Abstract:
A method of improving efficiency of an electrosurgical generator is presented, the method including controlling an output of an electrosurgical generator by converting a direct current (DC) to an alternating current (AC) using an inverter, and sensing a current and a voltage at an output of the inverter. The method further includes the steps of determining a power level based on the sensed voltage and the sensed current, determining an efficiency of the electrosurgical generator, and inserting a predetermined integer number of off cycles when the efficiency of the electrosurgical generator reaches a threshold power efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of and priority to U.S. Provisional Application Nos. 61/881,547 and 61/881,575, both filed on Sep. 24, 2013, the entire contents of which are hereby incorporated herein by reference. The present application is related to U.S. patent application Ser. No. 14/320,804 filed on Jul. 1, 2014. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to electrosurgery. More particularly, the present disclosure relates to systems and methods for improving efficiency of electrosurgical generators. 
     Background of Related Art 
     Electrosurgery involves the application of high-frequency electric current to cut or modify biological tissue during an electrosurgical procedure. Electrosurgery is performed using an electrosurgical generator, an active electrode, and a return electrode. The electrosurgical generator (also referred to as a power supply or waveform generator) generates an alternating current (AC), which is applied to a patient&#39;s tissue through the active electrode and is returned to the electrosurgical generator through the return electrode. The AC typically has a frequency above 100 kilohertz (kHz) to avoid muscle and/or nerve stimulation. 
     During electrosurgery, the AC generated by the electrosurgical generator is conducted through tissue disposed between the active and return electrodes. The tissue&#39;s impedance converts the electrical energy (also referred to as electrosurgical energy) associated with the AC into heat, which causes the tissue temperature to rise. The electrosurgical generator controls the heating of the tissue by controlling the electric power (i.e., electrical energy per time) provided to the tissue. Although many other variables affect the total heating of the tissue, increased current density usually leads to increased heating. The electrosurgical energy is typically used for cutting, dissecting, ablating, coagulating, and/or sealing tissue. 
     The two basic types of electrosurgery employed are monopolar and bipolar electrosurgery. Both of these types of electrosurgery use an active electrode and a return electrode. In bipolar electrosurgery, the surgical instrument includes an active electrode and a return electrode on the same instrument or in very close proximity to one another, usually causing current to flow through a small amount of tissue. In monopolar electrosurgery, the return electrode is located elsewhere on the patient&#39;s body and is typically not a part of the electrosurgical instrument itself. In monopolar electrosurgery, the return electrode is part of a device usually referred to as a return pad. 
     Some electrosurgical generators include a controller that controls the power delivered to the tissue over some period of time based upon measurements of the voltage and current near the output of the electrosurgical generator. These generators use a discrete Fourier transform (DFT) or polyphase demodulation to calculate the phase difference between measurements of the voltage and current for calculating real power and for performing calibration and compensation. 
     However, at low power levels, some electrosurgical generators exhibit low efficiencies. Thus, there is a need for improved methods of maintaining the efficiency of electrosurgical generators. 
     SUMMARY 
     A method for controlling an output of an electrosurgical generator includes the steps of converting a direct current (DC) to an alternating current (AC) using an inverter, and sensing a current and a voltage at an output of the inverter. The method further includes the steps of determining a power level based on the sensed voltage and the sensed current, determining an efficiency of the electrosurgical generator, and inserting a predetermined integer number of off cycles when the efficiency of the electrosurgical generator reaches a threshold power efficiency. 
     According to a further aspect of the present disclosure, an electrosurgical generator includes a radio frequency (RF) amplifier coupled to an electrical energy source and configured to generate electrosurgical energy, the RF amplifier including: an inverter configured to convert a direct current (DC) to an alternating current. The electrosurgical generator further includes a plurality of sensors configured to sense voltage and current of the generated electrosurgical energy and a controller coupled to the RF amplifier and the plurality of sensors. The generator may further determine a power level based on the sensed voltage and the sensed current, determine an efficiency of the electrosurgical generator, and insert a predetermined integer number of off cycles when the efficiency of the electrosurgical generator reaches a threshold power efficiency. 
