CA2554720C - Circuit and method for controlling an electrosurgical generator using a full bridge topology - Google Patents

Abstract

A system for controlling an electrosurgical generator using a full bridge topology is disclosed. The system includes a high voltage direct current power source which supplies power and a radio frequency output stage which receives power from the high voltage direct current power source and outputs radio frequency energy at a predetermined radio frequency set point. The system also includes one or more sensors which determine one or more parameters of radio frequency energy being applied to tissue and a microprocessor configured to receive one or more parameters and outputs the predetermined radio frequency set point to the radio frequency output stage as a function of one or more of the parameters of radio frequency energy.

Description

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Cl CUIT AND METHOD FOR CONTROLLING AN ELECTRO3URGICAL
GENERATOR USING A FULL BRtDGE TOPOLOGY
BACKGROUND
1_ Technical Field The present disclosure is directed to electrosurgicai systems, and, in particular, to a circuit and method for cQntrolGng an electrosurgical generator using a fuli bridge topology to control the radio frequency (RF) energy ouiput_ 2. Descriation Qf the Related Art An electrosurgical generator is used in surgical procedures to deliver electrical energy to the tissue of a patient. When an electrode is connected to the generator, the electrode can be used for cutting, coagulating or sealing the tissue of a patient with high frequency electrical energy. During normal operation, alternating electrical current from the generator flows between an active electrode and a return electrode by passing through the tissue and bodily fluids of a patient.

The electrical energy usually has its waveform shaped to enhance its ability to out, coagulate or seal tissue, Different waveforms correspond to different modes of operation of the generator, and each mode gives the surgeon various operating advantages. Modes may include out, coagulate, a blend thereof, desiccate, or spray. A
surgeon can easily select and change the different modes of operation as the surgical procedure progresses.

In each mode of operation, it is important to regulate the electrflsurgicaf power delivered to the patient to achieve the desired surgical effect. Applying more electrosurgical power than necessary results in tissue destruction and prolongs healing.
Applying less than the desired amount of alcfctrosurgical powrer inhibits the surgical procedure. Thus, it is desirable to control the output energy from the electrosurgical generator for the type of tissue being treated.

Dlfferent types of tissues will be encountered as the surgical procedure progn:sses and each unique tissue requires more or less power as a function of frequently changing tissue 9mpedance. As different types of tissue and bodily fluids are encountered, the impedance changes and the response time of the electrosurgical control of output power must be rapid enough to seamlessly permit the surgeon to treat the tissue. Moreover, the same tissue type can be desiccated during electrosurgica!

treatment and thus its impedance will change dramatically in the space of a very brtef time. The electrosurgical output power control has to respond to that impedance change as weq.

Two conventional types of power regulation are used in commercial electrosurgical generators. The most common type controls the DC power supply of the generator by limiting the amount of power provided from the AC mains to which the generator is connected. A feedback control loop regulates dutput voltage by oomparing a desired voltage with the output voltage supplied by the power supply.
Another type of power regulation in commercial electrosurgical generators controls the gain of the high-frequency or radio frequency amplifier. A feedback control loop compares the output power supplied from the RF amplifier for adjustment to a desired power level.
Generators that have feedback control are typically designed to haki a constant output voltage, and not to hold a constant output power.

U.S, Pat. Nos. 3,964,487; 3,980,085; 4,168,927 and 4,092,986 have circultry to reduce the output current in accordance with increasing load impedance. In those patents, constant vottage output is maintained and the current is decreased with increasing load impedance.

U.S. pat. No. 4,126,137 controls the power amplifier of the electrosurgical unit in accord with a non-linear compensation circuit applied to a feedback signal derived from a comparison of the power level reference signal and the mathematical product of two signals including sensed current and voltage in the unit.

U.S. Pat. No. 4,668,819 has an electrosurgicai generator which has a microprocessor controller based means for decreasing the output power as a func6on of changes in tissue impedance.

