Abstract:
A constant power control circuit for an electrosurgical generator and a method for maintaining the electrical power output of an electrosurgical generator at a generally constant value throughout a given tissue impedance range are disclosed. The constant power control circuit and the method recognize and use the unique and simple linear characteristics associated with certain electrosurgical generator designs to monitor and control the electrical power output without having to calculate or monitor the actual output power. The constant power control circuit includes a current sampling circuit, a linear conversion circuit, and a feedback correction circuit. The constant power control circuit may also include protection circuitry that prevents the electrosurgical generator from being over-driven during high and/or low impedance loading, and reduces the severity of exit sparking by providing a quick response to high impedance indications while nonetheless maintaining increased power levels throughout a preset, nominal impedance range. The constant power control circuit and method may be included as an integral part of the overall electrosurgical generator&#39;s circuitry, or may be embodied as a separate unit that connects to, and controls, an electrosurgical generator. The constant power control circuit and method may be embodied through a variety of analog and/or digital circuit components or arrangements, including software running on computational and memory circuitry.

Description:
RELATED APPLICATION INFORMATION 
     This application is a Continuation application of U.S. patent application Ser. No. 08/533,891 filed on Sep. 26, 1995, now U.S. Pat. No. 5,772,659. 
    
    
     FIELD OF THE INVENTION 
     A constant power control circuit for an electrosurgical generator and a method for maintaining the electrical power output of an electrosurgical generator at a generally constant level throughout a given tissue impedance range. 
     BACKGROUND OF THE DISCLOSURE 
     An electrosurgical generator is used in surgical procedures to deliver electrical energy to the tissue of a patient. An electrosurgical generator often includes a radio frequency generator and its controls. When an electrode is connected to the generator, the electrode can be used for cutting or coagulating the tissue of a patient with high frequency electrical energy. During normal operation, alternating electrical current from the generator flows between an active electrode and a return electrode by passing through the tissue and bodily fluids of a patient. 
     The electrical energy usually has its waveform shaped to enhance its ability to cut or coagulate tissue. Different waveforms correspond to different modes of operation of the generator, and each mode gives the surgeon various operating advantage. Modes may include cut, coagulate, a blend thereof, desiccate, or spray. A surgeon can easily select and change the different modes of operation as the surgical procedure progresses. 
     In each mode of operation, it is important to regulate the electrosurgical power delivered to the patient to achieve the desired surgical effect. Applying more electrosurgical power than necessary results in tissue destruction and prolongs healing. Applying less than the desired amount of electrosurgical power inhibts the surgical procedure. Thus, it is desirable to control the output energy from the electrosurgical generator for the type of tissue being treated. 
     Different types of tissues will be encountered as the surgical procedure progresses and each unique tissue requires more or less power as a function of frequently changing tissue impedance. Even the same tissue will present a different load impedance as the tissue is desiccated. 
     Two conventional types of power regulation are used in commercial electrosurgical generators. The most common type controls the DC power supply of the generator by limiting the amount of power provided from the AC mains to which the generator is connected. A feedback control loop regulates output voltage by comparing a desired voltage with the output voltage supplied by the power supply. Another type of power regulation in commercial electrosurgical generators controls the gain of the high-frequency or radio frequency amplifier. A feedback control loop compares the output power supplied from the RF amplifier for adjustment to a desired power level. Generators that have feedback control are typically designed to hold a constant output voltage, and not to hold a constant output power. 
     U.S. Pat. Nos. 3,964,487; 3,980,085; 4,188,927 and 4,092,986 have circuitry to reduce the output current in accordance with increasing load impedance. In those patents, constant voltage output is maintained and the current is decreased with increasing load impedance. 
     U.S. Pat. No. 4,126,137 controls the power amplifier of the electrosurgical unit in accord with a non linear compensation circuit applied to a feedback signal derived from a comparison of the power level reference signal and the mathematical product of two signals including sensed current and voltage in the unit. 
     U.S. Pat. No. 4,658,819 has an electrosurgical generator which has a microprocessor controller based means for decreasing the output power as a function of changes in tissue impedance. 
     U.S. Pat. No. 4,727,874 includes an electrosurgical generator with a high frequency pulse width modulated feedback power control wherein each cycle of the generator is regulated in power content by modulating the width of the driving energy pulses. 
     U.S. Pat. No. 3,601,126 has an electrosurgical generator having a feedback circuit that attempts to maintain the output current at a constant amplitude over a wide range of tissue impedances. 
     None of the aforementioned U.S. Patents include a constant power control circuit that provides for a generally constant output power while also providing a linear adjustment to account for the unique waveform crest factors associated with different operational modes. 
