Electrosurgical generator with leakage current detection

Description

[0001] The invention relates to an electrosurgical generator designed to deliver a high-frequency alternating voltage to an electrosurgical instrument. It comprises a high-voltage inverter that generates the high-frequency alternating voltage, which is routed via an output line to an output for connecting the electrosurgical instrument, and a leakage current detection device for the electrosurgical instrument connected to the output.

[0002] In electrosurgery, or high-frequency surgery, a high-frequency alternating current is applied to human body tissue using an electrosurgical instrument, such as an electrosurgical scalpel, primarily to cut or sever the tissue through the resulting heat. One advantage of this technique is that bleeding can be stopped simultaneously by closing the affected vessels, and electrosurgical instruments can be used for other applications, such as coagulation.

[0003] This requires considerable power, at frequencies from 200 kHz or higher up to 4000 kHz, typically around 400 kHz. At such frequencies, body tissue behaves like an ohmic resistor. However, the specific resistance depends heavily on the type of tissue; for example, the specific resistances of muscle, fat, and bone differ greatly from one another, up to a factor of 1000. This means that during operation, the load impedance of the electric scalpel can change dramatically and rapidly depending on the tissue being cut, ranging from almost infinity when the instrument is brought close to the tissue to a near short circuit. This places special and unique demands on the electrosurgical generator, and in particular its high-voltage supply, which do not occur in other areas of technology.

[0004] To meet these unique requirements, electrosurgical generators are typically designed with an inverter to supply rectified current to the electrosurgical instrument. The inverter is typically designed as a free-running single-ended generator with an LC resonant circuit. This design is proven. Furthermore, the applicant has developed a type of electrosurgical generator that incorporates a multilevel inverter as the inverter. This allows the frequency, amplitude, and waveform of the generated AC voltage to be largely freely adjusted. The necessary high voltages are achieved by an output transformer.

[0005] Given the high output voltage that electrosurgical generators can deliver, patient protection is of particular importance. This includes ensuring that the current flow to the patient is controlled, from one electrode of the electrosurgical generator to the other. The electrode connected to the electrosurgical instrument is typically referred to as the active electrode. Depending on the design, the other electrode can also be connected to the instrument or to the patient via a surface contact electrode to close the electrical circuit. It is important that the latter occurs completely and that no leakage currents occur. These can be dangerous for the patient.

[0006] For the safe operation of electrosurgical generators, it is therefore important to quickly and reliably detect the occurrence of leakage currents. For this purpose, current sensors are commonly used at the output terminal of the electrosurgical generator. These sensors should preferably be located in the immediate vicinity of the terminal to avoid interference from the generator's internal structure. Typically, in existing electrosurgical generators, a type of leakage current measurement is performed to measure the current. This determines the difference between the current flowing to the active electrode and the current returning via the surface electrode. Current sensors (current transformers) that operate according to the transformer principle are known for this purpose.Disadvantages of this arrangement are that the sensors are complex and have special installation space requirements, since the sensors typically have to be arranged in the immediate vicinity of the output to avoid incorrect measurements.

[0007] The invention is based on the object of providing an improved electrosurgical generator which enables simplified and reliable detection of leakage current.

[0008] The inventive solution lies in the features of the independent claim. Advantageous further developments are the subject of the dependent claims.

[0009] In an electrosurgical generator designed to deliver a high-frequency alternating voltage to an electrosurgical instrument, comprising a high-voltage inverter that generates the high-frequency alternating voltage, which is routed via an output line to an output for connecting the electrosurgical instrument, and a leakage current detection device for the electrosurgical instrument connected to the output, the invention provides that the leakage current detection device comprises a voltage measuring device, each of whose inputs is connected via a capacitive coupling to an active and neutral line of the output line and has a bipolar voltage divider with a predetermined fixed division ratio, which has an upper terminal and a lower terminal, to which the capacitive coupling is applied, as well as a center tap, and an asymmetry detector,which is designed to compare an upper voltage between the upper terminal and the center tap with a lower voltage between the lower terminal and the center tap, and to output an error signal for leakage current if the ratio of upper voltage to lower voltage deviates from the predetermined fixed division ratio.