     According to another aspect of the present disclosure a method of improving efficiency of an electrosurgical generator includes determining power levels based on sensed voltage and sensed current, determining an efficiency of the electrosurgical generator based on the detected power levels, and gradually dropping a predetermined integer number of output or off cycles when the efficiency of the electrosurgical generator reaches a threshold power efficiency, the predetermined integer number of output or off cycles being randomized via a random number generator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present disclosure are described with reference to the accompanying drawings wherein: 
         FIG. 1  is an illustration of an electrosurgical system including a generator, in accordance with embodiments of the present disclosure; 
         FIG. 2A  is a block diagram of an electrosurgical system including generator circuitry according to a combination of a modified-Kahn technique and a Class S generator topology, in accordance with one embodiment of the present disclosure; 
         FIG. 2B  is a block diagram of an electrosurgical system including generator circuitry according to the modified-Kahn technique, in accordance with another embodiment of the present disclosure; 
         FIG. 2C  is a block diagram of an electrosurgical system including generator circuitry according to the Class S device topology, in accordance with still another embodiment of the present disclosure; 
         FIG. 3  a schematic block diagram of a controller of the generator circuitry of  FIG. 2A , in accordance with an embodiment of the present disclosure; 
         FIG. 4  is a circuit diagram illustrating switching in different resonant components, in accordance with an embodiment of the present disclosure; and 
         FIGS. 5A and 5B  are graphs illustrating insertion of output cycles at predetermined time periods, in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an electrosurgical system  100  in accordance with embodiments of the present disclosure. The electrosurgical system  100  includes an electrosurgical generator  110  which generates electrosurgical energy to treat tissue of a patient. The electrosurgical generator  110  generates an appropriate level of electrosurgical energy based on the selected mode of operation (e.g., cutting, coagulating, ablating, or sealing) and/or the sensed voltage and current waveforms of the electrosurgical energy. The electrosurgical system  100  may also include a plurality of output connectors corresponding to a variety of electrosurgical instruments. 
     The electrosurgical system  100  further includes a monopolar electrosurgical instrument  120  having an electrode for treating tissue of the patient (e.g., an electrosurgical cutting probe or ablation electrode) with a return pad  125 . The monopolar electrosurgical instrument  120  can be connected to the electrosurgical generator  110  via one of the plurality of output connectors. The electrosurgical generator  110  may generate electrosurgical energy in the form of radio frequency (RF) energy. The electrosurgical energy is supplied to the monopolar electrosurgical instrument  120 , which applies the electrosurgical energy to treat the tissue. The electrosurgical energy is returned to the electrosurgical generator  110  through the return pad  125 . The return pad  125  provides a sufficient contact area with the patient&#39;s tissue so as to minimize the risk of tissue damage due to the electrosurgical energy applied to the tissue. In addition, the electrosurgical generator  110  and the return pad  125  may be configured to monitor tissue-to-patient contact to ensure that sufficient contact exists between the return pad  125  and the patient to minimize the risk of tissue damages. 
     The electrosurgical system  100  also includes a bipolar electrosurgical instrument  130 , which can be connected to the electrosurgical generator  110  via one of the plurality of output connectors. During operation of the bipolar electrosurgical instrument, electrosurgical energy is supplied to one of the two jaw members, e.g., jaw member  132 , of the instrument&#39;s forceps, is applied to treat the tissue, and is returned to the electrosurgical generator  110  through the other jaw member, e.g., jaw member  134 . 
     The electrosurgical generator  110  may be any suitable type of generator and may include a plurality of connectors to accommodate various types of electrosurgical instruments (e.g., monopolar electrosurgical instrument  120  and bipolar electrosurgical instrument  130 ). The electrosurgical generator  110  may also be configured to operate in a variety of modes, such as ablation, cutting, coagulation, and sealing. The electrosurgical generator  110  may include a switching mechanism (e.g., relays) to switch the supply of RF energy among the connectors to which various electrosurgical instruments may be connected. For example, when an electrosurgical instrument  120  is connected to the electrosurgical generator  110 , the switching mechanism switches the supply of RF energy to the monopolar plug. In embodiments, the electrosurgical generator  110  may be configured to provide RF energy to a plurality of instruments simultaneously. 
