Method for controlling a power supply system for a mild hybrid electric vehicle

Abstract

A method for controlling a power supply system for a mild hybrid electric vehicle, wherein the power supply system comprises a low-voltage power grid supplied by a low-voltage battery (30) and a high-voltage power grid supplied by a high-voltage battery (40), wherein a transfer of electrical energy between the low-voltage and the high-voltage power grid is enabled by a low-voltage DC-DC converter (LDC, 50), and wherein a high-voltage relay (43) interrupts a supply of electrical energy from the high-voltage battery (40) to the high-voltage power grid, characterized in that the method comprises the following method steps: Carrying out an ON sequence of the power supply system in that, when an ignition key is turned on, an electrical voltage of the low-voltage battery (30) is increased and thus also an electrical voltage of the high-voltage power network is increased, wherein the high-voltage relay (43) is closed when the electrical voltage of the high-voltage power network exceeds a predetermined electrical target voltage; Driving a mild hybrid electric vehicle in normal mode after completion of the ON sequence of the power system, wherein the mild hybrid electric vehicle is driven substantially dependent on operation by a driver; Implementing a limitation such that, when an electrical voltage drop occurs in the low-voltage power network, entry into a failure mode is limited and then it is determined whether the high-voltage relay (43) is closed or not; and Performing a release such that if the high voltage relay (43) is in the open state, then the ON sequence of the power supply system is performed again, while if the high voltage relay (43) maintains a closed state, then the restriction of entry into the failure mode is released and thus driving in the normal mode is continued.

Description

Field of the invention

[0001] The present invention relates to a method for controlling a power supply system for a mild hybrid electric vehicle, in particular wherein a temporary electrical voltage drop of a low-voltage power network can be prevented from being associated with an entry into a failure mode. State of the art

[0002] A mild hybrid electric vehicle is a hybrid vehicle in which an electric motor is driven on demand, thus supporting the power of an internal combustion engine. While a mild hybrid electric vehicle does not have a driving mode in which the vehicle is driven solely by electric motor torque, the electric motor can, depending on the driving situation, support the internal combustion engine torque, allowing a battery (e.g., a 48V battery) to be charged through regenerative braking. This means that by adding the electric motor torque to the internal combustion engine torque, the torque is counteracted by the driver's desired torque, ensuring optimal responsiveness and the use of electrical energy obtained through regenerative braking, which can improve fuel consumption.

[0003] In contrast, the power system for a mild hybrid electric vehicle (MHE) consists of two different power supply elements: a low-voltage power grid and a high-voltage power grid. The high-voltage power grid drives the electric motor or supplies the electrical power required to drive various systems, while the low-voltage power grid supplies electrical power to a starter motor. When such a high electrical power is immediately required, such as when driving the starter motor, the low-voltage power grid may be in a temporary low-voltage state. Such a low-voltage state is merely a temporary phenomenon and does not constitute an abnormality in the power system. Nevertheless, conventional power systems recognize such a state as a failure situation.This causes the vehicles to unnecessarily enter failure mode and thus impairs their drivability, thereby acting as a factor in reducing the commercial value of the vehicles.

[0004] Examples of previously known approaches can be found in DE 10 2010 043 551 A1, DE 10 2017 219 945 A1, DE 10 2017 217 850 A1 and US 10 164 522 B2. Description of the inventionObject of the invention

[0005] The present invention is based on the object of providing a method for controlling a power supply system for a mild hybrid electric vehicle, which prevents a vehicle provided with such a system from entering a failure mode when an electrical voltage drop phenomenon occurs in the power supply system, and by means of which the power supply system can again be operated normally. Solution to the problem of the invention

[0006] This object is achieved by a method for controlling a power supply system for a mild hybrid electric vehicle having the features of claim 1. Further embodiments emerge from the dependent claims. Effect of the invention

[0007] With the method for controlling a power supply system for a mild hybrid electric vehicle according to the invention, a temporary electrical voltage drop phenomenon occurring in the power supply system is prevented from being associated with entry into a failure mode, and the power supply system can be controlled in such a way that it resumes normal operation. Thus, unnecessary entry into the failure mode can be prevented and the reliability of the vehicle control can be improved. Short description of the drawings

[0008] They show: Fig. 1 shows a diagram of a power supply system for a mild hybrid electric vehicle; Fig. 2 a flowchart illustrating the method steps of a method for controlling an ON sequence of a system by means of the power supply system of Fig. 1; Fig. 3 different diagrams to illustrate the operating states of the system after the process steps from Fig. 2; Fig. 4 is a flowchart illustrating a control method when a low voltage occurs in a conventional mild hybrid electric vehicle; and Fig. 5 is a flowchart illustrating the method steps of a method for controlling the power supply system for a mild hybrid electric vehicle according to the present invention. Preferred embodiments of the invention

[0009] With respect to the embodiments of the present invention disclosed in this specification, the specific structural or functional explanations are given only for the purpose of explaining the embodiments of the present invention by way of example, and the embodiments of the present invention may be embodied in a variety of forms and should not be construed as being limited to the embodiments explained in this specification.

