Electrosurgical generator with multilevel inverter for hf high voltage

Abstract

To provide an electrosurgical generator for an electrosurgical instrument 16.SOLUTION: An electrosurgical generator comprises a DC voltage power source 2 and a high voltage inverter which generates a high frequency AC voltage having a variable voltage and a frequency to be outputted by a connection output 14 of an electrosurgical instrument 16. The inverter consists of a plurality of cascade-connected inverter cells consisting of multilevel inverters 4 and driven by a control device 41. A switching loss is reduced in a value (a voltage is divided and reduced) and also in a frequency (a switching frequency is reduced) by the cascade connection, and unbalance of the inverter cells is mitigated by a secondary relation between the voltage and a voltage loss. The inverter cells can be promptly and accurately adjusted with respect to a load impedance change, a dynamic range is also improved, a stable voltage is outputted even in a high pulse mode and even if a duty cycle is small, a crest factor is reduced to a negligible level.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electrosurgical generator for outputting a high frequency AC voltage to an electrosurgical instrument, the electrosurgical generator comprising a DC voltage source and a high voltage inverter powered from the DC voltage source for generating a high frequency AC voltage and applying it to an output for connection to the electrosurgical instrument. [Background technology]

[0002] In electrosurgery, high-frequency alternating currents are used, in particular for cutting or slicing tissue, as well as for the ablation of body tissue in the sense of thermal ablation (what is known as an electrocautery). The principle of operation is based on heating the tissue to be cut. The advantage of this is that, at the same time as the cutting, it is also possible to stop bleeding by closing the affected blood vessels (coagulation). For this, considerable power is required, specifically frequencies above 200 kHz and below 4000 kHz, typically around 400 kHz. At this frequency, body tissue behaves like an ohmic resistor. However, its resistivity strongly depends on the type of tissue, with the resistivity of muscle, fat, bone, etc. differing greatly from each other, specifically by a factor of 1000. For this reason, the load impedance of the electrocautery can change so quickly and so much depending on the tissue to be cut that it is virtually short-circuited. This places special and unique requirements on the electrocautery generator and its high-voltage source. In particular, there is a need for high voltage control suitable for high voltages in the range of several kilovolts, and for a wide range of high frequencies, typically from 200 kHz up to 4 MHz.

[0003] Depending on the tissue and impedance, the current can vary between a few milliamps to a few amps, and can vary in a very short time in a very dynamic way. The waveform of the output AC voltage can be a continuous sine wave or it can be modulated with a crest factor of up to 10% at a modulation frequency of up to 20 kHz. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] European Patent Publication No. 2514380 Summary of the Invention [Problem to be solved by the invention]

[0005] To meet these unique demands, electrosurgical generators are generally constructed with inverters for powering the electrosurgical instruments, with rectified current from the grid being fed at different voltages. The inverters, in turn, are typically constructed as free-oscillating single-ended generators with LC resonant circuits (see block 114 highlighted in dashed lines in prior art FIG. 13, which is fed by a power supply 112 to power the instrument 116). This construction is proven (see, for example, US Pat. No. 5,399,633). However, more recently, it is important to note that the number of modulation modes in which the electrosurgical instruments are clocked has increased. Examples of such modes are included in FIGS. 9a to 9e. The electrosurgical generator may thus output a voltage of, for example, 600 volts (RMS value) continuously in a cutting mode for cutting tissue (FIG. 9a), and in a coagulation mode, a modulated high voltage with a peak voltage of up to 4500 V, but spaced apart with a small duty cycle (FIG. 9e). Here, various other modes with other types of voltage / time profiles can be set (see Figs. 9b-9d). Modes with small duty cycles and large, fast voltage jumps in particular place high demands on the electrosurgical generator. Electrosurgical instruments with parallel resonant circuits are able to generate the required high frequencies, but they have some disadvantages. Firstly, they are inefficient due to large losses. Secondly, parallel resonant circuits generate large reactive currents, which lead to larger components and a poorer efficiency at low power. Furthermore, the output frequency is load-dependent and has a high crest factor, which makes them unsuitable for high modulation modes. Secondly, the output voltage regulation is relatively slow, which leads to poor matching to changes in the load impedance.

[0006] In other fields, for example in the case of audio amplifiers, it is known to provide inverters with structures according to digital amplifier technology, so-called class D amplifiers, as power stages. However, such structures are not used for high-frequency applications such as electrosurgical generators, since the output frequency is in the low-frequency range. This is because the power losses occurring when switching power semiconductors increase linearly with frequency and quadratically with voltage, so that at, for example, six times the frequency and six times the voltage, the power loss coefficient is (6 3 =) 216, which is unacceptable. This cannot be justified either in terms of losses in the power semiconductors or in terms of efficiency.

[0007] The invention is based on the object of improving an electrosurgical generator of the type mentioned at the outset with regard to its operating behavior, in particular in the case of the modulation mode. [Means for solving the problem]

[0008] The solution according to the invention is characterized in the independent claims. Advantageous developments are the subject of the dependent claims.

[0009] SUMMARY OF THE DISCLOSURE An electrosurgical generator configured to output a high frequency AC voltage to an electrosurgical instrument, said electrosurgical generator comprising a DC voltage source and a high voltage inverter powered from the DC voltage source to generate a high frequency AC voltage having a variable voltage and frequency applied to an output for connection to the electrosurgical instrument, wherein according to the invention the inverter is configured as a multi-level inverter and comprises a plurality of inverter cells in cascade connected together and driven by a control device.

[0010] The core concept of the present invention is to split the high voltage and high frequency to be output by the electrosurgical generator among several inverter cells. This reduces the switching losses occurring in the power semiconductors of the individual inverter cells. This means both the power losses due to the voltage drop and the frequency losses due to the lowering of the switching frequency. In particular, since the power losses increase quadratically with respect to the voltage thanks to the cascade, it is possible to achieve a reduction in the imbalance of the inverter cells with the multilevel inverter according to the present invention. If ten inverter cells are provided, then for example, a power loss of 1 / 100th of the power loss for each inverter cell (1 / 10) can be achieved. 2 This results in only one-tenth of the switching procedures of the . However, there are advantages not only in terms of voltage and frequency strength, but also in terms of dynamic range, since the inverter cell allows the output AC voltage to adapt quickly to changes, especially jumps in the load impedance. The waveform can then be practically freely selected. For this reason, the output AC voltage can also be modulated over a large range, including abrupt and high voltage peaks, without overloading the inverter cell and its power semiconductors. This allows a stable output voltage with a negligible crest factor, even at small duty cycles, especially in high-pulse mode.