     The electrosurgical generator  110  includes a user interface having suitable user controls (e.g., buttons, activators, switches, or touch screens) for providing control parameters to the electrosurgical generator  110 . These controls allow the user to adjust parameters of the electrosurgical energy (e.g., the power level or the shape of the output waveform) so that the electrosurgical energy is suitable for a particular surgical procedure (e.g., coagulating, ablating, tissue sealing, or cutting). The electrosurgical instruments  120  and  130  may also include a plurality of user controls. In addition, the electrosurgical generator  110  may include one or more display screens for displaying a variety of information related to operation of the electrosurgical generator  110  (e.g., intensity settings and treatment complete indicators). The electrosurgical instruments  120  and  130  may also include a plurality of input controls that may be redundant with certain input controls of the electrosurgical generator  110 . Placing the input controls at the electrosurgical instruments  120  and  130  allows for easier and faster modification of the electrosurgical energy parameters during the surgical procedure without requiring interaction with the electrosurgical generator  110 . 
       FIG. 2A  is a block diagram of generator circuitry  200  within the electrosurgical generator of  FIG. 1 . The generator circuitry  200  includes a low frequency (LF) rectifier  220 , a direct current-to-direct current (DC/DC) converter  225 , an RF amplifier  230 , a plurality of sensors  240 , analog-to-digital converters (ADCs)  250 , a controller  260 , a hardware accelerator  270 , a processor subsystem  280 , and a user interface (UI)  290 . The generator circuitry  200  is configured to connect to a power source  210 , such as a wall power outlet or other power outlet, which generates alternating current (AC) having a low frequency (e.g., 25 Hz, 50 Hz, or 60 Hz). The power source  210  provides the AC power to the LF rectifier  220 , which converts the AC to direct current (DC). Alternatively, the power source  210  and the LF rectifier  220  may be replaced by a battery or other suitable device to provide DC power. 
     The DC output from the LF rectifier  220  is provided to the DC/DC converter  225  which converts the DC to a desired level. The converted DC is provided to the RF amplifier  230 , which includes a DC-to-AC (DC/AC) inverter  232  and a resonant matching network  234 . The DC/AC inverter  232  converts the converted DC to an AC waveform having a frequency suitable for an electrosurgical procedure (e.g., 472 kHz, 29.5 kHz, and 19.7 kHz). 
     The appropriate frequency for the electrosurgical energy may differ based on electrosurgical procedures and modes of electrosurgery. For example, nerve and muscle stimulations cease at about 100,000 cycles per second (100 kHz) above which point some electrosurgical procedures can be performed safely, i.e., the electrosurgical energy can pass through a patient to targeted tissue with minimal neuromuscular stimulation. For example, typically, ablation procedures use a frequency of 472 kHz. Other electrosurgical procedures can be performed at pulsed rates lower than 100 kHz, e.g., 29.5 kHz or 19.7 kHz, with minimal risk of damaging nerves and muscles, e.g., Fulgurate or Spray. The DC/AC inverter  232  can output AC signals with various frequencies suitable for electrosurgical operations. 
     As described above, the RF amplifier  230  includes a resonant matching network  234 . The resonant matching network  234  is coupled to the output of the DC/AC inverter  232  to match the impedance at the DC/AC inverter  232  to the impedance of the tissue so that there is maximum or optimal power transfer between the generator circuitry  200  and the tissue. 
     The electrosurgical energy provided by the DC/AC inverter  232  of the RF amplifier  230  is controlled by the controller  260 . The voltage and current waveforms of the electrosurgical energy output from the DC/AC inverter  232  are sensed by the plurality of sensors  240  and provided to the controller  260 , which generates control signals from a DC/DC converter controller  278 , e.g., a pulse width modulator (PWM) or digital pulse width modulator (DPWM) to control the output of the DC/DC converter  225  and from a DC/AC inverter controller  276  to control the output of the DC/AC inverter  232 . The controller  260  also receives input signals via the user interface (UI)  290 . The UI  290  allows a user to select a type of electrosurgical procedure (e.g., monopolar or bipolar) and a mode (e.g., coagulation, ablation, sealing, or cutting), or input desired control parameters for the electrosurgical procedure or the mode. The DC/DC converter  225  of  FIG. 2A  may be fixed or variable depending on the power setting or desired surgical effects. When it is fixed, the RF amplifier behaves as a Class S device, which is shown in  FIG. 2C . When it is variable, it behaves as a device according to the modified-Kahn technique, which is shown in  FIG. 2B . 
