Abstract:
An electrosurgical generator may reduce unintended tissue damage by improving regulation of output power. The electrosurgical generator may control the power during a cycle, and react to a change in power if arcing occurs. Voltage sources, especially, demonstrate the tendency to have large, uncontrolled power excursions during normal electrosurgical use. The magnitude of the power excursions may be dependent on various factors. An exemplary electrosurgical generator control scheme reduces or minimizes the thermal spread by accurately supplying the specified power within a few cycles. Additionally, fast and accurate regulation provided by the constant voltage mode reduces or minimizes unintentional tissue charring. Thus, reduced thermal spread and charring should result in better surgical outcomes by reducing scarring and decreasing healing times. An electrosurgical generator controller may be configured to control both a DC-DC buck converter and a DC-AC boost inverter based in part on electrical parameters of the electrosurgical generator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional of U.S. Provisional Application No. 61/426,985, entitled “DUAL CURRENT-MODE CONTROLLER FOR REGULATION OF ELECTROSURGICAL GENERATOR OUTPUT POWER,” which was filed on Dec. 23, 2010. This application is also a non-provisional of U.S. Provisional Application No. 61/530,528, entitled “CONSTANT POWER SOURCE BY NONLINEAR CARRIER-CONTROL OF A BUCK CONVERTER FOR USE IN AN ELECTROSURGICAL GENERATOR,” which was filed on Sep. 2, 2011. All of the contents of the previously identified applications are hereby incorporated by reference for any purpose in their entirety. 
     BACKGROUND OF THE INVENTION 
     An electrosurgical generator is commonly used in surgical practice to perform arc cutting and coagulation. The electrosurgical generator produces a high-frequency electric current to cut tissue with limited blood loss and enhanced cutting control compared to a metal blade. Standard industry practice is for electrosurgical generators to measure and average the alternating current (AC) output power over several cycles and use a low-bandwidth control loop to adjust the duty cycle of a pulse width modulated (PWM) converter, modulating the carrier of a fixed-output-impedance resonant inverter to achieve the desired output characteristic. However, the feedback control loop and several cycles average gives rise to latency issues. 
     One example of an industry practice is for electrosurgical generators to mimic medium-frequency (MF) amplitude modulated (AM) broadcast transmitters via a method commonly called the Kahn Envelope Elimination and Restoration technique. Such generators typically use a class-D or class-E RF output stage operating with constant voltage amplitude at the electrosurgical analogy of a carrier frequency. In various known embodiments, the generators are combined with an efficient converter power supply amplitude modulator, sometimes referred to as a class-S modulator. The converter power supply amplitude modulator may be configured to regulate the RF output voltage, current, or power dissipated in the tissue load to a desired power versus impedance characteristic called a power curve. 
     The assumption of such a technique is that the tissue load changes at rates substantially lower than the audio frequency (AF) band. However, this assumption is not entirely accurate when viewed through the prism of arcing, which is the primary mechanism of cutting and coagulation in electrosurgery. Arcing in electrosurgery can extinguish and re-ignite in the middle of a cycle, and changes in its characteristics can occur on scales much broader than the AF. Therefore, this assumption may be one of convenience more so than fact, since the feedback of RE for purposes of control is well known to be very difficult due to the lag introduced by most common feedback controller techniques. 
     The commonly used envelope feedback regulation for electrosurgery is accomplished by measuring and averaging the alternating current (AC) output power and load impedance via voltage and current sensor feedback over many (sometimes hundreds) of cycles. This approach is complex, and its slow response during arcing leads to poor regulation of the AC output power, resulting in undesirable thermal spread or other well known tissue damage such as charring and scarring. Thus, a need exists for an electrosurgical generator that overcomes these and other deficiencies. 
     SUMMARY OF THE INVENTION 
     Using a high frequency inverter to form an arc between the output of an electrosurgical generator and tissue of a patient, a surgeon can induce joule heating in the affected cells; this causes the desired surgical effects of cutting, coagulation, and dissection. In an exemplary embodiment, the electrosurgery utilizes joule heating produced by the electrosurgical generator. The electrosurgical generator produces an accurate power source output characteristic, to which maximum voltage and current limits are added. The voltage and current limits of the electrosurgical generator contribute to the safety of the process. Furthermore, in an exemplary embodiment the voltage and current limits are configured to produce particular tissue effects which may be desirable in various surgical applications. 
     In an exemplary embodiment, an electrosurgical generator control system produces constant power output without measuring output voltage or output current, and regulates the output power with substantially deadbeat control. The electrosurgical generator control system performs near deadbeat control by regulating inductor current to a specified value, equal to a reference current. Thus, in an exemplary embodiment, the electrosurgical generator control system achieves a desired inverter output characteristic with an efficient and substantially deadbeat control method for AC output power. Furthermore, an exemplary electrosurgical generator control system switches between operating modes based in part on at least one of a measured output voltage, a measured inductor current, and by observing a duty cycle command generated by the control system. Additionally, an exemplary control system provides the ability to adjust the voltage and current limits and facilitate precision control of desired tissue effects. The desired tissue effects may include at least one of cut depth and the amount of surface hemostatis versus thermal spread. 
