Abstract:
An electrosurgical generator is disclosed that provides radio-frequency electrical waveforms for performing surgical operations on a tissue mass. The various aspects of the present invention are embodied in an electrosurgical generator that includes a DC regulator, an amplifier, an energy recovery circuit, and a method of controlling these components to generate a desired electrical waveform for an electrosurgical operation.

Description:
REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 08/702,282 entitled “IMPROVED ELECTROSURGICAL GENERATOR” filed Aug. 23, 1996, now U.S. Pat. No. 5,836,943, which prior application is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to electrosurgical generators in general and, in particular, to an improved electrosurgical generator capable of supplying a plurality of radio frequency electrical waveforms for surgical procedures. 
     BACKGROUND 
     Electrosurgery involves the application of radio-frequency electrical energy to tissue to produce a surgical operation. Electrosurgery is generally performed with a generator that converts electrical energy from a power source to a predetermined radio-frequency waveform that is delivered to the tissue through an active electrode and a return path. 
     There are essentially four main surgical operations that are electrically performed on tissue, depending on the radio-frequency waveform output by the generator. These operations are typically described as desiccation, fulguration, cutting and cutting with hemostasis. 
     For a desiccation operation, the generator outputs a radio-frequency waveform that heats the tissue, by electrical resistance heating due to current flowing through the tissue, sufficient to produce an area of necrosis. 
     For a fulguration operation, the generator typically outputs a burst waveform which has a high peak voltage but a low duty cycle. Due to the low duty cycle of the fulgurating waveform, the power per unit time applied to the tissue is low enough so that explosive vaporization of cell moisture is minimized. The burst waveform forms a radio-frequency spark or arc between the active electrode and the tissue, thereby delivering power over the area of the spark or arc tissue contact and providing coagulation of the tissue in the immediate vicinity of the spark or arc. 
     Other operations can be performed with still different waveforms output by an electrosurgical generator. Cutting occurs when sufficient power per unit time is delivered to the tissue to vaporize cell moisture. Cutting is typically performed with a repetitive voltage waveform, such as a sinusoid, which produces a cut with very little necrosis and little hemostasis. 
     It is also possible to achieve a combination of the above operations by varying the electrical waveform produced by the generator. In particular, a combination of cutting and desiccation (called cutting with hemostasis or blend) can be produced by periodically interrupting the continuous sinusoidal voltage normally used to produce an electrosurgical cut. 
     Known electrical generators which are capable of producing one or more of the above-described surgical operations are generally designed as in FIG.  1 . The AC power mains  200  provide AC electrical power to AC/DC converter  202 , which provides unregulated DC power to the DC regulator  206 . Under the control of clinician  208 , control and timing circuitry  210  causes the DC regulator  206  to produce DC power of a specified value to the tuned RF amplifier  212 . The control and timing circuitry  210  also produces RF signals for amplification by the tuned RF amplifier  212 . This results in RF power signals being delivered to the patient  214 . 
     Known electrosurgical generators are subject to one or more limitations. For example, some generators are limited in the degree to which they can generate more than one individual waveform without producing an admixture of inappropriate effects, thus they are limited in the number of electrical waveforms that are appropriate for surgical operations. 
     Another limitation is that known generators emit a substantial amount of electromagnetic interference to the environment. Electromagnetic interference poses a serious risk in operating rooms where it can cause malfunction or failure of electronic equipment. A primary source of the electromagnetic interference is the substantial pulsating currents which are created in electrosurgical generators circuits. 
     There are primarily two sources of electromagnetic interference (EMI) in known generators. Such EMI consists of conducted EMI, nearfield EMI and radiated EMI. A primary source of the conducted EMI, which is sent back into the AC power lines and carried to equipment at distant locations in the hospital and beyond, is produced by the substantially pulsating currents which are created in the DC regulator  206 . A primary source of the nearfield and radiated EMI is the harmonic content of the tuned amplifier output. The harmonic components couple much better to the environment, and are radiated away more effectively. As will be shown, a key aspect of this invention is the simultaneous reduction of conducted, nearfield and radiated EMI. 
     Another limitation of known electrosurgical generators is their relatively low efficiency in converting and amplifying electrical power from the power source to the tissue, resulting in the dissipation of electrical energy as heat. Heat dissipation by an electrosurgical unit (ESU) within an operating room is objectionable due to the generation of convective air currents and the associated circulation of airborne pathogens. The additional heat dissipation requirement increases the weight and volume of the ESU. Furthermore, the reliability of the electrosurgical unit typically decreases as the heat dissipation increases. 
