Power converter and operating method for a power converter

Abstract

In a galvanically connected power converter, for example in a drive unit or a charger, an H-bridge DC / DC converter is arranged parallel to the intermediate circuit and the time components for the two off states of the DC / DC converter are determined in such a way that the DC / DC converter generates common-mode voltages that at least partially compensate for the common-mode voltages in the power converter.

Description

[0001] The invention relates to a non-galvanically isolated power converter comprising an active rectifier, a DC link, and a load-side output power converter. Furthermore, the invention relates to an operating method for such a power converter.

[0002] Power converters of this type are candidates, for example, for charging devices for electric vehicles. They are typically two-stage power converters: In the active rectifier ("Active Front End", AFE, stage 1), the mains voltage is converted into DC voltage and stored in a DC link capacitor. In the DC-DC converter (stage 2), the DC link voltage is converted into the voltage required by the load (vehicle battery) (charging voltage).

[0003] Power converters of this type can also be part of a mains-fed drive unit with an electric motor. Instead of a DC / DC converter, the load-side output converter, i.e., the second stage, is, in this case, a three-phase inverter. This can be constructed similarly to the active rectifier, for example, with three parallel half-bridges, each with two semiconductor switches. It provides the three-phase AC current required by the motor at the required frequency.

[0004] Parasitic coupling and any Y capacitors present create a leakage current that flows through the housing of the power electronics unit and, if necessary, through the protective conductor toward the mains. This leakage current can lead to a violation of EMC limits and / or cause a false tripping of an upstream, mains-side residual current device (RCD). Furthermore, in the case of a defective protective conductor, a dangerous touch voltage can build up on the housing of the functional unit, which in turn drives a dangerous body current through a person who comes into contact with the housing. The driving source of the touch currents is the common-mode voltage generated by the active rectifier and / or the load-side output converter.It depends on the specific design of the pulse width modulation (PWM) and usually has two main components in the spectrum, namely at three times the fundamental frequency, i.e. at 150 Hz if the fundamental frequency is 50 Hz, and at the switching frequency.

[0005] Common-mode voltages can be counteracted, for example, by special control of the power converters involved. However, this makes them more complex and requires a specific design for the application.

[0006] The object of the invention is to provide a power converter of the type mentioned above that reduces or avoids the disadvantages mentioned above. A further object is to provide an operating method for a power converter that reduces or avoids the disadvantages mentioned above.

[0007] This object is achieved by a power converter having the features of claim 1. A further solution consists in the operating method having the features of claim 10.

[0008] The power converter according to the invention comprises an active rectifier, a DC link, a load-side output converter, and a DC / DC converter. The rectifier, the inverter, and the DC / DC converter are connected to the DC link.

[0009] The DC / DC converter comprises two half-bridges, each with two semiconductor switches, connected in parallel to each other and to the DC link. The DC / DC converter also features a selectable compensation load connected between the center terminals of the two half-bridges.

[0010] The power converter further comprises a controller for the DC / DC converter, wherein the controller is configured to determine a common-mode voltage to be compensated, generated by the rectifier and / or load-side output converter. Finally, the controller is configured to operate the DC / DC converter such that the thus determined common-mode voltage is at least partially compensated.

[0011] For this purpose, the controller is designed to operate the DC / DC converter with an alternating first switching state and a second switching state. The first switching state is selected from a first and second switching option, with the two upper switches of the half-bridges being switched on in the first switching option, and the two lower switches of the half-bridges being switched on in the second switching option. In the second switching state, however, diagonally arranged switches of the half-bridges are switched on.

[0012] The controller is further configured to determine first, second, and third time periods using the determined common-mode voltage, and to use the first switching option in the first time periods, the second switching option in the second time periods, and the second switching state in the third time periods. The time periods are determined such that the determined common-mode voltage is at least partially compensated.

[0013] In the operating method according to the invention for the power converter according to the invention, a common-mode voltage to be compensated, generated by the rectifier and / or load-side output power converter, is determined and the DC / DC converter is operated such that the common-mode voltage thus determined is at least partially compensated.

[0014] It is therefore advantageous to use a simple DC / DC converter to compensate for common-mode voltages. Its hardware, i.e., the semiconductor switch, is a standard component. According to the invention, its control is geared toward compensation and is simplified compared to operation as a load-side converter, as it does not have to provide a specific voltage on the output side, but can be operated in an optimized manner for compensation. The compensation load is therefore not a load that can change depending on the application, but is an integral part of the converter. This reduces the load on the other components of the converter, as they do not require special control to compensate for CM disturbances. Instead, standard components suitable for the application, including the control system, can be used here as well.

