GB2530132A

GB2530132A - Inverter circuit and method for operating an inverter circuit

Info

Publication number: GB2530132A
Application number: GB1509685.2A
Authority: GB
Inventors: Patrick Ebelshaeuser; Marco Koehler; Moritz Berberich
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2014-06-04
Filing date: 2015-06-04
Publication date: 2016-03-16
Also published as: GB201509685D0; FR3022090A1; DE102014210647A1

Abstract

An inverter circuit 110 comprises a series-connected arrangement of two switching elements 145, 147, connected in series between an input connection 150 and a first intermediate connection 152. Both switching elements have a parallel connected arrangement of a controllable switch T5, T6 and a diode D5, D6, where the first diode D5 is poled in a direction other than that of the second diode D6. A third switching element 155 is disposed between the first intermediate connection and the output connection 140, having a parallel connected arrangement of a third controllable switch T2 and a third diode D2 having its cathode connected to the first intermediate connection and its anode connected to the output connection. A fourth switching element 160 is disposed between the output connection 140 and a second intermediate connection 157, having a parallel connected arrangement of a fourth controllable switch T3 and a fourth diode D3 having its cathode connected to the output connection and its anode connected to the second intermediate connection. The inverter may be a multilevel inverter used for feeding renewably generated energy into an energy supply grid.

Description

Description Titie

Inverter circuit and methcd fcr operating an inverter circuit

Prior art

The present invention relates to an inverter circuit and a method for operating an inverter circuit, a corresponding control device and a corresponding computer program product.

In modern energy supply grids, the feed-in of renewably generated electrical energy is becoming increasingly important. Frequently, however, in such cases this renewably generated electrical energy is present as direct voltage, which has to be converted to an alternating voltage for feeding in to the public energy supply grids.

Tnverter circuits, which effect such a conversion, are used for this purpose. In such cases, however, it is particularly problematic that, in the case of variable DC voltage of greater than 1000 V, many of the multilevel inverter or converter circuits in the prior art themselves have a high energy demand, such that the efficiency of such an inverter circuit is impaired.

DE 10 2011 079 214 Al describes a converter circuit.

Disclosure of the invention

Against this background, the approach presented here presents an inverter circuit, a method for operating an inverter circuit, an inverter, additionally a control device that uses this method, and, finally, a corresponding computer program product, according to the main claims.

Advantageous designs are disclosed by the respective

dependent claims and the following description.

The approach presented here creates an inverter circuit having the following features: -an input connection, a first intermediate connection, a second intermediate connection and an output connection; -a series-connected arrangement of two switching elements, connected in series between the input connection and the first intermediate connection, a first switching element having a parallel-connected arrangement of a first controllable switch and a first diode, and the second switching element having a parallel-connected arrangement of a second controllable switch and a second diode, the first diode being poled in a direction other than that of the second diode in respect of a direction of flow in the series-connected arrangement; -a third switching element, which is disposed between the first intermediate connection and the output connection, and which has a parallel-connected arrangemenc of a third controllable switch and a third diode, the cathode of the third diode being connected to the first intermediate connection, and the anode of the third diode being connected to the output connection; and -a fourth switching element, which is disposed between the output connection and the second intermediate connection, and which has a parallel-connected arrangement of a fourth controllable switch and a fourth diode, the cathode of the fourth diode being connected to the output connection, and the anode of the fourth diode being connected to the second intermediate connection.

A connection is to be understood to mean, for example, an electrical tapping point or, generally, a conductor piece of equal electrical potential. A switch is to be understood to mean an electrical or electronic component that is able to control, release or block a flow of current. For example, such a switch may be a mechanical switch, or an electronic switch such as, for example, a thyristor or an IGBT. The inverter circuit in this case may be designed such that, when a voltage is applied to the input connection, it outputs a pulsed electrical voltage signal at the output connection, in order to generate an alternating voltage by means of this voltage signal as an output signal.

The approach presented here is based on the concept that a circuit that can be adapted in a highly flexible manner to differing application scenarios can be created with very few electrical or electronic components. Such a circuit offers the possibility of generating an alternating voltage, in a very simple and flexible manner, from a direct voltage to be applied to the input connection, whereas, in the prior art, a circuit having a flexibility corresponding to the inverter circuit presented here would be significantly more elaborate.

