ELECTROSURGICAL GENERATOR WITH INVERTER FOR GENERATING HF HIGH VOLTAGE

Abstract

An electrosurgical generator includes a power supply unit which, when operating, supplies a direct voltage circuit, and a high-voltage inverter supplied from it that generates a high-frequency alternating voltage that is applied to outputs for connection of the electrosurgical instrument. The inverter includes a clock-driven power switch and a zero-crossing detector that recognizes zero crossings of the oscillation generated by the inverter. A signal for the generated alternating voltage is applied to the zero-crossing detector via a voltage divider which is a capacitive voltage divider with at least one capacitor that is resistant to high voltage. Undesirable direct voltage components at the center tap in the presence of changes to the supply voltage can be avoided thereby, since charge reversals as a result of changes to the supply voltage occur on both sides, and their effects thus cancel out.

Claims

1. An electrosurgical generator designed to output a high-frequency alternating voltage to an electrosurgical instrument, comprising a power supply unit which, when operating, feeds a direct voltage circuit, and an inverter for high voltage that is fed from the direct voltage circuit and generates a high-frequency alternating voltage that is applied to outputs for connection of the electrosurgical instrument, wherein the inverter has a clock-driven power switch and a zero-crossing detector designed to detect zero crossings of the oscillation generated by the inverter, wherein a signal for the generated alternating voltage is applied to the zero-crossing detector by means of a first voltage divider via a signal line, wherein the voltage divider is designed as a capacitive voltage divider for alternating voltage with at least one capacitor resistant to high voltage.

2. The electrosurgical generator as claimed in claim 1, wherein the capacitive voltage divider has a division ratio between 1:20 and 1:4.

3. The electrosurgical generator as claimed in claim 1, wherein values of the capacitors of the capacitive voltage divider lie in the range between 50 pF and 10 nF.

4. The electrosurgical generator as claimed in claim 1, wherein the capacitive voltage divider is connected in parallel with the power switch.

5. The electrosurgical generator as claimed in claim 1, wherein the capacitive voltage divider is connected to the alternating voltage generated by the power switch directly or by means of a current limiting element.

6. The electrosurgical generator as claimed in claim 5, wherein an ohmic resistor, the resistance value of which is less than the impedance of the capacitive voltage divider, is provided as the current limiting element.

7. The electrosurgical generator as claimed in claim 1, wherein the signal line comprises a correction circuit for direct voltage offset, designed to minimize or remove a direct voltage potential in the signal line, wherein the correction circuit is implemented as a high-pass filter.

8. The electrosurgical generator as claimed in claim 1, wherein a variable reference is applied as a zero reference to the zero-crossing detector.

9. The electrosurgical generator as claimed in claim 8, wherein the variable reference is derived from the voltage in the direct voltage circuit.

10. The electrosurgical generator as claimed in claim 8, wherein the variable reference is generated by means of a second voltage divider that has a different type of construction from the first voltage divider.

11. The electrosurgical generator as claimed in claim 10, wherein an impedance converter is connected between the second voltage divider and the zero-crossing detector.

12. The electrosurgical generator as claimed in claim 11, wherein an offset circuit is provided in the reference line, being designed, in the absence of an input signal, to apply a defined reference to the zero-crossing detector via the reference line.

13. The electrosurgical generator as claimed in claim 12, wherein the offset circuit is integrated into the impedance converter.

14. The electrosurgical generator as claimed in claim 8, wherein limiting circuits are provided at the input to the zero-crossing detector for the signal line and/or the reference line.

Description

[0028] The invention is explained in more detail below with reference to an advantageous exemplary embodiment. In the figures:

[0029] FIG. 1 shows an electrosurgical generator according to one exemplary embodiment with an attached electrosurgical instrument;

[0030] FIG. 2 shows a schematic functional diagram of the electrosurgical generator according to FIG. 1;

[0031] FIG. 3 shows a block diagram of an inverter of the electrosurgical generator according to FIG. 1;

[0032] FIG. 4 shows an exemplary circuit diagram of the inverter with power switch and zero-crossing detector;

[0033] FIGS. 5a, b show graphs of voltage curves;

[0034] FIGS. 6a, b show graphs of voltage curves and zero crossings according to the prior art; and

[0035] FIG. 7 shows a circuit diagram of an inverter according to the prior art.

