electrical system branch, a first comparison unit which is designed to compare the detected instantaneous voltage Uact,1with a first upper voltage threshold value Uo,1and a first lower voltage threshold value Uu,1, where Uu,1<U1<U0,1, wherein the first actuating unit is designed to actuate the at least one DC/DC converter in such a way that energy is transferred from the first vehicle electrical system branch to the second vehicle electrical system branch if Uact,1>Uo,1, and that energy is transferred from the second vehicle electrical system branch to the first vehicle electrical system branch if Uact,1<Uu,1.

In a further embodiment, the first comparison unit has at least one comparator.

In a further embodiment, the first comparison unit is also designed to compare the detected instantaneous voltage Uact,1with a second upper voltage threshold value Uo,2and a second lower voltage threshold value Uu,2, where Uu,1<Uu,2<U1and U1<Uo,2<Uo,1.

In a further embodiment, the first actuating unit is also designed to actuate the at least one DC/DC converter in such a way that the transfer of energy from the first vehicle electrical system branch to the second vehicle electrical system branch is ended if Uact,1<Uo,2, and that the transfer of energy from the second vehicle electrical system branch to the first vehicle electrical system branch is ended if Uact,1>Uu,2.

In a further embodiment, the vehicle electrical system also includes a voltage-limiting switch.

In a further embodiment, the voltage-limiting switch has at least one freewheeling diode.

In a further embodiment, the vehicle electrical system further includes a second detection unit which is designed to detect an instantaneous voltage Uact,2of the second vehicle electrical system branch and a second comparison unit which is designed to compare the detected instantaneous voltage Uact,2with a third upper voltage threshold value Uo,3and a third lower voltage threshold value Uu,3, where Uu,3<U2<U0,3.

In a further embodiment, the first actuating unit is also designed to actuate the at least one DC/DC converter in such a way that energy is transferred from the second vehicle electrical system branch to the first vehicle electrical system branch if Uact,2>U0,3, and that energy is transferred from the first vehicle electrical system branch to the second vehicle electrical system branch if Uact,2<Uu,3.

In a further embodiment, the at least one DC/DC converter is embodied as a synchronous converter.

Another embodiment provides a vehicle having any of the vehicle electrical systems disclosed above.

Another embodiment provides a method for operating a vehicle electrical system, wherein the vehicle electrical system has a first vehicle electrical system branch with a first nominal voltage U1, a second vehicle electrical system branch with a second nominal voltage U2and at least one DC/DC converter for transferring energy between the first vehicle electrical system branch and the second vehicle electrical system branch, and wherein the method has the following steps: detecting a first value of an instantaneous voltage Uact,1of the first vehicle electrical system branch, comparing the detected first value of the instantaneous voltage Uact,1with a first upper voltage threshold value Uo,1and a first lower voltage threshold value Uu,1, where Uu,1<U1<Uo,1, and if Uact,1>Uo,1, actuating the at least one DC/DC converter in such a way that energy is transferred from the first vehicle electrical system branch to the second vehicle electrical system branch, and if Uact,1<Uu,1, actuating the at least one DC/DC converter in such a way that energy is transferred from the second vehicle electrical system branch to the first vehicle electrical system branch.

In a further embodiment, if Uact,1>Uo,1, the method also has the following steps after the actuation of the at least one DC/DC converter: detecting a second value of the instantaneous voltage Uact,1of the first vehicle electrical system branch, comparing the detected second value of the instantaneous voltage Uact,1with a second upper voltage threshold value Uo,2, where U1<Uo,2<Uo,1, and if Uact,1<Uo,2, ending the transfer of energy from the first vehicle electrical system branch to the second vehicle electrical system branch.

In a further embodiment, if Uact,1>Uo,1the transfer of energy from the first vehicle electrical system branch to the second vehicle electrical system branch is ended after a predetermined time period.

In a further embodiment, if Uact,1<Uu,1the method also has the following steps after the actuation of the at least one DC/DC converter: detecting a second value of the instantaneous voltage Uact,1of the first vehicle electrical system branch, comparing the detected second value of the instantaneous voltage Uact,1with a second lower voltage threshold value Uu,2, where Uu,1<Uu,2<U1, and if Uact,1>Uu,2ending the transfer of energy from the second vehicle electrical system branch to the first vehicle electrical system branch.

