CH704553A2 - Alternating current/direct current (AC/DC) converter for electric vehicle, has bridge circuit comprising three connection points that are connected to DC voltage side center through four-quadrant switches respectively - Google Patents

Abstract

An upper DC/DC converter (9) is connected between positive terminal of bridge circuit (1) and positive terminal (17) of AC/DC converter. A lower DC/DC converter (13) is connected between negative terminal of bridge circuit and negative terminal (18) of AC/DC converter. A lower connection point (11) of upper DC/DC converter and an upper connection point (15) of lower DC/DC converter are connected with each other to form DC voltage side center. Three phase connection points (3-5) of bridge circuit are connected to DC voltage side center through four-quadrant switches (6-8) respectively. An independent claim is included for method for controlling AC/DC converter.

Description

The invention relates to the field of power electronic circuits and relates to an AC / DC converter for power transmission from a three-phase AC side to a DC side or vice versa, and to a method for its control.

STATE OF THE ART

For rectification of a three-phase mains voltage are active in the demand for low system perturbations and regulated output voltage according to the prior art active, i. self-guided or hybrid, i. used rectifier circuits formed from line-guided and self-commutated circuit parts.

In principle, a distinction must be made between systems with a boost converter characteristic and systems with a buck converter characteristic. Three-phase buck converter rectifier systems allow a DC output voltage in the range of zero to 1.5 times the net phase voltage amplitude to generate, and therefore have high in connection with future 360 V DC (Direct Current) distribution systems for data centers or the battery charging of electric vehicles Meaning. Further advantages are the possibility of a current-limited operation at output short-circuit and a controlled, current-limited run-up starting from zero output voltage. The description of the prior art is for this reason and with regard to the subject matter of the invention limited to buck converter concepts. An active three-phase buck converter rectifier system is described in: M. Baumann, J.W. Kolar, "A Novel Control Concept for Reliable Operation of a Three-Phase Three-Switch Buck-Type Unity Power Factor Rectifier with Integrated Boost Output Under Heavily Unbalanced Mains Condition", IEEE Trans, on Industrial Electronics, vol. 52, no. 2, pp. 399-409, April, 2005. Similar to a direct current to direct current (DC / DC) buck, active three-phase buck converter rectifier systems, such as the cited output side, have inductance, i. an impressed by means of this inductance output current and subsequently an output capacitance, since a load to be fed i.a. an impressed output voltage must be offered. On the input side, mains phase voltages are impressed via a star or delta connection of filter capacitors. The rectifier section is realized by a modified three-phase diode bridge, wherein a turn-off electronic switch is placed in the flow direction of each diode. By switching on a switch so the flow of current can be released via a diode branch, the current transfer of the diode branch is then still dependent on the sign and magnitude of the voltage applied to this diode branch network phase voltage. The locking of the switch interrupts the flow of current, the three-phase diode bridge is therefore considered to be self-contained. As a result, the output current can be distributed sinusoidally pulse-width modulated on mains phases and, after low-pass filtering by the filter capacitors on the input side and upstream filter inductances, a sinusoidal profile of mains phase currents in phase with the respective mains phase voltage, i. ohmic fundamental network behavior of the three-phase buck converter rectifier system can be achieved. A voltage at the output of the modified three-phase diode bridge is determined by sections of phase voltage differences, i. Defined by sections of the interlinked mains voltages, which are switched by the switching on and off of the electronic switch to the positive and negative output of the self-commutated three-phase diode bridge, and corresponding to the network side, according to the sinusoidal current of the currents constant instantaneous power a constant short-term average (average over a pulse period) which comes to rest at one end of the output side inductance (output inductance) and is converted via the low pass filter effect of this inductance in conjunction with the output capacitance into a DC output voltage with a low switching frequency ripple.

Disadvantages of this concept are the complexity of the controller, a relatively high implementation cost - all diodes are designed for switching frequency commutation, and the power semiconductors are in the off state maximum claimed with the relatively high peak value of the concatenated mains voltage - and the relatively high conduction losses, there a phase current is always passed through a diode and an electronic switch.

The number of electronic switches can be reduced from six to three by changing the topology of the modified three-phase diode bridge. However, then six more, so a total of 12 diodes are required, and there are even higher conduction losses.

In addition to the active three-phase buck converter rectifier circuit described above, a hybrid three-phase rectifier circuit is still known, which allows for a load with a constant instantaneous power consumption, to achieve a sinusoidal line current waveform, or again ohmic fundamental network behavior. This is described in: H. Yoo, S.-K. Sul, "A New Circuit Design and Reduce Input Harmony Current for a Three-Phase AC Machine Drive System Having a Very Small DC-Link Capacitor", in Proc. 25th IEEE Applied Power Electronics Conference and Exposition APEC10, pp. 611-618, 2010.

However, the system does not allow output voltage regulation, i. at the output, the unregulated, six-pulse rectified three-phase mains voltage occurs.

The power section of the hybrid three-phase rectifier circuit is provided by an input-side, mains-driven diode bridge and two turn-off electronic switches with antiparallel freewheeling diodes, i. formed as a bridge branch semiconductor device, which is arranged in the flow direction between the positive and negative output terminal of the diode bridge. The three-phase AC supply voltage is applied directly to the inputs of the three-phase network-connected diode bridge. Further, branching from each input is an opposite series circuit of electronic switches, i. a four-quadrant switch (which can absorb blocking voltages of both polarities and can carry current in both directions) is arranged against a common switch star point. This switch star point is connected via an inductance with the center of the bridge branch. The output terminals of the line-commutated diode bridge at the same time also form the output terminals of the hybrid three-phase rectifier circuit.

If a constant power consumer is connected to the output of the system, an inverse to the six-pulse voltage variation varying consumer current, each from the mains phase with the highest potential (hereinafter referred to briefly as the highest phase) relative and in the network phase with the lowest potential ( lowest phase) is returned. In order to supply current to the third, in terms of the potential medium phase, now lying on this phase four-quadrant switch is turned on and the bridge branch clocked so that adjusts a sinusoidal current waveform in the inductance. The regulation of this current fed into the network then takes place in such a way that, in accordance with the goal of a sinusoidal current consumption of the overall system, a current lying in phase with the average phase voltage results. The current of the middle phase closes over the network and the highest and the lowest phase back and superimposed there the constant power current. As a recalculation shows, a sinusoidal, voltage-proportional current profile can be ensured in all phases.

