Vehicle electrical system with optimized through-switching function

Abstract

Motor vehicle electrical system with a voltage converter (9), a traction battery (8) and a traction network (7), characterized in that: The voltage converter (9) has a first operating state (30) in which the voltage converter (9) operates as a buck converter and reduces an electrical voltage (Ubatt) of the traction battery (8) to a voltage limit (Umax) of the traction network (7), and a second operating state (31) in which the voltage converter (9) operates as a through-switch and passes the electrical voltage (Ubatt) of the traction battery (8) through to the traction network (7) without reduction; wherein the voltage limit (Umax) is a maximum operating voltage of components on the traction network (7).

Description

[0001] The invention relates to a motor vehicle electrical system with a voltage converter, a traction battery and a traction network, as well as a method which is used in a motor vehicle electrical system with such a voltage converter.

[0002] A traction battery for a motor vehicle, particularly for an electric vehicle, a hybrid vehicle, or a fuel cell vehicle, contains a large number of battery cells, typically arranged in modules to meet the vehicle's power and range requirements. Individual battery cells or battery modules are electrically connected in series, so that the electrical voltages of the battery cells or battery modules add up.

[0003] The range of electric vehicles can be increased by carrying more stored electrical energy in the vehicle. The energy content of a traction battery depends on its electrical capacity and electrical voltage. A range increase can be achieved either by increasing the electrical capacity of the traction battery or by increasing the electrical voltage of the traction battery. However, the limitations of power electronics place limits on the series connection of battery cells or battery modules. For example, the current voltage limit for the vehicle's high-voltage infrastructure for traction drives is approximately 400–450 V. The electrical voltage cannot therefore be increased arbitrarily, as all components in the traction network have a maximum permissible voltage limit.

[0004] Typically, all components supplied by the traction network are designed so that the maximum voltage at the traction battery is below the component's upper voltage limit. This allows the traction battery outputs to be connected directly to the traction network. However, this has the disadvantage that, during normal operation, the voltage at the traction network in the middle range of the state of charge is well below the maximum permissible operating voltage of the components on the traction network.

[0005] It is also known to increase the range of a motor vehicle by connecting several battery module strings in parallel within a traction battery, thus increasing the electrical capacity of the traction battery. While this increases the range of the vehicle, the disadvantage of the significantly fluctuating voltage at the traction battery remains. Even with this solution, the voltage at the traction network during normal operation in the mid-state of charge range is well below the maximum permissible operating voltage of the components in the traction network.

[0006] European patent EP 1 889 349 B1 discloses a device for supplying power to a vehicle, in which a voltage converter boosts the voltage of a secondary battery. A connection unit enables switching between connecting and disconnecting the voltage boosted by the voltage converter.

[0007] German patent application DE 10 2012 200 577 A1 describes a method for controlling a battery. The battery comprises several battery module strings. The battery module strings have several battery modules, each battery module having a coupling unit. The coupling unit acts as a voltage converter and couples the battery modules to one another. The method enables a load to be supplied with a predetermined and essentially constant total power. Battery modules of the battery module strings can each be connected in series. This allows the output voltages of the battery modules in a series connection to be summed to produce a higher string voltage.

[0008] The German patent application DE 10 2007 024 567 A1 describes a high-voltage on-board power system architecture with integrated power electronics for a fuel cell vehicle with an electric motor, combined with a fuel cell unit as a complementary drive means of the vehicle, with an electrical energy storage device and a DC / DC converter as a boost converter and / or buck converter within the on-board power system architecture, wherein a DC / DC converter is provided on the electrical energy storage device and at least one additional DC / DC converter is provided on the fuel cell unit.

[0009] German patent application DE 10 2009 049 761 A1 describes a method for cold-starting a fuel cell system of a motor vehicle, which is intended to increase the operational readiness of the motor vehicle. According to the cited document, this is achieved by absorbing a specific amount of output power from the fuel cell system during the warm-up phase and / or the warm-up phase by a power absorption device, and converting the absorbed amount of output power into power loss by applying a suitable current to a winding of an electric motor that is to be energized.

