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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates generally to improvements in cutting signal
generators in electrosurgical devices, and more particularly pertains to
new and improved oscillator circuitry utilized for generating high
frequency cutting signals.
In the field of electrosurgical devices, it has been the practice to employ
various types of electrosurgical generators suitable for generating
cutting and coagulation currents. In some cases, the currents are provided
by separate generators, one generator providing a cutting current and the
other generator providing a coagulation current. In other cases, a single
generator is utilized to provide both a cutting and coagulating current,
or a combination of such currents. These prior art electrosurgical
instruments, whether they used separate signal generators or one signal
generator for producing the various types of signals required, either
ignore the problem of the patient acting as a varying impedance load, or
compensate for it by expensive and complicated means.
SUMMARY OF THE INVENTION
An object of this invention is to provide an electrosurgical device that
provides a relatively constant tissue cutting signal, in spite of
impedance changes in the load.
This object and the general purpose of this invention are accomplished by
providing a transistor regenerative feedback oscillator-amplifier that has
a load-coupling transformer in the collector circuit along with a feedback
coupling winding to provide regenerative feedback to the base circuit
which contains the frequency determining elements. The output signal from
this oscillator-amplifier is applied to human tissue for cutting purposes.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and many of the attendant advantages of this invention will
be readily appreciated as it becomes better understood by reference to the
following detailed description when considered in conjunction with the
accompanying drawings, in which, like reference numerals designate like
parts throughout the figures thereof and wherein:
FIG. 1 is a block diagram illustration of a typical electrosurgical device.
FIG. 2 is a circuit diagram of the preferred embodiment of the cutting
signal generator of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1, the organization of an electrosurgical device
that could utilize the cutting signal generator of the present invention
is illustrated. A power supply 11 supplies DC power to a switching device
15 by way of cable 13. The switching device 15 may be mechanically or
electromechanically actuated and is intercoupled electromechanically 31
with a switching device 29 at the output of a coagulating signal generator
21 and a cutting signal generator 23. Switching device 15 selectively
supplies energy from the power supply to the coagulating signal generator
21 over cable 17.
The coagulating signal generator 21 may be a spark-gap oscillator that
supplies high current, high frequency, damped oscillations over cable 25
to switching device 29. Switching device 29 may contain manually
adjustable signal controls and path selection devices to, for example,
switch the coagulating signal to line 33. Line 33 is then connected to a
surgical instrument. Line 39 may be connected to the grounding or
"indifferent" plate, contacting the patient and acts as a return path for
the electrosurgical signal.
If a cutting signal is desired, switching device 15 supplies DC energy from
the power supply 11 to the cutting signal generator 23 by way of cable 19.
The cutting signal generator 23, as will be more fully explained
hereinafter, is a medium frequency, high power oscillator-amplifier that
generates a continuous signal. This signal is supplied to switching device
29 over cable 29. Switching device 29 connects this signal to a cutting
terminal 35 which is connected to a surgical instrument (not shown).
Electrosurgical devices such as, illustrated by FIG. 1, are well known in
the prior art.
The cutting signal generator 23, illustrated in FIG. 2, is uniquely
designed to accommodate itself to the varying load conditions presented by
the human body. The load 59 connected across the cutting signal generator
output lines 27a, 27b may go from infinite impedance to zero impedance.
However, practically speaking, when operating in the human body, the load
can vary from several thousand ohms down to around 30 ohms.
The cutting signal generator 23 comprises an NPN transistor amplifier 53
shown to be configured in a commonemitter arrangement. The transistor
amplifier is illustrated as being made up of one transistor. However, it
should be understood that as many transistors as is necessary, connected
in parallel, may be utilized to achieve the desired power output. The
transistor amplifier 53 is connected as a regenerative feedback oscillator
by way of a feedback transformer having a primary winding 41 and a
secondary winding 43. Primary winding 47 of the load transformer is
connected in series with the primary winding 41 of the feedback
transformer. The secondary winding 45 of the load transformer is connected
to the load 59 over output lines 27a, 27b.
The transistor amplifier 53 is biased by a resistor 49, which keeps the
collector terminal at side 1 of resistor 49 positive with respect to the
base at side 2 of the resistor 49. It should be understood that this NPN
arrangement is only exemplary and that PNP transistors may be used as
well, the biasing being rearranged appropriately.
A DC voltage from the power supply 11 is supplied to side 1 of the primary
winding 41 over line 19. The secondary winding 43 of the transformer
inversely couples the signal in the primary winding 41 to the base of
transistor 53 by way of capacitor 51. One side of the secondary winding 43
is grounded, while the other side is connected to one side of capacitor
51.
The emitter of the transistor 53 is also grounded. The primary winding 47
and secondary winding 45 couple the signal in the collector circuit of
transistor 53, without inversion, to the load.
The effective inductance of the winding 43 in the feedback loop and the
capacitance of capacitor 51 in the feedback loop determine the time
constant of the regenerative feedback oscillator circuit illustrated. In
other words, the frequency of the output signal is determined by the value
of inductor 43 and the value of capacitor 51.
