Switching power supply for balancing an intermediate circuit

Description

[0001] The present invention relates to a switching power supply for balancing a plurality of intermediate circuit voltages that can be applied to an intermediate circuit capacitor of an intermediate circuit between a pair of input poles of the switching power supply, wherein the switching power supply is designed to output a DC output voltage to an output capacitor of the switching power supply connected between a first output pole and a second output pole of the switching power supply.Furthermore, the present invention relates to a method for balancing a plurality of intermediate circuit voltages, each of which is applied to an intermediate circuit capacitor of an intermediate circuit arranged between an associated input pole pair of a switched-mode power supply, wherein an output voltage is output at an output capacitor of the switched-mode power supply between a first output pole and a second output pole of the switched-mode power supply, and to an arrangement comprising a power electronic device with a plurality of intermediate circuit capacitors and a switched-mode power supply for balancing intermediate circuit voltages at the plurality of intermediate circuit capacitors.

[0002] Various power electronic devices, such as inverters, welding devices, chargers, etc., are known which have an intermediate circuit with a plurality of intermediate circuit capacitors, with intermediate circuit voltages being applied to each of the intermediate circuit capacitors. Due to asymmetrical loads on the intermediate circuit capacitors by an associated power electronic device and / or due to parasitic leakage currents, the intermediate circuit voltages may become asymmetrical, i.e., assume different values. It is desirable to symmetricalize, i.e., equalize, these intermediate circuit voltages. It may also be desirable to symmetricalize the intermediate circuit voltages of different power electronic devices.

[0003] To balance imbalances between intermediate circuit voltages, switched-mode power supplies can be provided which are designed to preferably draw energy from intermediate circuit capacitors with a higher intermediate circuit voltage and feed it into a load. EP 2 826 126 B1, for example, describes an inverter comprising several intermediate circuit capacitors. Boost converters are provided which first shift energy from the existing intermediate circuit capacitors into a specific intermediate circuit capacitor and, in a sense, concentrate it. This specific intermediate circuit capacitor is in turn connected to the input side of a switched-mode power supply, with the output side of the switched-mode power supply being connected to a load. Thus, the energy from the intermediate circuit capacitor is delivered to the load via the switched-mode power supply.

[0004] However, known switching power supplies are only capable of symmetrizing the intermediate circuit voltages of the intermediate circuit capacitors connected on the input side if the switching power supplies are connected to a load on the output side in order to dissipate the energy taken from the intermediate circuit capacitors to the load.

[0005] In addition, so-called "balancing concepts" are known from the state of the art, in which several energy storage devices are sometimes balanced. For example, DE 102011119904 A1 balances series-connected battery cells connected to inductively coupled coils, rather than DC link capacitors whose terminals are inductively decoupled. This severely limits the applicability of DE 10 2011 119 904 A1 for balancing inductively decoupled DC link capacitors. The same applies to documents CH 712 011 A2, CN 107 634 655 A, WO 2018 / 134827 A1 or CN 208 638 071 U, which, although they deal with switching power supplies, only inadequately address possible problems associated with balancing intermediate circuit capacitors.

[0006] It is therefore an object of the present invention to provide an improved balancing circuit for the intermediate circuit capacitors of an intermediate circuit.

[0007] This object is achieved according to the invention by a switching power supply having the features of independent claim 1. This describes a switching power supply for balancing a plurality of intermediate circuit voltages, each of which can be applied to intermediate circuit capacitors of an intermediate circuit.

[0008] Furthermore, the object is achieved by a method according to independent claim 7.

[0009] The DC link voltages at the DC link capacitors are thus balanced passively. Since the switched-mode power supply does not have an output diode or diodes connected in series with the primary windings, a negative current is also possible through the primary windings and through the secondary windings of the transformers. This allows energy to flow not only from the primary winding to the secondary winding, but also from the secondary winding to the primary winding. Each circuit section contains a transformer, although the transformers between the different circuit sections are not inductively coupled to one another. This makes it possible to charge energy from the DC link capacitor with the highest DC link voltage into the output capacitor via the respective circuit section and its transformer.Furthermore, energy from the output capacitor is charged via the respective circuit section and its transformer into the intermediate circuit capacitor(s) of the lower intermediate circuit voltage(s). Therefore, no load connected to the output terminals (first and second output poles) is required, to which the energy drawn from the intermediate capacitor with the highest intermediate circuit voltage is dissipated to balance the intermediate circuit voltages. However, if a load is connected to the output terminals, energy can of course be delivered to the load from the intermediate circuit capacitor with the highest intermediate circuit voltage. If the load-side energy demand is high, additional energy can also be delivered to the load from intermediate circuit capacitors with a lower intermediate circuit voltage.When the load-side energy demand is low, energy can be delivered from the DC link capacitor with the highest DC link voltage to the load and also to the DC link capacitor with the lowest DC link voltage. If at least three DC link voltages are present on at least three DC link capacitors, energy can of course also be delivered from one or more DC link capacitors with an DC link voltage that is neither the highest nor the lowest to one or more DC link capacitors with a lower DC link voltage and / or to the load.

[0010] Preferably, the transformers of the circuit components are of identical design, although the two circuit components can also be completely identical. If the transformers are of identical design, the intermediate circuit voltage of all switching power supplies is balanced to an identical voltage level. If the winding ratios and / or transformation ratios of the transformers differ, each intermediate circuit voltage is balanced to the voltage level set by the winding ratios and / or transformation ratio.

[0011] According to the invention, a first terminal of the secondary windings of the transformers is connected to the first output pole via a balancing switch, and a second terminal of the secondary windings of the transformers is connected to the second output pole. A control unit is provided that is configured to control the balancing switches to transfer energy from the input pole pair having the highest intermediate circuit voltage via the associated transformer to the output capacitance and from the output capacitance to the other input pole pairs. The use of the balancing switches eliminates the need for balancing diodes, thus also enabling negative currents.