[0015] The invention advantageously exploits the degree of freedom in controlling the DC / DC converter. Assuming that exactly one of the two semiconductor switches of each of the half-bridges of the DC / DC converter is always switched on, four different switching possibilities arise. In the two switching possibilities that belong to the active switching state, the diagonally arranged semiconductor switching elements of the two half-bridges are switched on. In this switching possibility, the intermediate circuit voltage is present at the output terminals, i.e., between the centers of the two half-bridges. However, in these two switching possibilities, there is no common-mode voltage.

[0016] In the inactive switching state, the two upper semiconductor switching elements of the two half-bridges are switched on in the first switching option, and the two lower semiconductor switching elements of the two half-bridges are switched on in the second switching option. This results in a common-mode voltage whose sign differs between the two switching options. The controller can therefore generate two common-mode voltages that are different from zero. These are used based on the time periods to achieve compensation of the common-mode voltage by the active rectifier and / or output converter.

[0017] The controller is expediently designed to control the active rectifier such that it switches between a plurality of rectifier switching states. It is further designed to control the load-side output converter such that it switches between suitable switching states to generate output voltage / output current from the intermediate circuit voltage. Different common-mode voltages are generated in this process.

[0018] The active rectifier can be a two-point rectifier, which, in a conventional manner, comprises three parallel-connected half-bridges, each with two semiconductor switching elements, such as IGBTs or MOSFETs. The semiconductor switching elements can comprise intrinsic or discrete parallel freewheeling diodes. However, the active rectifier can also have a different design.

[0019] The load-side output converter can, for example, be a three-phase inverter. This can be constructed analogously to an active rectifier, i.e., with three parallel half-bridges, each with two semiconductor switches. Such an output converter can be used, for example, in a drive unit with an electric motor.

[0020] Alternatively, the load-side output converter can also be a second DC / DC converter. This can be used, for example, to create a non-isolated charger for a rechargeable battery, such as the battery of an electric vehicle.

[0021] The DC link comprises one or more capacitors and is conveniently connected in parallel to the external terminals of the half-bridges of the active rectifier.

[0022] The controller conveniently includes a programmable control module, such as a microcontroller. It is connected to a higher-level controller, from which it receives signals that determine the operating state of the power converter. The controller calculates the switching times for the existing semiconductor switches and sends corresponding signals to the gate driver units assigned to the semiconductor switches.

[0023] Advantageous embodiments of the operating method and power converter according to the invention emerge from the dependent claims. The embodiment of the independent claims can be combined with the features of one of the subclaims or, preferably, with those of several subclaims. Accordingly, the following additional features can be provided: The common-mode voltage can be determined by measurement. Alternatively, the common-mode voltage can also be calculated. The calculation can be performed by the controller based on the switching operations performed by the controller. If a calculation is performed, the common-mode voltage is known in advance and without the need for additional measuring equipment. If a measurement is used, however, common-mode voltages that are not directly a result of the switching operations in the power converter can also be recorded and compensated.

[0024] Preferably, the time periods are portions of a respective switching period, the length of which is determined by the switching frequency. Thus, a rhythm of switching operations is maintained that follows the switching periods. However, the length of the time periods can vary from switching period to switching period, as the voltage supplied by the power converter to the load varies, for example, if it is an AC voltage or the operating point changes.

[0025] The time periods are preferably 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.

[0026] In an advantageous embodiment of the invention, the controller is designed to operate the DC / DC converter in such a way that in a first switching period exclusively the first or second switching option is used, and in a second switching period following the first, the remaining other switching option is used. In other words, in a first switching period only the freewheeling circuit in which the two upper semiconductor switches are switched on is used, and in the following switching period only the freewheeling circuit in which the two lower semiconductor switches are switched on is used. Alternatively, the switching options can also be swapped in the procedure, i.e. in a first switching period only the freewheeling circuit in which the two lower semiconductor switches are switched on is used, and in the following switching period only the freewheeling circuit in which the two upper semiconductor switches are switched on is used.This switching pattern reduces the number of switching operations used in the DC / DC converter, thereby reducing switching losses. At the same time, sufficient compensation of common-mode voltages is maintained by the DC / DC converter.