When a DC power source is supplied to terminal 1 of primary coil 41, by way
of line 19, the polarity of the primary winding will be plus at terminal 1
and minus at terminal 2. The polarity of primary winding 47 will be plus
on top and minus on the bottom. The polarity of resistor 49 will be plus
at terminal 1 and minus at terminal 2. This will bias the transistor 53 in
a forward direction causing base current 63 to start flowing. When such
base current starts flowing, collector current 61 and emitter current 65
will also start flowing in the direction shown by the arrows. The base
current illustrated is actually electron flow. It should be understood
that conventional current flows in a direction opposite to electron flow.
This electron flow causes an expanding electromagnetic field in primary
windings 47 and 41.
The electromagnetic field caused by primary winding 41 is coupled to
secondary winding 43 which is wound to cause polarity inversion. Thereby,
terminal 4 of winding 43 will be positive and terminal 3 will be negative
when terminal 1 of primary winding 41 is positive and terminal 2 is
negative. As electron flow increases in the collector circuit of
transistor 53, the current induced in secondary winding 43, as a result of
the expanding electromagnetic field in winding 41, will cause a positive
charge build-up on plate 1 of capacitor 51. The other side or plate 2 of
capacitor 51 will, therefore, have an increasing negative charge. This
increasing negative charge at plate 2 acts to further bias the transistor
53 in the forward direction, causing an increase in base current 63, which
in turn causes an increase in collector current 61 and emitter current 65.
This action will continue until the collector current 61 reaches a maximum
as determined by the elements 41, 47 in the collector circuit and the
transistor 53. At such time, since the current is no longer increasing,
lack of an electromagnetic flux change in the primary winding 41 causes a
lack of feedback current, by way of inductor 43 and capacitor 51. At such
time, plate 1 of capacitor 51 has reached its maximum positive charge, as
shown by the first half cycle of wave 55. The opposite plate 2 of
capacitor 51, at this time has reached its maximum negative charge as
shown by the first half cycle of wave 56.
The capacitor 51 will start discharging through inductor 43 to ground,
according to the particular time constant dictated by the values of
capacitor 51 and inductor 43. This discharge action causes the base
current 63 to decrease, thereby decreasing the collector current 61 and
the emitter current 65 of transistor 53. Since the collector current is
now collapsing, the feedback coupling between primary winding 41 and the
secondary winding 43, will aid this process, causing the capacitor to
discharge until the transistor 53 is driven to cut-off as exhibited by
practically zero emitter current and collector current.
At this time, plate 1 of capacitor 51 has reached its maximum negative
charge, as shown by the second half cycle of wave 53. The other plate 2 of
capacitor 51 has, in turn reached its maximum positive charge, as shown by
the second half cycle of wave 56.
The biasing resistor 49 maintains the transistor in a forward bias
condition, thereby causing base current 63 to again increase, in turn
increasing emitter current 65 and collector current 61. This action causes
a repeat of the first half cycle, as shown by waves 55 and 56. Oscillation
will continue in this manner until removal of the DC supply from side 1 of
the primary winding 41 of the feedback transformer.
The primary winding 47 and the secondary winding 45 couple the load 59 into
the collector circuit of the transistor 53. What in effect occurs by this
arrangement, is the load impedance of the load 59 is reflected back into
the primary 47 of the transformer so that, effectively, primary winding 47
may be replaced by the reflected impedance value of the load. Therefore,
primary winding 47, secondary winding 45 and the load 59 can be thought of
as a variable impedance in the collector circuit.
Bearing this in mind, it can be seen that as the value of the load
impedance in the collector circuit of transistor 53 decreases, the
collector current will increase. Assuming that the collector current 61 is
in the increasing part of its cycle, a decreasing load impedance will tend
to increase the collector current causing the voltage drop across the
primary winding 41 to become larger. This increasing voltage drop is
coupled from the primary winding 41 to the secondary winding 43, which in
turn increases the feedback signal to the capacitor 51 thereby further
forwarding biasing the transistor 53. This increased forward biasing
increases the base current 63, the collector current 61 and the emitter
current 65. The increasing collector current will compensate
commensurately for the decreasing reflected impedance of the load to keep
the voltage across the load from swinging sharply to a value below the
desired minimum.
Assume now that the load impedance is increasing and the collector current
is in an increasing cycle. The increasing impedance will cause the
collector current to decrease. This causes the voltage drop across the
primary winding 41 to decrease correspondingly, thereby decreasing the
feedback signal and causing the charge on capacitor 51 to decrease. This
decreases the forward bias of the transistor 53 which causes a decrease in
the base current 63 and a commensurate decrease in the collector current
61. Here, the combination of a decreasing collector current with an
increasing reflected load impedance keeps the voltage across the load from
swinging sharply to a value above the desired maximum.
Besides stabilizing the voltage swings across the load, the oscillator of
FIG. 2 controls the change in current through the load, thereby also
preventing erratic swings in load current.
In summary, what has been disclosed is an electrosurgical device that
utilizes an uncomplicated and inexpensive oscillator-amplifier circuit for
generating tissue cutting signals. The oscillator-amplifier is designed to
inexpensively adapt itself to impedance changes of the load. Obviously,
many modifications and variations of the foregoing disclosure, as
illustrated by the preferred embodiment, 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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