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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention presented herein relates to electrosurgical units and, more
specifically, to circuitry for providing automatic control of the output
power at a selected level over the usual impedance range of electrically
conductive tissue encountered when using such units.
2. Description of the Prior Art
A wide variety of electrosurgical units for generating various high
frequency output waveforms for electrosurgical procedures are known in the
art. A selected waveform output from an electrosurgical unit is applied to
a patient by the use of an active electrode placed in contact with the
patient at the point where a desired surgical procedure is to be carried
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 be high enough to
generate heat sufficient to accomplish the desired surgical procedure.
Since the impedance of the current path is a function of the different
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 impedance of the
electrosurgical unit to the median of the expected impedance range of the
tissue. This method is not acceptable since the output power decreases as
the impedance varies from the median or center designed value. In order
that adequate power be provided at both low or high impedance tissue with
such method, it is necessary to provide for a power level at the median of
the impedance range which may be excessive resulting in unnecessary tissue
damage.
U.S. Pat. No. 3,601,126 points out the power level problem involved due to
the range of impedance that is encountered 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 which is compared with a reference voltage
level with the difference that is detected being used to vary the output
of the electrosurgical unit. Such an arrangement, while providing 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.
SUMMARY OF THE INVENTION
This invention provides an improved electrosurgical 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 feedback control circuit
which includes a first sensing means for 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 a 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
mathematical 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 magnitude of which is selected by
the operator. The output of the comparator-amplifier is connected to the
controllable power amplifier of the electrosurgical unit whereby a
transfer 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 electrosurgical unit.
Should the power output not be as constant as desired, due possibly to
non-linearities introduced by various components selected for the circuit
just described, another embodiment of the invention provides for a
non-linear compensation circuit to correct for such problem. The
compensation circuit is provided in the feedback circuit as a part of the
function generator connected to the first and second sensing means. A
suitable compensation circuit may be used that is responsive to a signal
connected to a first input with another signal connected to a second input
for providing an output signal from the compensation circuit that is a
modification of the signal applied to the one input with the modification
being dependent on the magnitude of the signal provided to the second
input. The output signal of the compensation circuit is reduced as the
signal to the 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 indicative 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 indicative 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 delivered to the tissue, is applied to the second input. This
introduces an increase in power at the low end portion of the impedance
range that is greater than any increase introduced at the high end portion
of the impedance range. In each case, the output signal from the
compensation circuit and the signal connected to the second input are the
two signals from which the function generator provides a signal that is
directly related to the mathematical product of the two signals.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of electronic circuitry forming a preferred
embodiment of the electrosurgical unit of the present invention;
FIG. 2 is a schematic and block diagram showing of a portion of the block
diagram of FIG. 1;
FIG. 3 is a block diagram showing a modification of a portion of the block
diagram of FIG. 1; and
FIG. 4 is a schematic of a portion of the block diagram of FIG. 3.
DETAILED DESCRIPTION
Referring to FIG. 1 of the drawing, the electronic circuitry shown in block
diagram form consisting of the oscillator 10, the mode select 12, drive
circuit 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 representative of 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 10 is designed to generate electrosurgical currents, such as
cutting and coagulation currents, with the mode select 12 providing the
means operated by the operator to select the desired type of
electrosurgical current. The selected output from oscillator 10 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
amplify the desired electrosurgical current in accordance with a signal
from the power level selector 28 as selected by the operator to obtain 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 connected between the active electrode 24 and the return
or patient electrode 26.
It is known that the arrangement of FIG. 1 described to this point will not
deliver power at a substantially constant level over the range of
impedance of the various types of tissue that may be presented to the
active and return electrodes of the unit. The remaining portion of the
circuitry shown in FIG. 1, which includes a first sensing means provided
by the current to voltage sensing circuit 30, a second sensing means
provided 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 negative feedback circuit which is
effective to control the power amplifier 16 to adjust the power level
delivered to the tissue when presented between the active electrode 24 and
return electrode 26 so it is substantially constant over the range of
impedance usually presented by the various types of tissue encountered
during various surgical procedures that may be carried out when using the
electrosurgical unit.
The first sensing means provided by the current to voltage sensing circuit
30 provides an electrical signal to the multiplier 34 that is directly
related to the current flow to the primary winding 20 and, therefore,
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 38 and 40 of the primary winding 20 to provide a
voltage signal to the multiplier 34 that is directly related to the
voltage across the primary winding 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 transferred
to the load circuit by the primary winding 20. A voltage reference signal
is provided to the comparator-amplifier 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. If a difference exists between
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 difference detected is reduced to
approximately zero.