[0012] Preferably, the switched-mode power supply comprises a first circuit part of the plurality of circuit parts with a first transformer of the plurality of transformers provided in a circuit part and a first input pole pair of the plurality of input pole pairs, and a second circuit part of the plurality of circuit parts with a second transformer of the plurality of transformers provided in a circuit part and a second input pole pair of the plurality of input pole pairs, wherein the first circuit part is connectable on the input side via the first input pole pair to a first intermediate circuit capacitor having a first intermediate circuit voltage, and the second circuit part is connectable on the input side via the second input pole pair to a second intermediate circuit capacitor having a second intermediate circuit voltage, wherein the first circuit part is designedto charge energy from the first intermediate circuit capacitor into a primary winding of the first transformer and vice versa, and wherein the second circuit part is designed to charge energy from the second intermediate circuit capacitor into a primary winding of the second transformer and vice versa, wherein a secondary winding of the first transformer is connected to the output capacitor and a secondary winding of the second transformer is connected to the output capacitor in order to transfer energy from the first intermediate circuit capacitor via the first transformer to the output capacitance and from the output capacitance to the second intermediate circuit capacitor at a first intermediate circuit voltage greater than the second intermediate circuit voltage,and, when the second intermediate circuit voltage is greater than the first intermediate circuit voltage, transferring energy from the second intermediate circuit capacitor via the second transformer to the output capacitance and from the output capacitance to the first intermediate circuit capacitor. This describes a switched-mode power supply for balancing two intermediate circuit voltages across two intermediate circuit capacitors.

[0013] Accordingly, a first intermediate circuit voltage at a first intermediate circuit capacitor arranged between a first input pole pair and a second intermediate circuit voltage at a second intermediate circuit capacitor arranged between a second input pole pair can be symmetrized by transforming energy from the first intermediate circuit capacitor via a first primary winding of a first transformer to a secondary winding of the first transformer and transferring energy from the secondary winding of the first transformer to an output capacitance, and transferring energy from the output capacitance to a secondary winding of a second transformer of the switched-mode power supply,from the secondary winding of the second transformer to a primary winding of the second transformer and transferred from the primary winding of the second transformer to the second intermediate circuit capacitor. At a second intermediate circuit voltage greater than the first intermediate circuit voltage, energy from the second intermediate circuit capacitor can be transformed via the primary winding of the second transformer to the secondary winding of the second transformer and transferred from the secondary winding of the second transformer to the output capacitance, as well as energy and transferred from the output capacitance to the secondary winding of the first transformer.from the secondary winding of the first transformer to the primary winding of the first transformer and transferred from the primary winding of the first transformer to the first intermediate circuit capacitor. This describes a method for balancing two intermediate circuit voltages across two intermediate circuit capacitors.

[0014] Preferably, a first balancing switch arranged between a first terminal of the secondary winding of the first transformer and the first output pole is controlled, and a second balancing switch arranged between a second terminal of the secondary winding of the second transformer and the second output pole is controlled in order to transfer energy from the first intermediate circuit capacitor to the output capacitance via the first transformer and to transfer energy from the output capacitance to the second intermediate circuit voltage when the first intermediate circuit voltage is greater than the second intermediate circuit voltage, and in order to transfer energy from the second intermediate circuit capacitor to the output capacitance via the second transformer and to transfer energy from the output capacitance to the first intermediate circuit voltage when the second intermediate circuit voltage is greater than the first intermediate circuit voltage.

[0015] A first terminal of the secondary winding of the first transformer can be connected to the second output pole via a first balancing switch of the balancing switches for connecting a respective first terminal of the secondary windings of the transformers to the second output pole, and a first terminal of the secondary winding of the second transformer can be connected to the second output pole via a second balancing switch of the balancing switches for connecting a respective first terminal of the secondary windings of the transformers (T1, T2) to the second output pole, wherein a second terminal of the secondary winding of the first transformer and a second terminal of the secondary winding of the second transformer are connected to the first output pole, and a control unit is provided which is designed to control the first and second balancing switches,To transfer energy from the first intermediate circuit capacitor via the first transformer to the output capacitance and from the output capacitance to the second intermediate circuit capacitor when the first intermediate circuit voltage is greater than the second intermediate circuit voltage, and to transfer energy from the second intermediate circuit capacitor via the second transformer to the output capacitance and from the output capacitance to the first intermediate circuit capacitor when the second intermediate circuit voltage is greater than the first intermediate circuit voltage. The first and second balancing switches can also be an integral part of the switched-mode power supply or an already provided component, but can also be designed as a standalone component.

[0016] Preferably, the input pole pairs are connected in series. Thus, input pole pairs connected directly in series share a common input pole, which thus forms a common center point, particularly when two input pole pairs are provided.

[0017] RC circuits can be connected in series with the diodes. This allows any energy remaining due to stray inductance in the primary windings to be discharged via the corresponding diode and RC circuit.

[0018] The present invention is described below with reference to the Figuren 1 bis 8 which show exemplary, schematic and non-limiting advantageous embodiments of the invention. Fig.1 a switching power supply for the loaded balancing of an intermediate circuit (not part of the scope of protection), Fig.2 the switching power supply is designed as a flyback converter (not part of the scope of protection), Fig.3 an inventive switching power supply for balancing an intermediate circuit, Fig.4 the switching power supply according to the invention is designed as a flyback converter, Fig.5 Currents of the switching power supply for loaded balancing under load, Fig.6 Currents of the switching power supply for loaded balancing without load, Fig.7 Currents of the switching power supply according to the invention in idle mode, Fig.8 Currents of the switching power supply according to the invention under load.

[0019] An intermediate circuit of a power electronics system to be balanced comprises a first intermediate circuit capacitor C1 and a second intermediate circuit capacitor C2. A first intermediate circuit voltage UC1 is applied to the first intermediate circuit capacitor C1 and a second intermediate circuit voltage UC2 is applied to the second intermediate circuit capacitor C2. If the values ​​of the first intermediate circuit voltage UC1 and the second intermediate circuit voltage UC2 are at least approximately identical, the intermediate circuit voltages UC1, UC2 are symmetrical. If the intermediate circuit voltages UC1, UC2 differ, they are asymmetrical. If an asymmetry exists (ieIf (i.e., one intermediate circuit voltage UC1 or UC2 is lower than the other intermediate circuit voltage UC1 or UC2), the smaller intermediate circuit voltage UC1 or UC2 increases such that the difference between the intermediate circuit voltages UC1 and UC2 decreases, and / or the larger intermediate circuit voltage UC1 or UC2 decreases such that the difference between the intermediate circuit voltages UC1 and UC2 decreases, this is referred to as balancing of the intermediate circuit voltages UC1 and UC2. Preferably, complete balancing occurs, meaning that the difference between the intermediate circuit voltages UC1 and UC2 is reduced to zero.

