Electric drive system with two dc-side parallel or cascaded switchable inverters

Abstract

The present invention relates to an electric drive system (1) for a vehicle, comprising: at least two motors (3.1, 3.2) for driving the vehicle, each motor having three stator windings (L1 to L6); at least one high-voltage battery (2); and two inverters (4.1, 4.2) for converting the DC voltage of the high-voltage battery (2) into an AC voltage to power one of the motors (3.1, 3.2), wherein the inverters (4.1, 4.2) are connectable in series with each other and are controllable and / or regulated such that one of the inverters (4.1, 4.2) and its connected motor (3.1, 3.2) can operate as a bidirectional DC / DC converter to supply a voltage lower than the DC voltage of the high-voltage battery (2) to the other inverter (4.1, 4.2) or to recover energy from the motor (3.1, 3.2) connected to the other inverter (4.1, 4.2).

Description

[0001] The present invention relates to an electric drive system according to the preamble of claim 1, a method of operating the electric drive system according to the preamble of claim 3, and an electric drive vehicle according to claim 8.

[0002] When an electric vehicle is traveling at low power and / or speed, the efficiency of the inverter is determined by switching losses, which depend on the DC operating voltage.

[0003] The losses in an inverter are divided into switching losses and conduction losses. Switching losses are related to the high-voltage DC voltage being switched; that is, the higher the voltage, the greater the switching losses. Furthermore, during each switching operation, a certain amount of switching energy is released as heat as a loss. This clearly shows that the switching loss power of the inverter during operation is directly proportional to the inverter's clock frequency.

[0004] In the prior art, the following measures are known to reduce losses: - Semiconductors switch faster: Specifically, by using new semiconductors such as SiC and GaN transistors, the turn-on and turn-off processes can be designed to be faster, thereby reducing the product of the existing current and the applied voltage during switching operations. The disadvantages are: faster switching operations generate higher frequency EMC interference and place a greater burden on the insulation of the stator winding. Furthermore, SiC semiconductors (primarily used for 800V high-voltage DC voltages) are significantly more expensive than IGBTs.

[0005] - Changing the inverter's DC operating voltage via an upstream DC / DC converter: This converter allows for optimization of the inverter's operating voltage based on the operating point, thereby achieving efficiency advantages. However, the disadvantages are that this additional equipment is large, heavy, and expensive.

[0006] - Reducing the clock frequency at certain operating points: By reducing the clock frequency at certain operating points, the number of switching operations per unit time is reduced. This automatically leads to a reduction in switching losses (conduction losses remain unchanged). The downside is that as the clock frequency decreases, the voltage swing at the intermediate circuit capacitor increases (the AC component increases). This results in a larger voltage ripple in the inverter. Either all other components are designed to operate normally with increased voltage ripple, or the intermediate circuit capacitor in the inverter must be increased to still meet existing ripple requirements when the clock frequency is reduced.

[0007] DE 10 2021 003 882 A1 describes an electric drive system for a vehicle, the electric drive system having: - A first AC motor and a second AC motor for driving at least one drive shaft of the vehicle. - An energy storage device for supplying power to a first AC motor and a second AC motor during vehicle operation, wherein a first inverter of the first AC motor and a second inverter of the second AC motor are each coupled to the energy storage device. - A vehicle-side charging port for electrically coupling the energy storage device to an external vehicle charging unit, wherein - Depending on the first inverter and / or the second inverter, the charging voltage at the charging port on the vehicle side can be converted into a supply voltage for charging the energy storage device.

[0008] A vehicle drive system is known from DE 10 2022 134 499 A1, comprising a battery, at least one drive motor, and associated inverter circuitry. This vehicle drive system has two drive motors, each with its own inverter circuitry controlling one drive motor. A boost circuit is connected between the battery and the inverter circuitry. A switch is designed to toggle the connection state between the inverter circuitry and the boost circuitry between series and parallel connections.

[0009] DE 10 2017 212 853 A1 discloses a highly integrated converter system for a motor vehicle having a powertrain, wherein the powertrain has a first electric motor and at least one second electric motor. The highly integrated converter system has a first inverter for the first electric motor and a second inverter for the second electric motor. In the highly integrated converter system, the first inverter, having two input terminals, is connected to two potentials of a battery, and the two input terminals of the second inverter can be connected in parallel with the input terminals of the first inverter via a switch.

[0010] The purpose of this invention is to provide a novel electric drive system, a novel method for operating the electric drive system, and a novel electric drive vehicle.

