Method for operating a DC-DC converter of a motor vehicle, as well as DC-DC converter and motor vehicle

Abstract

Method for operating a DC-DC converter (13) of a motor vehicle (10), wherein a first on-board network (11) operated at a first voltage level (U1) is supplied with electrical energy via the DC-DC converter (13) from a second on-board network (12) operated at a second voltage level (U2) that is higher than the first voltage level (U1), and in a driving mode (F1) in which an internal combustion engine (18) of the motor vehicle (10) drives it, the energy is used to at least partially cover the energy demand of electrical consumers (14) of the first on-board network (11), wherein During driving operation (F1), a setpoint signal (21) of a voltage control unit (20) for the first vehicle electrical system (11) is set to a minimum value (W1) in the DC voltage converter (13), whereby an electrical energy storage device (15) of the first vehicle electrical system (11) discharges to the voltage value specified by the minimum value (W1), and in a recuperation mode (F2), in which the vehicle (10) coasts and an electric machine (17) of the second on-board network (12) is operated in generator mode, the setpoint signal (21) is set to a predetermined maximum value (W2) that is greater than the minimum value (W1), characterized by the fact that Each of the consumers (14) has a respective minimum voltage (Umin1, Umin2, Uminn) below which operation of the consumer (14) is impossible and the minimum value (W1) corresponds to the largest minimum voltage (Umin1, Umin2, Uminn) plus a predetermined tolerance value (25) so that all consumers (14) are operational.

Description

[0001] The invention relates to a method for operating a DC-DC converter of a motor vehicle. A first electrical system is supplied with electrical energy from a second electrical system via the DC-DC converter. This at least partially covers the energy demand of the electrical consumers in the first electrical system. The invention also includes said DC-DC converter and a motor vehicle equipped with the DC-DC converter.

[0002] A motor vehicle can have two electrical systems with different voltage levels. One voltage level can be, for example, medium voltage (e.g., in a voltage range of 30 to 60 volts) or high voltage (in the voltage range of 60 to 1000 volts). A low-voltage electrical system (e.g., in a voltage range below 30 volts) can be supplied with electrical power from this system via a DC / DC converter.

[0003] The step-down of the voltage from the high-voltage or medium-voltage electrical system to the low-voltage system is now performed regardless of how the energy in the high-voltage / medium-voltage system is generated. Energy can be generated there by a running combustion engine driving a generator, which means that the combustion engine will have increased fuel consumption. The electrical energy generated here is therefore purchased at the cost of fuel consumption. Another way to generate energy is through recuperation, in which the combustion engine and generator are propelled by the coasting vehicle. This form of energy generation requires no fuel consumption and is therefore free energy.

[0004] However, the electrical energy generated during recuperation must be absorbed quickly enough by an electrical energy storage device in the vehicle's electrical system, as it is only available when the vehicle is coasting. Especially with low-capacity energy storage devices, it is often not possible to utilize or absorb all the potential recuperated energy during a coasting phase, because the electrical system's storage capacity, which also powers the generator, cannot fully absorb the energy. This is typically the high-voltage or medium-voltage electrical system.

[0005] From DE 10 2010 029 299 A1, it is known to store electrical energy in a motor vehicle with two electrical systems of different voltage levels. This energy is recovered during recuperation by means of a generator in one voltage level and can also be stored in an energy storage device in the other voltage level. This is achieved by using a DC / DC converter to transfer the recovered electrical current to the other electrical system. However, this requires that the energy storage device in the other electrical system is also capable of receiving the recovered electrical energy.

[0006] From DE 10 2013 201 346 A1, it is known that in a motor vehicle with the two electrical systems described above, a DC-DC converter can be used to ensure that the electric battery in the low-voltage electrical system is permanently operated at its open-circuit voltage, thereby compensating for any load on the energy storage device. However, this is not suitable for absorbing recuperation energy.

[0007] From DE 11 2009 002 655 T5, it is known to maintain the charge status of a low-voltage battery series in an electric hybrid vehicle by recharging the low-voltage battery series using a high-voltage traction battery series, depending on a recharging trigger timer. However, the trigger timer is not coordinated with a recuperation operation.