U.S. Pat. No. 4,727,874 includes an eiectrosurgicai generator with a high frequency pulse width modulated feedback power control wherein each cycle of the generator is regulated in power content by modulating the width of the driving energy pulses.

U.S. Pat. No. 3,601,126 has an etectrosurgicaf generator having a feedback circuit that atfempts to maintain the output current at a constant amplitude over a wide range of tissue impedances.

SUMMARY
An eleotrosurgical generator that uses a full bridge topology as the RF output stage to control the output RF energy and a control method therefore are provided.
Most electrosurgical generators have a closed loop control signal that is derived from the difference of what is measured and what is desired. This control signal is feed back to the DC converter, which varies the DC voltage that feeds into the RF stage.
The closed loop of the full bridge topology of the present disclosure uses the control signal ._ ~

that is derived from the RF energy that is measured and what is desired, to drive the RF
output stage. The full bridge topology can be either pulse width driven or phase shift driven, in either case, as the control signal changes so will the amount of dme the full btidge will place voltage across the primary of the RF transformer. Varying the amount of time the voltage is imposed across the transformer's primary winding controls the amount of energy on the RF transformer. This voltage is a square wave whose pulse length varies in relation to the control signal. The corresponding energy on the seGondary side of the RF transformer is filtered, usually at the operating frequency, to produce a sinusoidal waveform. Using a full bridge topology, the RF energy can be controlled directly at the RF output stage instead of using a varying DC input This allows the DC input to be at a steady level or an unregulated level and, therefore. the DC converter can be removed from the closed loop of the generator.

The advantage of using a full bridge topology for the RF stage is that the DC
voltage no longer controls the output RF energy. This means the DC canverter can be removed from the electrosurgical generator, or the DC level can be preset such as the elosed loop of the electrosurgicat generator will not control it during operation. By removing the DC converter from the closed loop has the advantage of a much faster closed loop response of the electrosurgical generator. The other advantage of using the full bridge topology is the RF transformer is - driven symmetrically which makes the output stable to all loads.

According to one embodiment of the present disclosure, a system for controlling an electrosurgical generatvr using a fui( bridge topology is disclosed. The system includes a high voltage direct current power source which supplies power and a radio frequency output stage which receives power from the high voltage direct current power source and outputs radio frequency energy at a predetermined radio frequency set point, The system also includes one or more sensors which determine one or more parameters of radia frequency energy being applied to tissue and a micmprooessor configured to receive one or more parameters and outputs the predetermined radio frequency set point to the radio frequency output stage as a function of one or more of the parameters of radio frequency energy.

According to another embodiment of the present disclosure, a method for controlling an electrosurgical generator using a full bridge topology is disclosed. The method includes the steps of supplymg power from a high voltage direct current power source and receiving power from the high voltage direct current power source at a radio frequency output stage. The method also includes the steps of outputting radio frequency energy at a predetermined radio frequency set point and determining one or more parameters of radio frequency energy being applied to a load. The method further includes the step of receiving the one or more parameters at a microprocessor and outputting the predetermined radio frequency set point to the radio frequency output stage as a function of the one or more of the parameters.

BRIEF DESCRIPTIaN OF TTIEDRAWIN0.5 The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the foNowing detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of a radio frequency (RF) output control circuit of a conventional electrosurgicaf generator;

FIG. 2 is a block diagram of RF output control circuit of an electrosurgical generator according to the teachings of the present disclosure;

FIG. 3 is a schematic diagram of an RF output stage according to an embodiment of the present disciosure;

FIG. 4 is a schematic diagram of an over-current protection circuit;
FIG. 6 is a schematic diagram of a first over-voltage protection circuit;
FIG_ 6 is a schematic diagram of a se pnd over-voltage protection circuit;
FIG. 7 is a schematic diagram of a conventional RF output stage oircuit;

FIG. 8 illustrates several views of how the RF output stage can be modeled over time; and FIGS. 9A-B illustrate several views of output curves after experiencing arcing, DETAILF-D DESCRIPTION

Embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. In the foilowing description, well-known functions or constructions are not described in detail to avoid obscurirtig the invention in unnecessary detail. In the figures, like reference numerals represent like elements.