[0027] The compensation load may comprise a parallel circuit of a resistor and a capacitor. Furthermore, a filter circuit comprising an inductor and a resistor connected in series therewith may be arranged between each of the center terminals of the half-bridges of the DC / DC converter and a respective terminal of the compensation load.

[0028] The controller can be configured to control the DC / DC converter such that at least some of the switching operations of the DC / DC converter and the active rectifier occur simultaneously within a switching period. This advantageously also reduces or compensates for switching-frequency components of the common-mode voltage.

[0029] The time periods are preferably determined from the duration of the switching periods of the DC / DC converter, i.e., the switching frequency, the intermediate circuit voltage, and an instantaneous value for the common-mode voltage generated by the active rectifier. However, an output voltage to be set does not need to be taken into account, since the load driven by the DC / DC converter is only a dummy load.

[0030] Advantageously, the switching period and the intermediate circuit voltage are fixed or known to the controller. The instantaneous value of the common-mode voltage generated by the active rectifier can be determined from available control data. This allows the controller to 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 adjusted to the instantaneous value of the common-mode voltage.

[0031] The instantaneous value for the common-mode voltage generated by the rectifier can be determined as a value averaged over one switching period of the rectifier. This advantageously determines a single value for one switching period, which can also be used to control the DC / DC converter within one switching period. This avoids setting values for the common-mode voltage that vary within one switching period.

[0032] 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 and / or the output converter. The instantaneous level of the common-mode voltage 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 controller. The portions of the switching period are determined in the controller, as they are required to control the rectifier.

[0033] It is advisable to arrange the time periods for the DC / DC converter so that there are always third time periods between the first and second time periods, so that times in which the DC / DC converter is in one of the inactive switching modes and thus generates a common-mode voltage are interspersed with times in the active switching state. This avoids direct switching, and possibly even multiple switching between the switching modes of the first state.

[0034] The active rectifier and the DC / DC converter can operate at the same switching frequency. The load-side output converter can also operate at this 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.

[0035] The invention is described and explained in more detail below with reference to the exemplary embodiments shown in the figures. They show: Fig. 1 an electrical circuit diagram of a galvanically non-isolated charger with an active rectifier, a load-side DC / DC converter and a second DC / DC converter, Fig. 2an electrical diagram of a mains-fed drive unit with an active rectifier, a load-side inverter and a second DC / DC converter, Fig. 3 a temporal progression of the switching states of the active rectifier within a switching period, Fig. 4 to 10 time courses of the switching states in the second DC / DC converter for compensation of common-mode voltages.

[0036] In Fig.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 its 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 wallbox, a high-voltage connection specifically designed for charging an electrically powered vehicle.

[0037] The active rectifier 12 comprises, on the mains side, one inductor 18a...c per phase and three half-bridges, each with two semiconductor switching elements 20a...f, such as IGBTs or MOSFETs. The half-bridges are connected in parallel with their outer terminals in a known manner and connected to the inductors 18a...c with their center terminals. The active rectifier 12 thus formed is a two-point rectifier.

[0038] The outer 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 may also comprise 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.

[0039] 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 load-side 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—here the vehicle's accumulator, commonly referred to as the battery—is connected in parallel to the capacitor 27c.

[0040] The DC intermediate circuit 22 is further connected to the external terminals of two third half-bridges, each with two semiconductor switching elements 35a...d, which form a second DC / DC converter 36. The second DC / DC converter 36 is thus also an H-bridge DC / DC converter. The center terminals of the third half-bridges are each connected to an output filter 37a, b. The output filters 37a, b each comprise an inductance and a resistor connected in series with it. The two output filters 37a, b, together with a compensation load 39, form a series circuit. The compensation load 39 comprises a parallel circuit of a resistor and a capacitor.

[0041] The charger therefore comprises two DC / DC converters 14, 36. Although these are described here as being of the same type, details of their design do not necessarily have to be. For example, different types of semiconductor switching elements can be used. The control of the two DC / DC converters 14, 36 also differs.

[0042] Fig.Figure 2 shows an electrical circuit diagram of a drive unit 60. Elements of the drive unit 60 that correspond to the charger 10 are provided with the same reference numerals. The drive unit 60 comprises an active rectifier 12, a three-phase inverter 61, and an electric motor 62 connected to the inverter 61. Like the charger 10, the drive unit 60 is connected to a three-phase supply voltage 16 on the input side. The active rectifier 12 corresponds to that of the charger 10. The external connections of the half-bridges are also connected to a DC link 22.