     Compared to prior art electrosurgical generators, an exemplary electrosurgical generator reduces unintended tissue damage by improving regulation of output power. In accordance with an exemplary embodiment, an electrosurgical generator controls the power during a cycle, and reacts to a change in power if arcing occurs. Voltage sources, especially, demonstrate the tendency to have large, uncontrolled power excursions during normal electrosurgical use. The magnitude of the power excursions may be dependent on various factors. One factor is how far the surgeon is away from the tissue when an arc occurs in the sinusoidal cycle. Furthermore, in the prior art, the current sources may introduce long, unintended arcs, even if distance from the tissue was well controlled. Therefore, in an exemplary embodiment, the electrosurgical generator may be configured to control power within a carrier frequency cycle for full arc and plasma control throughout the cycle. Power control within the duration of a carrier frequency cycle is advantageous over the prior art systems because arcing occurs faster than typical voltage or current detection feedback mechanisms can respond. 
     Furthermore, the exemplary electrosurgical generator is less complex than prior art electrosurgical generators. Moreover, it is an objective of this application to present an inverter topology and control algorithm which combines current-mode and voltage-mode control to realize the desired output characteristic of an electrosurgical generator in a markedly simpler and more accurate fashion. By directing which of two conversion stages is to be current-mode controlled, constant power, constant current, and constant voltage outputs can be achieved with excellent regulation and fast transitions. 
     In an exemplary embodiment, effective regulation of an electrosurgical generator&#39;s output is important to achieving the desired clinical effects. If output power is allowed to exceed the desired value, excessive thermal spread may occur, unnecessarily damaging and scarring tissue and impeding healing. If maximum output voltage exceeds the limiting value, charring of tissue may occur, which is frequently undesirable as it may unnecessarily damage tissue and obscure the surgical field. Use of an exemplary electrosurgical generator control scheme in an electrosurgical generator can provide near-deadbeat regulation of output power. In addition, the electrosurgical generator control scheme tends to assure that thermal spread is minimized by accurately supplying the specified power within a few cycles. Additionally, in various embodiments, fast and accurate regulation provided by the constant voltage mode minimizes unintentional tissue charring. Thus, reduced thermal spread and charring should result in better surgical outcomes by reducing scarring and decreasing healing times. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       A more complete understanding of the present invention may be derived by referring to the detailed description and draft statements when considered in connection with the appendix materials and drawing figures, wherein like reference numbers refer to similar elements throughout the drawing figures, and: 
         FIG. 1  illustrates a schematic of an electrosurgical generator circuit, in accordance with various embodiments; 
         FIG. 2  illustrates a graphical representation of desired output characteristics, in accordance with various embodiments; 
         FIG. 3  illustrates a schematic of an electrosurgical generator circuit, in accordance with various embodiments; 
         FIG. 4  illustrates a schematic of an exemplary electrosurgical generator in constant power output mode, in accordance with various embodiments; 
         FIG. 5  illustrates a schematic of an exemplary electrosurgical generator circuit with buck converter and boost inverter control, in accordance with various embodiments; 
         FIG. 6  illustrates another graphical representation of desired output characteristics, in accordance with various embodiments; 
         FIG. 7  illustrates a schematic of an exemplary buck converter circuit with current programmed mode control, in accordance with various embodiments; 
         FIG. 8  illustrates a graphical representation of the interaction between the nonlinear carrier control current limit and measured inductor current, and the establishing of a corresponding duty cycle, in accordance with various embodiments; 
         FIG. 9  illustrates yet another graphical representation of desired output characteristics using duty cycle limits, in accordance with various embodiments; and 
         FIG. 10  illustrates a schematic of an exemplary non-dissipative snubber circuit, in accordance with various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     While exemplary embodiments are described herein in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that logical electrical and mechanical changes may be made without departing from the spirit and scope of the invention. Thus, the following detailed description is presented for purposes of illustration only. 
     In accordance with an exemplary embodiment, an electrosurgical generator controller operates with near-deadbeat control to maintain a desired AC output of an electrosurgical generator, which operates in at least one of a constant voltage mode, a constant current mode, and a constant power mode. The mode selection is generally based on the impedance associated with the tissue being cut. Different types of tissue, such as muscle and fat, have different impedances. In terms of electrosurgical operations, constant power output tends to uniformly vaporize tissue, resulting in clean dissection. Whereas constant voltage output tends to explosively vaporize or carbonize tissue (“black coagulation”), and constant current output tends to thermally coagulate tissue without vaporization (“White coagulation”). Carbonization is surgically useful if the surgeon wishes to rapidly destroy surface tissue, and thermal coagulation is regularly coupled with mechanical pressure to seal hepatic or lymphatic vessels shut. However, it is desirable for the surgeon to operate using constant power output and importantly, return to using constant power output as quickly as possible if there is deviation. 
     With reference to the schematic shown in  FIG. 1 , in an exemplary embodiment, an electrosurgical generator  100  comprises a DC-DC buck converter  101 , a DC-AC boost inverter  102 , an inductor  103 , a transformer  104 , and an electrosurgical generator (ESG) control system  110 . In the exemplary embodiment, a DC voltage source Vg is electrically coupled to DC-DC buck converter  101 . Furthermore, inductor  103  is electrically coupled between DC-DC buck converter  101  and DC-AC boost inverter  102 . The output of DC-AC boost inverter  102  transmits power to the primary winding of transformer  104 , which passes through the secondary winding of transformer  104  to the load Z. Additionally, the load Z changes because tissue impedances vary, and also changes because the cutting process is an arc process. The impedance of an arc varies as it goes through several “phases” of formation and eventual extinguishment within a carrier frequency cycle. 
