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Description  |
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FIELD OF THE INVENTION
This invention relates to electrosurgery in general and in particular to
electrosurgical generators which are capable of performing surgical
operations by means of radio-frequency electrical currents.
BACKGROUND OF THE INVENTION
In addition to performing surgical operations on animal tissues by means of
mechanical instruments such as scalpels or knives, surgery may also be
performed by passing radio-frequency current through animal tissues. Ihere
are essentially four main surgical operations that can be performed
depending on the voltage levels and the amount of power applied to the
tissue. These operations are typically designated as dessication,
fulgeration, cutting and cutting with hemostasis. Often, dessication is
referred to as coagulation and sometimes dessication and fulguration are
designated collectively as coagulation.
The radio-frequency current used in the performance of electrosurgical
operations is typically generated by means of a radio-frequency generator
connected to a power amplifier. The output of the power amplifier is in
turn connected to the tissue mass by means of two electrodes. Surgical
operations are performed by means of an "active" electrode which
introduces the radio-frequency current into the tissue mass. Since, as
mentioned above, electrosurgical effects are primarily dependent on the
power and voltage applied, the active electrode typically has a small
cross-section to concentrate the power and limit the surgical effects to a
small, controlled area. A return path from the tissue mass to the
generator for the radio-frequency current is provided by a "passive" or
"patient" plate which has a large area to prevent electrosurgical effects
from taking place at the current return location. Alternatively, a pair of
active electrodes may be used in a "bipolar" mode in which the
electrosurgical effects are confined to the sample of tissue between the
two electrodes.
A dessication operation is performed by holding the active electrode in
firm contact with the tissue. Radio-frequency current passes from the
electrode directly into the tissue to produce heating of the tissue by
electrical resistance heating. The heating effect destroys the tissue
cells and produces an area of necrosis which spreads radially from the
point of contact between the electrode and the tissue. Due to the nature
of the cell destruction, the necrosis is usually deep. The eschar produced
during the operation is usually light in color and soft. In order to
produce optimal results in a dessication operation, an electrosurgical
generator must be capable of providing several amperes (peak current) of
radio-frequency current to moist tissue which has an impedance of
approximately 100 ohms. Although the radio-frequency peak current density
is high, the power delivered to the tissue is relatively low because of
the low tissue impedance. In addition, the dessication waveform may be
interrupted to produce an overall low duty cycle which helps to reduce
cutting effects. Therefore, although the peak current values are high, the
RMS value of the current is low. During a dessication operation the
moisture in the tissue cells is driven off at a controlled rate and as the
moisture content in the tissue decreases its impedance increases.
Therefore, in order to keep the power applied to the tissue at a low value
and prevent a cutting effect, as described below, it is necessary to limit
the power output as the tissue impedance increases. Ideally, the power
decrease should be proportional to the impedance.
As the impedance of the tissue increases, depending on the output
characteristics of the electrosurgical generator, another surgical effect
can be produced. Cutting occurs when sufficient power per unit time is
delivered to the tissue to vaporize cell moisture. If the power applied is
high enough a sufficient amount of steam is generated to form a layer of
steam between the active electrode and the tissue. When the steam layer
forms, a "plasma" consisting of highly ionized air and water molecules
forms between the electrode and the tissue causing the tissue impedance,
as seen by the generator, to rise to approximately 1000 ohms. If the
electrosurgical generator can provide sufficient power to a 1000-ohm load
and has sufficiently high output voltage, a radio-frequency electrical arc
develops in the plasma. When this happens the current entering the tissue
is limited to an area equal to the cross-sectional area of the arc where
it contacts the tissue and thus the power density becomes extremely high
at this point. As a result of the locally high power density the cell
water volatizes into steam instantaneously and disrupts the tissue
architecture - literally blowing the cells apart. New steam is thereby
produced to maintain the steam layer between the electrode and the tissue.
If the power density delivered to the tissue mass is sufficient, enough
cells are destroyed to cause a cutting action to take place. A repetitive
voltage waveform, such as a sinusoid, delivers a continuous succession of
arcs and produces a cut with very little necrosis and hemostasis.