[0020] A switched-mode power supply (SNT) can be used to balance the intermediate circuit voltages UC1, UC2 of intermediate circuit capacitors C1, C2. An SNT switched-mode power supply outputs a DC voltage Ua on the output side to an output capacitor Ca, which is connected to an output pole pair C, D, and is fed on the input side by the intermediate circuit voltages UC1, UC2. For this purpose, the switched-mode power supply comprises a first input pole pair A1, B1 with a first upper input pole A1 and a second upper input pole B1, as well as a second input pole pair A2, B2 with a first lower input pole A2 and a second lower input pole B2. The input pole pair A1, B1 is connected to the first intermediate circuit capacitor C1, and the second input pole pair A2, B2 is connected to the second intermediate circuit capacitor C2.

[0021] A known way to symmetrize the intermediate circuit voltages UC1, UC2 is to extract more energy E1, E2 from the intermediate circuit capacitor C1, C2, which has the higher intermediate circuit voltage UC1, UC2, than from the intermediate circuit capacitor C1, C2, which has the lower intermediate circuit voltage UC1, UC2, and to load it into the output capacitor Ca. Depending on the load-side energy requirement and the extent of the asymmetry, it is also possible to transfer energy exclusively from the intermediate circuit capacitor C1, C2, which has the higher intermediate circuit voltage UC1, UC2, to the load Z.

[0022] In Fig. 1 A schematic of a two-part switched-mode power supply SNT according to the prior art is shown. The switched-mode power supply SNT thus comprises a first circuit part SNT1 and a second circuit part SNT2. A transformer T is provided in the switched-mode power supply SNT, which transformer comprises a first and a second primary winding L1, L2, and a secondary winding L3. The first primary winding L1 and the second primary winding L2 are each inductively coupled to the secondary winding L3. The first primary winding L1 is assigned to the first circuit part SNT1 and the second primary winding L2 to the second circuit part SNT2. The secondary winding L3, as part of the switched-mode power supply SNT, is assigned equally to both circuit parts SNT1, SNT2.

[0023] The first circuit part SNT1 is designed to transfer energy E1 from the first intermediate circuit capacitor UC1 into the first primary winding L1; the second circuit part SNT2 is designed to transfer energy E2 from the second intermediate circuit capacitor UC2 into the second primary winding L2. As is known, energy is stored in capacitors in an electric field, whereas energy is stored in inductors in a magnetic field. Energy E1 is thus transferred from the first intermediate circuit capacitor C1 into the first primary winding L1 by the first intermediate circuit voltage UC1 generating a first current is1 in the first primary winding L1. Analogously, energy E2 is transferred from the second intermediate circuit capacitor C2 into the second primary winding L2 by the second intermediate circuit voltage UC2 generating a second current is2 in the second primary winding L2.The first / second current is1, is2 in the first / second primary winding L1, L2 each generates a magnetic field. The energy E1, E2 is temporarily stored in this magnetic field in the transformer T. Furthermore, the first and second currents is1, is2 are each transformed into the output current ia in the transformer according to the associated transformation ratio. The energy E1, E2 stored in the transformer by the first and second currents is1, is2 is output by the transformer T via the secondary winding L3 in the form of the output current ia. The output current ia charges the output capacitor Ca connected between the output pole pair C, D (comprising a first and second output pole C, D) to an output voltage Ua, whereby the energy E1, E2 is charged into the output capacitor Ca. In summary, energy E1, E2 is transferred from the first and second intermediate circuit capacitors C1, C2 via the transformer T to the output capacitor Ca.

[0024] An output diode Da is provided in series with a terminal of the secondary winding L3, which allows only a positive output current ia. Accordingly, balancing diodes DS1, DS2 are provided in series with the first primary winding L1 and the second primary winding L2 in order to generate only a positive first and second current is1, is2 at the primary windings L1, L2.

[0025] If a higher intermediate circuit voltage UC1, UC2 is present at one of the intermediate circuit capacitors C1 or C2 than at the other intermediate circuit capacitor C1 or C2, more energy E1, E2 must be transferred from this intermediate circuit capacitor C1 or C2 to the output capacitor Ca in order to balance, i.e., symmetrize, the intermediate circuit voltages UC1, UC2. However, this is only possible if a load Z is connected to the output pole pair C, D to dissipate this excess energy E1, E2.

[0026] In Fig. 2 is the schematic switching power supply SNT made of Fig. 1 shown as a flyback converter, which, like the switching power supply SNT according to Fig.1 does not fall within the subject matter of the claim. Here, the first input pole pair A1, B1 and the second input pole pair A2, B2 and thus also the intermediate circuit capacitors C1, C2 are connected in series. This means that the second upper input pole B1 and the first lower input pole A2 are connected to a common center point M. This series connection of the first and second intermediate circuit capacitors C1, C2 thus connects the first upper input pole A1 and the second lower input pole B1, with the first intermediate circuit capacitor C1 being connected between the first upper input pole A1 and the center point M and the second intermediate circuit capacitor C1 being connected between the center point M and the second lower input pole B2.The first circuit part SNT1 of the flyback converter connects the center point M to the first upper input pole A1 in parallel with the upper intermediate circuit capacitor C1 and comprises (from the center point M towards the first upper input pole A1) a series circuit of a first switch S1, a first RC circuit RC1 (parallel circuit of a first resistor R10 and a first capacitor C10), and a first diode D1. The forward direction of the first diode D1 is connected to the first intermediate circuit voltage UC1, which means that the cathode of the first diode D1 is connected to the first upper input pole A1 and the anode of the first diode D1 is connected to the center point M via the first RC circuit RC1 and first switch S1.The second circuit part SNT2 of the flyback converter connects the second lower input pole B2 to the center point M in parallel with the second intermediate circuit capacitor C2 and advantageously comprises (from the second lower input pole B2 towards the center point M) a series connection of a second diode D2, a second RC circuit RC2 (parallel connection of a second resistor R20 and a second capacitor C20), and a second switch S2. The forward direction of the second diode D2 is connected to the first intermediate circuit voltage UC1, which means that the anode of the second diode D2 is connected to the second lower input pole B2 and the cathode of the second diode D2 is connected to the center point M via the second RC circuit RC2 and the second switch S2.

[0027] A first terminal of the first primary winding L1 is connected to the first upper input terminal A1 via a first balancing diode DS1. A second terminal of the first primary winding L1 is connected to the junction point of the RC circuit RC1 and the first switch S1. A first terminal of the second primary winding L2 is connected to the junction point of the second RC circuit RC2 and the second switch S2, and a second terminal of the first primary winding L1 is connected to the second lower input terminal B2 via a second balancing diode DS2. The terminals of the secondary winding L3 are, as already shown in Fig. 1 shown, connected to a series circuit of an output diode Da and an output capacitor Ca, with an output voltage Ua applied to the output capacitor Ca. A load Z connected in parallel to the output capacitor Ca is supplied by the output voltage Ua.