[0043] The DC intermediate circuit 22 is further connected to the external terminals of three half-bridges, each with two semiconductor switches 63a...f, which form the load-side inverter 61. The center terminals of the half-bridges are connected to the electric motor M via a respective output filter 64a...c. The output filters 64a...c are each formed by a series of an inductance and a resistor.

[0044] Analogous to the charger, the DC intermediate circuit 22 is additionally connected to the external terminals of two third half-bridges, each with two semiconductor switching elements 35a...d, which form the second DC / DC converter 36. While the charger 10 has two H-bridge DC / DC converters 14, 36 connected in parallel, the drive unit 60 comprises only one such DC / DC converter 36.

[0045] Both the charger 10 and the drive unit 60 comprise a controller, which is not shown in the figures. The controller is connected to a higher-level control device, which transmits commands regarding the overall status to the controller. The controller itself calculates switching times for the semiconductor switches 15a...d, 20a...f, 35a...d, 63a...f used in the charger 10 or the drive unit 60. At the determined switching times, signals are sent to gate driver units, also not shown in the figures, which then in turn cause the switching of the semiconductor switching elements 15a...d, 20a...f, 35a...d, 63a...f. The controller performs, among other things, 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 further calculation steps for the switching operations of the second DC / DC converter 36.

[0046] The active rectifier 12 is also referred to below as the AFE. The DC / DC converter 14 and the three-phase inverter 61 are collectively referred to below as the load converter. The second DC / DC converter 36 is also referred to as the compensator. The charger 10 and the drive unit 60 are collectively referred to as the converter.

[0047] Parasitic couplings against Ground 28 are in Fig.1 by the capacitive elements 29a, 29b. The vehicle-side capacitive couplings via Y-capacitors to PE 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 electrical equivalent circuit is arranged parallel to the symbolic switch 31 to represent touch voltages and touch currents. An analogous representation of parasitic couplings is also shown in Fig. 2 available.

[0048] One component of such contact voltages arises as 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 and is caused by the switching operations of the active rectifier 12. The time-dependent common-mode voltage u CM0 is defined for the active rectifier 12 as: uCM0=(uR0+uS0+uT0) / 3

[0049] This includes R0 , and S0 , and u T0 Components of the common mode voltage that Fig. 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 + / -U DC / 6 when a mixture of upper and lower switching elements in the active rectifier 12 is switched on.

[0050] If we designate the switching state with the upper switch switched on as 1 and the switching state with the lower switch switched on as 0 and write the switching states for the three half-bridges one after the other, the possible overall states, referred to as vectors, are as follows: vector Switching states RST Common mode voltage / U DC V0 000 -1 / 2 V1 100 -1 / 6 V2 110 +1 / 6 V3 010 -1 / 6 V4 011 +1 / 6 V5 001 -1 / 6 V6 101 +1 / 6 V7 111 +1 / 2

[0051] The currently used switching state changes several times within a switching period, as described below. The exact switching process depends on the selected pulse width modulation for the active rectifier 12, and the beat of the common-mode voltage at three times the frequency of the mains voltage is also determined by the PWM implementation.

[0052] For the concrete example of a symmetrical space vector PWM, a curve within one switching period results, which is Fig. 3 is shown in simplified form. Fig. 3 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 t a , t b and t0. In reality, these are not of equal length and depend on the current operating point of rectifier 12. They result from the pulse width modulation used.

[0053] The switching period is generally described as consisting of two zero vectors V0 and V0' and two active vectors V a and V bIn the case of the first sector, these are specifically the vectors V7 and V0 as well as V1 and V2. The switching times t a , t b and τ0 of the vectors are recalculated by the PWM unit of the rectifier 12 for each period.

[0054] The shape of the third harmonic depends on the DC link voltage and the selected PWM. It can be easily calculated as a short-term average value. CM,x a switching period from the control data of the respective inverter. It is assumed that the control is Fig. 3 for each switching period the switching times τ α and τ β for the two active vectors and τ0 for the zero vector(s).