[0010] The object of the present invention is to improve a motor vehicle electrical system in such a way that the above-mentioned disadvantages are at least partially overcome.

[0011] This object is achieved by the motor vehicle electrical system according to the invention as claimed in claim 1.

[0012] Further advantageous embodiments of the invention emerge from the subclaims and the following description of preferred embodiments of the present invention.

[0013] A motor vehicle electrical system according to the invention comprises a voltage converter, a traction battery, and a traction network. The motor vehicle electrical system can be, for example, a motor vehicle electrical system for an electric vehicle, a hybrid vehicle, or a fuel cell vehicle.

[0014] The voltage converter has a first operating state in which it operates as a step-down converter and steps down an electrical voltage of the traction battery to a voltage of the traction network, and a second operating state in which it operates as a switch and switches the electrical voltage of the traction battery through to the traction network without stepping down.

[0015] The voltage of a traction battery is not constant throughout its state of charge, but decreases as the state of charge decreases. Although the voltage of a fully charged traction battery may be too high for the components of the traction network, this does not necessarily mean that it will also be too high at a lower state of charge. The voltage converter can therefore be configured to reduce the voltage on the traction network only when the battery is fully charged, and to transfer the voltage of the traction battery to the traction network once the state of charge reaches a certain level.

[0016] For example, the voltage converter can be designed so that in the first operating state it reduces the voltage of the traction battery to an upper voltage limit of the traction network.

[0017] The operating state for switching can be optimized for the switching function so that losses during switching are reduced.

[0018] In plug-in hybrid vehicles, i.e., hybrid vehicles where the traction battery can be charged externally, hybrid operation generally only begins when the traction battery is discharged to a low state of charge. In hybrid operation, the voltage converter in this example application in a plug-in hybrid vehicle does not have to operate as a buck converter in the first operating mode, but only needs to switch through. In this example, the voltage converter can therefore essentially operate as a switch in the second operating mode during hybrid operation.

[0019] According to one embodiment, the voltage converter has a switch that enables switching between the first operating state, in which the voltage converter operates as a buck converter, and the second operating state, in which the voltage converter operates as a switch.

[0020] The switch which enables switching between the first operating state and the second operating state of the voltage converter can be designed, for example, as a relay, in particular as a contactor.

[0021] The switch, which enables switching between the first operating state and the second operating state of the voltage converter, can also be designed as a semiconductor switch, for example as a MOSFET or IGBT.

[0022] In a further embodiment, the switch which enables switching between the first operating state and the second operating state of the voltage converter can be functionally implemented in transistors of the voltage converter, which also perform the function of voltage conversion.

[0023] The voltage converter described above can thus enable optimized switching of the input voltage.

[0024] An exemplary motor vehicle electrical system comprises a traction battery, a traction network, and a voltage converter coupled between the traction battery and the traction network, as described above. The voltage converter can, for example, be connected directly to the terminals of a traction battery on the input side. On the output side, the voltage converter can directly feed a traction network and its loads.

[0025] The traction battery can advantageously have an electrical voltage at high state of charge values ​​that is higher than the limit voltage of the traction network. Thus, a traction battery can be used in the vehicle's electrical system that has a fully charged voltage that exceeds the upper voltage limit of the traction network. This makes it possible to design the maximum voltage at the traction battery so that it is above the maximum operating voltage of the components in the traction network.

[0026] The traction network may include power electronics (e.g. a converter) that converts electrical energy from the traction network to operate an electrical machine.

[0027] A low-voltage electrical system can also be coupled to the traction network via another voltage converter. This converter is connected between the traction network and the low-voltage electrical system, reduces the electrical voltage of the traction network to the voltage level of the low-voltage network, and supplies electrical energy from the traction network to the low-voltage electrical system. The low-voltage electrical system can be a conventional 12V electrical system, for example.

[0028] The traction network may also include other consumers that are coupled to the traction network, for example an air conditioning compressor or a heater.