In a further embodiment, if Uact,1<Uu,1the transfer of energy from the second vehicle electrical system branch to the first vehicle electrical system branch is ended after a predetermined time period.

DETAILED DESCRIPTION

Embodiments of the invention provide a vehicle electrical system, a vehicle having a vehicle electrical system and a method for operating a vehicle electrical system which permit further improved voltage stabilization.

One embodiment provides a vehicle electrical system having a first vehicle electrical system branch with a first nominal voltage U1and a second vehicle electrical system branch with a second nominal voltage U2. In addition, the vehicle electrical system has at least one DC/DC converter which is designed to transfer energy at least between the first vehicle electrical system branch and the second vehicle electrical system branch. Furthermore, the vehicle electrical system has a first actuating unit which is designed to actuate the at least one DC/DC converter. Furthermore, the vehicle electrical system has a first detection unit which is designed to detect an instantaneous voltage Uact,1of the first vehicle electrical system branch. In addition, the vehicle electrical system has a first comparison unit which is designed to compare the detected instantaneous voltage Uact,1with a first upper voltage threshold value Uo,1and a first lower voltage threshold value Uu,1, where Uu,1<U1<Uo,1. The first actuating unit is designed to actuate the at least one DC/DC converter in such a way that energy is transferred from the first vehicle electrical system to the second vehicle electrical system if Uact,1>Uo,1. Furthermore, the first actuating unit is designed to actuate the at least one DC/DC converter in such a way that energy is transferred from the second vehicle electrical system branch to the first vehicle electrical system branch if Uact,1<Uu,1.

The vehicle electrical system may permit further improved voltage stabilization in the first vehicle electrical system branch, in particular by providing the at least one DC/DC converter and the first actuating unit which is correspondingly designed to actuate the DC/DC converter. In this context, transferring energy from the first vehicle electrical system branch to the second vehicle electrical system branch if Uact,1>Uo,1permits an instantaneous overvoltage in the first vehicle electrical system branch to be reduced and/or compensated. Furthermore, transferring energy from the second vehicle electrical system branch to the first vehicle electrical system branch if Uact,1<Uu,1permits an instantaneous undervoltage in the first vehicle electrical system branch also to be reduced and/or compensated. As a result, the voltage in the first vehicle electrical system branch can advantageously be held at a value which corresponds essentially to the first nominal voltage U1. In particular, fluctuations in the vehicle electrical system while changes in load occur in the first vehicle electrical system branch can be compensated.

In one embodiment of the vehicle electrical system, the first comparison unit has at least one comparator. This permits the detected instantaneous voltage Uact,1to be easily compared with the first upper voltage threshold value Uo,1and the first lower voltage threshold value Uu,1.

In addition, the first comparison unit can be designed to compare the detected instantaneous voltage Uact,1with a second upper voltage threshold value Uo,2and a second lower voltage threshold value Uu,2, where Uu,1<Uu,2<U1and U1<Uo,2<Uo,1.

The first actuating unit may be designed here to actuate the at least DC/DC converter in such a way that the transfer of energy from the first vehicle electrical system to the second vehicle electrical system is ended if Uact,1<Uo,2. Furthermore, the first actuating unit in the specified embodiment is designed to actuate the at least one DC/DC converter in such a way that the transfer of energy from the second vehicle electrical system branch to the first vehicle electrical system branch is ended if Uact,1>Uu,2. The specified embodiments permit hysteresis behavior to be taken into account during the actuation of the at least one DC/DC converter by comparing the instantaneous voltage Uact,1of the first vehicle electrical system branch with the second upper voltage threshold value Uo,2and the second lower voltage threshold value Uu,2.

In a further embodiment, the vehicle electrical system also has a voltage-limiting switch. The voltage-limiting switch can have here at least one freewheeling diode. Furthermore, the voltage-limiting switch can be formed by an inherent body diode of a MOSFET. The provision of such voltage-limiting switches permits the voltage in the first vehicle electrical system branch to be stabilized further, in particular in the case of low overvoltage values.

In addition, the vehicle electrical system can have a second detection unit which is designed to detect an instantaneous voltage Uact,2of the second vehicle electrical system branch. In this embodiment, the vehicle electrical system also has a second comparison unit which is designed to compare the detected instantaneous voltage Uact,2with a third upper voltage threshold value Uo,3and a third lower voltage threshold value Uu,3, where Uu,3<U2<Uo,3.

