Power converter and control method for a power converter

Abstract

In a galvanically connected power converter or charging device having an active rectifier and H-bridge DC / DC converter at least part of the switching processes of rectifier and DC / DC converter are synchronized.

Description

[0001] 202304405 1 Description Power converter and control method for a power converter The invention relates to a galvanically non-isolated power converter with an active rectifier, a DC link, and a DC / DC converter. The invention further relates to an operating method for such a power converter. Power converters of the type mentioned are, for example, candidates for charging devices for electrically powered vehicles. They are typically two-stage power converters: in the active rectifier (“Active Front End”, AFE, 1st stage), the mains voltage is converted into DC voltage and stored in an intermediate circuit capacitor; in the DC-DC converter (2nd stage), the intermediate circuit voltage is converted into the voltage required by the load (vehicle battery) (charging voltage). Chargers that are not galvanically isolated do not require an isolating transformer between the mains and the vehicle’s high-voltage system.This design is advantageous in terms of the volume, weight, and cost of the charger. However, due to the high number of Y-capacitances installed in the vehicle, as well as parasitic capacitances, the coupling path to PE or earth is also significantly increased in these systems. The reason for this is the elimination of the series connection of the coupling capacitance of the isolation transformer with the aforementioned capacitances of the vehicle. During normal operation, this significantly increases the capacitive leakage currents to earth / PE, which can trigger the mains-side residual current device (RCD) and thus prevent the device from operating. During faulty operation, i.e., if the PE conductor breaks, detrimental touch voltages can build up on the body and / or the housing of the charger. For safety reasons, strict limits exist for the touch currents caused by this.The driving source of the touch currents is the common-mode voltage generated by the active rectifier. It depends on the specific design of the pulse-width control (PWM) and generally has two main components in the spectrum: at three times the fundamental frequency, i.e., at 150 Hz if the fundamental frequency is 50 Hz, and at the switching frequency of the active rectifier and DC / DC converter. Common-mode voltages can be counteracted, for example, by active and passive filters. However, these require the use of additional active and / or passive components, which has a detrimental effect on size and weight. The object of the invention is to provide a power converter of the type mentioned that reduces or avoids the disadvantages mentioned above.A further object is to specify an operating method for a power converter with which the disadvantages mentioned at the outset are reduced or avoided. This object is achieved by an operating method having the features specified in claim 1. A further solution consists in the power converter with the features of claim 10. In the operating method according to the invention for a power converter which comprises an active rectifier, a DC voltage intermediate circuit and a DC / DC converter, the active rectifier is controlled such that it changes between a plurality of rectifier switching states, wherein a change between two switching states brings about a change in the existing common-mode voltage.Furthermore, in order to generate a DC output voltage from the voltage of the intermediate circuit, the DC / DC converter switches between a first DC converter switching state and a second DC converter switching state, wherein this switch causes a change in the existing common-mode voltage. At least some of the switching operations of the DC / DC converter and rectifier within a switching period are controlled such that they occur at the same time. The power converter according to the invention comprises an active rectifier, a DC voltage intermediate circuit, a DC / DC converter and a control device for controlling the rectifier and the DC / DC converter. The control device is designed to control the active rectifier such that it switches between a plurality of rectifier switching states, wherein a switch between two switching states causes a change in the existing common-mode voltage.The control device is further configured to control the DC / DC converter such that, in order to generate a DC output voltage from the intermediate circuit voltage, it switches between a first DC converter switching state and a second DC converter switching state, wherein this switch causes a change in the existing common-mode voltage. Finally, the control device is configured to control at least some of the switching operations of the DC / DC converter and rectifier within a switching period such that they occur at the same time. In other