ELECTROSURGICAL GENERATOR HAVING AN HF HIGH-VOLTAGE MULTILEVEL INVERTER

Abstract

An electrosurgical generator for an electrosurgical instrument includes a DC voltage supply and a high-voltage inverter that generates a high-frequency AC voltage having a variable voltage and frequency that is output at an output for the connection of the electrosurgical instrument. The inverter is configured as a multilevel inverter and includes a plurality of inverter cells connected in a cascaded manner that are driven by a control device. Thanks to the cascading, switching losses incurred in the power semiconductors are reduced, both in terms of value (through the divided and thus lower voltage) and in terms of frequency (through the reduced switching frequency).

Claims

1. An electrosurgical generator that is designed to output a high-frequency AC voltage to an electrosurgical instrument, comprising a DC voltage supply and a high-voltage inverter that is fed from the DC voltage supply and generates a high-frequency AC voltage having a variable voltage and frequency that is applied to an output for the connection of the electrosurgical instrument, wherein the inverter is configured as a multilevel inverter and comprises a plurality of inverter cells connected in a cascaded manner that are driven by a control device.

2. The electrosurgical generator as claimed in claim 1, wherein the inverter cells have potential decoupling at output.

3. The electrosurgical generator as claimed in claim 2, wherein a respective transformer is connected at the output of the respective inverter cell with its primary side.

4. The electrosurgical generator as claimed in claim 3, wherein the transformers are each provided with a transformer unit as preamplifier for stepping up the voltage.

5. The electrosurgical generator as claimed in claim 1, wherein the inverter cells are fed from in each case one voltage source.

6. The electrosurgical generator as claimed in claim 1, wherein a plurality of, at least two groups of inverter cells are provided, wherein the inverters of the respective group are supplied jointly by one DC voltage source.

7. The electrosurgical generator as claimed in claim 1, wherein a plurality of, at least two groups of inverter cells are provided, wherein the groups are supplied with DC voltage of different values.

8. The electrosurgical generator as claimed in claim 7, wherein provision is made for in each case at least one DC-to-DC converter for supplying at least one of the groups with a different voltage.

9. The electrosurgical generator as claimed in claim 1, wherein DC voltage sources for supplying the inverter cells are galvanically coupled.

10. The electrosurgical generator as claimed in claim 1, wherein the DC voltage supply is designed as a fixed voltage supply.

11. The electrosurgical generator as claimed in claim 1, wherein the inverter cells are each configured with a type of structure with neutral point clamping at their DC voltage supply or with a floating capacitor.

12. The electrosurgical generator as claimed in claim 1, wherein the inverter cells are connected in series.

13. The electrosurgical generator as claimed in claim 1, wherein provision is made for a control signal generator for the multilevel inverter that is designed to generate a reference signal for driving the multilevel inverter.

14. The electrosurgical generator as claimed in claim 13, wherein the reference signal is a pattern for AC voltage to be output by the electrosurgical generator.

15. The electrosurgical generator as claimed in claim 13, wherein the control signal generator drives an inverter controller that is designed to drive the inverter cells such that they generate an output voltage in accordance with the reference signal.

16. The electrosurgical generator as claimed in claim 1, wherein the inverter cells are driven with a variable-frequency reference signal.

17. The electrosurgical generator as claimed in claim 1, wherein provision is made for an output transformer on the output line as a further galvanic isolation device.

18. The electrosurgical generator as claimed in claim 16, wherein provision is made, in the output line, for a low-pass filter.

19. The electrosurgical generator as claimed in claim 18, wherein provision is made for an active damping device for the low-pass filter.

20. The electrosurgical generator as claimed in claim 19, wherein the active damping device comprises a feedback system, wherein the feedback system has at least one current sensor on the low-pass filter.

21. The electrosurgical generator as claimed in claim 19, wherein an output signal from the active damping device acts on the multilevel inverter.

22. The electrosurgical generator as claimed in claim 1, wherein provision is made for at least one further output to which a further AC voltage generated by the multilevel inverter is applied.

23. The electrosurgical generator as claimed in claim 22, wherein the at least one further AC voltage has a lower frequency than the high-frequency AC voltage at the output for the connection of the electrosurgical instrument.

24. The electrosurgical generator as claimed in claim 22, wherein provision is made for at least one changeover device that is designed to selectively connect the multilevel inverter to one of the outputs.