[0011] According to the present invention, this objective is achieved by an electric drive system having the features of claim 1, a method for operating the electric drive system having the features of claim 3, and an electric drive vehicle having the features of claim 8.

[0012] Advantageous designs of the present invention are the subject of the dependent claims.

[0013] An electric drive system for a vehicle is proposed, comprising: at least two motors for driving the vehicle, each motor having three stator windings; at least one high-voltage battery; and two inverters for converting the DC voltage of the high-voltage battery into AC voltage to power one of the motors. According to the invention, the inverters can be connected in series with each other and can be controlled and / or regulated such that one of the inverters and its associated motor can operate as a bidirectional DC / DC converter to supply a voltage lower than the DC voltage of the high-voltage battery to the other inverter, or to recover energy from the motor connected to the other inverter.

[0014] According to the present invention, the first inverter has two input terminals, each of which is connected to one of the two high-voltage potentials of the high-voltage battery. The second inverter has two input terminals, one of which is also connected to one of the high-voltage potentials. The other input terminal of the second inverter can be selectively connected to the first high-voltage potential of the high-voltage battery via a first switching element. The first inverter has three half-bridges, each consisting of two semiconductor switches connected in series between the input terminals, i.e., one high-side switch and one low-side switch. Each of these half-bridges has a center tap, which is connected to one of the stator windings of the motor. A second switching element is connected to the center tap of one of these half-bridges, and this second switching element is connected to the same input terminal of the second inverter, just like the first switching element.

[0015] These switching elements can be designed, for example, as contactors, relays, or semiconductor switches.

[0016] In one implementation, the semiconductor switch is designed as a MOSFET or an IGBT with a freewheeling diode.

[0017] In addition, intermediate circuit capacitors can be connected between the input terminals of the inverter.

[0018] The second inverter can also have three half-bridges, each consisting of two semiconductor switches connected in series between the input terminals. Each of these semiconductor switches has a center tap, which is connected to one of the stator windings of the motor.

[0019] According to one aspect of the invention, a method for operating the aforementioned electric drive system is provided. According to the invention, in order to operate at higher power and / or speed, a first switching element is in a closed state or is closed, and a second switching element is in an open state or is open, wherein, in order to operate at lower power, the first switching element is in an open state or is open, and the second switching element is in a closed state or is closed.

[0020] By opening the first switching element and closing the second switching element, the first inverter and the motor connected thereto can be used as a bidirectional buck DC / DC converter. The output voltage reduced at the center tap of the half-bridge connected to the second switching element is used as the input voltage of the second inverter. In this way, the DC operating voltage at the second drive unit, which consists of the second inverter and the second motor, is reduced during operation.

[0021] In one embodiment, a first switching element is connected to the input of a first inverter, and its high-side switch is connected to the input of the first inverter. When the second switching element is closed and the first switching element is open, during buck operation, at least one high-side switch of the half-bridge not connected to the second switching element is operated in a clock manner. Alternatively, the first switching element may be connected to the input of the first inverter, and its low-side switch is connected to the input of the first inverter. When the second switching element is closed and the first switching element is open, during buck operation, at least one low-side switch of the half-bridge not connected to the second switching element is operated in a clock manner.

[0022] In one embodiment, a first switching element is connected to the input of a first inverter, and its high-side switch is connected to the input of the first inverter. In the boost operation, when the second switching element is closed and the first switching element is open, at least one low-side switch of the half-bridge not connected to the second switching element is operated in a clock manner. Alternatively, the first switching element is connected to the input of the first inverter, and its low-side switch is connected to the input of the first inverter. In the boost operation, when the second switching element is closed and the first switching element is open, at least one high-side switch of the half-bridge not connected to the second switching element is operated in a clock manner.

[0023] In one implementation, to reduce losses in a half-bridge (where one of the semiconductor switches operates in a clocked manner), when the clocked semiconductor switch is off and current is detected flowing through the freewheeling diode or body diode of the other semiconductor switch, the other semiconductor switch can be turned on.

[0024] In one implementation, the two high-side switches or the two low-side switches of the two half-bridges not connected to the first switching element can be operated in a time-biased clock manner.

[0025] According to one aspect of the invention, an electrically driven vehicle is provided, the electrically driven vehicle having the above-described electric drive system having two drive units, each of the two drive units including an inverter of an inverter and a motor of a motor.

[0026] In one embodiment, each of the two drive units may be individually coupled to and disconnected from the drive wheels and / or transmission via a clutch.

