Patent Publication Number: US-2017354455-A1

Title: Variable active snubber circuit to induce zero-voltage-switching in a current-fed power converter

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to systems and methods for controlling an electrosurgical generator. In particular, the present disclosure relates to a current source electrosurgical generator having a power converter and a variable active snubber circuit configured to achieve zero voltage switching within the power converter. 
     Background of Related Art 
     Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, desiccate, or coagulate tissue. In monopolar electrosurgery, a source or active electrode delivers radio frequency alternating current from the electrosurgical generator to the targeted tissue. A patient return electrode is placed remotely from the active electrode to conduct the current back to the generator. 
     In bipolar electrosurgery, return and active electrodes are placed in close proximity to each other such that an electrical circuit is formed between the two electrodes (e.g., in the case of an electrosurgical forceps). In this manner, the applied electrical current is limited to the body tissue positioned between the electrodes. Accordingly, bipolar electrosurgery generally involves the use of instruments where it is desired to achieve a focused delivery of electrosurgical energy between two electrodes. 
     Conventional electrosurgical generators may utilize voltage-fed or current-fed power converters. Current-fed power converters have a number of advantages over voltage-fed converters including control of arcs, desirable transient performance, and simplified control dynamics. However, current-fed power converters also present a number of problems, such as power dissipation, which limits their usability. Accordingly, there is a need for a system and method to control an electrosurgical generator including a current-fed power converter that overcomes these problems. 
     SUMMARY 
     The present disclosure provides for an electrosurgical generator including a power converter having a plurality of switching elements, such as FETs. The generator also includes a current source, which may be an inductor, and a variable active snubber circuit. The current source supplies current to the power converter, whereas the variable active snubber circuit mitigates some of the effects of the current source on the power converter to achieve zero voltage switching therein. 
     Electrosurgical generators according to the present disclosure may include a current source having a relatively large inductance to smooth the current supplied to the power converter. In particular, the inductance of the current source provides advantages over voltage-fed generators, as it naturally regulates and stabilizes the current to downstream elements, e.g., power converter. This allows for better control of arcing, e.g., generating or mitigating arcing, and minimizing transient performance. 
     Electrosurgical generators including voltage-fed converters operate based on switching on/off of switching-elements to control the power. In contrast, electrosurgical generators including current-fed converters control power delivery by shorting the current to ground, or some other return path. Thus, when the voltage-fed converter turns on all of the switching elements, this results in large power dissipation with destructive currents. To deal which these surges, at least one of the switching elements remains open or off at all times. For the current-fed converter, shorting or shunting the current source, e.g., an inductor, no significant power is dissipated while the current flow is maintained. The problem for the current-fed converters, however, is that if all of the switching elements are off at the same time, such that the current flow is interrupted, destructive voltages are generated, as the current flowing in the inductor is interrupted. Conversely, when the switching elements are turned on in the current-fed converter, the full normal load voltage is present across the switching elements. This results in significant power dissipation in the switching elements, large noise, voltage and current spikes, and potentially large radiated and conducted electromagnetic interference. In particular, sensitive circuits, e.g., sense circuits, may be impacted and measurement and control may be degraded and other equipment in the operating room may also be disrupted due to these surges. This is highly undesirable because switching losses can become significant enough to cause excessive heating of various components and require larger, more robust components and cooling. 
     The switching elements of the power converter operate as voltage-controlled resistive devices and due to their construction do not dissipate heat efficiently. Thus, even when the switching elements are turned off, large currents are passing through them even as the voltage across the switching elements remains very close to zero for the nanoseconds required for the switching element to actually stop conducting. However, when the switching element turns on, any voltage across the device is shunted very quickly as the switching element begins to conduct. This results in the large switching losses associated with current-fed generators as described above. 
     The generator according to the present disclosure provides for a variable active snubber circuit which induces zero-voltage switching in the power converter to deal with the above-described switching losses associated with current-fed power converters. Bringing the voltage across the switching elements to zero prior to turning them on, eliminates the large current spikes associated with the load voltage. Accordingly, this also overcomes the above-described disadvantages of using the current source in an electrosurgical generator. Previous attempts to deal with this problem involved the use of auxiliary circuits to provide shunting of the current by secondary elements. These configurations involve some form of pre-shunting of the current to reduce or eliminate the voltage before the switching elements are closed. Although these circuits mitigate the problem, they are not entirely effective, as they introduce additional current and noise spikes, albeit of diminished magnitude. 