The speed of any electrosurgical generator is governed by the slowest block or component in the control Ioop_ In conventional electrosurgical generator topology as shown in FIG. 1, the high voltage DC power source (HVDC) response time is 10 times slower than the intended microprocessQr (uP) loop speed, e.g., 2.5ms vs.
250us. This results in the microprocessor waiting for the HVDC to respond thereby not utilizing faster response times offered by electronic circuitry.

Referring to FIG. 2, an electrosurgicat generator according to the present disclosure is illustrated. The electrosurgical generator 10 includes a high voltage DC
power source (HVDC) 12 for supplying power to the generator, an RF output stage 14 for receiving power from the HVDC 12 and outputting RF power at a predetermined set point, sensors 16 for determining parameters, e.g., current and voltage, of power being 'v T

applied to a load, e.g., tissue, and a microprocessor 18 for controlling the overall operations of the eiectrosurgical generator 10 and for receiving the measured current and volfiage and outputting a set point to the RF output stage 14, ltt embodiments, the HVDC 12 is set to a fixed voltage level, The response of the HVDC 12 becomes a concern only at initialization of the generator 10. The RF
output stage's response time with a phase shift controlled topology is now 10 times faster than the microprocessor control loop speed, as wiil be described below. The limiting factor is no longer the HVDC, but the speed of the microprocessor control loop, which Is times faster than the intended microprocessor (uP) loop speed, e.g.. 2.5ms vs.
250us.
It is envisioned that the difference will be even larger as more advanced microprocessors are developed and implemented_ Referring to FIG. 3, the RF output stage is itlustrated in more detail. The RF
output stage 14 includes a transformer 20 and a plurality of transistors 22, 24, 28, 28 for adjusting the RF output power of the transformer 20 configured in a#ull bridge topotogy_ The RF output stage 14 further includes a pulse-width modulator 30 for driving the plurality of transistors 22, 24, 26, 28. The pulse width modulator (PWM) 30 has at least two drive outputs, e.g., Drive A and Drive B. Drive A is coupled to transistor 22 and Drive A shifted 180 degrees is coupled to transistor 24. Similarly. prive B is coupled to transistor 28 and Drive B shifted 180 degrees is cpupled to transistor 28. A
push-pull typology is used to accomplish voltage from DC to RF conversion. Two gate drive signals that are 180 out of phase are used to drive the plurality of transistors 22, 24, 26, 28, caiied T ON and T oN_1$0. The gate drive signals turn on each of the FET's at opposite times to deliver a wavefoml at the specified pawer. The pulse width modulator 30 Includes an input for receiving the RF output set point from microprocessor 18.
Aitemativeiy, the transistors may be phase shift drlven from a single control.

In embodiments, the RF set point is not proportional to the output level, Le,, the RF set point runs "open loop" control. For example, in the HVDC, which is a closed loop, 1V set point = 25V out, independent of load. In the generator employing the full bridge topology of the present disctosure, the RF output stage will have a set point that will range preferably from about 0.7V to about 3.7V. This is a range that is set up by the ramp and slope of the PWM 30. This also means that 1V set point into 100 ohms won't be the same vutput as 1 V set point into 1 k Ohm.

Since, the RF output stage 14 responds to a dynamic load ten times faster than the microprocessor, over-voltage and over-current conditions may occur at the load.
Therefore, three hardware solutions have been implemented to help minimize these conditions.

