CA1099789A

CA1099789A - Electrosurgical unit

Info

Publication number: CA1099789A
Application number: CA294,306A
Authority: CA
Inventors: Gerald K. Archibald
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1977-01-21
Filing date: 1978-01-04
Publication date: 1981-04-21
Also published as: US4126137A; JPS5396292A; AU3257478A; GB1584923A; DE2803017A1; AU515537B2

Abstract

ABSTRACT

An electrosurgical unit having a feedback control circuit for providing control of the power amplifier of the unit so power delivered to tissue presented between the active and return electrodes of the unit will be substan-tially constant over the range of impedance presented by various tissues that are encountered. The feedback signal to the power amplifier is determined from a comparison of a power level reference signal and the mathematical product of two signals which are derived from a sensed current in the unit that is directly related to current delivered to tissue and a sensed voltage that is directly related to voltage delivered to tissue. A non-linear compensation circuit in the feedback circuit corrects for non-linearities introduced by components selected for the circuit.

Description

~` ~13,390 97~

ELEC~OSURGICAL UNIT

The lnvention presented herein relates to electro-surgical units and, more specifically, to circuitry for pro-vidin~ automatlc control of the output power at a selected level over the usual impedance range of electrically conduc-tive tissue encountered when using such units.
A wide variety of electrosurgical units for generating various high ~requency output waveforms for electrosur~ical procedures are known in the art. A selected wavePorm output from an electrosurgical unit is applied to a patient by the use of an active electrode placed in con- -~
tact with the patient at the point where a desired surgical procedure is to be carrled out on tissue with current return provided via the patient and a plate or return electrode positioned in electrical contact with the patient. The active electrode provides a small area contact with the tissue of the patient to cause the current density at such ~ -' contact to ~e high enough to generate heat sufficient to accompllsh the desired surgical procedure~
Since the impedance of the current path is a function of the di~ferent electrically conductive tissue ; types that may be encountered, the power delivered by the electrosurgical unit will vary dependent on the -tissue encountered when the unit is set to provide a selected desired output. It has been a common practice to merely match the output impe~ance of the electrosurgical unit to the median of the expected impedance range of the tissueO
This method is not acceptable since the output power de-creases as the impedance varies ~rom the median or center designed value. In order that adequate power be prov~ded ., ~.

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at both low or high i~pedance tlssue with such method, it ls necessary to provide for a power level at the median of the impedance range which may be excessive resulting in unnecessary tissue darnage.
U. S. Patent 3,601,126 points out the power level problem involved due to the range of impedance that is en-countered when using an electrosurgical unit. The patent purports to solve this problem by monitoring the load current from the secondary of an output transformer by the use of a square law detector, the output of ~hich ~s com-pared with a reference voltage level with the dif~erence that is detected being used to vary the output of the electrosurgical unit. Such an arrangement, while provlding .
a constant load current~ produces in a linear increase in power to the load as the load impedance increases rather than providing a constant power output. Further, by sensing current flow on the secondary side of the output transformer, a degradation of high frequency isolation is presented.
This invention provides an improved electrosurgi-cal unit for providing electrosurgical currents to tissue from a controllable power amplifier coupled via an output transformer to an active electrode and return electrode, the improvement residing in a negative ~eedback control circuit which includes a first sensing means ~or sensing a current that is directly related to the current delivered to tissue presented between the active and return electrodes of the unit; a second sensing means for sensing à voltage that is directly related to the voltage presented across the tissue presented between the active and return electrodes of the unit with a function generator connected to the first and second sensing means for obtaining two signals from the sensings made by the first and second sensing means and providing a signal that is directly related to the mathe- ~`
matical product of the two signals which is applied to a comparator-amplifier circuit in the feedback circuit where it is compared to a power level reference signal, the magni-tude of ~hich is selected by the operator~ The output of the comparator-amplifier is connected to the controllable power amplifier of the electrosurgical unit whereby a trans fer of a constant power level in accordance with the selected power level reference signal is made to tissue when presented between the active and return electrodes of the electrosurgi-cal unit.
--The function generator includes a multiplier circuit means and a compensation circuit means. The multi-plier circuit means provides a signal that is directly related to the mathematical product of two signals applied to it, The compensation circuit provides one of the two signals that is a modification of the signal at one input of the compen-sation circuit with such modification a Function of the signal applied to a second input of the compensation circuit~ The other signal for the multiplier circuit is provided by one of the first and second signals of the first and second J
sensing means. The compensation circuit receives one of the first and second signals of the first and second sensing means at one input and the other of the first and second .
signals at its second input.--The compensation circuit means of the function generator provides correction for non-linearities introduced by various components selected for the circuit just described.