[0028] The first intermediate circuit voltage UC1 is thus applied to the first intermediate circuit capacitor C1 and thus between the first upper input pole A1 and the center point M. The first switch S1 is controlled by a control unit 1 according to a predetermined duty cycle. While the first switch S1 is closed, the potential difference caused by the first intermediate circuit voltage UC1 generates a first current is1, which flows between the first upper input pole A1 and the center point M via the first balancing diode Ds1, the first primary winding L1 and the first switch S1. The second switch S2 is controlled by the control unit 1 synchronously with the first switch S1, whereby when the second switch S2 is closed, the potential difference caused by the second intermediate circuit voltage UC2 generates a second current is2, which flows between the center point M and the second lower input pole B2, i.e.via the second switch S2, the second primary winding L2 and the second balancing diode Ds2. The magnetic field created in the first and second primary windings L1, L2 is induced in the secondary winding L3, whereby an output current ia flows through the secondary winding L3, which charges the output capacitance Ca. The output voltage Ua is applied to the output capacitance Ca. In the process, energy E1, E2 is transferred, as described above using . Fig. 1 As described above, the voltage is transferred from the intermediate circuit capacitors C1, C2 via the transformer T into the output capacitance Ca. This corresponds to the operation of a conventional flyback converter with two circuit sections SNT1, SNT2, where the first / second circuit section SNT1 / SNT2 comprises a first / second primary winding L1 / L2 and the secondary winding L3 is inductively coupled to both primary windings L1, L2 (called a double flyback converter).

[0029] If an asymmetry now occurs between the intermediate circuit voltages UC1, UC2, more energy E1, E2 is taken from the intermediate circuit capacitor C1, C2 with the higher intermediate circuit voltage UC1, UC2 than from the intermediate circuit capacitor C1, C2 with the lower intermediate circuit voltage UC1, UC2 and transformed to the secondary winding L3 in order to feed the load Z.

[0030] In the Fig. 5 und 6 The switching patterns of the switches S1, S2 are shown, where the switches S1, S2 are closed during the dashed marking and open outside of it. Furthermore, in Fig. 5 und 6 the first and second switch currents i1, i2, which flow through the associated switch S1, S2, as well as the first current is1, which flows through the first primary winding L1 of the transformer T and the second current is2, which flows through the second primary winding L2 of the transformer T. The first switch current i1 is congruent with the first current is1, and the second switch current i1 is congruent with the second current is1, as long as the associated switch S1, S2 is closed. While the switches S1, S2 are closed, the first and second switch currents i1, i2 increase, whereby energy E1 is charged from the first intermediate circuit capacitor C1 via the first primary winding L1 into the transformer T and energy E2 is charged from the second intermediate circuit capacitor C2 via the second primary winding L2 into the transformer T. The energy E1, E2 transported via the switch current i1, i2 is proportional to the area under the respective switch current i1, i2.

[0031] However, as soon as the first and second switches S1, S2 are opened, the first switch current i1 and the second switch current i2 decay rapidly. The first and second switch currents i1, i2 thus reach their maximum values ​​î1, î2 during the transition from the closed to the open state of the first and second switches S1, S2.

[0032] Due to stray inductances in the first and second primary windings L1, L2, energy remaining is discharged via the first and second diodes D1, D2 and the first and second RC circuits RC1, RC2 when the switches S1, S2 are open. Thus, the first and second switch currents i1, i2 decay more rapidly than the first and second currents is1, is2, since the first and second currents is1, is2 include the currents flowing into the first and second RC circuits RC1, RC2 in addition to the corresponding switch current i1, i2.

[0033] After the switches S1, S2 are opened, the output current ia initially increases during a commutation phase and decreases after reaching its peak value if the switches S1, S2 remain open. While the output current ia decreases, the energy E1, E2 is transferred from the transformer T to the output capacitance Ca, whereby the first and second currents is1, is2 are transformed into the output current ia. The peak value of the output current ia results from the sum of the maximum values ​​of the first and second switch currents î1, î2, each multiplied by the corresponding transformation ratio ü1, ü2, which is Fig. 5 und 6 are identical and are therefore referred to as the transmission ratio ü.

[0034] It is assumed that the second intermediate circuit voltage UC2 is greater than the first intermediate circuit voltage UC1. With respect to Fig. 5 It is assumed that the output pole pair C, D of the flyback converter is connected to a load Z. As can be seen, more energy E2 is drawn from the second intermediate circuit capacitor C2 via the second circuit part SNT2 of the flyback converter than from the first intermediate circuit capacitor C1 via the first circuit part SNT1. This means that the second switch current i2 is greater than the first switch current i1, with the respective transferred energy E1, E2 being proportional to the area under the corresponding switch current i1, i2. The output current ia is therefore largely transformed from the second switch current i2.

[0035] In relation to Fig. 6 It is also assumed that the second intermediate circuit voltage UC2 is greater than the first intermediate circuit voltage UC1, but the switched-mode power supply SNT is in no-load operation, ie the output terminals C, D are connected to a very small or negligible load Z. The output current ia is zero as long as the switches S1, S2 are closed. The switches S1, S2 are closed for a shorter time than in Fig. 5 , since the load Z is lower. Now only energy E2 is drawn from the second intermediate circuit capacitor C2 and stored in the transformer T. Thus, only a second switch current i2 occurs (in this phase, identical to the second current is2), whereby the first switch current i1 and accordingly also the first current is1 are zero.

[0036] Subsequently, switches S1, S2 are opened, causing the second switch current i2 to decrease rapidly and the second current is2 to decrease somewhat more slowly, as energy is again transferred from the stray inductance of transformer T into the RC circuit RC2. Meanwhile, the output current ia increases to its maximum during the commutation phase and then decreases while the output capacitor Ca is charged. It can be seen that the output current ia is lower than in Fig. 5 because energy E2 was only drawn from the second intermediate circuit capacitor C2.

[0037] The output capacitor Ca is only able to absorb the energy E2 originating from the second intermediate circuit capacitor C2 from the secondary winding L2 of the transformer T to a limited extent, since this absorbed energy E2 cannot be discharged to a sufficiently large load Z during no-load operation. Therefore, this circuit is not suitable for balancing the intermediate circuit voltages UC1, UC2 of the intermediate circuit capacitors C1, C2 during no-load operation. Fig. 1 and Fig. 2 However, they are suitable for balancing the intermediate circuit voltages UC1, UC2 if the output terminals C, D are connected to a sufficiently dimensioned load Z, since the energy is delivered to the load Z.