25. The electrosurgical generator as claimed in claim 23, wherein the inverter cells are divided in terms of circuitry such that at least one portion of the inverter cells is provided for connection to the at least one further output and another portion of the inverter cells furthermore supplies the output.

Description

[0042] The invention is explained in more detail below by way of example with reference to advantageous embodiments. In the figures:

[0043] FIG. 1 shows a schematic illustration of an electrosurgical generator according to one exemplary embodiment with a connected electrosurgical instrument;

[0044] FIG. 2a, b show block diagrams of exemplary embodiments for a multilevel inverter of the electrosurgical generator according to FIG. 1 with cascaded inverter cells;

[0045] FIG. 3 shows a schematic circuit diagram of two of the inverter cells;

[0046] FIG. 4a-c show diagrams of voltage and signal profiles for switching elements of the two inverter cells according to FIG. 3;

[0047] FIG. 5 shows an exemplary circuit diagram of the multilevel inverter having a plurality of cascaded inverter cells;

[0048] FIG. 6a, b show schematic circuit diagrams of alternative embodiments of the inverter cell;

[0049] FIG. 7a, b show voltage profiles at the output without feedback in the case of a high-resistance load or short circuit;

[0050] FIG. 8a, b show voltage profiles at the output with feedback in the case of a high-resistance load or short circuit;

[0051] FIG. 9a-e show an illustration of various voltage/time profiles in high-frequency surgery;

[0052] FIG. 10 shows a schematic illustration of an electrosurgical generator according to another exemplary embodiment;

[0053] FIG. 11 shows a schematic illustration of an electrosurgical generator according to a further exemplary embodiment;

[0054] FIG. 12 shows a schematic illustration of a variant of the further exemplary embodiment according to FIG. 11; and

[0055] FIG. 13 shows a circuit diagram of an inverter according to the prior art.

[0056] An electrosurgical generator according to one exemplary embodiment of the invention is illustrated in FIG. 1. The electrosurgical generator, referenced in its entirety by the reference numeral 1, comprises a housing 11 that is provided with a port 14 for an electrosurgical instrument 16; in the illustrated exemplary embodiment, this is an electrical scalpel. It is connected to the port 14 of the electrosurgical generator 1 via a connection plug 15 of a high-voltage connection cable. The power output to the electrosurgical instrument 16 may be changed via a power controller 12.

[0057] In order to supply power to the electrosurgical generator 1, provision is made for a DC voltage supply 2, which is able to be connected, via a mains connection cable (not illustrated), to the public grid and is fed therefrom. The DC voltage supply 2 in the illustrated exemplary embodiment is a high-voltage power supply unit (High Voltage Power Supply—HVPS). It comprises a rectifier and feeds a DC link circuit 20 with DC voltage, the value of which is preferably fixed and is for example 48 volts. However, it should not be ruled out that the DC voltage value is variable between 0 and around 400 volts, wherein the absolute value of the DC voltage may in particular depend on the set power, the type of electrosurgical instrument 16 and/or its load impedance, which in turn depends on the type of tissue being treated. However, an internal power supply unit is not necessary, meaning that the DC voltage supply may also be implemented by an external power supply unit, or provision is made for a direct DC feed, for example 24 volts in vehicles or 48 volts in stationary applications.

[0058] An inverter is fed by the DC link circuit 20 and generates, from supplied DC voltage, high-frequency AC voltage in the high-voltage range of a few kilovolts, at predefinable frequencies in the range between 200 kHz and 4 MHz. The inverter is designed in the structural form of a multilevel inverter 4, as will be explained in even more detail below. The frequency and curve form of the high-frequency AC voltage to be generated by the multilevel inverter 4 are in this case predefined by an inverter controller 41 on the basis of a reference signal generated by a control signal generator 40. The high-frequency high voltage generated by the multilevel inverter 4 is routed via a low-pass filter 8 and an output transformer 7, operating as an output transformer unit for stepping up the voltage, secured against undesirable DC current components by a blocking capacitor 17 arranged in series, and output at the port 14 in the form of Uout for connection to the electrosurgical instrument 16. The voltage and current of the high voltage generated and output by the multilevel inverter 4 are furthermore measured by way of a combined voltage and current sensor 18, and the measured signals are supplied to a processing unit 19, which applies the corresponding data about the output voltage, current and power as feedback to the control signal generator 40 and to an operating controller 10 of the electrosurgical generator 1. The power controller 12 is also connected to the operating controller 10. The operating controller 10 is furthermore designed to set various what are known as modes, which are typically stored voltage/time profiles. Provision is made for a selection switch 12′ for the user to select the mode. The operating controller 10 furthermore interacts with the control signal generator 40, which is designed to generate the reference signal for the AC voltage to be output, in particular with regard to amplitude, frequency, curve form and duty cycle.