The output signal of the compensation circuit is reduced as the signal to its second input increases. Accordingly, if any non-linearity that may be introduced shows a need to increase the power at the high end portion of the impedance range, the signal from the first sensing circuit, which is indica-tive of the current delivered to the tissue, is applied to the first input of the compensation circuit, while the signal from the second sensing circuit, which is indicative of the voltage presented across the tissue, is applied to the second input. This arrangement will also cause some increase in power at the low end power of the impedance range, but a greater increase will be introduced at the high end of the impedance range. Similarly, if any non linearity that may be introduced shows a need to increase the power at the low end portion of the impedance range, the signal From the second sensing circuit, which is in~
dicative of the voltage presented across the tissue, is applied to the first input, while the signal from the first sensing circuit, which is indicative of the current de- ~ !
livered to the tissue, is applied to the second input.
This introduces an increase in power at the low end por~
tion of the impedance range that is greater than any in~ ;
crease introduced at the high end portion of the impedance range. In each case, the output signal from the compensa-tion circuit and the signal connected to the second input of the compensation circuit are the two signals from which the multiplier circuit provides a signal that is directly related to the mathematical product cf the two signals.
Figure 1 is a block diagram of electronic cir- ;
cuitry forming one embodiment of the electrosurgical unit of the present invention;

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Figure 2 is schematic and block diagram showing of a portion of the block diagram of Figure l;
Figure 3 is a block ~liagram showing a modification __ __ ._ ___ _______ _ __ __ __ _ ~

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of a portion of the block diagram o~ Figure l; and Figure 4 is a schematic of a portion o~ the block diagram of Figure 3.
Referring to Figure l of the drawing, the elec-tronic circuitry shown in block diagram ~orm consisting ofthe oscillator lO~ the mode select 12, drive c1rcuit 14 controllable power amplifier 16, output transformer 18 having a primary winding 20 and a secondary winding 22, an active electrode 24 capacitively coupled to one end of the secondary winding 22 and a return or patient electrode 26 ;
capacitively coupled to the other end of the secondary winding 22 is representatlve Or circuitry found in a number of known electrosurgical units for providing electrosurgical currents to tissue presented between the active and return electrodes. A power level selector 28 is also shown which ~
generally, in known units, is connected directly to a power ~;
control in the power amplifier 16.
The oscillator lO is designed to generate electro- ~-surgical currents, such as cutklng and coagulation currents, ~ ;~
with the mode select 12 providing the means operated by the operator to select the desired type of electrosurgical cur-rent. The selected output from oscillator lO is applied to the drive circuit 14 which serves to amplify the output received from the oscillator with the controllable power amplifier 16 serving to further ampllfy the desired elec-trosurgical current ln accordance with a signal from the power level selector 28 as selected by the operator to ob-tain a desired power level for the selected current. The output transformer 18 is used to couple the output of the power amplifier 16 to the tissue of the patient which is 3 ~ 9!7 connected between the active electrode 24 and the return or patient electrode 26.
It is known that the arrangement of ~igure 1 described to this point will not deliver power at a sub-stantlally constant level over the range o~ i.mpedance o~the various types o~ tissue that may be presented to the active and return electrodes of the unit. The remaining portion of the circuitry shown in Figure 1, which includes a ~irst sensing means provided by the current to voltage sensing circuit 30g a second sensing means pro~ided by the voltage sensing circuit 32, a function generator provided by the multiplier 34 and comparator-amplifier 36 to which the power level selector 28 is connected, provides a nega-tive feedback circuit which is e.~fective to control the power amplifier 16 to adJust the power level deIlvered to the tissue when presented between the active electrode 24 and return electrode 26 so it is substantially constant over the range o~ impedance usually presented by the various types of tissue encountered during various surgical proce-dures that may be carried out when using the electrosurgicalunit.
The first sensing means provided by the current to voltage sensing circuit 30 provides an electr~cal signal : to the multiplier 34 that is directly related to the current Z5 flow to the primary winding 20 and, there~ore, directly ; related to the current delivered to tissue when presented to the active and return electrodes.
The second sensing means provided by the voltage sensing circuit 32 is connected at points 3~ and 40 o~ the primary winding 20 to provide a voltage signal to the 7~