[0010] The core of the invention is the idea of ​​measuring the voltages prevailing in the two lines of the output line. This is done using the voltage divider, whose division ratio is a predetermined fixed ratio, in the sense of a differential voltage measurement. This produces two measurement signals, one for each of the lines. These are fed to the asymmetry detector, which checks whether the two measured voltages are in the predetermined fixed ratio. If this is not the case, it means that one of the two measured voltages deviates from its intended value, which may be caused by a leakage current in the affected line.The occurrence of leakage current is an indication that there is a low-impedance path to earth (earth fault) on this line, for example if the patient to be operated on touches a metal part that is earthed (via a protective conductor), causing the potentially dangerous leakage current to occur.

[0011] The invention takes advantage of the finding that the occurrence of a leakage current shifts the result of the voltage measurement with a bipolar voltage divider. This shift is detected and signaled by the asymmetry detector. The signal serves as a warning to the user (surgeon) of the electrosurgical generator and can also serve as a start signal for a possible automated rapid shutdown of the delivered alternating voltage. This enables effective protection for the patient.

[0012] Thanks to the inventive voltage measuring device with a bipolar voltage divider, the measurement is differential in nature and thus extremely immune to interference. Unlike the prior art, it can be positioned anywhere along the output line and does not need to be located directly at the output. Complex measuring transducers, such as current transformers operating according to the transformer principle, are not required. This allows for greater design freedom in placement and simplifies the design of the electrosurgical generator as well as the integration of the inventive leakage current detection device. It is therefore more robust and reliable than previously conventional methods for detecting leakage current, further improving patient protection.

[0013] Some of the terms used are explained below: A voltage measuring device with a bipolar voltage divider is one that measures two voltages in the same way. Ideally, it is constructed symmetrically with respect to the components defining the measurement, so that similar relationships exist in terms of measurement gain and frequency response. A special case of this is a symmetrical voltage divider with identical impedances on both sides. A symmetrical voltage divider is not mandatory; in this case, it is sufficient if at least the impedances of the voltage divider are in a predetermined fixed ratio to one another (corresponding to the desired degree of division of the voltage divider). The impedances concerned here are, in particular, those arranged between the upper terminal and the center tap, as well as between the center tap and the lower terminal.The fixed ratio is the desired degree of division of the voltage divider and is defined by the ratio (e.g., 2:1) of the impedances of the bipolar voltage divider. If the impedances are equal, i.e., the voltage divider is symmetrical, then the ratio is 1:1. This special case, when the bipolar voltage divider is a symmetrical voltage divider, ensures a particularly clear circuit design and is therefore a preferred embodiment.

[0014] The asymmetry detector's comparison typically involves comparing the voltage level (amplitude and / or effective value), whereby the comparison is to be understood broadly in the sense that it not only checks for equality, but also, if necessary, for the presence of the predetermined fixed ratio. In this sense, asymmetry exists when equality or the predetermined fixed ratio is not present. The asymmetry detector is designed to check this (whether equality or the predetermined fixed ratio is present).

[0015] In the field of electrosurgical generators, "high frequency" typically refers to frequencies in the range of 200 kHz to 4000 kHz. "High voltage" typically refers to voltages up to 10 kV, preferably up to 5000 V.

[0016] The power provided by the electrosurgical generator typically ranges between 1 and 500 watts, whereby the load impedance can vary greatly and, accordingly, the output voltage and power output can change just as sharply and rapidly.

[0017] The asymmetry detector conveniently has a minimum threshold below which no error signal is output. The error signal is only output when the minimum threshold is exceeded. This allows a certain tolerance threshold to be defined so that the error signal is not triggered by even the smallest deviations, such as those that can occur with a still small, harmless leakage current. This prevents unnecessarily early shutdown.