[0059] The multilevel inverter 4 comprises a plurality of series-connected inverter cells 5 that are driven by an inverter controller 41. Reference is now made to FIG. 2a. In the exemplary embodiment illustrated there, a DC voltage source having a defined DC voltage is connected to the input (illustrated on the left in the drawing) of each of the inverter cells 5. The respective inverter cell 5 generates therefrom an AC voltage that is output at the output (illustrated on the right in the drawing) of the respective inverter cell 5 in the form of AC voltage. The number of inverter cells is not limited and is as desired per se. The inverter cells 5 are numbered consecutively in FIG. 2a with the designation “5-1”, “5-2” to “5-5”, wherein the number 5 is an example and any number of at least two inverter cells may be provided. The DC voltages applied at the input of the respective inverter cell 5 are optionally coupled in terms of potential via a busbar 50. The AC voltage output at the output of the respective inverter cell 5 is accordingly denoted “V_1”, “V_2” up to “V_5”. A series connection of the inverter cells 5 results in their output voltages being added, ultimately giving, as overall output voltage:

[00001]$V_{out}=\underset{i=1}{\overset{N}{.Math.}}\mathrm{V\_i}$

[0060] The number of voltage levels able to be achieved with the “N” inverter cells 5 is in this case at least

2N+1

[0061] assuming that the DC voltages “Vin_1”, “Vin_2” to “Vin_N” applied at the input of the inverter cells 5 are all of the same value. This thus results, for example in the case of a number of five inverter cells 5, in a total of eleven possible voltage levels for the overall output voltage Vout.

[0062] The number of voltage levels may be increased considerably for an identical number of inverter cells 5 when they are fed at least in groups with DC voltage of different values. Such a configuration is shown in FIG. 2a, b. Two groups of inverter cells are formed there: a first group I containing three inverter cells 5-1, 5-2 to 5-3, which are fed from a DC voltage source with a low DC voltage, in the example 12 V; and a second group II containing two inverter cells 5-4, 5-5, which are fed from another DC voltage source with a higher DC voltage, in the example 48 V. The first group I contains low-voltage inverter cells, and the second group II contains high-voltage inverter cells. The outlay in terms of DC voltage sources is increased here, because two are now required instead of one. However, to make up for this, the number of voltage levels is increased considerably, specifically starting from eleven to more than twice that with 23 voltage levels. The number of voltage levels thus able to be achieved follows the formula

2*(mHVc*r+nLVc)+1,

[0063] wherein mHVc represents the number of higher-DC-voltage inverter cells (in the above example m=2), nLVc represents the number of low-DC-voltage inverter cells (in the above example n=3) and r represents the ratio of higher to low DC voltage (in the above example r=4).

[0064] The two voltage sources do not need to be isolated from one another in terms of potential here, but rather they may share a common reference potential, as implemented in FIG. 2a by way of the busbar 50. This also makes it possible to generate the lower DC voltage from the higher DC voltage, which may be for example the DC voltage in the link circuit 20, by way of a DC-to-DC converter 42, in particular a DC-to-DC buck converter. In the present example, as illustrated in FIG. 2b, it would be designed for a ratiometric supply with a transmission ratio of 4:1. One advantage of this configuration by way of a ratiometric supply is that changes or fluctuations in the higher DC voltage are then reflected proportionally in the lower DC voltage, meaning that the relative gradation is maintained. This makes it possible for example to increase the output voltage by 12 V in two different ways: one conventionally by activating a further 12 V inverter cell, or by activating a 48 V inverter cell in combination with deactivating three 12 V inverter cells.