The approach presented here offers the advantage of supplying, for the purpose of providing an alternating voltage, an output signal that is generated with the inverter circuit having a high degree of flexibility and with the use of a very small number of necessary electrical or electronic components. This, in turn, offers the advantage that the generation of the output signal involves only a very small power loss. At the same time, since only a small number of electrical or electronic components is reguired, a very inexpensive inverter circuit can be realized.

Also advantageous, moreover, is an embodiment of the present invention in which there is additionally provided an intermediate diode, which is disposed between a reference potential connection, in particular a frame potential connection, and the first intermediate connection, and/or there is additionally provided a second intermediate diode, which is disposed between a reference potential connection, in particular a frame potential connection, and the second intermediate connection. A reference potential connection in this case may be understood to mean, for example, a connection that is connected to frame. Such an embodiment of the present invention offers the advantage that a negative power flow (i.e. a power flow into the output connection) can be captured, such that it becomes possible for an alternating voltage to be generated from the output signal in a reliable and technically very simple manner.

Additionally conceivable is an embodiment of the present invention in which there is provided a series-connected arrangement of two switching elements, connected in series between a reference voltage connection and the first output connection, a first reference switching element having a parallel-connected arrangement of a first controllable reference switch and a first reference diode, and the second reference switching element having a parallel-connected arrangement of a second controllable reference switch and a second reference diode, the first reference diode being poled in a direction other than that of the second diode in respect of a direction of flow in the series-connected arrangement. Likewise, such an embodiment of the present invention offers the advantage that a negative power flow (i.e. a power flow into the output connection) can be captured, such that it becomes possible for an alternating voltage to be generated from the output signal in a reliable and technically very simple manner.

An inverter circuit can be designed in a particularly flexible manner in that, according to an embodiment of the present invention, there is additionally provided a fifth switching element, which is disposed between a second input connection and the first intermediate connection, and which has a parallel-connected arrangement of a fifth controllable switch and a fifth diode, in particular an anode of the fifth diode being connected tc the first intermediate connection. Such an embodiment of the present invention offers the advantage that a plurality of voltage levels can be taken up at one input connection, or at a plurality of input connections, and a corresponding alternating voltage, or the corresponding output signal for generating such an alternating voltage, can be provided therefrom. It is thereby possible to realize an inverter circuit that can take up voltage in differing operating stages and, from this, can supply a base signal for generating a corresponding alternating voltage.

According to a further embodiment of the present invention, there may additionally be provided a sixth switching element, which is disposed between a third input connection and the second intermediate connection, and which has a parallel-connected arrangement of a sixth controllable switch and a sixth diode, in particular the cathode of the sixth diode being connected to the second intermediate connection. Such an embodiment of the present invention offers the advantage that even (direct) voltages with reversed polarity signs can be used for infeeding to the inverter circuit, and the inverter circuit can nevertheless be constructed with a very small number of electrical or electronic components, such that it can be produced inexpensively.

Particularly advantageous is an inverter circuit according to an embodiment of the present invention in which additionally a second series-connected arrangement of two seventh and eighth switching elements is provided, connected in series between a fourth input connection and the second intermediate connection, the seventh switching element having a parallel-connected arrangement of a seventh controllable switch and a seventh diode, and the eighth switching element having a parallel-connected arrangement of an eighth controllable switch and an eighth diode, the seventh diode being poled in a direction other than that of the eighth diode in respect of a direction of flow in the second series-connected arrangemenc. Such an embodiment of the present invention likewise offers the advantage that input voltages of differing polarity can be processed in the inverter circuit. In this case, the further, or second, series-connected arrangement likewise allows highly flexible generation of an output signal that, both in the case of a positive and a negative flow of current at the output connection, enables the differing direct voltages present to be converted to a common alternating voltage signal.

Also advantageous is an embodiment of the present invention as an inverter having the following features: -an inverter circuit according to a variant presented here; and -an inductor, which is coupled to the output connection, in particular in order to provide an alternating voltage at an end of the inductor that is opposite the output connection.

Such an embodiment of the present invention is particularly advantageous because of the compact structure of the inverter circuit with the inductor, at which the alternating voltage can be obtained from the output signal provided at the output connection.

