Abstract:
A high frequency surgery generator having an output circuit and a power oscillator for generating an HF output voltage. The power oscillator is designed to excite the output circuit at a first excitation frequency, which corresponds to a resonant frequency of the output circuit, in a first operating mode and to excite the output circuit at a second excitation frequency which differs from the first excitation frequency, which does not correspond to the resonant frequency of the output circuit, in a second operating mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is the U.S. national phase of PCT Application No. PCT/EP2010/066726 filed on Nov. 3, 2010, now published WO 2011/067061, which claims priority to DE Patent Application No. 102009044720.2 filed on Dec. 1, 2009 and DE Patent Application No. 102010000184.8 filed on Jan. 25, 2010, the disclosures of which are incorporated in their entirety by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to an electrosurgical generator for producing an RF output voltage. 
     BACKGROUND 
     These days, electrosurgery is used in many medical fields. In the process, a relatively high voltage, which is applied to a monopolar or bipolar electrode, is used to damage or cut biological tissue in a targeted manner. In order to produce these voltages, use is usually made of an electrosurgical generator (RF surgical generator). 
     Different types of modulation are used for cutting and/or coagulating the biological tissue. To this end, e.g. bursts must have a quick beginning and end. 
     EP 1 776 929 A1 describes an electrosurgical generator in which the output voltage of the RF surgical generator can quickly be reduced. In the process, if a threshold is exceeded, the output voltage of the RF surgical generator is quickly reduced by virtue of converting the energy from the power supply unit into heat by connecting an additional load component. A disadvantage of this is that the generated heat needs to be dissipated and that the RF surgical generator is technically complex and has poor cutting properties. 
     DE 102 18 895 A1 describes a further electrosurgical generator, wherein the output voltage of the electrosurgical generator can quickly be reduced. Here, the output voltage of the power supply unit is reduced by virtue of feeding the output capacitor energy back to the DC voltage supply. As a result, the energy stores on the output side of the DC voltage supply and in the power oscillator are quickly discharged. As a result of this, the output voltage of the electrosurgical generator is quickly reduced. A disadvantage of this is that the electrosurgical generator offers poor cutting properties and the circuit is complicated and technically complex. 
     US 2004/0193148 A1 describes an electrosurgical generator in which surges in the output voltage of the RF surgical generator can quickly be regulated down. In this case, the resonant frequency of the oscillator is modified by the addition of further components as soon as the output voltage of the RF surgical generator lies above a limit voltage. As a result, the unchanging excitation frequency no longer corresponds to the resonant frequency of the oscillator, as a result of which the output voltage of the RF surgical generator is reduced below the limit voltage, i.e. to the level of a normal output voltage, and sparking is prevented. It is disadvantageous in this case that the circuit is technically complex and sparking, which is advantageous for cutting, is prevented and hence the RF surgical generator offers poor cutting properties. 
     SUMMARY 
     The invention is based on the object of highlighting an RF surgical generator. 
     This object is achieved by an RF surgical generator. An essential point of the invention consists of the fact that the RF surgical generator comprises two operating modes: in the first operating mode, an output circuit is excited by an RF power oscillator with a resonant frequency of the output circuit. In a second operating mode, the output circuit is excited by the RF power oscillator with another frequency that does not correspond to the resonant frequency thereof. The output voltage of the RF surgical generator is quickly and abruptly reduced in the case of a transition from the first operating mode to the second operating mode. 
     As a result of this, output voltages with harmonics are produced and this results in improved cutting properties of the RF surgical generator. Moreover, the RF surgical generator is simple from a technical point of view and cost effective. 
     The RF power oscillator can comprise switching elements, more particularly transistors, for controlling a sign of an excitation voltage, by means of which switching elements the RF power oscillator excites the output circuit in a feedback operation in the first operating mode depending on a sign of an excitation current of the RF power oscillator, and for setting the second excitation frequency in a second, forced operating mode. As a result of this, in the first operating mode, the output circuit can be driven in a technically simple manner with as little energy loss as possible using a resonant frequency. The fact that the frequency of the excitation voltage in the second operating mode can be set in a technically simple manner is also advantageous in this case. 
     In one embodiment, the electrosurgical generator comprises a controllable operating-mode switching device for switching from the first operating mode to the second operating mode for producing an output voltage with harmonics. An advantage of this is that an output voltage with harmonics that is very advantageous for cutting the tissue can be set in a targeted manner. 
     The RF surgical generator can comprise an adjustable frequency control instrument for measuring a sign of the excitation current and for opening and/or closing the switching elements for controlling a sign of the excitation voltage in the first operating mode and for setting the second excitation frequency in the second operating mode. As a result of this, the sign of the excitation current can be captured in a technically simple manner and the sign of the excitation voltage can accordingly be set in the first operating mode. The fact that the excitation frequency can be set in a targeted manner in the second operating mode is also advantageous in this case. 