[0008] From the generic DE 10 2009 000 222 A1, it is provided for an on-board network of a motor vehicle that a DC voltage converter transfers electrical energy from a high-voltage level to a low-voltage level and in doing so initially regulates a voltage in the range of 12 V to 14 V in the low-voltage level and raises this to more than 14 V in the case of recuperation in order to use a reserve capacity of an accumulator of the low-voltage level for recuperation.

[0009] The invention is based on the objective of storing the electrical energy gained in a motor vehicle during recuperation operation.

[0010] The problem is solved by the subject matter of the independent claims. Advantageous embodiments of the invention are described by the dependent claims, the following description, and the figures.

[0011] The invention provides a method for operating a DC / DC converter for a motor vehicle. In a manner known per se, a first electrical system, operating at a first voltage level, is supplied with electrical current or energy from a second electrical system, operating at a higher voltage level than the first, via the DC / DC converter. Another name for a DC / DC converter is DC / DC converter. The first voltage level can be in a voltage range of less than 30 volts (low voltage). The second voltage level can be in a voltage range of 30 volts to 60 volts (medium voltage) or from 60 volts to 1000 volts (high voltage).

[0012] The described configuration, for example, involves the DC-DC converter operating in buck mode. This transfers electrical energy from the second electrical system to the first. During operation, when the vehicle's combustion engine is propelling it, this electrical energy at least partially covers the energy demand of electrical consumers in the first electrical system. In other words, the electrical consumers are at least partially powered by electrical energy from the second electrical system, which operates at a higher voltage level. Therefore, the electrical energy storage device in the first electrical system does not need to fully meet the energy demand itself. Instead, the electrical energy storage device in the first electrical system discharges to the voltage level determined by the minimum value.The energy storage device could, for example, be an arrangement with a battery and / or an electric accumulator and / or at least a double-layer capacitor.

[0013] To maximize the absorption of recuperated electrical energy during regenerative braking, the invention utilizes the fact that each consumer in the first electrical system has a minimum voltage below which operation is impossible, for example, because a control circuit of the consumer performs a reset or because an electrical circuit is operating unstably. During driving, a setpoint signal from a voltage control unit for the first electrical system is set to a value in the DC-DC converter, which is referred to here as the minimum value. The voltage control unit regulates the output voltage of the DC-DC converter, i.e., the electrical system voltage in the first electrical system. During driving, the minimum value is set for the setpoint signal of this voltage control unit.This corresponds to the highest minimum voltage of the consumers plus a predetermined tolerance value, ensuring that all consumers are operational. In other words, none of the consumers' minimum voltages are undershot. The first electrical system is therefore operated at the minimum possible voltage for stable operation.

[0014] The tolerance value has the advantage that if the voltage control unit cannot respond to an increase in the energy demand of the consumers with the same rate of change, but only with a lower rate of change than the energy demand itself, then the energy storage of the first electrical system can support the first electrical system with electrical energy without any of the minimum voltages being undershot. The tolerance value can also be 0.

[0015] In regenerative braking mode, where the vehicle coasts and an electric machine in the secondary electrical system operates as a generator, the setpoint signal is adjusted to a predetermined maximum value that is higher than the minimum value. During regenerative braking, a higher voltage is applied to the primary electrical system by the voltage regulator. This triggers the charging of the primary electrical system's energy storage device, meaning the device absorbs electrical energy. This energy is recovered during regenerative braking, thus advantageously storing the energy gained through recuperation in the primary electrical system's energy storage device.

[0016] The invention offers the advantage that, during driving, an electrical energy storage device in the first vehicle electrical system discharges to an open-circuit voltage predetermined by a minimum value. In other words, the open-circuit voltage of the energy storage device drops to the voltage value specified by the minimum value. The energy storage device is thus discharged to the extent necessary for the operation of all electrical consumers. During recuperation, the target value signal is then amplified to the maximum value, thereby triggering a charging process for the energy storage device. This allows the electrical energy recovered during recuperation to be stored in the energy storage device of the first vehicle electrical system. Since the energy storage device is discharged to the greatest extent possible during driving, its capacity to absorb recuperated energy is maximized.

[0017] The invention also includes embodiments that offer additional advantages.