The implementation cost of the system is low and the control is easily possible, however, remains the output voltage, as mentioned above, unregulated and must, for. be converted by a downstream DC / DC buck converter into a constant DC voltage value, which overall results in a relatively high production cost and low efficiency.

The object of the invention is therefore to provide a three-phase AC / DC converter, which for ensuring a mains voltage proportional current consumption and a regulated output voltage, compared with the prior art, both a low implementation cost and a low complexity of the controller having.

PRESENTATION OF THE INVENTION

This object is achieved by an AC / DC converter having the features of patent claim 1 and a method for controlling an AC / DC converter having the features of patent claim 9.

The basic idea of the invention is to realize a hybrid buck converter rectifier system which integrates a DC / DC converter function and at the same time also use this for the impressing of the current into a mean network phase. The requirement for the formation of a regulated output voltage and sinusoidal current consumption can thus be met with low complexity of the power and control part.

The AC / DC converter for power transmission from a three-phase AC side to a DC side with a positive terminal and a negative terminal and / or vice versa, has a three-phase bridge circuit with three connection points for each input phase on its AC side and one positive and a negative terminal on its DC side. It is between the positive connection point of the bridge circuit and the positive connection terminal of the AC / DC converter, an upper DC / DC converter with a lower connection point as reference point, and between the negative connection point of the bridge circuit and the negative connection terminal of the AC / DC converter, a lower DC / DC converter with an upper connection point as a reference point.

In this case, the lower and the upper connection point are connected to each other and form a DC side center. Each of the three connection points is in each case connected via a four-quadrant switch, that is to say a bidirectional switch, to the DC-side center.

Thus, it is possible, according to the method for controlling the AC / DC converter, to clock the upper DC / DC converter (ie, to turn on / off a switch of the DC / DC converter) that the current from the positive connection point of the bridge circuit to the upper DC / DC converter flows in the short-term mean sinusoidal and proportional to the voltage curve in the mains phase with the highest voltage, and analog clocking the lower DC / DC converter so that the current, which of the negative Connection point of the bridge circuit flows to the lower DC / DC converter, is sinusoidal in the short-term average and proportional to the voltage curve in the mains phase with the lowest voltage.

In each case of the three four-quadrant switches each one which is connected to the input phase with the average voltage or the input phase smallest amount of voltage, switched while the other two four-quadrant switch remain in the off state, which also results in a sinusoidal summation current in this input phase.

So that can be generated at all three input phases, or, after smoothing by a switched between input phases and network phases filter on all three network phases, a current which is in phase with the respective (mains) phase voltage.

The upper DC / DC converter and the lower DC / DC converter may be functionally completely symmetrical (i.e., only with respect to the nature of the components not with respect to their orientation) to each other. Alternatively, inductors of the upper DC / DC converter and the lower DC / DC converter may be realized together by a single device.

There form the positive terminal (17) and the negative terminal (18) has a common output. In this case, the upper and the lower DC / DC converters (9, 13) are interconnected and designed such that between an upper input of the upper DC / DC converter (9), formed by an input-side connection point (10) of the upper DC / DC converter (9) connected to the positive connection point of the bridge circuit and by the lower connection point (11) of the upper one DC / DC converter (9), and the common output, as well between a lower input of the lower DC / DC converter (13) formed by an input side terminal point (14) of the lower DC / DC converter (13) connected to the negative terminal of the bridge circuit and the upper terminal point (15) of the lower one DC / DC converter, and the common output, unidirectional power flow towards the common output or unidirectional power flow from the common output or bidirectional power flow is possible.

The two DC / DC converters are designed so that during operation of the converter over the capacitor (19) voltage applied (20) smaller, equal to or greater than a voltage between the input side terminal point (10) of the upper DC / DC converter and the input-side connection point (14) of the lower DC / DC converter is.

In one embodiment of the invention, the AC / DC converter is designed as a rectifier for a power flow from the AC side to the DC side, wherein the three-phase bridge circuit is a diode bridge circuit and the upper and the lower DC / DC converter for a power flow from positive and negative connection point of the bridge circuit to the positive and negative terminal of the AC / DC converter are formed.

In another embodiment of the invention, the AC / DC converter is designed as an inverter for a power flow from the DC voltage side to the AC side, wherein the three-phase bridge circuit is an inverter bridge circuit of electronic switches each having anti-parallel diodes and the upper and the lower DC / DC converter are designed for a power flow from the positive and negative terminal of the AC / DC converter to the positive and negative terminal of the bridge circuit.

In a further embodiment of the invention, the AC / DC converter is designed as a bidirectional converter for a power flow from the AC side to the DC side or vice versa, wherein the three-phase bridge circuit is designed as in the inverter, and the upper and the lower DC / DC converters allow bidirectional power flow.

In one embodiment, the upper of the lower DC / DC converter are each formed by parallel circuits of a plurality of partial converters of the same structure. An associated control device is designed for an offset clocking of the parallel sub-converter.

In one embodiment, regardless of the realization of the DC / DC converter circuit, the upper and the lower DC / DC converter are operated with the same frequency and with offset clocking. For example, for a pulse width modulation of the upper and lower switch equal-frequency and in-phase triangular or Sägezahnträgersignale be used.

To control a mid-phase current, which occurs in each of the input phase with the average voltage, the relative turn-on of the upper and lower DC / DC converter can be changed in opposite directions to each other.

To regulate the output voltage of the AC / DC converter to the output terminals, the relative on times of the upper and lower DC / DC converter can be changed in the same direction.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the subject invention will be explained in more detail with reference to preferred embodiments, which are illustrated in the accompanying drawings. Each show schematically: <Tb> FIG. 1: <sep> Topological basic structure of a hybrid three-phase rectifier system according to the invention, comprising a three-phase diode bridge 1, three mains-frequency clocked bi-directional switches 6, 7, 8 and an upper and a lower high-frequency clocked DC / DC converter 9,13. <Tb> FIG. 2: <sep> topology of a hybrid three-phase buck converter rectifier system according to the invention with sinusoidal input current and regulated output voltage, wherein electronic switches 6, 7, 8, 22, 26 are implemented by way of example by means of bipolar switches. <Tb> FIG. 3: <sep> topology of a hybrid three-phase buck converter rectifier system according to the invention with sinusoidal input current and regulated output voltage, wherein electronic switches 6, 7, 8, 22, 26 are implemented by way of example by means of unipolar switches. <Tb> FIG. 4: <sep> Exemplary illustrated topology of an upper inverting converter. <Tb> FIG. 5: <sep> Exemplary illustrated topology of an upper buck and boost converter. <Tb> FIG. 6: <sep> Exemplary illustrated topology of an upper boost converter. <Tb> FIG. 7: <sep> Exemplary illustrated topology of an upper Cuk converter (Cuk, name, no abbreviation) <Tb> FIG. 8: <sep> Exemplary topology of an upper SEPIC converter (SEPIC, Single Ended Primary Inductance Converter). <Tb> FIG. 9: <sep> Exemplary topology of an upper turn-on converter. <Tb> FIG. 10 <sep> Control structure of a hybrid three-phase buck converter rectifier system according to the invention with a superimposed output voltage control and a lower-level control of the currents in an upper output inductance 25 and a lower output inductance 29, based on pulse width modulation with a constant switching frequency. <Tb> FIG. 11 <sep> Control structure of a hybrid three-phase step-down converter rectifier system according to the invention with a superimposed output voltage regulation and a lower-level tolerance band regulation of the currents in an upper output inductance 25 and a lower output inductance 29.