It is also possible to achieve a combination of the above effects by
varying the electrical waveform applied to the tissue. In particular, a
combination of cutting and dessication (called cutting with hemostasis)
can be produced by periodically interrupting the continuous sinusoidal
voltage normally used to produce an electrosurgical cut. If the
interruption is of sufficient duration, the ionized particles in the
plasma located between the electrode and the tissue diffuse away, causing
the plasma to collapse. When this happens the electrode comes in contact
with the tissue momentarily until a new plasma layer is formed. During the
time that the electrode is in contact with the tissue it dessicates the
tissue thereby sealing off small blood vessels and other bleeders in the
vicinity of the electrode.
Another surgical effect called fulguration may be obtained by varying the
voltage and power per unit time applied by the electrosurgical generator.
Although fulguration is often confused with dessication, it is a
distinctly different operation. In particular, fulguration is typically
performed with a waveform which has a high peak voltage but a low duty
cycle. If an active electrode with this type of waveform is brought close
to a tissue mass and if the peak voltage is sufficient to produce a
radio-frequency arc (at an impedance of 5000 ohms before electrical
breakdown), fulguration occurs at the point where the arc contacts the
tissue. Due to the low duty cycle of the fulgurating waveform, the power
per unit time applied to the tissue is low enough so that cutting effects
due to explosive volatization of cell moisture are minimized. In effect,
the radio-frequency arc coagulates the tissue in the immediate vicinity of
the active electrode thereby allowing the operating surgeon to seal off
blood vessels in the vicinity of the electrode. The fulgurating electrode
never touches the surface of the tissue and a hard, dark eschar is formed
at the surface of the tissue mass in the fulgurated area. In contrast to
dessication, fulguration is a surface process and the area of necrosis is
confined to the surface. Therefore, fulguration can be used where the
tissue mass is very thin and the deep necrosis produced by a dessication
operation would damage underlying organs and accordingly, is a very useful
operation.
In order to perform the above four surgical operations properly, a
general-purpose electrosurgical generator must be capable of delivering
significant amounts of radio-frequency power into a tissue impedance which
varies over an order of magnitude (between approximately 100 ohms to
approximately 1000 ohms). In addition, the generator must be capable of
producing a sufficient peak voltage to initiate sparking in the
fulguration and cutting modes. These requirements necessitate that the
generator be capable of handling high internal radio-frequency voltages
and currents.
In order to meet the internal generator demands the earliest prior art
generators used oscillators and power amplifiers comprised of electron
tube circuits. These prior art units had a disadvantage in that they
dissipated large amounts of heat internally. In order to handle the
internal heat load the units were large and bulky and required ventilating
fans which exhausted non-sterile air into the operating room environment.
To reduce the heat problem, subsequent prior art units used semiconductor
components to generate the required radio-frequency power output. The
semiconductor devices inherently dissipated less heat than the electron
tube counterparts, but did not entirely eliminate the heat loading
problem. When the semiconductor devices were used in a linear mode they
still dissipated significant amounts of heat internally.
Other units utilized semiconductor switching circuits to produce
rectangular waveforms instead of the sinusoidal waveforms used by the
previous units. These rectangular waveforms could be generated more
efficiently than sinusoidal waveforms but still did not entirely eliminate
the heat problem. In particular, because the semiconductor devices in a
practical general purpose generator are required to handle both high
voltages and high currents, high power semiconductor switching devices
were often used. These devices were able to handle the required voltages
and currents, but had the disadvantage that their switching times were
slow. A slow switching time results in high internal heat dissipation.
Therefore, many prior art semiconductor devices still required large and
bulky heat-sinks or ventilating fans. Although semiconductor components
were available which had fast switching times and therefore low internal
heat dissipation, these devices were not used in prior art general purpose
electrosurgical generators because they were not inherently capable of
handling the high voltages and high currents required. In addition, the
use of a non-sinusoidal waveform produced significant amounts of
radio-frequency noise due to the high order harmonics in the output
signal.
Other prior art general purpose generators have attempted to overcome the
internal heating problem by using several separate semiconductor
generating circuits, each optimized for a particular electrosurgical
operation. This prior art approach allows the semiconductor circuitry to
be tailored to each output required for the associated electrosurgical
operation. The tailoring reduces the current and voltage requirements
placed on the semiconductors and thus semiconductors can be used which
have faster switching times and thus less internal power dissipation.