[0010] Furthermore, the present invention is susceptible to various modifications and takes a variety of forms, specific embodiments of which are indicated in the drawings and are intended to be particularly explained in this specification. However, the present invention should not be limited to any particular form disclosed, and all modifications, equivalents, and alternatives falling within the spirit and technical scope of the present invention are intended to be included within the scope of this invention.

[0011] Furthermore, while the terms "first," "second," and the like may be used to describe various components, the components should not be limited by the above terms. That is, such terms may be used only for the purpose of distinguishing one component from another. For example, a first component may be referred to as a second component, and similarly, the second component may also be referred to as the first component, without departing from the scope of the present invention.

[0012] Furthermore, if it is mentioned that a component is "connected" or "attached" to another component, then it may further be directly connected or attached to the other component, but it should also be understood that another component may be present between such two components. On the other hand, if it is mentioned that a component is "directly connected" or "attached" to another component, then it must be understood that no other component is present between such two components. Furthermore, other expressions describing the relationships between the components, such as "between the components" and "just between the components" or "adjacent to the component" and "immediately adjacent to the component" and the like, should also be understood in a manner described above.

[0013] Furthermore, the terms used in this application are used only to explain a particular embodiment, but are not intended to limit the present invention. The singular terms also include the plural meanings, unless the context requires otherwise. In this application, the terms such as "comprise", "include", "have" or the like further indicate only that embodied features, digits, steps, movements, components, accessories or combinations thereof are present in this invention. Therefore, the presence or addition of one or more other features, digits, steps, movements, components, parts or combinations thereof should not be construed as previously excluding it or them.

[0014] All terms used herein, including technical or scientific terms, have the same meanings as those commonly understood by one of ordinary skill in the art to which the present invention pertains, unless otherwise defined. Terms defined in commonly used dictionaries should be interpreted to have meanings consistent with those within the context of the technology in question and should not be interpreted in idealized or overly formal ways unless clearly defined in this application.

[0015] A preferred embodiment of the present invention will be explained in more detail below with reference to the accompanying drawings. Like reference numerals are used for like components in the drawings, and repeated explanations of like components are omitted.

[0016] Fig. Figure 1 shows a diagram of a power supply system for a mild hybrid electric vehicle. Fig. 1, a solid line represents a flow of a control signal, while a dotted line represents a flow of energy.

[0017] With reference to Fig. 1, the mild hybrid electric vehicle can be driven by means of the driving forces generated by an internal combustion engine 10 and a mild hybrid starter generator 20 (MHSG).

[0018] The combustion engine 10 is started by a starter motor 11 and, after starting, is driven by the combustion of fuel. Furthermore, the starter motor 11 can be driven by a low-voltage battery 30, which can be, for example, a 12V battery.

[0019] In addition, the MHSG (mild hybrid starter generator) 20 is powered by a high-voltage battery 40, thereby supporting the combustion engine 10 so that it can generate motive power. The operation of the high-voltage battery 40 can be controlled by a battery management unit 41, whereby electrical energy from the high-voltage battery 40 can be supplied to the MHSG 20 via a converter 21. During regenerative braking, however, electrical energy generated by the MHSG 20 can also be supplied to the high-voltage battery 40 via the converter 21.

[0020] Accordingly, the operating states of the MHSG 20 can be expressed as OFF mode, neutral mode, operating mode, or the like. The OFF mode can be understood as a state in which the supply of electrical energy to the MHSG 20 is interrupted. The operating mode can be understood as a state in which the vehicle is being driven, i.e., an operating state in which the MHSG 20 generates the motive power or power is generated through regenerative braking. The neutral mode can also be understood as an intermediate step in which the OFF mode transitions to the operating mode, or vice versa.