     Low efficiency in ESU&#39;s are caused by a number of effects: 
     (1) Topology selected, which determines intrinsic efficiency (maximum achievable efficiency under optimum conditions); 
     (2) Loading, which determines extrinsic efficiency (efficiency achieved with given topology into a given load); 
     (3) Component selection, which determines realized efficiency (efficiency with a given topology, load and selection of components). 
     Ideally, a topology is selected which maximizes the extrinsic and realized efficiency over a wide range of conditions. In known ESU&#39;s, in order to achieve cutting with a minimum of hemostasis, AC ripple voltage present on the DC regulator output should be minimized. At the same time, the conducted EMI should be reduced as much as possible. To do this, large size capacitors are sometimes added to the AC/DC convertor  202 , in FIG. 1, in an attempt to smooth the current pulses, reducing conducted EMI, while at the same time large capacitors are added to the output of the DC regulator  206  to reduce output ripple and hence reduce hemostasis. These capacitors filter the current by passing the ripple component to ground through the ESR of the capacitor, thereby wasting power. This loss and bulk would be greatly reduced if less AC ripple were generated, and hence less power wasted. 
     Control devices, such as transistors, are often used in both the DC regulator  206  and the RF amplifier  212  circuits to synthesize and regulate the electrical waveform applied to tissue. These control devices may be used in a variety of ways. A very common method in prior art has been to use the control devices as variable impedance current sources which results in the simultaneous application of voltage and current across the transistor and thereby a dissipation of power within the transistor. Control devices are also used as alternating low impedance (i.e., closed) and high impedance (i.e., open) switches. In prior art, some generator circuits dissipate a substantial amount of power in such switches due to transitioning the switches to low impedance while a voltage exists across the switch and thereby dissipating power due to the simultaneous presence of voltage and current in the switch. Some topologies of generator circuits which contain transistors often cannot tie the biasing of the transistors to a common reference node, thereby requiring relatively complicated level shifting circuitry. 
     Some known electrosurgical generators&#39; topologies convert the input voltage to an output voltage through a process that includes storing input energy inductively in the form of a DC magnetic field during one interval and releasing the energy as an oscillating voltage across a load during a subsequent interval. This process of storage and release of energy results in a waveform in the form of a damped sinusoid which has a significant amplitude remaining at the time of the next storage cycle. For some output waveforms, such as pulsed energy waveforms, energy not sent to the load by the end of the pulse remains in the generator where it is dissipated as heat, decreasing the generator&#39;s efficiency. 
     Consequently, there is a need for a generator that addresses such limitations of known electrosurgical generators. 
     SUMMARY OF THE INVENTION 
     Accordingly, objectives of the present invention include the following: 
     To provide an electrosurgical generator with reduced generation of electromagnetic interference. 
     To provide an electrosurgical generator with improved efficiency. 
     To provide an electrosurgical generator with current isolation between an input power source and an output load. 
     To provide an electrosurgical generator with a reduced number and size of electrical components. 
     To provide a switching DC regulator for an electrosurgical generator wherein input and output current ripple is substantially reduced. 
     To provide a switching DC regulator for an electrosurgical generator with an adjustable output DC voltage that can be increased (step-up) or decreased (step-down) relative to the input DC voltage. 
     To provide an amplifier for an electrosurgical generator that converts a DC input voltage to a radio-frequency signal that provides surgical effects on tissue with reduced generation of electromagnetic interference and increased efficiency. 
     To provide an energy recovery circuit for an electrosurgical generator that selectively stores and releases energy within the generator to increase the efficiency of energy delivery to the tissue. 
     To provide an electrosurgical generator whereby the flow of energy to the tissue is controlled in response to a sensed tissue condition to provide improved surgical effects. 
     One or more of the above objectives are addressed by providing a generator that comprises an inventive DC regulator, amplifier, and energy recovery circuit. These generator components can be controlled in an inventive manner to convert energy from a power source to a range of predetermined radio-frequency waveforms to provide electrosurgical operations, e.g., desiccation, fulguration, cutting, or cutting with hemostasis. 
     The DC regulator and the amplifier are connected in series between a power source (e.g., a battery or AC-to-DC converter) and the tissue. Generally, the power source provides a DC voltage to the DC regulator. The DC regulator converts the input DC voltage to a range of DC output voltages that can be greater (step-up) or lesser (step-down) than the DC input voltage. The DC output voltage flows into the amplifier where it is converted to a range of radio-frequency voltage waveforms which are delivered to the tissue. The energy recovery circuit stores and releases energy generated by the amplifier to increase the efficiency with which energy is transferred from the power source to the tissue. 