According to a further embodiment of the present invention, presented in this case is a method for operating an inverter circuit according to a variant described here, the method having, in a time interval, at least the following steps: -applying a direct voltage to the input connection; -operating the third switch, and that switch of the first or second switching element whose diode is poled in the direction of flow between the input connection and the first intermediate connection, in an open switching state; and -opening the switch of the first or second switching element whose diode is poled in the direction of flow between the first intermediate connection and the input connection, when the fourth switch has been closed or is closed, and/or closing the switch of the first or second switching element whose diode is poled in the direction of flow between the first intermediate connection and the input connection, when the fourth switch has been opened or is opened.

Such an embodiment of the present invention also makes it possible, advantageously, to operate an inverter circuit having a small number of electrical or electronic components such that an output signal, fron which an alternating voltage can be generated, is provided at the output connection in a highly efficient manner and with very low loss.

Also advantageous is an embodiment of the present invention in which there is proposed a method, according to an embodiment described here, for operating an inverter circuit according to a particular embodiment described here, the method having, in at least one further time interval, the following steps: -operating the third switch, and that switch of the first or second switching element whose diode is poled in the direction of flow between the first intermediate connection and the input connection, in an open switching state; and -opening the switch of the first or second switching element whose diode is poled in the direction of flow between the input connection and the first Intermediate connection, when the fifth switch has been closed or is closed, and/or closing the switch of the first or second switching element whose diode is poled in the direction of flow between the first intermediate connection and the input connection, when the fifth switch has been opened or is opened.

Such an embodiment of the present invention offers the advantage that, in the case of differing input voltages being present, in particular direct voltages, these input voltages can be processed in a highly efficient manner and with very low loss, with the use of only a small number of electrical or electronic components, into an output signal, present at the output connection, that can be transformed in a technically very simple manner to produce an alternating voltage. Such an embodiment of the present invention thus renders possible a flexible circuit design for converting differing direct voltages into a (single) alternating voltage.

Also particularly advantageous is an embodiment of the present invention in which, in the step of opening, at least one switch is controlled in such a manner that a flow of current is effected, in the form of a PWM signal, through the respective switch, and/or in the step of opening, at least one switch being controlled by use of a PWM signal. Such an embodiment of the present invention offers the advantage that it is possible to use switches that are technically very simple and that can therefore be provided inexpensively, which only have to be able to perform the function of switching on/off.

Additionally advantageous is an embodiment of the present invention as a control device, which is designed to perform or control the steps of a method according to a variant presented here in corresponding items of equipment. Such an embodiment of the present invention offers the advantage -10 -that the knowledge of the technically very simple circuit topology makes it possible, by advantageous control of the individual switching elements, or switches, for an output signal to be produced with very low loss from one or more direct voltage levels, for the purpose of converting to an alternating voltage. This embodiment variant of the invention, in the form of a control device, likewise enables the object on which the invention is based too be achieved in a rapid and efficient manner.

In the present case, a control device may be understood to mean an electrical device that processes sensor signals and outputs control and/or data signals in dependence thereon.

The control device may have an interface, which may be designed as hardware and/or software. In the case of design as hardware, the interfaces may be, for example, part of a so-called system ASIC, which includes a great variety of functions of the control device. It is also possible, however, for the interfaces to be discrete, integrated circuits, or to be composed, at least partially, of discrete components. In the case of design as software, the interfaces may be software modules that are present, for example, on a microcontroller, in addition to other software modules.

Also advantageous is a computer program product, having program code that can be stored on a machine-readable medium such as a semiconductor memory, a hard-disk memory or an optical memory, and that is used to perform the method according to one of the embodiments described above when the program product is executed on a computer or an appliance.