[0018] It is advantageous for the minimum threshold to be adjustable, preferably depending on the operating mode of the electrosurgical generator and / or the connected instrument. This takes advantage of the knowledge that different operating modes of the electrosurgical generator, in particular different programmed so-called modes, pose different levels of risk to the patient. For example, in clocked modes with a high duty cycle, high pulse-like peak voltages are reached, which pose a potentially greater risk and therefore require a lower minimum threshold setting. For other modes, a correspondingly more generous setting can be made. The same applies to differently designed electrosurgical instruments, whereby the different design of the electrosurgical instrument and thus the risk to the patient can also be taken into account when determining the minimum threshold.In particular, the minimum threshold can be set instrument-dependently, preferably in a memory associated with the instrument in which a value for the minimum threshold is stored. The memory can be provided on the instrument or in the electrosurgical generator.

[0019] The output of the output line often includes a connection for an active electrode and a neutral electrode. Typically, the neutral electrode is intended to be connected to the patient over a large area, depending on the application. In this case, it is expedient if different minimum thresholds can be set for the active electrode leading to the instrument and the large-area neutral electrode leading to the patient. This is preferably possible in the voltage measuring device according to the invention by setting different minimum thresholds.

[0020] According to a further particular advantage of the invention, the asymmetry detector is further provided with a polarity detector for the leakage current. This makes it possible to identify at which of the two lines of the output line the leakage current is occurring. In particular, it can be quickly signaled whether the leakage current is occurring at the active electrode or the neutral electrode, which makes it easier for the surgeon to initiate appropriate countermeasures. For this purpose, the asymmetry detector preferably cooperates with a display device that accordingly signals whether the leakage current is occurring at the active electrode or the neutral electrode.

[0021] A practical embodiment of the asymmetry detector has a comparator with two inputs, the upper terminal being connected to one of the inputs and the lower terminal to the other input. The comparator can be used to detect asymmetry in a reliable and inexpensive manner. In particular, it is possible to implement the comparator using analog technology. This offers the advantage of fast processing and high reliability. This is particularly true when the comparator is implemented using an operational amplifier. Another practical implementation of the asymmetry detector is when it has a difference calculation unit with a downstream threshold switch, in particular a Schmitt trigger.

[0022] The asymmetry detector preferably has an analog / digital converter (ADC). This offers the advantage of digital signal processing. This provides advantages in terms of interference immunity compared to analog signals (particularly important for small, low-amplitude signals such as those for leakage currents) and thus increases both measurement accuracy and flexibility in the arrangement of downstream signal processing units, as these are now supplied with interference-free digital signals. This increased flexibility applies in particular to the structural arrangement in the electrosurgical generator, which is a significant practical advantage given the limited space at the output terminals. Previously necessary compromises between (analog) signal quality and structural arrangement are no longer necessary, as the high (digital) signal quality is maintained regardless of the spatial arrangement.

[0023] In particular, an arrangement of the analog / digital converters (ADCs) on the input side of the asymmetry detector is preferred. An arrangement directly at the upper and lower terminals, possibly after pre-amplification, is particularly preferred, in order to achieve conversion to digital signals early in the signal path. Two analog / digital converters or a dual analog / digital converter are expediently provided for this purpose. This has the advantage that the transition to the interference-free digital signals takes place immediately after the leakage current signal has been obtained, as soon as possible after the voltage divider. This expediently takes place before asymmetry detection in order to be able to evaluate the asymmetry at the digital level. This enables the advantages of digital signal processing to be utilized for asymmetry detection.

[0024] Furthermore, performing analog-to-digital conversion as early as possible in the signal chain offers the advantage that advanced analysis functions, particularly spectral analysis or frequency-domain analyses, can be implemented, either in the asymmetry detector and / or in other downstream signal processing units. Such spectral analyses can also evaluate higher-order frequency components. For this purpose, the asymmetry detector can be configured for advanced analysis functions, particularly spectral analysis or frequency-domain analyses. This not only ensures improved evaluation and thus detection accuracy, but also increases flexibility, particularly with regard to different types of electrosurgical instruments.