     According to one aspect of the invention, an inventive switched DC regulator is provided that achieves increased efficiency, a reduced generation of electromagnetic interference, and a reduced number of circuit components. The switched DC regulator converts a first DC signal from a power source into a second DC signal having a predetermined voltage. 
     The switched DC regulator includes an input inductor means (e.g., one or more inductors) for reducing current ripple in the first DC signal, capacitor means (e.g., one or more capacitors) for capacitively storing and releasing energy, first switch means (e.g., bipolar transistor, diode, insulated gate bipolar transistor, or field effect transistor) for alternately charging the capacitor means with the first DC signal and second switch means for discharging the capacitor means to generate the second DC signal, and output inductor means (e.g., one or more inductors) for reducing current ripple in the second DC signal. 
     The input inductor means is connected in series between the power source and the capacitor means. The capacitor means is connected in series between the input inductor means and the output inductor means. Energy is capacitively transferred from the input inductor means to the output inductor means by the first switch means charging the capacitor means with the first DC signal and the second switch means discharging the capacitor means through the output inductor means to generate the second DC signal. The voltage of the second DC signal is controlled by adjusting the duty ratio of the first and second switch means, i.e., adjusting the ratio of the time that the capacitor means is charged to the total time over which the capacitor means is charged and discharged. The second DC signal voltage can be higher (step-up) or lower (step-down) than the first DC signal voltage. 
     The current ripples in the first DC signal and the second DC signal are further reduced by properly magnetically coupling the input inductor means and the output inductor means. Proper magnetic coupling is achieved by considering the coefficient of coupling K and the turns ratio N of a transformer. Moreover, proper magnetic coupling occurs when K substantially equals N for the transformer. Such substantial equivalence may be obtained either by using a transformer designed such that K is substantially equal to N or by using a transformer in conjunction with one or more auxiliary inductances, such auxiliary inductances selected so that K is substantially equal to N. DC isolation between the first DC signal and the second DC signal is achieved by the capacitor means including a first and a second capacitor with an isolation transformer interposed between the capacitors. Current ripples in the first DC signal and the second DC signal are substantially reduced to zero by magnetically coupling the input and output inductor means and the isolation transformer. 
     The efficiency of the DC regulator is substantially improved by selecting the input inductor means, the output inductor means, and the capacitor means to provide a substantially zero voltage across the switch means and a substantially zero instantaneous rate of change of voltage across the switch means prior to the switch means closing to charge the capacitor means. In this manner, energy dissipation in the switch means is substantially eliminated by avoiding the simultaneous application of a voltage across the switch means and a current through the switch means. 
     According to another aspect of the present invention, an inventive amplifier is provided that converts the second DC signal generated by the DC regulator into a radio-frequency output signal having a predetermined frequency appropriate for achieving electrosurgical effects. The inventive amplifier yields increased efficiency and a reduced generation of electromagnetic interference. 
     The amplifier includes input inductor means in series with the DC regulator or another DC source (e.g., AC-to-DC converter, or battery), a resonant circuit connected in series with the input inductor means, and switch means in parallel with the resonant circuit. The input inductor means reduces the current ripple in the second DC signal and thereby reduces the radiated electromagnetic interference. The resonant circuit includes an inductor, a capacitor, and the tissue. The switch alternately connects (closed switch) and disconnects (open switch) a junction between the input inductor and the resonant circuit to a current return path for the amplifier, thereby periodically charging the resonant circuit with the second DC signal and discharging energy as the output signal. The magnitude and frequency of the output signal is regulated by adjusting the duty ratio and period of the switch, i.e., adjusting the ratio of the time that the resonant circuit is charged to the total time over which the resonant circuit is charged and discharged. 
     The components of the resonant circuit are selected to provide a substantially zero voltage and zero rate of change of voltage across the switch prior to the switch closing to charge the input inductor. In this manner, energy dissipation in the switch is substantially eliminated by avoiding the simultaneous application of a voltage potential across the switch and a current through the switch, and the sensitivity of the amplifier circuit to component tolerances is substantially reduced. 
     According to another aspect of the present invention, an energy recovery circuit is provided for use in an electrosurgical generator that improves the efficiency of energy delivery to the tissue. An electrosurgical generator synthesizes varying width bursts/waveforms of radio-frequency energy to create the various types of surgical operations. At the end of a burst type output signal, the energy that has not been delivered to the tissue remains within the generator where it is dissipated as heat. The energy dissipated within the generator can be quite high when the resistance of the tissue is relatively high. The energy recovery circuit substantially reduces these loses by recovering the energy remaining within the electrosurgical generator at the end of a burst/waveform. The energy recovery circuit generally includes an energy storage device(s) (e.g., capacitor, inductor, or combination thereof), a switch(es) (e.g., bipolar transistor, insulated gate bipolar transistor, or field effect transistor) that alternatively stores and releases energy between the energy storage device(s) and the electrosurgical generator, and a switch controller that regulates the storing and release of energy. 