     The plurality of sensors  240  sense voltage and current at the output of the RF amplifier  230 . The plurality of sensors  240  may include two or more pairs or sets of voltage and current sensors that provide redundant measurements of the voltage and current. This redundancy ensures the reliability, accuracy, and stability of the voltage and current measurements at the output of the RF amplifier  230 . In embodiments, the plurality of sensors  240  may include fewer or more sets of voltage and current sensors depending on the application or the design requirements. The plurality of sensors  240  may also measure the voltage and current output from other components of the generator circuitry  200  such as the DC/AC inverter  232  or the resonant matching network  234 . The plurality of sensors  240  may include any known technology for measuring voltage and current including, for example, a Rogowski coil. 
     The sensed voltage and current waveforms are fed to analog-to-digital converters (ADCs)  250 . The ADCs  250  sample the sensed voltage and current waveforms to obtain digital samples of the voltage and current waveforms. This is also often referred to as an Analog Front End (AFE). The digital samples of the voltage and current waveforms are processed by the controller  260  and used to generate control signals to control the DC/AC inverter  232  of the RF amplifier  230  and the DC/DC converter  225 . The ADCs  250  may be configured to sample the sensed voltage and current waveforms at a sample frequency that is an integer multiple of the RF frequency. 
     As shown in the embodiment of  FIG. 2A , the controller  260  includes a hardware accelerator  270  and a processor subsystem  280 . As described above, the controller  260  is also coupled to a UI  290 , which receives input commands from a user and displays output and input information related to characteristics of the electrosurgical energy (e.g., selected power level). The hardware accelerator  270  processes the output from the ADCs  250  and cooperates with the processor subsystem  280  to generate control signals. 
     The hardware accelerator  270  includes a dosage monitoring and control (DMAC)  272 , an inner power control loop  274 , a DC/AC inverter controller  276 , and a DC/DC converter controller  278 . All or a portion of the controller  260  may be implemented by a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), a digital signal processor (DSP), and/or a microcontroller. 
     The DMAC  272  receives samples of the sensed voltage and current waveforms from the ADCs  250  and calculates the average real power and the real part of the tissue impedance. The DMAC  272  then provides the real power and the real part of the impedance of the tissue to the inner power control loop  274 , which generates a control signal for the DC/AC inverter controller  276  based on one or more of the real power and the real part of the impedance of the tissue. The DC/AC inverter controller  276  in turn generates a first pulse-width modulation (PWM) control signal to control the output of the DC/AC inverter  232 . 
     The processor subsystem  280  includes an outer power control loop  282 , a state machine  284 , and a power setpoint circuit  286 . The processor subsystem  280  generates a second PWM control signal based on the output of the DMAC  272  and parameters (e.g., electrosurgical mode) selected by the user via the UI  290 . Specifically, the parameters selected by the user are provided to the state machine  284  which determines a state or mode of the generator circuitry  200 . The outer power control loop  282  uses this state information and the output from the DMAC  272  to determine control data. The control data is provided to the power setpoint circuit  286 , which generates a power setpoint based on the control data. The DC/DC converter controller  278  uses the power setpoint to generate an appropriate PWM control signal for controlling the DC/DC converter  225  to converter the DC output from the LF rectifier  220  to a desired level. If the user does not provide operational parameters to the state machine  284  via the UI  290 , then the state machine  284  may maintain or enter a default state. 
       FIG. 3  shows a more detailed diagram of the hardware accelerator  270  of  FIG. 2A . The hardware accelerator  270  implements those functions of the generator circuitry  200  that may have special processing requirements such as high processing speeds. The hardware accelerator  270  includes the DMAC  272 , the inner power loop control  274 , the DC/AC inverter controller  276 , and the DC/DC converter controller  278  shown in  FIG. 2A . 