[0011] First, some terminology that will be used will be explained.

[0012] In the field of electrosurgical generators, "high frequency" frequencies are typically understood to be in the range of 200 kHz to 4000 kHz. Optionally, in advantageous embodiments, the ultrasonic range may also be covered. The ultrasonic range is understood to mean the frequency range between 20 kHz and 200 kHz.

[0013] "High voltage" is typically understood to mean voltages of up to 10 kV, preferably up to 5000V.

[0014] The power provided by an electrosurgical generator typically ranges between 1-500 Watts. The load impedance can vary widely, resulting in large and rapid changes in output voltage and power.

[0015] A multilevel inverter is an inverter that generates an AC voltage from a DC voltage and is capable of generating two or more voltage levels other than zero at its output.

[0016] The inverter cells advantageously have a potential difference decoupling at their respective outputs. The invention now makes use of the fact that an AC voltage is by definition present at the outputs of the inverter cells. This means that with little expenditure (compared to isolated individual DC voltage sources as would be required at the inputs) it is possible to achieve reliable potential separation of the voltages finally output by the inverter cells using simple and inexpensive transformers. From the point of view of decoupling, it is desirable to provide a transformer at the output of the inverter cells. The transformer of each inverter cell ensures that the output voltage finally output by each inverter cell is potential free. Advantageously, a transformer is connected at each primary side at the output of each inverter cell. The secondaries of the transformers are connected to sum the secondary voltages of the respective transformers, and the summed voltage is passed via an output line to an output for connection of an electrosurgical instrument.

[0017] This results in an overall improved switching behavior and a significant reduction in expenditure. The arrangement of the transformer at the output of the inverter cell means that it is no longer possible to output a DC voltage, but in the field of electrosurgical generators this is not a disadvantage - as the present invention also specifies - but rather an advantage.

[0018] Patient safety is thus additionally improved, since the entire inverter arrangement loses its essential ability to output direct current, which is dangerous for the patient. In this respect, the potential decoupling at the output of the inverter cells, in particular the transformer, serves as another protective shield for the patient.

[0019] The transformers preferably each comprise a transformer unit as a preamplifier for boosting the voltage. In this way, the voltages output by the inverter cells can be amplified and at the same time the current flowing in the output lines can be reduced. It is particularly expedient that the transformers are constructed so as to be structurally integrated with the respective transformer unit. This allows the two functions of galvanic isolation on the one hand and voltage amplification on the other hand to be combined in a particularly inexpensive and space-saving manner.

[0020] The inverter cells are preferably fed from respective DC voltage sources. The DC voltage sources may in this case be insulated from one another or separated from one another in terms of potential. However, this is not necessary, but may instead be provided as an optional connection via a reference potential. In this way, isolation of complex potential differences of the DC voltage sources at the inputs of the inverter cells is not necessary.

[0021] However, multiple, at least two, groups of inverter cells can also be provided. The inverters of each group are fed in common by a single DC voltage source. This grouping allows efficient use of the DC voltage source and reduces expenditure. However, as will be explained below, other advantages can also be achieved by a common power source. By "group" it is meant that it consists of at least one inverter cell.

[0022] The DC voltage sources are advantageously fed jointly from a DC voltage source. The DC voltage sources are in particular constituted by a DC link circuit, which is fed, for example, directly from a power supply or from external means. In this way, the complex task of providing separate, possibly separated DC voltage sources for the inverter cells is no longer necessary. This not only simplifies the supply of the DC voltage required for the operation of each inverter cell, but also greatly simplifies the construction. The construction can also be greatly simplified, since during the operation of the multilevel inverter, a situation may occur in which the power flow direction is reversed in at least one inverter cell, i.e. a situation in which power flows back into the DC voltage source. For this reason, what are called bidirectional DC voltage sources are required, which are more complex than normal DC voltage sources. If a large number of DC voltage sources are required, for example one for each inverter, the expenditures are considerably increased. However, the present invention eliminates the need to separate the DC voltage sources, but rather allows them to be switched together, so that the power flowing back to the DC voltage source through one of the inverter cells is compensated by another inverter cell, which has a regular forward flow of power. This becomes even more pronounced when inverter cells are combined to form groups of inverter cells. This ultimately results in little or no backflow of power. However, even if a backflow of power does occur, only one DC voltage source needs to be configured bidirectionally, rather than multiple DC voltage sources configured bidirectionally as in the past.

[0023] Advantageously, a plurality of, at least two groups of inverter cells are provided. These groups are then fed with DC voltages of different values. Preferably, one of the groups is fed with a DC voltage that is at least twice as high as another of the groups. Here, "group" should be understood to mean consisting of at least one inverter cell. By feeding different DC voltages to the groups of inverter cells, it is possible to increase the maximum number of voltage levels that can be output, compared to the case where the same number of inverter cells are all fed with the same voltage. This allows the gradation of the output AC voltage to be finer. Furthermore, the number of switchings can be further reduced by devising the driving of the inverter cells, thereby reducing power losses. Advantageously, there are three or more groups of inverter cells. In that case, the values ​​of the DC voltages fed to them are different in each case. Advantageously, it can be provided in particular such that the different DC voltages fed follow a geometric order. The division of the supplied DC voltage preferably has a ratio of 1:3:9, so as to make it possible to achieve the highest possible number of different levels by three groups.

[0024] In this case, advantageously, at least one, in particular ratiometric, DC-DC converter is provided for supplying in each case at least one of the groups with a different voltage. The ratio of the DC voltages thus supplied remains constant even if the absolute value of the DC voltage varies. It is particularly advantageous if the DC-DC converter is bidirectional, for example, so that it is possible to convert 12 volts DC into 48 volts DC or vice versa. This allows for flexible use in particular in environments with DC power supplies, for example in traditional vehicles with a 12 volt on-board power supply or in modern vehicles with a powerful 48 volt on-board power supply. The bidirectional DC-DC converter can perform a step-up conversion from 12 volts to 48 volts or a step-down conversion from 48 volts to 12 volts.

[0025] Thus, the present invention has succeeded in easily and conveniently resolving all at once the difficulties that arise in multilevel inverters with respect to DC voltage sources, specifically the potential difference and its bidirectionality (i.e., including power consumption capability), by a measure that at first glance appears surprisingly simple.