Basically, the same parts are provided with the same reference numerals in the figures.

WAYS FOR CARRYING OUT THE INVENTION

In detail, the system according to FIG. 1 by a three-phase mains-driven diode bridge 1 with input side filter capacitors 2 in star or delta connection, one of connection points 3, 4, 5 of three input phases with the three-phase diode bridge branching star circuit of three electronic four-quadrant switches (bidirectional switches) 6,7,8, an upper DC / DC converter (-Steller) 9 with an input-side connection point 10 (left), a lower connection point 11 as a reference point and an output-side connection point 12 (right), a lower DC / DC converter 13 with an input-side connection point 14 (left), an upper connection point 15 as a reference point and an output-side connection point 16 (right) and between the output-side connection points 12 and 16 of the two DC / DC converter, which with a positive respectively a negative output terminal 17, 18 of the system, lying output capacitance 19 to S Use of the output voltage 20 is formed. A load 99 is connected between the output terminals 17, 18. The output capacitance 19 may be present as part of a load, or may be implemented as part of the AC / DC converter. The input-side terminal 10 of the upper DC / DC converter 9 is connected to the positive terminal or output of the three-phase diode bridge in the current flow direction and the input-side terminal 14 of the lower DC / DC converter 13 to the negative terminal or output of the three-phase diode bridge in the current flow direction connected. Further, the lower terminal point 11 of the upper DC / DC converter 9 and the upper terminal point 15 of the lower DC / DC converter 13 with each other, forming a DC side center, and connected to the neutral point of the four-quadrant switch (switch star point) 21. For the hybrid buck converter rectifier system according to the invention, the DC / DC converters 9 and 13 are designed as step-down converters, the upper DC / DC converter 9 also being referred to as the upper step-down converter and the lower DC / DC converter 13 also as the lower step-down converter.

The three-phase bridge circuit, which allows bidirectional power transmission in the general case by adding electronic switches in anti-parallel to the diodes of the three-phase diode bridge 1, can be considered as AC / DC sub-converter of the entire AC / DC converter, and forms together with the two DC / DC controllers the entire AC / DC converter.

Fig. 2 shows the circuit topology of the hybrid buck converter rectifier system according to the invention by way of example with bipolar electronic switches and Fig. 3 exemplified with unipolar electronic switches. The following description of the circuit topology is based on FIG. 2, but applies analogously to the embodiments of FIGS. 1 and 3.

Branching from the input side terminal point 10 of the upper buck converter 9, an electronic switch 22 of the upper buck converter (the upper switch) is placed in the current flow direction against the cathode of an upper freewheeling diode 23, the anode to the lower terminal 11 of the upper buck converter and consequently with the Star point (switch star point) 21 of the four-quadrant switch is connected. From the connection point (or upper connection point) 24 of the upper switch 22 and the upper freewheeling diode 23 branches off an upper Ausgangsinduktivität 25 whose other end is connected via the output-side terminal 12 of the upper buck converter to the positive output terminal 17 of the system. Similarly, for the lower buck converter, a lower electronic switch 26 in the current flow direction with a first terminal against the input side terminal 14 of the lower buck converter 13 and thus against the negative output of the three-phase diode bridge 1 and a second connection to the anode of a lower freewheeling diode 27th whose cathode is connected to the upper terminal 15 of the lower buck converter and thus to the switch star point 21. From the lower connection point 28 of lower switch 26 and lower freewheeling diode 27 branches off a lower Ausgangsinduktivität 29 whose other end is connected via the output-side terminal 16 of the lower buck converter to the negative output terminal 18 of the system.

The star connection of four-quadrant switches is driven the same as for known hybrid dc systems, i. E. only the four-quadrant switch connected to the mean phase in each case is switched through. The timing of the upper switch 22 is such that, assuming a constant current in the upper Ausgangsinduktivität 25 in the phase with the highest voltage sets a sinusoidal, phase-voltage proportional current profile in the short-term average. In the same sense, the lower switch 26 is clocked so that, assuming a constant current in the lower output inductance 29 also in the phase with the 'lowest voltage results in a short-term average sinusoidal, phase voltage proportional current flow. It should be noted that by the timing of the upper switch pulse width modulated sections of the current in the upper output inductance and the timing of the lower switch sections of the current in the lower output inductance over the respective conductive diodes of the upper and lower half of the three-phase diode bridge are performed. These sections are smoothed by the arranged at the input side of the three-phase diode bridge filter capacitors 2 in conjunction with optional, to the network between the input phases and the mains phases 34, 35, 36 arranged filter Vorschaltinduktivitäten, short filter inductances, 30 and so in a continuous Current with a, the short-term average same course in power phase terminals 31,32, 33 of the system transformed. This impression of two network currents leads to a current flow in the third phase due to the zero-imposed sum of the currents of the network phases 34, 35, 36. For example, this is the mean network phase 35. The said current flow closes in this case via the switched-through central four-quadrant switch 7 and the upper and lower freewheeling diode 23, 27 in the system. This ensures a sinusoidal course of all network phase currents.

As a closer analysis shows, by the above-described timing of the upper and lower switches 22, 26, the output voltage of the three-phase diode bridge 1, i. the voltage between the input-side terminal 10 of the upper buck converter 9 and the input-side terminal 14 of the lower buck converter 13, converted into a pulse width modulated, occurring between the upper and lower connection point 24, 28 voltage with a constant short-term average, which after low-pass filtering on the upper and lower Output inductance 25, 29 and the output capacitance between the output terminals 17, 18 of the system sets as output voltage. The value of the output voltage can be adjusted via corresponding same-direction change in the relative duty cycle of the upper and lower switches 22, 26. So there is a regulated output voltage.