     The preferred constant power control circuit and method provided herein allows for output power control by way of a unique and simple linear conversion circuit coupled with protection circuitry that prevents the electrosurgical generator from being over-driven during high and/or low impedance loading. The preferred constant power control circuit also reduces the severity of exit sparking by responding quickly to high impedance indications while nonetheless maintaining substantially increased power levels throughout a predetermined patient tissue impedance range. 
     SUMMARY OF THE INVENTION 
     A constant power control circuit for use with an electrosurgical generator. The constant power control circuit and method may be included as an integral part of the overall electrosurgical generator&#39;s circuitry, or may be designed as a separate unit that connects to, and controls, an electrosurgical generator. The constant power control circuit and method may be embodied through a variety of analog and/or digital circuit components or arrangements, including software running on computational and memory circuitry. 
     The constant power control circuit and method maintain the output power of the electrosurgical current at a generally constant level over a finite patient tissue impedance range. The preferred patient tissue impedance range is about 300 to 2500 ohms. 
     The constant power control circuit and method provide the capability to control the output power of the electrosurgical generator without having to actually monitor the amplitude of both the output current and output voltage. This allows for a simple constant power control circuit and method which operate to control the power output without having to calculate the actual power output of the electrosurgical generator. 
     While the constant power control circuit may be used to control electrosurgical generators of varying designs, it is preferred that the electrosurgical generator includes a power selection system wherein the user may initialize, set, monitor, and/or control the operation of the electrosurgical generator. It is also preferred that the power selection system produces a control voltage signal that acts to control a high voltage direct current supply which in turn acts to supply a high voltage signal to an output switching radio frequency stage. Then output switching radio frequency stage creates an electrosurgical energy between two output electrodes. The preferred electrosurgical generator need not be limited to these three functional elements, for example the electrosurgical generator could also include additional safety, monitoring, signal modification/conditioning, and/or feedback circuitry or functional elements/processes. The actual electrosurgical generator&#39;s design may include the use of digital components and signaling and/or analogue components and signaling, or may be embodied, completely or partially within a software process running on hardware components. 
     The constant power control circuit includes a current sampling circuit, a linear conversion circuit, and a feedback correction circuit. The current sampling circuit is coupled to one of the output electrodes, and functions so as to produce a sampled current signal that is proportional to the average current flowing through the output electrode. 
     The linear conversion circuit which is connected to the current sampling circuit internally generates one or more multiplier reference signals and one or more offset reference signals, each of which is used to modify the sampled current signal in accord with the crest factor associated with the electrosurgical energy output by the electrosurgical generator; the modified signal being a linear converted signal. 
     The feedback correction circuit which is electrically connected to receive the linear converted signal from the linear conversion circuit and the control voltage signal from the power selection system functions to produce a feedback control signal which it then supplies to the power selection system, within the electrosurgical generator, so as to cause the power selection system to control the amount of electrosurgical energy created. The feedback correction circuit functions so as to determine the difference in amplitude between the control voltage signal and the linear converted signal and to then add this difference to the control voltage signal to produce a feedback control signal. The feedback correction circuit may also be connected to the primary transformer winding within the output switching radio frequency stage, or its equivalent, thereby allowing the feedback correction circuit to detect high impedance loading between the output electrodes and to reduce the amplitude of the feedback control signal to protect the circuitry and/or the patient from excessive current and/or voltage levels. A high impedance load is generally considered to be above 2500 ohms. The feedback correction circuit may also include circuitry or processes that substitute another signal for the feedback control signal when the impedance loading between the output electrodes is calculated as being low. A low impedance load is generally considered to be below 300 ohms. Both high and low impedance limits may be adjusted to match the instruments, processes, and/or procedures as necessary. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 presents an electrosurgical generator interfaced to a constant power control circuit having a current sampling circuit, linear conversion circuit and feedback correction circuit. 
     FIG. 2 is the preferred embodiment of the linear conversion circuit shown in FIG.  1 . 
     FIG. 3 is the preferred embodiment of the feedback correction circuit shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For an electrosurgical generator  101  having a high voltage direct current (DC) supply  103  which is electrically connected to control an output switching radio frequency (RF) stage  105 , a unique linear relationship exists between the control voltage supplied to the high voltage DC supply  103  and the root-mean-square (RMS) current generated by the electrosurgical generator  101 . This unique linear relationship can be used to design a constant power control circuit  107  that functions as a feedback control loop to control the electrosurgical generator  101 . The following mathematical derivations define this unique linear relationship. 
     It can be shown that: 
     