[0025] Flexibility with regard to different types of electrosurgical instruments (including future ones that will only be available after the respective electrosurgical generator is manufactured) can be further increased by different thresholds for each electrosurgical instrument, which can be taken into account in a particularly simple and expedient manner using digital signal processing. This can be achieved, for example, by means of threshold values ​​stored individually for each instrument in the electrosurgical generator or by means of corresponding minimum thresholds stored as a value in the instrument's memory. An analog-to-digital converter thus also enables the easy implementation of different minimum thresholds, even for future instruments. This increases the flexibility and future-proofing of the electrosurgical generators.

[0026] The capacitive coupling is advantageously designed with a high impedance compared to the voltage divider, preferably by at least a factor of 100, in particular at least 1000. The capacitive coupling expediently comprises capacitors in the picofarad range, preferably no more than 20 picofarads. The high-impedance capacitive coupling also offers the advantage that the voltage divider located behind the coupling—as seen from the electrode lines (active and neutral lines)—is "high-voltage-free," i.e., is not subjected to high voltage. It therefore requires no high-voltage-resistant components (especially capacitors). This is a significant practical advantage, since there is often a conflict of objectives between high-voltage resistance on the one hand and tightly toleranced, precise specification of the division ratio on the other.

[0027] The voltage divider is advantageously designed as a capacitive voltage divider. This prevents the influence of direct current and enables a favorable frequency response, especially in conjunction with the capacitive coupling. Furthermore, the capacitive design provides additional isolation compared to a resistive voltage divider, especially against unwanted direct current.

[0028] Advantageously, the center tap of the voltage divider is connected to a fixed potential. This creates a virtual ground for the voltage measurement, thus providing a clear reference potential. In particular, the center tap can be connected to a protective conductor and thus electrically to earth (protective earth). This has the advantage that the center tap is connected to the same potential to which any leakage currents flow. For this purpose, the ground of the measuring electronics is also conveniently connected to earth (protective earth).

[0029] Furthermore, it can be provided that the leakage current detection device interacts with a separate device for determining a current magnitude, in particular the leakage current. First and foremost, it is important, particularly for patient safety, to quickly and reliably detect that a leakage current is present. Secondly, it represents a further advantage to be able to determine a measure of the magnitude of the leakage current in order to provide valuable additional information for locating the fault leading to the leakage current. For example, such a device is designed to interact with the operating control of the generator.Based on measured values ​​for the current in the output line, which are usually already available in the control system, it can compare the measured values ​​of the total current delivered immediately before and after the leakage current is detected by the asymmetry detector, and from this determine a measure of the magnitude of the leakage current. Document US 4 102 341 A relates to an electrosurgical generator with leakage current detection.

[0030] The invention will be explained in more detail below with reference to an advantageous embodiment of the invention and the accompanying drawings. They show: Fig. 1 is a schematic representation of an electrosurgical generator according to an embodiment with a connected electrosurgical instrument; Fig. 2 is a block diagram of the electrosurgical generator according to Fig. 1; Fig. 3 an exemplary circuit diagram for a sensor with a leakage current detection device; Fig. 4a, b Variants for an asymmetry detector to Fig. 3 ; Fig. 5 a diagram of measured voltages without leakage current; and Fig. 6 a diagram of measured voltages with leakage current.

[0031] An electrosurgical generator according to an embodiment of the invention is shown in Figure 1The electrosurgical generator, designated as a whole by the reference numeral 1, comprises a housing 11 provided with a connection 14 for an electrosurgical instrument 16. In the illustrated embodiment, the instrument is an electric scalpel. It is connected to the connection 14 of the electrosurgical generator 1 via a high-voltage connecting cable 15. A further connecting cable 15' is led to an operating table 98 and connected there to a counter electrode 16'. This counter electrode is designed to be arranged over a large area on the patient 99 to be operated on. The power output to the electrosurgical instrument 16 can be varied via a power controller 12.