     In an exemplary embodiment, ESG control system  110  is in communication with both DC-DC buck converter  101  and DC-AC boost inverter  102 . The ESG control system  110  is configured to control the duty cycle d 1  of DC-DC buck converter  101  and the duty cycle d 2  of DC-AC boost inverter  102 . Additionally, ESG control system  110  is configured to measure power characteristics of electrosurgical generator  100 , and control electrosurgical generator  100  based at least in part on the measured power characteristics. Examples of the measured power characteristics include the current through inductor  103  and the voltage at the output of DC-AC boost inverter  102 . In various embodiments of control modes, ESG control system  110  controls buck converter  101  by generating duty cycles based on a combination and/or selection of duty cycle inputs from various controllers depending on the mode of operation (e.g., constant current, constant power, or constant voltage). 
     With respect to the AC output of the electrosurgical generator and in exemplary embodiments, “constant power” is defined to mean the average power delivered in each switching cycle is regulated to a substantially fixed value. Likewise, “constant voltage” and “constant current” are defined as the rms value of the AC voltage or current, respectively, being regulated to a substantially fixed value. In various embodiments, the substantially fixed values of the constant power, constant voltage, and constant current may be selected by a user or selected from a lookup table. In accordance with an exemplary embodiment, ESG control system  110  comprises a current-mode controller  111 , a voltage-mode controller  112 , a mode selector  113 , and steering logic  114 . In one exemplary embodiment, mode selector  113  compares the output voltage V out (t) and the inductor current i L (t) to “predetermined limits” (discussed in further detail herein) in order to determine the desired mode of operation of electrosurgical generator  100 . An exemplary graphical representation of the desired output characteristics is illustrated in  FIG. 2 . In an exemplary embodiment, as the load impedance increases and causes the voltage to increase, the corresponding increasing output voltage triggers the transitioning of the operating mode from constant current (A) to constant power (B) to constant voltage (C). Similarly, in an exemplary embodiment, as the load impedance decreases and causes the current to increase, the corresponding decreasing output voltage triggers the opposite transitioning from constant voltage (C) to constant power (B) to constant current (A) operating modes. 
     In various embodiments, a constant power mode may be maintained by varying just the duty cycle of a DC-AC boost inverter. With reference to  FIG. 3 , an ESG control system  310  comprises a current-mode controller  311 , a voltage-mode controller  312 , a mode selector  313 , and steering logic  314 . In this exemplary embodiment, current-mode controller  311  compares the inductor current i L (t) to a control current limit i C . In an exemplary embodiment, the control current limit i C  is set by a user, or provided by a look-up table. In an exemplary embodiment, current-mode controller  311  uses a latch circuit to generate a switching waveform δ(t) with a duty cycle d 1 . The inputs of the latch circuit are the current comparison and a clock signal. In an exemplary embodiment, the switching waveform δ(t) is switched “high” at the start of a switching period if the inductor current i L (t) is lower than control current limit i C . Furthermore, in the exemplary embodiment, the switching waveform δ(t) is switched “low” in response to the inductor current i L (t) exceeding the control current limit i C . In other words, a comparison of the inductor current i L (t) to control current limit i C  facilitates adjusting the inductor current i L (t) to match the control current limit i C . For small inductor current ripple, in other words Δi L &lt;&lt;i L , the current-mode controller regulates the inductor current i L (t) to an approximately constant value, substantially equal to control current limit i C . 
     In various embodiments and with continued reference to  FIG. 3 , voltage-mode controller  312  comprises a comparator  321 , a compensator  322 , and a pulse-width modulator  323 . Furthermore, in various embodiments, voltage-mode controller  312  compares the output voltage v out (t) with a reference voltage V max  at comparator  321 . The output of comparator  321  is communicated to compensator  322  which in turn outputs an error signal that drives PWM  323 . In the various embodiments, the output of compensator  322  is an input signal to PWM  323 , which sets the duty cycle d 2  of the signal. 
     Furthermore, in various embodiments, mode selector  313  comprises an encoder and performs multiple comparisons. The output voltage v out (t) is compared with a first voltage limit V limit   _   1  to generate “signal a”. The output voltage v out (t) is compared with a second voltage limit V limit   _   2  to generate “signal b”. Similarly, the inductor current i L (t) is compared with a first current I limit   _   1  to generate a “signal c”. The inductor current i L (t) is compared with a second current limit I limit   _   2  to generate a “signal d”. In one exemplary embodiment and with reference to Table 1, the mode selection is set by mode selector  313  based on the above described comparisons. Table 1 lists comparison outcomes and corresponding mode. In an exemplary embodiment, Table 1 lists a “1” value if the output voltage or inductor current is greater than the compared limit, and a “0” value if the output voltage or inductor current is less than the compared limit. For example, if output voltage v out (t) exceeds both the first voltage limit V limit   _   1  and the second voltage limit V limit   _   2 , then the encoder selects the constant voltage mode. Further, the second voltage limit V limit   _   2  is equivalent to reference voltage V max , the same used in the comparison at voltage-mode controller  312 . 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 a 
                 b 
                 c 
                 d 
                 Mode 
               