[0055] The following applies to the short-term CM mean value of the symmetrical Space Vector PWM: u¯CM,SymSV=UDC6⋅(tb−ta), at S=S1, S3, S5 u¯CM,SymSV=UDC6⋅(ta−tb), at S=S2, S4, S6

[0056] S stands for the sector in the pulse width modulation of the active rectifier 12. For a discontinuous Space Vector PWM, however, the following applies: u¯CM,DiscSV=UDC2⋅(−13ta+13tb+t0), at S=S1, S3, S5 u¯CM,DiscSV=UDC2⋅(+13ta−13tb+t0), at S=S2, S4, S6

[0057] Depending on the desired compensation, the compensator must provide the following CM counter voltage: u¯CM,Comp=−gAFE⋅u¯CM,AFE,Compensation of the AFE u¯CM,Comp=−gL⋅u¯CM,LK,Load converter compensation u¯CM,Comp=−gres⋅(u¯CM,AFE+u¯CM,LK), compensation of AFE and load converter

[0058] The weighting factor g expresses the need to consider the relative sizes of the parasitic capacitances and, if applicable, filter capacitances and the coupling capacitors of the compensator. Furthermore, the resulting CM equivalent circuit of the specific application, including the inductive components, is also incorporated.

[0059] For the switching operations of the second DC / DC converter 36, a scheme can be set up similarly to that of the active rectifier 12, although it only has four different states, since only two half-bridges are present. The common-mode voltage u CM,DC is calculated for the second DC / DC converter 36 as follows: uCM,DC=(uA0+uB0) / 2

[0060] The following switching states are possible with the DC / DC converter: Condition Switching states AB Common mode voltage u CMDC Output voltage uAB Corresponding switching time DC0 10 0 +U DC t on DC1 00 -1 / 2 U DC 0 t off 1 DC2 11 +1 / 2 U DC 0 t off 2 DC3 01 0 -U DC t on

[0061] The second DC / DC converter 36 only generates 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. If the off time of a switching period is evenly distributed between the switching states DC1 and DC2, the DC / DC converter generates no common-mode voltage on average for this switching period. An uneven distribution, on the other hand, generates a corresponding common-mode voltage value that averages over the switching period. This can be used to counteract and compensate for the low-frequency common-mode beat of the active rectifier 12.

[0062] The average value of the CM voltage generated by the compensator is generally: u¯CM,Comp=UDC2(Toff,2−Toff,1)⋅1Tcomp

[0063] In order to operate the compensator with the lowest possible switching losses, it is recommended to use only one freewheeling state per period, depending on the CM mean value to be compensated: u¯CM,Comp=UDC2⋅Toff,2Tcomp for ru¯CM,Comp>0 u¯CM,Comp=−UDC2⋅Toff,1Tcomp fu¨ru¯CM,Comp<0

[0064] Since the compensator only drives the compensation load 39, i.e. a pseudo load, at the output, the selected output voltage u AB be arbitrary and thus dependent on the CM voltage to be compensated: uAB=don⋅UDC don=1−(Toff,1+Toff,2)⋅1Tcomp

[0065] If the switching actions of the active rectifier 12 and / or the load converter and the compensator are synchronized, this can be used to compensate for the switching frequency component of the common-mode interference.

[0066] For synchronization, the off states DC1 and DC2 of the second DC / DC converter 36 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 36. Here, T OFF10 + T OFF1x = T OFF1 and T OFF20 + T OFF2x = T OFF2 .

[0067] Fig. Figure 4 shows an example of how this synchronization is performed. The switch-on times τ a, τ b For simplicity, τ and τ0 are shown as equal lengths. In real implementation, however, these are typically of different lengths and subject to constant change.

[0068] As from Fig.As can be seen in Figure 4, each switching operation of the rectifier 12 is associated with a switching operation of the DC / DC converter 36, 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 36 of 1 / 2 U DC be assigned.

[0069] It is important to understand that, due to the chosen common-mode definition, the active rectifier 12 and the DC / DC converter 36 operate in opposite directions with respect to the leakage current in the PE conductor. To compensate for the switching-frequency component of the current in the PE conductor, the synchronized voltage changes therefore always occur in the same direction.

[0070] Depending on the converter's operating point, this results in eight modes for the second DC / DC converter 36, i.e., the compensator, between which the control device switches. If the entire off-time of the second DC / DC converter 36 cannot be mapped within the zero vector phase τ0 of the AFE, the remaining off-time of the second DC / DC converter 36 must be calculated based on the switching times t a and t b the active vectors of the rectifier 12.