Over-current protection -s implemented by an over-current proteetion circuit shown in FIG. 4. Referring ta F1G_ 4, current sense transformer TX1 located on the sense board 16 measures the output current to the load_ The sensed current is input to comparator 40 which is then compared to a reference voltage Vref. If a fault condition occurs, such as arcing, the comparator 40 will determine that the sensed current is greater than the reference voltage Vref and will trigger the RF set point to be pulled low through transistor M1 via connection point TP26. A timer 42, e.g., a 555 timer, will keep the RF set point at OV for a predetermined peciod of tirne, e.g., 150uS, to effectively reduce the RF output from the RF output stage 14. Altematively, the timer could be replaced by a flip flop that will pull down the RF set point by halt and send an alarm to the microprocessor 18. Once the microprocessor 18 handles the alarm, the microprocessor 18 would reset the flip flop.

Over-voltage condibons may occur If low impedance is sensed, e.g., when a surgical Instrument is removed from the operative site, and the generator attempts to increase the output voltage. Referring to FIG_ S. over-voltagQ proteetlon is achieved by adding an inductor Lp to the primary side of the RF transformer 20. As the voltage on the output fioreases so will the current flowing through the output capacitor Vout. The prirrtary current (Ip) equals the secondary current because it is dominated by the capac'itor current (lc). Aa Ip increases, the voltage that Is across the primary Inductor Lp will also increase. The voltage across the inductor will subtract from the voltage across the transformcir which witl mduce the amount of voltage to the secondary and as that is reduced, so is the amount of output voltage. This protection works cycle by cycle, will prevent a runaway condition of the output voltage and prevant a full-bore output from occurring at the Idad.

Additionally, over-voltage protection will be implemented to IImit the amount of overshoot while an instrument goes from a low to high impedance. This over-voltage protection scheme is implemented in an over-voltage protection circuit 17 shvwn in FIG.
6, Transformer TXZ monitors the current on the primary winding of the RF
transformer 20, or alternatively, will monitor the output vottage from the sense board. As described above, when the primary current Increases so'wili the voltage at the output.
Since the capacitance is a constant, the primary current can be calculated such that a certain voitage will be across the output capacitance. Once this level goes above a specific value, e.g_, Vref, comparator 60 will start to pull down the RF set point through dlode Dl via connection point TP26. This protection has the capability of changing the level of overshoot by changing the value of variable resistor R2, which can be switched in. The speed of this protection is about 10Khz, or 10Dus. This is limited to about 10% over the voltage limit which will allow the microprocessor to see the error and adjust the set point accordingiy.

The generator employing a full bridge topology of the present disclosure has several advantages over conventional generators which control the RF output by varying the high voltage power supply (HVDC)_ For exampie, the stored energy on the HVDC does not effect the output of the generator. Currently, the generator output is left on for a few milliseconds after the end of an activation to help bleed off the voltage to prevent excess energy storage in the genergtor_ The higher the impedance present at the output, the longer it takes to dissipate the voltage on the HVDC. The danger here is that the RF board may get turned off but the HVDC may still have stored energy, causing inadvertent errors to be reported. Additionally, as RF power requirements increase so will the capacitor off of the HVDC. Currently, this stored voltage in the capacitor needs to be discharged after each activation. Because the set point turns the RF off at the PWM instead of the HVDC, residual voltage on the HVDC isn't an Issue at the end of an activation.

Furthermore, an RF gmplifier of the electrosurgical generator will have no tuned elements. The advantage of having no tuned elements on the RF amplifier is that the operating frequency of the output stage module will be completely independent of the operating frequency of the RF ampiifter, Frequency, instead, becomes dapendertit on the resonance of each of the separate output modes of the generator, e.g., coagulate, eut blend, etc. The advantage of this eonflauration is that the output frequency can be changed without re-tuning the RF output stage only re-tuning the output module relating to a particular mode, facilitating any modifications or troubleshooting. This delegation of resonating components places much tess stress on the FETs durtng arcing and the RF
output stage will be able to force the output to be more stable as wili be describad below.