[0038] A switching power supply SNT according to the invention is in Fig. 3 shown schematically. Instead of a transformer T with two primary windings L1, L2 on the primary side and a common secondary winding L3 on the secondary side, two inductively uncoupled transformers T1, T2 are used. The first transformer T1 belongs to the first circuit part SNT1 and comprises a primary winding L11 and a secondary winding L12, which is inductively coupled to the associated primary winding L11. The second transformer T2 belongs to the second circuit part SNT2 and comprises a primary winding L21 and a secondary winding L22, which is inductively coupled to the associated primary winding L21.

[0039] In contrast to the circuit according to Fig. 1 are in Fig. 3 Furthermore, no first and second balancing diodes DS1, DS2 are provided and an output diode Da is omitted. Instead, a first balancing switch SS1 is provided between a first connection of the first secondary winding L12 and the second output pole D, and a second balancing switch SS2 is provided between a first connection of the second secondary winding L22 and the second output pole D. The secondary winding L12 of the first transformer T1 and the secondary winding L22 of the second transformer T2 are connected in parallel via the balancing switches SS1 and SS2 to the output capacitor Ca, to which the output DC voltage Ua is applied. The balancing switches SS1 and SS2 are preferably also controlled by the control unit 1.

[0040] Since an output diode Da is omitted and output-side balancing switches SS1, SS2 are provided instead of the primary-side balancing diodes DS1, DS2, negative switch currents i1, i2 and a negative output current ia are also possible. These required negative switch currents i1, i2 result from the fact that the balancing switches SS1, SS2 each remain switched on longer than the respective switch current i1, i2 needs to drop to zero. For this reason, not only a transformation of energy E1 from the primary winding L11 of the first transformer T1 to the secondary winding L12 of the first transformer T1 is possible, but also a transformation of energy E1 from the secondary winding L12 of the first transformer T1 to the primary winding L11 of the first transformer T1.Analogously, not only a transformation of energy E2 from the primary winding L21 of the second transformer T2 to the secondary winding L22 of the second transformer T2 is possible, but also a transformation of energy E2 from the secondary winding L22 of the second transformer T2 to the primary winding L21 of the second transformer T2.

[0041] It is thus possible, in a situation in which the first intermediate circuit voltage UC1 is greater than the second intermediate circuit voltage UC2, to charge excess energy E1 from the first intermediate capacitor C1 via the first transformer T1 into the output capacitance Ca and further to charge energy E2 from the output capacitance Ca via the second transformer T2 into the second intermediate circuit capacitor C2.

[0042] In a situation where the second intermediate circuit voltage UC2 is greater than the first intermediate circuit voltage UC1, it is also possible to charge excess energy E2 from the second intermediate capacitor C2 via the second transformer T2 into the output capacitance Ca and further to charge energy E1 via the first transformer T1 into the first intermediate circuit capacitor C1.

[0043] Fig 1 bis 8 describe a balancing of two intermediate circuit voltages UC1, UC2 across two intermediate circuit capacitors C1, C2. Of course, more than two intermediate circuit voltages UC1, UC2 can also be balanced, with an additional circuit section being provided for each additional intermediate circuit voltage. An additional transformer is provided for each additional circuit section, the primary side of which is connected to the associated additional intermediate circuit voltage and the secondary side of which is connected in parallel to the secondary sides of the first and second transformers T1, T2. The operation of the additional circuit sections can correspond to the operation of the first and second circuit sections SNT1, SNT2 described here.

[0044] The method according to the invention is described only by way of example using the symmetrization of two intermediate circuit voltages and can also be applied to any number of intermediate circuit voltages.

[0045] In Fig. 4 is the schematic switching power supply SNT made of Fig. 2 shown as a flyback converter. The primary winding L11 of the first transformer T1 is connected in parallel with the series circuit of the first diode D1, and the primary winding L12 of the second transformer T2 is connected in parallel with the second diode D2.

[0046] The first circuit part SNT1 of the flyback converter connects as in Fig. 2 the center point M with the first upper input pole A1 and comprises (from the center point M towards the first upper input pole A1) a series circuit of a first switch S1, a first RC circuit RC1 (parallel circuit of a first resistor R10 and a first capacitor C10), and a first diode D1. The forward direction of the first diode D1 is connected to the first intermediate circuit voltage UC1, which means that the cathode of the first diode D1 is connected to the first upper input pole A1 and the anode of the first diode D1 is connected to the center point M via the first RC circuit RC1 and first switch S1. The second circuit part SNT2 of the flyback converter connects the second lower input pole B2 to the center point M.

[0047] Preferably, the second circuit part SNT2 (from the second lower input pole B2 toward the center point M) comprises a series connection of a second diode D2, a second RC circuit RC2 (parallel connection of a second resistor R20 and a second capacitor C20), and a second switch S2. The forward direction of the second diode D2 is connected to the first intermediate circuit voltage UC1, which means that the anode of the second diode D2 is connected to the second lower input pole B2, and the cathode of the second diode D2 is connected to the center point M via the second RC circuit RC2 and the second switch S2.

[0048] The first / second RC circuits RC1 / RC2 serve to dissipate residual energy stored in the stray inductance of the first / second transformer T1 / T2. Instead of the first / second RC circuit RC2, another energy dissipation circuit, such as a series connection of a diode and transil diode, etc., can be provided.

[0049] The output current ia results from the sum of the first balancing current iss1 and the second balancing current iss2. The first balancing current iss1 corresponds to the first current is1 multiplied by the transformation ratio ü1 of the first transformer T1, the second balancing current iss2 corresponds to the second current is2 multiplied by the transformation ratio ü2 of the second transformer T2, which preferably corresponds to the transformation ratio ü1 of the first transformer T1 and therefore in the Figuren 7 and 8 generally referred to as the gear ratio ü.

[0050] While the first and second switches S1, S2 are closed, the balancing switches SS1, SS2 are open. It is important to avoid switching S1, S2 and balancing switches SS1, SS2 being closed simultaneously. However, phases may occur in which switches S1, S2 and balancing switches SS1, SS2 are open simultaneously.

[0051] The switches S1, S2 and the balancing switches SS1, SS2 alternate between the switching phases P1, P2, P3, and P4. In the first switching phase P1, both the switches S1, S2 and the balancing switches SS1, SS2 are open. In the second switching phase P2, the switches S1, S2 are closed and the balancing switches SS1, SS2 are open. In the third switching phase P3, both the switches S1, S2 and the balancing switches SS1, SS2 are open. In the fourth switching phase P4, the switches S1, S2 are open and the balancing switches SS1, SS2 are closed. The fourth switching phase P4 is therefore complementary to the second switching phase P2.