       
         V control =V dc /K ps ; 
       
     
     where, 
     V control =a control voltage supplied to the high voltage DC supply, 
     V dc =the output voltage signal of the high voltage DC supply, and 
     K ps =a feedback ratio of the high voltage DC supply. 
     It can further be shown that: 
     
       
         V dc   2 ×K a =P out ; 
       
     
     where, 
     P out =the output power of the electrosurgical generator  101 , and 
     K a =a linear constant (which can be empirically derived). 
     Therefore, the output power of the electrosurgical generator  101  is directly proportional to the square of the output voltage signal of the high voltage DC supply. 
     Thus, by substitution: 
     
       
         (V control ×K ps ) 2 ×K a =P out , or 
       
     
     
       
         V control   2 ×K g =P out ; 
       
     
     where, 
     
       
         k g =K ps   2×K   a . 
       
     
     Therefore, the output power of the electrosurgical generator  101  is proportional to the square of the control voltage supplied to the high voltage DC supply. 
     Examining the output of the generator we have: 
     
       
         P out =V rms ×I rms ; 
       
     
     where, 
     V rms =output RMS voltage of the electrosurgical generator  101 , and 
     I rms =output RMS current of the electrosurgical generator  101 . 
     Accordingly, at a given load impedance=R: 
     
       
         V rms =I rms ×R, 
       
     
     and by substitution 
     
       
         P out =I rms   2 ×R. 
       
     
     By allowing R to equal a ‘matched’ load impedance we have 
     
       
         V control   2 ×K g =I rms   2 ×R, 
       
     
     and therefore 
     
       
         V control   2 =I rms   2 ×R/K g . 
       
     
     Consequently, for a given impedance K r =R/K g  the equation can be simplified to: 
     
       
         V control   2 =I rms   2 ×K r . 
       