     Generally, to reduce the energy lost at the end of a burst output signal, the switch controller toggles the switch to alternately store energy in the energy recovery circuit near the end of a burst and then to release the stored energy during a subsequent burst. In this manner, the energy can be selectively stored and later released to increase the efficiency of energy transfer to the tissue. As can be appreciated, the energy recovery circuit can be controlled to store and release energy at any time and is thereby not limited to storing energy at any time and is thereby not limited to storing energy at particular times, such as near the end of a burst output signal. 
     According to another aspect of the present invention, a method for operating an electrosurgical unit is provided whereby the flow of energy to the tissue is controlled in response to a sensed tissue condition to provide improved surgical effects, e.g., desiccation, fulguration, cutting, or cutting with hemostasis. It has been found that the complex impedance of tissue provides information about the condition of the tissue and thereby the condition of a surgical effect. 
     The complex impedance of tissue includes a resistance and a capacitance. Generally, tissue includes cells and fluid. Tissue resistance is created by the electrical conduction path through the fluid. Tissue capacitance is created by the cell membranes which provide an electrical insulating effect around the electrically conducting fluid within the cells. Cell membranes puncture/burst when a sufficient voltage is applied across the tissue. After the cell membrane bursts, the capacitive effect of the membrane is substantially reduced and the associated complex impedance of the tissue becomes more resistive and less capacitive. The complex impedance of the tissue is further changed when sufficient energy is dissipated in the tissue to vaporizes some of the fluid thereby causing an increase in resistance. Additional changes in the complex impedance are created through effects such as the denaturing and recombining of proteins in response to heating. 
     It has further been found that the complex impedance of the tissue can be measured over a time period to observe the extent, if any, of cellular membrane resealing. For example, cells which have not been destroyed by electrosurgical energy can reseal small holes in the cell membrane over a period of about a millisecond to a second. Measuring the change and rate of change of tissue&#39;s complex impedance in between or during delivery of electrosurgical energy provides information about the condition of the tissue and the associated surgical effect. 
     The present method for operating an electrosurgical unit includes controlling the delivery of energy to the tissue in response to the sensed tissue&#39;s complex impedance and/or rate of change of the complex impedance to provide improved surgical effects. More particularly, a sensor that uses an impedance controller for use with an electrosurgical generator to sense the complex impedance of the tissue is included in the present invention where the impedance controller regulates the output, for example the voltage that is converted to a RF signal, from the generator circuit in response to the change in the measured complex tissue impedance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and further advantages thereof, reference is now made to the following Detailed Description, taken in conjunction with the Drawings, in which: 
     FIG. 1 is a prior art embodiment of an electrosurgical generator; 
     FIG. 2 is a block diagram illustrating the components of the generator; 
     FIG. 3 is a schematic of a DC regulator embodiment according to the present invention; 
     FIG. 4 is a schematic of an isolated DC regulator embodiment according to the present invention; 
     FIG. 5 is a schematic of an amplifier embodiment according to the present invention; 
     FIG. 6 is a schematic of one embodiment of an energy recovery circuit; 
     FIG. 7 is a schematic of an energy recovery circuit embodiment in combination with the amplifier of FIG. 5 according to the present invention; 
     FIG. 8 a  is a distributed complex tissue impedance model of a tissue sample; 
     FIG. 8 b  is a sample of a tissue structure; and 
     FIG. 9 is a block diagram of an tissue impedance controller according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 2, there is shown a block diagram of an electrosurgical generator constructed according to the principles of the present invention. The electrosurgical generator includes a DC regulator  10 , an amplifier  100 , an energy recovery circuit  90 , and a controller  70 . 
     The DC regulator  10  receives an input DC voltage from a power source  12  and converts the input DC voltage into an output DC voltage that is provided to the amplifier  100 . The amplifier  100  converts the DC voltage from the regulator  10  into a radio-frequency output signal that is provided to a tissue mass  11 . The energy recovery circuit  90  alternately stores energy from the amplifier  100  and releases energy back to the amplifier  100  to increase the efficiency at which energy is transferred from the power source  12  to the tissue  11 . The controller  70  regulates the DC regulator  10 , the amplifier  100 , and the energy recovery circuit  90  to create a predetermined radio-frequency output signal from the electrosurgical generator that is operative for performing a desired electrosurgical operation, e.g., desiccation, fulguration, cutting, or cutting with hemostasis. 