[0036] An electrosurgical generator according to one exemplary embodiment of the invention is illustrated in FIG. 1. The electrosurgical generator, identified as a whole with reference sign 1, comprises a housing 11 provided with a terminal 14 for an electrosurgical instrument 16 which, in the exemplary embodiment illustrated, is an electrical scalpel. It is connected via a high-voltage connecting cable 15 to the terminal 14 of the electrosurgical generator 1. The power output to the electrosurgical instrument 16 can be changed by means of a power controller 12. A mains connecting cable 13, which can be connected to the public electricity mains, is provided for the supply of electrical power to the electrosurgical generator 1.

[0037] A schematic functional diagram of the electrosurgical generator 1 is illustrated in FIG. 2. It comprises a power supply unit 3 that is supplied with electrical power by the mains connecting cable 13 (see FIG. 1). The power supply unit 3 is a high-voltage power supply unit (HVPS). It comprises a rectifier and feeds a DC link 4 with direct voltage, the level of which can vary between 0 and about 300 volts in the embodiment illustrated, wherein the absolute level of the direct voltage depends 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.

[0038] An inverter 5 that generates high-frequency alternating current in the high-voltage range of a few kilovolts is fed from the DC link 4. The inverter 5 is of the type with a free-running single-ended generator. The high-frequency high voltage output at the terminal 14 is measured by means of voltage and current sensors 17, 18, and the measurement signals are supplied to a processing unit 19 that applies the corresponding data regarding the voltage, current and power that are output to an operating controller 10 of the electrosurgical generator 1 to which the power controller 12 is also connected.

[0039] In a free-running single-ended generator, as is typically used in the inverter 5 for electrosurgical generators 1, it is necessary for the sake of stable operation that the zero crossing of the oscillation generated is detected correctly. For this purpose, a zero-crossing detector 7 is provided which makes a signal for the zero crossing available at its output via a line 70 which is applied to an oscillation control unit 51.

[0040] This is illustrated in more detail in FIG. 3, which shows a block diagram of the inverter 5 with its power stage. A parallel resonant circuit 54 comprises a high-voltage capacitor 55 and an inductor 57 that is preferably the primary winding of a transformer 56 whose secondary side is connected to the output terminal 14. The upper terminal of the parallel resonant circuit 54 is connected to the upper potential of the direct voltage circuit 4, while its lower terminal is connected via a power switch 53 to the lower potential of the direct voltage circuit 4. The semiconductor power switch 53 is clock-driven by an oscillation control unit 51 via a driver 52 for decoupling and amplification. The power switch 53 is a power semiconductor, particularly of the MOSFET type, although other types of fast-switching power semiconductors may also be used. Through the fast, periodically clocked driving of the power switch 53, a corresponding alternating voltage is generated across the capacitor 54, which is then output via the transformer 56 at the terminal 14 as a high-frequency high voltage. The frequency of the periodically clocked driving can be changed and is largely determined by the parallel resonant circuit 54.

[0041] To detect the zero crossings, the voltage at the drain terminal of the power switch 53, i.e., at the connection between the power switch 53 and the parallel resonant circuit 54, is tapped off by means of a voltage divider 6.