-ii -The approach presented here is explained exemplarily in greater detail in the following, on the basis of the appended drawings. There are shown in: Fig. 1 a circuit diagram of an inverter according to an exemplary embodiment of the present invention, having an inverter circuit according to an exemplary embodiment of the present invention; Fig. 2 a circuit diagram of an inverter circuit according to an exemplary embodiment of the present invention; Fig. 3 a diagram of an output voltage for feeding into an energy supply grid, by use of an inverter circuit according to an exemplary embodiment of the present invention; Fig. 4 a diagram of a voltage of the output connection, of the output current through the output connection, and of the voltage of the grid, over time; Fig. 5 a plurality of diagrams, in which are represented exemplary embodiments of the switching behaviour of the individual switches Ti to TB over time; Fig. 6 a plurality of diagrams, in which are represented exemplary embodiments of voltages at the individual switches Ti to TB over time; Fig. 7 & plurality of diagrams, in which are represented exemplary embodiments of current characteristics -12 -through the individual switches Ti to T8 over time; Fig. 8 a plurality of diagrams, in which are represented exemplary embodiments of voltage characteristics at the individual diodes Dl to D8 over time; Fig. 9 a plurality of diagrams, in which are represented exemplary embodiments of current characteristics through the individual diodes Dl to D8 over time; Fig. 10 a sequence diagram of an exemplary embodiment of a method for operating an inverter circuit according to an exemplary embodiment of the present invention; Fig. 11 a circuit diagram of an inverter according to a further exemplary embodiment of the present invention, having an inverter circuit; Fig. 12 a circuit diagram of an inverter circuit according to a further exemplary embodiment of the present invention; Fig. 13 a diagram of an output voltage for feeding into an energy supply grid, by use of an inverter circuit according to a further exemplary embodiment of the present invention; and Fig. 14 a diagram of a voltage of the output connection, of the output current through the output connection, and of the voltage of the grid, over time.

-13 -In the following description of advantageous exemplary embodiments of the present invention, the same or similar references are used fcr the elements that are represented in the various figures and that act in a similar way, and

description of these elements is not repeated.

Fig. 1 shows a circuit diagram of an inverter 100, having an inverter circuit 110 according to an exemplary embodiment of the present invention. The exemplary embodiment of an inverter 100 presented here, and a method for operating the inverter 100, presented further below, enable electrical energy to be fed into an alternating voltage grid 130 having the alternating voltage Vc, from direct voltage sources 120 (which are represented here only in the form of connection contacts, at which a respective direct voltage can be tapped) . For reasons of simplicity, in Fig. 1 the alternating voltage grid 130 is represented merely as single-phase, it being known to persons skilled in the art that, through appropriate design (for example, multiplication of the approach presented here for differing phases), an alternating voltage can also be provided for differing phases of the alternating voltage grid 130.

Fields of application of the inverter 100 presented here are, for example, inverters for photovoltaic systems 135, application in conjunction with fuel cells, batteries or other direct voltage sources being likewise conceivable.

The exemplary embodiment of the inverter 100, presented in the form of a circuit diagram in Fig. 1, provides for a multilevel topology (i.e. generation of a plurality of voltage levels) . It is based on feeding the energy from, for example, four direct voltage sources 120 (for example, having differing voltage levels) into an existing -14 -alternating voltage grid 130. Through the use of, for example, appropriate DC-DC converters 125 (for providing the voltage levels) , the energy from a DC source such as, for example, the photovoltaic system 135, can thus be fed into the AC grid 130 in a highly effective manner.

The AC topology of the inverter 100 feeds the energy from the various series-connected DC sources 120 according to the level of the instantaneous AC voltage. Consequently, switching is only ever effected to the voltage level that is just sufficient to magnetize the choking coil L on the grid side. Consequently, the choking coil L on the grid side can be significantly smaller in its dimensions, thereby reducing costs and weight (in comparison with 2-level or 3-level topologies) In addition, it is possible to work with higher system voltages, it being possible to use components or switches (for example IGBTs or thyristors) having a lesser electric strength. This, in turn, has a positive effect upon the power loss. The topology may be realized both as single-phase and as three-phase.

The circuit diagram of an inverter 100 according to an exemplary embodiment of the present invention shown in Fig. 1 comprises, as its core, the inverter circuit 110, which, at an output connection 140, provides an output signal, not described in further detail in the following, which can be fed to the inductor L in order to provide the alternating voltage VA: for the alternating voltage grid 130. The inverter circuit 110 in this case comprises a first switching element 145 and a second switching element 147, which are connected in series between an input connection 150 and a first intermediate connection 152.

-15 - The first switching element 145 comprises a parallel-connected arrangement of a first switch IS, which may be, for example, an electrical or electronic component, or semiconductor components, such as an IGBT, and a first diode 55. If an IGET is used for the first switch T5, the collector of this IGBT may be connected to the input connection 150, and the emitter of this 1551 may be connected to the second switching element 147. Moreover, the cathode of the first diode PS is connected to the input connection 150, and the anode of the first diode D5 is connected to the second switching element 147. The second switching element 147 comprises a parallel-connected arrangement of a second switch 16, which, for example, may likewise be an electrical or electronic component, or semiconductor components such as an IGBT or a thyristor, and a second diode 56. If an TGBT is used for the second switch T6, the collector of this 1551 may be connected to the first intermediate connection 152, and the emitter of this IGBT may be connected to the first switching element 145. Moreover, the cathode of the second diode P6 is connected to the first intermediate connection 152, and the anode of the second diode D6 is connected to the first switching element 145.