     The inventive DC regulator  10  is described first. That description is followed by a description of the inventive amplifier  100  and then the inventive energy recovery circuit  90 . Finally, a tissue impedance controller  109  (FIG. 9) is described that controls the flow of energy to the tissue in response to a sensed tissue condition to provide improved surgical effects. 
     In one aspect of the invention the DC regulator  10  (FIG. 3) converts an input DC voltage from the power source  12  to an output DC voltage that can be higher or lower than the first DC voltage. The DC regulator  10  achieves this conversion with a higher efficiency, a substantially reduced radiation of electromagnetic interference, and a lower number and smaller size of components than known electrosurgical generators. 
     The DC regulator  10  includes an input inductor  16  in series with the DC power source  12 , an output inductor  18  in series with the amplifier circuit  100 , and an energy transferring circuit  20 . The energy transferring circuit  20  includes a storage capacitor  24 , a switch  26  (e.g., insulated gate bipolar transistor) for alternately connecting (i.e., closed switch and disconnecting (i.e., open switch) a first junction  28  between the input inductor  16  and the storage capacitor  24  to a current return path  30  of the power source  12 , and a diode  32  for alternately connecting (i.e., diode forward biased) and disconnecting (i.e., diode reversed biased) a second junction  34  between the storage capacitor  24  and the output inductor  18  to the current return path  30 . A filter capacitor  36  is connected across the output of the DC regulator  10 . 
     During the interval when the switch  26  is open, the diode  32  is forward biased and the capacitor  24  is charging through the input inductor  16  which reduces the input current ripple and radiated electromagnetic interference. During the interval when the switch  26  is closed, the capacitor  24  is connected across the diode  32 , thereby reverse biasing the diode  32 . The capacitor  24  discharges through the output inductor  18  and the amplifier  100 . The output inductor  18  reduces the output current ripple and radiated electromagnetic interference. The switching cycle is then repeated by the switch  26  opening to forward bias the diode  32  and to recharge the capacitor  24  through the input inductor  16 . 
     In this manner, the DC regulator  10  capacitively transfers energy from the power source  12  to the amplifier  100 . Capacitive energy transfer is substantially more effective on a per unit size and weight basis than the inductive energy transfer used in prior electrosurgical generators. For example, a capacitor of 1 microfarad charged to 50 V has a stored energy of 1.25 mJ, equal to an inductor of 2.5 mH passing 1 A. The size of a 1 microfarad 50 V capacitor, however, is considerable smaller than a 2.5 mH 1 A inductor. Furthermore, capacitive energy transfer is more efficient than inductive energy transfer which has a relatively high loss of transferred energy due to resistive heating of the inductor. 
     The DC output voltage from the DC regulator  10  can be higher or lower than the DC input voltage from the power source  12  and is adjusted according to the following formula: 
     
       
           V output/ V input= D/D′   
       
     
     where: 
     Voutput is the DC output voltage; 
     Vinput is the DC input voltage; 
     D is the fractional time that the switch  26  is closed (i.e., the time that the switch  26  is closed divided by the time for one cycle between the switch closing a first and then a second time); and 
     D′ is the fractional time that the switch is open (i.e., D′=(1−D)). 
     In this manner, the output voltage can be adjusted lower than the input voltage (step-down conversion) for D&lt;0.5 or above the input voltage (step-up conversion) for D&gt;0.5. 
     The controller  70  adjusts the output voltage by opening the switch  26  (i.e., biasing the transistor to achieve low impedance) and closing the switch  26  (i.e., biasing the transistor to achieve high impedance) according the formula above. The controller  70  in FIG. 3 performs current feedback control by sensing the output current at node  38  and adjusting the duty cycle of the switch  26  to maintain the voltage and/or current of the DC regulator  10  within a predetermined range to provide a desired surgical effect. 
     Energy dissipation in the switch is substantially eliminated by the controller  70  closing the switch  26  when substantially zero voltage and zero rate of change of voltage are present across the switch  26 , thereby avoiding the simultaneous application of a voltage across the switch  26  and a current through the switch  26 . The frequency at which the switch  26  can be operated under these zero voltage conditions can be increased by selecting the storage capacitor  24  and the inductors  16  and  18  to provide a rapid discharge of the capacitor  24  through the output inductor  18  and the amplifier  100 . 
     In another embodiment, current ripple in the input inductor  16  and/or output inductor  18  is further reduced by magnetically coupling the inductors  16  and  18 . Magnetic coupling  39  is provided by winding the inductors together on a magnetic core. With coupled inductors, energy transfers from the source to the load through the storage capacitor  24  (i.e., by the electric field) and directly through the coupled inductors  39  (i.e., by the magnetic field). The total DC magnetizing current in the magnetic core is the sum of the input and output currents. The turns ratio and coupling coefficient of the inductors  16  and  18  can be selected so that the current ripple in either, but not both, is reduced to zero. 