     The DMAC  272  includes four analog-to-digital converter (ADC) controllers  312   a - 312   d , a digital signal processor  314 , RF data registers  316 , and DMAC registers  318 . The ADC controllers  312   a - 312   d  control the operation of the ADCs  250  ( FIG. 2A ), which convert sensed voltage and current waveforms into digital data. The digital data is then provided to the digital signal processor  314  that implements various filtering and other digital signal processing functions. 
     The sensed voltage and current are the digital input to the ADCs  250 , which sample the sensed voltage and current. The ADC controllers  312   a - 312   d  provide operational parameters, including a predetermined sampling rate, to the ADCs  250  so that the ADCs sample the sensed voltage and current synchronously at a predetermined sampling rate, i.e., a predetermined number of samples per second, or predetermined sampling period that is coherent with the RF inverter frequency, i.e., an integer multiple sampling frequency to the RF inverter frequency. The ADC controllers  312   a - 312   d  control the operation of the ADCs  250 , which convert sensed voltage and current waveforms into digital data. The digital data is then provided to the digital signal processor  314  that implements various filtering and other digital signal processing functions. 
     The sensed voltage and current are input to the ADCs  250 , which sample the sensed voltage and current. The ADC controllers  312   a - 312   d  provide operational parameters, including a predetermined sampling rate, to the ADCs  250  so that the ADCs sample the sensed voltage and current synchronously at a predetermined sampling rate, i.e., a predetermined number of samples per second, or predetermined sampling period. The ADC controllers  312   a - 312   d  may be configured to control the ADCs  250  so that the sampling period corresponds to an integer multiple of the RF frequency of the voltage and current waveforms. This is often referred to as coherent sampling. 
     The digital data obtained by sampling the voltage and current waveforms is provided to the digital signal processor  314  via the ADC controllers  312   a - 312   d . The digital signal processor  314  uses the digital data to calculate a complex voltage V comp , a complex current I comp , a real power P real , and a real part of the tissue impedance Z real . Generally, tissue impedance is real or resistive, but can have a small capacitive component after the tissue is “cooked.” Further, a cable between the electrosurgical generator and the tissue also has resistive and reactive components. For these reasons, electrosurgical generators typically include controls systems that compensate for these parasitics to more accurately measure the tissue impedance. These control systems, however, require complex computations that are computationally inefficient, which results in additional cost to perform the tissue impedance calculations in a timely manner or at update rates commensurate to the capabilities of the RF control loop calculations. 
     In alternative embodiments depicted in  FIGS. 2B and 2C , the hardware accelerator is not available and many of the primary RF measurement and control functions just described reside instead entirely within a programmable device called an application specific standard product (ASSP) integrated circuit that includes at least a DSP core processor and multiple digital pulse width modulators (DPWM) that are substantially similar in function to the hardware accelerator and its DSP and/or microcontroller core. 
     In other embodiments, there may also be a second microprocessor core available within the ASSP that contains additional ADCs which may be connected to the sensors for performing the redundant dosage monitoring functions separately from the RF control functions. The second processor may also perform user interface functions such as receiving and requesting power settings, activation requests, and so forth for the user from the RF controller. The ASSP may also utilize only one RF control loop (or compensator loop), instead of two “inner” and “outer” compensator loops, for controlling directly any of the following: power, voltage, current, temperature, or impedance. This loop may use a single proportional-integral-derivative compensator that changes between these process variables using bumpless transfer methods and saturable limits. 
       FIG. 2B  shows an electrosurgical system including generator circuitry according to the modified-Kahn technique  201 . The generator circuitry  201  includes an RF amplifier  241  and a controller  251  for controlling the RF amplifier  241  to deliver electrosurgical energy having desired characteristics to tissue  247  being treated. The RF amplifier  241  receives AC or DC from the power source  210 . The RF amplifier includes an AC/DC or DC/DC converter  242 , which converts the AC or DC provided by the power source  210  into a suitable level of DC. As in  FIG. 2A , the RF amplifier  241  also includes a DC/AC inverter  232  which converts the DC to AC. The RF amplifier  241  also includes a single- or dual-mode resonant matching network  244  and mode relays  248  for switching modes of the resonant matching network  244 . 