     According to one embodiment of the present disclosure, an electrosurgical generator is provided. The electrosurgical generator includes: a power supply configured to output a direct current; a current source coupled to the power supply and configured to output source current based on the direct current; and a power converter coupled to the current source, the power converter including at least one power switching element operated at a switching waveform. The power converter is configured to generate a converted waveform based on the source current. The electrosurgical generator also includes a controller coupled to the power converter and configured to modulate the switching waveform and a snubber circuit coupled to the current source and the power converter. The snubber circuit is configured to return the voltage at the at least one power switching element to zero after the power converter generates at least a portion of the converted waveform. 
     According to another embodiment of the present disclosure, an electrosurgical generator is provided. The electrosurgical generator includes: a power supply configured to output direct current; a current source coupled to the power supply and configured to output source current based on the direct current; and a power converter coupled to the current source, the power converter including four power switching elements arranged in an H-bridge topology and operated at a switching waveform. The power converter is configured to generate a converted waveform based on the source current. The electrosurgical generator also includes a controller coupled to the power converter and configured to modulate the switching waveform and a snubber circuit coupled to the current source and the power converter. The snubber circuit is configured to return the voltage at each of the power switching elements to zero after the power converter generates at least a portion of the converted waveform. 
     According to an aspect of any of the above embodiments, the snubber circuit may include a snubber inductor, a snubber capacitor, and a snubber catch diode, all of which are interconnected in series. The snubber circuit may further include at least one snubber switching element coupling the snubber inductor with the snubber capacitor and the snubber catch diode. Voltage at the snubber inductor rises and is clamped by the snubber catch diode and charges the snubber capacitor in response to deactivation of the power converter. Current in the snubber inductor is reversed after the snubber capacitor is charged and the current from the snubber inductor causes ring-back to counteract capacitance of the power converter. 
     According to another aspect of any of the above embodiments, the controller is coupled to the snubber circuit and is configured to control the at least one snubber switching element to maintain a desired voltage in the snubber capacitor. 
     According to a further embodiment of the present disclosure a method for controlling an electrosurgical generator is provided. The method includes: activating a first pair of power switching elements and a second pair of power switching elements of a power converter; increasing current at a current source coupled to the power converter; deactivating the first pair of the power switching elements to generate a radio frequency pulse; deactivating at least one power switching element of the second pair of the power switching elements; and activating a snubber circuit coupled to the current source and the power converter to return the voltage at each of the power switching elements to zero prior to reactivating the first pair of power switching elements and the second pair of power switching elements. 
     According to an aspect of the above embodiment, activating the snubber circuit may include increasing voltage at a snubber inductor of the snubber circuit. 
     According to another aspect of the above embodiment, activating the snubber circuit may further include clamping the current at the snubber inductor by a snubber catch diode of the snubber circuit in response to deactivating the at least one power switching element of the second pair of the power switching elements. 
     According to a further aspect of the above embodiment, activating the snubber circuit may further include charging a snubber capacitor of the snubber circuit. 
     According to yet another aspect of the above embodiment, activating the snubber circuit may further include controlling the at least one snubber switching element of the snubber circuit to maintain a desired voltage in the snubber capacitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be understood by reference to the accompanying drawings, when considered in conjunction with the subsequent, detailed description, in which: 
         FIG. 1  is a perspective view of a surgical system according to an embodiment of the present disclosure; 
         FIG. 2  is a front view of an electrosurgical generator of  FIG. 1  according to an embodiment of the present disclosure; 
         FIG. 3  is a schematic diagram of the electrosurgical generator of  FIG. 2  according to an embodiment of the present disclosure; 
         FIG. 4  is a schematic diagram of a variable active snubber circuit of the electrosurgical generator of  FIG. 2  according to the present disclosure; 
         FIG. 5  is a plot of a switching waveform, a generator output waveform, a power converter waveform, and a current source waveform according to the present disclosure; and 
         FIG. 6  is a flow chart of a method for operating the electrosurgical generator of  FIG. 2  according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Particular embodiments of the present disclosure will be described below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. Those skilled in the art will understand that the present disclosure may be adapted for use with either an endoscopic instrument, a laparoscopic instrument, or an open instrument. It should also be appreciated that different electrical and mechanical connections and other considerations may apply to each particular type of instrument. 
     A generator may be used in monopolar and/or bipolar electrosurgical procedures, including, for example, cutting, coagulation, ablation, and vessel sealing procedures. The generator may include a plurality of outputs for interfacing with various ultrasonic and electrosurgical instruments (e.g., ultrasonic dissectors and hemostats, monopolar instruments, return electrode pads, bipolar electrosurgical forceps, footswitches, etc.). Further, the generator may include electronic circuitry configured to generate radio frequency energy specifically suited for powering ultrasonic instruments and electrosurgical devices operating in various electrosurgical modes (e.g., cut, blend, coagulate, division with hemostasis, fulgurate, spray, etc.) and procedures (e.g., monopolar, bipolar, vessel sealing). 