A third switching element 155 is connected between the first intermediate connection 152 and the output connection 140. The third switching element 155 comprises a parallel-connected arrangement of a third switch T2, which, for example, may likewise be an electrical or electronic component, or semiconductor components such as an IGBT or a thyristor, and a third diode D2. If an IGBT is used for the third switch T2, the emitter of this IGBT may be connected to the output connection 140, and the collector -16 -of this IGBT may be connected to the first intermediate connection 152. Moreover, the cathode of the third diode 57 is connected to the first intermediate connection 152, and the anode of the third diode D2 is connected to the output connection 140.

The inductor S is connected in circuit between the output connection 140 and the alternating voltage grid 130.

Furthermore, a fourth switching element 160 is connected between the output connection 140 and a second intermediate connection 157. The fourth switching element 160 comprises a parallel-connected arrangement of a fourth switch T3, which, for example, may likewise be an electrical or electronic component, or semiconductor components such as an TGBT or a thyristor, and a fourth diode 53. Tf an TGBT is used for the fourth switch T3, the emitter of this TGBT may be connected to the second intermediate connection 157, and the collector of this IGBT may be connected to the output connection 140. Moreover, the cathode of the fourth diode D3 is connected to the output connection 140, and the anode of the fourth diode 53 is connected to the second intermediate connection 157.

Tn addition, the inverter circuit 110 comprises a reference potential connection 165, to which a frame potential is, or may be, applied when the inverter circuit 110 is in operation. A first intermediate diode 59 is connected between this reference potential connection 165 and the first intermediate connection 152, the anode of the first intermediate diode D9 being connected to the reference potential connection 165, and the cathode being connected to the first intermediate connection 152. According to the -17 -circuit diagram of an exemplary embodiment of an inverter that is represented in Fig. 1, a second intermediate diode 510 is also connected between the reference potential connection 165 and the second intermediate connection 157, the cathode of the second intermediate diode 510 being connected to the reference potential connection 165, and the anode being connected to the second intermediate connection 157.

Furthermore, a fifth switching element 170 is connected between the first intermediate connection 152 and a second input connection 150. The fifth switching element 170 comprises a parallel-connected arrangement of a fifth switch Ti, which, for example, may likewise be an electrical or electronic component, or semiconductor components such as an IGBT or a thyristor, and a fifth diode Dl. If an IGIRT is used for the fifth switch Ti, the emitter of this IGBT may be connected to the first intermediate connection 152, and the collector of this IGBT may be connected to the second input connection 168.

Moreover, the cathode of the fifth diode Dl is connected to the second input connection 168, and the anode of the fifth diode Dl is connected to the first intermediate connection 152.

Furthermore, a sixth switching element 177 is connected between the second intermediate connection 157 and a third input connection 175. The sixth switching element 177 comprises a parallel-connected arrangement of a sixth switch T4, which, for example, may likewise be an electrical or electronic component, or semiconductor components such as an IGBT or a thyristor, and a sixth diode D4. If an 101ST is used for the sixth switch T4, the -18 -emitter of this IGBT may be connected to the third input connection 175, and the collector of this IGBT may be connected to the second intermediate connection 157.

Moreover, the cathode of the sixth diode D4 is connected to the second intermediate connection 157, and the anode of the sixth diode 154 is connected to the third input connection 175.

A series-connected arrangement of a seventh switching element 185 and an eighth switching element 187 is connected between the second intermediate connection 157 and a fourth input connection 180.

The seventh switching element 185 comprises a parallel-connected arrangement of a seventh switch T7, which, for example, may likewise be an electrical or electronic component, or semiconductor components such as an TGBT or a thyristor, and a seventh diode D7. If an IGBT is used for the seventh switch 17, the emitter of this IGBT may be connected to the eighth switching element 187, and the collector of this IGBT may be connected to the fourth input connection 180. Moreover, the cathode of the seventh diode 157 is connected to the fourth input connection 180, and the anode of the seventh diode D7 is connected to the eighth switching element 187.