[0021] Since the power axes of the internal combustion engine 10 and the MHSG 20 are connected to each other via a belt, etc., the MHSG 20 can be driven by the output torque of the internal combustion engine 10. The MHSG 20 can function as a generator that charges the high-voltage battery 35. In this way, charging the high-voltage battery 35 using the output torque of the internal combustion engine 10 can be referred to as regenerative braking. In this case, the high-voltage battery 40 can be charged. Furthermore, the operations of the MHSG 20 and the inverter 21 can be controlled by a motor control unit 80 (MSE).

[0022] In this case, the battery management unit 41 can manage the state of charge (SOC) of the high-voltage battery 40. This means that the battery management unit 41 can detect the state of charge of the high-voltage battery 40 and selectively manage the charging and discharging so that the high-voltage battery 40 is not overcharged or overdischarged. In this case, the battery management unit 41 can represent, for example, a battery management system (BMS).

[0023] This is achieved by the high-voltage battery 40 transferring electrical energy to the low-voltage battery 30 via a low-voltage DC-DC converter 50 (LDC), allowing this low-voltage battery 30 to be charged. The high-voltage battery 40 can be a 48V battery, for example.

[0024] Accordingly, the power supply system for a mild hybrid electric vehicle according to the present invention may include a low-voltage power network in which the low-voltage battery 30 functions as a center; a high-voltage power network in which the high-voltage battery 40 functions as a center; a low-voltage DC-DC converter 50 that transfers electric power between the low-voltage and high-voltage power networks; and a vehicle control unit 70 that controls the operations of the above components.

[0025] The low-voltage power grid can be understood as a system that, for example, has the low-voltage battery 30, an electrical resistor 31, and the like, and is driven by the low voltage (e.g., 12V). The high-voltage power grid, on the other hand, can be understood as a system that, for example, has the high-voltage battery 40, the MHSG 20, the converter 21, a high-voltage relay 43, the battery management unit 41, and the like, and is driven by the high voltage (e.g., 48V). The supply of electrical energy to the high-voltage power grid by the high-voltage battery 40 can be interrupted by the high-voltage relay 43. That is,When the high-voltage relay 43 is closed, the electrical energy of the high-voltage battery 40 can be supplied to the high-voltage power grid, while when the high-voltage relay 43 is opened, the electrical energy of the high-voltage battery 40 supplied to the high-voltage power grid can be interrupted.

[0026] The low-voltage DC-DC converter 50 can mediate the transfer of electrical energy between the high-voltage and low-voltage power grids. For example, the low-voltage DC-DC converter 50 can have a voltage boost mode and a voltage drop mode. In the voltage boost mode, the low-voltage DC-DC converter 50 can boost the low-voltage electrical voltage of the low-voltage power grid and then transfer the boosted electrical voltage to the high-voltage power grid. In the voltage drop mode, however, the low-voltage DC-DC converter 50 can step down the high-voltage electrical voltage of the high-voltage power grid and then transfer the reduced electrical voltage to the low-voltage power grid.

[0027] Accordingly, the operating states of the low-voltage DC-DC converter 50 can include an OFF mode, a test mode, a voltage boost mode, a voltage drop mode, a termination mode, and the like. The OFF mode can be understood as an operating state in which the low-voltage DC-DC converter 50 is not supplied with electrical power. The test mode can be performed before the low-voltage DC-DC converter 50 is supplied with electrical power and represents a step that checks whether the state of the low-voltage DC-DC converter 50 is normal or not. Furthermore, the voltage boost mode can be understood as an operating state in which the low-voltage electrical voltage of the low-voltage power grid is increased and then the increased electrical voltage is transferred to the high-voltage power grid.In contrast, the voltage reduction mode can be understood as an operating state in which the high-voltage electrical voltage of the high-voltage power grid is reduced and then the reduced electrical voltage is transferred to the low-voltage power grid. Finally, the termination mode represents an intermediate step in which the voltage reduction mode transitions to the OFF mode, in which case operations such as self-diagnosis, fault storage, etc., can also be performed. In this termination mode, the transmission of electrical energy between the low-voltage and high-voltage power grids can also be interrupted.

[0028] Therefore, the drive forces generated by the internal combustion engine 10 and the MHSG 20 can be transmitted to the wheels 90 via a transmission 60. A transmission clutch 61 is provided between the internal combustion engine 10 and the transmission 60, wherein this transmission clutch 61 can interrupt the transmission of the drive force from the internal combustion engine 10 to the transmission 60. This means that when the transmission clutch 61 is engaged, the drive force of the internal combustion engine 10 can be transmitted to the wheels 90 via the transmission 60, while when the transmission clutch 61 is disengaged, the transmission of the drive force to the wheels 90 can be blocked.