[0026] The DC voltage sources of the inverter cells are advantageously galvanically coupled. The galvanic coupling makes it possible to connect the DC voltage sources to the inverter cells simply and generally uncomplicated. This makes it possible, in particular, to realise the concept of providing a single power source for feeding the inverter cells. The DC link circuit of the electrosurgical generator advantageously operates as a DC voltage source in each case. This allows a conceptually simple and robust construction to be realised.

[0027] The DC voltage source is particularly advantageously configured as a fixed voltage source and is particularly advantageously configured with a DC link circuit having a fixed voltage level. An in particular ratiometric DC-DC converter can be optionally connected thereto. Such a fixed voltage source allows for a significant simplification compared to the type of construction that requires a complex DC voltage with a variable voltage and a DC link circuit with a correspondingly variable voltage. For the present invention, a power supply (feed) of a fixed value DC voltage is sufficient, and the multilevel inverter according to the present invention takes over all the remaining voltage regulation in the output voltage range, which can range from several hundred to several thousand volts.

[0028] The DC voltage source may be internal or external. It may be configured as a power supply for connection to a power grid, in particular a three-phase or AC grid, or it may be configured to directly supply a DC voltage. The last case is particularly advantageous for mobile applications in automobiles (24 VDC, 48 VDC in modern automobiles) or other environments powered by DC (e.g. 48 VDC).

[0029] With regard to the configuration of the multiple inverter cells, the present invention is not limited to one type of structure. Thus, the multiple inverter cells may be configured, for example, with a type of structure with a neutral point clamp or with a type of structure with a floating capacitor. Cells with a neutral point clamp have a relatively simple configuration, especially in that the configuration with clamp diodes is convenient. Therefore, only two diodes are required in a three-level inverter. Further multi-stage configuration is possible, as a result of which finer gradations can be realized and the voltage load of each diode is reduced. However, the number of levels is limited for practical reasons, since the number of diodes required increases quadratically with the number of levels. In this respect, inverter cells of a type of structure using a floating capacitor are more suitable. This has the same advantages as those using diodes, but the number of capacitors required does not increase significantly with increasing the number of levels. However, a configuration in which the inverter cells are connected in series is preferable. In particular, each is configured as an H-bridge. This allows the number of inverter cells to be set arbitrarily, and as a result, the voltage load on each inverter cell decreases in inverse proportion to the number of inverter cells. This reduces not only the voltage loss through the power semiconductors but also the switching loss. In addition, in the case of a cascade connection, the number of switching times per inverter cell is reduced, which also contributes to reducing switching losses.

[0030] In order to drive the multilevel inverter with a plurality of inverter cells, it is advantageous to provide a control signal generator which generates a reference signal for driving the multilevel inverter. This allows precise control of the type of AC voltage to be output, with great advantages in particular in that different modes can be set. The reference signal is advantageously a pattern for the AC voltage to be output by the electrosurgical generator, in particular a pattern with regard to amplitude, frequency, waveform and / or duty cycle. Here, the waveform can preferably be freely set as desired. This makes it possible in particular to impress the frequency and possibly also the waveform of the AC voltage generated by the plurality of inverter cells. Advantageously, the control signal generator drives an inverter control device arranged to drive the plurality of inverter cells, such that the plurality of inverter cells generate an output voltage according to the reference signal. The amplitude, waveform and / or duty cycle of the generated AC voltage are more particularly according to the reference signal.

[0031] Moreover, it is advantageous for the inverter cells to be driven at a variable frequency. This allows a faster and more direct reaction to different demands by changing the reference signal. The frequency of the AC voltage generated by the inverter cells can therefore be rapidly adjusted according to the demands of the tissue to be treated by the electrosurgical instrument. It is also possible in this way to rapidly and harmonically change between different types of modulation.

[0032] The inverter controller is preferably configured as a high-speed controller. It is configured to generate drive signals for the inverter cells at a frequency of at least 150 MHz, preferably 200 MHz. This makes it possible to minimize distortion of the output signal. In order to be able to supply drive signals at such high speeds, it is preferable that the inverter controller is configured as an FPGA (Field-Programmable Gate Array).

[0033] The output lines of the electrosurgical generator, in particular in the region of the output port, are advantageously provided with an output transformer. This serves as a galvanic isolation device in order to further ensure that the AC voltage output for the connection of the electrosurgical instrument is potential-free to protect the user and the patient. The output transformer is in particular configured as an output transformer unit and can therefore operate as a mains voltage amplifier. A series capacitor is preferably additionally provided at the output port on the secondary side of the output transformer. This acts as a direct current blocking capacitor and therefore prevents harmful direct currents from flowing into the electrosurgical instrument and from there to the patient.

[0034] Advantageously, at the end of the output line, a low-pass filter is provided, which is preferably configured as at least a second-order filter, in particular as an LC filter. This low-pass filter makes it possible to eliminate high-frequency interferences due to the high switching frequency of the inverter cells. This filter is advantageously configured so that its resonant peak is in the region between the maximum frequency of the output AC voltage and the effective switching frequency of the inverter cells. This second-order filter makes it possible to achieve sufficient smoothing of the signal at the output of the electrosurgical generator. The low-pass filter may preferably be configured in two parts (two stages), whereby advantageously one stage is provided upstream and another stage is provided downstream of the output transformer. In this way, the advantages of smoothing close to the power supply and smoothing close to the output are finally realized in combination.

[0035] In general, in filters, there is a risk that the resonant frequencies of the filter are excited due to high-frequency components of the control signal, nonlinearities of the system and (rapid) changes in the load impedance. To avoid this, a damping device is provided, which is advantageously configured as an active damping device. This allows a sufficient damping of the low-pass filter to be achieved, in particular without the undesirable power losses in the output signal associated with passive damping measures. It is pointed out that band-pass filters can also function as low-pass filters in this sense, provided that the upper frequency limit of the passband is high enough to eliminate high-frequency interference.

[0036] According to a particularly advantageous embodiment, which may merit independent protection, the active damping device is composed of a feedback system with at least one current sensor, preferably on a low-pass filter. If this is an LC filter, the current sensor is advantageously provided in series on the output port of the low-pass filter. This allows active damping by measuring the current that actually flows through the capacitor of the LC filter. This allows the filter to be tuned (adjusted or tuned) more accurately than in the case of conventional passive damping, thus improving the impulse behavior considerably. It will be appreciated that an even finer tuning of the filter is possible by adding other variables (state variables). By way of example, for this purpose a second current sensor may be provided, which is arranged to determine the current at the output. It is advisable to provide the feedback system with a transverse current detector, which allows the (possibly parasitic) current flow in the filter and / or in the transformer to be determined. In particular, it is advantageous to provide respective sensors upstream and downstream of the transformer. This allows current losses caused by parasitic transverse capacitances to be detected and possibly compensated, especially in the output transformer. "Lateral" is understood herein to mean the current flow or capacitance between two AC voltage conductors of an output line or output of an electrosurgical generator.