[0065] The structure of the individual inverter cells 5 and their interaction are illustrated by way of example in the schematic circuit diagram according to FIG. 3. A total of two inverter cells 5-1 and 5-2 are shown there in a cascaded arrangement, in order thus also to illustrate their mutual interconnection. The common DC voltage source 31, having a supply voltage Vin of 12 volts, is illustrated on the left-hand edge of the image. It is assigned a stabilization capacitor 33. These supply the two inverter cells 5-1 and 5-2. Reference is first of all made below to the switching of the inverter cell 5-1.

[0066] Provision is made for four power switches that operate as current valves and are arranged in an H-bridge configuration. The power switches are power semiconductor switches, for example configured as IGBTs, MOSFETs or GaNFETs. The power switches 51, 53 are connected in series and form a first branch, and the power semiconductors 52, 54 are likewise connected in series and form a second branch. The center taps of the two branches are guided out and connected to both ends of a primary winding 61 of a first transformer 6-1. The transformer 6-1 furthermore has a secondary winding 62 and is used for potential isolation, wherein it may optionally furthermore have a transmission ratio for pre-amplifying the voltage; this is 1:1.5 in the illustrated example (it is pointed out that a different transmission ratio may be provided, for example with a transmission ratio of 1:1, in particular when no pre-amplification is intended to be achieved. An output line 13 is connected to the secondary winding 62 and leads to the output 14 of the electrosurgical generator 1 (possibly via a low-pass filter, not illustrated in FIG. 3).

[0067] The two power switches 51, 53 of the first branch are driven by a common signal C1.a, wherein this signal is supplied to the power switch 53 in inverted form. The two power switches 52, 54 of the second branch are accordingly likewise driven by a common signal C1.b, wherein this signal is supplied to the power switch 52 in inverted form. This means that, in the event of a HIGH signal of C1.a, the power switch 51 is put into the on state and the power switch 53 is put into the off state, that is to say the first power branch applies a positive potential to the upper connection of the primary winding 61 of the transformer 6-1. Accordingly, in the event of a HIGH signal of C2.b, the power switch 54 is put into the on state, while the power switch 52 is put into the off state in the second power branch. The second power branch thus applies a negative potential to the lower connection of the primary winding 61. In the event of a LOW signal of C1.a or C1.b, this accordingly applies vice versa, that is to say the polarity at the primary winding 61 is reversed. An AC voltage is thus generated by the inverter cell 5-1 and applied to the primary winding 61 of the transformer 6-1.

[0068] The second inverter cell 5-2 has an identical structure, and is supplied from the DC voltage source 31 in the same way as the first inverter cell 5-1. The same reference numerals are therefore used for identical elements in the figure. It is driven by the control signals C2.a and C 2.b in a manner corresponding to that described above. It thus likewise outputs, at its output, an AC voltage that is applied to a primary winding 61 of a second transformer 6-2. Since the two inverter cells 5-1 and 5-2 are fed from the same DC voltage source 31, they are connected in terms of potential. This means that the AC voltages output directly by the inverter cells 5-1 and 5-2 are not readily able to be added, since they are linked to one another in terms of their potential. However, since this output AC voltage is supplied to each of the transformers 6-1 and 6-2, the AC voltages output by the transformers 6-1 and 6-2 are each potential-free and are readily able to be added to one another to give a common output voltage that is applied to the output line 13.

[0069] The switching behavior of the power switches 51 to 54 under the effect of the control signals C1.a, C1.b, C2.a and C2.b, as are generated by the inverter controller 41 for example by way of PWM control, which is known per se, is illustrated in FIG. 4. FIG. 4a shows the obtaining of the control signals C1.a, C1.b, C2.a and C2.b. The inverter controller 41 provides a sawtooth-shaped carrier signal having a frequency of 1 MHz for each of the control signals, which are phase-offset equally from one another by 90°. These four carrier signals are illustrated by four offset sawtooth profiles in FIG. 4a. Also illustrated is the modulation signal required for the PWM modulation, in this case formed by the reference signal in the form of a sinusoidal oscillation having a frequency of 200 kHz. The signal sequences resulting from the modulation for the four control signals C1.a, C1.b, C2.a and C2.b, as are output by the inverter controller 41 for the inverter cells 5-1 and 5-2, are illustrated in FIG. 4b. These are pure rectangular-wave signal sequences that each know only a 1-bit switching state. If the power switches 51 to 54 of the two inverter cells 5-1 and 5-2 are driven with these signal sequences for the control signals in the manner described above, and the voltages respectively output by the two inverter cells 5-1 and 5-2 are added by the transformers 6-1 and 6-2, then this results in the voltage profile ultimately illustrated in FIG. 4c at the output 14. An approximately sinusoidal output voltage having five voltage levels is thus generated from the four 1-bit control signals.

