Electric drive system having two inverters which can be connected in parallel or in cascade on the DC side

Abstract

The invention relates to an electric drive system (1) for a vehicle, comprising at least two electric machines (3.1, 3.2), each electric machine having three stator windings (L1 to L6) for driving the vehicle; at least one high-voltage battery (2); and two inverters (4.1, 4.2) for converting a DC voltage of the high-voltage battery (2) into an AC voltage for supplying one of the electric machines (3.1, 3.2) in each case, wherein the inverters (4.1, 4.2) can be connected in series with one another and can be controlled and / or regulated in such a way that one of the inverters (4.1, 4.2) can be operated, together with the electric machine (3.1, 3.2) connected thereto, as a bidirectional DC / DC converter for supplying the other inverter (4.1, 4.2) with a voltage which is lower than the DC voltage of the high-voltage battery (2) or for recovering energy from the electric machine (3.1, 3.2) connected to the other inverter (4.1, 4.2).

Description

[0001] ELECTRIC DRIVE SYSTEM WITH TWO DC-SIDED PARALLEL OR CASCADED SWITCHABLE INVERTERS

[0002] The invention relates to an electric drive system according to the preamble of claim 1, a method for its operation according to the preamble of claim 3 and an electrically driven vehicle according to the preamble of claim 8.

[0003] When driving electrically powered vehicles with low power and / or speeds, the efficiency in the inverter is determined by the switching losses, which depend on the DC operating voltage.

[0004] Losses in the inverter are divided into switching losses and conduction losses. Switching losses depend on the high-voltage DC voltage being switched; that is, the higher this voltage, the greater the switching losses. Furthermore, a certain amount of switching energy is released as heat during each switching operation. This also shows that the switching power loss of the inverter during operation is proportional to the inverter's clock frequency.

[0005] The following measures are known in the state of the art to reduce losses:

[0006] Faster switching on and off of semiconductors:

[0007] The use of new semiconductors such as SiC and GaN transistors in particular allows for faster switching on and off, reducing the product of the existing current and the applied voltage over the switching time. The disadvantage is that faster switching processes generate higher-frequency EMC interference and place greater strain on the insulation of the stator windings. Furthermore, SiC semiconductors (mainly used for 800V HV DC voltages) are considerably more expensive than IGBTs. - Varying the inverter's DC operating voltage using an upstream DC / DC converter: The converter allows the inverter's operating voltage to be optimized depending on the operating point, thus achieving an efficiency advantage in the inverter. The disadvantage is that this additional device is large, heavy, and expensive.

[0008] - Reducing the switching frequency at certain operating points: By reducing the switching frequency at certain operating points, a smaller number of switching operations are performed per unit of time. This automatically leads to a reduction in switching losses (on-state losses remain unchanged). The disadvantage of this is that lowering the switching frequency increases the voltage swing at the DC link capacitor (increases the AC component). This leads to a larger voltage ripple generated by the inverter. Either all other components are designed to function at increased voltage ripple, or the DC link capacitor in the inverter must be increased in size to still meet the existing ripple requirements at the reduced switching frequency.

[0009] DE 102021 003 882 A1 describes an electric drive system for a vehicle, with

[0010] - a first three-phase electric machine and a second three-phase electric machine for driving at least one drive axle of the vehicle,

[0011] - an electrical energy storage device for supplying the first and second three-phase electric machines during driving operation of the vehicle, wherein a first inverter of the first three-phase electric machine and a second inverter of the second three-phase electric machine are each coupled to the electrical energy storage device, and

[0012] - a vehicle-side charging connection for electrically coupling the electrical energy storage device with a vehicle-external charging unit, wherein

[0013] - depending on the first and / or second inverter, a charging voltage of the vehicle-side charging connection can be converted into a supply voltage for charging the electrical energy storage device.

[0014] DE 102022 134499 A1 discloses a vehicle drive system comprising a battery and at least one drive motor and an associated inverter circuit. The vehicle drive system has two drive motors, each inverter circuit configured to control a drive motor. A boost circuit is connected between the battery and the inverter circuits. A switch is configured to switch the connection state of the inverter circuits to the boost circuit between a series connection and a parallel connection.