[0029] In addition, the vehicle control unit 70 represents a higher-level control unit that controls the operation of the vehicle and can control the output torques of the internal combustion engine 10 and the MHSG 20. For example, the vehicle control unit 70 can represent an electronic control unit (ECU).

[0030] Depending on the driver's desired torque and vehicle condition, the vehicle control unit 70 can selectively drive the internal combustion engine 10 and the MHSG 20. Specifically, based on the specified map data, the vehicle control unit 70 can determine a driver-desired torque distribution ratio between the internal combustion engine 10 and the MHSG 20 depending on the driving range, driver-desired torque, charge state, etc. For example, if the driver's desired torque is low, power is generated solely by the internal combustion engine 10. As the driver's desired torque increases, the internal combustion engine is driven together with the MHSG 20, thus enabling power generation.

[0031] As mentioned above, the power system for the mild hybrid electric vehicle can supply the necessary electrical power to a group of components requiring low voltage and another group of components requiring high voltage. Furthermore, the supply of electrical power to the high-voltage power grid can be interrupted by the high-voltage relay 43. That is, while the vehicle is being driven, the high-voltage relay 43 is closed, thus supplying the electrical power from the high-voltage battery 40 to the high-voltage power grid. After the engine is turned off, the high-voltage relay 43 is opened, thus blocking the supply of electrical power to the high-voltage power grid.Thus, when an ignition key is turned on, one sequence is required to stably close the high-voltage relay 43, while after the ignition key is turned off, another sequence is also required to stably open the high-voltage relay 43.

[0032] Fig. 2 shows a flowchart illustrating the method steps of a method for controlling an ON sequence of a system, and Fig. 3 different diagrams to illustrate the operating states of the system after the process steps from Fig. 2.

[0033] When an ignition key is turned on at a first time t1 (S100), it is determined whether a high-voltage relay 43 is in the open state (S110). This is because a series of processes for closing the high-voltage relay 43 are not necessary if the high-voltage relay 43 is already closed.

[0034] If the high-voltage relay 43 is in the open state, the condition check of a low-voltage DC-DC converter 50 is performed first (S120). Since only a very short period of time is sufficient from the ignition key being turned on until the start of the condition check of the low-voltage DC-DC converter 50, this can be understood as starting the condition check of the low-voltage DC-DC converter 50 simultaneously with the ignition key being turned on.

[0035] Specifically, it is provided that a test signal is sent from the vehicle control unit 70 to the low-voltage DC-DC converter 50, wherein, if this low-voltage DC-DC converter 50 enters a specific mode, e.g., test mode, in response to the test signal, then it can be decided that the low-voltage DC-DC converter 50 is in the normal state (S130). This takes place between the first time t1 and a second time t2 in Fig. 3. If the low-voltage DC-DC converter 50 does not enter test mode even though the vehicle control unit 70 has sent the test signal, it can be determined that the low-voltage DC-DC converter 50 is in failure. In this case, the system is diagnosed as faulty (S190), and the vehicle can then enter a failure mode (S191). This failure mode can be the one intended to operate the vehicle restrictively in an emergency situation, such as a failure.

[0036] If the low-voltage DC-DC converter 50 is in the normal state, then the MHSG 20 is caused to enter a neutral mode (S140). This is at a third time t3 in Fig. 3. That is, step S140 may represent the step that occurs before the MHSG 20 is properly actuated and checks the operating state of the MHSG 20.

[0037] Thereafter, the low-voltage DC-DC converter 50 is caused to enter a voltage boost mode, so that the electrical voltage of the high-voltage power grid is increased (S150). This is between a fourth time t4 and a fifth time t5 in Fig. 3 shown.

[0038] In the OFF state of the system, the high-voltage relay 43 is open. Thus, the electrical energy of a high-voltage battery 40 cannot be transferred to the high-voltage power grid, and no voltage is generated in the high-voltage power grid. Thus, the electrical voltage of a low-voltage battery 30 is increased by the low-voltage DC-DC converter 50, and the increased electrical voltage is then transferred to the high-voltage power grid, so that the electrical voltage of the high-voltage power grid can also be increased.

[0039] If the electrical voltage of the high-voltage power grid exceeds a predetermined target voltage (S160), then it can be decided that the power system is in the normal state (S170).