[0037] The active damping device is preferably configured to act on the multilevel inverter with an output signal, in particular to be coupled to the drive of the inverter cells. In this case, the drive of the inverter cells can be superimposed with a suitable correction signal, thereby influencing the power output by the inverter cells. In this way, the output signal from the damping device can directly act on the power supply to suppress to a certain extent the occurrence of unwanted oscillations and unwanted impulse movements. From a technical point of view, this is advantageously achieved in that the output signal from the damping device is applied to the drive of the inverter cells, a reference signal for the inverter drive is modified, from which appropriately modified drive signals for the current valves of the inverter cells are determined. In this way, the drive of the inverter cells is dynamically changed by the active damping device. Thereby, the output voltage of the multilevel inverter is controlled in a manner that depends on the output signal from the damping device.

[0038] In a further advantageous embodiment of the invention, preferably, a separate output is provided, the multilevel inverter being further configured to generate a separate AC voltage which is applied to the separate output. The separate AC voltage advantageously has a lower frequency than the high-frequency AC voltage at the output for connection of an electrosurgical instrument. This lower frequency is advantageously in the ultrasonic range. The spectrum of use of the electrosurgical generator is thus considerably broadened, since ultrasonic surgical instruments can also be connected and operated. This opens up the possibility for the surgeon to change to an ultrasonic surgical instrument when necessary with very little effort, without having to provide and use another generator for this purpose. It is also possible to use instruments which are operated ultrasonically and at high frequency simultaneously.

[0039] It is advantageous to provide at least one switching device adapted to selectively connect the multilevel inverter to one of the outputs, which allows the surgeon to quickly and rapidly change the output, including during surgery, and thus allows for a quick and easy optimal adaptation of the surgical instrument to the patient's specific requirements depending on the conditions found on the spot.

[0040] Conveniently, the inverter cells are divided in terms of the circuit on the multilevel inverter, where at least a portion of the inverter cells is provided for connection to at least one further output, and another portion of the inverter cells further feeds the (first) output. This allows the further outputs to be operated simultaneously, so that it is possible to operate both an electrosurgical instrument on the (first) output and an ultrasonic surgical instrument on the further output. Furthermore, the separate inverter cells for the further output have the advantage that apart from the different frequencies of the output AC voltage, it is possible to adapt the AC voltage output on the further output to other voltage or current requirements of the ultrasonic surgical instrument. It is therefore also possible to safely and reliably drive ultrasonic surgical instruments with different characteristics, for example with significantly different internal resistances.

[0041] The invention is explained in more detail below, by way of example, with reference to advantageous embodiments. [Brief description of the drawings]

[0042] [Figure 1] 1 is a schematic diagram of an electrosurgical generator according to one exemplary embodiment having an electrosurgical instrument connected thereto; [Figure 2a] FIG. 2 is a block diagram of an exemplary embodiment for a multi-level inverter of the electrosurgical generator according to FIG. 1 having multiple inverter cells cascaded. [Figure 2b] FIG. 2 is a block diagram of an exemplary embodiment for a multi-level inverter of the electrosurgical generator according to FIG. 1 having multiple inverter cells cascaded. [Diagram 3] FIG. 2 is a schematic circuit diagram of two of a plurality of inverter cells. [Figure 4] FIG. 4 shows voltage and signal profiles of the switching elements of two inverter cells according to FIG. [Diagram 5]FIG. 1 is an example circuit diagram of a multilevel inverter having multiple cascaded inverter cells. [Figure 6a] FIG. 13 is a circuit schematic diagram of an alternative embodiment of an inverter cell. [Figure 6b] FIG. 13 is a circuit schematic diagram of an alternative embodiment of an inverter cell. [Figure 7] FIG. 13 shows the voltage profile at the output without feedback in the case of a high resistive load or a short circuit. [Figure 8] 4 shows the voltage profile at the output with feedback in the case of a high resistive load or a short circuit. [Figure 9] FIG. 1 is an illustration showing various voltage / time profiles in high frequency surgery. [Figure 10] 1 is a schematic diagram illustrating an electrosurgical generator according to another exemplary embodiment. [Figure 11] 1 is a schematic diagram illustrating an electrosurgical generator according to another exemplary embodiment. [Figure 12] FIG. 12 is a schematic diagram showing a variant of another exemplary embodiment according to FIG. [Figure 13] FIG. 1 is a circuit diagram of an inverter according to the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] An electrosurgical generator according to one exemplary embodiment of the present invention is shown in Figure 1. The electrosurgical generator, generally referred to by the reference numeral 1, comprises a housing 11 with a port 14 for an electrosurgical instrument 16; in the exemplary embodiment shown this is an electrocautery. This is connected to the port 14 of the electrosurgical generator 1 via a connection plug 15 of a high voltage connecting cable. The power output to the electrosurgical instrument 16 can be varied via a power controller 12.

[0044] To supply power to the electrosurgical generator 1, a DC voltage source 2 is provided which can be connected to and is powered from the public grid via a mains connection cable (not shown). The DC voltage source 2 in the illustrated exemplary embodiment is a High Voltage Power Supply (HVPS). It comprises a rectifier and supplies the DC link circuit 20 with a DC voltage, the value of which is preferably fixed, for example 48 volts. However, it should not be denied that the value of the DC voltage can be variable between 0 and about 400 volts. Here, the absolute value of the DC voltage depends in particular on the set power, the type of electrosurgical instrument 16 and / or its load impedance, which in turn can depend on the type of tissue to be treated. However, an internal power supply is not necessary, i.e. the DC voltage source may also be implemented by an external power supply or provided for a direct DC power supply, for example 24 volts in a vehicle or 48 volts in stationary applications.