     
     Therefore, the square of the control voltage supplied to the high voltage DC supply  103  is directly proportional to the square of the output RMS current of the electrosurgical generator  101 . It can also be shown by similar derivation that the square of the control voltage supplied to the high voltage DC supply  103  is directly proportional to the square of the output RMS voltage of the electrosurgical generator  101 . 
     Thus, the above derivation implies that if either the output RMS current or voltage is sampled properly (I sample  &amp; V sample  respectively) the control voltage supplied to the high voltage DC supply  103  may be used as a reference value in a feedback control loop to keep either the output RMS current or output RMS voltage constant. When the linear relationship of I rms  to I sample  is ‘mapped’ into the linear relationship of V control  to I rms  then a linear relationship can be derived between V control  and I samples . When the scaling is done properly for a given power setting, V control  will equal I sample  at the ‘matched’ load impedance. Therefore, in a feedback circuit designed with the above mapping a feedback loop which keeps I sample  equal to V control  will by definition keep I rms  constant. 
     In accord with the above presented mathematical derivation, we have designed a constant power control circuit  107  for the electrosurgical generator  101 , shown in FIG. 1, having a power selection system  109  that produces a control voltage signal to control a high voltage direct current supply  103  which supplies a high voltage signal to an output switching radio frequency stage  105  thereby creating an electrosurgical energy between two output electrodes  111 . The preferred electrosurgical generator  101  has a plurality of operational modes selectable within the power selection system  109 , and a primary transformer winding  113  within the output switching radio frequency stage  105 , as shown in FIG.  1 . 
     The constant power control circuit  107 , shown in FIG. 1, includes a current sampling circuit  115 , a linear conversion circuit  117  and a feedback correction circuit  119 . 
     In the preferred embodiment, the current sampling circuit  115  is inductively coupled to one of the output electrodes  111 , as shown in FIG.  1 . Alternatively the current sampling circuit  115  could be actively coupled, in circuit, with the output electrode. 
     The current sampling circuit  115  produces a sampled current signal that is proportional in amplitude to the average current flowing from the electrosurgical generator  101  through the one output electrode, an impedance load  121 , and returning to the electrosurgical generator  101  through another output electrode. 
     The preferred embodiment of the current sampling circuit  115  includes an inductive coil element, similar in design and function to that of a secondary winding of a current transformer. Additional circuit elements function to transform the induced current into a proportional voltage signal and include a voltage drop resistor, a calibrating variable resistor, and elements that rectify and average the sampled current signal. 
     The current sampling circuit  115  supplies the sampled current signal to the linear conversion circuit  117 . However, before the sampled current signal can be used as a feedback term, the mode crest factor for the selected electrosurgical generator  101  operational mode, needs to be compensated for. The linear conversion circuit  117 , in FIGS. 1 and 2, compensates for the linear relationship between the sampled current signal and a ‘true’ sampled RMS value, which is of the form I rms =m×I sample +b, where I rms  is a signal which is directly proportional to the RMS current, and m and b are given constants derived for a given crest factor. While electrosurgical generators  101  have a wide variety of different output wave shapes with varying crest factors, it is preferred that the crest factor for a given mode be significantly constant over a finite patient tissue impedance range, such as between 300 and 2500 ohms. 
     Accordingly, the linear conversion circuit  117  first multiples the sampled current signal by the gain, m, and then adds the offset to it, b. When the values of m and b are chosen properly the resulting linear converted signal is directly proportional to the output RMS current of the electrosurgical generator  101 . The preferred method for determining the proper values of m and b for a given operational mode and electrosurgical generator  101  includes collecting empirical data on the control voltage supplied to the high voltage DC supply  103  and the resulting output RMS current of the electrosurgical generator  101  and solving the linear equation, for m and b, by substitution. 
     The linear conversion circuit  117 , shown in FIGS. 1 and 2, is electrically connected to the current sampling circuit  115 . In the preferred embodiment, the linear conversion circuit  117  is also electrically connected to the power selection system  109  such that the operational mode of the electrosurgical generator  101  can be determined based on this connection. The linear conversion circuit  117  generates a linear converted signal and supplies this signal to the feedback correction circuit  119 . 
     The preferred embodiment includes a linear multiplier generating means  201  within the linear conversion circuit  117 , see FIG.  2 . The linear multiplier generating means  201  generates a plurality of unique multiplier reference signals (i.e., a factor ‘m’). There is preferably one, unique, multiplier reference signal for each operational mode. The preferred embodiment, of the linear multiplier generating means  201  includes several resistive components connected to voltage sources, across which a predetermined voltage is maintained. 