     The output from the RF amplifier  241  is provided to sensors  246 , which may include voltage sensors, current sensors, and temperature sensors. The sensor signals output from sensors  246  are provided to the controller  251  via an analog front end (AFE)  252  of the controller  251 . The AFE conditions and samples the sensor signals to obtain digital sensor data representing the sensor signals. The controller  251  also includes a signal processor  253 , a mode state control and bumpless transfer unit  254 , a compensator or PID controller  255 , a pulse width modulator (PWM) or digital pulse width modulator (DPWM)  256 , and a voltage-controlled oscillator or numerically-controlled oscillator  257 . 
     The signal processor  253  receives the digital sensor data and performs the calculations and other functions of the systems and methods according to the present disclosure. Among other things, the signal processor  253  calculates the real and imaginary parts of the sensed voltage and current, the impedance, and/or the power, and performs functions to control one or more of the voltage, current, power, impedance, and temperature. The signal processor  253  also generates and provides process variables to the mode state control and bumpless transfer unit  254  and a compensator or PID controller  255 . The mode state control and bumpless transfer unit  254  controls the mode relays  248  for the single or dual mode resonant matching network  244  according to the tissue effect algorithm, and generates and provides coefficients and setpoints to the compensator or PID controller  255 . 
     The compensator or PID controller  255  generates controller output variables and provides them to the pulse width modulator (PWM) or digital pulse width modulator (DPWM)  256 . The pulse width modulator (PWM) or digital pulse width modulator (DPWM)  256  receives an oscillator signal from the voltage-controlled oscillator or the numerically-controlled oscillator  257  and generates a control signal for controlling the AC/DC or DC/DC converter  242 . The voltage-controlled oscillator or the numerically-controlled oscillator  257  also generates control signals for controlling the DC/AC inverter  232 . 
     Like the generator circuitry  200  of  FIG. 2A , the generator circuitry  201  includes a user interface  290  through which a user can control and/or monitor the functions of the generator circuitry  201  via a controller application interface  258  of the controller  251 . 
       FIG. 2C  shows an electrosurgical system including generator circuitry according to a Class S device topology  202 . Unlike the generator circuitry  201  of  FIG. 2B , the generator circuitry  202  does not include the AC/DC or DC/DC Converter  242 . An external low-frequency (LF) rectifier  220  or battery provides an appropriate level of DC to the DC/AC Inverter  232  of the RF amplifier  241 . As shown in  FIG. 2C , the PWM or DPWM  256  receives an oscillator signal from the VCO or NCO  257  and generates a control signal for controlling the DC/AC Inverter  232 . 
     The output of the digital signal processor  314  is provided to the processor subsystem  280  of  FIG. 2A  via RF data registers  316  (see  FIG. 3 ). The DMAC  272  also includes DMAC registers  318  that receive and store relevant parameters for the digital signal processor  314  (see  FIG. 3 ). The digital signal processor  314  further receives signals from a PWM module  346  of the DC/AC inverter controller  276 . 
     The DMAC  272  provides a control signal to the inner power control loop  274  via signal line  321  and to the processor subsystem  280  via signal line  379 . The inner power control loop  274  processes the control signal and outputs a control signal to the DC/AC inverter controller  276 . The inner power control loop  274  includes a compensator  326 , compensator registers  330 , and VI limiter  334 . The signal line  321  carries and provides a real part of the impedance to the compensator  326 . 
     When there is a user input, the processor subsystem  280  receives the user input and processes it with the outputs from the digital signal processor  314  via a signal line  379 . The processor subsystem  280  provides control signals via a compensator registers  330  to a VI limiter  334 , which corresponds to the power setpoint circuit  286  in  FIG. 2A . The VI limiter  334  then provides a desired power profile (e.g., a minimum and a maximum limits of the power for a set electrosurgical mode or operation) based on the user input and the output of the digital signal processor  314 , the compensator registers  330  also provide other control parameters to the compensator  326 , and then the compensator  326  combines all control parameters from the compensator registers  330  and the VI limiter  334 , to generate output to the DC/AC inverter controller  276  via signal line  327 . 