[0070] An exemplary circuit diagram of the multilevel inverter 4 and its connection to adjacent components is illustrated in FIG. 5. It is possible to see the multilevel inverter 4 with its multiplicity of inverter cells, illustrated by the inverter cell 5-1 up to the inverter cell 5-n. They apply the AC voltage that they generate in each case to primary windings 61 of the transformers 6-1 to 6-n assigned thereto. In this exemplary embodiment, the transformers are configured such that their secondary windings 62′ have a higher number of turns than the primary winding 61. They are therefore designed as combined transformers and transformer units and thus ensure not only potential decoupling but also additional voltage amplification. The secondary windings 62′ are connected in series, such that their amplified voltages sum to give an increased overall voltage.

[0071] The overall voltage is output to the output line 13, at the end of which the low-pass filter 8 is arranged. This is configured as a second-order filter and comprises an inductor 81 and a capacitor 82 connected in series therewith. It is pointed out that stray inductances of the transformers 6-1 to 6-n also contribute to the inductance of the inductors 81 of the low-pass filter, and may possibly at least partially replace them. The low-pass filter 8 is tuned such that interference in the generated AC voltage due to the switching frequency of the power switches in the inverter cells of the multilevel inverter 4 is filtered out. The output of the low-pass filter 8 is applied to a primary winding 71 of an output transformer 7, which brings about galvanic isolation of the port 14 connected to the secondary winding 72. Provision is furthermore made for a blocking capacitor 17. This serves for preventing the output of DC current components to the surgical instrument 16.

[0072] The low-pass filter 8 is provided with active damping. This comprises a feedback system 9 to which the current sensor 83 is connected at input. The current sensor 83 is arranged in the same branch as the capacitor 82 of the low-pass filter 8 and thus defines the current flow through the capacitor 82. By defining the current, an appropriate signal proportional to the measured current is able to be fed back through the feedback system 9. This implements a transfer function that is selected depending on the desired behavior of the low-pass filter 8, which is now actively damped. In the simplest case, the transfer function may be configured as a proportional member. The output signal from the feedback system 9 is switched onto a negative input of a differential member 91 in order to modify the reference signal that is generated by the control signal generator 40 and connected to the positive input of the differential member 91. The reference signal modified in this way is output at the output of the differential member 91 and is applied to an input of the inverter controller 41 as drive signal for the multilevel inverter 4. The output voltage of the multilevel inverter 4 is thereby able to be controlled in a manner dependent on the feedback system 9. Undesirable resonances are thus already able to be prevented to some extent. Provision may furthermore alternatively or additionally be made for a current sensor 84 that is arranged on the primary-side port of the output transformer 7 or in series with the blocking capacitor 17 and thus defines the current flow through the output transformer 7. By defining the current, an appropriate signal proportional to the measured current is likewise able to be fed back through the feedback system 9. The feedback system implements an (appropriately expanded) transfer function that is selected according to the desired behavior, which is now actively damped, of the LC filter formed by the inductor 81 and the blocking capacitor 17.

[0073] The effect of the feedback system 9 on the voltage and current profiles at the output 14 is illustrated in FIGS. 7a, b and FIG. 8a, b. In both cases, the multilevel inverter 4 generates a pulsed AC voltage signal consisting of an individual sinusoidal oscillation (as illustrated in FIG. 9e). In the case illustrated in FIG. 7a, the load at the output is assumed to be high-resistance (in the region of 100 kOhm). The output voltage (dashed line) of the sinusoidal oscillation generated by the inverter cells 5 of the multilevel inverter 4 is thereby additionally overlaid with a resonant oscillation. This results from the resonant frequency of the LC filter formed by the inductors 81 and the capacitor 82 in accordance with the known formula

[00002]$f=\frac{1}{2.Math.\pi \sqrt{L.Math.C}}.$

The resultant overlaid output signal is illustrated by the solid line. It is possible to see a considerable deformation of the curve and pronounced reverberation. The complementary case of a short circuit is illustrated in FIG. 7b. It is again possible to see the (identical) sinusoidal oscillation generated by the inverter cells 5 of the multilevel inverter 4 (dashed line). In addition to this, there is an overlap from the resonant oscillation, which results from the filter inductor 81, which resonates with the blocking capacitor 17 at the output 14. The current profile that results in this case is illustrated with the solid line at the output 14. It may be seen that a considerable interfering oscillation builds up.