     The preferred embodiment includes a linear offset generating means  203  within the linear conversion circuit  117 , see FIG.  2 . The linear offset generating means  203  generates a plurality of unique offset reference signals (i.e., a factor ‘b’). There is preferably one, unique, offset reference signal for each operational mode. The preferred embodiment of the linear offset generating means  203  includes several resistive components connected to voltage sources, across which a predetermined voltage is maintained. 
     The preferred embodiment also includes a plurality of multipliers  205 , within the linear conversion circuit  117 , see FIG.  2 . There is preferably one, corresponding, multiplier  205  for each operational mode. Each multiplier  205  is electrically connected to receive the sampled current signal and one unique multiplier reference signal from the linear multiplier generating means  201 . Each multiplier  205  multiplies the sampled current signal and the unique multiplier reference signal associated with one operational mode to produce a unique multiplied signal for that operational mode. The preferred embodiment of the multiplier  205  includes a plurality of operational amplifiers. 
     The preferred embodiment includes a plurality of summers  207 , within the linear conversion circuit  117 , see FIG.  2 . There is preferably one, corresponding, summer  207  for each operational mode. Each summer  207  is electrically connected to receive a unique multiplied signal and one unique offset reference signal from the linear offset generating means  203 . Each summer  207  sums the offset reference signal associated with one operational mode and the unique multiplied signal associated with that operational mode to produce a unique linear converted signal for that operational mode. The preferred embodiment of the summer  207  includes configuring the plurality of operational amplifiers used as multipliers  205  to also function as summers  207 . 
     The preferred embodiment includes a mode monitor  209 , within the linear conversion circuit  117 , see FIG.  2 . The mode monitor  209  is electrically connected to the power selection system  109 , for identifying the operational mode of the electrosurgical generator  101  and producing an identified operational mode signal therefrom. 
     Closely associated with the mode monitor  209 , is a signal selector  211  that is also within the linear conversion circuit  117 , see FIG.  2 . The signal selector  211  is electrically connected to receive the identified operational mode signal and the unique linear converted signal from each of the summers  207 . The signal selector  211  selects the unique linear converted signal associated with the identified operational mode, and causes that linear converted signal to be supplied to the feedback correction circuit  119 . In the preferred embodiment the mode monitor  209  and signal selector  211  are embodied within a circuit including a digital processing component that activates and/or deactivates a plurality of electronic switching elements. 
     The feedback correction circuit  119 , shown in FIGS. 1 and 3, is electrically connected to receive the linear converted signal from the linear conversion circuit  117 , the control voltage signal from the power selection system  109 , and the voltage signal across the primary transformer winding  113 . The feedback correction circuit  119  produces a feedback control signal and supplies the feedback control signal to the power selection system  109  so as to control the amount of electrosurgical energy created by the electrosurgical generator  101 . 
     The feedback correction circuit  119  includes a subtractor  301 , see FIG.  3 . The subtractor  301  is electrically connected to receive the linear converted signal from the linear conversion circuit  117  and the control voltage signal which is generated by the power selection system  109  and supplied to the high voltage DC supply, see FIGS. 1 and 3. The subtractor  301  determines the difference in amplitude between the control voltage signal and the linear converted signal, and produces a delta signal proportional to the difference. The preferred embodiment of the subtractor  301  includes an operational amplifier component. 
     Also included in the feedback correction circuit  119  is an adder  303 , see FIG.  3 . The adder  303  is electrically connected to receive the delta signal and the control voltage signal. The adder  303  adds the delta signal to the control voltage signal to produce the feedback control signal. The preferred embodiment includes an operational amplifier component. 
     Since holding the output RMS current constant for all impedances would be a physical impossibility based on the design limitations of the high voltage DC supply  103  and the output switching RF stage  105 , it is preferred that the feedback control signal to the high voltage DC supply  103  be limited as a function of the impedance load  121  between the output electrodes  111 . 
     In the preferred embodiment, the feedback correction circuit  119  includes a maximum control voltage reference generator  305  for generating a maximum control voltage reference signal, see FIG.  3 . The preferred embodiment uses an operational amplifier component connected to the control voltage signal to establish a maximum control voltage reference signal based thereon. 
     The maximum control voltage reference signal is supplied to a switcher  307  within the preferred feedback correction circuit  119 , see FIG.  3 . The switcher  307  is also electrically connected to receive the feedback control signal from the adder  303 . The switcher  307  substitutes the maximum control voltage reference signal for the feedback control signal when the feedback control signal is greater in amplitude than the maximum control voltage reference signal, thereby limiting the electrosurgical generator&#39;s  101  output current through the output electrodes  111  when the impedance load  121  is at a low impedance level. The preferred embodiment of the switcher  307  includes an AND circuit created with diodes that passes the lower of the two signals as the feedback control signal. 