The first actuating unit may be designed here to actuate the at least one DC/DC converter in such a way that energy is transferred from the second vehicle electrical system branch to the first vehicle electrical system branch if Uact,2>U0,3. In addition, the first actuating unit is preferably designed to actuate the at least one DC/DC converter in such a way that energy is transferred from the first vehicle electrical system branch to the second vehicle electrical system branch if Uact,2<Uu,3. The specified embodiments advantageously permit improved voltage stabilization in the second vehicle electrical system branch. In this context, overvoltages in the second vehicle electrical system branch can be reduced and/or compensated by transferring energy to the first vehicle electrical system branch, and undervoltages in the second vehicle electrical system branch can be reduced and/or compensated by transferring energy from the first vehicle electrical system branch.

The at least one DC/DC converter may be embodied as a synchronous converter for a bidirectional transfer of energy between the first vehicle electrical system branch and the second vehicle electrical system branch.

Another embodiment provides a vehicle which has a vehicle electrical system as disclosed herein. The vehicle is, for example, a motor vehicle, in particular a passenger car or a truck, and can be embodied as a hybrid vehicle or vehicle with pure internal combustion engine drive.

Another embodiment provides a method for operating a vehicle electrical system, wherein the vehicle electrical system has a first vehicle electrical system branch with a first nominal voltage U1, a second vehicle electrical system branch with a second nominal voltage U2and at least one DC/DC converter for transferring energy at least between the first vehicle electrical system branch and the second vehicle electrical system branch. The method has the following steps. A first value of an instantaneous voltage Uact,1of the first vehicle electrical system branch is detected. In addition, the detected first value of the instantaneous voltage Uact,1is compared with a first upper voltage threshold value Uo,1and a first lower voltage threshold value Uu,1, where Uu,1<U1<U0,1. If Uact,1>U0,1, the at least one DC/DC converter is actuated in such a way that energy is transferred from the first vehicle electrical system branch to the second vehicle electrical system branch. If Uact,1<Uu,1, the at least one DC/DC converter is actuated in such a way that energy is transferred from the second vehicle electrical system branch to the first vehicle electrical system branch.

The method may provide any or all of the same advantages as discussed in relation to the vehicle electrical system.

If Uact,1>U0,1, in one embodiment the method also has the following steps after the actuation of the at least one DC/DC converter. A second value of the instantaneous voltage Uact,1of the first vehicle electrical system branch is detected. In addition, the detected second value of the instantaneous voltage Uact,1is compared with a second upper voltage threshold value Uo,2, where U1<U0,2<U0,1. If Uact,1<Uo,2, the transfer of energy from the first vehicle electrical system branch to the second vehicle electrical system branch is ended.

In a further embodiment, if Uact,1>U0,1the transfer of energy from the first vehicle electrical system branch to the second vehicle electrical system branch is ended after a predetermined time period.

In a further embodiment, if Uact,1<Uu,1the method also has the following steps after the actuation of the at least one DC/DC converter. A second value of the instantaneous voltage Uact,1of the first vehicle electrical system branch is detected. Furthermore, the detected second value of the instantaneous voltage Uact,1is compared with a second layer voltage threshold value Uu,2, where Uu,1<Uu,2<U1. If Uact,1>Uu,2, the transfer of energy from the second vehicle electrical system branch to the first vehicle electrical system branch is ended.

If Uact,1<Uu,1, in a further embodiment of, the transfer of energy from the second vehicle electrical system the method to the first vehicle electrical system branch is ended after a predetermined time period.

In the abovementioned embodiments, the first nominal voltage U1can be higher or lower than the second nominal voltage U2. Furthermore, the first nominal voltage U1can correspond to the second nominal voltage U2.

The voltage values and voltage threshold values specified in the application are understood here to mean in each case the absolute value of the voltage, that is to say the specified voltages each have a non-negative sign.

FIG. 1Ashows a block circuit diagram of a vehicle electrical system8according to a first embodiment of the application. The vehicle electrical system8can be, for example, a component of a motor vehicle (not illustrated in more detail), in particular of a passenger car or of a truck.

The vehicle electrical system8has a first vehicle electrical system branch1with a first nominal voltage U1, which can also be referred to as Vsys1, and a second vehicle electrical system branch2with a second nominal voltage U2, which can also be referred to as Vsys2.