     In terms of the method, the invention is achieved by virtue of the fact that the RF power oscillator excites the output circuit in a first operating mode with a first excitation frequency, which corresponds to a resonant frequency of the output circuit, for producing the RF output voltage and that the RF power oscillator excites the output circuit in a second operating mode with a second excitation frequency that differs from the first excitation frequency and does not correspond to the resonant frequency of the output circuit for the accelerated reduction of the RF output voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Preferred embodiments of the invention emerge from the dependent claims. In the following text, the invention will be explained in more detail on the basis of drawings of exemplary embodiments. In this case: 
         FIG. 1  shows a schematic view of an RF surgical generator and 
         FIG. 2  shows a schematic view of a circuit design in a power supply unit and in a power oscillator of an RF surgical generator. 
     
    
    
     In the following description, the same reference signs are used for equivalent and functionally equivalent parts. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic view of an RF surgical generator  1 . The RF surgical generator  1  comprises a power supply unit  20 , which is connected to a current and voltage supply (not illustrated) via connection lines and which supplies a power oscillator  10  with electrical energy via a connection line  12 . The power oscillator  10  is connected to an output circuit  35  via a connection line  21 . The power oscillator  10  drives the output circuit  35  with an excitation frequency. The output circuit  35  is connected to one of the outputs  45  of the RF surgical generator  1  via a connection line  11 . A monopolar or bipolar electrode  40 , to which an output voltage of the RF surgical generator  1  is applied, can be connected to the outputs  45 . 
     In the first operating mode, the output circuit  35  is excited by the power oscillator  10  with a first excitation frequency which corresponds to a resonant frequency or the resonant frequency of the output circuit  35 . At a resonant frequency of the output circuit  35 , there is a maximum of the amplitude transfer function |A(f)|=|u.sub.out/u.sub.in| (where u.sub.in is the voltage which is applies at the output of the power oscillator and u.sub.out is the voltage applied at the output of the output circuit, i.e. at the output of the RF surgical generator  1 ). The maximum power is transmitted at the resonant frequency. In general, this is also the work point with the highest energetic efficiency. 
     This excitation in the first operating mode of the output circuit  35  with an excitation frequency that corresponds to the resonant frequency of the latter or one of the resonant frequencies of the latter is brought about as follows: the frequency control instrument  30  measures the sign of the output current of the power oscillator  10 , i.e. the phase in which the output current currently is, with the aid of an ammeter via a connection line  33 . Depending on this, the frequency control instrument  30  accordingly sets the sign of the output voltage of the power oscillator  10  via a connection line  32  such that the output voltage of the power oscillator  10  is in phase with the output current of the power oscillator  10 . 
     In the second operating mode, the output current  35  is excited by the power oscillator  10  with a second excitation frequency, which does not correspond to the first excitation frequency, by virtue of, independently of the phase of the output current of the power oscillator  10 , setting the output voltage of the power oscillator  10  to a frequency which corresponds to the second excitation frequency. The second excitation frequency is set by the frequency control instrument  30 . The second excitation frequency is not in the vicinity of the resonant frequency of the output circuit  35 , i.e. the second excitation frequency differs not only insubstantially from the resonant frequency, because the amplitude transfer function is near its maximum at a frequency close to the resonant frequency. 
     In the case of this second excitation frequency, the amplitude transfer function is not at its maximum and far from it. As a result, the output voltage and output power of the output circuit  35  reduces and hence the output voltage of the RF surgical generator  1  is quickly reduced or lowered. Moreover, the generator internal resistance of the RF surgical generator  1  abruptly increases as a result of this. 
     When operating the RF surgical generator  1  in the second operating mode, the output circuit  35  supplies output current forms and output voltage forms with substantial harmonics. This promotes sparking at the electrode  40 . Sparking at the electrode  40  is particularly desirable if biological tissue is intended to be cut using the electrode  40 . The second operating mode makes it possible to produce output voltages or output voltage forms with harmonics in the case of an otherwise low-distortion surgical generator. 
     An operating-mode switching device  25  is connected to the frequency control instrument  30  via a connection line  31 . As a result of this it is possible to switch between a first operating mode and a second operating mode or from the first operating mode to the second operating mode. This can be used in a targeted manner to set the second operating mode, in which tissue can be cut. 
     Moreover, an emergency button can for example be connected to the operation switching device  25  such that, in the case of an emergency, the output voltage of the RF surgical generator  1 , i.e. the voltage at the electrode  40 , can be rapidly reduced if the surgeon or another person actuates the emergency button. 
     The RF surgical generator  1  can comprise an indicator, for example in the form of indicator lamps, which shows which operating mode the RF surgery generator  1  is currently in. 