multipl~er 3L~ that is directly related to the voltage acrosæ
the primary windin~ 20 and, therefore, directly related to the voltage provided across the tissue when presented to the active and return electrodes. The multiplier 34 provides an output voltage signal on conductor 42 that is applied to the comparator-amplifier 36 which is proportional to the mathematical product of the signals from the sensing circuits 30 and 32 and, therefore, proportional to the power trans-ferred to the load circuit by the primary winding 20. A
voltage re~erence signal is provided to the comparator-ampli~ier 36 from the power selector 28, which is set by -the operator for a desired power level at the active and passive electrodes 24 and 26, respectively. I~ a difference exists bekween the voltage signal from the multiplier 34 and the voltage reference signal from the power level selector 28, such difference is applied to the power control in the controllable power amplifier 16 to change the current flow in the primary winding 20 so the dif~erence detected is reduced to approximately zero.
Figure 2 is a schematic and block diagram showing of the negative feedback circuit that has been described in connectlon with Figure 1, with details provided with respect to suitable circuits usable as the voltage sensing circuit 32, the current to voltage sensing circuit 30 and the power level selector 28. The multiplier 34, which is the function generator needed for the circuit of Figure 1, is not shown in detail since it may, for example, be pro-vided by a standard commercially available multipller -circuit available as type AD533 from Analog Devices~ Inc.
Norwood, Massachusetts~

The comparator-amplifier 36 can be provided b~ a dif~erential amplifier connected as a comparator with the output of the comparator appropriately amplified to provide the necessary drive signal to the power control portion of the power amplifier 16.
An exemplary current to voltage sensing circuit 30 is shown connected for sensing the current through a resistor 44. The resistor 44 is provided in the power ampli~ier 16 and is selected as a reslstor that carries all or a known proportional part of the current that ~lows through the primary winding 20~ so the circuik 30 will provide a voltage signal that is dlrectly related to the current flow in the primary winding 20 and in the secondary winding 22. The voltage appearing across the resistor 1~4 causes current to flow to a holding circuit portion which includes the diode 11~ capacitor 13 and a discharge resistor 15 for the capaci-tor 13. The voltage at the capacitor 13 is applied to the ; two series connected resistors 17 and 19 which connect between one side of the capacitor 13 and ground. The voltage appearing across resistor 19 is applied to the positive in-put terminal of a differential amplifier 21 which has two series connected resistors 23 and 25 connected between the negative input of the ampli~ier 21 and a negatlve voltage.
A diode 27 is connected between ground and the connection common to resistors 23 and 25 and serves to compensate for the voltage drop that occurs across diode 11. ~ resistor 29 is connected between the output 31 of the amplifier 21 and its negative input terminal. Therefore, the different~al amplifier circuit as described~ amplifies the di~ference between the voltage at the cathodes of diodes 11 and 27~