[0015] DE 10 2017212 853 A1 discloses a highly integrated power converter system for a motor vehicle with a drive train, wherein the drive train comprises a first electric motor and at least one second electric motor. The highly integrated power converter system comprises a first inverter for the first electric motor and a second inverter for the second electric motor. In the highly integrated power converter system, the first inverter is connected to the two potentials of a battery via two inputs, and the two inputs of the second inverter can be connected in parallel with the inputs of the first inverter via switches.

[0016] The invention is based on the object of providing a novel electric drive system, a novel method for its operation and a novel electrically driven vehicle.

[0017] The object is achieved according to the invention by an electric drive system having the features of claim 1, a method for its operation having the features of claim 3 and an electrically driven vehicle having the features of claim 8.

[0018] Advantageous embodiments of the invention are the subject of the subclaims.

[0019] An electric drive system for a vehicle is proposed, comprising at least two electric machines, each with three stator windings, for driving the vehicle, at least one high-voltage battery, and two inverters for converting a direct voltage from the high-voltage battery into an alternating voltage to supply each of the electric machines. According to the invention, the inverters can be connected in series with one another and controlled and / or regulated such that one of the inverters, with the electric machine connected to it, can be operated as a bidirectional DC / DC converter for supplying the other inverter with a voltage that is lower than the direct voltage of the high-voltage battery or for recuperating energy from the electric machine connected to the other inverter.

[0020] According to the invention, a first inverter has two inputs, each of which is connected to one of two high-voltage potentials of the high-voltage battery, wherein the second inverter has two inputs, one of which is also connected to one of the high-voltage potentials, wherein the other input of the second inverter can be selectively connected to the first high-voltage potential of the high-voltage battery via a first switching element, wherein the first inverter has three half-bridges each consisting of two semiconductor switches connected in series between the inputs, that is to say one high-side switch and one low-side switch, each having a center tap to which a stator winding of the electric machine is connected, wherein a second switching element is connected to the center tap of one of the half-bridges and is connected to the same input of the second inverter as the first switching element.

[0021] The switching elements can, for example, each be designed as a contactor, a relay or a semiconductor switch.

[0022] In one embodiment, the semiconductor switch(es) is / are designed as a MOSFET or as an IGBT with a freewheeling diode.

[0023] Furthermore, an intermediate circuit capacitor can be connected between the inputs of the inverters.

[0024] The second inverter can also have three half-bridges, each consisting of two semiconductor switches connected in series between the inputs, each having a center tap to which a stator winding of the electric machine is connected.

[0025] According to one aspect of the present invention, a method for operating the electric drive system described above is proposed. According to the invention, for operation at higher power levels and / or speeds, the first switching element is or will be closed and the second switching element will be opened, while for operation at lower power levels, the first switching element is or will be opened and the second switching element will be closed.

[0026] By opening the first switching element and closing the second switching element, the first inverter, together with the electrical machine connected to it, can be used as a bidirectional buck DC / DC converter. Its reduced output voltage at the center tap of the half-bridge connected to the second switching element is used as the input voltage for the second inverter. Thus, while driving, the DC operating voltage on the second drive formed by the second inverter and the second electrical machine is reduced. In one embodiment, the first switching element is connected to the input of the first inverter, to which its high-side switch is connected. When the second switching element is closed and the first switching element is open, the high-side switch of at least one of the half-bridges to which the second switching element is not connected is operated in a clocked manner in buck operation.Alternatively, the first switching element can be connected to the input of the first inverter to which its low-side switches are connected, wherein, when the second switching element is closed and the first switching element is open, the low-side switch of at least one of the half-bridges to which the second switching element is not connected is operated in a clocked manner in a buck operation.

[0027] In one embodiment, the first switching element is connected to the input of the first inverter, to which its high-side switches are connected, wherein, when the second switching element is closed and the first switching element is open in boost mode, the low-side switch of at least one of the half-bridges to which the second switching element is not connected is operated in a clocked manner. Alternatively, the first switching element is connected to the input of the first inverter, to which its low-side switches are connected, wherein, when the second switching element is closed and the first switching element is open in boost mode, the high-side switch of at least one of the half-bridges to which the second switching element is not connected is operated in a clocked manner.

[0028] In one embodiment, in order to reduce losses in half-bridges, of which one of the semiconductor switches is operated in a clocked manner, the other semiconductor switch can be switched on when the clocked semiconductor switch is switched off and a current flow through a freewheeling diode or body diode of the other semiconductor switch is detected.