     In the second operating mode, the power oscillator  10  always drives the output circuit  35  with the same low-resistance source impedance. 
     This ensures that the output frequency of the output circuit  35  always corresponds to the set frequency of the power oscillator  10 . 
     The cutting properties of the RF surgical generator are also improved by the RF surgical generator  1  highlighted here in the case of surroundings of the connected electrode  40  consisting of a conductive liquid, such as e.g. physiological saline, as a result of the output voltage with harmonics in the second operating mode. 
       FIG. 2  shows a circuit design of a second embodiment of an RF surgical generator  1 . The power oscillator  10  is connected to a power supply unit (not shown) via connection lines  22 . The power supply unit supplies a voltage U 0 , which is applied across a first capacitor  75 . The power oscillator  10  is connected to the output circuit  35  via a transformer  70 . There is galvanic isolation between the electrical circuits of the RF power oscillator  10  and the output circuit  35  as a result of the transformer  70 . The transformer  70  comprises a first inductor  71  and a second inductor  72 , which are coupled across a magnetic core  73 . The power oscillator  10  comprises four transistors  50 ,  55 ,  60 ,  65 . A voltage U 1  is applied to the transformer on the side of the power oscillator  10 . A frequency control instrument  30  measures the sign of a current I 1  at the output of the power oscillator  10  with the aid of an ammeter  34 . 
     The inputs of the first transistor  50  and the third transistor  60  are connected to the power supply unit via one of the connection lines  22 . The output of the first transistor  50  is connected to a connector of the first inductor  71  and to the input of the second transistor  55 . The output of the third transistor  60  is connected to an output of the ammeter  34  and to the input of the fourth transistor  65 . The outputs of the second transistor  55  and the fourth transistor  65  are connected to the power supply unit via one of the other connection lines  22 . 
     The frequency control instrument  30  is connected to a frequency generator (not shown here) via a connection line  85 . The frequency control instrument  30  is respectively connected to the control connector of the first  50 , second  55 , third  60  and fourth transistor  65 . 
     If the value of I 1 , measured by the ammeter  34 , is greater than zero, the frequency control instrument  30  turns the first transistor  50  and the fourth transistor  65  on. If the frequency control instrument  30  determines that I 1  is less than or equal to zero, the frequency control instrument  30  turns the third transistor  60  and the second transistor  55  on. The other two transistors are respectively turned off. 
     The output circuit  35  furthermore comprises a third inductor  74  and a second capacitor  80 . These are connected to one of the outputs  45  of the RF surgical generator  1 . The radiofrequency voltage UHF is applied to the outputs  45 . As a result of control by the frequency control instrument  30 , the output voltage U 1  and the output current I 1  of the RF power oscillator are in phase and the amplitude transfer function is at its maximum or in the vicinity thereof. 
     In the second operating mode, the output circuit  35  is excited by a resonant-distant excitation frequency. A resonant-distant frequency should be understood to mean a frequency which does not equal the resonant frequency of the output circuit  35  and which also does not only deviate insubstantially from the resonant frequency. In the second operating mode, the first transistor  50  and the fourth transistor  65  and also the second transistor  55  and the third transistor  60  are alternately turned on with respectively half the period duration of the desired second excitation frequency, independently of the sign of the excitation current I 1 . The respective other two transistors are turned off. This second excitation frequency is transmitted to the frequency control instrument  30  by means of the frequency generator. The second excitation frequency transmitted by the frequency generator can be set. The amplitude transfer function is (further) away from its maximum in the second operating mode. 
     At this point, reference is made to the fact that all parts described above, more particularly the details illustrated in the drawings, are claimed as essential to the invention, either when considered on their own or in any combination. A person skilled in the art is aware of modifications of this. 
     LIST OF REFERENCE SIGNS 
     
         
         
           
               1  RF surgical generator 
               10  RF power oscillator 
               11  Output circuit—output connection line 
               12  Power supply unit—RF power oscillator connection line 
               20  Power supply unit 
               21  RF power oscillator—output circuit connection line 
               22  Power supply unit—current source connection line 
               25  Operating-mode switching device 
               30  Frequency control instrument 
               31  Frequency control instrument—operating-mode switching device connection line 
               32  Frequency control instrument—RF power oscillator connection line 
               33  Frequency control instrument—ammeter connection line 
               34  Ammeter 
               35  Output circuit 
               40  Electrode 
               45  Outputs of the RF surgical generator 
               50  First transistor 
               55  Second transistor 
               60  Third transistor 
               65  Fourth transistor 
               70  Transformer 
               71  First inductor 
               72  Second inductor 
               73  Magnetic core 
               74  Third inductor 
               75  First capacitor 
               80  Second capacitor 
               85  Frequency generator—frequency control connection line