     In electrosurgery it is advantageous to have DC isolation between a power source and the tissue/output load  11 . Such isolation is advantageous, for example, due to the substantial variation in resistance of the tissue/output load  11  (e.g., varying from essentially zero to infinity). The present invention is easily extended to achieve such isolation. With reference now to FIG. 4, an embodiment is shown that achieves both isolation between the power source  12  and the amplifier circuit  100  and further reduction of the current ripple in the input and output current ripple. Isolation is provided by dividing the storage capacitor  24  of FIG. 3 into two capacitors  40  and  42  and interposing an isolation transformer  44  between them. The isolation transformer  44  includes a primary winding  46  and a secondary winding  48 . One capacitor  40  is connected in series with the input inductor  16  and the primary winding  46 . The other capacitor  42  is connected in series with the output inductor  18  and the secondary winding  48 . 
     The inductors  16  and  18  and the isolation transformer  44  can be magnetically coupled  50  to reduce the input and output current ripple. Under certain conditions, both the input and output current ripple can be reduced to zero. The input current ripple can be reduced to zero under the following condition: 
     
       
           Le   1 = L   11  [ N   2 / N   1 −1] 
       
     
     where: 
     Le 1  is the leakage inductance of the input inductor  16 ; 
     N 1  is the number of turns of the input inductor  16 ; 
     N 2  is the number of turns of the output inductor  18 ; and 
     L 11  is the self-inductance of the input inductor  16 . 
     Here, input ripple current may be reduced to zero by having N 1 , the number of winding turns in the input inductor, substantially equivalent to N 2 , the number of winding turns in the output inductor. In one embodiment, N 1  and N 2  need only be approximately equivalent to produce a reduction in the input current ripple. 
     The output current ripple can be reduced to zero under the following condition: 
     
       
           Le   2 = L   11  ( N   2 / N   1 ) 2    [N   2 / N   1 −1] 
       
     
     where: 
     Le 2  is the leakage inductance of the output inductor  18 ; 
     N 1  is the number of turns of the input inductor  16 ; 
     N 2  is the number of turns of the output inductor  18 ; and 
     L 11  is the self-inductance of the input inductor  16 . 
     Similarly, the output ripple current may also be reduced to zero by having N 2  substantially equivalent to N 1 . Again, in one embodiment N 2  and N 1  need only be approximately equal to produce a reduction in the output ripple current. 
     In another aspect of the invention an improved amplifier  100  is provided for converting the DC output voltage from the DC regulator  10  to a radio-frequency output signal provided to the tissue  11 . The amplifier  100  achieves this conversion with high efficiency and substantially reduced radiation of electromagnetic interference. 
     With reference now to FIG. 5, the amplifier  100  includes an input inductor  62  for reducing input current ripple, a resonant circuit  64  connected in series with the input inductor  62 , and a transistor switch  66  for alternately connecting (closing) and disconnecting (opening) a current return path  68  of the amplifier  100  to a junction of the input inductor  62  and the resonant circuit  64 . 
     The resonant circuit  64  generally includes an inductor  72 , a capacitor  74 , and the complex impedance of the tissue impedance  11 . The controller  70  adjusts the frequency and magnitude of the radio-frequency output voltage of the amplifier  100  by opening the switch  66  (i.e., biasing the transistor to achieve low impedance) and closing the switch  66  (i.e., biasing the transistor to achieve high impedance). The controller  70  for the amplifier  100  can include simple oscillating circuits or a more complex feedback controller to regulate the switch  66 . 
     As the switch  66  is cyclically operated by the switch controller  70 , the input signal from the DC regulator  10  is converted into an output signal corresponding to the switching frequency. The magnitude and frequency of the output signal is regulated by adjusting the duty ratio the switch  66 , i.e., adjusting the ratio of time that the resonant circuit  64  is charged to the total time over which the resonant circuit  64  is charged and discharged. During the time that the switch  66  is closed, the voltage across the switch  66  is essentially zero and the input current flows through the input inductor  62  to ground. The input inductor  62  is sufficiently large so as to act as a source of substantially constant current. When the switch  66  is opened, the input current flows through the resonant circuit  64 . The transient response of the resonant circuit  64  is the response of a damped second-order system created by the series connection of the inductor  72 , the capacitor  74 , and the tissue impedance  11 . Energy within the resonant circuit  64  is dissipated during a resonant transient across the resistive component of the tissue impedance  11 . DC isolation is provided between the amplifier  100  and the tissue impedance  11  by an isolation transformer  76  and DC filter capacitors  78  and  80 . 
