Circuit and method for reducing stored energy in an electrosurgical generator

[0001] The present application is a continuation of U.S. application Ser. No. 11/256,374, filed on Oct. 21, 2005, now U.S. Pat. No. 8,734,438, the entire contents of which are hereby incorporated by reference herein.

[0002] 1. Field

[0003] The present disclosure relates generally to electrosurgical system, and more specifically, to a system and method for discharging excess energy of a high voltage direct current (HVDC) power supply of an electrosurgical generator.

[0004] 2. Description of the Related Art

[0005] Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, seal, ablate, or coagulate tissue. In monopolar electrosurgery, a source or active electrode delivers radio frequency energy from the electrosurgical generator to the tissue and a return electrode carries the current back to the generator. In monopolar electrosurgery, the source electrode is typically part of a surgical instrument held by the surgeon and applied to the tissue to be treated. A patient return electrode is placed remotely from the active electrode to carry the current back to the generator.

[0006] In bipolar electrosurgery, a hand-held instrument typically carries two electrodes, e.g., electrosurgical forceps. One of the electrodes of the hand-held instrument functions as the active electrode and the other as the return electrode. The return electrode is placed in close proximity to the active (i.e., current supplying) electrode such that an electrical circuit is formed between the two electrodes. In this manner, the applied electrical current is limited to the body tissue positioned between the two electrodes.

[0007] Conventional electrosurgical generators include a high voltage direct current (HVDC) power connected to a radio frequency (RF) output stage, which converts DC energy generated by the HVDC into RF energy. The high voltage direct current (HVDC) power supply includes an output filter which smoothes the switching of the HVDC into a DC level. This filter can store large amount of energy and under light loads and high impedance, the discharge of the output filter is slow. As a result, the generator response time is significantly lowered thereby limiting the generator's ability to pulse energy rapidly or respond quickly during light loads.

[0008] Therefore, there is a need for an electrosurgical generator which can discharge energy in a consistent and rapid manner under various load conditions, including light loads and high impedance.

[0009] The present disclosure provides for an electrosurgical generator which includes a circuit for discharging stored energy and a high voltage power supply. The active discharge circuit includes a pulse width modulator, a load having a resistive element and a switching circuit, and an error signal generating circuit. The error generating circuit determines a difference between and output set point voltage and feedback voltage and generates an error signal. If the error signal is less than a first predetermined threshold the switching circuit switches in a load and sinks current supplied by the high voltage power supply through the load. If the signal is above a second predetermined threshold the pulse width modulator is switched on. This ensures that the pulse width modulator and the load are not active simultaneously.

[0010] According to one embodiment of the present disclosure a circuit for discharging stored energy in an electrosurgical generator is disclosed. The circuit includes a pulse width modulator for controlling a high voltage power supply, an error signal generating circuit configured for delivering an error signal as a difference between an output signal voltage with a feedback voltage generated by the high voltage power supply. The error signal is transmitted to the pulse width modulator when the error signal is large enough the pulse width modulator turns on. The circuit further includes a switching circuit configured to switch in a load in parallel with an output of the high voltage power supply when the error signal is lesser than a first predetermined threshold to discharge the output.

[0011] According to another embodiment of the present disclosure an electrosurgical generator is disclosed. The generator includes a high voltage power source for generating direct current, a radio frequency output stage for converting direct current into radio frequency energy, and a circuit for discharging stored energy. The circuit includes a pulse width modulator for controlling a high voltage power supply, an error signal generating circuit configured for delivering an error signal as a difference between an output signal voltage with a feedback voltage generated by the high voltage power supply. The error signal is transmitted to the pulse width modulator. The circuit further includes a switching circuit configured to switch in a load in parallel with an output of the high voltage power supply when the error signal is lesser than a first predetermined threshold to discharge the output.

[0012] According to a further aspect of the present disclosure a method for discharging energy stored in a circuit in an electrosurgical generator is disclosed. The method comprises the steps of deriving an error signal as a difference between an output setpoint voltage with a feedback voltage generated by a high voltage power supply, comparing the error signal with a first predetermined threshold, switching on a load in parallel with an output of the high voltage power supply when the error signal is lesser than a first predetermined threshold to discharge the output, and switching on a pulse width modulator if the error signal is above a second predetermined threshold.

[0019] Preferred embodiments of the present disclosure will be described herein 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.