words, the rectifier and the DC / DC converter are controlled such that some of their switching operations are synchronized.This advantageously ensures that jumps in the common-mode voltage, which occur at the switching frequency due to switching operations of the active rectifier, are at least partially compensated by the synchronized switching operations of the DC / DC converter. This takes advantage of the fact that a change in the switching state of the DC / DC converter also results in a jump in the generated common-mode voltage. The direction of the change in the switching state of the DC / DC converter is expediently selected such that the resulting jumps in the common-mode voltage compensate one another. The active rectifier can be a two-point rectifier which, in a manner known per se, comprises three half-bridges connected in parallel, each with two semiconductor switching elements such as IGBTs or MOSFETs. The semiconductor switching elements can comprise intrinsic parallel freewheeling diodes or parallel freewheeling diodes implemented as discrete components.The active rectifier can also have a different design. The DC link comprises one or more capacitors and is expediently connected in parallel to the external terminals of the half-bridges of the active rectifier. The control device expediently comprises one or more programmable control modules such as FPGAs and microcontrollers. It is connected to a higher-level controller from which it receives signals that determine the operating state for the power converter. The control device calculates times for switching operations for the semiconductor switching elements of the rectifier and the DC / DC converter and sends corresponding signals to gate driver units assigned to the semiconductor switching elements. Advantageous embodiments of the operating method and power converter according to the invention are set out in the dependent claims.The embodiment of the independent claims can be combined with the features of one of the subclaims or 202304405 5 preferably also with those of several subclaims. Accordingly, the following additional features can be provided: In an advantageous embodiment of the invention, the DC / DC converter is controlled such that for each switching operation of the rectifier it changes once, in particular exactly once, from a first DC converter switching state to a second DC converter switching state and back. As a result, each switching operation of the rectifier can also be contrasted with a switching operation of the DC / DC converter. It is advantageous if the active rectifier and the DC / DC converter are operated at the same switching frequency. This makes calculating the synchronized switching times for the DC / DC converter significantly easier than with different switching frequencies.In a further advantageous development of the invention, a first of the two switching states of the DC / DC converter has a plurality of switching options for representing the switching state, in which different common-mode voltages are generated. In a period in which the first switching state exists, a first switching option is used for a first portion of the period and a second switching option is used for the remaining, second portion of the period. The portions are determined such that the common-mode voltage generated by the DC / DC converter is equal in magnitude to a common-mode voltage generated by the rectifier. For this purpose, the DC / DC converter is expediently an H-bridge DC / DC converter with two half-bridges connected in parallel. This advantageously utilizes the fact that there is a degree of freedom in the control of the DC / DC converter.Assuming that exactly one of the two semiconductor switching elements of each half-bridge is always switched on, four different switching options result. In the two 202304405 6 switching options that belong to an on position, the diagonally arranged semiconductor switching elements of the two half-bridges are switched on. In this switching option, the intermediate circuit voltage is present at the output terminals, i.e. between the centers of the two half-bridges. However, there is no common-mode voltage in these two switching options. In the first switching state, which represents an off position, in the first switching option, the two upper semiconductor switching elements of the two half-bridges are switched on, and in the second switching option, the two lower semiconductor switching elements of the two half-bridges are switched on.This offers the advantageous use of a degree of freedom consisting of two different switching options for the first switching state, which differ in the common-mode voltage generated. The proportions of the time period during which the first and second switching options are used are preferably determined from the switching period of the DC / DC converter, the intermediate circuit voltage, the DC output voltage, and an instantaneous value for the common-mode