In the embodiment shown, a generator10, at least an electrical load12and an electrical energy storage device13, for example in the form of a 12 volt accumulator, are arranged in the first vehicle electrical system branch1. The generator10is connected via a mechanical coupling11, for example a V-ribbed belt, to an engine9, wherein the engine9is embodied as an internal combustion engine.

In the embodiment shown, an electrical energy storage device14, for example in the form of a 12 volt accumulator, and at least one electrical load15, are arranged in the second vehicle electrical system branch2.

A DC/DC converter3is arranged between the first vehicle electrical system branch1and the second vehicle electrical system branch2. The DC/DC converter3is embodied as a bidirectional direct voltage converter which can, in particular, convert the first nominal voltage U1into the second nominal voltage U2, and vice versa. For this purpose, the DC/DC converter3is embodied as a synchronous converter in the embodiment shown. Furthermore, it is possible to provide a first DC/DC converter, which is embodied as a step-up converter or boost converter, and a second DC/DC converter, which is embodied as a step-down converter or block converter.

The vehicle electrical system8also has a first actuating unit4which is designed to actuate the at least one DC/DC converter3. The first actuating unit4is coupled or connected here to the positive terminal of the first vehicle electrical system branch1. In the embodiment shown, the first actuating unit4has a first detection unit5which is designed to detect an instantaneous voltage Uact,1of the first vehicle electrical system branch1. Furthermore, the first actuating unit4has a first comparison unit6which is designed to compare the detected instantaneous voltage Uact,1with a first upper voltage threshold value Uo,1and a first lower voltage threshold value Uu,1, where Uu,1<U1<Uo,1. The first comparison unit6has, for example, at least one comparator for this purpose.

The first actuating unit4is designed to actuate the DC/DC converter3in such a way that energy is transferred from the first vehicle electrical system branch1to the second vehicle electrical system branch2if Uact,1>U0,1. The first actuating unit4is further designed in such a way that energy is transferred from the second vehicle electrical system branch2to the first vehicle electrical system branch1if Uact,1<Uu,1.

Further, the vehicle electrical system8has a voltage-limiting switch7which is embodied as a MOSFET in the embodiment shown, and a second actuating unit16for actuating the voltage-limiting switch7. The voltage-limiting switch7is connected to the positive paths of the first vehicle electrical system branch1and of the second vehicle electrical system branch and is connected electrically in parallel with the DC/DC converter3.

Furthermore, the second actuating unit16is coupled to the positive path of the first vehicle electrical system branch1.

In addition, the vehicle electrical system8has a power switch17and a control unit18for actuating the power switch17. The power switch17is embodied here as a MOSFET in the embodiment shown, wherein the inherent body diode of the MOSFET inFIG. 1Ais not illustrated in more detail. The power switch17is connected to the voltage-limiting switch7and to the positive path of the electrical energy storage device14of the second vehicle electrical system branch2.

During a nominal operating state of the vehicle electrical system8, i.e. operation within the abovementioned voltage threshold values, the voltage-limiting switch7is closed and the power switch17is opened. Further details of the voltage-limiting switch7and of the power switch17are explained in more detail below.

Fluctuations in the vehicle electrical system during the occurrence of changes in load can be compensated by means of the embodiment shown. The changes in load relate here to an electrical system having at least one electronic switch in the form of the voltage-limiting switch7and at least one voltage converter module in the form of the DC/DC converter3. The system is in a nominal system state here.

Requirements such as, for example, guaranteed switch-on resistance, continuity resistance, which is also referred to as Rds,on, assistance of the linear operation, which is also referred to as the linear mode, are typically made of the voltage-limiting switch7. The DC/DC converter3is typically subject to requirements such as, for example, the exchange of energy between the two energy systems in the form of the first electrical vehicle system branch1and the second electrical vehicle system branch2.

In this context, the DC/DC converter3controls the current between the two energy systems. The DC/DC converter3is, in the embodiment shown, in a discharge mode, which is also referred to as MD1, or in a charge mode, which is also referred to as MD2. The specified operating modes are typically provided for long-term charging or discharging of the second energy system in the form of the second vehicle electrical system branch2or of the first energy system in the form of the first vehicle electrical system branch1. Furthermore, the DC/DC converter3can be in a quiescent state, which is also referred to as the standby mode or MD0.