[0071] The operating mode is selected based on the ratios between the switching times of the second DC / DC converter (T OFF10 , T OFF1x , T OFF20 , T OFF2x ) and the AFE (t a , t b and t0). Table 1: Size ratios for odd sectors (S1, S3, S5) fashion T off,20 T off,10 T off,2x T off,1x 0 < t0 / 2 < t0 / 2 < t b 0 1 < t0 / 2 ≥ t0 / 2 < t b < t a 2 ≥ t0 / 2 < t0 / 2 < t b 0 3 ≥ t0 / 2 ≥ t0 / 2 < t b < t a 4 ≥ t0 / 2 ≥ t0 / 2 ≥ t b < t a 5 ≥ t0 / 2 ≥ t0 / 2 < t b ≥ t a 6 < t0 / 2 ≥ t0 / 2 ≥ t b < t a 7 < t0 / 2 ≥ t0 / 2 < t b ≥ t a Table 2: Size ratios for even sectors (S2, S4, S6) fashion T off,20 T off,10 T off,2x T off,1x 0 < t0 / 2 < t0 / 2 < t a 0 1 < t0 / 2 ≥ t0 / 2 < t a < t b 2 ≥ t0 / 2 < t0 / 2 < t a 0 3 ≥ t0 / 2 ≥ t0 / 2 < t a < t b 4 ≥ t0 / 2 ≥ t0 / 2 ≥ t a < t b 5 ≥ t0 / 2 ≥ t0 / 2 < t a ≥ t b 6 < t0 / 2 ≥ t0 / 2 ≥ t a < t b 7 < t0 / 2 ≥ t0 / 2 < t a ≥ t b

[0072] The calculation of the individual switching times of the freewheeling phases is carried out according to Tables 3, 4 and 5. Table 3: Switch-on times for odd sectors (S1, S3, S5) fashion T off,20 T off,2x T off,2y 0 = t off2- t b / 2 = t b / 2 - 1 = t0 / 2 = t off2 -t0 / 2 - 2 = t off2- t b / 2 = t b / 2 - 3 = t0 / 2 = t off2 -t0 / 2 - 4 = t0 / 2 = t b * 0,95 = t off2 - t off,20 -t off,2x 5 = t0 / 2 = t off2 -t0 / 2 - 6 = t off2- t b / 2 = t b * 0,95 = t off2 - t off,20 -t off,2x 7 = t off2- t b / 2 = t b / 2 - Table 4: Switch-on times for even sectors (S2, S4, S6) fashion T off,20 T off,2x T off,2y 0 = t off2- t a / 2 = t a / 2 - 1 = t0 / 2 = t off2 -t0 / 2 - 2 = t off2- t a / 2 = t a / 2 - 3 = t0 / 2 = t off2 -t0 / 2 - 4 = t0 / 2 = t a * 0,95 = t off2 -t off,20 -t off,2x 5 = t0 / 2 = t off2 -t0 / 2 - 6 = t off2- t a / 2 = t a * 0,95 = t off2 - t off,20 -t off,2x 7 = t off2- t a / 2 = t a / 2 - Table 5: Additional switch-on times for all sectors fashion T off,10 T off,1x T off,1y 0 = t off1 - - 1 = t0 / 2 = t off1 -t0 / 2 - 2 = t off1 - - 3 = t0 / 2 = t off1 -t0 / 2 - 4 = t0 / 2 = t off1 -t0 / 2 - 5 = t0 / 2 = t off1 -t0 / 2 = t off1 -t off,10 -t off,1x 6 = t0 / 2 = t off1 -t0 / 2 - 7 = t0 / 2 = t off1 -t0 / 2 = t off1 -t off,10 -t off,1x

[0073] Another example is given below using Fig. 5. In this embodiment, a reduction in the number of switching operations of the second DC / DC converter 36 within one clock period is achieved compared to the previous example; in other words, it is a simpler method that achieves similar results with regard to compensation.

[0074] Here, the lower off-state DC2 is arranged symmetrically to the center of the clock period. The upper off-state is divided equally between the beginning and end of the clock period. Thus, the CM voltage time surfaces generated by the second DC / DC converter 36 are aligned parallel to the maximum and minimum amplitudes of the common-mode voltage of the AFE. This achieves partial compensation of the switching frequency component of the common-mode voltage. The third harmonic is compensated by unevenly dividing the on-time of the upper and lower off-states DC1 and DC2, as shown in Fig. 5. If the duty cycle of the two freewheeling circuits is equally distributed, the compensation is limited to switching frequency components.

[0075] A further reduction in switching operations can be achieved if only one off state DC1, DC2 is used per clock period. Such a switching pattern is shown in Fig. 6 shown.