       FIG. 1  is a perspective view of the components of one illustrative embodiment of an electrosurgical system  10  according to the present disclosure. The system  10  may include one or more monopolar electrosurgical instruments  20  having one or more active electrodes  23  (e.g., electrosurgical cutting probe, ablation electrode(s), etc.) for treating tissue of a patient. Electrosurgical alternating RF current is supplied to the instrument  20  by a generator  200  via a supply line  24  that is connected to an active terminal  230  ( FIG. 3 ) of the generator  200 , allowing the instrument  20  to cut, coagulate, thermally or non-thermally ablate and/or otherwise treat tissue. The alternating current is returned to the generator  200  through a return electrode pad  26  via a return line  28  at a return terminal  232  ( FIG. 3 ) of the generator  200 . For monopolar operation, the system  10  may include a plurality of return electrode pads  26  that, in use, are disposed on a patient to minimize the chances of tissue damage by maximizing the overall contact area with the patient. In addition, the generator  200  and the return electrode pads  26  may be configured for monitoring tissue-to-patient contact to ensure that sufficient contact exists therebetween. 
     The system  10  may also include one or more bipolar electrosurgical instruments, for example, a bipolar electrosurgical forceps  30  having one or more electrodes for treating tissue of a patient. The electrosurgical forceps  30  includes a housing  31  and opposing jaw members  33  and  35  disposed at a distal end of a shaft  32 . The jaw members  33  and  35  have one or more active electrodes  34  and a return electrode  36  disposed therein, respectively. The active electrode  34  and the return electrode  36  are connected to the generator  200  through cable  38  that includes the supply and return lines  24 ,  28 , which may be coupled to the active and return terminals  230 ,  232 , respectively ( FIG. 3 ). The electrosurgical forceps  30  is coupled to the generator  200  at a port having connections to the active and return terminals  230  and  232  (e.g., pins) via a plug disposed at the end of the cable  38 , wherein the plug includes contacts from the supply and return lines  24 ,  28  as described in more detail below. 
     With reference to  FIG. 2 , a front face  240  of the generator  200  is shown. The generator  200  may include a plurality of ports  250 - 262  to accommodate various types of electrosurgical instruments (e.g., monopolar electrosurgical instrument  20 , electrosurgical forceps  30 , etc.). 
     The generator  200  includes a user interface  241  having one or more display screens  242 ,  244 ,  246  for providing the user with variety of output information (e.g., intensity settings, treatment complete indicators, etc.). Each of the screens  242 ,  244 ,  246  is associated with a corresponding port  250 - 262 . The generator  200  includes suitable input controls (e.g., buttons, activators, switches, touch screen, etc.) for controlling the generator  200 . The screens  242 ,  244 ,  246  are also configured as touch screens that display a corresponding menu for the instruments (e.g., electrosurgical forceps  30 , etc.). The user then adjusts inputs by simply touching corresponding menu options. 
     Screen  242  controls monopolar output and the devices connected to the ports  250  and  252 . Port  250  is configured to couple to a monopolar electrosurgical instrument (e.g., electrosurgical instrument  20 ) and port  252  is configured to couple to a foot switch (not shown). The foot switch provides for additional inputs (e.g., replicating inputs of the generator  200 ). Screen  244  controls monopolar and bipolar output and the devices connected to the ports  256  and  258 . Port  256  is configured to couple to other monopolar instruments. Port  258  is configured to couple to a bipolar instrument (not shown). 
     Screen  246  controls the electrosurgical forceps  30  that may be plugged into one of the ports  260  and  262 , respectively. The generator  200  outputs energy through the ports  260  and  262  suitable for sealing tissue grasped by the electrosurgical forceps  30 . In particular, screen  246  outputs a user interface that allows the user to input a user-defined intensity setting for each of the ports  260  and  262 . The user-defined setting may be any setting that allows the user to adjust one or more energy delivery parameters, such as power, current, voltage, energy, etc. or sealing parameters, such as energy rate limiters, sealing duration, etc. The user-defined setting is transmitted to a controller  224  ( FIG. 3 ) where the setting may be saved in memory. In embodiments, the intensity setting may be a number scale, such as for example, from one to ten or one to five. In embodiments, the intensity setting may be associated with an output curve of the generator  200 . The intensity settings may be specific for each electrosurgical forceps  30  being utilized, such that various instruments provide the user with a specific intensity scale corresponding to the electrosurgical forceps  30 . The active and return terminals  230  and  232  may be coupled to any of the desired ports  250 - 262 . In embodiments, the active and return terminals  230  and  232  may be coupled to the ports  250 - 262 . 