Unfortunately, the multiplicity of components necessary for this approach
produces expensive and bulky units.
Still other units have solved the problem by optimizing the generator for
one or two electrosurgical operations. These units are small and compact
but typically produce poor results in surgical operations other than those
for which they were designed.
SUMMARY OF THE INVENTION
The foregoing and other problems inherent in the prior art are solved by a
general-purpose electrosurgical generator which utilizes high speed
semiconductor switching circuitry to produce optimal voltage and current
requirements for cutting, dessication, cutting with hemostasis and
fulguration while maintaining low internal heat dissipation. Therefore, no
large and bulky heat sinks or fans are required.
Specifically, the electrosurgical generator described in the illustrative
embodiment herein utilizes a single output switching circuit and three
output circuits. The switching circuit consists of four field effect
transistor semiconductor switches driven by a common timing circuit. The
interconnection of these semiconductor switches may be internally
reconfigured under operator control so that, depending on the
electrosurgical operation being performed, the switches may be connected
in a bridge configuration or in a series configuration. In each
configuration, the semiconductor switches are connected to handle the
particular voltage and current requirements necessary to produce the
optimal output power for the associated surgical operation. The timing
waveforms produced by the timing circuitry which drive the switching
circuit are also changed during each electrosurgical operation to produce
optimal output waveforms.
More specifically, in configuring the unit to perform cutting and
dessication operations, the four semiconductor switches are arranged in a
bridge circuit. A filter circuit and output transformer are connected
across the bridge configuration. The timing circuitry causes the
semiconductor switches to operate as a full-wave bridge to provide
rectangular pulses to the filter circuitry. These rectangular pulses can
be generated efficiently, but, advantageously, the filter circuit converts
the rectangular pulses into a sinusoidal waveform which efficiently drives
the transformer and reduces radio-frequency noise which might otherwise be
caused by high-order harmonics in the rectangular wave signal.
When the unit is configured to perform a fulguration operation, the four
semiconductor switches are arranged in series to produce the high voltage
necessary for satisfactory fulguration. The semiconductor switches are
controlled by the timing circuit to act as a single switch in order to
generate the waveforms to produce optimal fulguration results.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a block diagram of the inventive electro surgical generator.
FIG. 2 discloses the detailed circuitry of the radio-frequency driver
circuits and semiconductor switches.
FIG. 3 is a block schematic diagram of a timing circuit suitable for use
with the illustrative embodiment.
FIGS. 4 and 5 are detailed schematic diagrams of the circuitry shown in
block form in FIG. 3.
FIG. 6 shows selected electrical waveforms produced by the circuitry of
FIGS. 4 and 5.
FIG. 7 shows a circuit diagram of the output filter for use with the
monopolar output of the illustrative embodiment.
FIG. 8 shows a circuit diagram of the output filter for use with the
bipolar output of the illustrative embodiment.
FIG. 9 shows output waveforms produced by the illustrative generator
circuitry.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 of the drawing shows the schematic diagram of the output section of
an illustrative electrosurgical generator. The generator has a separate
output terminal set for use with monopolar electrodes (an electrode set
which uses a single active electrode and a "patient" or return plate) and
an output set for use with bipolar electrodes (where both electrodes are
active). As will hereinafter be described more fully, each electrode set
may be connected to output circuitry which produces optimal electrical
waveforms for each electrosurgical operation. Specifically, the bipolar
electrodes may be provided with waveforms which are optimized for
dessication. The monopolar electrodes may be provided with waveforms that
are optimized for dessication, cutting, cutting with hemostasis and
fulguration.
According to the invention, the output circuitry is reconfigured
automatically by internal generator circuitry according to the surgical
operation selected by the surgeon.
Specifically, output waveforms produced by the generator are provided by
three output transformers, one of which is selected according to the
surgical operation desired transformer 175 provides output to bipolar
electrodes 195, via leads 186; transformer 170 provides cutting, cutting
with hemostasis and coagulation waveforms to monopolar electrodes 192, 193
or 194 via leads 181; and transformer 116, in turn, provides a fulguration
waveform, via leads 130, to monopolar electrodes 192, 193 or 194.