[0020] The present disclosure provide for an electrosurgical generator including a high voltage power supply for supplying a DC voltage, an output filter, and an active discharge circuit for effectively discharging stored energy in the output filter. The active discharge circuit switches in a load in parallel with the output filter so that energy stored in the output filter is discharged in consistent manner regardless of the resistance of the external load.

[0021] The generator according to the present disclosure can be used with bipolar and monopolar electrosurgical devices. FIG. 1 is a schematic illustration of a monopolar electrosurgical system 1. The system 1 includes an active electrode 14 and a return electrode 16 for treating tissue of a patient P. Electrosurgical RF energy is supplied to the active electrode 14 by a generator 10 via a cable 18 allowing the active electrode 14 to ablate, cut or coagulate the tissue. The return electrode 16 is placed at the patient P to return the energy from the patient P to the generator 10 via a cable 15.

[0022] The generator 10 may include input controls (e.g., buttons, activators, switches, etc.) for controlling the generator 10. The controls allow the surgeon to adjust power of the RF energy, waveform, and other parameters to achieve the desired waveform suitable for a particular task (e.g., cutting, coagulating, etc.). Disposed between the generator 10 and the active electrode 14 on the cable 18 is a hand piece 12, which includes a plurality of input controls which may be redundant with certain input controls of the generator 10. Placing the input controls at the hand piece 12 allows for easier and faster modification of RF energy parameters during the surgical procedure without having the surgeon divert his attention from the surgical site and returning to the generator 10.

[0023] FIG. 2 shows a schematic block diagram of the generator 10 having a controller 26, a high voltage DC power supply (HVDC) 28, and an RF output stage 30. The controller 26 includes a microprocessor and an output port of the microprocessor is electrically connected to the HVDC 28. The HVDC 28 is configured to supply DC power to the RF output stage 30. The controller 26 receives input signals from the generator 10 and/or hand piece 12, e.g., a set point, and the controller 26 in turn adjust power outputted by the generator 10, more specifically the HVDC 28, and/or performs other control functions thereon.

[0024] The RF output stage 30 converts DC power into RF energy and delivers the RF energy to the active electrode 14. In addition, the RF output stage 30 also receives RF energy from the return electrode 16. The power of the HVDC 28 can be varied to modify RF magnitude (e.g., amplitude) thereby adjusting the power of the RF energy delivered to the tissue. This allows for accurate regulation of the power of delivered RF energy.

[0025] Regulation of output energy is controlled by the controller 26 (e.g., a microprocessor) using algorithms and/or software. The controller 26 forms a closed-control loop with a sensor 29 which senses various tissue and output energy properties and reports the properties data to the controller 26. The closed-control loop allows for real-time adjustment of output energy based on the properties sensed by the sensor 29. More specifically, the closed-control loop can process signals from the sensor 29 and make corresponding adjustments in about 250 μs. The HVDC 28 is capable of supplying and discharging the current at similar rates (e.g., sourcing at about 300 V/ms or faster and discharging at about 7 V/ms or faster).

[0026] With reference to FIG. 3, discharging of current is accomplished using an active discharge circuit (ADC) 31—a component of the HVDC 28—which is a circuit that switches a load 36 (e.g., one or more resistors) in parallel with an output capacitor 34. The HVDC 28 includes a diode 33 on an output connection 35 which allows current to flow away from the HVDC 28 into a filter 32. The load 36 discharges the energy stored in the capacitor 34. During discharge, the diode 33 prevents the discharged current to flow back into the HVDC 28 thereby directing the current toward the active electrode 14.

[0027] FIG. 4 shows the ADC 31 in more detail and other components of the generator 10. The output and input connections 35, 37 include a plurality of diodes 33 which block the output current from returning to the HVDC 28 during discharge. The HVDC 28 is connected to a pulse width modulator 38, which may be a Pulse Width Modulator UCC3895 available from Texas Instruments, for controlling the output of the HVDC 28. The pulse width modulator 38 implements control of a full-bridge power stage 33 by phase shifting the switching of one half-bridge with respect to the other. It allows constant frequency pulse-width modulation in conjunction with resonant zero-voltage switching to provide high efficiency at high frequencies and can be used either as a voltage mode or current mode controller.