The output 31 of the amplifier 21 is connected to the multiplier 34.
An exemplary circuit for the voltage sensing cir-cuit 32 includes a sensing transformer that has its primary 33 connected across the primary 20 of the output transformer 18. ~ sensing transformer of the step-down type is suitable.
The output of the secondary 35 of the sensing transformer is connected to a full~wave rectifier 37, the output of which is connected across a capacitor 39. Two series connected resistors 41 and 43 connect across the capacitor 39 with !;
the connection 45 common connected to the multiplier 34 to provide it with a voltage signal that is directly related to the voltage presented across the primary winding 20 and, therefore, directly related to the voltage between active electrode 24 and return electrode 26.
An exemplary circuit for the power level selector 28 is shown in Figure 2. It includes a potentiometer 47 connected between a voltage source and ground with the ad-justable connection of the potentiometer connected to ground via a resistor 49 and to the comparator-amplifier 36 to provide the reference input signal to the comparator-ampli-fier 36.
~ hile a selection of the various components for the circuits described is not considered critical, non-linearities may be introduced by the various circult com-ponents selected for the circuits described to cause the power output level not to be as constant as desired. The non-linearity that is introduced may be compensated by the use of a non-linearity compensatlon circuit in the feedback circuit. Figure 3 shows how such a compensation circuit 46 7~3~

is connected in the circuitry of ~igure 2 to provide the needed compensation. The compensation circuit 46 and the multiplier 34 are viewed as providing a function generator.
An exemplary circuit for use as a non-linearity compensation circuit 46, as indicated irl Figure 3, ls shown in detail in ~igure 4. If the circuitry per Figure l, i.e., without the compensation circuit 46, indicates there is need for compensation to raise the power level at the high im-pedance end portion of the impedance range~ the resistor 53, which provides a ~lrst input for the circuit 46 is connected to the output 31 of the current to voltage sensing circuit 30. The base electrode of transistor 51, which provides a second input for the circuitry 46, is connected to the con-nection 45 of the voltage sensing circult 32, which is also ; 15 connected to the multiplier 34 to provide one input to the multiplier 34. If there is a need for compensation to raise the power level at the low end portion rather than at the hlgh end portion Or the impedance range, the output 31 of the current to voltage sensing circuit 30 is connected to 20 the second input (transistor 51) for the compensation clr-cuitry 46 and to one input of the multiplier 34 with the voltage sensing circuit having the polnt 45 connected to the flrst lnput of the compensation circuit 46 provided at the resistor 53. In each of the two situations mentioned, 25 the second input to the multipller 3ll is obtained from the emitter of the transistor 55. In the case of the block diagram showing in ~lgure 3, the two possible connections are ind~cated by using reference numerals 31 and 45 for one situation with reference numerals (31) and (45) used for the 30 second situation.

In addition to the components mentloned, the compensation circuit of Figure 4 includes a resistor 57 connected between the emitter of transistor 51 and a posi-tive voltage source with a resistor 59 provided between the 5 collector of transistor 51 and a negat:Lve voltage source.
The resistors 57 and 5~ are of the same magnitude and the positive and negatlve voltage sources are of the same magni-tude, so the transistor 51, which has collector connected to the emitter follower connected transistor 61, serves to provide a voltage at the emitter of transistor 61 that is the inverse of the voltage presented to the base of transis-tor 51. The collector of transistor 61 is connected to the negative voltage source with its emitter connected via a resistor 63 to the emitter of transistor 65. The collector 15 of transistor 65 is connected to the resistor 53 with base biased at ~. 7 volts via its connection to the diode 67.
Diode 67 is connected to ground at one end and to the posi-tive voltage source via a resistor 69. The voltage provided ~ ~-by the diode 67, which is about +.7 volts, serves to com-pensate for the . 7 volt drop that occurs between the baseand emitter Junction of transistor 65. The signal present at the collector of transistor 65 is coupled by the trans-istor 71, that is connected as an emitter-follower, to the transistor 55, the emitter of which is connected to provide 25 one input to the multlpl~er 34. A resistor 72 is connected between the emitter of trans~stor 71 and the negative volt-age source while its collector is connected to the positive voltage source. The collector of transistor 55 is connected to the negative voltage source while its emitter ls connected to the positive voltage source via a resistor 73O