The vehicle electrical system8can be embodied in a most cost-effective and voltage-stable fashion through the use of the recharging unit in the form of the DC/DC converter3or of the at least one switch in the form of the voltage-limiting switch7, and a braking torque which is produced by a change in load and which acts on the internal combustion engine, i.e. the engine9, can be minimized. In particular, the functionality of the DC/DC converter3is thereby extended. This permits a predefined voltage state of the vehicle electrical system8to be obtained in the case of changes in load in the first vehicle electrical system branch1. Changes in load can bring about undervoltages or overvoltages in the vehicle electrical system8in this context, wherein these can be reduced by actuating the DC/DC converter3or the voltage-limiting switch7, as is explained further below.

In this respect, a vehicle electrical system state with overvoltage will firstly be considered. In this state, the generator10cannot independently compensate a vehicle electrical system overvoltage due to a lack of loads. In a vehicle electrical system state with overvoltage in the first vehicle electrical system branch1, wherein there is a random magnitude of the vehicle electrical system overvoltage, the DC/DC converter3is actuated for voltage stabilization. The DC/DC converter3can be used here as a second stage or as a single stage. As a result, what is referred to as load dumping, i.e. the occurrence of voltage peaks, can be eliminated or suppressed, and the amplitude and mean value thereof can be reduced.

In this respect, for example the vehicle electrical system voltage in the first vehicle electrical system branch1is measured by means of a comparator circuit in the first actuating unit4and compared with the first upper voltage threshold value Uo,1. When the voltage threshold value, i.e. the first upper voltage threshold value Uo,1, is exceeded, the DC/DC converter3is operated as a load sink.

The DC/DC converter3is for this purpose activated by means of the first actuating unit4, typically in the microsecond to millisecond range in a “sink/source” mode, which is also referred to as MD3, and in the process transports excessive energy or portions of the energy from the first vehicle electrical system branch1to the second vehicle electrical system branch2. In this way, a reduction in the overvoltage within the first vehicle electrical system branch1and a more controlled voltage increase in the second vehicle electrical system branch2is brought about. In addition, a typically small part of the overvoltage energy from the first vehicle electrical system branch1is converted or dissipated in the form of heat in the DC/DC converter3.

When a hysteresis value in the form of a second upper voltage threshold value Uo,2is undershot, where U1<Uo,2<U0,1, the DC/DC converter3is changed back into a quiescent mode. The operation of the DC/DC converter3in the sink mode is typically chronologically limited and is therefore configured, in particular, for transient fluctuations of the vehicle electrical system. If this chronological limit is exceeded, the first actuating unit4switches the DC/DC converter3into the quiescent mode until a renewed requirement, for example by means of a microcontroller, changes the DC/DC converter3back into the sink or source mode.

An undervoltage vehicle electrical system state is typically operation in which a vehicle electrical system undervoltage can not be dynamically compensated independently by a combination of the generator10and electrical energy storage device13, for example due to a lack of engine torque, generator control behaviour, power capacity or excessively high intrinsic or vehicle electrical system impedances Z. Undervoltages are typically caused by sudden changes in load, in particular in conjunction with an unfavorable engine torque.

An undervoltage state can in turn be compensated or reduced by means of the DC/DC converter3. In this context, the magnitude of the vehicle electrical system undervoltage is typically random. A transient voltage dip can be eliminated or suppressed, or the amplitude and mean value thereof can be reduced.

For this purpose, the vehicle electrical system voltage in the first vehicle electrical system branch1is measured by means of, for example, a comparator circuit in the first actuating unit4and is compared with a first lower voltage threshold value Uu,1. If the voltage threshold value in the form of the first lower voltage threshold value Uu,1is undershot, the DC/DC converter3is operated as a source.

The DC/DC converter3is for this purpose typically actuated in the microsecond range to millisecond range in the “sink/source” mode, which, as already explained, is also referred to as the MD3, by means of the first actuating unit4, and in this context transports a corresponding portion of the stored energy from the second vehicle electrical system branch2to the first vehicle electrical system branch1. As a result, a reduction is brought about in the undervoltage within the first vehicle electrical system branch1, and a controlled drop in the voltage is brought about in the second vehicle electrical system branch2.