voltage generated by the active rectifier. Advantageously, the switching period and the intermediate circuit voltage are fixed or known to the control device. Likewise, the DC output voltage, i.e. the charging voltage in the case of a battery in an electrically powered vehicle, is known to the control device, since it has to set this charging voltage. The instantaneous value for the common-mode voltage generated by the active rectifier can be determined from available control data.This instantaneous value corresponds to the average common-mode voltage determined over one switching period. The control device can thus calculate the components from known and available data and repeat this calculation for each switching period, so that the components in each switching period are adapted to the instantaneous common-mode voltage. A value averaged over one switching period of the rectifier can be determined as the instantaneous value for the common-mode voltage generated by the rectifier. This advantageously determines a single value for one switching period, which can then be used to control the DC / DC converter, also within one switching period. This avoids setting values ​​for the common-mode voltage that vary within one switching period.The instantaneous value can be determined using the switching period, the intermediate circuit voltage, and portions of the switching period assigned to the switching states of the active rectifier. The instantaneous level of the common-mode voltage of the rectifier can be advantageously determined and fixed using these values. The intermediate circuit voltage and the switching period of the active rectifier are known in the control device. The portions of the switching period of the rectifier are determined in the control system, as they are required to control the rectifier. It is expedient to separate the first portion of the period and the second portion of the period in time by placing a second period between them in which the second switching state is used.This avoids direct switching, and possibly even multiple switching between the switching options of the first state. The active rectifier and the DC / DC converter can be operated at the same switching frequency. The switching frequency used is significantly higher than the fundamental frequency of the AC input voltage. For example, the switching frequency can be 16 kHz, 25 kHz, or 48 kHz or more. 202304405 8 The power converter is particularly preferably a charger for an electrically powered vehicle. The invention is described and explained in more detail below with reference to the exemplary embodiments illustrated in the figures.They show: Figure 1 is an electrical circuit diagram of a galvanically non-isolated charger with an active rectifier and a DC / DC converter, Figure 2 is a time curve of the switching states of the active rectifier within a switching period, Figure 3 is a frequency representation of a resulting touch current in the event of a fault without compensation, Figure 4 is an example of synchronization of switching edges of the rectifier and the DC / DC converter, Figure 5 is a time curve of a low-frequency common-mode beat, Figure 6 is a first example of operation with compensation of both the common-mode beat and a synchronized switching sequence. Figure 7 is a second example of operation with compensation of both the common-mode beat and a synchronized switching sequence. Figure 8 is a frequency representation of a resulting touch current in the event of a fault with compensation.Figure 1 shows an electrical circuit diagram of a non-galvanically isolated charger 10 with an active rectifier 12 and a DC / DC converter 14. The charger 10 is connected on the input side to a three-phase supply voltage 16, for example, the 400 V local grid. The connection can be made, for example, via a so-called wall box, i.e., a high-current connection specifically designed for charging an electrically powered vehicle. The active rectifier 12 comprises, for each phase on the grid side, an inductor 18a…c and three half-bridges, each with two semiconductor switching elements 20a…f, for example, IGBTs or MOSFETs. The half-bridges are connected in parallel with their outer terminals in a known manner and connected with their center terminals to the inductors 18a…c. The active rectifier 12 thus formed is a two-point rectifier.The external terminals of the half-bridges are connected to a DC intermediate circuit 22. In this example, the DC intermediate circuit 22 comprises two series capacitors 23a, 23b, but in other examples, it can also have one or more capacitors. Between the capacitors 23a, 23b is a node 24, which is subsequently used as a reference point for common-mode voltages. The DC intermediate circuit 22 is further connected to the external terminals of two further half-bridges, each with two semiconductor switching elements 15a...d, which form the DC / DC converter 14. The DC / DC converter 14 is thus an H-bridge DC / DC