[0045] The inverter is fed from the DC link circuit 20 and generates from the fed DC voltage a high-frequency AC voltage in the high-voltage range of several kilovolts at a predetermined frequency in the range of 200 kHz to 4 MHz. This inverter is configured in the structural form of a multilevel inverter 4, as will be explained in more detail below. The frequency and waveform of the high-frequency AC voltage to be generated by the multilevel inverter 4 are in this case predefined by the inverter control device 41 on the basis of a reference signal generated by the control signal generator 40. The high-frequency high voltage generated by the multilevel inverter 4 passes through a low-pass filter 8 and an output transformer 7 acting as an output transformer unit for boosting the voltage, is secured against undesired DC current components by a blocking capacitor 17 arranged in series, and is 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 multi-level inverter 4 are further measured by a composite voltage and current sensor 18, and the measurement signals are transmitted to a processing unit 19, which applies corresponding data regarding the output voltage, current and power as feedback to a control signal generator 40, which feeds the motion controller 10 of the electrosurgical generator 1. The power controller 12 is also connected to the motion controller 10. The motion controller 10 is further configured to set various, typically stored voltage / time profiles, known as modes. A selection switch 12' is provided for a user to select the mode. The motion controller 10 is further configured to interact with the control signal generator 40 and generate reference signals for the AC voltage to be output, in particular with respect to amplitude, frequency, waveform and duty cycle.

[0046] The multilevel inverter 4 is composed of a number of series-connected inverter cells 5 driven by an inverter controller 41. Reference is now made to FIG. 2a. In the exemplary embodiment shown therein, a DC voltage source having a defined DC voltage is connected to the input of each of the inverter cells 5 (illustrated on the left side of the drawing). Each inverter cell 5 generates therefrom an AC voltage, which is output in the form of an AC voltage at the output of each inverter cell 5 (illustrated on the right side of the drawing). The number of inverter cells is not particularly limited and is as desired per se. The inverter cells 5 are numbered consecutively in FIG. 2a with the designations "5-1", "5-2" to "5-5". Here, the number 5 is an example, and any number of inverter cells of at least two may be provided. The DC voltages applied at the input of each inverter cell 5 are optionally potential-coupled via a bus bar 50. The AC voltages output at the output of each inverter cell 5 are accordingly designated "V_1", "V_2" to "V_5". By connecting a plurality of inverter cells 5 in series (connecting them in a chain), their output voltages are added together, and finally, the total output voltage is expressed by the following equation.

[0047]

number

[0048] The number of voltage levels that can be achieved by the "N" inverter cells 5 is, in this case, at least 2N+1 This is assuming that the DC voltages "Vin_1", "Vin_2" to "Vin_N" applied to the inputs of the inverter cells 5 are all the same value. For this reason, if there are five inverter cells 5, for example, a total of 11 voltage levels are possible for the overall output voltage Vout.

[0049] If the same number of inverter cells 5 are fed with DC voltages of different values ​​at least per group, the number of voltage levels can be increased considerably. Such a configuration is shown in Fig. 2a and Fig. 2b. There, two groups of inverter cells are formed. A first group I including three inverter cells 5-1, 5-2 to 5-3 is fed from a DC voltage source with a low DC voltage, for example 12V. And a second group II including two inverter cells 5-4, 5-5 is fed from another DC voltage source with a higher DC voltage, for example 48V. The first group I includes low voltage inverter cells, and the second group II includes high voltage inverter cells. The number of DC voltage sources increases from one to two. However, the number of voltage stages increases significantly from 11 to 23, more than double. The number of voltage levels that can be realized in this way is expressed by the following formula:

[0050] 2*(mHVc*r+nLVc)+1

[0051] Here, mHVc represents the number of high DC voltage inverter cells (m=2 in the above example), nLVc represents the number of low DC voltage inverter cells (n=3 in the above example), and r represents the ratio of high to low DC voltages (r=4 in the above example).

[0052] Here, the two voltage sources do not need to be electrically isolated from each other, but may share a common reference potential, as implemented by the busbar 50 in FIG. 2a. This also allows the DC-DC converter 42, in particular the DC-DC step-down converter, to generate a lower DC voltage from a higher DC voltage, which may be, for example, the DC voltage of the link circuit 20. In this embodiment, as shown in FIG. 2b, a ratiometric power supply with a transmission ratio of 4:1 is configured. One advantage of this configuration with a ratiometric power supply is that changes or fluctuations in the higher DC voltage are reflected proportionally to the next lower DC voltage, i.e., the relative gradation is maintained. This allows the output voltage to be increased in two ways, for example by activating a 48V inverter cell and deactivating three 12V inverter cells, instead of increasing the output voltage by 12V by activating an additional 12V inverter cell as in the past.

[0053] The structure of the individual inverter cells 5 and their interaction are shown 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, thus also in order to illustrate their interconnection. A common DC voltage source 31 with a supply voltage Vin of 12 volts is shown at the left edge of the image. It is assigned a stabilizing capacitor 33. These supply the two inverter cells 5-1, 5-2. In the following, first the switching of the inverter cell 5-1 is mentioned. Four power switches operating as current valves are provided and 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 to form a first branch, and the power semiconductors 52, 54 are likewise connected in series to form a second branch. The center taps of the two branches are led out and connected across the primary winding 61 of the first transformer 6-1. The transformer 6-1 further comprises a secondary winding 62, which serves to isolate the potential difference. Here, it may optionally further comprise a transmission ratio for preamplifying the voltage; this is 1:1 in the example shown. It is pointed out that a different transmission ratio may be provided, for example a transmission ratio of 1:1, in particular if no preamplification is intended. An output line 13 is connected to the secondary winding 62 and is connected to the output 14 of the electrosurgical generator 1 (possibly via a low-pass filter not shown in FIG. 3).

[0054] The two power switches 51, 53 of the first branch are driven by a common signal C1.a, which is now transmitted in inverted form to the power switch 53. The two power switches 52, 54 of the second branch are accordingly driven by a common signal C1.b, which is now transmitted in inverted form to the power switch 52. This means that in the case of a high signal on C1.a, the power switch 51 is turned on and the power switch 53 is turned off, i.e. the first power branch applies a positive potential to the upper connection of the primary winding 61 of the transformer 6-1. Thus, in the case of a high signal on C2.b, in the second power branch, the power switch 54 is turned on and the power switch 52 is turned off. In this way, the second power branch applies a negative potential to the lower connection of the primary winding 61. In the case of a low signal on C1.a or C1.b, this is accordingly applied inversely, i.e. the polarity at the primary winding 61 is reversed. In this manner, an AC voltage is generated by the inverter cell 5-1 and applied to the primary winding 61 of the transformer 6-1.