               
                   
               
             
             
               
                 0 
                 0 
                 1 
                 1 
                 I 
               
               
                 1 
                 0 
                 1 
                 0 
                 P 
               
               
                 1 
                 1 
                 0 
                 0 
                 V 
               
               
                   
               
             
          
         
       
     
     Constant Power Output 
     In various embodiments, constant AC power output is achieved by setting duty cycle d 1  to a fixed value, and running the DC-AC boost inverter stage as a current-programmed boost inverter by varying duty cycle d 2 . As previously mentioned, electrosurgical generator controller  310  performs near deadbeat control by regulating inductor current to an approximately constant value, equal to a control current limit i C . For illustration purposes,  FIG. 4  represents an exemplary schematic of the electrosurgical generator in constant power output mode. 
     In steady-state, the average voltage of v 1 (t) is constant in response to the input voltage Vg being constant, the DC-DC buck converter being bypassed by being set to 100% duty cycle, and no average voltage being able to exist across inductor L. The use of current programmed mode control results in the average current of i 1 (t) being regulated to an approximately fixed value with deadbeat or near-deadbeat control. In order to regulate i 1 (t), duty cycle d 2  is varied by the current mode controller to maintain i 1 (t) at a fixed value. Given the fixed voltage v 1  and current i 1 , the power at input of DC-AC boost circuit  102  (i.e., a switch network) is also constant. In an exemplary embodiment, the switch network is nearly lossless, resulting in the output power being approximately equal to the input power. Since the input power is constant, the output power of DC-AC boost circuit  102  is also constant. 
     Constant Voltage Output 
     In various embodiments and with renewed reference to  FIG. 3 , constant voltage output is achieved by setting duty cycle d 1  of DC-DC buck converter  101  to a fixed value, and using voltage-mode control for duty cycle d 2  of DC-AC boost circuit  102 . In an exemplary embodiment, the voltage-mode control involves measuring the output voltage v out (t) of DC-AC boost circuit  102  with a sensor network, feeding the sensed output voltage v out (t) to a control loop in voltage-mode controller  312 , and adjusting the converter&#39;s duty cycle command based on the relative difference between the measured output voltage v out (t) and the reference output voltage V max . In other words, the duty cycle d 2  is set to increase or decrease the output voltage to match V max . In an exemplary embodiment, V max  may be set by a user or based on values in a look-up table. 
     Constant Current Output 
     In an exemplary embodiment, constant current output is achieved by operating DC-AC boost circuit  102  at a fixed duty cycle d 2  and current-mode controlling DC-DC buck converter  101 . In an exemplary embodiment, the current-mode control accurately controls the average inductor current such that the output of buck converter  101  is a constant current. In one embodiment, current-mode controller  111  compares inductor current i L (t) to control current limit i C , where the control current limit i C  is a desired fixed value. In other words, electrosurgical generator controller  310  is configured to vary duty cycle d 1  in order to maintain inductor current i L (t) at the fixed value. In various exemplary embodiments, as with v out (t), i L (t) is measured with a sensor and not an estimated value. As a result, the constant current output mode produces an AC output current whose magnitude is regulated with near-deadbeat speed. 
     Mode Transition Via Direct Measurement 
     In various embodiments, an electrosurgical generator system implementing the three modes of constant power, constant voltage, or constant current produces a very fast, very accurate regulation of the AC output characteristic. Various modes are impacted by measured characteristics, while other modes do not need to respond to the same measured characteristics. Specifically, electrosurgical generator controller  310  may switch between operating modes based in part on measured output voltage v out (t). Furthermore, electrosurgical generator controller  310  may adjust the operating parameters in the constant voltage mode based on the measured output voltage v out (t). In other words, the selection of which stage of the converter to current-mode control may be achieved with minimal feedback and without a need for extraneous measurements, averaging, or feedback of the output. 
     Transitioning between the three modes, in an exemplary embodiment, is determined by monitoring the voltage of the primary winding of transformer  104  and the inductor current. As previously described, in accordance with one exemplary embodiment, the transition from one mode to the next is summarized in Table 1. An exemplary ESG transitions modes from constant current to constant power to constant voltage as the output voltage v out (t) increases. Specifically, in an exemplary embodiment, electrosurgical generator  300  operates in the constant current mode if the output voltage v out (t) is less than a first voltage limit V limit   _   1 . If the output voltage v out (t) exceeds the first voltage limit, electrosurgical generator  300  transitions to the constant power mode. If the output voltage v out (t) exceeds a second voltage limit V limit   _   2 , electrosurgical generator  300  transitions to the constant voltage mode, where the output voltage v out (t) is limited and held constant. In an exemplary embodiment, the first voltage limit V limit   _   1  and the second voltage limit V limit   _   2  are set by a user or from a look-up table. 
     Similarly, electrosurgical generator  300  transitions from constant voltage mode to constant power mode to constant current mode as inductor current i L (t) increases. Specifically, in an exemplary embodiment, electrosurgical generator  300  operates in the constant voltage mode if the inductor current i L (t) does not exceed a first current I limit   _   1 . If the inductor current i L (t) does exceed the first current I limit   _   1 , then the mode transitions to the constant power mode. If the inductor current i L (t) exceeds a second current limit I limit   _   2 , electrosurgical generator  300  transitions to the constant current mode, where the inductor current i L (t) is limited and held constant. In an exemplary embodiment, the first current limit I limit   _   1  and the second current limit I limit   _   2  are set by a user or from a look-up table. 
     ESG with Buck Converter and Boost Inverter Control 
     In accordance with various embodiments and with reference to  FIG. 5 , an electrosurgical generator  500  having an ESG control system  510  comprises a current-mode controller  511 , a voltage-mode controller  512 , a mode selector  513 , and steering logic  514 . In various embodiments, the operational mode of electrosurgical generator  500  is one of constant (or maximum) current I max , constant power P 1  from a buck converter, constant power P 2  from boost inverter, or constant (or maximum) voltage V max . These modes are illustrated in an exemplary embodiment with reference to  FIG. 6 . The output selection of mode selector  513  is communicated to steering logic  514 . In an exemplary embodiment, steering logic  514  controls which of at least one of current-mode controller  511  and voltage-mode controller  512  are enabled. Furthermore, steering logic  514  may select which conversion stage receives the output of current-mode controller  511  and/or voltage-mode controller  512 , in various embodiments, steering logic  514  switches between operating either DC-DC buck converter  101  or DC-AC boost inverter  102  with current-mode control for constant power, depending on which portion of constant power regions (P 1  or P 2 ) is currently the operating mode. For example, the voltage mode controller  512  and/or current mode controller  511  may adjust the duty cycles d 1  and/or d 2  for the operating mode (constant current mode, constant voltage mode, constant power P 1 , or constant power P 2 ). Furthermore, steering logic  514  selects the duty cycle that each of DC-DC buck converter  101  and/or DC-AC boost inverter  102  receives. 
     In various embodiments, the current-mode controller  511  compares the inductor current i L (t) to a nonlinear carrier control current limit i C (t). In an exemplary embodiment, the nonlinear carrier control current limit i C (t) is set by the selection of Pset, which may be done by a user, or provided by a look-up table. In an exemplary embodiment, current-mode controller  511  uses a latch circuit to compare inductor current i L (t) to control current limit i C (t), comprising either a current limit signal (I) or a power limit signal (P 1 ). The control signal for a P/I switch is the mode signal, which is communicated from mode selector  513 . The inputs of the latch circuit are a clock signal and the comparison of control current limit i C (t) and inductor current i L (t), comprising one of the current limit signal (I) or a power limit signal (P 1 ). The selection of the current-mode controller  511  output is in response to the current mode of the electrosurgical generator  500 . The operating mode of the electrosurgical generator  500  may be communicated from the output of mode selector  513 . In an exemplary embodiment, the switching waveform δ(t) is switched “high” at the start of a switching period if the inductor current i L (t) is lower than nonlinear carrier control current limit i C (t). Furthermore, in the exemplary embodiment, the switching waveform δ(t) is switched “low” in response to the inductor current i L (t) exceeding the nonlinear carrier control current limit i C (t). In other words, a comparison of the inductor current i L (t) to nonlinear carrier control current limit i C (t) facilitates adjusting pulse duration of buck converter&#39;s  101  duty cycle, as previously described. 