[0042] Before the embodiment according to the invention is explained in more detail, reference will first be made to the implementation of this topology according to the prior art, as is illustrated in the circuit diagram according to FIG. 7. The input for the supply voltage from the direct current circuit 4, together with smoothing capacitors 41′, can be seen at the top left. The input for the clocked oscillation signal that acts on the driver 52′, which in turn drives the power switch 53′ via a protective resistor 58′, can also be seen at the left-hand edge. This is connected to the resonant circuit 54′ which comprises a capacitor 55′ and an inductor 57′. A voltage divider 6′ is connected to the drain terminal of the power semiconductor 53′, in order to tap off the voltage for detection of the zero crossing. The voltage divider 6′ is formed by a high-pass filter with a capacitor 64′ that is connected to the drain terminal of the power switch 53′, and a resistor 65′ that connects the capacitor 64′ to the lower potential of the direct voltage circuit. At its output, the voltage divider 6′ outputs the voltage U.sub.Null which is output via a voltage limiting circuit comprising a resistor 71′ and diodes 73′, 74′ connected antiparallel, and is applied to a negative input of the comparator 77′ that acts as the zero-crossing detector. A second voltage divider 81′ with the two resistors 82′, 83′ is connected to the other, positive input of the comparator 77′. They form the zero reference against which the comparator 77′ compares the voltage signal measured by the voltage divider 6′. The resistors 82′ and 83′ are dimensioned in the exemplary embodiment illustrated in such a way that a small, positive offset voltage, which is thus not exactly at zero, results. In this way, it is ensured that, even in the absence of a signal from the voltage divider 6′, the comparator 77′ always outputs a defined signal, namely a positive output voltage, and an undefined state therefore cannot arise. A pull-up resistor 79′ is provided there, again to avoid undefined states at the output of the comparator 77′. In regular operation, when a high-frequency signal is generated by the electrosurgical generator 1 (typically in the range between 300 and 600 kHz) the output of the comparator 77′ continuously changes in time with the voltage at the output of the comparator 77′ tapped off by the voltage divider 6′, between 0 V when the voltage U.sub.Null present at the negative input exceeds the reference set by the voltage divider 81′ and a positive output voltage when the voltage U.sub.Null falls below the set reference. In this way, in the settled state, the zero crossing of the alternating voltage generated by the electrosurgical generator can be detected and processed further.

[0043] As already explained at the outset, the disadvantage of this circuit is relevant in particular when the voltage with which the inverter is supplied is changed. This can happen intentionally by adjusting the power controller 12, but also through what may be a very fast change in the load impedance. If the supply voltage in the direct voltage circuit 4 changes, then the direct voltage component of the generated alternating voltage, as is also present at the voltage divider 6′, necessarily also changes. The result of this is that with each change in the supply voltage, the capacitor 64′ in the voltage divider 6′ is charged up or discharged in accordance with the changed direct voltage component, and this charge compensation leads to a direct current component. This additional direct current component leads to faulty detection of the zero crossing, which can then consequently lead to the oscillation stalling and/or to an incorrect switching of the power switch.

[0044] This is shown visually in FIG. 6. The regular settled state, in which the zero crossings are correctly detected at regular intervals, is illustrated in FIG. 6a. FIG. 6b shows that the supply voltage is increased as the oscillation continues. As a result of the direct component from the charge reversal of the capacitor, the alternating voltage curve, unchanged in itself, now rises to a higher potential, which has the consequence of a significant shift in the zero crossings. In FIG. 6b this shift can be seen in the discrepancy A between the vertical dashed line indicating the zero crossing time that is, in itself, correct, and the actual zero crossing time of the solid curve, which differs from it significantly. It can be seen straight away that the detection is significantly distorted.

[0045] The improved version according to the invention is described with reference to the circuit diagram of FIG. 4. The voltage supply and the driver 52 in the left-hand region of the circuit diagram, including the power switch 53 and the parallel resonant circuit 54, are as described above for FIG. 7. A different voltage divider is provided according to the invention, namely a capacitive voltage divider 6 that contains two capacitors 61, 62 that are resistant to high voltage. To protect them from any current peaks that may occur, the connection to the power switch 53 is made via a current limiting element 2, which, in the exemplary embodiment illustrated, is realized as a low-ohm resistor (in the range between 2 and 20 ohms). Due to this low resistance, influence on the capacitive voltage divider is extremely small, and can be disregarded. In the exemplary embodiment illustrated, the capacitors 61, 62 are dimensioned such that a division ratio of 1 to 6 results, i.e., the voltage across the power switch 53 is divided down by the voltage divider 6 to one-sixth of the value. This voltage signal is transmitted via a signal line 60 from the capacitive voltage divider 6 to the zero-crossing detector 7 or, put more precisely, to a negative input 76 of a comparator 77 of the zero-crossing detector 7.