     When the impedance load  121  between the output electrodes  111  is high, the preferred constant power control circuit  107  should limit the output voltage of the electrosurgical generator  101  so as protect the electrosurgical generator  101 , and reduce leakage currents and exit sparking. 
     In the preferred embodiment, the feedback correction circuit  119  shown in FIG. 3, includes a high impedance reference generator  309  for generating a high impedance reference signal. The high impedance reference generator  309  is electrically connected to receive the control voltage signal. The preferred high impedance reference generator  309  establishes the high impedance reference signal by linearly converting the control voltage signal with an operational amplifier. 
     In the preferred embodiment a connector  311  is used for electrically connecting a comparator  313 , within the feedback correction circuit  119 , to the primary transformer winding  113 , see FIGS. 1 and 3. The connector  311  provides the comparator  313  with the voltage across the primary transformer winding  113 . The comparator  313  is also electrically connected to receive the high impedance reference signal. The comparator  313  compares the amplitude of the high impedance reference signal to the voltage across the primary transformer winding  113  and produces a high impedance detection signal that indicates the results of this comparison. In the preferred embodiment the comparator  313  includes an operational amplifier component. 
     The high impedance detection signal is received by a reducer  315 , shown in FIG. 3 of the preferred embodiment, which is electrically connected to the comparator  313  and to the switcher  307 . The reducer  315  reduces, to an internally generated preset reduced voltage level signal, the amplitude of the feedback control signal from the switcher  307  when the voltage across the primary transformer winding  113  is greater than the high impedance reference signal as indicated by the high impedance detection signal. In the preferred embodiment, the reducer  315  includes a logic driven switched circuit and an adjustable resistor providing a reduced voltage level signal. The reducer  315  supplies the resulting feedback control signal to the power selection system  109 . 
     Associated with the constant power control circuit  107  is a method for maintaining a generally constant output power from an electrosurgical generator  101  having a power selection system  109  that produces a control voltage signal to control a high voltage direct current supply  103  which supplies a high voltage signal to an output switching radio frequency stage  105  thereby creating an electrosurgical energy between two output electrodes  111 . 
     The method includes the steps of inductively coupling to one output electrode, sensing the current flowing through the output electrode  111  and producing a sampled current signal proportional to the average current flowing through the output electrode. The method then continues with the steps of generating a multiplier reference signal, generating an offset reference signal, multiplying the sampled current signal and the multiplier reference signal, and then summing the offset reference signal to the product to producing a linear converted signal. 
     The method continues with the steps of connecting to the control voltage signal from the power selection system  109 , determining the difference in amplitude between the control voltage signal and the linear converted signal, adding the difference determined by the subtraction means to the control voltage signal to produce a feedback control signal, and then supplying the feedback control signal to the power selection system  109  to control the amount of electrosurgical energy created. 
     To protect the electrosurgical generator  101  and the patient when the impedance load  121  is high, the method can include the steps of generating a high impedance reference signal, connecting to the primary transformer winding  113 , comparing the amplitude of the high impedance reference signal to the voltage across the primary transformer winding  113 , and reducing the amplitude of the feedback control signal when the voltage across the primary transformer winding  113  is greater than the high impedance reference signal. 
     To protect the electrosurgical generator  101  and patient when the impedance load  121  is low, the method can include the steps of generating a maximum control voltage reference signal and substituting the maximum control voltage reference signal for the feedback control signal when the feedback control signal is greater in amplitude than the maximum control voltage reference signal. 
     For electrosurgical generators  101  having a plurality of operational modes, the method can be modified to include the steps of generating a plurality of unique linear multiplier reference signals, one for each operational mode, and generating a plurality of unique linear offset reference signals, one for each operational mode. The method would then include the steps of multiplying the sampled current signal, separately and concurrently, with each of the unique multiplier reference signals to produce a plurality of unique multiplied signals, one for each operational mode, and then summing each of the unique multiplied signals with the offset reference signal associated with the same operational mode to produce a plurality of unique linear converted signals, one for each operational mode. The method would continue with the steps of connecting to the power selection system  109  to identify the operational mode selected, selecting the unique linear converted signal that matches the identified operational mode, and then causing that linear converted signal to be supplied to the feedback correction circuit  119 .