     To generate and control a constant current from electrosurgical generator  500 , the average value of inductor current i L (t) is controlled to be substantially equal to fixed control current limit K*Pset, which is a fixed, non-time varying value. For small inductor current ripple, in other words Δi L &lt;&lt;i L , the current-mode controller regulates the inductor current i L (t) to an approximately constant value, substantially equal to the fixed control current limit. 
     With respect to using a buck converter to generate substantially constant power (e.g., constant power P 1 ), implementation of a nonlinear carrier control current limit is further described. In addition to generating a constant power source based on varying just the duty cycle of a DC-AC boost inverter, a buck converter may also be configured to generate substantially constant power output. In accordance with various exemplary embodiments, substantially constant power output of a buck converter may be achieved by adjusting a duty cycle&#39;s active period for the buck converter, in an exemplary embodiment and with reference to  FIG. 7 , a buck converter system comprises a power source Vg, a buck converter circuit  710 , a controller  720 , and a load  730 . The impedance of the load may be static or dynamic. In the various embodiments, the controller  720  receives a feedback signal  711  representative of the output of the buck converter  710 . In an exemplary embodiment, the feedback signal  711  is a measurement of the current passing through an inductor  712  coupled to buck converter circuit  710 . 
     In various embodiments, controller  720  receives real time feedback of the inductor current i L (t) from the buck converter. The feedback signal  711  is used by controller  720  to adjust the duration of the active and non-active portions of the duty cycle. Adjustment of the duty cycle portions in real time, or substantially in real time, may be configured to produce a constant power source from buck converter  710 . In various embodiments, two characteristics of the inductor feedback signal  711  are used to make the determination of duty cycle adjustments. The two characteristics are, first, the value of inductor current i L (t) and second, the slope of the change in the inductor current i L (t). These two characteristics may be used to provide implied information regarding the current and voltage of the output power into load  730 , and this implied information may be used to adjust the magnitude of the duty cycle in real time and produce substantially constant power output. 
     The pulse duration of the duty cycle of DC-DC buck converter  710  is varied using current mode controller  720 . The varying pulse duration of the duty cycle controls the inductor current i L (t), which is responsive to load  730  in contact with buck converter  710 . As the impedance of load  730  varies, the voltage across inductor  712  also varies, and the current through inductor  712  varies as well. 
     Described in more detail, at the beginning of the buck converter duty cycle, the active portion (also referred to as the pulse duration of the pulse period or the “on” portion) of the duty cycle is initiated. With respect to a buck converter, the active portion of the pulse period closes a switch between a power source and an inductor, thereby allowing power to flow through the inductor. In various embodiments and with reference to  FIG. 8 , the inductor feedback signal i L (t) is compared to a nonlinear carrier control current i C (t). The nonlinear carrier control current i C (t) is a time-varying, nonlinear control signal that may be set for customized uses based on the desired output power. In response to the inductor feedback signal i L (t) exceeding the control current i C (t), the duty cycle switches to the non-active portion (also referred to as the “off” portion). The duty cycle stays in the non-active portion until the end of the pulse period. At the end of the pulse period, the cycle begins again with another pulse duration. 
     In various embodiments, the switching cycle has a fixed time period. Comparison of the inductor feedback signal i L (t) and the nonlinear carrier control current i C (t) is able to facilitate substantially constant power output based on a variable division of active and non-active portions of the duty cycle. As briefly described and with continued reference to  FIG. 8 , the inductor current value and the slope of the change in the inductor current i L (t) are used to adjust the duty cycle. By way of example and without limitation, the inductor current slope affects the timing of how long the inductor current i L (t) is less than the nonlinear carrier control current i C (t). A lower slope value indicates that the inductor current i L (t) is increasing at a slower rate, and therefore it will take a longer period of time until the inductor current i L (t) exceeds the control current i C (t). In other words, the more time is takes for the inductor current i L (t) to exceed the control current i C (t), the longer the corresponding pulse duration. For example, see the comparison between the pulse duration at 2 T S  and 3 T S . A higher slope value of inductor current i L (t) indicates that the inductor current is increasing at a quicker rate, and therefore it will take a shorter period of time until the inductor current i L (t) exceeds the control current limit i C (t). The shorter period of time results in the duty cycle staying in the active portion for a shorter period and having shorter pulse duration. 
     The nonlinear carrier control current i C (t) is part of a nonlinear carrier control (NLC) technique. In various embodiments, the NLC technique applied to the buck converter is based on a nonlinear time dependent variable, which is the nonlinear carrier. In various embodiments, the nonlinear time dependent variable is determined by the input voltage Vg, period of the switching cycle, and the desired power output. The application of NLC technique and production of substantially constant power output creates a buck converter that is a power source. In other words, the buck converter may implement NLC techniques to generate a fixed amount of power and be a power source. In contrast, prior art use of NLC techniques was typically configured to cause a converter to absorb a fixed amount of power and be a power sink. One of the benefits of using NIX control techniques is that a buck converter in combination with a boost inverter can produce a constant power source over a wider impedance range than using just a boost inverter alone. For example, an electrosurgical generator as described herein is capable of operating over an impedance range of about 64 to 4000 ohms. Using both a boost inverter and buck converter to source constant power facilitates operating over the wide impedance range without unreasonably high peak voltages. 
     In accordance with various exemplary methods, producing constant power output in a buck converter with a load having variable resistance includes turning on a switch of the buck converter at the beginning of the duty cycle to initiate a pulse, and monitoring the current through the inductor. The inductor current linearly increases while the buck converter is operating in the active portion of the duty cycle. The exemplary method may further include comparing, at a control circuit, the inductor current i L (t) to a nonlinear carrier control current i C (t), and turning off the switch of the buck converter in response to the magnitude of the inductor current meeting or exceeding the magnitude of the nonlinear carrier control current. In response to turning off the switch of the buck converter, the inductor current ramps down during the non-active portion of the duty cycle. The changing inductor current slope corresponds to the changing impedance of the load, which may be used to adjust the pulse duration of the duty cycle in order to produce substantially constant power output from the buck converter. In various embodiments, the nonlinear carrier control current is derived from the following equation: 
                   i   C     ⁡     (   t   )       =       P   Vg     *     Ts   t         ,         
where P is power at the load, Ts is the switching cycle period, Vg is the input DC voltage source magnitude, and t is the time (assuming t=0 occurs at the start of the switching cycle). Additionally, as is understood by one in the art, the inductor current has minor fluctuation during each cycle due to turning the buck converter on and off, and the minor fluctuation may not be due to any change in the load impedance. In various embodiments, changes to the load impedance result in a change in inductor current slopes and a change to the average value of the inductor current.
 