[0027] According to the invention, during operation, the DC operating voltage at the other inverter is reduced via one inverter and the motor connected to that inverter. This is possible if the vehicle is moving at low power and / or low speed (e.g., in urban traffic and on suburban roads) because, on the one hand, only one drive unit is used for operation, and on the other hand, the induced voltage in the other drive unit remains low. By reducing the voltage, switching losses in the other drive unit are reduced. Therefore, the solution according to the invention makes it possible to operate the electrically driven vehicle more efficiently at low operating voltages without affecting the behavior of the drive unit.

[0028] Embodiments of the present invention will now be explained in more detail with reference to the accompanying drawings.

[0029] in: Figure 1 A schematic view of an electric drive system for an electric vehicle; Figure 2 This is a schematic view of an electric drive system in buck operation; and Figure 3 This is a schematic view of the electric drive system during boost operation.

[0030] Corresponding parts in all the accompanying figures are labeled with the same reference numerals.

[0031] Figure 1 This is a schematic view of an electric drive system 1 for an electrically driven vehicle. The electric drive system 1 includes: at least one energy storage device 2, particularly a high-voltage battery 2; and at least two motors 3.1, 3.2, each having three stator windings L1, L2, L3 and L4, L5, L6, which can supply energy from the high-voltage battery 2 to these stator windings via an inverter 4.1, 4.2 to drive the vehicle.

[0032] High-voltage battery 2 has a positive high-voltage potential HV+ and a negative high-voltage potential HV-.

[0033] The vehicle can be at least partially electric, such as a hybrid vehicle or an electric vehicle, particularly a passenger car, commercial vehicle, or bus.

[0034] The motors 3.1 and 3.2 used to drive the vehicle can each be designed as AC motors. Specifically, these AC motors are electric motors. In particular, the three-phase motor can operate in motor mode, and therefore functions as an electric motor. To enable the three-phase motor to operate in motor mode, AC voltage, especially high-voltage AC voltage, can be supplied to it via its phases. The phases of the AC motor can be interconnected, for example, via a common star point.

[0035] In order to supply AC voltage to motors 3.1 and 3.2, the electric drive system 1, and therefore the vehicle, has at least one high-voltage battery 2. The high-voltage battery 2 can supply electrical energy to motors 3.1 and 3.2, and, where appropriate, to other vehicle components and / or vehicle systems and / or the on-board electrical network.

[0036] The high-voltage battery 2 can provide battery voltage. Specifically, the vehicle can be a battery-powered vehicle with a voltage of 800 volts. Here, the battery voltage can provide a voltage of approximately 800 volts.

[0037] Motors 3.1 and 3.2 require AC voltage for their operation. This AC voltage can be provided by corresponding inverters 4.1 and 4.2. Here, the battery voltage is converted to AC voltage. Specifically, the AC voltage for motors 3.1 and 3.2 is provided by the corresponding main or primary functions of inverters 4.1 and 4.2.

[0038] For example, inverters 4.1 and 4.2 can be connected or arranged between high-voltage battery 2 and the corresponding motors 3.1 and 3.2.

[0039] Inverters 4.1 and 4.2, together with their respective connected motors 3.1 and 3.2, form a drive unit. These two drive units can be located nearby, for example, mounted on the front and / or rear axles. The type of inverters 4.1 and 4.2 (e.g., two-level B6 type, three-level T type, NPC type, flying capacitor type, etc.) is irrelevant. It can be configured such that each of the two drive units can be individually disconnected from the drive wheel and / or transmission via a clutch. The first inverter 4.1 has two input terminals E1 and E2, each connected to one of the high-voltage potentials HV+ and HV- of the high-voltage battery 2. The second inverter 4.2 also has two input terminals E3 and E4, one of which (input terminal E4) is also connected to one of the high-voltage potentials HV+ and HV- (input terminal HV-). The other input terminal E3 is selectively connected to the first high-voltage potential HV+ or HV- of the high-voltage battery 2 via a first switching element S1 (e.g., a contactor, relay, or semiconductor switch).