The eighth switching element 187 comprises a parallel-connected arrangement of an eighth switch T8, which, for example, may likewise be an electrical or electronic component, or semiconductor components such as an IGBT or a thyristor, and an eighth diode D8. If an IGBT is used for the eighth switch T8, the emitter of this IGBT may be connected to the seventh switching element 185, and the -19 -collector of this IGBT may be connected to the second intermediate connection 157. Moreover, the cathode of the eighth diode D8 is connected to the second intermediate connection 157, and the anode of the eighth diode D8 is connected to the seventh switching element 185.

A first capacitor Cl is connected between the second input connection 168 and the input connection 150. A second capacitor 02 is connected between the input connection 150 and the reference potential connection 165. A third capacitor 03 is connected between the reference potential connection 165 and the fcurth input connection 180. A fourth capacitor 04 is connected between the fourth input connection 180 and the third input connection 175.

A first output of the energy source 135, which, for example, represents a photovoltaic module, is connected to the input connection 150 and to an input of a first DC-DC converter 125a. An output of the first DC-DC converter 125a is connected to the second input connection 168. A second output of the energy source 135 is connected to the fourth input connection 180 and to an input of a second DC-DC converter 125b. An output of the second DC-DC converter 125b is connected to the third input connection 175.

The exemplary embodiment of an inverter circuit 110 represented in Fig. 1 thus comprises 8 semiconductor switches Tl to T8 (e.g. IGBT), having corresponding parallel-connected reverse diodes Dl to D8, and two clamping diodes D9 and D10. At the output 140 there is an inductor L. Control of the switches (IGBT) is effected by a control unit 190, which generates a corresponding pulse pattern (for example, a PWM pattern) for controlling the -20 -individual switches, for example according to the procedure described in greater detail in the following.

Four series-connected DC sources 120 (for example, corresponding to the capacitors Cl to 04) may be used as an input voltage. The sum of the four voltages should be greater than the peak-peak voltage of the AC voltage of the grid 130 to be fed. The DC voltages are symmetrical in relation to the mid-point 165, which represents the reference potential connection, or frame connection.

The level of the two inner voltages (V2; V3) represented in the circuit diagram of the inverter circuit 110 from Fig. 2 is not limited (it being the case that V2 «= Vi and V3 »= V4) . Switching can be effected between 5 voltage levels (Vi; V2, 0, V3; V4) Fig. 3 shows a diagram of an alternating-voltage signal, represented as a voltage characteristic over time, that can be obtained from feeding the output signal from the output connection 140 to the inductor. In this case, time intervals and voltage levels have already been entered in the diagram from Fig. 3, in order better to explain the principle of functioning of the inverter circuit 110 in the

following description.

In order to feed a grid-compatible current into a connected grid 130, a voltage (VAC) that is influenced by the output signal that can be tapped at the output connection 140 should be generated at the output of the inductor L. This voltage may be generated by corresponding switch positions, with the use, for example, of PWM signals, for opening or closing the individual switches Tl to T8 of the inverter -21 -circuit 110. In this case, the voltage VAC represents a low-pass filtered output voltage V of the voltage of the output signal present at the output connection 140 of the inverter circuit 110.

If the voltage VAC > 0 and < V2, the switches 13 and Tb are controlled in a complementary manner, the switches P2 and 16 being switched on throughout the entire portion (tO to tl, and t2 to t3) Switching-on of the switch P5 (T3 blocks) causes the output to be connected to the potential V2. The choking coil L becomes magnetized, causing the current to increase. The current in this case flows via Tb, P6 and 12. Switching-off of the switch IS (13 switches on) causes the flow of current to be interrupted. The current carried in the choking coil commutates to the diode D9 and drops off. If the current becomes negative, the current is routed via the diode 510 and the switch P3.

If the switch 13 is opened again (15 switches on) , the potential V2 is connected to the output, and the current rises again. If the current is negative at this instant, it is taken over by the diodes 55 and 52 and by the switch 16. If the direction of current changes, the current flows through IS, 56 and 12.

If the voltage VAC > V2 and K Vi, the switches Ti and P6 are controlled in a complementary manner, the switches P2 and IS being switched on throughout the entire portion (tl-t2) By switching-on of the switch Ii, the potential Vi can be connected (via Ti and 12) to the output 140, and the current in the choking coil increases.