[0052] In the second switching phase P2, energy is first transferred from the first transformer T1 to the first intermediate circuit capacitor C1 and from the second transformer T2 to the second intermediate circuit capacitor C2. This is Fig. 7 and 8This can be seen from the negative (rising) first and second currents is1, is2. After the zero crossing of the currents i1, i2, a transfer of energy E1 from the first intermediate circuit capacitor C1 into the first transformer T1 takes place, as well as a transfer of energy E2 from the second intermediate circuit capacitor C2 into the second transformer T2. This is shown in Fig. 7 and 8 This can be seen from the positive (rising) first and second currents is1, is2. Throughout the (positive and negative) rising section of the second switching phase P2, the first current is1 corresponds to the first switch current i1, and the second current is2 corresponds to the second switch current i2.

[0053] The maximum value of the second current î2 is greater than the maximum value of the first current î1, since the energy E2 stored in the second intermediate circuit capacitor C2 is greater than the energy E1 stored in the first intermediate circuit capacitor C1.

[0054] During the third switching phase P3, the first and second switch currents i1, i2 rapidly drop to zero because the first and second switches are open and the primary windings L11, L21 of the transformers T1, T2 are no longer supplied with energy E1, E2 from the associated intermediate circuit capacitor C1, C2. Meanwhile, residual energy from the stray inductances of the primary windings L11, L21 of the transformers T1, T2 discharges via the currents is1, is2 into the associated RC circuit RC1, RC2, which is why the currents is1, is2 do not drop as quickly as the switch currents is1, is2. With the start of the fourth switching phase P4, the first and second currents i1, i2 and the first and second switch currents is1, is2 no longer flow.

[0055] In the third switching phase P3 (commutation phase), the primary currents is1, is2 are commutation to the secondary side of the transformers T1, T2, whereby the balancing currents iss1, iss2 increase.

[0056] The output current ia reaches its maximum at the end of the third switching phase P3 together with the balancing currents iss1, iss2, since the output current ia represents the sum of the balancing currents iss1, iss2. In the third switching phase P3, the balancing currents iss1, iss2 are opposite to the switching currents is1, iss2, so that the output current ia is zero at the end of the third switching phase P3.

[0057] Then, in the fourth switching phase P4, i.e. after the third switching phase, which serves as the commutation phase, energy E1 stored in the first transformer T1 is charged into the output capacitor Ca via the first balancing current iss1, and energy E2 stored in the second transformer T2 is charged into the output capacitor Ca via the second balancing current iss2. Since the energy E1 stored in the first transformer T1 is lower than the energy E2 stored in the second transformer T2, the first balancing current iss1 drops below zero more quickly than the second balancing current iss2. However, after the first balancing current iss1 has reached zero, it becomes negative, which means that energy is again charged from the output capacitor Ca into the first transformer T1.After the second balancing current iss2 has reached the zero line, it also becomes negative, which also charges a small amount of energy from the output capacitor Ca into the second transformer T2.

[0058] The subsequent first switching phase P1, like the third switching phase P3, represents a commutation phase. The behavior is the same as for P3, except that the signs of the currents i1, i2, is1, is2, iss1, iss2 are reversed. The commutation phases (first and third switching phases P1, P3) are shown in great detail and schematically in the figures.

[0059] It is again the case that the second intermediate circuit voltage UC2 is greater than the first intermediate circuit voltage UC1, whereby again a symmetrization of the intermediate circuit voltages UC2 UC1 is the goal.

[0060] The energy E2 stored in the second intermediate circuit capacitor C2 is greater than the energy E1 stored in the first intermediate circuit capacitor C1 because the second intermediate circuit voltage UC2 is greater than the first intermediate circuit voltage UC1. This means that in the second switching phase P2 the negative first switch current is1 is greater in magnitude than the second switch current is2. However, the positive first switch current is1 is less than the positive second switch current is2 in the second switching phase P2 because more energy E2 is charged from the second intermediate circuit capacitor UC2 into the primary winding T21 of the second transformer T2 than energy E1 is charged from the first intermediate circuit capacitor UC1 into the primary winding T11 of the first transformer T1.This naturally results in a higher positive second balancing current iss2 compared to the first balancing current iss1 in the fourth switching phase P4, since more energy E2 is present in the second transformer T2 than in the first transformer T1. Likewise, in the fourth switching phase P4, a larger first negative balancing current iss1 results compared to the second balancing current iss2, since more energy is charged from the output capacitor Ca into the first transformer T1 (i.e., into its secondary winding T12) than into the second transformer T2.

[0061] In total, more energy is charged from the second DC link capacitor C2 into the output capacitor Ca than from the first DC link capacitor C1. Conversely, more energy is charged from the output capacitor Ca into the first DC link capacitor C1 than into the second DC link capacitor C2 in order to balance the DC link voltages UC1, UC2.

[0062] Fig. 7 represents the no-load case, ie no or a negligible load Z is connected to the output capacitor Ca, whereas in Fig. 8 a significant load Z is provided, e.g. in the form of lighting, a display, a fan, etc.

[0063] In the absence of a significant load Z, Fig. 7 the balancing of the intermediate circuit voltages UC1, UC2 by recharging between the intermediate circuit capacitors C1, C2, which, however, takes place via the output capacitor Ca. In Fig. 8 on the other hand, energy E1, E2 is also delivered from the output capacitor Ca to the load Z.