converter. The center terminals of the further half-bridges are connected to an output filter 26. The output filter 26 comprises two inductors 27a, b and a capacitor 27c, which are connected between the two center terminals as an LCL series.The load, i.e., here the vehicle's accumulator, commonly referred to as the battery, is connected in parallel to the capacitor 27c. The charger 10 further comprises a control device, which is not shown in Figure 1. The control device is connected, on the one hand, to a higher-level controller, which forwards commands regarding the overall status of the charger 10 to the 202304405 10 control device. The control device itself calculates switching times for the semiconductor switching elements 15a...d, 20a...f used in the charger 10. At the determined switching times, signals are sent to gate driver units, also not shown in Figure 1, which then in turn cause the switching of the semiconductor switching elements 15a...d, 20a...f. The control device performs the calculation steps described below. These are the calculation steps for the pulse width modulation for the active rectifier 12.Their results are incorporated into the subsequent calculation steps for the switching operations of the DC / DC converter 14. Parasitic couplings to ground 28 are symbolized in Figure 1 by the capacitive elements 29a, 29b. The vehicle-side capacitive couplings to PE via Y capacitors are symbolized by the capacitive elements 30a and 30b. The body and the housing of the charger are assumed to be ideally grounded, i.e. the PE conductor and ground 28 have the same potential. A break in the PE conductor corresponds to the opening of the symbolic switch 31. Such a break allows the generation of capacitively coupled touch voltages and thus touch currents. The human body or its equivalent electrical circuit is arranged parallel to the symbolic switch 31 to represent touch voltages and touch currents. One component of such touch voltages arises from the switching operations of the active rectifier 12.As soon as the rectifier changes switching state and the common-mode voltage of the new state is different from the common-mode voltage of the previous state, a jump in the common-mode voltage occurs, which can be observed in a frequency response analysis (FFT) at the switching frequency and its harmonics. The time-dependent common-mode voltage, etc. CM0 is defined for the active rectifier 12 as: 202304405 11 u CM0 = (u R0 +u S0 +u T0 ) / 3 This includes R0 , and S0 , and u T0 Components of the common-mode voltage, which are also shown in Figure 1. These components are each either +U DC / 2 or –U DC / 2, depending on whether the upper or lower switching element of the respective half-bridge is switched on. In total, the common-mode voltage is therefore u CM0 = + / -U DC / 2, if all upper or all lower switching elements are switched on or + / -UDC / 6, when a mixture of upper and lower switching elements is switched on in the active rectifier 12. If the switching state with the upper switch switched on is designated as 1 and the switching state with the lower switch switched on as 0 and the switching states for the three half-bridges are written one after the other, the possible overall states, referred to as vectors, are as follows: Vector Switching states Common mode RST voltage / U DCV0000 -1 / 2 V1100 -1 / 6 V2110 +1 / 6 V3010 -1 / 6 V4011 +1 / 6 V5001 -1 / 6 V6101 +1 / 6 V7111 +1 / 2 The currently used switching state changes several times within a switching period, as described below. The exact course of the switching operations depends on the selected pulse width modulation for the active rectifier 12. Thus, the exact position of the switching edges in the common-mode voltage is also determined by the implementation of the PWM. 202304405 12 For the specific example of a symmetrical space vector PWM, a course within a switching period results, which is shown in simplified form in Figure 2. Figure 2 shows the individual components of the common-mode voltages u R0 , and S0 , and u T0 and the total common-mode voltage u CM0 over the time of a switching period. The switching period of length T = 1 / f with f = 48 kHz is divided into switching times, which are denoted by τ a , τ band τ0. In reality, these are not of equal length and depend on the current operating point of the rectifier 12. They result from the pulse width modulation used. The switching period is generally described as consisting of two zero vectors V0 and V 0‘ and two active vectors V a and V b In the case of the first sector, these are specifically the vectors V7 and V0 as well as V1 and V2. The switching times τ a , τ b and τ0 of the vectors are recalculated for each period by the PWM unit of the rectifier 12. Depending on the selected pulse width modulation of the rectifier 12, the magnitude of the possible jumps in the common-mode voltage varies; changes in u CM0 in the range of ±1 / 3 U DC up to ±U DCThe touch current, i.e. for the simulation the