[0074] The same cases are illustrated in FIG. 8a, b, wherein the filter 8 is damped by way of the feedback system 9. FIG. 8a again shows the case with a high-resistance load. The original output signal from the inverter cells 5 of the multilevel inverter 4 is also illustrated with a dashed line as reference. The actual output signal overlaid with interference is fed back via the feedback system 9 using measured signals from the current sensor 83 and changes the signal supplied to the multilevel inverter 4 by acting on the differential member 91. This reference signal is bent in a targeted manner, as it were, giving rise to a modified reference signal, which is actually applied to the inverter controller 41 as control signal in order to drive the inverter cells 5. The resulting output signal (see dashed line, which illustrates a smoothed profile) is “bent” in a targeted manner such that the overlaid oscillation at the output is counteracted in a targeted manner. The actual output signal ultimately resulting from the generated voltage, which is “bent” in a targeted manner, of the multilevel inverter 4 and from the resonant oscillation of the filter 8 is illustrated by the solid line. It is readily able to be seen through comparison with FIG. 7a that the actual output signal is a substantially more harmonic sinusoidal oscillation.

[0075] The same applies to the short-circuit case using the feedback system 9. This case is illustrated in FIG. 8b. The original drive signal, as generated as reference signal by the control signal generator 40, is again illustrated with a dashed line. The modified reference signal ultimately generated under the effect of the feedback system 9 using measured signals from the current sensor 84 is used to drive the inverter cells 5.

[0076] The resulting output signal is illustrated (following smoothing) by the dashed line. It is surprisingly small in relation to the voltage amplitude, the reason for which is that the undesirable resonant frequency lies very close to the frequency of the AC voltage generated by the multilevel inverter 4. Only a very small actual drive signal for the inverter controller 41 is thus required. The actual current profile that then results at the output 14 is again illustrated with the solid line. It is readily able to be seen through comparison with FIG. 7a that the actual output signal is a substantially more harmonic sinusoidal oscillation. It may clearly be seen, through comparison with FIG. 7b, that the actual sinusoidal oscillation is reproduced significantly more accurately (interval up to 2 μs) and parasitic reverberation is then effectively suppressed (no “ringing” effect). The feedback using the measured signals from the current sensor 84 thus ensures a considerably better and lower-harmonic sinusoidal output signal in spite of the critical LC filter 8 with the blocking capacitor 17 at the output 14.

[0077] As a result, the multilevel inverter 4 according to the invention may be used to finely and precisely predefine the AC voltage profiles to be output. The multilevel inverter 4 driven by the reference signal in particular gives full control of the curve form, specifically in particular including in the case of modulated output signals. Modulated output signals are thus able to be generated accurately and in a reproducible manner, as illustrated in FIGS. 9a to 9e. In order to ensure a constant energy output, the multilevel inverter 4 according to the invention furthermore makes it possible, in highly modulated modes with a shorter duty cycle, to increase the value of the output voltage to the extent that, in spite of the short switch-on time, the same energy is output to the electrosurgical instrument 16 as in the modes with a longer switch-on time or in the continuous mode.

[0078] The invention thus allows more dynamic and more accurate control of the output high-frequency AC voltage, specifically including and specifically in pulsed modes. The modes are again able to be kept considerably more precise thanks to the optional feedback.

[0079] It is furthermore pointed out that the invention is not restricted to inverter cells 5 with an H-bridge configuration. Provision may also be made for other topologies for the inverter cells 5. FIGS. 6a and 6b show examples of these and illustrate alternative topologies, specifically likewise each having four switching elements 51′ to 54′ and 51″ to 54″. FIG. 6a thus shows a configuration of the inverter cell with a type of structure with neutral point clamping by way of diodes 55, 56, and FIG. 6b with a type of structure with a floating capacitor 57. Similarly to the inverter cells in an H-bridge configuration, these may likewise be cascaded in order to achieve a higher number of voltage levels.