Claims

1. Switched-mode power supply (SNT) for balancing a plurality of intermediate circuit voltages (UC1, UC2) which can each be present at a respective intermediate circuit capacitor (C1, C2) of a respective intermediate circuit between a respective input terminal pair (A1, B1, A2, B2) of the switched-mode power supply (SNT), wherein the switched-mode power supply (SNT) is designed to output a DC output voltage (Ua) at an output capacitor (Ca) of the switched-mode power supply (SNT) connected between a first output terminal (C) and a second output terminal (D) of the switched-mode power supply (SNT), characterized in that for each input terminal pair (A1, B1, A2, B2) exhibiting an intermediate circuit voltage (UC1, UC2), the switched-mode power supply (SNT) comprises a circuit part (SNT1, SNT2) each having a transformer (T1, T2), which transformers (T1, T2) between the different circuit parts (SNT1, SNT2) are not inductively coupled to one another, and wherein the input terminal pairs (A1, B1, A2, B2) of the circuit parts (SNT1, SNT2) are each connectable to the intermediate circuit capacitor (C1, C2) which provides the intermediate circuit voltage (UC1, UC2) and exhibits the intermediate circuit voltage (UC1, UC2), and the circuit parts (SNT1, SNT2) are designed to transfer energy (E1, E2) via the input terminal pairs (A1, B1, A2, B2) to a primary winding (L11, L21) of the associated transformer (T1, T2) and vice versa, in that secondary windings (L12, L22) of the transformers (T1, T2) are each connected to the output capacitor (Ca), in that, in each case, a first terminal connection of the secondary windings (L12, L22) of the transformers (T1, T2) is connected to the second output terminal (D) via a balancing switch (SS1, SS2), in that, in each case, a second terminal connection of the secondary windings (L12, L22) of the transformers (T1, T2) is connected to the first output terminal (C), in that, in each case, in the circuit parts (SNT1, SNT2) the primary winding (L11, L21) of the associated transformer (T1, T2) is connected in parallel to an associated diode (D1, D2), wherein the diodes (D1, D2) are each connected in series to an associated switch (S1, S2) between the associated input terminal pair (A1, B1), in that, in each circuit part (SNT 1, SNT2), a corresponding energy annihilation circuit is connected in series to the diodes (D1, D2) in order to annihilate residual energy stored in a stray inductance of the associated transformer (T1) and in that a control unit (1) is provided which is designed to control the balancing switches (SS1, SS2) and the switches (S1, S2) in successive switching phases (P1, P2, P3, P4) and to open both the switches (S1, S2) and the balancing switches (SS1, SS2) in a first switching phase (P1), to close the switches (S1, S2) and to open the balancing switches (SS1, SS2) in a second switching phase (P2) following the first switching phase (P1), to open both the switches (S1, S2) and the balancing switches (SS1, SS2) in a third switching phase (P3) following the second switching phase (P2), and to open the switches (S1, S2) and to close the balancing switches (SS1, SS2) in a fourth switching phase (P4) following the third switching phase (P3) in order to transfer energy (E1, E2) from an input terminal pair (A1, B1, A2, B2) having an intermediate circuit voltage (UC1) via the associated transformer (T1, T2) to the output capacitance (Ca) and from the output capacitance (Ca) to at least one input terminal pair (A1, B1, A2, B2) having a lower intermediate circuit voltage.

2. Switched-mode power supply (SNT) according to claim 1, characterized in that the switched-mode power supply (SNT) comprises a first circuit part (SNT1) of the plurality of circuit parts (SNT1, SNT2) which has a first transformer (T1) of the plurality of transformers (T1, T2) that are provided in a circuit part (SNT1, SNT2) and a first input terminal pair (A1, B1) of the plurality of input terminal pairs (A1, B1, A2, B2), as well as a second circuit part (SNT2) of the plurality of circuit parts (SNT1, SNT2) which has a second transformer (T2) of the plurality of transformers (T1, T2) that are provided in a circuit part (SNT1, SNT2) and a second input terminal pair (A2, B2) of the plurality of input terminal pairs (A1, B1, A2, B2), wherein the first circuit part (SNT1) is connectable on the input side via the first input terminal pair (A1, B1) to a first intermediate circuit capacitor (C1) exhibiting a first intermediate circuit voltage (UC1), and the second circuit part (SNT2) is connectable on the input side via the second input terminal pair (A2, B2) to a second intermediate circuit capacitor (C2) exhibiting a second intermediate circuit voltage (UC2), wherein the first circuit part (SNT1) is designed to transfer energy (E1) from the first input terminal pair (A1, B1) to a primary winding (L11) of the first transformer (T1) and vice versa, and wherein the second circuit part (SNT2) is designed to transfer energy (E2) from the second input terminal pair (A2, B2) to a primary winding (L21) of the second transformer (T2) and vice versa, and in that a secondary winding (L12) of the first transformer (T1) as well as a secondary winding (L22) of the second transformer (T2) are connected to the output capacitor (Ca) in order to transfer energy (E1) from the first input terminal pair (A1, B1) via the first transformer (T1) to the output capacitance (Ca) and from the output capacitance (Ca) to the second input terminal pair (A2, B2) when a first intermediate circuit voltage (UC1) is greater than the second intermediate circuit voltage (UC2), and to transfer energy (E2) from the second input terminal pair (A2, B2) via the second transformer (T2) to the output capacitance (Ca) and from the output capacitance (Ca) to the first input terminal pair (A1, B1) when a second intermediate circuit voltage (UC2) is greater than the first intermediate circuit voltage (UC1).

3. Switched-mode power supply (SNT) according to claim 2, characterized in that a first terminal connection of the secondary winding (L12) of the first transformer (T1) is connected to the second output terminal (D) via a first balancing switch (SS1) of the balancing switches (SS1, SS2) for connecting a corresponding first terminal connection of the secondary windings (L12, L22) of the transformers (T1, T2) to the second output terminal (D), and a first terminal connection of the secondary winding (L22) of the second transformer (T2) is connected to the second output terminal (D) via a second balancing switch (SS2) of the balancing switches (SS1, SS2) for connecting a corresponding first terminal connection of the secondary windings (L12, L22) of the transformers (T1, T2) to the second output terminal (D), in that a second terminal connection of the secondary winding (L12) of the first transformer (T1) and a second terminal connection of the secondary winding (L22) of the second transformer (T2) are connected to the first output terminal (C), and in that a control unit (1) is provided which is designed to control the first and the second balancing switch (SS1, SS2) in order to transfer energy (E1) from the first input terminal pair (A1, B1) via the first transformer (T1) to the output capacitance (Ca) and from the output capacitance (Ca) to the second input terminal pair (A2, B2) when a first intermediate circuit voltage (UC1) is greater than the second intermediate circuit voltage (UC2), and to transfer energy (E2) from the second input terminal pair (A2, B2) via the second transformer (T2) to the output capacitance (Ca) and from the output capacitance (Ca) to the first input terminal pair (A1, B1) when a second intermediate circuit voltage (UC2) is greater than the first intermediate circuit voltage (UC1).

4. Switched-mode power supply (SNT) according to any of claims 1 to 3, characterized in that the input terminal pairs (A1, B1, A2, B2) are connected in series.

5. Switched-mode power supply (SNT) according to any of claims 1 to 4, characterized in that the switched-mode power supply (SNT) is designed as a flyback converter.

6. Arrangement consisting of an electronic power device comprising a plurality of intermediate circuit capacitors (C1, C2), each connected to the input terminal pairs (A1, B1, A2, B2) of a switched-mode power supply (SNT) according to any of claims 1 to 5, for balancing intermediate circuit voltages (UC1, UC2) present to the plurality of intermediate circuit capacitors (C1, C2).