Due to the sinusoidal network currents, a temporally constant power is taken from the network, which, assuming loss of system freedom, ultimately via a corresponding output current, i. E. a corresponding current in the upper and lower output inductances 25, 29 is supplied to the output. With a constant short-time mean value of the output voltage, this current must also have a constant short-term average for the formation of constant power, which confirms the assumption made above for the consideration of the sinusoidal form of the mains current.

The input-side frequency is typically a network frequency such as 50 Hz, 60 Hz or 360 Hz to 800 Hz (Bordnetze of aircraft). The switching timing of the two DC / DC converters is high frequency, i. typically at frequencies above 500 Hz, typically above 1 kHz or above 5 kHz. The above-mentioned short-time mean value is to be understood in this context: the high-frequency clocked switches lead to currents which, after smoothing with their short-time average, follow a mains-frequency course. The term clocking refers to switching on and off of a switch for pulse width modulation, wherein a fixed clock frequency may be specified, but also in the context of a modulation without fixed clock frequency, for example in a tolerance band control.

Topological variations

With regard to the realization of the system, various advantageous topological Modifikations- or expansion options, which will be described briefly below.

Execution of the four-quadrant switches

A four-quadrant switch (switch structures 6, 7, 8) can each be realized by an opposing series circuit of two unipolar with reverse voltage loaded electronic switches, for reasons of protection and to enable independent of the specific practical realization of the electronic switch two current flow directions, about each Switch is placed an antiparallel diode or backward conductive switches are to be used.

Alternatively, a four-quadrant switch can also be realized by a single-phase diode bridge and only one arranged between the positive and negative output terminal of the single-phase diode bridge electronic switch, wherein the alternating voltage terminals of the single-phase diode bridge forming the two switch terminals. A further alternative is the anti-parallel connection of two bipolar with reverse voltage loaded but only unidirectional current flow permitting electronic switches. In the future, moreover, the realization by means of a currently available monolithic four-quadrant switch will be possible.

All of the above-mentioned embodiments are known, it should therefore only be emphasized that each of the described embodiments can be used in conjunction with the inventive system.

Arrangement of filter capacitors

In the embodiment of the inventive system described above, the low-pass filtering of the upper or lower switch 22, 26 from the input, or to the input flowing pulse width modulated sections of the currents in the upper and lower Ausgangsinduktivität 25, 29 by filter capacitors 2 at the input of the three-phase diode bridge, ie by AC-side reactive elements, which lead to a corresponding fundamental reactive power, or load the network independently of a load to be fed via the rectifier system, ie even at idle, with reactive current. In another embodiment, therefore, the filter capacitors are at least partially realized on the output side of the three-phase diode bridge. Specifically, for example, an upper filter capacitor from the input side terminal 10 of the upper buck converter is placed against the switch star point 21 and switched from the switch star point 21 a lower filter capacitor against the input side terminal 14 of the lower buck converter.

Capacitive coupling of the output voltage center and the star point of the filter capacitors / filtering of the common-mode voltage

Three-phase rectifier systems have i.a. missing connection of the output voltage center point and the neutral point of the three-phase network, a common-mode component of the output voltage, i. the output voltage center shifts in terms of potential with three times the mains frequency or switching frequency with respect to the mains neutral point. Through this common mode voltage flow across parasitic capacitances of the output circuit common mode interference currents to earth or the network star point, which must not exceed a defined by EMC regulations Mass, or can be disturbed by the potential shift of the output voltage a subsequent, powered by the system consumers.

According to the invention, the switching-frequency component of the common-mode voltage, in the present case by a capacitive coupling of the output circuit and the network, can now be suppressed. For this purpose, e.g. from the positive and negative output terminal 17, 18, filter capacitors (coupling capacitors) against the neutral point of the AC side filter capacitors (filter capacitor star point) 37 to lay. Alternatively, a capacitive connection of both output terminals 17, 18 to each of the mains phases 34, 35, 36, or an output voltage center formed by an example, capacitive voltage divider with the filter capacitor star point 37 done.

In each case, a low-pass filtering of the common-mode voltage is achieved with the common-mode filter inductance being the parallel connection of the upper and lower output inductances 25, 29 and the filter capacitance being the parallel connection of the coupling capacitors. Supporting this can be inserted between the upper and lower output inductance and the output capacitance, a common mode filter inductance, which has a very high impedance for common mode noise. As a result, only the low-frequency component of the common-mode voltage, which is i.a., remains above the coupling capacitances or for the potential movement of the output circuit. leads to no disturbance.

Arrangement of only one output inductance

In the basic form of the system according to the invention, a separate output inductor 25, 29 is arranged both for the upper and the lower step-down converter 9, 13, with which an advantage is given with regard to the common-mode voltage occurring at the output of the system. However, neglecting parasitic currents flowing in the upper and lower output inductances, the same current flows, so that without good common-mode filter characteristics, one of the two output inductances 25, 29 can be omitted without impairing the basic function of the system.

Realization of the upper and lower buck converter by means of several parallel-connected and phase-shifted clocked subsystems

The upper and lower buck converter 9, 13 are in the simplest case in each case by an electronic switch 22, 26, a freewheeling diode 23, 27 and an output inductance 25, 29 realized. In one embodiment of the invention, each of the two buck converters 9, 13 realized by a plurality of parallel units of the same structure, in which case the electronic switches of an upper, consisting of n subsystems Tiefsetzstellergruppe a same time, but each set by 360 ° / n phase drive signal where 360 ° corresponds to the length measured in degrees of a pulse period. Analogously, the implementation and control of the lower buck converter group to make.

Replacement of the buck converters by other DC / DC converter basic circuits

In the implementation described above, aimed at a three-phase buck converter rectifier system according to the invention, a step-down converter is used in each case for the upper and lower DC / DC converters 9, 13. Alternatively, an upper and a lower inverter converter or an upper and a lower buck and boost converter can be provided. This then also an increase in the output voltage 20 is possible. The connection points 10, 11, 12, 14, 15, 16 of the DC / DC converter 9, 13 remain unchanged.