[0029] In one embodiment, two high-side switches or two low-side switches of two half-bridges not connected to the first switching element can be operated in a time-shifted manner.

[0030] According to one aspect of the present invention, an electrically powered vehicle is proposed, which comprises the above-described electric drive system with two drives, each comprising one of the inverters and one of the electric machines. In one embodiment, each of the two drives can be separately coupled to and detachable from a driven wheel and / or a transmission via a clutch.

[0031] According to the invention, while driving, one of the inverters and the electric machine connected to it reduce the DC operating voltage at the other inverter. This is possible when the vehicle is moving at low power levels and / or speeds (e.g., in city traffic and cross-country driving), since, on the one hand, only one drive is used, and, on the other hand, the induced voltage in the other drive is still low. The voltage reduction reduces the switching losses in the other drive. The inventive solution therefore enables more efficient operation of an electrically powered vehicle at lower operating voltages without affecting the behavior of the drive.

[0032] Embodiments of the invention are explained in more detail below with reference to drawings.

[0033] Showing:

[0034] Fig. 1 is a schematic view of an electric drive system for an electrically powered vehicle,

[0035] Fig. 2 is a schematic view of the electric drive system in buck mode, and

[0036] Fig. 3 is a schematic view of the electric drive system in boost mode.

[0037] Corresponding parts are provided with the same reference numerals in all figures.

[0038] Figure 1 is a schematic view of an electric drive system 1 for an electrically powered vehicle. The electric drive system 1 has at least one electrical energy storage device 2, in particular a high-voltage battery 2, and at least two electric machines 3.1, 3.2, each with three stator windings L1, L2, L3, and L4, L5, L6, which can be supplied with energy from the high-voltage battery 2 to drive the vehicle via an inverter 4.1, 4.2. The high-voltage battery 2 has a positive high-voltage potential HV+ and a negative high-voltage potential HV-.

[0039] The vehicle may be an at least partially electrically powered vehicle such as a hybrid vehicle or electric vehicle, in particular a passenger car, a commercial vehicle or a bus.

[0040] The electrical machines 3.1, 3.2 for driving the vehicle can each be designed as three-phase electrical machines. In particular, this three-phase electrical machine is an electric motor. In particular, the three-phase electrical machine can be operated in motor mode and thus as an electric motor. In order to operate the three-phase electrical machine in motor mode, the three-phase electrical machine can be supplied with an electrical alternating voltage, in particular with a high-voltage electrical alternating voltage, via its phases. The phases of the three-phase electrical machine can be connected to one another, for example, via a common star point.

[0041] To ensure that the electric motors 3.1, 3.2 can be supplied with an alternating voltage, the electric drive system 1, and thus the vehicle, has at least one high-voltage battery 2. Using the high-voltage battery 2, the electric motors 3.1, 3.2 and, if applicable, other vehicle components and / or vehicle systems and / or on-board electrical systems can be supplied with electrical energy.

[0042] A battery voltage can be provided using the high-voltage battery 2. In particular, the vehicle can be a battery-powered vehicle with a voltage level of 800 volts. A voltage of essentially 800 volts can be provided using the battery voltage.

[0043] The electric machines 3.1, 3.2 require an alternating voltage for their operating state. This alternating voltage can be provided by the respective inverter 4.1, 4.2. This involves converting the battery voltage into an alternating voltage. In particular, the alternating voltage for the electric machines 3.1, 3.2 is provided by a respective primary function or main function of the inverter 4.1, 4.2. For example, the inverter 4.1, 4.2 can be connected or arranged between the high-voltage battery 2 and the respective electric machine 3.1, 3.2.

[0044] The inverters 4.1, 4.2 form a drive with the respective connected electric machine 3.1, 3.2. The two drives can be located in close proximity, for example, by being installed on a front axle and / or a rear axle. It is irrelevant which type of inverter 4.1, 4.2 is used (2-Level B6, 3-Level T-Type, NPC, Flying Cap, etc.). It can be provided that each of the two drives can be separated separately from a driven wheel and / or transmission via a clutch. The first inverter 4.1 has two

[0045] Inputs E1, E2, each of which is connected to one of the high-voltage potentials HV+, HV- of the high-voltage battery 2. The second inverter 4.2 also has two inputs E3, E4, one of which, here input E4, is also connected to one of the

[0046] High-voltage potentials HV+, HV- (here HV-) are connected. The other input E3 is optionally connected to the first high-voltage potential HV+, HV- of high-voltage battery 2 via a first switching element S1 (e.g., a contactor, a relay, or a semiconductor switch).