[0055] The second inverter cell 5-2 has an identical structure and is powered by the DC voltage source 31 in the same way as the first inverter cell 5-1. Therefore, in the figures, the same reference numerals are used for the same elements. It is driven in a manner corresponding to that described above by the control signals C2.a and C2.b. It therefore outputs at its output an AC voltage which is applied to the primary winding 61 of the second transformer 6-2 in the same way. The two inverter cells 5-1, 5-2 are powered by the same DC voltage source 31, so that they are connected potential-wise. That is to say, the AC voltages directly output by the inverter cells 5-1, 5-2 cannot be easily added together, since they are connected potential-wise to each other. However, since this output AC voltage is supplied to each of the transformers 6-1, 6-2, the AC voltages output by the transformers 6-1, 6-2 are each potential-free and can be easily added together to give a common output voltage which can be applied to the output line 13.

[0056] The switching behavior of the power switches 51-54 under the influence of the control signals C1.a, C1.b, C2.a, C2.b as generated by the inverter controller 41, for example, by PWM control known per se, is shown in Fig. 4. Fig. 4a shows the acquisition of the control signals C1.a, C1.b, C2.a, C2.b. The inverter controller 41 transmits a sawtooth carrier signal having a frequency of 1 MHz for each of the control signals, which are equally phase offset by 90 degrees from each other. These four carrier signals are illustrated in Fig. 4a by four offset sawtooth profiles. Also illustrated is the modulation signal required for the PWM modulation, which in this case is formed by a reference signal of a sinusoidal oscillatory waveform having a frequency of 200 kHz. The signal sequence resulting from the modulation on the four control signals C1.a, C1.b, C2.a, C2.b as output by the inverter controller 41 for the inverter cells 5-1, 5-2 is illustrated in Fig. 4b. These are pure square wave signal trains each knowing only one bit of switching state. By using these control signal trains to drive the power switches 51-54 of the two inverter cells 5-1 and 5-2 in the above-mentioned manner, and adding the voltages output by the two inverter cells 5-1 and 5-2 in the transformers 6-1 and 6-2, the final voltage profile at the output 14 is as shown in FIG. 4c. In this way, an approximately sinusoidal output voltage having five voltage levels is generated from the four one-bit control signals.

[0057] An exemplary circuit diagram of a multilevel inverter 4 and its connections to adjacent components are shown in FIG. 5. It is possible to see the multilevel inverter 4 with its multiplexed inverter cells, exemplified by inverter cell 5-1 to inverter cell 5-n. They apply the AC voltages generated in each case to the primary windings 61 of the transformers 6-1 to 6-n assigned to it. In this exemplary embodiment, the transformers are configured such that their secondary windings 62' have a higher number of turns than the primary windings 61. They are therefore configured as a composite transformer and a transformer unit, thus ensuring additional voltage amplification as well as potential decoupling. The secondary windings 62' are connected in series such that their amplified voltages are summed to give an increased overall voltage. The overall voltage is output on an output line 13, at the end of which a low-pass filter 8 is provided. It is configured as a secondary filter and consists of an inductor 81 and a capacitor 82 connected in series with it. It is pointed out that the stray inductances of the transformers 6-1 to 6-n may also contribute to, and possibly at least partially replace, the inductance of the inductor 81 of the low-pass filter. The low pass filter 8 is adjusted so that interference of the generated AC voltage due to the switching frequency of the power switches of the inverter cells of the multilevel inverter 4 is filtered. The output of the low pass filter 8 is applied to the primary winding 71 of the output transformer 7, which provides galvanic isolation of the port 14 connected to the secondary winding 72. Furthermore, a blocking capacitor 17 is provided, which is intended to prevent the output of a DC current component to the surgical instrument 16.

[0058] The low-pass filter 8 is provided with active damping. This constitutes a feedback system 9 to which a current sensor 83 is connected at the input. The current sensor 83 is arranged in the same branch as the capacitor 82 of the low-pass filter 8 and thus defines and detects the current flowing through the capacitor 82. By defining the current, a suitable signal proportional to the measured current can be fed back via the feedback system 9. This implements a transfer function that is selected depending on the desired behavior of the low-pass filter 8, which transfer function 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 generated by the control signal generator 40 and is switched to the negative input of the differential member 91 in order to modify a reference signal that is connected to the positive input of the differential member 91. The reference signal thus modified is output at the output of the differential member 91 and applied to the input of the inverter control device 41 as a drive signal for the multilevel inverter 4. This allows the output voltage of the multilevel inverter 4 to be controlled in a manner that depends on the feedback system 9. Unwanted resonances can thus already be prevented to a certain extent. Alternatively or additionally, a current sensor 84 can be provided, arranged at the primary port of the output transformer 7 or in series with the blocking capacitor 17, thus detecting the current flow through the output transformer 7. By defining the current, a suitable signal proportional to the measured current can likewise be fed back via the feedback system 9. The feedback system implements a (suitably extended) transfer function selected according to the desired behavior (now actively damped) of the LC filter formed by the inductor 81 and the blocking capacitor 17.

[0059] The effect of the feedback system 9 on the voltage and current profile at the output 14 is shown in Figures 7a, 7b and 8a, 8b, in each case the multilevel inverter 4 generates a pulsed AC voltage signal consisting of individual sinusoidal oscillations (as shown in Figure 9e). In the case shown in Figure 7a, the load at the output is assumed to be highly resistive (in the region of 100 kΩ). This results in a further resonant oscillation being superimposed on the sinusoidally oscillating output voltage (dashed line) generated by the inverter cells 5 of the multilevel inverter 4. This is because the resonant frequency of the LC filter formed by the inductors 81 and capacitors 82 is given by the well-known formula:

number

[0060] The same case is shown in Fig. 8a and Fig. 8b. Here, the filter 8 is damped by a feedback system 9. Fig. 8a shows again the case of a high resistive load. Also, the original output signal from the inverter cell 5 of the multilevel inverter 4 is shown in dashed lines as a reference. The actual output signal with the interference superimposed is fed back via the feedback system 9 using the measurement signal from the current sensor 83, acting on the differential element 91 to modify the signal transmitted to the multilevel inverter 4. This reference signal is, as it were, bent in a targeted manner, resulting in a modified reference signal, which is applied as a control signal to the inverter control device 41 in order to actually drive the inverter cell 5. As a result, the output signal (see dashed line showing the smoothed profile) is "bent" in a targeted manner, so that the superimposed oscillations at the output are cancelled in a targeted manner. The actual output signal, which is finally obtained from the generated voltage of the multilevel inverter 4, "bent" in a targeted manner, and the resonant oscillations of the filter 8, is shown in solid lines. Through comparison with Fig. 7a, it is easy to see that the actual output signal is substantially a more harmonic sinusoidal oscillation.