[0040] The first inverter 4.1 has three half-bridges HB1, HB2, and HB3, each consisting of two semiconductor switches connected in series between high-voltage potentials HV+ and HV-, i.e., one high-side switch HS1 to HS3 and one low-side switch LS1 to LS3. Each of these semiconductor switches has a center tap, which is connected to one of the stator windings L1 to L3 of the motor 3.1. Additionally, an intermediate circuit capacitor C1 can be connected between the high-voltage potentials HV+ and HV-. A second switching element S2 (e.g., a contactor, relay, or semiconductor switch) is connected to the center tap of one of the three half-bridges HB1 to HB3 (here, HB3). The second switching element S2 is connected to the same input terminal E3 of the second inverter 4.2, just like the first switching element S1. Through the second switching element S2, the input terminal E3 of the second inverter 4.2 can be coupled to the center tap of half-bridge HB3 of the first inverter 4.1.

[0041] By opening switch element S1 and closing switch element S2, the first inverter 4.1 and its connected motor 4.1 can be used as a bidirectional buck DC / DC converter. The reduced output voltage at the center tap of half-bridge HB3 is used as the input voltage of the second inverter 4.2. This reduces the DC operating voltage at the second drive unit, which consists of the second inverter 4.2 and the second motor 3.2, during driving, significantly reducing switching losses in inverter 4.2. The result is improved efficiency during driving. This is possible, for example, when the vehicle is moving at low power and / or low speed (e.g., urban traffic, suburban road driving, slow highway driving).

[0042] The second inverter 4.2 can also have three half-bridges HB4, HB5, and HB6, each consisting of two semiconductor switches connected in series between the input terminals E3 and E4. Each of these semiconductor switches has a center tap, which is connected to one stator winding L4 to L6 of the motor 3.2. In addition, an intermediate circuit capacitor C2 can be connected between the high-voltage input terminals E3 and E4.

[0043] Each semiconductor switch in a semiconductor switch can be designed, for example, as a MOSFET or an IGBT with a freewheeling diode.

[0044] In the first state, switching element S1 is closed and switching element S2 is open. This allows the first inverter 4.1 and the second inverter 4.2 to be directly coupled to the high-voltage battery 2. The two inverters 4.1 and 4.2 can act as drive inverters. The DC operating voltages of the two inverters 4.1 and 4.2 correspond to the battery voltage of the high-voltage battery 2 (minus the voltage drop across the battery's internal resistance or the circuit). This corresponds to the typical operation of inverters 4.1 and 4.2.

[0045] In the second state, switching element S1 is open and switching element S2 is closed. The first inverter 4.1, together with the inductance of at least one stator winding of the motor 3.1, particularly the inductance of two or three stator windings L1 to L3, constitutes a DC / DC converter. This DC / DC converter uses its output voltage to control and / or regulate the operating voltage of the second inverter 4.2. The input voltage of the first inverter 4.1 corresponds to the battery voltage. The first inverter 4.1 can transfer power from the high-voltage battery 2 to the second inverter 4.2, and also can transfer power from the second inverter 4.2 back to the high-voltage battery 2. Therefore, the first inverter is bidirectional. Therefore, these two operating states will be described in more detail below.

[0046] Figure 2 This is a schematic view of the electric drive system 1 in a second state during buck operation, where power is transferred from the high-voltage battery 2 to the second inverter 4.2 (as shown). Figure 1 (As shown).

[0047] In this example, the high-side switch HS1 of the half-bridge HB1, which is not connected to the switching element S2, is operated in a clock-mode, and the output voltage is drawn from the center tap of the third half-bridge HB3. When the high-side switch HS1 is closed, the current I1 flows from the high-voltage battery 2 (positive high-voltage potential HV+) through the stator windings L1 and L3 to the second inverter 4.2, and then returns from there to the high-voltage battery 2 through the negative high-voltage potential HV-. Since the voltage across the intermediate circuit capacitor C2 of the second inverter 4.2 is lower than the battery voltage of the high-voltage battery 2 in this example, the current flowing through the stator windings L4 to L6 and the voltage of the intermediate circuit capacitor C2 of the second inverter 4.2 increase over time. If a predetermined upper limit value is reached here (e.g., a target voltage upper limit at the second inverter 4.2 or a target current upper limit for the choke current flowing through one of the stator windings L1 to L3), the high-side switch HS1 is opened. The energy stored in the inductors of stator windings L1 and L3 further drives the current flow in the inductors. Here, the energy gradually dissipates, and the current decreases accordingly. This freewheeling path of current I2 does not pass through the high-voltage battery 2, but rather through the freewheeling diode or body diode of the low-side switch LS1 of the same half-bridge HB1, in which a clock-operated high-side switch HS1 is arranged. To reduce losses, the low-side switch LS1 can be turned on once current flows through its freewheeling diode or body diode.