The current topology is a class E, wh re the FET resonates with the tank, capacitor C6, capacitor C3, and inductor L3, as shown in FIG. 7. When an arcing condition hits the tank, this can short out the capacitor C3 thereby causing the resonance of this system to change dramatically. Any stored energy in inductor L6, the RF choke, will be released causing the voFtage across the FET to increase.
This voltage will eventually be clamped by the avalanche mode of the FET, for example, in conventional topology this is around 570V for a 500V rated FET. In the topology of the present disclosure, since there Is no tank or RF choke, the voltage will not increase.
Because of this, the generator is able to use 200V rated FETs. The lower voltage rated FETs have an advantaige with lower Rds on and gate charge, which have less dissipatation and are less expensive than higher voltage rated FETs.

Additionally, the current conventional class E topology is efficient only in a narrow range of load impedances. As the load changes so will the waveform across the FET.
This will' cause symmetry issues of the waveform at the output The topology of the present disclosure has an output fffter which is optimized for certain voltages and currents. Because the Fl*Ts themsetves are not tuned, the RF ampfifier to the filter is unaffected by load changes.

FIG. d illustrates how the transformor voltage, or output of the RF, is induced on to the output filter. During time A, the voltage positively charges the inductor Lf and capacitor Cf. During time B, the inductor Lf and capacitor Cf are of the opposite polarity.
Tme C shows that when there is no output voltage, the inductor Lf is in parallel with the capacitor Cf which will keep the same resonant frequency. If the load is shorted, the max current = (1l"t)/L.

As mentioned above, an arcing condition in the existing class E topology causes the resonant frequency to change, subsequently changing ttle output frequency as illustrated in FIG. 9A_ Because the topology of the present disclosure is using a transformer that is syrnmetrlcal on the primary side, it forces the secondary to be 11.

symrnetrical even during arcing, as shown in 1=IG. 9B. When. the output capacitor is shorted on the new topology, the series inductance acts as a resistor and it limits the current to the output.

Furthermore, the generator of the present disclosure reduces leakage current.
Leakage current is dependarit on several variables, The component that contributes most to leakage current is aapacitor coupling from the primary to secondary winding of the RF transfonner. As the voltage increases across the transformer, the capacitance will start to conduct current. The advantage of the full bridge RF topology is that lt employs a 1 to-1 transformer and only has a maximum voltage of 15DV, compared to 550 VRMS or 778 Vpeak of the transformers used in conventional generators.
Because of the reduced voltage, the number of tums is reduced to about 5 turns thereby reducing the coupling capacitance.

While several rnbodiments of the disclosure have been shown in the drawings, it is not intended that the disclosure be limited thereto, as it is intended that the disclosures be as broad in scope as the art will allow and that the specification be read I'ikewise, Therefore, the above description should not be construed as limiting, but merely as exemptifiestions of preferred embodiments.

Claims (16)

1. A
system for controlling an electrosurgical generator using a full bridge topology, comprising:
a high voltage direct current power source which supplies power;
a radio frequency output stage which receives power from the high voltage direct current power source and outputs radio frequency energy at a predetermined radio frequency set point;
at least one sensor which determines at least one parameter of the radio frequency energy being applied to a load;
a microprocessor configured to receive the at least one parameter of the radio frequency energy and output the predetermined radio frequency set point to the radio frequency output stage as a function of the at least one parameter of the radio frequency energy, the predetermined radio frequency set point is disproportional to the outputted radio frequency energy; and an over-current protection circuit including a first current sense transformer that measures output current to the load and a first comparator, the first comparator, in turn, compares the output current to a reference voltage, wherein if the output current is greater than the reference voltage the microprocessor sets the radio frequency set point to 0V for a predetermined period of time and automatically resets the radio frequency set point to the predetermined value at the expiration of the predetermined period of time.

2. The system according to claim 1, wherein the high voltage direct current power source is set to supply power at a fixed voltage level.