When a hysteresis value above the first lower voltage threshold value Uu,1is exceeded, the DC/DC converter3in the embodiment shown is changed again into the quiescent state. The operation of the DC/DC converter3in the source mode is typically chronologically limited and therefore configured, in particular, for transient fluctuations in the vehicle electrical system. If this chronological limit is exceeded, the first actuating unit4switches the DC/DC converter3in the embodiment shown into the quiescent state until a renewed request, for example via a microcontroller, changes the DC/DC converter3back into the sink or source mode.

Therefore, in the case of undervoltage states the DC/DC converter transports energy in the form of electrical charge from the second vehicle electrical system branch2to the first vehicle electrical system branch1if a request by means of a corresponding system variable is present.

In addition to an undershooting of the first lower voltage threshold value Uu,1, it is for this purpose additionally possible to use a change in current, measured by the first actuating unit4, in the first vehicle electrical system branch1or the definition of a decreasing engine rotational speed with the simultaneous request for a constant or accelerated engine rotational speed. Undervoltages which occur in the first vehicle electrical system branch1are therefore prevented or reduced. Furthermore, the voltage in the second vehicle electrical system branch2decreases.

In the case of overvoltage states, the DC/DC converter3transports energy in the form of electrical charge from the first vehicle electrical system branch1to the second vehicle electrical system branch2if a request by means of a corresponding system variable is present. In addition to exceeding of the first upper voltage threshold value Uo,1, it is for this purpose additionally possible to use a change in current, measured by the first actuating unit4, in the first vehicle electrical system branch1or the detection of an increasing engine rotational speed with a simultaneous request for a constant or reduced engine rotational speed. Overvoltages which occur in the first vehicle electrical system branch1are therefore prevented or reduced. Furthermore, the voltage in the second vehicle electrical system branch2increases.

In the mode which is also referred to as MD3, in this context the decision is made as to whether the DC/DC converter3is operated in the energy sink or “sink” operating mode or in the energy source or “source” operating mode, by means of the corresponding comparators of the comparison unit6. In both specified operating modes or states energy is transported by the DC/DC converter3.

A cost advantage can be achieved by means of the embodiment shown by virtue of the possibility of being able to use, with system adjustment in accordance with the generator10, electrical energy storage device13, electrical load12and the power impedances, the available energy from the second vehicle electrical system branch2. As a result, in particular the generator10and the electrical energy storage device13can be implemented in relatively small power classes.

The application can be applied here in potential-isolated and non-potential-isolated systems as well as in switching topologies in the ground path as well as in the positive path.

The first upper voltage threshold value U0,1and the first lower voltage threshold value Uu,1as well as the further voltage threshold values can, in a further embodiment, be adapted dynamically, for example to a changed temperature or a changed setpoint value of the voltage which is generated by the generator10.

FIG. 1Bshows a block circuit diagram of a vehicle electrical system8according to a second embodiment of the application. Components with the same functions as inFIG. 1Aare characterized by the same reference symbols and not explained once more below.

In the second embodiment which is shown, the vehicle electrical system8also has a power switch17and a control unit18for actuating the power switch17. The power switch17is embodied here in the embodiment shown as a MOSFET, wherein the inherent body diode of the MOSFET is not illustrated in more detail inFIG. 1B. The power switch17is connected to the ground path of the electrical energy storage device13of the first vehicle electrical system branch1and to the positive path of the electrical energy storage device14of the second vehicle electrical system branch2, which is embodied in the second embodiment as, for example, a 5 volt accumulator, and said power switch17thereby permits, given corresponding actuation by the control unit18, a series connection of the two energy storage devices and therefore allows the voltage in the first vehicle electrical system branch1to be raised. This is advantageous, in particular, if the electrical load12is embodied as a high current load. The control unit18is coupled here to the positive path of the first vehicle electrical system branch1.

Furthermore, in the second embodiment shown the voltage-limiting switch7is arranged in the ground path of the electrical energy storage device13. The second actuating unit16of the voltage-limiting switch7is coupled to the positive path of the first vehicle electrical system branch1.

During a nominal operating state of the vehicle electrical system8, i.e. operation within the above-mentioned voltage threshold value, the voltage-limiting switch7is closed and the power switch17is opened.