[0048] In this example, the high-side switch HS2 of half-bridge HB2 can also be turned on in parallel with the high-side switch HS1. This increases the total power transmission capacity. Here, in addition to the low-side switch LS1, the choke freewheeling can also be additionally implemented in parallel via the low-side switch LS2 of the other half-bridge HB2. Furthermore, interleaved operation, i.e., clock operation that time-biases the high-side switches HS1 and HS2, can be envisioned. This can reduce the current ripple of the high-voltage battery 2 and the current ripple at the output of the second inverter 4.2. In principle, the design of the first inverter 4.1 can freely choose which half-bridge from HB1 to HB3 to draw the output voltage for the second inverter 4.2.

[0049] Figure 3 This is a schematic view of the electric drive system 1 in a second state during boost operation, where power is transferred from the second inverter 4.2 to the high-voltage battery 2.

[0050] Power flows from the second inverter 4.2 to the high-voltage battery 2 via the first drive unit (consisting of the first inverter 4.1 and the first motor 3.1), which is used as a DC / DC converter, for example when the vehicle recovers energy. In this case, the voltage across the intermediate circuit capacitor C2 of the second inverter 4.2 is lower than the battery voltage. In the operating mode shown, the low-side switch LS1 is operated in a clock-driven manner. With the second switching element S2 on (switching element S1 off), the voltage of the second inverter 4.2 is applied to the center tap of the third half-bridge HB3 of the first inverter 4.1. When the low-side switch LS1 is closed, current I1 flows from the second inverter 4.2 (with a positive potential at input terminal E3) through the stator windings L3 and L1, the closed low-side switch LS1, and the negative high-voltage potential HV- back to the second inverter 4.2. In this case, the current flowing through the stator windings L3 and L1 increases over time, while the voltage across the intermediate circuit capacitor C2 of the inverter 4.2 decreases over time. If a limit is reached (e.g., a predetermined target lower voltage limit at the second inverter 4.2 or a target upper current limit for the choke current flowing through stator windings L1 and L3), the low-side switch LS1 is disconnected. The energy stored in the inductors of stator windings L1 and L3 further drives the current flow in the inductors. Here, the energy gradually dissipates, and the current I2 decreases accordingly. This freewheeling path of current I2 passes through the freewheeling diode or body diode of the high-side switch HS1 and through the high-voltage battery 2. Once current flow through the body diode or freewheeling diode is detected, the high-side switch HS1 can be turned on to reduce losses.

[0051] In this example, the low-side switch LS2 of half-bridge HB2 can also be turned on in parallel with the low-side switch LS1. This increases the total power transmission capacity. Here, in addition to the high-side switch HS1, the choke freewheeling can also be additionally implemented via the high-side switch HS2 of the other half-bridge HB2 in parallel. Furthermore, interleaved operation, i.e., clock operation that time-biases the low-side switches LS1 and LS2, can be envisioned. This can reduce the current ripple of the high-voltage battery 2 and the current ripple at the output of the second inverter 4.2. In principle, the design of the first inverter 4.1 can freely choose which half-bridge from HB1 to HB3 to draw the output voltage for the second inverter 4.2.

[0052] List of reference signs 1. Drive System 2. Energy storage device, high-voltage battery 3.1, 3.2 Motors 4.1, 4.2 Inverters C1, C2 intermediate circuit capacitors E1 to E4 input terminals HB1 to HB6 half-bridge HS1 to HS6 high-side switches HV+ high voltage potential, positive high voltage potential HV - High voltage potential, negative high voltage potential I1, I2 currents LS1 to LS6 low-side switches L1, L2, L3, L4, L5, L6 stator windings S1 and S2 switching elements