     The DC/AC inverter controller  276  receives a control parameter and outputs control signals that drives the DC/AC inverter  232 . The DC/AC inverter controller  276  includes a scale unit  342 , PWM registers  344 , and the PWM module  346 . The scale unit  342  scales the output of the compensator registers  330  by multiplying and/or adding a number to the output. The scale unit  342  receives a number for multiplication and/or a number for addition from the PWM registers  344  via signal lines,  341   a  and  341   b . The PWM registers  344  store several relevant parameters to control the DC/AC inverter  232 , e.g., a period, a pulse width, and a phase of the AC signal to be generated by the DC/AC inverter  232  and other related parameters. The PWM registers  344  send signals  345   a - 345   d  to the PWM module  346 . The PWM module  346  receives output from the PWM registers  344  and generates four control signals,  347   a - 347   d , that control four transistors of the DC/AC inverter  232  of the RF amplifier  230  in  FIG. 2A . The PWM module  346  also synchronizes its information with the information in the PWM registers  344  via a register sync signal  347 . 
     The PWM module  346  further provides control signals to the compensator  326  of the inner power control loop  274 . The processor subsystem  280  provides control signals to the PWM module  346 . In this way, the DC/AC inverter controller  276  can control the DC/AC inverter  232  of the RF amplifier  230  with integrated internal input (i.e., processed results from the plurality of sensors by the DMAC  272 ) and external input (i.e., processed results from the user input by the processor subsystem  280 ). 
     The processor subsystem  280  also sends the control signals to the DC/DC converter controller  278  via signal line  373 . The DC/DC converter controller  278  processes the control signals and generates another control signals so that the DC/DC converter  225  converts direct current to a desired level suitable for being converted by the RF amplifier  230 . The DC/DC converter controller  278  includes PWM registers  352  and a PWM module  354 . The PWM registers  352  receive outputs from the processor subsystem  280  via signal line  373  and stores relevant parameters as the PWM registers  344  does. The PWM registers  352  send signals  353   a - 353   d  to the PWM module  354 . The PWM module  354  also sends a register sync signal to the PWM registers  352  and generates four control signals,  355   a - 355   d , that control four transistors of the DC/DC converter  225  in  FIG. 2A . 
       FIG. 4  is a circuit diagram  400  illustrating switching in different resonant components, in accordance with an embodiment of the present disclosure. The circuit diagram  400  illustrates mode relays  248  and matching network  244 . The mode relays  248  allow a user to switch between different operating modes. For example, the top mode relay  248  allows a user to switch between a cut mode and a spray mode, whereas the bottom mode relay  248  allows a user to switch between a ligature mode and a blend mode. One skilled in the art may contemplate a plurality of relays for switching between a plurality of operating modes. Additionally, the capacitors  410  and the inductors  420  are appropriately sized for the selected mode. The matching network  244  includes two transformers  430  to vary the relative voltage of the circuit  400  and provide for patient isolation. 
     The preceding description provides a detailed account of the components and devices for controlling the output of an electrosurgical generator  110 . Typically, the manner in which average power output from a DC/AC inverter (and then applied to a patient) is reduced is by reducing the pulse width of the PWM signal output by the DC/AC inverter controller  276  ( FIG. 2A ). However, power control in this manner can result in a loss of efficiency when operating a low power setting. 
       FIG. 5A  depicts an output signal of the DC/AC inverter controller  276  ( FIG. 2A ) and particularly the PWM module  346  ( FIG. 3 ), which control the DC/AC inverter  232 . In  FIG. 5A , the initial signal is a continuous wave (CW) controlling the DC/AC inverter  232 . The pulses (i.e., the high and low signals) of the CW have a short pulse width T PW     High   . By shortening the pulse width, the average power output by the DC/AC inverter  232 , and ultimately applied to the patient, is reduced. As noted above, however, mere reduction in the pulse width when using a CW can result in efficiency losses as the average power output is reduced. 
     According to one embodiment of the present disclosure, the efficiency of the DC/AC inverter  232  may be increased by lengthening the pulse width to T PW     Low   , and transitioning from a CW to a pulsed wave (PW) with a 50% duty cycle. In other words, pulses are only sent to the DC/AC inverter  232  during 50% of a period T. In the example of  FIG. 5A , during the period T, four cycles of pulsed signals are produced during a period T on  followed by no signal being produced for a period T off , which is also four cycles, thus T on  and T off  are equal (i.e., represent the same period of time). Further, in this example, by lengthening the pulse width T PW     Low    to approximately twice T PW     High   , the average power output by the DC/AC inverter  232  may be maintained. However, because of the decrease in the number of switchings that occur at the DC/AC inverter  232 , an increase of the time between such switchings (i.e., T PW     Low   &gt;T PW     High   ), and a period T off  where no switchings occur, an increase in efficiency is achieved as compared to simply reducing the pulse width of a CW. 