[0047] The first inverter 4.1 has three half-bridges HB1, HB2, HB3, each consisting of two semiconductor switches connected in series between the high-voltage potentials HV+, HV-, i.e., one high-side switch HS1 to HS3 and one low-side switch LS1 to LS3, each having a center tap to which a stator winding L1 to L3 of the electric machine 3.1 is connected. Furthermore, an intermediate circuit capacitor C1 can be connected between the high-voltage potentials HV+, HV-. A second switching element S2 (for example, a contactor, a relay, or a 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 E3 of the second inverter 4.2 as the first switching element S1. The input E3 of the second inverter 4.2 can be coupled to the center tap of the half-bridge HB3 of the first inverter 4.1 via the second switching element S2.

[0048] By opening switching element S1 and closing switching element S2, the first inverter 4.1, together with the connected electric motor 4.1, can be used as a bidirectional buck DC / DC converter. Its reduced output voltage at the center tap of half-bridge HB3 is used as the input voltage for the second inverter 4.2. Thus, while driving, the DC operating voltage at the second drive formed by the second inverter 4.2 and the second electric motor 3.2 is reduced, which significantly reduces the switching losses in its inverter 4.2. The result is an increase in driving efficiency. This is possible, for example, in situations where the vehicle is driven at low power levels and / or speeds (e.g., city traffic, cross-country driving, slower motorway driving).

[0049] The second inverter 4.2 can also have three half-bridges HB4, HB5, HB6, each consisting of two semiconductor switches connected in series between the inputs E3, E4. Each half-bridge has a center tap to which a stator winding L4 to L6 of the electric machine 3.2 is connected. Furthermore, an intermediate circuit capacitor O2 can be connected between the high-voltage potentials of the inputs E3, E4.

[0050] Each of the semiconductor switches can be designed, for example, as a MOSFET or as an IGBT with a freewheeling diode.

[0051] In a first state, switching element S1 is closed and switching element S2 is open. Thus, the first inverter 4.1 and the second inverter 4.2 are directly coupled to the high-voltage battery 2. Both inverters 4.1, 4.2 can function as drive inverters. The DC operating voltage of both inverters 4.1, 4.2 corresponds to the battery voltage of the high-voltage battery 2 (minus any voltage drop across the internal resistance of the battery or across the cables, etc.). This corresponds to the typical operation of inverters 4.1, 4.2.

[0052] In a second state, the switching element S1 is open and the switching element S2 is closed. The first inverter 4.1 now provides the electrical

[0053] Machine 3.1 represents 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 is capable of transferring power from the high-voltage battery 2 to the second inverter 4.2 and vice versa, from the second inverter 4.2 to the high-voltage battery 2. It is therefore bidirectional. These two operating states are therefore described in more detail below. Figure 2 is a schematic view of the electric drive system 1 in the second state in buck operation, in which power is transferred from the high-voltage battery 2 to the second inverter 4.2 (shown in Figure 1).

[0054] In this example, the high-side switch HS1 of one of the half-bridges HB1, to which the switching element S2 is not connected, is operated in a pulsed manner, and the output voltage is tapped off at the center tap of the third half-bridge HB3. When the high-side switch HS1 is closed, a current I1 flows from the high-voltage battery 2 (positive high-voltage potential HV+) via the stator windings L1 and L3 to the second inverter 4.2 and from there via the negative high-voltage potential HV- back to the high-voltage battery 2. Since the voltage across the intermediate circuit capacitor C2 of the second inverter 4.2 in this example is lower than the battery voltage of the high-voltage battery 2, the current through the stator windings L4 to L6 and also the voltage of the intermediate circuit capacitor C2 of the second inverter 4.2 increases over time. If a specified upper limit is reached (for example, a target voltage upper limit on the second inverter 4.2 or a target current upper limit of a choke current through one of the stator windings L1 to L3), the high-side switch HS1 is opened. The energy stored in the inductances of the stator windings L1 and L3 drives the current flow in the inductances even further. The energy gradually dissipates and the current also reduces. This freewheeling path of the current I2 does not occur via the high-voltage battery 2 but via the freewheeling diode or body diode of the low-side switch LS1 of the same half-bridge HB1 in which the clocking high-side switch HS1 is arranged. To reduce losses, the low-side switch LS1 can be switched on as soon as a current flows through its freewheeling diode or body diode.