     The efficiency of the amplifier  100  is enhanced by selecting the inductor  72  and capacitor  74  in the resonant circuit  64  to provide a damped response with zero voltage and zero rate of change of voltage across the switch substantially simultaneous to switch  66  closing. Zero voltage switching can further be enabled by an anti-parallel diode  67  connected across the switch  66 . The anti-parallel diode  67  turns on for negative switch  66  current independent of the switch being open or closed, and hence more easily and automatically maintains the zero voltage switching described above. 
     In this manner, energy dissipation in the switch  66  is substantially eliminated by avoiding the simultaneous application of a voltage across the switch  66  and a current through the switch  66 . The zero rate of change of voltage across the switch  66  substantially simultaneous to the switch closing permits an increased range of tissue impedances (i.e., a range of second order responses) for which zero voltage switching will be achieved. 
     According to another aspect of the present invention the operating condition of the output transformer can be sensed. Such optical sensing may be done using a sense winding  81  that provides a voltage signal  82  to controller  70 . 
     According to yet another aspect of the present invention, an energy recovery circuit is provided for use in an electrosurgical generator to improve the efficiency of energy delivery to the tissue. The energy recovery circuit generally includes at least one energy storage device (e.g., capacitor, inductor, or combination thereof) and at least one switch (e.g., bipolar transistor, insulated gate bipolar transistor, or field effect transistor) that alternatively stores and releases electrical energy in the electrosurgical generator. 
     Referring to FIG. 6, an energy recovery circuit  150  is shown including inductive storage means  156 , having inductance L, and capacitive storage means  154 , having capacitance C, where both storage means are for storing electrical energy. In addition, circuit  150  also includes substantially DC power supply  152  having voltage V and resistive load  158  having a complex impedance Z that represents a patient. In operation, energy recovery circuit  150  has a state where first switch  160  is closed and inductor  156  is allowed to charge to a stored energy of ½LI 2 , where I is the current that passes through inductor  156 . When first switch  160  opens inductor  156  transfers energy to capacitor  154  due to the action of diode  166 . When diode  166  is conducting, second switch  162  may be closed. When second switch  162  is opened, all energy remaining in the circuit  150  will be stored in capacitor  154  rather than being dissipated as heat. When another energy delivery cycle is initiated, the voltage, V c , across the capacitor  154  is measured and inductor  156  is charged with current where: 
     
       
         Energy per period=½ LI   2 +½ CV   c   2   +∫   o   τ   |V   R   2   /Z|dt   
       
     
     where: 
     V R  is the RMS voltage on the patient  158   
     τ is the period of an energy delivery cycle. 
     Therefore, the energy that was not dissipated from the circuit  150  and stored in capacitor  154  is used in the next energy delivery cycle rather than being dissipated as heat. 
     With reference now to FIG. 7, an embodiment of an energy recovery circuit  90  is shown in combination with the amplifier  100  of FIG.  5 . The use of an energy recovery circuit  90  in combination with the amplifier  100  of FIG. 5 is intended only to illustrate the operation of the energy recover circuit  90  and not to limit its use in combination with an amplifier  100 . The energy recovery circuit  90  includes a transistor switch  92 , an energy storage inductor  96 , and a diode  94  in series with the amplifier  100  and in parallel with the energy storage inductor  96 . The controller  70  regulates the switch  92  to selectively store and release energy between the energy recovery circuit  90  and the amplifier  100 . 
     As previously described, the transient response of the output signal delivered by the amplifier  100  to the tissue impedance  11  for certain electrosurgical operations is that of a damped second-order system. Power within the resonant circuit  64  is transferred as a burst to the tissue that decays over a time constant defined by the inductor  72 , the capacitor  74 , and the impedance of the tissue  11 . At the end of a burst from the amplifier  100 , the energy that has not been transferred to the tissue  11  generally remains within the generator where it is dissipated as heat. 
     To avoid this loss of energy, the controller  70  stores some of the energy in the amplifier  100  by opening the switch  92  and passing the current through the energy storage inductor  96 . At the end of a burst, when the controller  70  opens the amplifier switch  66 , the controller  70  closes the switch  92  to trap the stored energy in a closed-circuit path connecting the energy storage inductor  96 , the diode  94 , and the switch  92 . During a subsequent burst (i.e., after the controller  70  closes the amplifier switch  66 ), the controller  70  opens the switch  92 , thereby transferring the energy remaining in the energy storage inductor  96  to the amplifier  100 . Power dissipation in the switch  92  is minimized by including an anti-parallel diode  93  across the switch  92 . The anti-parallel diode  93  turns on for negative voltages across the switch  92  to assist in obtaining zero voltage switching of the switch  92 . 