[0040] Eg as in Fig. 3 (d) and Fig. As shown in Fig. 3 (e), the low-voltage DC-DC converter 50 performs the voltage boosting mode, so that when the electric voltage of the high-voltage power grid is increased to the target electric voltage V at the fifth time t5 H is increased, then it can be diagnosed that the power system is in normal condition. The target electrical voltage can be, for example, 48V.

[0041] If the power system is normal, the high-voltage relay 43 is closed (S171), which can cause the MHSG 20 to enter the operating mode (S172) and the low-voltage DC-DC converter 50 to enter the voltage-reduction mode (S173). In this way, the electrical voltage of the high-voltage power grid is sufficiently reduced to the target voltage V H increased and then the high voltage relay 43 is closed, which is why the shock due to the application of high voltage can be prevented.

[0042] Conversely, if the high-voltage power grid voltage is not increased above the target voltage even though the low-voltage DC-DC converter 50 has been driven sufficiently in the boost mode for more than the first predetermined target time (S180), it can be diagnosed that the system is in a failure state (S190). In this case, a failure signal is output, and the vehicle enters the failure mode (S191), thus preventing further damage to the power system.

[0043] When such an ON sequence of the system is completed, the vehicle can perform normal operation. For example, the internal combustion engine 10 and the MHSG 20 can be driven, allowing the vehicle to be driven, or the high-voltage battery 40 can be charged through regenerative braking. This state is used as the opposite of the failure mode, and such a state is also referred to as 'normal mode' in this specification. In such a normal mode, the power can be determined according to the driver's desired moment, allowing the driver to use all the functions provided in the vehicle. In contrast, in the failure mode, most functions, except for those required for emergency measures, can be made unusable, limiting the vehicle's power to a minimum.

[0044] However, the low-voltage power grid may enter a temporary low-voltage electrical state after the vehicle enters normal mode. For example, if the starter motor 11 is driven to restart the internal combustion engine 10, a momentarily high electrical energy may be required, and the electrical voltage of the low-voltage power grid may temporarily decrease. In this case, the conventional method for controlling a power system has a problem in that such a case is recognized as a failure state. This is described in Fig. 4 shown.

[0045] With reference to Fig. 4, after the system ON sequence (S10) has been performed, the vehicle is driven in normal mode (S20). If an electrical voltage drop occurs in the low-voltage power grid for any reason (S30), the high-voltage relay 43 is opened (S40). The vehicle then enters the failure mode (S50), and the failure signal is output (S60). In this case, the vehicle's functions are limited, requiring the driver to visit a repair shop. However, during the process of driving high-power components, such as the starter motor 11, etc., an electrical voltage drop in the low-voltage power grid may temporarily occur, but this cannot be recognized as a failure situation. Thus, the vehicle may enter the failure mode (S50) unnecessarily, which may consequently act as a factor leading to a reduction in the vehicle's commercial value.Therefore, the invention aims to prevent the entry into failure mode in the temporary low voltage situation, so that the reduction of the commercial value of the vehicle is also to be avoided.

[0046] Fig. 5 shows a flowchart illustrating the method steps of a method for controlling the power supply system for a mild hybrid electric vehicle according to the present invention.

[0047] With reference to Fig. 5 the vehicle is then driven in normal mode (S110) if the Fig. 2 shown ON sequence of the system is completed (S100).

[0048] The electrical voltage status of the low-voltage power grid is then monitored. If a voltage drop in the low-voltage power grid occurs while the vehicle is moving (S120), a preliminary reset decision is made (S130), and entry into the failure mode is prohibited (S140).

[0049] Specifically, it is the case in Fig. 4, the conventional control method is provided such that the high-voltage relay 43 is opened and the vehicle enters the failure mode as soon as the electrical voltage drop occurs in the low-voltage power grid, while the present invention differs from the conventional control method in that the entry into the failure mode is prevented and the preliminary decision on the reset is made. Such a reset means that the Fig. 2 described system ON sequence is performed again. This means that instead of entering the failure mode, the invention intends that the power system can be operated normally by rebooting if necessary.

[0050] If the voltage of the low-voltage power grid drops below a specified reference voltage, it can also be determined that the voltage drop occurred in the low-voltage power grid. The reference voltage can be properly selected based on the technical specifications of the power system or as needed. For example, if the voltage of the low-voltage power grid drops below 5V, it can be determined that the voltage drop occurred.

[0051] Depending on whether the closed state of the high voltage relay 43 is maintained, it is then determined whether the reset must be performed or not (S150).