[0032] For the following explanation of the structure of the electrosurgical generator 1, particular reference is made to Figure 2A DC voltage supply 2 is provided to supply power to the electrosurgical generator 1. It can be connected to the public power grid via a power cable 13 and fed via a high-voltage power supply (HVPS). The power supply 22 comprises a rectifier and, in the illustrated embodiment, feeds a DC voltage intermediate circuit 24. It should be noted that supply from a power supply 22 is not mandatory; other types of DC voltage supply 2 are also possible, for example, a direct supply of direct current, particularly for electrosurgical generators installed in vehicles or for those provided in mobile or temporary hospitals.

[0033] The DC voltage is typically between 10 and approximately 500 volts, often 48 volts in modern electrosurgical generators. It can be fixed or variable, depending in particular on the design of the inverter generating the high voltage. The absolute DC voltage can depend in particular 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.

[0034] An inverter 3 is fed from the DC voltage supply 2, which generates high-frequency AC voltage in the high-voltage range of several kilovolts from the supplied DC voltage, with frequencies in the range between 200 kHz and 4 MHz. The inverter 3 can, for example, be a so-called single-ended converter controlled by an oscillator in a free-running manner; these are usually supplied by a DC voltage supply 2 with a variable voltage. This design has the advantage of conceptual simplicity and usually leads the high voltage it generates directly via an internal device output line 4 to the output connection 14 for the electrosurgical instrument 16. Alternatively, the inverter 3 can also be designed as an inverter.In this case, the power and the voltage to be delivered are adjusted via the inverter itself, so that a variable DC voltage supply 2 is not required; one with a fixed voltage (e.g., 48 V) is sufficient. The inverter has power semiconductor switches as so-called current valves, which are controlled by an inverter controller 31 in a conventional manner, for example, using the well-known pulse width modulation as PWM control, to generate a high-frequency high voltage. The high-frequency high voltage generated by the inverter is thus almost freely adjustable in terms of frequency and waveform. The high-frequency voltage generated by the inverter is typically output via a low-pass filter and an output transformer (not shown) to boost the voltage to the generator's internal output line 4, which leads to the connection 14 for the electrosurgical instrument 16.

[0035] Furthermore, the voltage and current of the high voltage generated by the inverter 3 are measured using a voltage sensor 17 and a current sensor 18, and the measurement signals are fed to a processing unit 19, which applies the corresponding data on the delivered voltage, current, and power to an operating control 10 of the electrosurgical generator 1. The power controller 12 is also connected to the operating control 10. The operating control 10 is further configured to set various so-called modes, which are typically stored voltage / time curves; however, they can also be specifications for the waveform of the high-frequency high voltage to be delivered.

[0036] The generated high-frequency high voltage is fed to the output terminal 14 via the output line 4 with its lines 41 for an active electrode and line 42 for a neutral electrode. Due to the high frequency of the voltage delivered by the electrosurgical generator, parasitic capacitances act on the lines 41, 42. They are in Figure 3 represented by the capacitive elements 48, 49.

[0037] The surgical instrument 16 is connected to the output connection 14. In the illustrated embodiment, this is a monopolar instrument connected to the line 41 with the active electrode; however, the use of bipolar instruments (not shown) can also be provided. To close the circuit, a counterelectrode 16' is connected to the line 42 via the connecting cable 15'. The counterelectrode 16' is located on the operating table 98, on which the patient 99 lies for the operation. There, the counterelectrode 16' is connected to the patient 99 over a large area at a suitable location (this is shown only symbolically in Figure 1). If the surgeon now applies the electrosurgical instrument 16 to the patient 99, the circuit is closed via the body tissue of the patient 99.The body tissue immediately adjacent to the instrument tip 16 is heated as a result of the contact resistance prevailing there and is cut, cauterized, coagulated, etc., depending on the electrosurgical instrument 16 used.