     In an alternative or additional embodiment, the efficiency at low power levels may be improved by dropping or deactivating at least some predetermined integer number of output cycles. As shown in  FIG. 5B , the duty cycle of the PW which is supplied to the DC/AC inverter  232  remains at 50%. However, rather than a PW where a period T on  is followed by a period T off , where periods T on  and T off  are equal, the periods of T off  may be randomly dispersed in the period T, as represented by the periods of no signal  510 . The aggregate time for the periods of no signal  510  are equivalent to T off  (shown in  FIG. 5A ), and result in the same output from the DC/AC inverter, when the amplitude of the current remains constant. 
     The result of the signaling schemes depicted in  FIGS. 5A and 5B  is that two time parameters are employed in achieving a desired average power. The first is the overall duty cycle (shown as 50%) whereby no signal is supplied during T off  or half of the period T. The other is the pulse width (e.g., T PW     Low   ) of the signal supplied during T on , resulting in greater control and greater efficiency when low powers are desired for use by a clinician. As an example, lower power may be 10% of rated power of the electrosurgical generator  110 . 
     It is noted that the efficiency may be determined by the controller  260 . However, in certain circumstances, the efficiency may not be determined by the controller  260 , but with other external efficiency computing components/elements. Reference is made to U.S. Provisional Patent No. 61/838,753 entitled “DEAD-TIME OPTIMIZATION OF RESONANT INVERTERS,” the entire contents of which are hereby incorporated by reference, for alternative online and/or offline methods of determining efficiency. For example, the efficiency of the system may be characterized offline and the off cycles may be inserted when a predetermined threshold is reached for a control parameter, such as, duty cycle or phase, or when a sensed power drops below a certain threshold. 
     Moreover, the dropped cycles, within any given time period, may be randomized by using a pseudo-random sequence determined by a random number generator. The random number generator may be, for example, a Galois sequence that spreads out the spectrum in order to mitigate any undesirable frequencies. As a result, a useful range of operation of the electrosurgical generator  110  may be extended, while maintaining reasonable energy conversion efficiency. 
     It is to be understood that the disclosed embodiments are merely exemplary of the disclosure, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the disclosure. 
     It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. In this document, the terms “a” or “an”, as used herein, are defined as one or more than one. The term “plurality,” as used herein, is defined as two or more than two. The term “another,” as used herein, is defined as at least a second or more. The terms “including” and/or “having,” as used herein, are defined as comprising (i.e., open language). The term “coupled,” as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. Relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “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. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
     As used herein, the term “about” or “approximately” applies to all numeric values, whether or not explicitly indicated. These terms generally refer to a range of numbers that one of skill in the art would consider equivalent to the recited values (i.e., having the same function or result). In many instances these terms may include numbers that are rounded to the nearest significant figure. In this document, the term “longitudinal” should be understood to mean in a direction corresponding to an elongated direction of the object being described. Finally, as used herein, the terms “distal” and “proximal” are considered from the vantage of the user or surgeon, thus the distal end of a surgical instrument is that portion furthest away from the surgeon when in use, and the proximal end is that portion generally closest to the user. 
     It will be appreciated that embodiments of the disclosure described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits and other elements, some, most, or all of the functions of ultrasonic surgical instruments described herein. The non-processor circuits may include, but are not limited to, signal drivers, clock circuits, power source circuits, and user input and output elements. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic, or in a field-programmable gate array (FPGA) enabling the use of updateable custom logic either by the manufacturer or the user. Of course, a combination of the three approaches could also be used. Thus, methods and means for these functions have been described herein. 
     From the foregoing, and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications may also be made to the present disclosure without departing from the scope of the same. While several embodiments of the disclosure have been shown in the drawings and/or described herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.