current through a body simulation with low-pass weighting, in the event of a fault (PE conductor break), i.e. when the symbolic switch 31 is open, manifests itself in the symmetrical space vector PWM as shown in the frequency domain in Figure 3. The switching frequency amplitude of approximately 0.47 A is clearly visible. A similar scheme can be set up for the switching operations of the H-bridge DC / DC converter 14, although this only has four different states since only two half-bridges are present. The common-mode voltage u CM,DC is calculated for the DC / DC converter 14 as follows: u CM,DC = (u A0 +u B0 ) / 2 202304405 13 The following switching states are possible with the DC / DC converter: State Switching states Common mode AB Voltage / U DCDC010 0 DC100 -1 / 2 DC211 +1 / 2 DC301 0 The DC / DC converter 14 therefore only forms a common-mode voltage amplitude in the two off states DC1 and DC2, i.e. when both half-bridges are switched up or down simultaneously. The transition from the active state DC0(u AB = U DC ) into the off states DC1 and DC2 also generates a change in the common-mode voltage u CM,DC of ±U DC / 2, a change from off state DC1 to DC2 or vice versa correspondingly even ±U DC If the change of the DC / DC converter 14 from state DC0 to DC1 or DC2 is synchronized with a common-mode jump of the rectifier 12, the switching frequency component of the common-mode voltage of the rectifier 12 can be minimized. The DC / DC converter 14 sets the intermediate circuit voltage U DC down to the voltage U required by the battery BATThis occurs by switching between the on state DC0 and an off state at a switching frequency, which in this example corresponds to that of the rectifier 12. In the on state DC0, the intermediate circuit voltage U DC at the output filter 26. In the off state DC1 / DC2, a voltage of 0 V is applied to the output filter 26. The required voltage is set by dividing the switching period T DC between a duration for the on state T ON and a duration for the off state T OFF (It is T DC = T ON + T OFF ). The share of ON by T DC , in which the on state is used, is calculated as d ON = U BAT / U DC , the share for the off state to d OFF = 1 – d ON . The durations are T ON = d ON * T DC and T OFF = d OFF * T DC. 202304405 14 In the on state DC0, no common-mode voltage can be generated. When switching to the off state, however, the jump in the common-mode voltage is set by selecting the off state used, DC1 or DC2. At the same time, staying in the respective off state results in an average common-mode voltage for the DC / DC converter 14. If this is to be 0 V on average, then the two off states DC1 and DC2 must be used in equal parts, i.e. T OFF1 = T OFF2 = T OFF / 2, where always: T OFF1 + T OFF2 = T OFF For synchronization, the off states DC1 and DC2 of the DC / DC converter 14 are divided into smaller intervals T OFF10 , T OFF1x , T OFF20 , T OFF2x divided in order to be able to counteract each common-mode voltage jump of the rectifier 12 with a common-mode voltage change of the DC / DC converter 14. Here, T OFF10 + T OFF1x = T OFF1 and T OFF20 + TOFF2x = T OFF2 Figure 4 shows a first example of how this synchronization is carried out. Here, the average common-mode voltage of the DC / DC converter is 140 V and the smaller intervals T OFF10 , T OFF1x , T OFF20 , T OFF2x are of equal length. The switching times τ a , τ b and τ0 are shown as being of equal length for the sake of simplicity, but are typically of different lengths and subject to constant change. As can be seen from Figure 4, each switching operation of the rectifier 12 is associated with a switching operation of the DC / DC converter 14, so that the switching-frequency jumps in the common-mode voltage are at least reduced. The amplitude of the jumps may not be the same; for example, a jump in the common-mode voltage of the rectifier 12 of 1 / 3 U DC a jump of the DC / DC converter 14 of 1 / 2 U DCIt is important to understand that the active rectifier 12 and the DC / DC converter 14 operate in opposite directions due to the selected common-mode definition with respect to the leakage current in the PE conductor. In order to compensate for the switching-frequency component of the current in the PE conductor, the synchronized voltage changes therefore always take place in the same direction. Depending on the operating point of the charger 10, two modes result for the DC / DC converter 14, between which the control device switches. If the entire off-time of the DC / DC converter 14 cannot be mapped within the zero vector phase τ0 of the AFE, the remaining off-time of the DC / DC converter 14 must be allocated to the switching times τ a and τ b of the active vectors of the rectifier 12. Mode T OFF2 T OFF1 T OFF20 T OFF2x T OFF10 T OFF1x 0 < τ0 / < τ0 / = 0.99 = 0.01 = T OFF1 2 2 T OFF2 TOFF2 3 >= τ0 / >= τ0 / = τ0 / = T OFF2 = τ0 / = T OFF1 2 2 2 - τ0 / 2 - τ0 / 2 2 A further improvement can be achieved if another component of the current in