-22 -If the switch Ti Is switched off (T6 switches on) , the current is rcuted via 15, D6 and 12. if the direction of current changes (current becomes negative) , it flows via the switch T6 and the diodes PS and P2.

If the switch T6 is then closed (Ti switches cn, potential Vi is again present at the output OUT, and the current increases. If the current is negative at this moment, it flows via the diodes Di and P2. After the direction of current has changed, the current is taken over by the switches Ti and 12.

The switching operations are effected in an analogous manner in the negative half-wave.

If rapid changes in current are required, switching can be effected directly to the higher, or lower, potentials.

Furthermore, if necessary, switchover can be effected from 5-level operation to 3-level and 2-level operation.

An advantage of this exemplary circuit topology, as compared with the prior art, is that fewer switches are required to generate the 5 voltage levels. Furthermore, the current is only ever routed via a maximum of 3 semiconductors, this having a positive effect upon the power loss.

A further advantage is that, even in the case of a system voltage of i500 V, semiconductors having a reverse voltage of 1200 V can be used.

-23 -The functioning of the exemplary inverter circuit 110 described above can be explained by the diagrams described more fully below.

Represented in Fig. 4 in this case is a diagram of the voltage V_CUT of the output connection 140, of the output current lOUT through the output connection 140, and the voltage VAC of the grid 130, over time.

Fig. 5 shows a plurality of diagrams, in which the switching behaviour of the individual switches Ti to T8 are represented over time.

Represented in Fig. 6 are a plurality of diagrams of voltages at the individual switches Ti to T8 over time.

Represented in Fig. 7 are a plurality of diagrams of current characteristics through the individual switches Ti to T8 over time.

Represented in Fig. 8 are a plurality of diagrams of voltage characteristics at the individual diodes Dl to D8 over time.

Represented in Fig. 9 are a plurality of diagrams of current characteristics through the individual diodes Di to US over time.

Fig. 10 shows a sequence diagram of an exemplary embodiment of a method 1000 for operating an inverter circuit according to an exemplary embodiment of the present invention, the method having, in a time interval, at least one step 1010 of applying a direct voltage to the input -24 -connection. In addition, the method 1000 has a step 1020 of operating the third switch, and that switch of the first or second switching element whose diode is poled in the direction of flow between the input connection and the first intermediate connection, in an open switching state.

Finally, the method comprises a step 1030 of opening the switch of the first or second switching element whose diode is poled in the direction of flow between the first intermediate connection and the input connection, when the fourth switch has been closed or is closed, and/or closing the switch of the first or second switching element whose diode is poled in the direction of flow between the first intermediate connection and the input connection, when the fourth switch has been opened or is opened.

As an alternative or in addition to the AC topology described with reference to the preceding figures, the following topology, according to the representation from Fig. ii, may also be used. Likewise, this is a 5-level topology. This offers the advantage that the mid-point can be switched directly, this advantage, however, being conditional upon a greater number of switches. Otherwise, the circuit offers the same advantages as the topology already disclosed with reference to the figures described above.

Fig. 11, in addition, shows a circuit diagram of an inverter circuit 100 that, apart from the diodes D9 and PlO, corresponds to the inverter circuit 100 from Fig. 1.

However, the diodes DO and PlO have been replaced by a series-connected arrangement of two switching elements 1110 and 1120, respectively, connected in series between the reference potential connection 165 and the first output -25 -connection 140, a first reference switching element 1110 having a parallel-connected arrangement of a first controllable reference switch T9 and a first reference diode D9, and the second reference switching element 1120 having a parallel-connected arrangement of a second controllable reference switch 110 and a second reference diode 510, the first reference diode D9 being poled in a direction other than that of the second diode 510 in respect of a direction of flow in the series-connected arrangement.

The circuit comprises ten semiconductor switches (e.g. IGBT) having corresponding parallel-connected reverse diodes, as marked by the circuit structure 1130 designated in Fig. 11. Again, the inductor S is present at the output.

The principle of functioning of the inverter circuit according to the representation from Fig. 11 is explained more fully with reference to the following Figures 12 to 14. Fig. 12 in this case shows a circuit diagram of an inverter circuit according to a further exemplary embodiment of the present invention. Fig. 13 shows a diagram of an output voltage for feeding into an energy supply grid, by use of an inverter circuit according to a further exemplary embodiment of the present invention, and Fig. 14 shows a diagram of a voltage of the output connection, of the output current through the output connection, and of the voltage of the grid, over time.