[0052] Specifically, it is intended that if the high voltage relay 43 is already open due to the electrical voltage drop (S120), then the reset is decided (S160) and then the ON sequence of the system in Fig. 2 is performed again. If the electrical voltage drop (S120) is not a temporary phenomenon, then the vehicle may enter the failure mode during the process of performing the system ON sequence, whereas if such a voltage drop is only a simply temporary phenomenon, then the vehicle may stably return to the normal mode (S180).

[0053] However, if the high-voltage relay 43 maintains the closed state, the system's ON sequence does not need to be persistently repeated. In this case, the pre-reset decision is canceled (S170) and a prohibition measure against entering the failure mode is released (S175).

[0054] As described above, the method for controlling a power supply system for a mild hybrid electric vehicle according to the present invention can prevent a temporary voltage drop of a low-voltage power grid from being associated with entry into a failure mode and cause the power supply system to be controlled so that it can resume normal operation. Thus, unnecessary entry into the failure mode can be prevented and the reliability of vehicle control can be improved. List of reference symbols 10 Combustion engine 20 Mild Hybrid Starter Generator (MHSG) 30 low-voltage battery 40 high-voltage battery 43 high-voltage relays 50 low-voltage DC-DC converters 60 gearboxes 70 Vehicle control unit 80 Engine Control Unit (MSE) 90 wheels

Claims

[1] A method for controlling a power supply system for a mild hybrid electric vehicle, wherein the power supply system comprises a low-voltage power network supplied by a low-voltage battery (30) and a high-voltage power network supplied by a high-voltage battery (40), wherein a transfer of electrical energy between the low-voltage and the high-voltage power network is enabled by a low-voltage DC-DC converter (LDC, 50), and wherein a high-voltage relay (43) interrupts a supply of electrical energy from the high-voltage battery (40) to the high-voltage power network, characterized by that the procedure comprises the following procedural steps: Carrying out an ON sequence of the power supply system in that, when an ignition key is turned on, an electrical voltage of the low-voltage battery (30) is increased and thus also an electrical voltage of the high-voltage power network is increased, wherein the high-voltage relay (43) is closed when the electrical voltage of the high-voltage power network exceeds a predetermined electrical target voltage; Driving a mild hybrid electric vehicle in normal mode after completion of the ON sequence of the power system, wherein the mild hybrid electric vehicle is driven substantially dependent on operation by a driver; Implementing a limitation such that, when an electrical voltage drop occurs in the low-voltage power network, entry into a failure mode is limited and then it is determined whether the high-voltage relay (43) is closed or not; and Performing a release such that if the high voltage relay (43) is in the open state, then the ON sequence of the power supply system is performed again, while if the high voltage relay (43) maintains a closed state, then the restriction of entry into the failure mode is released and thus driving in the normal mode is continued. [2] Method according to claim 1, characterized by that the method step for performing an ON sequence of the power supply system, a checking step in which an operating state of the low-voltage DC-DC converter (LDC, 50) is checked when the ignition key is turned on; a voltage boosting mode step in which the low-voltage DC-DC converter (LDC, 50) increases the electrical voltage of the low-voltage battery (30) when the low-voltage DC-DC converter (LDC, 50) is in the normal state, and thus the increased electrical voltage is supplied to the high-voltage power grid, so that the electrical voltage of the high-voltage power grid is also increased; a closing step in which the high-voltage relay (43) is closed when the electrical voltage of the high-voltage power network exceeds the electrical target voltage; and a voltage reduction mode step in which the increase in the electrical voltage by the low-voltage DC-DC converter (LDC, 50) is interrupted and the electrical voltage of the high-voltage battery (40) is reduced and thus the reduced electrical voltage is supplied to the low-voltage power grid. [3] Method according to claim 2, characterized by in that the method step of performing an ON sequence of the power supply system further comprises an operating step in which a mild hybrid starter generator (MHSG, 20) is operated by means of the high-voltage battery (40) when the high-voltage relay (43) is closed. [4] Method according to claim 2, characterized byin that the method step of performing an ON sequence of the power supply system further comprises an output step in which a failure signal of the power supply system is output when the electrical voltage of the high-voltage power network is less than the electrical target voltage despite the expiration of a predetermined setting time after the voltage boost mode step. [5] Method according to claim 1, characterized by that the method step of applying the limitation is carried out in such a way that when the electrical voltage of the low-voltage power network is reduced below a predetermined electrical reference voltage, then it is decided that the electrical voltage drop occurred.

Images