the PE conductor is taken into account. This is shown in Figure 5. This is a beat of common-mode voltages at three times the fundamental frequency, i.e. in this example at 150 Hz, starting from a fundamental frequency (mains frequency) of 50 Hz. This beat is also caused by the switching operations of the active rectifier 12. The instantaneous value of the common-mode beat e for the current switching period is calculated from the pulse pattern of the rectifier 12. This is done by comparing the phase output voltages u R0 , and S0 and u T0 of the rectifier 12 for each switching period as a voltage-time product u R0,Vt , and S0,Vt and u T0,Vt According to the definition for u CM0the mean value is then calculated: 202304405 16 After insertion and dissolution we get: Where S stands for the sector in the pulse width modulation and T AFEfor the period of one switching period of the active rectifier 12. The low-frequency common-mode beat manifests itself in a simulation as shown in Figure 5 as an approximately triangular wave. It can be seen that the amplitude is slightly more than 40 mA. The period is 1 / 150 Hz, i.e. approximately 6 ms. The use of the off states DC1 and DC2 of the DC / DC converter 14 provides a degree of freedom. If the off time of a switching period is evenly divided between the switching states DC1 and DC2, the DC / DC converter 14 will, on average, not generate a common-mode voltage for this switching period. An uneven distribution, on the other hand, generates a corresponding common-mode voltage value that exists on average over the switching period. This can be used to counteract and compensate for the low-frequency common-mode beat of the active rectifier 12.If e is not 0, then a suitable common-mode voltage is set so that the time periods T. OFF1 and T OFF2 for the off states, whereby the summed time for the off state (d OFF * T DC = T OFF = T OFF1 + T OFF2 ) must be maintained, as it is determined by the battery voltage. 202304405 17 The proportion of CM of the period that should be used to generate the required level of common-mode voltage is d CM = e / (0.5 * U DC ). The frequency of the beat is very low at 150 Hz compared to the switching frequency of, for example, 1 / T DC = 48 kHz. Therefore, the common-mode beat has a practically fixed instantaneous voltage value e for each switching period, which can be negative or positive. Depending on the sign of the common-mode voltage, the off-state DC1 or DC2 must be used for the component d CMof the period duration. The remaining part of the off state d OFF,RES = d OFF – d CM is divided equally between the two off states DC1 and DC2. This results in the following times for the two off states for a positive common-mode voltage to be balanced: ^^ைிி^ ൌ 0.5 ^^ைிிଶ ൌ ൫ ^^^ெ ^ 0,5 For a negative common-mode voltage to be compensated, the two off states must be exchanged, ie the following times for the two off states result: ^^ைிிଶ ൌ 0.5 ^^ைிி^ ൌ ൫ ^^^ ^ 0,5 If an additional compensation of the common-mode beat is carried out, then usually T T OFF2 This creates two additional operating modes for the DC / DC converter 14. The operating modes are as follows: 202304405 18 Mode T OFF2 T OFF1 T OFF20 T OFF2x T OFF10 T OFF1x 0 < τ0 / < τ0 / = 0.99 = 0.01 = T OFF1 2 2 TOFF2 T OFF2 1 < τ0 / >= τ0= 0.99 = 0.01 = τ0 / 2 = T OFF1 - 2 / 2 T OFF2 T OFF2 τ0 / 2 2 >= τ0< τ0 / = τ0 / 2 = T OFF2 - = T OFF1 / 2 2 τ0 / 2 3 >= τ0>= τ0= τ0 / 2 = T OFF2 - = τ0 / 2 = T OFF1 - / 2 / 2 τ0 / 2 τ0 / 2 Figure 6 shows a first example of operation with a compensation of both the common-mode beat shown in Figure 5 and a synchronized switching sequence. Here, mode 0 is shown, but T OFF1 T OFF2 . Depending on the application and operating point, further operating modes can be added, for example if the required common-mode voltage cannot be provided by the DC / DC converter 14 within one PWM period because the off-time T OFF is too short. Figure 7 shows a second example of operation, where the operating mode shown here is mode 3. The prerequisite for mode 3 is that the two components for the off-time TOFF1 and T OFF2 both are greater than τ0 / 2, so the required charging voltage is rather low. As can be seen in Figure 7, the time range V0' is used entirely for the off state DC1, whereby both switching edges for this range are directly synchronized between rectifier 12 and DC / DC converter 14. In addition, the DC / DC converter must switch to the DC1 state at other times. The same applies to the DC2 state. The result of the compensation is shown in Figure 8 again in frequency representation using the touch current in the event of a fault, i.e. when the symbolic switch 31 is open. The current flows via a hypothetical human body, i.e. in the simulation via its equivalent circuit. A significant reduction in the switching frequency component by around 66% from approx. 0.47 A to approx. 0.15 A can be seen. The third harmonic, i.e. the common-mode beat, is also practically completely eliminated.