[0046] A limiting circuit for the magnitude of the signal is provided along the signal line 60. It is realized by a series resistor 71 and two diodes 74, 75 arranged antiparallel between the signal line 60 and a reference line 80.

[0047] In respect of the reference signal transmitted on the reference line 80 to the comparator 77 of the zero-crossing detector 7, it is provided according to a particularly advantageous optional aspect of the invention that this is not fixed but is derived in a variable manner from the supply voltage. A second voltage divider 81, comprising two resistors 82, 83, is provided for this purpose. A measuring line 40 that applies the upper potential of the direct voltage circuit 4 to the upper terminal of the second voltage divider 81 is provided for this. Its lower terminal is connected to ground, and thus to the lower potential of the direct voltage circuit. In this way, a reference that follows the voltage level in the direct voltage circuit 4, and is therefore variable, can be generated. It is passed via an impedance converter 8 with a buffer amplifier 86 that is configured as a voltage follower. The voltage signal tapped off from the second voltage divider 81 is applied to the positive input of the buffer amplifier 86, while a pull-up resistor 84 and a capacitor 85 are furthermore provided to improve the signal. The pull-up resistor 84 ensures a positive initial voltage even when no measurement signal is transmitted from the second voltage divider 81. Feedback from the output is connected in the manner known per se to the negative input of the buffer amplifier 86. The output of the impedance converter 8 is applied via a resistor 72, which serves for signal limitation, to a positive input 78 of the comparator 76, in order there to form a variable reference for the zero threshold.

[0048] With this circuit, the reference for detection of the zero crossing as the generator supply rises is shifted upwards by a small amount (about 3% of the voltage in the DC link in the exemplary embodiment illustrated). This reduces the risk of incorrect detection of the zero crossings, in particular in the presence of load-dependent decay of the generator and of critical damping. At the other end of the spectrum, however, namely when the generator voltage is very small, the zero crossings can again be detected reliably as a result of the variable reference. This is advantageous in particular in the case of very low-impedance loads, since, due to the low voltage level that now prevails, the zero crossings can still be reliably detected. This is illustrated in FIG. 5. FIG. 5b there shows operation with regular voltage, while FIG. 5a shows operation with low voltage in which the reference threshold (dashed line) is lowered with respect to that of FIG. 5b.

[0049] To increase the detection reliability further, a correction circuit 9 against a DC voltage offset in the signal line 60 is also provided at the signal line 60. In terms of the alternating voltage, the position of the tap at the capacitive voltage divider 6 is strictly defined, but this does not apply to the direct voltage potential. In order to prevent the direct voltage potential from drifting away, and thus potentially undefined states at the comparator 76 of the zero-crossing detector 7, the correction circuit is provided with a resistor 90 that connects the signal line 60 to ground through a high resistance. The values for this resistor 90, and also those for the capacitors 61, 62, are selected here in such a way that the cut-off frequency of a possible parasitic high-pass filter is low enough that there is no longer any practical influence in the frequency range of a few 100 kHz that is of interest for the high-frequency application. A pull-up resistor 79 is provided, again to avoid undefined states at the output of the zero-crossing detector 7.

[0050] Overall, significant improvements result from the exemplary embodiment according to the invention, so that even at very low output powers of up to 5 W or less, the generator oscillation does not stall, and the zero crossings of the high-frequency signal output can be detected significantly more accurately and quickly. The operational safety is also significantly improved by the design according to the invention in respect of significant load impedance jumps.