       FIG. 3  shows a schematic block diagram of the generator  200 , which includes a controller  224 , a power supply  227 , and a power converter  228 . The power supply  227  may be a high voltage, DC power supply connected to an AC source (e.g., line voltage) and provides high voltage, DC power to the power converter  228 , which then converts high voltage, DC power into RF energy and delivers the energy to the active terminal  230 . The energy is returned thereto via the return terminal  232 . In particular, electrosurgical energy for energizing the monopolar electrosurgical instrument  20  and/or electrosurgical forceps  30  is delivered through the active and return terminals  230  and  232 . The active and return terminals  230  and  232  may be coupled to the power converter  228  through an isolation transformer  229 . The isolation transformer  229  is optional and the active and return terminals  230  and  232  may be coupled directly to converter  228 . 
     The generator  200  also includes a DC-DC buck converter  234  coupled to the power supply  227 . Furthermore, a current source  236  is electrically coupled to the DC-DC buck converter  234  and the power converter  228 . The current source  236  may be an inductor having an inductance which smoothes the current supplied to the power converter  228 . The current source  236  is configured to supply current to the power converter  228 . The output of power converter  228  transmits current through an isolation transformer  229  to the load “Z”, e.g., tissue being treated. 
     The power converter  228  is configured to operate in a plurality of modes, during which the generator  200  outputs corresponding waveforms having specific duty cycles, peak voltages, crest factors, etc. It is envisioned that in other embodiments, the generator  200  may be based on other types of suitable power supply topologies. Power converter  228  may be a resonant RF amplifier or a non-resonant RF amplifier. A non-resonant RF amplifier, as used herein, denotes an amplifier lacking any tuning components, e.g., conductors, capacitors, etc., disposed between the power converter and the load “Z.” 
     The controller  224  includes a processor (not shown) operably connected to a memory (not shown), which may include one or more of volatile, non-volatile, magnetic, optical, or electrical media, such as read-only memory (ROM), random access memory (RAM), electrically-erasable programmable ROM (EEPROM), non-volatile RAM (NVRAM), or flash memory. The processor may be any suitable processor (e.g., control circuit) adapted to perform the operations, calculations, and/or set of instructions described in the present disclosure including, but not limited to, a hardware processor, a field programmable gate array (FPGA), a digital signal processor (DSP), a central processing unit (CPU), a microprocessor, and combinations thereof. Those skilled in the art will appreciate that the processor may be substituted for by using any logic processor (e.g., control circuit) adapted to perform the calculations and/or set of instructions described herein. 
     The controller  224  includes an output port that is operably connected to the power supply  227  and/or power converter  228  allowing the processor to control the output of the generator  200  according to either open and/or closed control loop schemes. A closed loop control scheme is a feedback control loop, in which a plurality of sensors measure a variety of tissue and energy properties (e.g., tissue impedance, tissue temperature, output power, current and/or voltage, etc.), and provide feedback to the controller  224 . The controller  224  then controls the power supply  227  and/or power converter  228 , which adjusts the DC and/or power supply, respectively. 
     The generator  200  according to the present disclosure may also include a plurality of sensors (not shown). The sensors may be coupled to the power supply  227 , the current source  234 , and/or power converter  228  and may be configured to sense properties of DC current supplied to the power converter  228  and/or RF energy outputted by the power converter  228 , respectively. Various components of the generator  200 , namely, the power converter  228 , the current and voltage sensors, may be disposed on a printed circuit board (PCB). The controller  224  also receives input signals from the input controls of the generator  200 , the instrument  20  and/or electrosurgical forceps  30 . The controller  224  utilizes the input signals to adjust power outputted by the generator  200  and/or performs other control functions thereon. 
     The DC-DC buck converter  234  includes a switching element  234   a  and power converter  228  includes a plurality of switching elements  228   a - 228   d  arranged in an H-bridge topology. In embodiments, power converter  228  may be configured according to any suitable topology including, but not limited to, half-bridge, full-bridge, push-pull, and the like. Suitable switching elements include voltage-controlled devices such as transistors, field-effect transistors (FETs), combinations thereof, and the like. 