[0002] 202304405 20 Reference numerals 10 Charger 12 Active rectifier 14 DC / DC converter 15a…d Semiconductor switch 16 Supply voltage 18a…c Inductance 20a…f IGBTs 22 DC link 23a, b Intermediate link capacitor 24 Node 26 Output filter 27a, b Filter inductance 27c Filter capacitor 28 Ground 29a, b Parasitic capacitance 30a, b Y capacitor PE Protective Earth U DC DC link voltage U BAT Battery voltage

Claims

202304405 21 claims 1. Operating method for a power converter (10) comprising an active rectifier (12), a DC voltage intermediate circuit (22) and a DC / DC converter (14), in which - the active rectifier (12) is controlled such that it changes between a plurality of rectifier switching states, wherein a change between two rectifier switching states causes a change in the existing common-mode voltage, - the DC / DC converter (14) changes between a first DC converter switching state and a second DC converter switching state to generate a DC output voltage from the voltage of the intermediate circuit (22), wherein this change causes a change in the existing common-mode voltage, - at least some of the switching operations of the DC / DC converter (14) and rectifier (12) within a switching period are controlled such that they occur at the same time. 2.Operating method according to claim 1, wherein the DC / DC converter (14) is switched once from a first DC converter switching state to a second DC converter switching state and back for each switching operation of the rectifier (12).

3. Operating method according to claim 1 or 2, wherein the active rectifier (12) and the DC / DC converter (14) are operated at the same switching frequency.

4. Operating method according to one of the preceding claims, wherein - a first of the two switching states of the DC / DC converter (14) comprises a plurality of switching options for representing the switching state, in which different common-mode voltages are generated, - in a time period (T. OFF ), in which the first switching state is present, a first switching possibility (DC1) for a first 202304405 22 Share of period (T OFF1 ) and a second switching option (DC2) for the remaining, second part of the period (T OFF2) is used, - the shares (T OFF1 , T OFF2 ) are determined such that the common-mode voltage generated by the DC / DC converter (14) is equal in magnitude to a common-mode voltage generated by the rectifier (12).

5. Operating method according to claim 4, wherein the components (T OFF1 , T OFF2 ) from the switching period (T DC ) of the DC / DC converter (14), the intermediate circuit voltage (U DC ), the DC output voltage and an instantaneous value (e) for the common-mode voltage generated by the active rectifier (12).

6. Operating method according to claim 4 or 5, wherein the instantaneous value (e) for the common-mode voltage generated by the rectifier (12) is a switching period (T AFE ) of the rectifier (12) is determined.

7. Operating method according to claim 6, wherein the instantaneous value (e) is determined using the switching period (T AFE ), the intermediate circuit voltage (U DC) and portions of the switching period duration that are assigned to switching states of the active rectifier (12).

8. Operating method according to one of claims 4 to 7, wherein the first portion of the period and the second portion of the period (T OFF1 , T OFF2 ) are separated from each other in time by placing a second period between them, in which the second switching state is used.

9. Operating method according to one of the preceding claims, in which the active rectifier (12) is operated with a symmetrical space vector pulse width modulation.

10. Power converter (10) with an active rectifier (12), a DC voltage intermediate circuit (22), a DC / DC converter 202304405 23 (14) and a control device for controlling the rectifier (12) and the DC / DC converter (14), wherein the control device is designed to - control the active rectifier (12) such that it switches between a plurality of rectifier switching states, wherein a switch between two switching states causes a change in the existing common-mode voltage, - control the DC / DC converter (14) such that it switches between a first DC converter switching state and a second DC converter switching state to generate a DC output voltage from the voltage of the intermediate circuit (22), wherein this switch causes a change in the existing common-mode voltage, - control at least some of the switching operations of the DC / DC converter and rectifier within a switching period such that they occur at the same time. 11.Power converter (10) according to claim 10, wherein the active rectifier (12) is a two-point rectifier (12).

12. Power converter (10) according to claim 10 or 11, wherein the DC / DC converter (14) is an H-bridge DC / DC converter with two parallel-connected half-bridges.

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