[0029] In the case of an electric vehicle or a plug-in hybrid, the traction battery can also be charged with electrical energy using an external charger. In this case, the voltage converter according to the invention can be used to boost the charger voltage to the voltage of the traction battery.

[0030] A method for operating a motor vehicle electrical system with a traction battery, a traction network and a voltage converter as described above, which is coupled between the traction battery and the traction network, comprises: determining the electrical voltage of the traction battery, controlling the voltage converter such that it is placed in a first operating state in which the voltage converter operates as a step-down converter and steps down an electrical voltage of the traction battery to a voltage limit of the traction network if the electrical voltage of the traction battery is above the voltage limit of the traction network, and controlling the voltage converter such that it is placed in a second operating state in which the voltage converter operates as a switch and switches the electrical voltage of the traction battery through to the traction network without stepping down,if the electrical voltage of the traction battery is less than or equal to the upper voltage limit of the traction network. The method can be executed by a controller. The voltage converter can receive corresponding control signals, for example, from a battery management system or another control center such as an engine control unit.

[0031] Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 schematically shows an embodiment of a motor vehicle with an electrical system; Fig. 2 schematically shows a voltage converter according to an embodiment of the invention; Fig. 3 shows schematically the voltage curve of a traction battery depending on its state of charge; and Fig. 4 schematically shows an exemplary driving mode selection of a plug-in hybrid vehicle depending on the state of charge of a traction battery.

[0032] An embodiment of a motor vehicle in which the voltage converter according to the invention can be used is shown schematically in Fig. 1. The motor vehicle 1 is constructed here, by way of example, according to the principle of a parallel hybrid. In a parallel hybrid, both an internal combustion engine 2 and an electric motor 3 are mechanically coupled to a drive axle 5 via a transmission 4. The mechanical energy output by the internal combustion engine 2 is converted into electrical energy by the electric motor 3 and fed into a traction battery 8 with the aid of power electronics, in particular a converter 6, via a first voltage converter 9 according to the invention. The traction battery 8 can supply electrical energy to the electric machine 3 via the traction network 7. Conversely, mechanical energy output by the internal combustion engine 2 (or mechanical energy recuperatively obtained from the drive axle 4) can be fed to the traction battery 8.A low-voltage electrical system 11, which is embodied here as a 12V electrical system, is also coupled to the traction network 7 via a second voltage converter 10. Further high-voltage consumers 12, such as an air conditioning compressor, are also coupled to the traction network 7. The traction network 7, the power electronics 6, and the high-power consumers 12 are powered by the traction battery 8 and are at a voltage level specified by the first voltage converter 9. The first voltage converter 9 of this exemplary motor vehicle 1, which is coupled between the traction battery 8 and the traction network 7, can preferably be implemented using the voltage converter according to the invention.

[0033] The exemplary motor vehicle 1 can be operated in various operating states. In a first operating state, the motor vehicle 1 is driven using combustion power from the internal combustion engine. Part of the energy generated by the internal combustion engine is used to generate electrical energy. This electrical energy generated by the generator is used to supply low-voltage consumers of the low-voltage network 11 and high-voltage consumers 12. In a second operating state, the motor vehicle 1 is driven using combustion power from the internal combustion engine, and electrical energy generated by the generator is supplied to the traction battery 8 to charge it. In a third operating state, the motor vehicle 1 is driven purely electrically using energy from the traction battery 8.In a fourth operating mode, the motor vehicle 1 is driven "combustion-based" using power from the combustion engine, and electrical energy from the traction battery 8 is used to support combustion-based driving (boosting). In a fifth operating mode, electrical energy is recovered and fed to the traction battery 8.