     The controller  224  is in communication with both DC-DC buck converter  234  and power converter  228 , in particular, the switching elements  234   a  and  228   a - 228   d,  respectively. Controller  224  is configured to output control signals, which may be a pulse-width modulated signal, to switching elements  234   a  and  228   a - 228   d  as described in further detail in co-pending application published as US 2014/0254221, filed on Dec. 4, 2013 by Johnson et al., the entire contents of which are incorporated by reference herein. In particular, controller  224  is configured to modulate a control signal d 1  supplied to switching element  234   a  of DC-DC buck converter  234  and control signals d 2  supplied to switching elements  228   a - 228   d  of power converter  228 . Additionally, controller  224  is configured to measure power characteristics of generator  200 , and control generator  200  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 power converter  228 . 
     With reference to  FIG. 4 , the power converter  228  of the generator  200  is shown as a current-source power converter, which achieves zero-voltage-switching using a variable active snubber circuit  300 . The snubber circuit  300  is coupled in parallel with the current source  236 . The snubber circuit  300  includes a first node  301   a  disposed between the power supply  227  and the current source  236  and a second node  301   b  coupled to the power converter  228 . The snubber circuit  300  includes a snubber inductor  302 , a first switching element  304   a,  and a second switching element  304   b.  The first switching element  304   a  is connected to a ground and the second switching element  304   b  is connected in series with the power converter  228 . The snubber circuit  300  also includes a snubber capacitor  306  and a snubber catch diode  308 . The first and second switching elements  304   a  and  304   b  are configured to be switched at a fixed duty cycle by the controller  204  to establish the desired voltage at the snubber capacitor  306 . The voltage in the snubber capacitor  306  is constantly being controlled via the first and second switching elements  304   a  and  304   b  and the snubber inductor  302  to maintain the desired capacitor voltage, which produces the desired ring-back at the second node  301   b  as described in more detail below. 
     Although  FIG. 4  does not show the DC-DC buck converter  234 , the snubber circuit  300  may be implemented with the DC-DC buck converter  234 . In which case, the first node  301   a  would return to the DC-DC buck converter  234  rather than the power supply  227  as shown in  FIG. 4 . 
       FIG. 5  illustrates a plurality of waveforms, namely, a switching waveform  500  for switching the switching elements  228   a - 228   d,  a waveform  502  generated by the current source  236 , a converted waveform  504  generated by the power converter  228 , and an RF waveform  506  at the load “Z.” A method for operating the generator  200 , and in particular the snubber circuit  300 , is shown as a flow chart  600  in  FIG. 6  and is described below with reference to  FIG. 5 . 
     Initially, during period  510  as shown in  FIG. 5 , all of the switching elements are turned on by the switching waveform  500  and current in the current source  236  ramps up to a desired predetermined level. Once the desired current is achieved, which may be determined by sensors (not shown) coupled to the controller  224 , or by timing of the ramp time, one pair of the switching elements  228   a - 228   d,  e.g., switching element  228   a  and  228   d  or  228   b  and  228   c,  are turned off. This generates a first RF pulse (e.g., positive half cycles) that is supplied to the load “Z” during period  512 . 
     At a predetermined time, during period  514 , one of the high side switching elements, namely, switching element  228   a  or  228   b,  of the pair of the switching elements  228   a  and  228   d  or  228   b  and  228   c,  is also tuned off. As a result, all but one of the switching elements  228   a - 228   d  is turned off, namely, one of the low side switching elements  228   c  or  228   d  remains on. In embodiments, all of the remaining activated switching elements may also be turned off. In response to turning off the switching elements  228   a - 228   d,  the voltage at the output of the snubber inductor  302  rises very rapidly and is clamped by the snubber catch diode  308 , which then feeds the energy into the snubber capacitor  306 . In further embodiments, the power supply  227  feeding the current source  236  may also be turned off depending on specific requirements of downstream elements. 
     Due to the higher voltage at the snubber inductor  302 , the energy in the snubber inductor  302  quickly dissipates and the current in the snubber inductor  302  reverses, causing ring-back due to the intended and incidental stray capacitance of the H-bridge circuitry of the power converter  208 . In particular, the current from the snubber inductor  302  of the snubber  300  counteracts stray capacitance of the power converter  208 . As the ring-back occurs, the voltage at the power converter  208  returns to zero. 
     During period  516 , all of the switching element  228   a - 228   d  turn on to start the next cycle, which results in a reverse RF pulse (e.g., the negative half of the waveform  506  as compared to the pulse generated previously during period  512 ). The application of switching waveform  500  is repeated indefinitely to generate the desired waveform  506 . In this implementation all forward or downstream current paths are turned off at the same time. As described above, this would be a catastrophic state for a conventional current-fed power supply were it not for the snubber circuit  300  according to the present disclosure. 
     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 of the claims appended hereto.