[0038] It is critical if the circuit is (also) closed via other locations, especially parts of the patient's body. Uncontrolled leakage currents occur there. This poses a significant risk to the safety of the patient, and possibly also to medical personnel. A leakage current detection device 6 is provided to detect this.

[0039] For further explanation, please refer to Figure 3Reference is made there. In the left part of the image, the inverter 3 is symbolically shown as the source of the alternating voltage. This is output via the output line 4 with its lines 41, 42 for the active and neutral electrodes, respectively, to the connecting cable 15 for the surgical instrument 16. The circuit is closed via the further connecting cable 15'. The electrosurgical instrument 16 and the counter electrode 16' are in Figure 3 symbolically represented by a load resistor.

[0040] The leakage current detection device 6 comprises a bipolar voltage measuring device 7 with a bipolar voltage divider 73 and an asymmetry detector 8. The voltage measuring device 7 is designed to perform a voltage measurement on the lines 41, 42 of the output line 14 for the active and neutral electrodes. In the exemplary embodiment, the bipolar voltage divider of the voltage measuring device 7 is designed, for example, as a symmetrical voltage divider 73 with a division ratio of 1:1, which is implemented as a capacitive voltage divider with an upper measuring capacitance 74 and a lower measuring capacitance 77, which are connected to one another at a center tap 77. The respective other terminal of the measuring capacitance 74, 77 is arranged at an upper terminal 78 or a lower terminal 79 of the voltage divider 73. The size of the measuring capacitance is in the nanofarad range, for example, approximately 10 nF.

[0041] To ensure the most non-interfering measurement of the voltages in lines 41, 42, the symmetrical voltage divider 73 is connected to lines 41, 42 via a high-impedance coupling. The high-impedance coupling is implemented using two low-capacitance capacitors: a capacitor 71 as a connection between line 41 and the upper terminal 78 of the voltage divider 73, and a second capacitor 72 as a connection between line 42 and the lower terminal 79 of the voltage divider 73. The two capacitors 71, 72 have a capacitance in the picofarad range, for example, approximately 3 pF.

[0042] The upper terminal 78 and the lower terminal 79 are routed out of the voltage divider 7 as output terminals. They are connected to inputs 81, 82 of the downstream asymmetry detector 8. This is designed to compare the two voltages coming from the voltage divider 73 at the upper terminal 78 and the lower terminal 79 with each other and to check whether they have the predetermined fixed ratio (in the example, this is 1:1, since the voltage divider is symmetrical), i.e., they have the same amplitudes (or RMS value). It should be noted that for the implementation of the voltage measuring device 7 and the asymmetry detector 8, it is not essential that both voltages be exactly the same; rather, this could also be implemented such that both voltages have a previously defined fixed ratio.For the sake of simplicity, only the case where both voltages are to be equal is explained below; the same applies to other predefined fixed ratios. Inputs 81 and 82 of the asymmetry detector 8 are connected to the positive and negative inputs, respectively, of a comparator 83. This comparator is designed to compare the magnitude of the two applied voltages and to output a corresponding output signal that indicates whether one voltage is higher than the other or not. Depending on the design of the comparator 83, this output signal can be proportional to the deviation or digital, i.e. it only provides information about whether equality exists or not.

[0043] In the latter case, the output of the comparator 83 can function directly as an error signal and be applied to a signaling device, for example to an acoustic signal generator in the form of a warning horn 9.

[0044] In normal operation, the voltages output by the symmetrical voltage divider 73 at the upper terminal 78 and lower terminal 79 are the same, as in Figure 5 is shown. There, the curve labeled I shows the course of the voltage UH applied to the upper terminal 78 and curve II shows the course of the voltage UL applied to the lower terminal 79. In the example shown, the two voltages are of equal magnitude and correspond to half the voltage V1 output by the inverter 3, provided that no fault situation exists and in particular there is no earth fault in the area of ​​the lines 15, 15' of the electrosurgical instrument 16 or on the patient 99 with his or her support 98. The comparator 83 determines that the voltages are of equal magnitude and does not output an error signal.