[0018] One embodiment provides that if, during driving, the charge level of the electrical energy storage device in the first vehicle electrical system drops to such an extent that its open-circuit voltage matches or falls to its minimum value, the energy demand of the consumers is fully met by energy from the second vehicle electrical system via the DC-DC converter, thus keeping the energy storage device de-energized. In other words, the power or energy demand of all consumers in the first vehicle electrical system is met by electrical energy from the second vehicle electrical system via the DC-DC converter. Therefore, no charging or discharging of the energy storage device in the first vehicle electrical system occurs during driving. This prevents charging and discharging cycles of the energy storage device from occurring during driving.

[0019] One embodiment provides that the DC-DC converter operates in the aforementioned buck-stop mode, thus preventing bidirectional conversion. In other words, the DC-DC converter operates unidirectionally, transferring energy only from the second electrical system to the first, and not vice versa. After a recuperation phase, this prevents the energy storage of the first electrical system from discharging into the second via the DC-DC converter, because the DC-DC converter, with its voltage regulation unit, must regulate back to the minimum value. This reduces converter losses.

[0020] One embodiment provides that the aforementioned minimum value is less than 11 volts, in particular less than 10 volts. This results in an energy boost or charging boost in a so-called 12-volt electrical system, which corresponds to the amount of energy that can be stored in the energy storage device from recuperation.

[0021] One embodiment provides that the aforementioned tolerance value is less than 2 volts, in particular less than 1 volt. This further increases the energy swing.

[0022] One embodiment provides that the tolerance value is set depending on the control dynamics of the voltage control unit and / or the temporal dynamics of the energy demand. In other words, the control speed or a time constant of the voltage control unit and / or the possible temporal variation or gradient of the energy demand of the consumers are taken into account to determine the tolerance value. The tolerance value is set such that a time offset between the maximum possible temporal variation in energy demand, on the one hand, and the adjustment of the energy supplied by the DC-DC converter to the energy demand, on the other hand, results in a current from the energy storage device into the first vehicle electrical system, such that the voltage of the first energy storage device is maintained above, or reaches, the maximum minimum voltage of the consumers.This prevents the voltage from dropping below the maximum minimum voltage, ensuring stable operation of all electrical consumers. The time lag in adjusting the vehicle electrical system voltage can occur because the control dynamics of the voltage regulator are less than the temporal dynamics of the energy demand. By adjusting the tolerance value to the control dynamics and / or the temporal dynamics of the energy demand, the lowest possible minimum value for the voltage regulator is set.

[0023] One embodiment provides that the maximum value intended for recuperation operation corresponds to a maximum permissible value for the energy storage of the first vehicle electrical system. In other words, when the voltage control unit is operated at its maximum value, the electrical energy storage of the first vehicle electrical system can be charged to its maximum possible energy content without being damaged.

[0024] One embodiment provides that the maximum value lies within a voltage range of 12 volts to 16 volts. Thus, the energy storage system of a 12-volt electrical system can be fully charged.

[0025] The invention also includes the aforementioned DC-DC converter, which comprises a control unit configured to carry out an embodiment of the method according to the invention. For this purpose, the control unit may comprise at least one microprocessor and / or at least one microcontroller and / or control logic or logic circuitry. The steps of the method may be implemented as program code that can be executed by the control unit. The program code may be stored in a data memory of the control unit.

[0026] Finally, the invention also includes a motor vehicle in which a first electrical system operating at a first voltage level is coupled to a second electrical system operating at a second voltage level that is higher than the first via a DC-DC converter. The DC-DC converter is an embodiment of the DC-DC converter according to the invention. The motor vehicle according to the invention is, in particular, designed as a motor vehicle, preferably a passenger car or truck.

[0027] The following are exemplary embodiments of the invention described. This is illustrated by: Fig. 1 a schematic representation of an embodiment of the motor vehicle according to the invention during driving operation; Fig. 2 a schematic representation of the motor vehicle of Fig. 1 during a recuperation operation; Fig. 3 a diagram with a graph of an electrical voltage regulated by a voltage control device of a DC-DC converter of the motor vehicle; Fig. 4. A diagram illustrating the charging process; and Fig. 5 a flowchart to illustrate an embodiment of the method according to the invention.