Transformers 170 and 116 share a common return plate 192. However, the
active side of leads 181 and 130 is connected to the appropriate electrode
of electrodes 193 and 194 by high-voltage switch 190. High-voltage switch
190 may consist of a set of high-voltage relays which is controlled by
either a hand or foot switch in accordance with well-known generator
design.
Advantageously, in accordance with the invention, transformers 116, 170 and
175 are electrically driven by means of a single switching network which
consists of four semiconductor switches 150-153. Switches 150-153 may be
illustratively configured in a "bridge" configuration to provide full
bipolar signals to drive transformers 170 and 175. In an alternate
configuration, switches 150-153 may be connected in "series" to drive
fulguration output transformer 116 as will be hereinafter fully described.
Switches 150-153 are, in turn, controlled by radio-frequency drivers 140
and 141. In order to drive the switches in a bridge configuration, driver
140 is connected to switches 150 and 153 via leads 144 and 145. Similarly,
driver 141 is connected to switches 151 and 152, via leads 147 and 146,
respectively. Drivers 140 and 141 are controlled to operate switches
150-153 in either of the two configurations by means of timing circuit 100
which operates drivers 140 and 141 by means of lead sets 142 and 143,
respectively. Specifically, timing circuit 100 provides timing pulses via
lead sets 142 and 143 which pulses, in turn, cause drivers 140 and 141 to
close appropriate ones of switches 150-153 in order to provide carefully
defined switching waveforms to the output transformers.
In addition to providing timing signals to the radio-frequency drivers,
timing circuit 100 also controls mode relay 105 and bipolar relay 106.
Mode relay 105 controls the electrical configuration of switches 150-153
by means of transfer contacts as will hereinafter be described. Bipolar
relay 106 selects an appropriate one of transformers 170 and 175 to be
activated by the bridge circuitry to produce an output.
Each of the active modes for the three output transformers will now be
described in detail. Assume, for purposes of illustration, that the
operating surgeon has selected a cutting or dessication waveform to be
produced on monopolar electrodes 192 and 193 or 194. Illustrative cutting
waveforms and coagulation waveforms are shown in FIG. 9, lines A and B,
respectively. In this case, the output waveform will be provided by
transformer 170 under control of timing circuit 100. Specifically, under
control of panel switches located on the generator front panel or remote
foot or hand switches operated by the surgeon, timing circuit 100 releases
both mode relay 105 and bipolar relay 106. In FIG. 1, these relays are
shown with detached contacts. Normally-closed contacts are shown by lines
drawn perpendicular to the associated electrical path. Alternatively,
normally-open contacts are shown by crosses located on the associated
electrical path. With this notation, released relay 105 opens its contacts
M-1 and M-2, thereby disconnecting transformer 116 from power source 111
and the switching circuitry. Transformer 116 is thereby removed from
operation. Similarly, released relay 106 opens its B-2 contact, thereby
disconnecting transformer 175 from the circuitry.
Therefore, transformer 170 is the only transformer connected to the
switching circuitry. Power is provided from source 162, via contact M-3,
to semiconductor switch 150 via lead 161 and from source 162 to
semiconductor switch 152 via contact M-4 and lead 154. Source 162 may
illustratively be a regulated D.C. supply which produces a D.C. voltage of
between 0 and 200 volts D.C. Any regulated supply may be used. For
example, a supply which is suitable for use with the illustrative
embodiment is a Sorensen Model DCR 600-3B. The power supply may be
controlled from controls on the control panel of the electrosurgical
generator or may be controlled by a feedback network to provide constant
power under varying load conditions. A feedback network suitable for use
with the illustrative embodiment is disclosed in U.S. Pat. No. 3,601,126
issued on Aug. 24, 1971 to J.R. Estes.