[0045] The situation is different if a fault situation occurs, for example, if the patient 99 touches a grounded component (grounded metal part) with one of his body parts. This creates a parasitic impedance 97 to ground, through which a leakage current flows. As a result, the voltages measured by the voltage measuring device 73 at the upper terminal 78 and lower terminal 79 are no longer the same. As shown in Figure 6 As shown, the voltage in line 41 and thus at the upper terminal collapses due to the leakage current. The resulting voltage curve for UH is shown by curve I, which runs close to the zero line. In contrast, the voltage UL at the lower terminal 79, shown by curve II, remains constant. As shown in Figure 6As can be clearly seen, when a leakage current occurs, the voltages are not equal, but rather exhibit considerable differences. This is determined by the comparator 83 of the asymmetry detector 8. It outputs a corresponding error signal, which is fed to the warning horn 9, thus providing the surgeon with a clearly perceptible warning signal of the presence of the leakage current. At the same time, the signal can be connected to the operating control 10 of the electrosurgical generator 1, so that the generator can react to the detected error situation, for example, by quickly shutting down the inverter 3.

[0046] If an error signal should not be triggered even at the slightest deviation, the Schmitt trigger 85 can be configured as a threshold switch. This has an adjustable threshold 85'. This allows you to set how large the difference between the voltages at the upper terminal 78 and the lower terminal 79 may be before the error signal is triggered.

[0047] A polarity detector 87 is also expediently provided. Its input is connected to comparator 83. Thus, the sign of the result of comparator 83 can be used to determine whether the leakage current is flowing from the upper line 41 with the active electrode or from the connecting line 15 (so-called "AE" error), or whether the leakage current is flowing from the lower line 42 with the neutral electrode or the connecting line 15' (so-called "NE" error). Depending on the situation, polarity detector 87 activates a signal light 89, 89', which accordingly signals the presence of an AE or NE error.

[0048] Alternative designs for the asymmetry detector 8 are described in Figure 4a) and b). In the variant according to Figure 4a), two analog / digital converters 84, 84' are provided. The upper connection 78 is connected to the analog / digital converter 84, so that this converter converts a voltage signal for the voltage of the active electrode in the connecting line 15 into a corresponding first digital signal. The lower connection 79 is connected to the analog / digital converter 84', so that this converter converts a voltage signal for the voltage of the neutral electrode, as it is present in the line 15', into a corresponding digital signal. These two digital signals are applied to the inputs of a differential element 86. It is implemented using digital technology, for example, microprocessor technology. The difference between the two digitally converted voltage signals is checked. If it exceeds a threshold value, which can also be set digitally (in Figure 4anot shown), the outputs of the differential element 86 are activated. It has one output for connection to the, preferably acoustic, signaling device 9 as well as two further outputs, to which displays 89, 89' are connected, which indicate whether the leakage current is to be attributed to the active electrode "AE" or the neutral electrode "NE".

[0049] In Figure 4b ) shows a simplified variant. In this case, the signals coming from the voltage divider 73 for the voltage at the upper terminal 78 and the voltage at the lower terminal 79 are applied to a Schmitt trigger 85 with an integrated difference former 88, for example a comparator circuit. Here, too, as with the Schmitt trigger 85 in Figure 3 , the switching threshold of the Schmidt carrier can be set via an adjustable threshold signal 85'.

[0050] In this way, the invention enables reliable detection of leakage current with minimal additional effort. Furthermore, it can determine the polarity—i.e., whether the leakage current occurs at the active electrode (AE) or the neutral electrode (NE)—with minimal effort.