[0055] In this example, the high-side switch HS2 of the half-bridge HB2 can also be switched on in parallel with the high-side switch HS1. This increases the total current transmitted. The choke freewheeling can then take place via the low-side switch LS2 of the other half-bridge HB2 in addition to the low-side switch LS1. Interleaved operation is also conceivable, i.e. staggered clocking of the high-side switches HS1 and HS2. This way, the current ripple in the high-voltage battery 2 and also at the output to the second inverter 4.2 can be reduced. In principle, the design of the first inverter 4.1 can freely select which half-bridge HB1 to HB3 is used to tap the output voltage for the second inverter 4.2. Figure 3 is a schematic view of the electric drive system 1 in the second state in boost mode, in which power is transferred from the second inverter 4.2 to the high-voltage battery 2.

[0056] The power flows from the second inverter 4.2 via the first drive consisting of the first inverter 4.1 and the first electric machine 3.1, used as a DC / DC converter, to the high-voltage battery 2, for example during vehicle recuperation. 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 pulsing manner. The voltage from the second inverter 4.2 is present at the center tap of the third half-bridge HB3 of the first inverter 4.1 because the second switching element S2 is switched on (switching element S1 is open). When the low-side switch LS1 is closed, a current I1 flows from the second inverter 4.2 (positive potential at input E3) via 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. Over time, the current through the

[0057] Stator windings L3, L1 and the voltage of the intermediate circuit capacitor C2 of inverter 4.2 is reduced. If a limit is reached (for example, a specified lower target voltage limit on the second inverter 4.2 or an upper target current limit for the inductor current through the stator windings L1, L3), the low-side

[0058] Switch LS1 is open. The energy stored in the inductances of the stator windings L1 and L3 drives the current flow in the inductances even further. This energy gradually dissipates, and current I2 also decreases. This freewheeling path of current I2 runs through the freewheeling diode or body diode of the high-side switch HS1 and the high-voltage battery 2. The high-side switch HS1 can be switched on to reduce losses as soon as a current is detected via the body diode or freewheeling diode.

[0059] In this example, the low-side switch LS2 of the half-bridge HB2 can also be switched on in parallel with the low-side switch LS1. This increases the total transmitted current. The choke freewheel can then be switched on in parallel with the high-side switch HS1 and also via the high-side switch HS2 of the other half-bridge.

[0060] Half-bridge HB2. Interleaved operation is also conceivable, meaning staggered timing of the low-side switches LS1 and LS2. This reduces the current ripple at the high-voltage battery 2 and also at the output to the second inverter 4.2. In principle, the design of the first inverter 4.1 allows for the choice of which of the half-bridges HB1 to HB3 is used to tap the output voltage for the second inverter 4.2.