For the modification to an inverse converter, the upper and the lower freewheeling diode 23, 27 are to be replaced by an upper and a lower inverter transformer inductance. Further, the upper output inductance 25 is directed toward the upper connection point 24 through a terminal-side terminal 12 of the upper inverter. upper output diode and the lower output inductance 29 by a, to be replaced by the lower connection point 28 against the output-side terminal point 16 of the lower inverting converter, lower output diode to replace. The polarity of the voltage at the positive pole now at the cathode of the lower output diode and the negative pole at the anode of the upper output diode lying output capacitance 19 is then inverse to that of the three-phase buck converter rectifier circuit of Fig. 1, Fig. 2 and Fig. 3. However, the control of the electronic switch can be done similarly analogously, and only dependent on the Aussteuergrad (the average duty cycle of the electronic switch) can also be set above the 1.5-fold value of the mains phase voltage amplitude output voltage. 4 shows, by way of example, the circuit topology of the upper inverting converter.

In a further development of the basic concept, the potential-separated form of the inverted converter known as flyback converter (flyback converter) can also be used.

An extension of the output voltage range of the grandform, i. the buck converter form of the inventive system upward is also possible by supplementing the circuit structure by an upper and a lower electronic boost converter switch and an upper and a lower boost converter diode, resulting in a system with low and high boost characteristics. In this case, the upper output inductor 25 is to be detached from the output-side connection point 12 of the upper buck converter and to insert an upper boost converter diode from the now free terminal of the output inductance in the current flow direction against the output-side terminal 12 of the upper buck converter. In addition, an upper boost converter transistor is to be laid from the anode of the upper boost converter diode in the current flow direction against the switch star point 21. Similarly, the lower output inductor 29 is to be disconnected from the output side terminal 16 of the lower buck converter and a lower boost converter diode from the output side terminal 16 of the lower buck converter in current flow direction against the now free terminal of the lower output inductance and finally a lower boost converter transistor from the switch star point 21 against the cathode of the lower Hochsetstellerdiode to lay. Fig. 5 shows by way of example the circuit topology of the upper buck and boost converter.

For pure buck functions of the system, the lower and upper boost converter transistors then remain off, and only the electronic switches 22, 26 of the buck converters are clocked. For boost converter operation, the electronic switches 22, 26 of the buck converters remain turned on and the boost converter transistors are clocked.

If only one boost converter operation of the system is required, a simplification of the circuit just described is possible. It can then, starting from the above-described buck and boost converter circuit, the upper and lower freewheeling diode 23, 27 of the buck converter omitted without replacement, and the upper and lower electronic switches 22, 26 of the buck converter are replaced by short-circuit connections, whereby the upper connection point 24 directly at the input-side connection point 10 of the upper buck converter and the lower connection point 28 comes to rest directly on the input-side connection point 14 of the lower buck converter. 6 shows, by way of example, the circuit topology of the upper boost converter.

Other alternative circuit variants with buck and boost function are by replacing the upper and lower DC / DC Bucksetzstellers by an upper and lower DC / DC Cuk converter (Cuk, name, no shortcut) or an upper and lower DC / DC -SEPIC converter (Single Ended Primary Inductance Converter SEPIC) possible.

For the Cuk variant, an upper inductance is applied from the input-side connection point 10 of the upper Cuk converter, starting from a first upper circuit point, and from this an upper switch is connected against the lower connection point 11 of the upper Cuk converter. Furthermore, from the first upper circuit point, an upper coupling capacitor is placed against a second upper circuit point and fed by this upper output inductance to the output side terminal 12 of the upper Cuk converter, and additionally from the second upper circuit point, an upper diode in the current flow direction against the lower connection point eleventh arranged directed the upper Cuk-converter. Likewise, from the input-side connection point 14 of the lower CUK converter, a lower inductance is applied against a first lower circuit point, and a lower switch in the current flow direction is switched from the upper connection point 15 of the lower Cuk converter. Furthermore, from the first lower node, a lower coupling capacitor is placed against a second lower node and passed from this a lower output inductance against the output side terminal 16 of the lower Cuk converter, and additionally a lower diode in the current flow direction from the upper terminal 15 of the lower Cuk converter directed directed against the second lower circuit point. Fig. 7 shows by way of example the circuit topology of the upper Cuk converter.

For the SEPIC variant, starting from the Cuk converter, the upper output inductance of the Cuk system is controlled by an upper output diode directed towards the output terminal 12 of the upper Cuk converter and the lower output inductance by a lower connection point 16 of the lower Cuk converter. Converter away directed lower output diode replaced. Furthermore, instead of the upper diode of the upper Cuk converter, an upper output-side inductance and, in place of the lower diode of the lower Cuk converter, a lower output-side inductance are inserted. Fig. 8 exemplifies the circuit topology of the upper SEP IC converter.

Reversal of power flow direction - Inverter operation and bidirectional power exchange with the grid

Three-phase rectifier systems are used to power a load such as load 99 from the three-phase network. In connection with the grid feed of regenerative energy, however, there is increasingly the demand for the possibility of a return of energy from the DC to the AC side (AC, Alternating Current) or even after a bidirectional power transfer, for example, when batteries are to be charged from the network , or subsequently the batteries to support the network, ie to be used for power delivery to the network. The load 99 is a source of power in such a case.

If only one injection of power into the three-phase network, i. a power transfer from the DC to the AC side take place, the switching frequency operated active elements (upper and lower switches 22, 26 and diodes, without the bidirectional switches 6, 7, 8) of the previous circuits to exchange such that instead of an electronic switch is a diode with inverse current flow direction and instead of a diode to provide an electronic switch with inverse current flow direction. Furthermore, an electronic switch is to be arranged in anti-parallel to each diode of the three-phase diode bridge 1, which is only turned on when the associated diode can take over a current flow due to the AC-side potential conditions. The rest of the system can remain unchanged.

On the other hand, to provide the possibility of a bidirectional power exchange between AC and DC side, in addition to the switching frequency operated active elements of the rectifier circuits described so far more active elements are to be arranged. Specifically, antiparallel to each electronic switch (apart from the bi-directional switches 6, 7, 8) is to place a diode and an anti-parallel to each diode an electronic switch. In addition, an electronic switch is to be arranged in antiparallel to each diode of the three-phase diode bridge 1, which is only turned on when the associated diode can take over a current flow due to the mains voltage side potential conditions.

Bidirectional power exchange with the network

The previous description has described operation with output capacitor 19, i. based on impressed output voltage. In particular, in the field of drive technology, however, applications are also known where, for example, an inverter stage is fed with impressed current (current source inverter) and this current is impressed in the motor phases in the form of a full block clocking or pulse width modulated. For such applications, the output capacitor 19 can be omitted, or the value of the output capacitance can be reduced to the capacity value that may still be required for the limitation of overvoltages. This results in a mixed form of power source inverter and voltage link converter with variable DC link voltage.