[0051] The error signal output by the asymmetry detector 8 can be applied, in addition to the warning horn 9, to a separate device 80 that interacts with the operating control 10. The device 80 is expediently designed to determine the magnitude of the leakage current when the asymmetry detector 8 detects a leakage current. This can be done, for example, by using the operating control 10 to compare current measured values ​​of the total output current immediately before and after the leakage current is detected by the asymmetry detector 8, based on the measured values ​​for the current in the output line already available there, such as those from the measuring sensor 18, and optionally outputting them via a display device.

Claims

1. Electrosurgical generator configured to output a high-frequency AC voltage to an electrosurgical instrument (16), comprising an inverter (3) for high voltage, which generates the high-frequency AC voltage that is passed via an output line (4) to an output (14) for connection of the electrosurgical instrument (16), and a leakage current detecting device (6) for the electrosurgical instrument (16) connected to the output (14), characterized in that the leakage current detecting device (6) comprises a voltage measuring device (7), which is connected by its inputs in each case via a capacitive coupling (71, 72) to an active and a neutral line (41, 42) of the output line (4) and has a bipolar voltage divider (73) having a predetermined fixed ratio, which has an upper connection (78) and a lower connection (79), to which the capacitive coupling is applied, and also a centre tap (77), and an asymmetry detector (8) configured to compare an upper voltage between upper connection (78) and centre tap (77) with a lower voltage between lower connection (79) and centre tap (77), and to output a fault signal for leakage current in the case of deviation of the ratio of upper voltage to lower voltage from the predetermined fixed ratio.

2. Electrosurgical generator according to Claim 1, characterized in that the asymmetry detector (8) has a minimum threshold (85'), below which a fault signal is not yet output.

3. Electrosurgical generator according to the preceding claim, characterized in that the minimum threshold (85') is adjustable, preferably depending on an operating mode of the electrosurgical generator (1) and / or the connected instrument (16).

4. Electrosurgical generator according to Claim 2 or 3, characterized in that different minimum thresholds (85') are provided for the active electrode and the neutral electrode.

5. Electrosurgical generator according to Claim 3 or 4, characterized in that the minimum threshold (85') is defined in an instrument-dependent manner, preferably in a memory which is assigned to the instrument and in which a value for the minimum threshold (85') is stored.

6. Electrosurgical generator according to any of the preceding claims, characterized in that the asymmetry detector (8) is provided with a polarity detector (87) for the leakage current, wherein preferably the polarity detector (87) interacts with a display device (89, 89'), which signals whether a leakage current occurs at the active electrode or the neutral electrode.

7. Electrosurgical generator according to any of the preceding claims, characterized in that the asymmetry detector (8) has a comparator (83) having two inputs, wherein the upper connection (78) is connected to one of the inputs and the lower connection (79) is connected to the other input, wherein preferably the comparator (83) is embodied using analogue technology, preferably by means of an operational amplifier.

8. Electrosurgical generator according to any of the preceding claims, characterized in that the asymmetry detector (8) is embodied as a difference calculating unit (88) with a threshold value switch, in particular Schmitt trigger (85), connected downstream.

9. Electrosurgical generator according to any of the preceding claims, characterized in that the asymmetry detector (8) has an analogue / digital converter (84, 84').

10. Electrosurgical generator according to the preceding claim, characterized in that the analogue / digital converter (84, 84') is arranged on the input side of the asymmetry detector (8).

11. Electrosurgical generator according to any of the preceding claims, characterized in that the coupling (71, 72) is at high impedance relative to the voltage divider (73), preferably by at least a factor of 100, in particular at least 1000, wherein preferably the voltage divider (73) is kept free of high voltage.

12. Electrosurgical generator according to any of the preceding claims, characterized in that the voltage divider (73) is capacitive.

13. Electrosurgical generator according to any of the preceding claims, characterized in that the centre tap (77) is connected to a fixed potential, preferably to ground potential.

14. Electrosurgical generator according to any of the preceding claims, characterized in that the leakage current detector (6) interacts with a device (80) for determining a magnitude of the current, in particular of the leakage current.

15. Electrosurgical generator according to any of the preceding claims, characterized in that the voltage divider (73) is a symmetrical voltage divider.