[0028] The exemplary embodiments described below are preferred embodiments of the invention. In these exemplary embodiments, the described components each represent individual features of the invention that can be considered independently of one another. Each of these features further develops the invention independently and can therefore be considered part of the invention individually or in a combination other than that shown. Furthermore, the described embodiments can also be supplemented by other features of the invention already described.

[0029] In the figures, functionally identical elements are each provided with the same reference symbols.

[0030] Fig. Figure 1 shows a motor vehicle 10, which can be a car, in particular a passenger car or truck. The motor vehicle 10 can have a first electrical system 11 and a second electrical system 12. The first electrical system 11 can, for example, be a low-voltage system. The first electrical system 11 can operate at a voltage level U1, which can be in the range of, for example, 9 volts to 16 volts. A voltage level U2 of the second electrical system 12 can be higher than the voltage level U1. The electrical system 12 can, for example, be a medium-voltage system or a high-voltage system. The voltage level U2 can be in the range of 20 volts to 60 volts or 60 volts to 1000 volts.

[0031] The on-board networks 11, 12 can be coupled via a DC voltage converter 13, which can transfer electrical energy from the on-board network 12 to the on-board network 11 in a buck converter operation or as a buck converter.

[0032] The vehicle electrical system 11 may include electrical consumers 14 and an electrical energy storage device 15, for example an electric battery or an electric accumulator. An example of an electrical consumer 14 could be an infotainment system and / or power steering and / or the vehicle's lighting system 10.

[0033] The on-board network 12 can include an electrical energy storage device 16 and an electric machine 17.

[0034] The vehicle electrical system 11 illustrates a total of n electrical consumers, where n represents the number of electrical consumers 14. When the electrical consumers 14 are in operation, each consumes a current Iv1, Iv2, ..., Ivn. During operation, when electrical energy is generated in the vehicle electrical systems 11 and 12 by means of an internal combustion engine 18, for example by driving the electric machine 17 in generator mode, the currents of the consumers 14 can be supplied from the vehicle electrical system 12 by means of the DC-DC converter 13. The DC-DC converter 13 transfers the electrical energy from the vehicle electrical system 12 to the vehicle electrical system 11 by means of a total current Ib, where the current Ib is distributed among the electrical consumers 14.For driving operation F1, it is provided that the energy storage device 15 is kept without current, i.e., a storage current or battery current Ib is less than a predetermined threshold (for example, less than 1 ampere) and preferably 0 amperes.

[0035] Fig. Figure 2 shows the vehicle 10 in a recuperation mode F2, which differs from driving mode F1. In recuperation mode F2, the electric machine 17 and the combustion engine 18 are towed, meaning the momentum of the vehicle 10 is used to rotate or drive the electric machine 17. The electric machine 17 operates as a generator and thus produces electrical energy in the vehicle electrical system 12. This decelerates the vehicle 10. The electrical energy can also be transferred to the vehicle electrical system 11 via the DC-DC converter 13, so that the current Ib continues to flow and the consumers 14 are supplied with their currents. Additionally, to store excess electrical energy from recuperation mode F2, the battery current Ib can be set to greater than zero. This charges the energy storage device 15 with electrical energy.To trigger this charging process, a control unit 19 of the DC-DC converter 13 can set the output voltage of the DC-DC converter 13 to a different setpoint for driving operation F1 on the one hand and for recuperation operation F2 on the other. For this purpose, the control unit 19 can specify a setpoint signal 21 for a voltage control unit 20 of the DC-DC converter 13. The voltage control unit 20 regulates the output voltage of the DC-DC converter 13 in the vehicle electrical system 11. The setpoint signal 21 specifies the setpoint to which the output voltage is to be regulated.