[0061] The case of a short circuit using the feedback system 9 is similar. This case is shown in FIG. 8b. The original drive signal generated by the control signal generator 40 as a reference signal is again shown in dashed lines. The modified reference signal finally generated under the effect of the feedback system 9 with the measurement signal from the current sensor 84 is used to drive the inverter cells 5. The resulting output signal is shown (after smoothing) in dashed lines. It is surprisingly small relative to the voltage amplitude, because the undesired resonant frequency is very close to the frequency of the AC voltage generated by the multilevel inverter 4. Therefore, only a very small actual drive signal for the inverter controller 41 is needed. The actual current profile resulting at the output 14 is again shown in solid lines. Through comparison with FIG. 7a, it can be easily seen that the actual output signal is substantially a more harmonic sinusoidal oscillation. In comparison with FIG. 7b, it can be clearly seen that the actual sinusoidal oscillation is reproduced more accurately (with intervals of up to 2 μs) and that the parasitic reverberation is effectively suppressed (no "ringing" effect). Feedback using the measurement signal from the current sensor 84 ensures a fairly good, low-harmonic sinusoidal output signal despite the significant LC filter 8 with blocking capacitor 17 at the output 14 .

[0062] As a result, the multilevel inverter 4 according to the invention can be used to precisely predefine the AC voltage profile to be output. In particular the multilevel inverter 4 driven by a reference signal gives full control of the waveform, including in particular the case of modulated output signals. Thus, modulated output signals can be generated in a precise and reproducible manner, as shown in Figures 9a to 9e. To ensure a constant energy output, the multilevel inverter 4 according to the invention is furthermore able to increase the value of the output voltage in a high modulation mode with a shorter duty cycle, to such an extent that, despite the short switch-on time, the same energy is output to the electrosurgical instrument 16 as in a mode with a longer switch-on time or in continuous mode.

[0063] Therefore, the present invention can control the output high frequency AC voltage more dynamically and more accurately, particularly in pulse mode, and thanks to optional feedback, the mode can again be kept much more accurately.

[0064] It is further pointed out that the invention is not limited to inverter cells 5 having an H-bridge configuration. Other topologies can also be adopted for the inverter cells 5. Figures 6a and 6b show examples of these and illustrate alternative topologies, specifically also having four switching elements 51'-54' and 51"-54", respectively. Thus, Figure 6a shows an inverter cell configuration having a type of structure with neutral clamping via diodes 55, 56, and Figure 6b shows an inverter cell configuration having a type of structure with a floating capacitor 57. As with the H-bridge inverter cells, these may also be cascaded to achieve a higher number of voltage levels.

[0065] FIG. 10 shows an alternative exemplary embodiment to the exemplary embodiment according to FIG. 1. Identical or same type elements are indicated using the same reference numerals. In the alternative exemplary embodiment, it is essentially different in that the low-pass filter 8 has a two-stage configuration. A first stage 8′ of the low-pass filter is provided directly at the output of the multilevel inverter 4 in order to further smooth the generated AC voltage. A second stage 8″ of the low-pass filter is provided at the output side of the output transformer 7. In this way, another smoothing is performed just before the output, 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 and possibly at least partially replace the inductance of the inductor 81 of the second stage 8″ of the low-pass filter.

[0066] In the embodiment according to Fig. 10, a dual blocking capacitor 17, 17' is provided for added safety, it being understood that such a dual arrangement may also be provided in other exemplary embodiments.

[0067] An advantageous alternative arrangement of a current sensor for the feedback system is also explained using the example of this exemplary embodiment according to FIG. 10. This can also be provided in other exemplary embodiments. In this case, a current sensor 18' is provided in series with the low-pass filter 8, more precisely with the output of the first stage 8'. The combined current and voltage sensor 18 serves as a second current sensor. On the basis of these signals, it is possible to measure the actual output current, which is transmitted to the motion control device 10 via a processing unit 19, as well as the current flow at the input side of the output transformer 7. A transverse current detector is also formed, which is intended to determine, from the current difference occurring here, the magnitude of the current flowing through the capacitor 82 of the low-pass filter, here the second stage 8" of the low-pass filter, which can be obtained in the feedback system 9 and compensated for by modifying the drive of the multilevel inverter 4. This also makes it possible to detect and compensate for current losses caused by parasitic transverse capacitances, which cannot otherwise be measured directly, in particular of the output transformer 7 or of the low-pass filter 8 with its stages 8', 8". Another exemplary embodiment of an electrosurgical generator according to the invention is shown in FIG. 11. This is based on the exemplary embodiment shown in FIG. 1, but with the second output 14 * and a switching device 3. A multilevel inverter 4 is connected to the input of said switching device, and an output line 13 is connected to one of its outputs, which is connected via a low pass filter 8 and an output transformer 7 to a (first) output 14 for an electrosurgical instrument 16. * The second output line 13 * , second low-pass filter 8 * and via a second output transformer 7' to the other output of the switching device 3. The second output is connected to a connection plug 15 for a second instrument (not shown). *Here, the second instrument may in particular be an ultrasonic surgical instrument, such as an ultrasonic scalpel. As in the exemplary embodiment shown in FIG. * Each of these is provided with at least one further blocking capacitor 17 (not shown).

[0068] The switching device 3 selectively outputs the AC voltage generated by the multilevel inverter 4 at an output 14 to an instrument connected thereto, in particular an electrosurgical instrument 16, or to an output 14. * 12, the inverter cells 5 are arranged to selectively output to an instrument connected thereto, in particular an ultrasonic surgical instrument. Using the same electrosurgical generator 1, it is thus possible to use an electrosurgical instrument, such as an electrocautery, or an ultrasonic surgical instrument, such as an ultrasonic dissection scissors, as the surgeon wishes. The exchange between the instruments is considerably facilitated and may even be performed during the operation. In this way, the field of application of the electrosurgical generator is considerably expanded. Alternatively or additionally, in one variant as shown in FIG. 12, it is also possible that a plurality of inverter cells 5 are provided to be divided in the circuit. In this case, at least one (but not all) of the plurality of inverter cells 5 provides a second output 14. * and feeding it with, for example, an AC voltage in the ultrasonic frequency range, while the remaining inverter cells 5-1 to 5-4 continue to feed a high frequency AC voltage to output 14. This makes it possible to operate two electrosurgical instruments in parallel (including in different modes), or easily to operate an instrument that uses both ultrasonic and high frequency energy.