     In this manner, energy is selectively stored and released between the energy recovery circuit  90  and the amplifier  100  to increase the efficiency of energy transfer to the tissue. The energy recovery circuit  90  provides the further advantage of rapidly damping the output power of the generator at the end of a pulse. 
     Referring to FIGS. 8 a  and  8   b , the present inventors believe a distributed complex tissue impedance model may be obtained from a tissue structure that is undergoing an electrosurgical procedure. More particularly, the complex impedance  300  of tissue  400  includes a resistance  310  and a capacitance  320 . Generally, tissue  400  includes cells,  404  and  405 , and fluid  402 . Tissue resistance  310  is created by the electrical conduction path through the fluid  402 . Tissue capacitance  320  is created by the cell membranes  408  which provide an electrical insulating effect around the electrically conducting fluid  410  within the cells. Cell membranes puncture/burst, as shown by  406 , when a sufficient voltage is applied across the tissue  400 . After the cell membrane bursts  406 , the capacitive effect of the membrane  406  is substantially reduced, as shown by short circuit  330 , and the associated complex impedance  300  of the tissue  400  becomes more resistive and less capacitive. The complex impedance  300  of the tissue  400  is further changed when sufficient energy is dissipated in the tissue  400  to vaporize some of the fluid  402  thereby causing an increase in resistance, as shown by the additional resistor  340 . Additional changes in the complex impedance  300  are created through effects such as the denaturing and recombining of proteins in response to heating. 
     It has further been found that the complex impedance of the tissue can be measured over a time period to observe the extent, if any, of cellular membrane resealing. For example, cells which have not been destroyed by electrosurgical energy can reseal small holes in the cell membrane over a period of about a millisecond to a second. Measuring the change and rate of change of tissue&#39;s complex impedance in between or during delivery of electrosurgical energy provides information about the condition of the tissue and the associated surgical effect. 
     With reference now to FIG. 9, a tissue impedance controller  109  for use in an electrosurgical generator is illustrated according to another aspect of the present invention. The tissue impedance controller  109  includes a generator circuit  110 , an impedance measurement device  130 , and a controller  120  responsive to the impedance measurement device  130 . The generator circuit  110  synthesizes radio-frequency pulses that are applied across tissue to produce electrosurgical effects. The impedance measurement device  130  measures the complex impedance of the tissue  11 . The controller  120  regulates the generator circuit  100  in response to the measured tissue complex impedance  11  and the rate of change of impedance to provide improved electrosurgical effects. 
     In one embodiment as shown in FIG. 9, tissue impedance  11  is measured in between electrosurgical pulses. Between electrosurgical pulses, the controller  120  regulates the generator circuit  110  to apply a predetermined measurement signal across the tissue  11  for use by the impedance measurement device  130 . The impedance measurement device  130  measures the complex impedance of the tissue  11  (i.e., by dividing the voltage signal across the tissue by the current through the tissue). The controller  120  analyzes the measured impedance and/or the rate of change of the measured impedance over a predetermined time period to determine the present condition of the tissue  11 . The controller  120  compares the present tissue condition with a desired surgical effect and regulates the generator circuit  110  to obtain the desired surgical effect. 
     In another embodiment, tissue impedance is measured periodically or continuously during electrosurgical pulses. The impedance measurement device  130  applies a predetermined frequency voltage across the tissue  11  that has a different frequency than the signals synthesized by the generator circuit  110  for electrosurgical effects. The impedance measurement device  130  measures the current through the tissue  11  at the predetermined frequency to determine the complex impedance of the tissue  11  and thereby the tissue condition. The controller  120  then regulates the generator circuit  110  in response to the measured tissue condition to obtain a desired surgical effect. 
     The DC regulator  10 , the amplifier  100 , and the energy recovery circuit  90  of the present invention are each advantageous for use in the present generator circuit  110 . The DC regulator  10  and the amplifier  100  enable the controller  120  to rapidly vary the characteristics of the output signal, including frequency, magnitude, and pulse width in response to the measured tissue complex impedance  11 . The complex tissue impedance  11  can be more rapidly and accurately measured in between pulses by the energy recovery circuit  90  which efficiently captures the energy remaining in the generator circuit  110  at the end of a pulse and thereby rapidly dampens the output signal from the generator circuit  110  and allows rapid application of an impedance measurement signal to the tissue  11 . 
     While various embodiments of the present invention have been described in detail, it is apparent that further modifications and adaptations of the invention will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.