Claims

1. An electric drive system (1) for a vehicle, the electric drive system comprising: at least two motors (3.1, 3.2) for driving the vehicle, each motor having three stator windings (L1 to L6); at least one high-voltage battery (2); and two inverters (4.1, 4.2) for converting the DC voltage of the high-voltage battery (2) into AC voltage to power one of the motors (3.1, 3.2). in, The inverters (4.1, 4.2) can be connected in series with each other, and the first inverter (4.1) has two input terminals (E1, E2), each of which is connected to one of the two high-voltage potentials (HV+, HV-) of the high-voltage battery (2). The second inverter (4.2) has two input terminals (E3, E4), one of which is also connected to one of the high-voltage potentials (HV+, HV-) of the high-voltage battery (2). The other input terminal (E3, E4) of the second inverter (4.2) can be selectively connected to the first high-voltage potential (HV+, HV-) of the high-voltage battery (2) via a first switching element (S1). Its features are, The inverters (4.1, 4.2) can be controlled and / or regulated such that one of the inverters (4.1, 4.2) and its connected motor (3.1, 3.2) can operate as a bidirectional DC / DC converter to supply a voltage lower than the DC voltage of the high-voltage battery (2) to the other inverter (4.1, 4.2), or to recover energy from the motor (3.1, 3.2) connected to the other inverter (4.1, 4.2). The first inverter (4.1) has three half-bridges (HB1, HB2, HB3), each consisting of two semiconductor switches connected in series between the input terminals (E1, E2), namely one high-side switch (HS1 to HS3) and one low-side switch (LS1 to LS3). Each semiconductor switch has a center tap, and connected to the center tap is one stator winding (L1 to L3) of the motor (3.1). The center tap of one of the half-bridges (HB1 to HB3) is connected to a second switching element (S2), which is connected to the same input terminal (E3) of the second inverter (4.2) as the first switching element (S1).

2. The electric drive system (1) according to claim 1. Its features are, The semiconductor switch is designed as a MOSFET or an IGBT with a freewheeling diode.

3. A method of operating the electric drive system (1) according to any one of the preceding claims, Its features are, To operate at higher power, the first switching element (S1) is in a closed state or the first switching element is closed, and the second switching element (S2) is in an open state or the second switching element is open, wherein to operate at lower power, the first switching element (S1) is in an open state or the first switching element is open, and the second switching element (S2) is in a closed state or the second switching element is closed.

4. The method according to claim 3, Its features are, The first switching element (S1) is connected to the input terminal (E1) of the first inverter (4.1). The high-side switches (HS1 to HS3) of the first inverter are connected to the input terminal of the first inverter. When the second switching element (S2) is closed and the first switching element (S1) is open, in buck operation, the high-side switches (HS1, HS2) of at least one half-bridge (HB1, HB2) not connected to the second switching element (S2) are operated in a clock manner. Alternatively, the first switching element (S1) is connected to the input terminal (E2) of the first inverter (4.1). The low-side switches (LS1 to LS3) of the first inverter are connected to the input terminal of the first inverter. When the second switching element (S2) is closed and the first switching element (S1) is open, in buck operation, the low-side switches (LS1, HS3) of at least one half-bridge (HB1, HB2) not connected to the second switching element (S2) are operated in a clock manner. LS2).

5. The method according to claim 3 or 4, Its features are, The first switching element (S1) is connected to the input terminal (E1) of the first inverter (4.1), and the high-side switches (HS1 to HS3) of the first inverter are connected to the input terminal of the first inverter. When the second switching element (S2) is closed and the first switching element (S1) is open, during boost operation, the low-side switches (LS1, LS2) of at least one half-bridge (HB1, HB2) not connected to the second switching element (S2) are operated in a clock manner. Alternatively, the first switching element (S1) is connected to the input terminal (E2) of the first inverter (4.1), and the low-side switches (LS1 to LS3) of the first inverter are connected to the input terminal of the first inverter. When the second switching element (S2) is closed and the first switching element (S1) is open, during boost operation, the high-side switches (HS1, LS2) of at least one half-bridge (HB1, HB2) not connected to the second switching element (S2) are operated in a clock manner. HS2).

6. The method according to claim 4 or 5, Its features are, In a half-bridge (HB1, HB2) where one of the semiconductor switches operates in a clocked manner, when the clocked semiconductor switch is off and a current is detected flowing through the freewheeling diode or body diode of the other semiconductor switch, the other semiconductor switch is turned on.

7. The method according to any one of claims 4 to 6, Its features are, The two high-side switches (HS1, HS2) or the two low-side switches (LS1, LS2) of the two half-bridges (HB1, HB2) that are not connected to the first switching element (S2) are operated in a clock-biased manner.

8. An electrically driven vehicle, Its features are, According to any one of claims 1 or 2, the electric drive system (1) has two drive units, each including one of the inverters (4.1, 4.2) and one of the motors (3.1, 3.2), wherein one of the inverters (4.1, 4.2) and the motor (3.1, 3.2) connected thereto can operate as a bidirectional DC / DC converter of the other inverter (4.1, 4.2).

9. The electric vehicle according to claim 8, Its features are, Each of the two drive units can be individually coupled to and disconnected from the drive wheels and / or transmission via a clutch.

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