[0034] Fig. Figure 2 schematically shows an exemplary embodiment for the first voltage converter 9 of the motor vehicle of Fig. 1, which is located between the traction battery (8 in Fig. 1) and traction network (7 in Fig. 1) and represents an embodiment of the present invention. The voltage converter 9 receives a source voltage Ubatt via terminals 21. The terminals 21 are connected, for example, to the two poles of a traction battery (8 in Fig. 1). The voltage converter 9 has a first switching transistor T1 and an anti-parallel connected first diode D1, which together form a first switching element 22. The voltage converter 9 further has a second switching transistor T2 and an anti-parallel connected second diode D2, which together form a second switching element 23. The two switching elements 22, 23 are controlled by means of control signals 24 from a controller 20. The two transistors T1 and T2 can each be designed, for example, as a field-effect transistor (FET), in particular as a MOSFET, or as an insulated-gate bipolar transistor (IGBT). As an alternative to the transistors T1, T2, turn-off thyristors could also be used. The transistors T1, T2 are controlled by the controller 20 with a control signal 24 in a manner known to those skilled in the art, whereby they alternate between the conducting and insulating states at very short intervals.The control signals 24 can, for example, implement a control method for voltage converters known to those skilled in the art using pulse width modulation (PWM) or pulse frequency modulation (PFM). Depending on the switching state of the switching units 22 and 23, a coil L1 is supplied with electrical energy either from the source voltage Ubatt (e.g., from the traction battery 8 in . Fig. 1) or electrical energy is taken from the coil L1 and applied to a load (e.g. the traction network 7 from Fig. 1) is provided at an output 25 of voltage converter 9 with an output voltage U1. Voltage converter 9 can thus operate as a buck converter.

[0035] In a first switching state, transistor T1 is closed and transistor T2 is open. In this first switching state, current flows from the traction battery (8 in Fig. 1) via coil L1 into the traction network (7 in Fig. 1). In a second switching state, both transistors T1 and T2 are open. In this second switching state, current flows through the coil L1 and diode D2 into the traction network (7 in Fig. 1), whereby high losses occur at diode D2. In a third switching state, transistor T1 is open and transistor T2 is closed. In this third switching state, current flows through coil L1 and transistor T2 without losses into the traction network (7 in Fig. 1). In a fourth switching state, both transistors T1 and T2 are open. This fourth switching state corresponds to the second switching state.

[0036] By means of the control signals 24, a duty cycle can be controlled which, in a buck converter, determines the proportionality factor with which the source voltage Ubatt is converted into the output voltage U1.

[0037] For additional smoothing of the output voltage U1, a smoothing capacitor C1 is connected in parallel to the output 25 of the voltage converter 9 in the embodiment.

[0038] The voltage converter 9 of this exemplary embodiment has a switch S1 that makes it possible to switch between a first operating state and a second operating state. In the first operating state (open switch S1), the voltage converter 9 operates as a step-down converter and reduces the source voltage Ubatt to an output voltage U1. In the second operating state (closed switch S1), the voltage converter 9 operates as a through-switch and switches the source voltage Ubatt through to the output 25 without stepping down, so that the output voltage U1 is equal to the source voltage Ubatt. In this exemplary embodiment, the switch S1 is designed as a contactor that can be switched by means of an electrical control signal. Alternatively, the switch S1 could also be implemented as a field-effect transistor, in particular as a MOSFET. A MOSFET has the advantage that the switch S1 is highly efficient.

[0039] In yet another alternative embodiment, the switch S1 can also be functionally controlled by the transistors (T1, T2 in Fig. 1) of a voltage converter, which takes over the function of stepping down the source voltage. In this embodiment, transistor T1 is permanently closed and transistor T2 is opened. In this case, the step-down ratio with which the source voltage Ubatt is converted into the output voltage U1 is determined by the losses at coil L1.

[0040] The switch S1 is controlled by means of electrical signals from a control unit (not shown). This control unit can, for example, also specify the specifications for the voltage conversion for the control 20, for example the proportionality factor of the voltage conversion. The voltage converter can receive corresponding control signals, for example, from a battery management system or another control center such as an engine control unit. The corresponding control signals can be provided to the voltage converter via an electronic communication path such as a vehicle bus system in a manner known to those skilled in the art. The control center can control the voltage converter depending on other state variables, in particular depending on the state of charge of the traction battery, which is known to the battery management system.