[0061] List of reference symbols

[0062] 1 drive system

[0063] 2 energy storage, high-voltage battery

[0064] 3.1, 3.2 electrical machine

[0065] 4.1, 4.2 Inverter

[0066] C1 , C2 DC link capacitor

[0067] E1 to E4 input

[0068] HB1 to HB6 half bridge

[0069] HS1 to HS6 high-side switches

[0070] HV+ high voltage potential, positive high voltage potential

[0071] HV high-voltage potential, negative high-voltage potential

[0072] 11 , I2 current

[0073] LS1 to LS6 low-side switch

[0074] L1, L2, L3, L4, L5, L6 stator winding

[0075] S1, S2 switching element

Claims

Mercedes-Benz Group AG Patent claims 1. Electric drive system (1) for a vehicle, with at least two electric machines (3.1, 3.2) each with three stator windings (L1 to L6) for driving the vehicle, at least one high-voltage battery (2) and two inverters (4.1, 4.2) for converting a direct voltage of the high-voltage battery (2) into an alternating voltage for supplying one of the electric machines (3.1, 3.2), wherein the inverters (4.1, 4.2) can be connected in series with one another and a first inverter (4.1) has two inputs (E1, E2), each of which is connected to one of two high-voltage potentials (HV+, HV-) of the high-voltage battery (2), wherein the second inverter (4.2) has two inputs (E3, E4), one of which is also connected to one of the high-voltage potentials (HV+, HV-) of the high-voltage battery (2), wherein the other input (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), characterized in that the inverters (4.1, 4.2) can be controlled and / or regulated in such a way that one of the inverters (4.1, 4.2) with the electrical machine (3.1, 3.2) connected thereto can be operated as a bidirectional DC / DC converter for supplying voltage to the other inverter (4.1, 4.2) with a voltage which is lower than the direct voltage of the high-voltage battery (2) or for recuperating energy from the electrical machine (3.1, 3.2) connected to the other inverter (4.1, 4.2), wherein the first inverter (4.1) three half-bridges (HB1, HB2, HB3) each comprising two semiconductor switches connected in series between the inputs (E1, E2), i.e. one high-side switch (HS1 to HS3) and one low-side switch (LS1 to LS3), each having a center tap to which a stator winding (L1 to L3) of the electrical machine (3.1) is connected, wherein a second switching element (S2) is connected to the same input (E3) of the second inverter (4.2) as the first switching element (S1) at the center tap of one of the half-bridges (HB1 to HB3).

2. Electric drive system (1) according to claim 1, characterized in that the semiconductor switch(es) is / are designed as a MOSFET or as an IGBT with a freewheeling diode.

3. Method for operating the electric drive system (1) according to one of the preceding claims, characterized in that for operation with higher powers the first switching element (S1) is or becomes closed and the second switching element (S2) is or becomes open, wherein for operation with lower powers the first switching element (S1) is or becomes open and the second switching element (S2) is or becomes closed.

4. Method according to claim 3, characterized in that the first switching element (S1) is connected to the input (E1) of the first inverter (4.1) to which its high-side Switches (HS1 to HS3) are connected, wherein when the second switching element (S2) is closed and the first switching element (S1) is open in a buck operation, the high-side switch (HS1, HS2) of at least one of the half-bridges (HB1, HB2), to which the second switching element (S2) is not connected, is operated in a clocking manner, or that the first switching element (S1) is connected to the input (E2) of the first inverter (4.1), to which its low-side Switches (LS1 to LS3) are connected, wherein when the second switching element (S2) is closed and the first switching element (S1) is open, the low-side switch (LS1, LS2) of at least one of the half-bridges (HB1, HB2) to which the second switching element (S2) is not connected is operated in a clocked manner in buck operation.

5. Method according to claim 3 or 4, characterized in that the first switching element (S1) is connected to the input (E1) of the first inverter (4.1) to which its high-side Switches (HS1 to HS3) are connected, wherein when the second switching element (S2) is closed and the first switching element (S1) is open in a boost mode, the low-side switch (LS1, LS2) switches at least one of the Half bridges (HB1, HB2) to which the second switching element (S2) is not connected, is operated in a clocked manner, or that the first switching element (S1) is connected to the input (E2) of the first inverter (4.1), to which its low-side switches (LS1 to LS3) are connected, wherein when the second switching element (S2) is closed and the first switching element (S1) is open in a boost operation, the high-side switch (HS1, HS2) of at least one of the half-bridges (HB1, HB2), to which the second switching element (S2) is not connected, is operated in a clocked manner.

6. Method according to claim 4 or 5, characterized in that in half bridges (HB1, HB2), of which one of the semiconductor switches is operated in a clocked manner, the other semiconductor switch is switched through when the clocked semiconductor switch is switched off and a current flow through a freewheeling diode or body diode of the other semiconductor switch is detected.

7. Method according to one of claims 4 to 6, characterized in that two high-side switches (HS1, HS2) or two low-side switches (LS1, LS2) of two half-bridges (HB1, HB2) not connected to the first switching element (S2) are operated in a time-offset manner.

8. Electrically driven vehicle, characterized by an electric drive system (1) according to one of claims 1 or 2 with two drives, each comprising one of the inverters (4.1, 4.2) and one of the electric machines (3.1, 3.2), wherein one of the inverters (4.1, 4.2) with the electric machine (3.1, 3.2) connected thereto can be operated as a bidirectional DC / DC converter of the other inverter (4.1, 4.2).

9. Electrically driven vehicle according to claim 8, characterized in that each of the two drives can be coupled to and separated from a driven wheel and / or a transmission separately via a clutch.

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