Control of the switches P from Fig. 11 is effected by a control unit 190, which generates a corresponding pulse pattern (PWM) . Four series-connected DC sources Cl to 04 -26 -are advantageous as an input voltage. The sum of the four voltages should be greater than the peak-peak voltage of the AC voltage of the grid to be fed. The DC voltages should be symmetrical in relation to the mid-point 165.

The level of the two inner voltages (V2; V3) is not limited (it being the case that V2 «= Vi and V3 »= V4) . Switching can be effected between five voltage levels (Vi; V2, 0, V3; V4).

In order to feed a grid-compatible current into a connected grid, a voltage (VD analog) should be generated at the output (OUT) . This voltage may be generated by corresponding switch positions, by use of a PWM (V01 analog is the low-pass filtered output voltage. If the voltage (V3 analog) > 0 and < V2, the switches T5 and 110 are controlled in a complementary manner, the switches T2, T6 and T9 being switched on throughout the entire portion (t-t1 and t2-t) Switching-on of the switch T5 (TiC blocks) causes the output CUT to be connected to the potential V2. The choking coil L becomes magnetized, causing the current to increase. The current in this case flows via T5, P6 and T2. Switching-off of the switch TO (TiC is switched on) causes the flow of current to be interrupted. The current carried in the choking coil P commutates to 19 and DiG and drops off. If the current becomes negative, the current is routed via the diode P9 and the switch Tic.

If the switch Tic is opened again (T5 is switched on) , the potential V2 is connected to the output, and the current rises again. If the current is negative at this instant, it is taken over by the diodes P2 and DO and by the switch -27 - 16. If the direction of current changes, the current flows through 15, D6 and 12.

If the voltage (you: analog) > V2 and < Vi, the switches Ti and T6 are controlled in a complementary manner, the switches T2, Tb and 19 being switched on throughout the entire portion (t-t2) . Switching-on of the switch Ti causes the potential Vi to be connected (via Ti and P2) to the output 140, or OUT, and the current in the choking coil L increases.

If the switch Ti is switched off (T6 is switched on) , the current is routed via Tb, D6 and P2. If the direction of current changes (current becomes negative) , it flows via the switch 16 and the diodes US and D2.

If the switch T6 is then switched off (Ti is switched on) potential Vi is again present at the output 140, or OUT, and the current increases, If the current is negative, it flows via the diodes Dl and D2. After the direction of current has changed, the current is taken over by the switches Ti and P2.

If rapid changes in current are reguired, switching can be effected directly to the higher, or lower, potentials.

Currently, in the case of solar inverters, a two-level or three-level AC inverter topology is normally used. In -28 -order to increase the grid-side AC voltage of the inverter to, for example, 690 V, the solar generator voltage should be boosted by means of a Dc/DC converter. This, however, reduces the efficiency.

By means of a 5-level AC inverter topology, having a 4-part intermediate circuit, it is possible to directly connect the DC connection of the solar generator 135 to the middle intermediate-circuit capacitors, and thereby to provide a major part of the energy directly to the AC stage.

Presented here is a circuit that has a 5-point inverter topology for feeding energy from a direct current/voltage source into a grid. Five voltage levels, for example, may be used on the DC side. The additional voltage levels are connected in as soon as the voltage of the direct voltage source 135 is less than the instantaneous voltage value of the grid.

The approach presented here offers all advantages of conventional circuits, but additionally has the advantage that the power loss can be reduced (by reduction of the components that carry current) . Furthermore, fewer components are used, which reduces the resource requirement, and, in addition, there is the possibility (in certain circumstances) of switching over to a 2-level or 3-level mode.

The exemplary embodiments described and shown in the figures have been selected merely as examples. Differing exemplary embodiments may be combined with each other, either in their entirety or in respect of individual -29 -features. Mi exemplary embodiment may also be supplemented by features of a further exemplary embodiment.

Further, the method steps presented here may be executed repeatedly, and in a sequence other than that described.

If an exemplary embodiment includes an "and/or/" link between a first feature and a second feature, this is to be read such that the exemplary embodiment according to one embodiment has both the first feature and the second feature, and according to a further embodiment has either only the first feature or only the second feature.