     Although a buck converter with substantially constant power output is described in terms of implemention in an electrosurgical generator, such a buck converter may also be implemented in various applications, such as are welding and gas-discharge lamps (i.e. street lamps). 
     In an exemplary embodiment and with renewed reference to  FIG. 5 , voltage-mode controller  512  comprises a comparator  521 , a compensator  522 , and a pulse-width modulator (PWM)  523 . Furthermore, in an exemplary embodiment, voltage-mode controller  512  compares the measured output voltage v out (t) with a reference voltage V max  at comparator  521 . The output of comparator  521  is communicated to compensator  522  which in turn outputs an error signal that drives PWM  523 . In the exemplary embodiment, the output of compensator  522  is an input signal to PWM  523 , which sets the duty cycle d 2  of the signal in certain modes. 
     In various embodiments, constant voltage output may also be achieved by setting duty cycle d 1  of DC-DC buck converter  101  to a fixed value, and limiting the duty cycle d 2  of DC-AC boost inverter  102  to a maximum duty cycle d max . Implementing a duty cycle limit on DC-AC boost inverter  102  during the constant voltage output generally amounts to running DC-AC boost inverter  102  in an open-loop. In various embodiments, limiting the duty cycle d 2  of DC-AC boost inverter  102  to a maximum duty cycle d max  results in poorer steady-state output voltage regulation in comparison to mode transitions using direct measurement, but provides the significant advantage of limiting the peak output voltage on a per-cycle basis, with little or no risk of transient overshoot. For various electrosurgical applications, the steady-state value of the maximum output voltage v out (t) is of lesser importance, as it would be unusual to operate in this output mode for any length of time. Per-cycle transient voltage limiting, however, may be highly useful as a means to limit potential undesirable arcing. Additionally, in various embodiments, a maximum duty cycle may be easily varied without the need to linearize an output voltage measurement or tune a compensator, and in this exemplary embodiment no sensor is required on the output since no direct measurement is taken. 
     Furthermore, configurations such as exemplary electrosurgical generator  500  may have additional inputs into the mode selection. In another exemplary embodiment and with reference to  FIG. 5 , mode selector  513  comprises an encoder and performs multiple comparisons. The output voltage v out (t) is compared with three separate voltage limits (V limit   _   1 , V limit   _   2 , V limit   _   3 ) to generate three voltage comparison signals. Similarly, the inductor current i L (t) is compared with three separate current limits (I limit   _   1 , I limit   _   2 , I limit   _   3 ) to generate three current comparison signals. With reference to  FIG. 6 , in various embodiments, mode selector  513  uses the voltage comparison signals and the current comparison signals to determine whether electrosurgical generator  500  is operating in the constant current output region (A), the region P 1  of the constant power output region (B), the region P 2  of the constant power output region (B), or the constant voltage output region (C). Furthermore, the output mode signal from mode selector  513  controls the switch position in steering logic  514 . Moreover, the output mode signal from mode selector  513  controls the switch position in current-mode controller  511 . For example, if output voltage V out (t) exceeds the first voltage limit V limit   _   1 , the second voltage limit V limit   _   2 , and the third voltage limit V limit   _   3 , then the encoder selects the constant voltage mode. The constant voltage mode signal from mode selector  513  would cause the switches&#39; position of steering logic  514  to be “V”. As another example, if output voltage v out (t) exceeds the first voltage limit V limit   _   1  but does not exceed the the second voltage limit V limit   _   2 , and inductor current i L (t) exceeds first current limit I limit   _   1  and second current limit I limit   _   2 , but does exceed I limit   _   3 , then mode selector  513  determines that the operating mode is constant power P 1 . The constant power P 1  mode signal from mode selector  513  would cause the switches&#39; position of steering logic  514  to be “P 1 ” as illustrated in  FIG. 5  and Table 2. The values “1” and “0” represent any fixed value between 0% and 100% that is not closed-loop controlled. In other words, there is no feedback signal actively changing the fixed values represented by “1” and “0”. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Duty cycle of buck and boost conversion stages by operating mode 
               