Claims

1. An electrosurgical generator configured to output an AC voltage to an electrosurgical instrument (16), comprising: a DC voltage source (2); a high voltage inverter supplied with power from the DC voltage source (2) and adapted to generate a high frequency AC voltage having a variable voltage and frequency for application to an instrument output (14) for connection of the electrosurgical instrument (16); The DC voltage source (2) is a variable voltage source, The inverter is configured as a multi-level inverter (4) and comprises a plurality of cascaded inverter cells (5) driven by a controller (41).

2. An electrosurgical generator as described in claim 1, wherein the plurality of inverter cells (5) have potential decoupling at each cell output of the plurality of inverter cells (5).

3. An electrosurgical generator as described in claim 2, wherein each transformer (6) is connected to its primary side at each cell output of the plurality of inverter cells (5).

4. An electrosurgical generator as described in claim 3, wherein the secondary sides of multiple transformers (6) are connected in a chain form to add the secondary voltages of each transformer (6).

5. An electrosurgical generator as described in claim 4, wherein the added voltage is transmitted to the instrument output (14) via an output line (13) connecting the plurality of inverter cells (5) to the instrument output (14) for connection of an electrosurgical instrument (16).

6. Each of the plurality of transformers (6) includes a transformer unit as a preamplifier for boosting voltage, The electrosurgical generator according to claim 3, wherein the plurality of transformers (6) are configured to be structurally integrated with each transformer unit.

7. An electrosurgical generator as described in any one of claims 1 to 6, wherein each inverter cell (5) is powered from a single voltage source (31).

8. A plurality of, at least two groups of inverter cells (5) are provided, An electrosurgical generator according to any preceding claim, wherein each group of inverters is jointly powered by a single DC voltage source.

9. A plurality of inverter cells (5) of at least two groups (I, II) are provided, Each group is supplied with a different DC voltage; 7. An electrosurgical generator according to any one of claims 1 to 6, wherein one of the plurality of groups is powered with a DC voltage that is at least twice that of another of the plurality of groups.

10. An electrosurgical generator as described in claim 9, wherein at least one ratiometric DC-DC converter (42) is provided to supply different voltages to at least one of the plurality of groups.

11. An electrosurgical generator as described in Claim 10, wherein the DC-DC converter (42) is configured to be bidirectional.

12. An electrosurgical generator as described in any one of claims 1 to 6, wherein the DC voltage source for powering the plurality of inverter cells (5) is galvanically coupled.

13. An electrosurgical generator as claimed in any one of claims 1 to 6, wherein an optional DC-DC converter (42) is connected to the DC voltage source (2).

14. An electrosurgical generator as described in any one of claims 1 to 6, wherein each of the multiple inverter cells (5) is configured with a type of structure having a neutral point clamp (55, 56) in its DC voltage source section, or a type of structure having a floating capacitor (57).

15. An electrosurgical generator as described in any one of claims 1 to 6, wherein the multiple inverter cells (5) are connected in series and each inverter cell (5) is configured in an H-bridge configuration.

16. An electrosurgical generator as described in any one of claims 1 to 6, further comprising a control signal generator (40) for the multilevel inverter configured to generate a reference signal for driving the multilevel inverter (4).

17. An electrosurgical generator as described in claim 16, wherein the AC voltage output by the electrosurgical generator (1) is defined by the reference signal, and the waveform of the AC voltage output can be freely set as desired by the reference signal.

18. An electrosurgical generator as described in claim 16, wherein the control signal generator (40) drives a control device (41) configured to drive the plurality of inverter cells (5) so that the plurality of inverter cells (5) generate output voltages corresponding to the reference signal.

19. An electrosurgical generator as described in any one of claims 1 to 6, wherein the plurality of inverter cells (5) are driven by a variable frequency reference signal.

20. An electrosurgical generator as described in any one of claims 1 to 6, wherein the AC voltage output from the electrosurgical generator is a modulated voltage having a predefinable crest factor.

21. An output transformer (7) is provided on an output line (13) connecting the plurality of inverter cells to the appliance output (14), The output transformer (7) interacts with a capacitor (17) on the appliance output (14) as a DC current blocker; Electrosurgical generator according to any one of claims 1 to 6, wherein the output transformer (7) is configured as an output transformer unit for mains voltage amplification.

22. A low-pass filter (8) configured as at least a second-order filter is provided in an output line (13) connecting the plurality of inverter cells to the appliance output (14), Electrosurgical generator according to any one of claims 1 to 6, further comprising a two-way arrangement of the low pass filter (8', 8").

23. An electrosurgical generator as described in claim 22, wherein an active damping device is provided for the low-pass filter (8).

24. The active damping device comprises a feedback system; The feedback system comprises: a first current sensor (83) for detecting a first current flowing through the low-pass filter (8); a second current sensor (84) that detects a second current flowing through the output transformer (7); The detected first and second currents are fed back to the multilevel inverter (4) to provide active attenuation to the low pass filter (8).

24. An electrosurgical generator according to claim 23.

25. An output signal from the active damping device acts on the multilevel inverter (4), 24. An electrosurgical generator according to claim 23, wherein the output signal from the active damping device is coupled to drive a plurality of inverter cells (5).

26. An electrosurgical generator as described in any one of claims 1 to 6, provided with at least one other instrument output (14*) to which at least one other AC voltage generated by the multilevel inverter (4) is applied.

27. ​​An electrosurgical generator as described in claim 26, wherein the at least one other AC voltage has a lower frequency than the high frequency AC voltage at the instrument output (14) for connection of the electrosurgical instrument (16) and is in the ultrasonic range.

28. An electrosurgical generator as described in claim 26, provided with at least one switching device (3) for selectively connecting the multilevel inverter (4) to the instrument output (14) and one of the at least one further instrument output (14*).

29. The plurality of inverter cells (5) are arranged such that at least a portion of the plurality of inverter cells are provided for connection to the at least one other appliance output (14*); 28. An electrosurgical generator according to claim 27, wherein another portion of the plurality of inverter cells is further divided from a circuit standpoint to power the instrument output (14).

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