Semiconductor switches 150 and 152 are, in turn, connected directly to
switches 151 and 153 by means of leads 160 and 159 and leads 155 and 156,
respectively. Switches 151 and 153 are connected to ground by means of
leads 158, contact M-9 and lead 157, respectively. The switches and leads
form an electrical bridge network with one switch located in each arm of
the bridge. The primary of output transformer 170 is connected across the
center of the bridge by means of lead 163, contact M-6, lead 166, contact
M-7 and contact B-1. As will be hereinafter fully described, timing
circuit 100 operates drivers 140 and 141 to in turn operate the
semiconductor switches so that diagonally-opposite switches are
simultaneously enabled. The switches then operate as a full-wave bridge
which provides alternating signals to transformer 170. Specifically,
timing circuit 100 controls driver 140 by means of leads 142 to first
close switches 150 and 153. Current then flows from source 162 to ground
via the following path: contact M-3, lead 161, switch 150, lead 160, lead
163, contact M-6, primary of transformer 170, contact B-1, contact M-7,
lead 166, lead 156, switch 153, lead 157 to ground. After a predetermined
time interval, timing circuit 100 causes driver 140 to open switches 150
and 153 interrupting the current flow. Subsequently, driver 141 is
controlled to close switches 151 and 152. Current then flows (in the
opposite direction) in the primary of transformer 170 via the following
path: source 162, contact M-3, contact M-4, lead 154, switch 152, lead
155, lead 166, contact M-7, contact B-1, primary of transformer 170,
contact M-6, lead 163, lead 159, switch 151, lead 158, contact M-9 to
ground.
In this manner, timing circuit 100 alternately controls switches 150, 153
and switches 151, 152 to provide alternating signals to the primary of
transformer 170. The above-described bridge arrangement of switches
150-153 has several advantages. Advantageously, although a voltage equal
to twice the full supply voltage from supply 162 appears across the
primary of transformer 170 only the supply voltage appears across any one
switch. This feature allows very fast FET switches to be used as the
semiconductor switches. Thus, the efficiency of the switching circuits is
improved but high-voltage breakdown problems which are typically
encountered with the use of such switches are avoided. In addition, since
full-wave operation is used there is no center-tap on the primary of
transformer 170. This configuration avoids residual flux coupling on the
opposite section of the center-tapped transformer which is a major cause
of radio-frequency noise that is undesirable in an operating room
environment.
Also, advantageously, according to the invention, the signals that are
provided to the primary of transformer 170 are square wave pulses which
are efficiently generated by switches 150-153. These pulses are in turn
coupled to secondary 171 of transformer 170 and are provided to filter
circuit 180. Filter circuit 180 is illustratively a bandpass filter
circuit as shown in FIG. 7.
Filter circuit 180 provides two functions in accordance with the invention.
First, it filters out the high-frequency components of the switching
waveform coupled to the secondary 171 of transformer 170. The output of
filter circuit 180 provided on leads 181 is therefore approximately a sine
wave with a fundamental frequency of 500 kilohertz. The removal of the
high-frequency components of the signal by filter circuit 180 prevents the
generation of radio-frequency noise and allows easier control and
elimination of radio-frequency leakage paths from the active electrode to
ground or from the patient to ground. However, the rectangular waveform
provided to filter circuit 180 may be efficiently generated by means of
transistor switching circuitry operating in a "class D" mode. When
operating in a class D mode the semiconductor bridge switching circuitry
dissipates less power and therefore no fan is needed to cool the internal
circuitry of the generator. Since a fan is one source of contamination in
the operating room environment, its elimination results in a unit which is
safer for the patient's health.
Secondly, in accordance with the invention, when the output circuitry is
operating in the coagulation mode with an interrupted waveform, filter
circuit 180 produces a voltage doubling effect. The increased output
voltage in the coagultion mode helps to increase the electrical arc length
resulting in an optimum output waveform.
As previously described, one of leads 181 is connected to patient return
plate 192 and the other lead is connected, via high-voltage switch 190, to
active output electrode 194 or 193.
Assume now for the purposes of illustration, that the operating surgeon
desires to use bipolar electrode 195. In this case, in response to
actuation of panel or remote switches (not shown) by the operating
surgeon, timing circuit 100 operates bipolar relay 106. Operated relay 106
opens its contact B-1 and closes contact B-2. Transformer 170 is thereby
disconnected from the bridge circuitry and transformer 175 is connected.
Operation of the circuit then proceeds in the manner exactly analogous to
that previously described. Bipolar pulses provided to the primary of
transformer 175 are coupled to secondary 176 and applied to filter circuit
185 which is shown in more detail in FIG. 8. The filtered output is
provided to bipolar electrodes 195.