7. Method for balancing a plurality of intermediate circuit voltages (UC1, UC2), each of which is present at an intermediate circuit capacitor (C1, C2) of an intermediate circuit arranged between an associated input terminal pair (A1, B1, A2, B2) of a switched-mode power supply (SNT), wherein an output voltage (Ua) is output at an output capacitor (Ca) of the switched-mode power supply (SNT) between a first output terminal (C) and a second output terminal (D) of the switched-mode power supply (SNT), characterized in that energy (E1, E2) is transformed from the intermediate circuit capacitor (C1, C2) having an intermediate circuit voltage (UC1, UC2) to a secondary winding (L12, L22) of the associated transformer (T1, T2) and transferred to an output capacitance (Ca) via a primary winding (L11, L12) of an associated transformer (T1, T2) of a corresponding circuit part (SNT1, SNT2), wherein the transformers (T1, T2) are not inductively coupled to one another between the different circuit parts (SNT1, SNT2), and energy (E1, E2) is transferred from the output capacitance (Ca) to the other intermediate circuit capacitor or the other other intermediate circuit capacitors (C1, C2) having a lower intermediate circuit voltage by the energy (E1, E2) being transferred to secondary windings (L2) of the other intermediate circuit capacitor(s) (C1, C2) via transformers (T2) of the switched-mode power supply (SNT) associated with a lower intermediate circuit voltage, transformed in each case to a primary winding (L21) of the other transformers (T2) and transferred to the associated intermediate circuit capacitor (UC1, UC2), in that, in each case, in the circuit parts (SNT1, SNT2) the primary winding (L11, L21) of the associated transformer (T1, T2) is connected in parallel to an associated diode (D1, D2), wherein the diodes (D1, D2) are each connected in series to an associated switch (S1) between the associated input terminal pair (A1, B1), in that the switch (S1) and balancing switches (SS1, SS2) which are arranged in each case between a first terminal connection of the secondary winding (L12, L22) of the transformers (T1, T2) and the first output terminal (C) are controlled in successive switching phases (P1, P2, P3, P4), by both the switches (S1, S2) and the balancing switches (SS1, SS2) being opened in a first switching phase (P1), the switches (S1, S2) being closed and the balancing switches (SS1, SS2) being opened in a second switching phase (P2) following the first switching phase (P1), both the switches (S1, S2) and the balancing switches (SS1, SS2) being opened in a third switching phase (P3) following the second switching phase (P2), and the switches (S1, S2) being opened and the balancing switches (SS1, SS2) being closed in a fourth switching phase (P4) following the third switching phase (P3) in order to transfer energy (E1, E2) from an input terminal pair (A1, B1, A2, B2) having an intermediate circuit voltage (UC1) via the associated transformer (T1, T2) to the output capacitance (Ca) and from the output capacitance (Ca) to at least one input terminal pair (A1, B1, A2, B2) having a lower intermediate circuit voltage, and in that, in an energy annihilation circuit of the corresponding circuit part (SNT1, SNT2) connected in series to the diodes (D1, D2), residual energy stored in a stray inductance of the associated transformer (T1, T2) is annihilated.

8. Method according to claim 7, characterized in that a first intermediate circuit voltage (UC1) at a first intermediate circuit capacitor (C1) arranged between a first input terminal pair (A1, B1) of the input terminal pairs (A1, B1, A2, B2) of the switched-mode power supply (SNT) and a second intermediate circuit voltage (UC2) at a second intermediate circuit capacitor (C2) arranged between a second input terminal pair (A2, B2) of the input terminal pairs (A1, B1, A2, B2) of the switched-mode power supply (SNT) are balanced by way of energy (E1) being transformed from the first intermediate circuit capacitor (C1) via a first primary winding (L11) of a first transformer (T1) to a secondary winding (L12) of the first transformer (T1) and transferred from the secondary winding (L12) of the first transformer (T1) to an output capacitance (Ca) when a first intermediate circuit voltage (UC1) is greater than the second intermediate circuit voltage (UC2) and energy (E2) being transferred from the output capacitance (Ca) to a secondary winding (L2) of a second transformer (T2) of the switched-mode power supply (SNT) and transformed from the secondary winding (L2) of the second transformer (T2) to a primary winding (L21) of the second transformer (T2) and transferred from the primary winding (L21) of the second transformer (T2) to the second intermediate circuit capacitor (U2), or energy (E2) being transformed from the second intermediate circuit capacitor (C2) via the primary winding (L21) of the second transformer (T2) to the secondary winding (L22) of the second transformer (T2) and transferred from the secondary winding (L22) of the second transformer (T2) to the output capacitance (Ca) when a second intermediate circuit voltage (UC2) is greater than the first intermediate circuit voltage (UC1) and energy (E1) being transferred from the output capacitance (Ca) to the secondary winding (L12) of the first transformer (T1) and transformed from the secondary winding (L21) of the first transformer (T1) to the primary winding (L11) of the first transformer (T1) and transferred from the primary winding (L11) of the first transformer (T1) to the first intermediate circuit capacitor (UC1).

9. Method according to claim 8, characterized in that a first balancing switch (SS1) of the balancing switches (SS1, SS2) arranged in each case between a first terminal connection of the secondary winding (L12, L22) of the transformers (T1, T2) and the second output terminal (D), which first balancing switch is arranged between a first terminal connection of the secondary winding (L12) of the first transformer (T1) and the second output terminal (D), is switched, and a second balancing switch (SS2) of the balancing switches (SS1, SS2) arranged in each case between a first terminal connection of the secondary winding (L12, L22) of the transformers (T1, T2) and the second output terminal (D), which second balancing switch is arranged between a first terminal connection of the secondary winding (L22) of the second transformer (T2) and the second output terminal (D), is switched, in order to transfer energy (E1) from the first intermediate circuit capacitor (C1) via the first transformer (T1) to the output capacitance (Ca) and to transfer energy (E2) from the output capacitance (Ca) to the second intermediate circuit voltage (U2) when a first intermediate circuit voltage (UC1) is greater than the second intermediate circuit voltage (UC2), and in order to transfer energy (E2) from the second intermediate circuit capacitor (C2) via the second transformer (T2) to the output capacitance (Ca) and to transfer energy (E1) from the output capacitance (Ca) to the first intermediate circuit voltage (UC1) when a second intermediate circuit voltage (UC2) is greater than the first intermediate circuit voltage (UC1).