[0028] The pulse width modulator 38 is configured to receive an error signal which is generated by an error signal generating circuit, a first comparator 52. The error signal is derived from the difference between the output set point of the HVDC 28 (e.g., ECON)—the intended output voltage—and the voltage feedback of the HVDC 28 (VFB)—actual output voltage generated by the HVDC 28. If VFB is higher than ECON, without the ADC 31, the signal would be 0V. This causes the loop to be delayed as it waits for the signal to increase in order to activate the pulse width modulator 38. The ADC 31 avoids that problem and maintains the signal from dropping too low because the ADC 31 discharges the output of the HVDC 28 faster than the signal may drop. The error signal is sent to the shift controller 38 which compares the error signal with a second predetermined signal. If the signal drops below the second predetermined threshold, about 0.7V, the pulse width modulator 38 shuts down and the ADC 31 becomes operational. When the pulse width modulator 38 shuts down, the HVDC 28 stops sourcing current.

[0029] The signal is also compared against a first predetermined threshold, about 0.5V, at a second comparator 50. Thus, if the signal is below this level the ADC 31 will turn on. This ensures that the pulse width modulator 38 will not be turned on when the ADC 31 is on thereby reducing chance of HVDC 28 driving into the ADC 31. The signal feeding into the second comparator 50 is not filtered, this allows for a relatively fast response from the ADC 31. The time period between pulse width modulator 38 shutting down and the ADC 31 starting up, or vice versa, is about 5 μs.

[0030] As discussed above, the capacitor 34 is in parallel with the load 36 which is used to discharge the current. The load 36 provides a gate drive voltage and includes a switching component 40 and a resistive element 42. The switching component 40 can be a transistor, such as a field-effect transistor (FET), metal-oxide semiconductor field-effect transistor (MOSFET), insulated gate bipolar transistor (IGBT), relay, and the like. The resistive element 42 is in series with the switching component 40 and to ground 44, which is known as a source follower. The source follower limits amount of current which flows through the resistive element 42. As amount of current flowing through the resistive element 42 increases, the voltage across the resistive element 42 increases as well. This voltage subtracts from the gate drive voltage as the current reaches a predetermined threshold causing the switching component 40 to turn off thereby acting as a variable resistor. The resistive element 42 has a resistance, such as about 5 Ohms, which will limit the current to less than about 2 Amps. For instance, the resistive element 42 has a turn on around 2V and 2 A and will subtract 10B (2 A*5 Ohms) from the 12V gate drive. This reduces the stress on HVDC 28 and other output components.

[0031] FIGS. 5A-5E are graphs of HVDC 28 output. FIG. 5A shows output response of the HVDC 28 without the ADC 31 while FIG. 5B shows output response of the HVDC 28 equipped with the ADC 31. Waveform 60 represents ECON voltage (e.g., the intended output voltage) that fluctuates from about 0.5V and about 1V. Waveform 62 is the actual signal voltage output (e.g., voltage feedback) which is a 1 KHz sine waveform of the HVDC 28 into a load of about 200 Ohms. In FIG. 5A, waveforms 60, 62 do not track each other, representing delay in discharging of the current. In FIG. 5B, waveforms 60, 62 closer track each other due to faster discharging caused by the ADC 28.

[0032] FIGS. 5C and 5D shows a waveform 64 which represents gate drive of the switching component 40. When the gate drive waveform 64 shows a rise it is representative of the switching component 40 being on thereby activating a 5 Ohm load across the output. In FIG. 5C, the gate drive pulses on and off very rapidly, such as during the downward sloping portion of the waveforms 60, 62 the gate drive is on and off during the upward sloping portion of the waveforms 60, 62. FIG. 5D shows an expanded view of the gate drive shows the rate of the pulsing, which is about 230 KHz. This demonstrates that the ADC 28 is maintaining the signal from falling below 0.5V.

[0033] FIGS. 5E and 5F show time differences between the waveforms 60, 62, 64. More specifically, FIG. 5E shows the time period between the pulse width modulator 28 turning off and the ADC 31 turning on is about 4 μs, while FIG. 5F shows the time period between the pulse width modulator 28 turning on and the ADC 31 turning off is also about 15 μs. This demonstrates that the ADC 31 and the pulse width modulator 28 are not activated at the same time, which reduces the risk of over-stressing components of the generator 10.

[0034] The described embodiments of the present disclosure are intended to be illustrative rather than restrictive, and are not intended to represent every embodiment of the present disclosure. Various modifications and variations can be made without departing from the spirit or scope of the disclosure as set forth in the following claims both literally and in equivalents recognized in law.