For the sake of completeness, mention should be made here of the simplest form of a possible bidirectional DC / DC converter, which will be referred to below as a switch-on converter. For this purpose, a connection is inserted between the input-side connection point 10 and the output-side connection point 12 of an upper turn-on converter and an electronic switch in the current flow direction from the upper connection point 24, starting with an anti-parallel diode to the lower connection point 11 of the upper Einschalt converter. Correspondingly, a connection is inserted between the input-side connection point 14 and the output-side connection point 16 of the lower turn-on converter, and an electronic switch in the current flow direction from the upper connection point 15 with an anti-parallel diode is connected to the lower connection point 28 of the lower turn-on converter. Furthermore, for conventional clock frequencies, an additional commutation inductance is to be inserted between the switch star point 21 and the connection between the lower connection point 11 of the upper turn-on converter and the upper connection point 15 of the lower turn-on converter. 9 shows, by way of example, the circuit topology of the upper turn-on converter.

Basic control concept for three-phase buck converter rectifier system Control of four-quadrant switches

According to the invention, the potential of the associated mains phase terminal 31, 32, 33 is conducted to the switch star point 21 by switching through the four-quadrant switch 6, 7, 8 connected to the middle phase. In a symmetrical three-phase system, the middle phase is always the phase with the smallest amount of voltage and vice versa. Now change the network-side potential relationships, i. is to switch to a new middle phase, this switching takes place according to the invention preferably in the de-energized state of the four-quadrant switches 6, 7, 8. This can be achieved by turning off the electronic switches 22, 26 of the two buck converter, since then the current of the upper and lower Ausgangsinduktivität 25, 29 via the upper and lower freewheeling diode 23, 27 is guided, whereby due to the equality of both currents no current flow through the middle phase occurs. Here and elsewhere, the term "middle" or "highest" or "deepest" phase, etc., refers to the medium, highest, lowest phase in terms of voltage. The four-quadrant switch connected to the previous middle phase can then be switched off and closed after a safety time of the four-quadrant switch of the new middle phase. The safety time reliably prevents a short circuit between two network phases. Subsequently, then the regular timing of the upper and lower electronic switch 22, 26 are resumed.

It should be noted that depending on the synchronization of the timing of the upper and lower buck converter, a simultaneous freewheeling both systems can also set periodically operational and then only a corresponding extension of this freewheeling phase is required to allow a safe switching of the four-quadrant.

Synchronous or antiphase clocking of the upper and lower step-down converter

The size of power electronic systems is largely determined by the volume of the passive filter components. It makes sense, therefore, to choose the controller so that e.g. the ripple of the current in the output inductances 25, 29 is minimized, or for a given permissible ripple the lowest possible inductance value is to be provided.

According to the invention this can be achieved in that the pulse width modulation of the upper and lower electronic switches 22, 26 takes place with equal frequency, but offset by half a clock period triangular or sawtooth carrier signals. However, then occurs due to the phase-shifted conduction of the upper and lower freewheeling diode 23, 27, a high switching frequency ripple of the current in the middle phase, or is to provide a high filtering effort for the smoothing of the current.

According to the invention, the filtering effort can be minimized if 26 equal-frequency and in-phase triangular or Sägezahnträgersignale be used for the pulse width modulation of the upper and lower switches 22. In this case, however, results in a relatively high ripple of the current in the Ausgangsinduktivitäten 25, 29th

As previously described, filter capacitors 2 are used for smoothing the pulse-shaped current at the input of the three-phase diode bridge 1. The voltage of these filter capacitors consequently results in a switching frequency fluctuation, i. higher or lower values than the purely sinusoidal mains voltage, so that just before the previous upper or previous lower phase is replaced by a new phase, the new upper or the new lower phase can already take over power, resulting in a deviation of the mains current from the sinusoidal shape can.

According to the invention, this current distortion, which results in a corresponding reduction in the filter capacitor voltage ripple, is prevented by raising the clock frequency of the buck converter whose input voltage transitions to a new phase. The frequency increase of one of the two buck converters thus always takes place when a change takes place between upper and middle phase or middle and lower phase.

regulation

The regulation of the three-phase buck converter rectifier system is preferably carried out in a double-ended manner. Fig. 10 shows a corresponding controller structure.

The setpoint-actual difference 38 between an output voltage setpoint value 39 and a measured output voltage 20 is here applied to the input of an output voltage regulator 40 and dynamically evaluated or amplified and converted into a setpoint value of the output current (output current setpoint value) 41 for lower-level output. Current control loops transformed. For this purpose, an actual value of the load current which can be detected by measurement in the load supply line can still be added in the sense of a precontrol.

Control based on pulse width modulators, constant switching frequency

According to the invention, the output current value 41 is now applied to the input of two output current control loops, i. an upper output current control loop for regulating the current in the upper output inductance 25 and a lower output current control loop for controlling the current in the lower output inductance 29. The upper output current control circuit has on the input side an upper desired-actual comparison 42 (by means of a subtractor), at whose first positive input the output current value 41 and at its second negative input a measured actual value of the current 43 in the upper output inductance is present and its output in the form the control deviation 44 of the upper output current is fed to the input of an upper output current regulator 45 and dynamically evaluated and amplified by this. The upper output current regulator then forms at the output a voltage 46 to be set at the upper connection point 24 for bringing the current actual value to the desired value, which is applied to the first positive input of an upper summing element 47, to whose negative input an upper triangular or sawtooth pulse-width modulation carrier signal 48 is applied and at the second positive input of which a high voltage 50 measured and scaled at the positive output of the three-phase diode bridge is applied. The scaled upper voltage 50 is formed at the output of an upper scaling element (multiplier) 51, at the first input of which the voltage measured at the positive output of the three-phase diode bridge 49 and at its second input to the output voltage setpoint and the mains voltage amplitude proportional factor 52 is applied. The output 53 of the upper summing 47 is then applied to an upper comparator 54 with comparison threshold zero and thus formed at the output of the upper comparator a pulse width modulated drive signal 55 for the upper electronic switch 22, which leads to a voltage formation at the upper connection point 24 such that the Actual value of the current in the upper output inductance 25 follows the reference value of the upper output current. On the other hand, sinusoidal modulation of the relative turn-on time of the upper electronic switch 22 is achieved by the voltage 50 lying at the second positive input of the upper summing element 47 in such a way that a current with a sinusoidal short-time average flows through the respective suffering diode of the upper half of the three-phase diode bridge 1. This current then occurs, as already mentioned, after low-pass filtering by the input filter capacitors 2 and the filter inductances 30 arranged upstream of the network as a mains phase current.