In an overvoltage application, in the case of low vehicle electrical system overvoltages, also referred to as Vov1, the voltage-limiting switch7can reduce, in a linear operation in a first stage, an occurring overvoltage component by the voltage Vsw1with 0 V to Vd, wherein Vdis the diode flow voltage of the inherent body diode of the voltage-limiting switch7which is embodied as a MOSFET in the embodiment which is shown. As a result, closed-loop or open-loop control can be performed of an overvoltage target value of the vehicle electrical system voltage. When the voltage threshold value is exceeded, closed-loop control is performed of the voltage-limiting switch7in the linear operating mode by means of the second actuating unit16with a first closed-loop control target value. When a hysteresis value underneath the first closed-loop control target value is undershot, the voltage-limiting switch7is changed back into the on mode.

The operation of the voltage-limiting switch7in the linear mode is not chronologically limited here.

Therefore, in the case of overvoltage states the voltage-limiting switch7also functions alongside the DC/DC converter3as a controllable power sink. In this context, the voltage-limiting switch7operates in the linear operating mode by means of the closed-loop control unit or the second actuating unit16. As a result of the body diode which is inherently contained in the voltage-limiting switch7in the form of a MOSFET it is possible to eliminate very quickly overvoltages of up to typically 0.7 V. In the linear operating mode of the voltage-limiting switch7, the effective voltage at said switch is negative and the system overvoltage which occurs can therefore be reduced. The effective linear voltage range of the voltage-limiting switch7is typically between 0 V and 0.7 V. The limiting effect is here the flux voltage of the body diode, which is typically approximately 0.7 V. It is relevant for the position of the voltage-limiting switch7in the system here that the voltage-limiting switch7can isolate the system load at least from the electrical energy storage device13and at least in one direction of the current.

The voltage-limiting switch7according to the first embodiment shown inFIG. 1Acan also function as a power sink in overvoltage states, wherein for this purpose the power switch17is closed in this embodiment.

FIG. 2shows a basic circuit diagram of a vehicle electrical system8according to the application. Components with the same functions as in the preceding figures are characterized by the same reference symbols and will not be explained once more below.

As is illustrated schematically inFIG. 2, energy can be exchanged between the first vehicle electrical system branch1and the second vehicle electrical system branch2by means of a control unit18. The control unit18has for this purpose at least one DC/DC converter which is not presented in more detail as well as an actuating unit for actuating the DC/DC converter. The transfer of energy between the first vehicle electrical system branch1, the control unit18and the second vehicle electrical system branch2is illustrated schematically here by means of arrows A and B.

FIG. 3shows the voltage/time diagrams in a first vehicle electrical system branch of a vehicle electrical system. In this context, the time profile of the instantaneous voltage of the first vehicle electrical system branch is plotted.

The profile of the voltage which occurs without the actuation of the DC/DC converter explained above is illustrated schematically with a continuous line in the upper voltage/time diagram inFIG. 3. Between the times t1and t2as well as t5and t6there is an overvoltage present in the first vehicle electrical system branch, whereas between the times t3and t4there is an undervoltage present in the first vehicle electrical system branch. The time intervals between t1and t2as well as t3and t4constitute, for example, voltage fluctuations in the millisecond range and the time interval between t5and t6constitutes, for example, a voltage fluctuation in the microsecond range. The overvoltages here exceed a first upper voltage threshold value Uo,1, and the undervoltages undershoot a first lower voltage threshold value Uu,1.

As is illustrated schematically by a dashed line in the lower voltage/time diagram inFIG. 3, it is possible, by actuating the DC/DC converter in such a way that energy is transferred from the first vehicle electrical system branch to the second vehicle electrical system branch when the first upper voltage threshold value Uo,1is exceeded, for the amplitude of the overvoltage to be limited to this voltage threshold value. Furthermore, by actuating the DC/DC converter in such a way that in the case of the first lower voltage threshold value Uu,1being undershot, energy is transferred from the second vehicle electrical system branch to the first vehicle electrical system branch, it is possible for the amplitude of the undervoltage to be limited to this threshold value. Overall, there is therefore stabilization of the voltage profile in the first vehicle electrical system branch.

In this context, system lag times, in particular of the DC/DC converter are to be taken into account, which lag times are do not occur between a changeover of the operating modes “sink” and “source” and an associated reversal of direction of the transportation of energy between the two vehicle electrical system branches within which overvoltages and undervoltages which occur are typically not completely compensated. Such system lag times can, in particular, be configurable.

LIST OF REFERENCE SYMBOLS