             
          
           
               
                   
                 Constant Current 
                 Constant Power 
                 Constant Power 
                 Constant Voltage 
               
               
                   
                 I max   
                 P 1   
                 P 2   
                 V max   
               
               
                   
                   
               
             
          
           
               
                 Buck 
                 ESG controlled with fixed 
                 ESG controlled with nonlinear 
                 1 
                 1 
               
               
                 Converter 
                 control current limit 
                 carrier control current limit 
               
               
                 Boost 
                 0 
                 0 
                 ESG controlled with fixed 
                 Voltage mode 
               
               
                 Inverter 
                   
                   
                 control current limit 
                 controlled 
               
               
                   
               
             
          
         
       
     
     Constant Power Output 
     In an exemplary embodiment, constant AC power output is achieved by setting one or both of duty cycle δ 1  and duty cycle δ 2  to desired values. Moreover, electrosurgical generator  500  operates with constant AC power output in either a first constant power region P 1  or a second constant power region P 2 . In various embodiments, the converter switches between generating constant power using boost inverter  102  or buck converter  101 , depending on the impedance of the load. Moreover, in various embodiments, electrosurgical generator  100  may operate both boost inverter  102  and buck converter  101  at the same time, which results in a constant power output having a high voltage and low power. 
     In steady-state and operating in first constant power region P 1 , inductor current i L (t) is compared to a nonlinear carrier control current i C (t) in current-mode controller  511 . The pulse duration of the duty cycle of the DC-DC buck converter is varied using the current mode controller  511 . The varying pulse duration of the duty cycle controls the inductor current i L (t), which is responsive to the load in contact with the buck converter. As the impedance of the load varies, the voltage across the inductor v L (t) also varies, and the current through the inductor i L (t) varies as well. As previously described, at the beginning of the duty cycle, the active portion of the duty cycle is initiated. In response to the inductor current i L (t) exceeding the nonlinear carrier control current i C (t), the duty cycle switches to the non-active portion. The duty cycle stays in the non-active portion until the end of the duty cycle, upon which the next duty cycle begins in the active portion, in alternative embodiments, during the comparison of the inductor feedback signal i L (t) and the nonlinear carrier control current i C (t), once the control current exceeds the inductor current, the duty cycle switches to the active portion. In accordance with the exemplary embodiment, electrosurgical generator  500  generates constant power using buck converter  101  during first constant power region P 1 . 
     In steady-state and operating in second constant power region P 2 , the average voltage of v 1 (t) is constant in response to the input voltage Vg being constant, the DC-DC buck converter being bypassed by being set to 100% duty cycle, and no average voltage being able exist across inductor  103 . The use of current programmed mode control results in the average current of i 1 (t) being regulated to an approximately fixed value with deadbeat or near-deadbeat control. In order to regulate i 1 (t), duty cycle δ 2  is varied by the current mode controller to maintain i 1 (t) at a fixed value. Given the fixed voltage and current, the power at input of DC-AC boost inverter (i.e., a switch network) is also constant. In an exemplary embodiment, the switch network is nearly lossless, resulting in the output power being approximately equal to the input power. Since the input power is constant, the output power of DC-AC boost inverter  102  is also constant. 
     Constant Voltage Output 
     In an exemplary embodiment, constant voltage output is achieved by setting duty cycle δ 1  of DC-DC buck converter  101  to a fixed value, and duty cycle δ 2  of DC-AC boost inverter  102  is voltage-mode controlled. In an exemplary embodiment, the voltage-mode control involves measuring the output voltage v out (t) of DC-AC boost inverter  102  with a sensor, feeding the sensed output voltage to a control loop in voltage-mode controller  512 , and adjusting the converter&#39;s duty cycle command based on the relative difference between the measured output voltage and the reference output voltage. In other words, the duty cycle δ 2  is set to increase or decrease the output voltage to match V max . In an exemplary embodiment, V max  may be set by a user or based on values in a look-up table. In an alternative embodiment, the boost inverter is run at a fixed duty cycle with no feedback of the output voltage. 
     Constant Current Output 
     In an exemplary embodiment, constant current output is achieved by operating DC-AC boost inverter  102  at a fixed duty cycle δ 2  and current-mode controlling DC-DC buck converter  101 . In an exemplary embodiment, the current-mode control accurately controls the average inductor current such that the output of buck converter  101  is a constant current. In one constant current embodiment, current-mode controller  511  compares inductor current i L (t) to a control current limit i C (t). In various embodiments, control current limit i C (t) may be a selected, fixed value or may be set by K*Pset, where K*Pset is a constant current set by the user during use. In various embodiments, Pset is set during the design stage. In other words, ESG control system  510  is configured to vary duty cycle δ 1  in order to maintain inductor current i L (t) at the fixed value. As a result, the constant current output mode produces an AC output current whose magnitude is regulated with near-deadbeat speed. 
     Electrosurgical Generator Modes 
     Similar to the transition of modes in electrosurgical generator  300 , in an exemplary embodiment, electrosurgical generator  500  also implements the three modes of constant power, constant voltage, or constant current to produce a very fast, very accurate regulation of the AC output characteristic. Various modes are impacted by measured characteristics, while other modes do not need to respond to the same measured characteristics. Specifically, ESG control system  510  switches between operating modes based in part on measured characteristics, such as inductor current and voltage. In other words, the selection of which stage of the converter to current-mode control is achieved with minimal feedback and without a need for extraneous measurements, averaging, or feedback of the output. Also, and as previously mentioned, the ESG control system  510  performs near deadbeat control by regulating inductor current to an approximately constant value, equal to a reference current. 
     Mode Transition Via Direct Measurement 
     Transitioning between the three modes, in an exemplary embodiment, is determined by monitoring the voltage of the primary winding of transformer  104  and the inductor current. Furthermore, the determination of transitioning between the modes may also based on the voltage and current of the primary winding of transformer  104 . In various embodiments, ESG control system  510  transitions modes from constant current to constant power to constant voltage as the output voltage v out (t) increases. 
     Specifically, in various embodiments, electrosurgical generator  500  operates in the constant current mode if the output voltage v out (t) is less than a first voltage limit (V limit   _   1 ). If the output voltage v out (t) exceeds the first voltage limit, electrosurgical generator  500  transitions to a first constant power mode (P 1 ). If the output voltage v out (t) exceeds a second voltage limit (V limit   _   2 ), electrosurgical generator  500  transitions to a second constant power mode (P 2 ). If the output voltage v out (t) exceeds a third voltage limit (V limit   _   3 ), electrosurgical generator  500  transitions to the constant voltage mode. Where the output voltage v out (t) is limited and held constant. In an exemplary embodiment, the first voltage limit (V limit   _   1 ), the second voltage limit (V limit   _   2 ), and the third voltage limit (V limit   _   3 ) are set by a user or from a look-up table. 