A fully analog current control arrangement (lower output current control loop) is provided for the regulation of the current 56 in the lower output inductance 29. The only difference is that there the voltage 57 measured at the negative output of the three-phase diode bridge is inverted to the first input of a lower scaling element 58, which is explained by the fact that this voltage value naturally has a negative value which is equal to one Processing as for the upper output current control loop must be inverted.

The last part of the inventive device for controlling the system is finally a Netzspannungsauswerteschaltung 59, at the input of the three input phase voltages at the connection points 3, 4, 5 are laid and their function is to detect the input phase with the smallest amount of voltage in terms of absolute value and at the output three drive signals 60, 61, 62 for the three four-quadrant switches 6, 7, 8 to be formed such that in each case connected to the phase smallest voltage amount four-quadrant switch, is turned on and the other two four-quadrant switch remain in the off state.

The arrangement of two output current control loops is particularly advantageous when multiple pulse rectifier systems are connected in parallel on the input and output side, since then otherwise circular currents could form between the systems. If the upper and lower output currents are regulated, these are reliably prevented from leading to unbalanced loading of the parallel systems.

If only one pulse rectifier system is present, since no alternative path for the recirculating current is possible, the currents in the upper and lower output inductances 25, 29 have the same value, and the output current control can take place by means of only one output current regulator. If e.g. maintained upper output current controller 45, so the lower target-actual comparison 63 and the lower output current controller 64 can be omitted, and it is only in place of the output signal 65 of the lower output current controller, the output signal 46 of the upper output current regulator 45 to the first positive input of the lower summer 67th to lay.

Control based on tolerance band control of the output currents

In addition to a linear, based on pulse width modulation control of the upper and lower output current 43,56, the subordinate to the output voltage control, control circuits for the upper and lower output current can also be realized in the form of tolerance band controls, as shown in Fig. 11.

According to the invention occurring at the output of the upper target-actual comparison 42 control deviation 44 of the upper output current 43 is applied to the first positive input of an upper Offsetsummiergliedes 68, at the second positive input, from the device for controlling the current in the middle phase (middle phase current control) formed offsetsignal 69 is applied and whose output 70 is fed to an upper Hystereseschaltglied 71, which directly generates the drive signal 55 for the upper electronic switch 22 at its output. Furthermore, occurring at the output of the lower target-actual comparison 63 control deviation 72 of the lower output current 56 is applied to the first positive input of a lower Offsetsummiergliedes 73, applied to the second negative input, formed by the Mittenphasenstromregelunges, offset signal 69 and its output 74 is led to a lower hysteresis 75, which generates at its output directly the drive signal 76 for the lower electronic switch 26.

The two hysteresis switching elements 71, 75 actuate the electronic switches such that the upper and lower output currents are conducted in a tolerance band corresponding to the hysteresis width along the output current value. The offset signal 69 has no direct influence on the current conduction. This is explained by the fact that the positive inclusion of the offset in the upper output current control loop and the negative inclusion in the lower output current control loop requires, for example, a positive offset value after a current increasing in the upper output inductance 25 and decreasing in the lower output inductance 29, a current situation which 1. Kirchhoff's law is not adjustable. The offset signal thus only leads to an opposite change in the relative switch-on times of the upper and lower electronic switches (which, on average, has no influence on the differential voltage between the upper and lower connection points 24, 28 and thus on the current conduction) and thus causes the formation of a corresponding one Difference of the current in the upper and lower freewheeling diode 23, 27, which occurs as current in the middle phase. Thus, a possibility of sinusoidal control of the current in the middle phase (middle phase current) 77 is given by the offset signal 69.

The desired value of the middle phase current is formed by a functional unit 78, at the input of which the three mains phase voltages are applied. This functional unit determines the respective minimum voltage amount having phase and switches the time course of this phase voltage 79 at its output. The output of this functional unit is then fed to the first input of a scaler 80 for mid-phase current control, to whose second input a signal 81 proportional to the output current setpoint 41 is applied, so that at the output of the scaler for the mid-phase current control 80 the set point of the middle phase current (middle phase current setpoint) 82 is formed, that equivalency of the power transfer caused by the output current and of the sinusoidal network phase currents and thus also of the middle phase current is given. The mid-phase current setpoint value 82 is now fed to the positive input of a middle-current desired-actual comparison 83, at the negative input of which a measured middle phase actual value 85, which has been freed of switching-frequency spectral components via a low-pass filter 84 (LP, Low Pass). The output of the center current desired-actual comparison, i. a control deviation 86 of the center phase current, is then applied to a mid-phase current controller 87, at the output of which the above-described offset signal 69 occurs, via which, as described above, the desired course of the center phase current 77 can be adjusted.

A respective dynamic of the various regulators, such as output voltage regulator 40, output current regulator 45, center-phase current regulator 87 is denoted by the transfer functions R (s), G + (s), G. (s), K | (s) etc. and corresponds for example to each a P, PI, or PID or higher order regulator, and is determined by conventional control engineering design techniques.

Claims (15)