[0036] Fig. Figure 3 illustrates the intended setpoint values ​​for the setpoint signal 21. Fig. Figure 3 illustrates the open-circuit voltage UII of the energy storage device 15 using its state of charge (SOC) as a percentage of the maximum state of charge. The open-circuit voltage UII is the voltage present at the energy storage device 15 when its electrical terminals or connections are connected with infinite resistance or are in open-circuit mode (no connection). It corresponds to the voltage that must be set at the energy storage device 15 to ensure that the battery current Ib is 0 amperes. For driving operation F1, a minimum value W1 can be provided for the setpoint signal 21 by the control unit 19. The minimum value W1 is preferably the lowest possible voltage value for the vehicle electrical system 11 at which all consumers 14 still operate stably. This means that at a lower voltage value, at least one of the consumers 14 would operate unstably, for example, its control unit would perform a reset.For recuperation mode F2, the control unit 19 can specify a maximum value W2 for the setpoint signal 21, which charges the energy storage device 15 to a maximum permissible state of charge (SOC = 100 percent). This results in the largest possible charge range 22 between the minimum value W1 and the maximum value W2. This means that in recuperation mode F2, the energy storage device 15 can absorb the maximum possible amount of energy without causing unstable operation of the consumers 14 during driving mode F1. During driving mode W1, after the electrical energy recovered through recuperation has been consumed, the energy storage device 15 returns to the minimum possible state of charge (SOCmin). Since the DC-DC converter 13 operates in buck converter mode during driving mode F1, this energy is consumed by the consumers 14 and not transferred back to the vehicle electrical system 12 via the DC-DC converter 13.Then, when the energy storage device 15 has reached its minimum state of charge SOCmin again, the following results in connection with . Fig. 1 described operation, that is, the battery current Ib drops to 0 amperes and the consumers 14 are again supplied exclusively via the DC voltage converter 13.

[0037] Fig. Figure 4 illustrates once again, in recuperation mode F2, the charging process of the energy storage device 15 and the charging process of the energy storage device 16 of the vehicle electrical system 12. The electric machine 17 operates in generator mode and generates the electrical energy. Using the electrical energy in recuperation mode F2, the energy storage device 16 can be charged from its current state of charge 23 to a maximum state of charge 24. Using the DC-DC converter 13, a portion of the electrical energy from recuperation mode F2 can be transferred via the current Ib to the energy storage device 15, which until then was operated or maintained at its minimum state of charge (SOCmin) in driving mode F1 and can now be charged to its maximum state of charge. This maximizes the energy that the energy storage device 15 can absorb in recuperation mode F2.

[0038] Fig.Figure 5 illustrates a method that can be carried out by the control unit 19 of the DC-DC converter 13. The control unit 19 can, for example, comprise a microcontroller or a microprocessor.

[0039] In step S10, the first electrical system 11, operating at the first voltage level U1, is supplied with electrical energy via the DC-DC converter 13 from the second electrical system 12, which operates at the second voltage level U2 (higher than the first voltage level U1). This energy can be transmitted by means of the current Ib. During driving operation F1, in which the combustion engine 18 of the vehicle 10 propels it, this energy at least partially covers the energy demand of the electrical consumers 14 of the first electrical system 11 in step S11. The remaining energy can come from the energy storage device 15, which discharges to the minimum state of charge (SOCmin).

[0040] For the next step S12, it is utilized that each of the consumers has a respective minimum voltage Umin1, Umin2, Uminn, below which operation of the respective consumer 14 is impossible. In step S11, during driving operation F1, the setpoint signal 21 of the voltage control unit 20 for the first on-board power supply 11 is set to the minimum value W1 in the DC-DC converter 13, where the minimum value W1 corresponds to the highest minimum voltage Umin1, Umin2, Uminn plus a predetermined tolerance value 25, so that all consumers 14 are operational and the energy storage 15 of the first on-board power supply 11 discharges to the voltage value specified by the minimum value M1 (UII 0 W1) and thus has the minimum state of charge SOCmin.

[0041] In step S12, during recuperation mode F2, in which the vehicle 10 coasts and the electric machine 17 of the second on-board network 12 is operated in generator mode, the setpoint signal 21 is set to the predetermined maximum value W2, which is greater than the minimum value W1.

[0042] This procedure ensures that no current Ib flows into the energy storage device 15 during driving mode F1, and all the low-level energy available for the vehicle electrical system 11 is used to power the consumers 14. Electrical losses in the energy storage device 15 are minimized, and its service life increases, especially if it is a battery, such as a lithium-ion battery. Furthermore, the state of charge in the energy storage device 15 is minimized, so that the charge level 22 is available during recuperation mode F2.