With the circuit 46 of ~lgure 4 as described, a signal applied to the ~irst input (resistor 53) will cause transistor 71 to conduct and in turn cause transistor 55 to conduct so a slgnal that is dlrectly related to the signal at the first input will be presented to the multiplier 34.
Such description~ however, disregards the compensation action that is initiated by the remainder of the circuitr~.
When a signal is presented to the second input (base of transistor 51)~ transistor 51 conducts and with a signal also present at the first input (resistor 53), transistors J
65 and 61 conduct to increase the voltage drop across resistor 53, thereby decreasing the signal to transistor 71 to cause a decrease in the output to the multiplier 34 so that it is receiving a signal that, due to the signal to transistor 51, is less than that due to ~ust the signal applied to the first input of compensation circuit 46. The output of the multiplier 311 will then be decreased, which when compared at the comparator-amplifier 36 with the refer-ence signal from the power level selector 28, will provide a control signal to the power amplifier to cause the power delivered by the power amplifier to increase. The decrease in the output of the multiplier 34 becomes greater as the signal to the second input increases. When the signal to the first input tresistor 53) to the compensation circuit 46 is obtained from khe current to voltage sensing circuit 30 with the signal to the second input (transistor 51) o~
circuit 46 obtained from the voltage sensing circuit 32, there will be a s~all lncrease in the power at lower end portion of the impedance range, but the compensation pro-vided will be the most effective at the upper end portion of the impedance range. Conversely~ when the signal to the first input to the circuit 46 is obtained from the vol.tage sensing circuit 32 with the signal to the seeond input of eircuit 46 obtained from the current to voltage sensing eireuit 30, the compensation will be the most effective at the lower end portion of the impedanee range.
Obviously, many modifieations and variations of the foregoing disclosure are possible in light of the above teachings. It is~ therefore, to be understood that within the scope of the appended claims, the invention may be practieed otherwise than as speeifically described.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved electrosurgical unit for providing electrosurgical currents to electrically conductive tissue from a controllable power amplifier connected to the primary winding of an output transformer with the secondary winding of the transformer connected to an active electrode and return elec-trode between which the tissue to receive an electrosurgical current is positioned, the improvement therein being a feedback control circuit comprising:
a first sensing means connected for sensing a current in the unit and providing a first signal in response thereto that is directly related to the current delivered to tissue when presented between the active electrode and return electrode;
a second sensing means connected for sensing a voltage in the unit and providing a second signal in response thereto that is directly related to the voltage presented across the tissue when presented between the active electrode and return electrode;
a multiplier circuit means for receiving two signals and providing a signal that is directly related to the mathematical product of said two signals;
a compensation circuit means having a first and a second input and responsive to signals applied to said first and second inputs for providing one of said two signals for said multiplier circuit means that is a modification of the signal applied to said first input, said modifica-tion being a function of the signal applied to said second input, said compensation circuit means connected to said multiplier circuit means to provide said one of said two signals to said multiplier circuit means, said multiplier circuit means connected for receiving one of said first and second signals as the other of said two signals, said compensation circuit means connected to said first and second sensing means for receiving said one of said first and second signals at said second input and the other of said first and second signals at said first input;
a power level selector means for providing a reference signal; and a comparator-amplifier circuit means connected for receiving said signal provided by said multiplier circuit means and connected for receiving said reference signal and having an output connected to said controllable power amplifier, said comparator-amplifier circuit means providing a signal at said output in response to said signal provided by said multiplier circuit means and said reference signal for controlling the controllable power amplifier wherein a transfer of power at a substantially con-stant level is made to tissue when presented between the active electrode and return electrode.

2. An improved electrosurgical unit according to claim 1 wherein said one of said first and second signals is said first signal.

3. An improved electrosurgical unit according to claim 1 wherein said one of said first and second signals is said second signal.

4. An improved electrosurgical unit according to claim 1 wherein said first sensing means is connected to the controllable power amplifier.

5. An improved electrosurgical unit according to claim 1 wherein said second sensing means is connected across the primary winding of the ouput transformer.

6. An improved electrosurgical unit according to claim 1 wherein said first sensing means is connected to the controllable power amplifier and second second sensing means is connected across the primary winding of the output transformer.