[0076] The upper and lower off states DC1 and DC2 are used alternately per clock period of the rectifier 12. The switching pattern of the second DC / DC converter thus repeats every two clock periods of the rectifier 12. Here, too, for optimal synchronization with the rectifier 12, the upper off state DC1 is aligned symmetrically to the period center, and the lower off state DC2 is divided between the beginning and end of the clock period.

[0077] As in the previous example, the third harmonic is compensated by unevenly dividing the times of the upper and lower off states DC1, DC2.

[0078] Fig. Figure 7 shows a circuit diagram with which a simplified CM compensation of the load DC-DC converter 14 in an embodiment according to Fig.1 is reached. Here, it is assumed that the load DC-DC converter 14 only uses its lower freewheeling circuit. The turn-on time is at the beginning of T SW , the switch-off time at the end of the clock period.

[0079] For compensation, the second DC / DC converter 36 is operated synchronously with the first DC / DC converter 14, meaning they use the same clock frequency and phase. This approach is analogously applicable to the upper freewheeling circuit of the DC / DC converter 14 and the second DC / DC converter 26, should the DC / DC converter 14 use this instead of the lower one. The duty cycle of the compensation voltage is calculated as follows: u¯CM,Last=−UDC2⋅ToffTSW dCM=u¯CM,Last0.5⋅UDC Toff1=dCM⋅TSW=Toff; Toff2=0 ton=(1−dCM)⋅TSW

[0080] Fig.Figure 8 shows a circuit diagram that enables simplified compensation of the summed common-mode voltage of both the rectifier 12 and the load-side DC / DC converter 14.

[0081] Here, the assumption is made that the turn-off time of the load-side DC / DC converter 14 is symmetrical around the center of the clock period T SW Rectifier 12 and load-side DC / DC converter 14 operate at the same clock frequency, and their phases are synchronized. The negative half-wave of the common-mode voltage of rectifier 12 is thus synchronized with the lower freewheeling circuit of the load-side DC / DC converter 14, which already leads to a partial compensation of the resulting common-mode voltage.

[0082] The common mode voltage to be compensated by the second DC / DC converter 36 is obtained by including the formulas introduced above and: u¯CM,res=u¯CM,symSV−u¯CM,Last dCM=u¯CM,res0.5⋅UDC

[0083] Here, the turn-on times of the second DC / DC converter are also arranged symmetrically around the center of the clock period, with the turn-on time per clock period divided in two by an off time with the lower freewheeling circuit. The upper freewheeling circuit is used for the remaining part of the clock period.

[0084] Fig. Figure 9 shows a circuit diagram with which a simplified CM compensation for the inverter 61 in an embodiment according to Fig. 2 is achieved. It is assumed that the rectifier 12 and the inverter 61 operate at the same switching frequency and synchronized phase. Both inverters operate with the symmetrical SVPWM. The phase shift between the common-mode voltages of rectifier 12 and inverter 61 results from the definition of the common-mode voltages in Fig. 2.

[0085] For compensation, the second DC / DC converter 36 is synchronized to the inverter 61. Since it is Fig. 2 is parallel to the inverter 61, the compensation is achieved by the inverse arrangement of the positive half-wave of the inverter 61 and the negative half-wave of the second DC / DC converter 36 and vice versa. The duty cycle of the compensation voltage is calculated analogously to the examples already given. The circuit diagram for the second DC / DC converter 36 corresponds to the one used in the implementation of Fig. 8. In an analogous manner, the summed common-mode voltage of the rectifier 12 and the inverter 61 can also be compensated, as in Fig. 10 shown. Reference symbol 10 Charger 12 active rectifiers 14 DC / DC converters 15a... d, 35a... d, 63a...f semiconductor switches 16 Supply voltage 18a... inductance 20a...f IGBTs 22 DC intermediate circuit 23a, b DC link capacitor 24 Junction 26 output filters 27a,b Filter inductance 27c filter capacitor 28 Ground 29a, b parasitic capacitance 30a, b Y-capacitor 36 second DC / DC converter 37a, b Output filter 39 Compensation load 51 DC output voltage 52 Common mode voltage 60 drive unit 61 three-phase inverter 64 a...c filter inductance M electric motor PE Protective Earth U DC DC link voltage U BAT Battery voltage u R0 , and S0 , and T0 Phase voltages of the AFE u U0 , and V0 , and W0 Phase voltages of the load inverter u A0 , andB0 Phase voltages of the second DC / DC converter uAB Bridge output voltage of the second DC / DC converter u 0G Midpoint voltage u CM0 Common mode voltage of the AFE u CMDC Common mode voltage of the second DC / DC converter R, S, T Bridge center point of the AFE U, V, W Bridge center point of the load inverter A, B Bridge center point of the second DC / DC converter i R , i S , i T Phase currents on the grid side i PE Current in the PE conductor