[0080] FIG. 10 illustrates an alternative exemplary embodiment to the exemplary embodiment according to FIG. 1. Elements that are identical or of the same type are denoted using the same reference numerals. It differs essentially in that the low-pass filter 8 has a two-stage configuration in the alternative exemplary embodiment. A first stage 8′ of the low-pass filter is furthermore arranged directly at the output of the multilevel inverter 4 in order to smooth the generated AC voltage. A second stage 8″ of the low-pass filter is arranged on the output side of the output transformer 7. Further smoothing thus takes place just before the output, in order in particular also to detect interference caused by the output transformer 7. It is pointed out that the stray inductance of the output transformer 7 may also contribute to the inductance of the inductors 81 of the second stage 8″ of the low-pass filter, and may possibly at least partially replace them.

[0081] In the embodiment according to FIG. 10, provision is made for dual blocking capacitors 17, 17′ for increasing safety. It will be understood that such a dual arrangement may also be provided in the other exemplary embodiments.

[0082] An expedient alternative arrangement of the current sensors for the feedback system is also illustrated using the example of this exemplary embodiment according to FIG. 10; this may also be provided in the other exemplary embodiments. Provision is made in this case for a current sensor 18′ in series on the low-pass filter 8, more precisely on the output of the first stage 8′. The combined current and voltage sensor 18 functions as second current sensor. Based on these signals, it is possible to measure the actual output current (which is transmitted to the operating controller 10 via the processing unit 19) along with the current flow on the input side of the output transformer 7. A transverse current detector is also formed. This is designed to determine, from a current difference that results here, the magnitude of a current through a capacitor 82 of the low-pass filter (here the second stage 8″ of the low-pass filter). This may be acquired by the feedback system 9 and compensated for by changing the driving of the multilevel inverter 4. It is thereby also possible to detect and compensate for current losses caused by parasitic transverse capacitance that is not otherwise able to be measured directly, in particular of the output transformer 7 or of the low-pass filter 8 with its stages 8′, 8″.

[0083] A further exemplary embodiment of an electrosurgical generator according to the present invention is illustrated in FIG. 11. This is based on the exemplary embodiment illustrated in FIG. 1, but differs therefrom in that provision is made for a second output 14* and a changeover device 3. The multilevel inverter 4 is connected to the input of said changeover device and the output line 13 is connected to one of its outputs and leads, via the low-pass filter 8 and the output transformer 7, to the (first) output 14 for the electrosurgical instrument 16. A second output 14* is connected to the other output of the changeover device 3 via a second output line 13*, a second low-pass filter 8* and a second output transformer 7′. A connection plug 15* for a second instrument (not illustrated) may be connected to said second output, wherein the second instrument may be in particular an ultrasonic surgical instrument, such as for example an ultrasonic scalpel. Provision is made for another at least one blocking capacitor 17 (not illustrated) at each of the outputs 14, 14*, as in the exemplary embodiment shown in FIG. 1.

[0084] The changeover device 3 is designed to output the AC voltage generated by the multilevel inverter 4 selectively at the output 14 to the instrument 16 connected there, in particular the electrosurgical instrument 16, or at the output 14* to the instrument connected there, in particular the ultrasonic surgical instrument. Using the same electrosurgical generator 1, it is thus possible, as the surgeon wishes, to use an electrosurgical instrument, such as for example an electrocauter, or an ultrasonic surgical instrument, such as for example ultrasonic dissecting scissors. The change between the instruments is made considerably easier and may even take place in an intraoperative manner. The field of application for the electrosurgical generator is thus broadened considerably. As an alternative or in addition, in one variant as illustrated in FIG. 12, provision may also be made for the inverter cells 5 to be divided in terms of circuitry. In this case, at least one (but not all) of the inverter cells 5 is connected to the second output 14* and is able to supply same for example with an AC voltage in the ultrasonic frequency range, while the rest of the inverter cells 5-1 to 5-4 continue to supply the output 14 with high-frequency AC voltage. It is thereby also possible to operate two electrosurgical instruments in parallel (including in different modes), or it is also readily possible to operate an instrument that uses both ultrasound and high-frequency energy.