[0043] For this purpose, the energy management system provided by the control unit 19 for the DC-DC converter 13 sets the minimum open-circuit voltage UII of the energy storage device 15, i.e., the minimum value W1. This then becomes the setpoint for the DC-DC converter 13, i.e., the setpoint signal 21, to which the output voltage of the DC-DC converter 13 in the vehicle electrical system 11 is regulated during driving operation F1. As soon as the energy storage device 15 has discharged to the point where it has reached the minimum state of charge (SOCmin), no current Ib flows, i.e., the energy storage device 15 is de-energized. During the recuperation phase or in recuperation mode F2, however, the setpoint signal 21 is set to the maximum value W2, and thus the energy storage device 15 is charged. Additional capacity of the energy storage device 15 is therefore provided (charge stroke 22), and thus the electrical energy generated in recuperation mode F2 without fuel consumption is stored or saved.This energy recovered through recuperation would otherwise be lost unused if the storage capacity of the energy storage devices 15 and 16 were exhausted and would then have to be dissipated, for example, via the friction brakes of the vehicle 10. Instead, the additional stored energy can now be used to reduce the number of recharging phases for the energy storage devices 15 and 16 while the combustion engine 18 is running. This directly saves fuel. This is made possible by the highest possible charging stroke 22, without compromising the stability of the vehicle's electrical system 11.

[0044] Overall, the examples show how the invention can provide intelligent energy distribution during recuperation for drives with multiple voltage levels.

Claims

[1] Method for operating a DC-DC converter (13) of a motor vehicle (10), wherein a first on-board network (11) operated at a first voltage level (U1) is supplied with electrical energy via the DC-DC converter (13) from a second on-board network (12) operated at a second voltage level (U2) higher than the first voltage level (U1), and in a driving mode (F1) in which an internal combustion engine (18) of the motor vehicle (10) drives the motor vehicle (10), an energy requirement of electrical consumers (14) of the first on-board network (11) is at least partially covered by means of the energy, wherein during driving operation (F1) in the DC-DC converter (13) a setpoint signal (21) of a voltage control unit (20) for the first on-board network (11) is set to a minimum value (W1), wherein an electrical energy store (15) of the first on-board network (11) is discharged to the voltage value predetermined by the minimum value (W1), and in a recuperation mode (F2), in which the motor vehicle (10) rolls to a stop and an electrical machine (17) of the second vehicle electrical system (12) is operated in generator mode, the setpoint signal (21) is set to a predetermined maximum value (W2) which is greater than the minimum value (W1), characterized by , that each of the consumers (14) has a respective minimum voltage (Umin1, Umin2, Uminn), below which operation of the consumer (14) is impossible, and the minimum value (W1) corresponds to the largest minimum voltage (Umin1, Umin2, Uminn) plus a predetermined tolerance value (25) so that all consumers (14) are operational. [2] Method according to claim 1, wherein in the event that during driving operation (F1) a state of charge (SOC) of the electrical energy store (15) of the first vehicle electrical system (11) has dropped to such an extent that an open-circuit voltage (UII) of the energy store (15) has dropped to the minimum value (W1), the energy requirement of all consumers (14) of the first vehicle electrical system is covered by means of electrical energy from the second vehicle electrical system (12) via the DC-DC converter (12). [3] Method according to one of the preceding claims, wherein the DC-DC converter (13) is operated in step-down mode. [4] Method according to one of the preceding claims, wherein the minimum value (W1) is less than 11 volts, in particular less than 10 volts. [5] Method according to one of the preceding claims, wherein the tolerance value (25) is less than 2 volts, in particular less than 1 volt. [6] Method according to one of the preceding claims, wherein the tolerance value (25) is set as a function of a control dynamic of the voltage control unit (20) and / or a temporal dynamic of the energy requirement. [7] Method according to one of the preceding claims, wherein the maximum value (W2) corresponds to a maximum value permissible for the energy storage device (15) of the first vehicle electrical system (11). [8] Method according to one of the preceding claims, wherein the maximum value (W2) lies in a voltage range of 12 volts to 16 volts. [9] DC-DC converter (13) with a control device (19) which is designed to carry out a method according to one of the preceding claims. [10] Motor vehicle (10), in which a first on-board network (11) operated at a first voltage level (U1) is coupled to a second on-board network (12) operated at a second voltage level (U2) higher than the first voltage level (U1) via a DC-DC converter (13), characterized by that the DC-DC converter (13) is designed according to claim 9.

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