1. AC / DC converter for power transmission from a three-phase AC side to a DC side with a positive terminal (17) and a negative terminal (18) and / or vice versa, comprising a three-phase bridge circuit (1) with three terminals (3, 4, 5) for each one input phase on its AC voltage side and each having a positive and a negative connection point on its DC voltage side, characterized in that - is connected between the positive terminal of the bridge circuit and the positive terminal (17) of the AC / DC converter, an upper DC / DC converter (9) with a lower connection point (11) as a reference point, and - is connected between the negative terminal of the bridge circuit and the negative terminal (18) of the AC / DC converter, a lower DC / DC converter (13) having an upper terminal point (15) as a reference point, and - Between the positive terminal (17) and the negative terminal (18), a capacitor (19) is arranged, wherein the lower and upper terminal points (11, 15) are connected to each other and thereby form a DC-side center, and each of the three connection points (3, 4, 5) via a four-quadrant switch (6, 7, 8) directly or via a Commutation inductance is connected to the DC side center. 2. AC / DC converter according to claim 1, wherein the upper DC / DC converter (9) and the lower DC / DC converter (13) each comprise both buck converters, inverters, buck and boost converter, boost converter, Cuk converter, SEPIC converter or power-on converter are. 3. AC / DC converter according to claim 1 or 2, wherein the upper DC / DC converter (9) and the lower DC / DC converter (13) are constructed symmetrically to each other. 4. AC / DC converter according to claim 1 or 2, wherein inductances of the upper DC / DC converter (9) and the lower DC / DC converter (13), are realized together by a single component. 5. AC / DC converter according to one of the preceding claims, wherein the positive terminal (17) and the negative terminal (18) form a common output, and the upper and the lower DC / DC converter (9, 13) with each other so interconnected and designed that between an upper input of the upper DC / DC converter (9), formed by an input-side connection point (10) of the upper DC / DC converter (9) connected to the positive connection point of the bridge circuit and by the lower connection point (11) of upper DC / DC converter (9), and the common output, as well between a lower input of the lower DC / DC converter (13), formed by an input-side connection point (14) of the lower DC / DC converter (13) connected to the negative connection point of the bridge circuit and by the upper connection point (15) of lower DC / DC converter, and the common output, a unidirectional power flow leading to the common output or a unidirectional power flow starting from the common output or a bidirectional power flow is possible, - In the operation of the converter over the capacitor (19) voltage applied (20) smaller, equal to or greater than a voltage between the input-side connection point (10) of the upper DC / DC converter and the input-side connection point (14) of the lower DC / DC converter is. 6. AC / DC converter according to one of the preceding claims, which is designed as a rectifier for a power flow from the AC voltage side to the DC side, wherein the three-phase bridge circuit (1) is a diode bridge circuit and the upper and the lower DC / DC converter (9 , 13) are designed for a power flow from the positive and negative terminal of the bridge circuit to the positive and negative terminal (17, 18) of the AC / DC converter. 7. AC / DC converter according to one of the preceding claims, which is designed as an inverter for a power flow from the DC voltage side to the AC side, wherein the three-phase bridge circuit (1) is an inverter bridge circuit of electronic switches each having anti-parallel diodes and the upper and the lower DC / DC converter (9, 13) are designed for a power flow from the positive and negative terminal (17, 18) of the AC / DC converter to the positive and negative terminal of the bridge circuit. 8. AC / DC converter according to one of the preceding claims, which is designed as a bidirectional converter for a power flow from the AC side to DC side or vice versa, wherein the three-phase bridge circuit (1) is an inverter bridge circuit of electronic switches with respective anti-parallel diodes and the upper and the lower DC / DC converter (9, 13) is designed for bidirectional power flow from the positive and negative terminal (17, 18) of the AC / DC converter to the positive and negative terminal of the bridge circuit or in the opposite direction. 9. AC / DC converter according to one of the preceding claims, wherein the upper of the lower DC / DC converter (9, 13) are each formed by parallel circuits of a plurality of partial converters of the same structure. 10. AC / DC converter according to one of the preceding claims, wherein between the output-side connection points (12, 16) of the upper and lower DC / DC converter (9, 13) and connection points of the capacitor (19) is inserted a common mode inductance and a capacitive Coupling of the positive and negative terminal (17, 18) of the AC / DC converter takes place with the AC side, wherein - A voltage center formed by a capacitive voltage divider between the positive and the negative terminal (17, 18) is connected to an AC side filter capacitor star point (37), and / or - Coupling capacitors between the positive and the negative terminal (17, 18) and the filter capacitor star point (37) are connected, or - Coupling capacitors between the positive and the negative terminal (17, 18) and each network phase (34, 35, 36) are connected. 11. AC / DC converter according to one of the preceding claims, wherein between a connected to the positive connection point of the bridge circuit input side terminal point (10) of the upper DC / DC converter (9) and the DC side center respectively switch star point (21) of the four-quadrant switch ( 6, 7, 8), and filter capacitors are connected between the switch star point (21) and an input-side connection point (14) of the lower DC / DC converter (13) connected to the negative connection point of the bridge circuit. 12. A method for controlling an AC / DC converter according to one of the preceding claims, wherein - The upper DC / DC converter (9) is clocked so that the current flowing from the positive connection point of the bridge circuit to the upper DC / DC converter (9) in the short-term mean sinusoidal and proportional to the voltage curve in the mains phase with the highest Tension is, and - The lower DC / DC converter (13) is clocked so that the current flowing from the lower DC / DC converter (13) to the negative terminal of the bridge circuit, sinusoidal in the short-term average and proportional to the voltage curve in the mains phase with the lowest Tension is, and - Of the three Vierquadrantenschaltern (6, 7, 8) in each case the one which is connected to the input phase with the average voltage or the input phase least voltage is switched, during which the other two four-quadrant switch remain in the off state, whereby a likewise sinusoidal summation current in This input phase results. 13. The method according to claim 12, wherein the upper and the lower DC / DC converter (9, 13) with the same frequency and - be operated with staggered timing by using for a pulse width modulation of the upper and lower switch (22, 26) equal frequency and phase offset by half a clock period triangular or Sägezahnträgersignale, or - be operated with in-phase clocking by equal-frequency and in-phase triangular or Sägezahnträgersignale be used for a pulse width modulation of the upper and lower switch (22, 26). 14. The method according to claim 12 or 13 with pulse width modulation, - wherein for the pre-control of the turn-on of the upper switch (22) at least the desired value (39) of an output voltage between the positive terminal (17) and the negative terminal (18), a mains voltage on the AC side and a positive terminal of the three-phase bridge circuit (1 ) tapped and referenced to a net star point voltage, and - For pre-control of the turn-on of the lower switch (26) at least the desired value (39) of the output voltage, the mains voltage and at the negative connection point of the three-phase bridge circuit (1) tapped and used to a network star point voltage can be used. 15. The method according to claim 12, wherein for controlling a middle phase current (77), which occurs in each case the input phase with the average voltage with respect to the other input phases, - A setpoint (82) of the center phase current (77) from the time course (79) of the current minimum voltage amount having phase and a current setpoint (41) proportional signal (81) is determined, and - At the input of a mid-phase current regulator (87) a difference from the desired value (82) of the middle phase current and a measured and passed through a low-pass filter (84) center phase current value from which at the output of the middle phase current regulator (87) an offset signal (69) is generated, and - Ansteuersignale (55, 76) of the upper and lower switch (22, 26) by means of a respective upper and lower Hystereseschaltgliedes (71, 75) are generated, and - an input signal (70) of the upper hysteresis switching element (71) is formed by adding the offset signal (69) to a control deviation (44) of an upper output current and an input signal (74) of the lower hysteresis switching element (75) by subtracting the offset signal (69) from a control deviation (72) of a lower output current is formed.