[0041] The voltage converter 9 of this embodiment can be operated bidirectionally. In one direction, the voltage converter 9 reduces a high voltage of the traction battery 8 to a lower voltage of the traction network 7 and feeds electrical energy from the traction battery 8 into the traction network 7. In a reverse direction, the voltage converter 9 increases a low voltage of the traction network 7 to a higher voltage of the traction battery 8 and feeds electrical energy from the traction network to the traction battery 8.

[0042] Fig. Figure 3 shows a schematic diagram of the voltage curve of a traction battery depending on its state of charge. The state of charge value (also called “state of charge”, or SoC for short) is a parameter for the state of charge of batteries. The state of charge value indicates the amount of charge still available in a battery in relation to the nominal value, i.e. the amount of charge available when fully charged. The state of charge value is given as a percentage of the nominal value. A state of charge value of 50%, for example, means that the battery still has a residual charge of 50% relative to a full charge of 100%. A battery module with a nominal capacity of 28 Ah, for example, only has a removable charge of 14 Ah at a state of charge value of 50%.The state of charge of a battery module can be determined in a manner known to those skilled in the art, for example, by voltage measurement, current integration, impedance measurement, chemical electrolyte measurements for batteries with liquid electrolyte, or, for NiMH batteries, by the dependence of the state of charge on the internal pressure. The voltage measurement method uses predefined discharge curves to determine the state of charge. With the current integration method, the state of charge is determined via the battery current during an integration time. Furthermore, a combination of two or more of the aforementioned methods is also possible.

[0043] At maximum state of charge SoC (equal to or close to 100%), the voltage of the traction battery is, for example, at a final charging voltage of Ubattmax = 550 V. This final charging voltage Ubattmax is greater than the upper voltage limit Umax of a traction network, which is assumed here to be Umax = 400 V. In this range of high state of charge values, the voltage converter operates in an operating mode in which it steps down the voltage Ubatt to the maximum permitted limit voltage Umax. As the state of charge SoC decreases, the voltage Ubatt of the traction battery drops. At a state of charge of around 70%, the voltage Ubatt of the traction battery reaches the upper voltage limit Umax of the traction network of 400 V. From this point on, stepping down the voltage Ubatt of the traction battery is no longer necessary. The voltage converter is switched to its second operating mode in which it switches the traction voltage Ubatt through to the traction network without stepping down.As the traction battery becomes increasingly discharged, the voltage Ubatt of the traction battery continues to drop until, when the traction battery is completely discharged, it reaches a final discharge voltage of Ubattmin, which in this case is 300 V, for example. A voltage converter according to the invention therefore operates in two operating states. When the traction battery voltage is greater than the upper voltage limit of the traction network, the voltage converter operates in a first operating state in which it reduces the voltage of the traction battery to the upper voltage limit of the traction network. This operating state is shown in the diagram of the . Fig. 3 by a double arrow 30. When the voltage of the traction battery is lower than (or equal to) the upper voltage limit of the traction network, the voltage converter operates in a second operating state in which it connects the voltage Ubatt of the traction battery to the traction network without stepping down. This operating state is shown in the diagram of the Fig. 3 is marked by double arrow 31. In Fig. 3 also shows a nominal voltage Ubattnom of the traction battery, which here is, for example, Ubattnom = 384 V.