3. The system according to claim 1, wherein the radio frequency output stage includes:
a plurality of transistors configured to adjust the radio frequency energy of a transformer; and a pulse-width modulator configured to drive the plurality of transistors, wherein the pulse width modulator includes at least a first drive output and a second drive output, the pulse width modulator further comprises an input for receiving the predetermined radio frequency set point from the microprocessor.

4. The system as in claim 1, wherein the predetermined radio frequency set point is independent of the fixed voltage level.

5. The system as in claim 1, wherein the predetermined radio frequency set point is from about 0.7V to about 3.7V.

6. The system as in claim 1, wherein the radio frequency output stage further comprises:
a radio frequency transformer having a primary and a secondary side;

an output capacitor operatively coupled to the radio frequency output stage at the secondary side of the radio frequency transformer, the output capacitor being configured to maintain equality between primary current at the primary side and secondary current at the secondary side; and an inductor operatively coupled to the primary side of the radio frequency transformer configured to subtract primary voltage at the primary side from voltage crossing the transformer between the primary and secondary thereby reducing output voltage.

7. The system as in claim 6, further comprising:
an over-voltage protection circuit including a second current sense transformer configured to measure primary current at the primary side of the radio frequency transformer and a second comparator configured to compare the primary current to a reference voltage, wherein if the output current is greater than the reference voltage, the microprocessor adjusts the predetermined radio frequency set point.

8. A method for controlling an electrosurgical generator using a full bridge topology comprising the steps of:
supplying power from a high voltage direct current power source;
receiving power from the high voltage direct current power source at a radio frequency output stage;
outputting radio frequency energy at a predetermined radio frequency set point;
determining at least one parameter of the radio frequency energy being applied to a load; and receiving the at least one parameter of radio frequency energy at a microprocessor and outputting the predetermined radio frequency set point to the radio frequency output stage as a function of the at least one parameter of the radio frequency energy, the predetermined radio frequency set point is disproportional to the outputted radio frequency energy, wherein the radio frequency stage includes an over-current protection circuit including a first current sense transformer that measures output current to the load and a first comparator, the first comparator, in turn, compares the output current to a reference voltage, wherein if the output current is greater than the reference voltage the microprocessor sets the radio frequency set point to 0V for a predetermined period of time and automatically resets the radio frequency set point to the predetermined value at the expiration of the predetermined period of time.

9. The method according to claim 8, wherein the high voltage direct current power source is set to supply power at a fixed voltage level.

10. The method according to claim 8, wherein the radio frequency output stage includes:
a plurality of transistors configured to adjust the radio frequency energy of a transformer; and a pulse-width modulator configured to drive the plurality of transistors, wherein the pulse width modulator includes at least a first drive output and a second drive output, the pulse width modulator further comprises an input for receiving the predetermined radio frequency set point from the microprocessor.

11. A method according to claim 8, wherein the predetermined radio frequency set point is independent of the fixed voltage level.

12. A method according to claim 8, wherein the predetermined radio frequency set point is from about 0.7V to about 3.7V.

13. The method according to claim 8, wherein the radio frequency output stage further comprises:
a radio frequency transformer having a primary and a secondary side;
an output capacitor operatively coupled to the radio frequency output stage at the secondary side of the radio frequency transformer, the output capacitor being configured to maintain equality between primary current at the primary side and secondary current at the secondary side; and an inductor operatively coupled to the primary side of the radio frequency transformer configured to subtract primary voltage at the primary side from voltage crossing the transformer between the primary and secondary thereby reducing output voltage.

14. A method according to claim 13, wherein the radio frequency stage further comprises:
an over-voltage protection circuit including a second current sense transformer configured to measure primary current at the primary side of the radio frequency transformer and a second comparator configured to compare the primary current to a reference voltage, wherein if the output current is greater than the reference voltage, the microprocessor adjusts the predetermined radio frequency set point.

15. The system in accordance with claim 1, wherein the predetermined period of time is about 150µS.

16. The method in accordance with claim 8, wherein the predetermined period of time is about 150µS.