FIG. 2 is a schematic and block diagram showing of the negative feedback
circuit that has been described in connection with FIG. 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 FIG. 1, is not shown in detail since it may, for example,
be provided by a standard commercially available multiplier circuit
available as type AD533 from Analog Devices, Inc., Norwood, Mass.
The comparator-amplifier 36 can be provided by a differential 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 amplifier 16 and is selected as a resistor that carries all or a
known proportional part of the current that flows through the primary
winding 20, so the circuit 30 will provide a voltage signal that is
directly related to the current flow in the primary winding 20 and in the
secondary winding 22. The voltage appearing across the resistor 44 causes
current to flow to a holding circuit portion which includes the diode 11,
capacitor 13 and a discharge resistor 15 for the capacitor 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 input
terminal of a differential amplifier 21 which has two series connected
resistors 23 and 25 connected between the negative input of the amplifier
21 and a negative 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. A resistor 29 is connected
between the output 31 of the amplifier 21 and its negative input terminal.
Therefore, the differential amplifier circuit as described, amplifies the
difference 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 circuit 32 includes a sensing
transformer that has its primary 33 connected across the primary 20 of the
output transformer 18. A 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 FIG. 2. It
includes a potentiometer 47 connected between a voltage source and ground
with the adjustable 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-amplifier 36.
While a selection of the various components for the circuits described is
not considered critical, situations may arise wherein non-linearities are
introduced by the various circuit components that have been selected 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 compensation circuit in the feedback circuit. FIG. 3 shows
how such a compensation circuit 46 is connected in the circuitry of FIG. 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 in FIG. 3, is shown in detail in FIG. 4. If the circuitry per
FIG. 1, i.e., without the compensation circuit 46, indicates there is need
for compensation to raise the power level at the high impedance end
portion of the impedance range, the resistor 53, which provides a first
input for the circuitry 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
connection 45 of the voltage sensing circuit 32, which is also 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 high end portion of the impedance range, the output 31
of the current to voltage sensing circuit 30 is connected to the second
input (transistor 51) for the compensation circuitry 46 and to one input
of the multiplier 34 with the voltage sensing circuit having the point 45
connected to the first input of the compensation circuit 46 provided at
the resistor 53. In each of the two situations mentioned, the second input
to the multiplier 34 is obtained from the emitter of the transistor 55. In
the case of the block diagram showing in FIG. 3, the two possible
connections are indicated by using reference numerals 31 and 45 for one
situation with reference numerals (31) and (45) used for the second
situation.
In addition to the components mentioned, the compensation circuit of FIG. 4
includes a resistor 57 connected between the emitter of transistor 51 and
a positive voltage source with a resistor 59 provided between the
collector of transistor 51 and a negative voltage source. The resistors 57
and 59 are of the same magnitude and the positive and negative voltage
sources are of the same magnitude, 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 transistor 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 of transistor 65 is connected to the resistor 53 with
base biased at +0.7 volts via its connection to the diode 67. Diode 67 is
connected to ground at one end and to the positive voltage source via a
resistor 69. The voltage provided by the diode 67, which is about +0.7
volts, serves to compensate for the 0.7 volt drop that occurs between the
base and emitter junction of transistor 65. The signal present at the
collector of transistor 65 is coupled by the transistor 72, that is
connected as an emitter-follower, to the transistor 55, the emitter of
which is connected to provide one input to the multiplier 34. A resistor
72 is connected between the emitter of transistor 71 and the negative
voltage 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 is connected to the positive voltage
source via a resistor 73.
With the circuit 46 of FIG. 4 as described, a signal applied to the first
input (resistor 53) will cause transistor 71 to conduct and in turn cause
transistor 55 to conduct so a signal that is directly 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 circuitry. 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 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 just the signal applied
to the first input of compensation circuit 46. The output of the
multiplier 34 will then be decreased, which when compared at the
comparator-amplifier 36 with the reference 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 (resistor 53)
to the compensation circuit 46 is obtained from the current to voltage
sensing circuit 30 with the signal to the second input (transistor 51) of
circuit 46 obtained from the voltage sensing circuit 32, there will be a
small increase in the power at lower end portion of the impedance range,
but the compensation provided 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 voltage sensing circuit 32
with the signal to the second input of circuit 46 obtained from the
current to voltage sensing circuit 30, the compensation will be the most
effective at the lower end portion of the impedance range.
Obviously, many modifications 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 practiced otherwise than as specifically described.
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