     Moreover, an exemplary ESG control system  510  transitions from constant voltage mode to constant power mode to constant current mode as inductor current i L (t) increases. Specifically, in an exemplary embodiment, electrosurgical generator  500  operates in the constant voltage mode if the inductor current i L (t) does not exceed a first current limit (I limit   _   1 ) if the inductor current i L (t) does exceed the first current limit (I limit   _   1 ), then the mode transitions to the second constant power mode (P 2 ). If the inductor current i L (t) exceeds a second current limit (I limit   _   2 ), then the mode transitions to the first constant power mode (P 1 ). If the inductor current i L (t) exceeds a third current limit (I limit   _   3 ), electrosurgical generator  500  transitions to the constant current mode, where the inductor current i L (t) is limited and held constant. In an exemplary embodiment, the first current limit (I limit   _   1 ), the second current limit (I limit   _   2 ), and the third current limit (I limit   _   3 ) are set by a user or from a look-up table. 
     Mode Transition Via Duty Cycle 
     In various alternative embodiments, the selection of operating modes may be based in part on the duty cycle. For example, if the electrosurgical generator is operating in constant power mode using the buck converter and the duty cycle reaches 100% active, the controller may be configured to switch to the constant power mode using the boost inverter. The switch to the boost inverter enables the electrosurgical generator to operate over a higher range of impedances. 
     In various embodiments, duty cycle limits may be used in the electrosurgical generator controller to control the mode transitions. With reference to  FIG. 9 , in various embodiments, an exemplary mode selector may use duty cycle comparison signals to determine whether electrosurgical generator  500  is operating in the constant current output region (A), the region P 1  of the constant power output region (B), the region P 2  of the constant power output region (B), or the constant voltage output region (C). 
     In an exemplary embodiment, the duty cycle comparison signals are generated from the comparison of the buck converter duty cycle d buck  (also referred to as d 1  herein) and the boost inverter duty cycle d boost  (also referred to as d 2  herein) to at least four separate duty cycle limits (d limit   _   1 , d limit   _   2 , d limit   _   3 , and d limit   _   4 ). For example, if the buck converter duty cycle d buck  exceeds the first duty cycle limit d limit   _   1  and the second duty cycle limit d limit   _   2 , and also the boost inverter duty cycle d boost  exceeds the third duty cycle limit d limit   _   3 , then the electrosurgical generator operates in the constant voltage mode and constant voltage output region (C). Similarly, if the boost inverter duty cycle d boost  is less than the third duty cycle limit d limit   _   3 , and the fourth duty cycle limit d limit   _   4 , and the buck converter duty cycle d buck  is less than the first duty cycle limit d limit   _   1 , then the electrosurgical generator operates in the constant current mode and constant current output region (A). Further, as is illustrated in  FIG. 9 , the duty cycle comparison signals may also result in the electrosurgical generator operating in the region P 1  of the constant power output region (B), or the region P 2  of the constant power output region (B). Therefore, in one exemplary embodiment, mode selector  513  is configured to determine the operating mode basd at least in part on comparisons of the buck converter duty cycle d buck  and boost inverter duty cycle d boost  to the duty cycle limits and to generate mode output signals to control steering logic  514  and/or current mode controller  511 . 
     In accordance with an exemplary embodiment, both the current-mode control  311  and the current-mode controller  511  may be able to maintain an approximately constant value of inductor current i L (t) by adjusting the current within 1-2 cycles. In another exemplary embodiment, the current-mode controller adjusts the inductor current within 1-10 cycles. In yet another embodiment, the current-mode controller adjusts the inductor within 10-100 cycles. Any of these examples may comprise a “low cycle” adjustemtn. This low cycle adjustment can be considered “deadbeat control” or “near-deadbeat control”. In accordance with an exemplary embodiment, near-deadbeat control minimizes unintentional charring by ensuring that only the requested quantum of power is delivered to the electrosurgical instrument. In the prior art, slow transient response of the converter to changes in load impedance may result in excessive delivery of power that may not be detected for 500 cycles or more. Stated another way, in an exemplary embodiment, an electrosurgical generator has an operating bandwidth of 100-500 kHz, compared to the prior art bandwidth of 1-10 kHz. 
     Although the mode transitions operate with near-deadbeat control, it still takes at least 1-2 cycles to change modes, and in some embodiments up to 100 cycles. Thus, should the load impedance suddenly increase while in either constant power mode, the converter will continue to supply constant power for the remainder of at least one cycle before transitioning to the constant voltage mode. In accordance with an exemplary embodiment and with reference to  FIG. 10 , an electrosurgical generator further comprises a non-dissipative voltage snubber circuit  1000  to prevent undesirable voltage spikes. The snubber circuit  1000  may be coupled to an electrosurgical generator such as electrosurgical generator  300  or electrosurgical generator  500 . The non-dissipative voltage snubber circuit  1000  is coupled to the primary winding of the transformer  104 . In an exemplary embodiment, a duty cycle d S  of snubber circuit  1000  is varied to maintain v CS (t) at a fixed value. Furthermore, instruments used for electrosurgery typically have leads that are several meters long. The long leads can result in an inductive load to the electrosurgical generator. Therefore, snubber circuit  1000  may further be configured to damp voltage spikes generated when switching the inductive load. 
     In general, any number of current, voltage, or duty cycle limits, and any number of subdivisions of constant current, constant power, or constant voltage modes may be used to facilitate operating mode selection and transition in order to provide near deadbeat control of an electrosurgical generator. The electrosurgical generator may include any electrosurgical generator control system comprising a mode selector that determines the current operating mode, steering logic that selects from the possible operating modes of constant current, constant power, or constant voltage, where the operating mode is based in part on the outputs of a current mode controller and a voltage mode controller. The operating mode and transitions between operating mode are configured to provide near deadbeat control of an electrosurgical generator having both a DC-DC buck converter and a DC-AC boost inverter. 
     Failure to maintain either accurate regulation of output power or sufficient means of voltage limiting may lead to higher output voltages, leading to unintentional charring, or higher output power, leading to unintentional thermal spread. The exemplary embodiments of the electrosurgical generators described herein accurately and quickly maintain the proper power characteristics, and allow a user to control the cutting process. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of any or all the draft statements. As used herein, the terms “includes,” “including,” “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, no element described herein is required for the practice of the invention unless expressly described as “essential” or “critical.”