Claims

[1] Power converter (10, 60) with an active rectifier (12), a DC voltage intermediate circuit (22), a load-side output power converter (14, 61) and a DC / DC converter (36), wherein - the rectifier (12), the load-side output converter (14, 61) and the DC / DC converter (36) are connected to the DC intermediate circuit (22), - the DC / DC converter (36) comprises two half-bridges, each with two semiconductor switches (35a...d), which are connected in parallel to each other and to the DC voltage intermediate circuit (22), - the DC / DC converter (36) comprises a definable compensation load (39) connected between the center terminals of the two half-bridges, - a control for the DC / DC converter (36) is provided, wherein the control is designed, - to determine a common-mode voltage to be compensated, generated by the rectifier (12) and / or load-side output converter (14, 61), - to operate the DC / DC converter (36) with an alternately used first switching state and a second switching state, wherein the first switching state is selected from a first and a second switching possibility, wherein in the first switching possibility the two upper switches (35a, c) of the half-bridges are switched on and in the second switching possibility the two lower switches (35b, d) of the half-bridges are switched on and wherein in the second switching state diagonally arranged switches (35a...d) of the half-bridges are switched on, - to determine first, second and third time periods using the determined common-mode voltage and to use the first switching possibility in the first time periods, the second switching possibility in the second time periods and the second switching state in the third time periods, wherein the time periods are determined such that the determined common-mode voltage is at least partially compensated. [2] Power converter (10, 60) according to claim 1, wherein the controller is designed to operate the DC / DC converter (36) such that the common-mode voltage generated by the DC / DC converter (36) is equal in magnitude to the common-mode voltage generated by the rectifier (12) and / or load-side output power converter (14, 61). [3] Power converter (10, 60) according to claim 1 or 2, wherein the controller is designed to operate the DC / DC converter (36) such that in a first switching period exclusively the first or second switching possibility is used and in a second switching period following the first switching period the remaining other switching possibility is used. [4] Power converter (10, 60) according to one of the preceding claims, wherein the active rectifier (12) is a two-point rectifier (12). [5] Power converter (10, 60) according to one of the preceding claims, wherein the load-side output power converter (14, 61) is a three-phase inverter (61). [6] Power converter (10, 60) according to one of claims 1 to 4, wherein the load-side output power converter (14, 61) is a DC / DC converter (14). [7] Power converter (10, 60) according to one of the preceding claims, wherein the compensation load (39) comprises a parallel connection of a resistor and a capacitor. [8] Power converter (10, 60) according to one of the preceding claims, in which a filter circuit (37a, b) with an inductance and a resistor connected in series therewith is arranged between each of the center terminals of the half-bridges and a respective terminal of the compensation load (39). [9] Power converter (10, 60) according to one of the preceding claims, in which the controller is designed to control the DC / DC converter (36) such that at least some of the switching operations of the DC / DC converter (36) and the active rectifier (12) occur at the same time within a switching period. [10] Operating method for a power converter (10, 60) according to one of the preceding claims, in which a common-mode voltage to be compensated for, generated by the rectifier (12) and / or load-side output power converter (14, 61), is determined and the DC / DC converter (36) is operated such that the common-mode voltage thus determined is at least partially compensated. [11] Operating method according to claim 10, wherein the active rectifier (12), the load-side output converter (14, 61) and the DC / DC converter (36) are operated at the same switching frequency. [12] Operating method according to claim 10 or 11, wherein the time periods are determined from the switching period of the DC / DC converter (36), the intermediate circuit voltage, the DC output voltage and an instantaneous value for the common-mode voltage generated by the active rectifier (12). [13] Operating method according to one of claims 10 to 12, in which a value averaged over a switching period of the rectifier is determined as the instantaneous value for the common-mode voltage generated by the active rectifier (12). [14] Operating method according to claim 13, wherein the instantaneous value is determined using the switching period duration, the intermediate circuit voltage and portions of the switching period duration which are assigned to the switching states of the active rectifier (12).

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