[0044] Fig. Figure 4 schematically shows an example of a driving mode selection for a plug-in hybrid vehicle depending on the state of charge (SoC) of a traction battery. The traction battery is charged by external charging up to a maximum state of charge of 95%. Purely electric driving mode is used down to a state of charge of 25%. This pure electric driving mode is Fig. 4 is indicated by the double arrow 32. Between a charge level of 25% and a minimum charge level of 10%, hybrid driving mode occurs. This hybrid driving mode is Fig. 4 is marked by a double arrow 33. A comparison of the driving conditions of the Fig. 4 with the switching ranges of the voltage converter Fig. 3 shows that a voltage converter according to the invention can be maintained in its second operating state (31) in hybrid driving mode, in which it operates as a switch and transfers the voltage of the traction battery to the traction network without stepping down. List of reference symbols 1 motor vehicle 2 combustion engine 3 electric machine 4 gearboxes 5 drive axle 6 inverters (power electronics) 7 Traction network 8 Traction battery 9 first voltage converter 10 second voltage converter 11 Low-voltage electrical system 12 high-voltage consumers (e.g. air conditioning compressor) 20 Control 21 connections for source voltage Ubatt 22 Switching unit 23 Switching unit 24 control signals 25 terminals for output voltage U1 T1 transistor T2 transistor D1 diode D2 diode C1 capacitor L1 coil S1 switch (e.g. contactor) U1 source voltage (e.g. from traction battery) U2 output voltage (e.g. for traction network) 30 first operating state (buck converter) 31 second operating state (through switch) 32 electrical operation 33 hybrid operation

Claims

[1] Motor vehicle electrical system with a voltage converter (9), a traction battery (8) and a traction network (7), characterized by , that: the voltage converter (9) has a first operating state (30) in which the voltage converter (9) operates as a step-down converter and steps down an electrical voltage (Ubatt) of the traction battery (8) to an upper voltage limit (Umax) of the traction network (7), and a second operating state (31) in which the voltage converter (9) operates as a switch and switches the electrical voltage (Ubatt) of the traction battery (8) through to the traction network (7) without stepping down; wherein the upper voltage limit (Umax) is a maximum operating voltage of components on the traction network (7). [2] Motor vehicle electrical system according to the preceding claim, wherein the voltage converter (9) further comprises a switch (S1) which enables switching between the first operating state (30), in which the voltage converter (9) operates as a step-down converter, and the second operating state (31), in which the voltage converter (9) operates as a switch. [3] Motor vehicle electrical system according to claim 2, wherein the switch (S1) of the voltage converter (9) is designed as a contactor, or as a field effect transistor, in particular as a semiconductor switch, in particular as a MOSFET or IGBT. [4] Motor vehicle electrical system according to claim 2, wherein the switch (S1) is functionally implemented in transistors (T1, T2) of the voltage converter (9), which also perform the function of voltage conversion. [5] Motor vehicle electrical system according to one of the preceding claims, wherein the voltage converter (9) is coupled between the traction battery (8) and the traction network (7). [6] Motor vehicle electrical system according to claim 5, in which the traction battery (8) has an electrical voltage (Ubatt) at high values ​​of the state of charge (SoC) which is higher than the upper voltage limit (Umax) of the traction network (7). [7] Motor vehicle electrical system according to claim 5 or 6, in which the traction network (7) comprises power electronics (6) which converts electrical energy from the traction network (7) for the operation of an electrical machine (3). [8] Motor vehicle electrical system according to one of claims 5-7, further comprising a low-voltage electrical system (11) and a further voltage converter (10) which couples the low-voltage electrical system (11) to the traction network (7), reduces the electrical voltage (U1) of the traction network (7) to the voltage level of the low-voltage network and supplies electrical energy from the traction network (7) to the low-voltage electrical system (11). [9] Method for operating a motor vehicle electrical system with a traction battery (8), a traction network (7) and a voltage converter (9) according to one of claims 1-5, which is coupled between the traction battery (8) and the traction network (7), comprising Determining the electrical voltage of the traction battery (8), Controlling the voltage converter (9) in such a way that it is placed in a first operating state (30) in which the voltage converter (9) operates as a step-down converter and reduces an electrical voltage (Ubatt) of the traction battery (8) to an upper voltage limit (Umax) of the traction network (7) if the electrical voltage (Ubatt) of the traction battery is above the upper voltage limit (Umax) of the traction network (7), and Controlling the voltage converter (9) in such a way that it is placed in a second operating state (31) in which the voltage converter (9) operates as a switch and switches the electrical voltage (Ubatt) of the traction battery (8) through to the traction network (7) without stepping down if the electrical voltage (Ubatt) of the traction battery (8) is less than or equal to the upper voltage limit (Umax) of the traction network (7).

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