Assume now that by suitable switches, the operating surgeon choses a
fulguration output to be produced on monopolar electrodes 192 and 193 or
194. An illustrative fulguration output is shown in line C of FIG. 9. In
this case, timing circuit 100 is controlled by panel or remote switches
(not shown) to actuate mode relay 105 and release bipolar relay 106.
Operated relay 105 opens contacts M-6 and M-7, thereby isolating
transformers 170 and 175 from the switching circuitry. Operated relay 105
also opens contact M-3, disconnecting power supply 162 from the switching
circuitry, and closes contacts M-1 and M-2 thereby connecting transformer
116 in series with power supply 111 and switches 150-153. Finally,
operated relay 105 closes contacts M-5 and M-8 and opens contacts M-4 and
M-9, thereby connecting switches 150-153 in series with transformer 116
and the power supply ground.
Also, as will hereinafter be explained in detail, timing circuit 100 is
conditioned in response to the operator of the panel or remote switches to
control drivers 140 and 141 to close and open switches 150-153
simultaneously, thus effectively repetitively connecting the primary of
transformer 116 between power supply 111 and ground. In particular,
primary 124 of transformer 116 is connected in parallel with capacitor 115
and forms a high-frequency resonant "tank" circuit. When the lower end of
primary winding 124 is connected to ground, current flows from power
supply 111 through contact M-1 and primary 124 of the transformer.
Subsequently, when timing circuit 100 causes drivers 140 and 141 to, in
turn, open switches 150-153, the tank circuit "rings down" to produce a
damped sinusoidal output (as show in line C of FIG. 9). Since timing
circuit 100 periodically closes and opens switches 150-153, a periodic
damped sinusoidal waveform is produced. This periodic waveform is coupled
to the secondary 125 of transformer 116 and is provided (through
capacitors 120 and 121 and leads 130) to the monopolar electrodes
consisting of patient plate 192 and output electrode 193 or 194.
The components of the high-frequency tank circuit are chosen to produce a
waveform which is optimized for a surgical fulguration operation. The
particular details of the fulguration output circuitry do not form a
portion of the invention described herein and will not be described in
further detail. A detailed explanation of fulguration circuitry which may
be used with the illustrative embodiment disclosed herein is given in a
copending patent application entitled "Electrosurgical Generator" filed
July 6, 1981 by Francis T. McGreevy, designated Serial No. 281,005 and
assigned to the same assignee as the present invention.
Specifically, in order to excite the high-frequency tank circuitry, timing
circuit 100 repetitively opens and closes switches 150-153 by means of
driver circuits 140 and 141. Current then flows from source 111 through
the following pathway: contact M-1, primary 124 of transformer 116,
resistor 117, contact M-2, lead 161, switch 150, leads 160 and 159, switch
151, lead 158, contact M-8, contact M-5, lead 154, switch 152, leads 155
and 156, switch 153, lead 157 to power supply ground. When switches
150-153 are opened, a relatively high voltage is produced by the "back
E.M.F." induced in primary winding 124 of transformer 116. However, since
switches 150-153 are connected in a series configuration, the high voltage
is divided across all four switches, thus preventing a secondary breakdown
of any one of the switches even though FET switches are used.
In order to further protect the FET switches, a current sensing circuit
consisting of resistor 117 and transformer 110 has been provided. Current
flowing through the circuit develops a voltage across resistor 117 which
voltage is, in turn, coupled, via transformer 110 and leads 107, to timing
circuit 100. As will be hereinafter described, an increase in current
flowing through resistor 117 above a predetermined threshold causes the
"on" time of switches 150-153 to be reduced, thereby reducing the average
current flowing through the switches and preventing any damage caused by
overcurrent.
The detailed circuitry of the semiconductor switches and driver circuits
140 and 141 is shown in FIG. 2. Each of driver circuits 140 and 141
contains identical circuitry so, for clarity of description only one
driver circuit will be shown in detail. Likewise, each semiconductor
switch 150-153 is identical and therefore only one switch will be
described in detail. Referring to FIG. 2, the circuitry of the driver
circuits is shown in detail. The driver circuits are controlled by the
signals .0. and .0. provided from the control circuit via terminals 200
and 201 shown at the left side of FIG. 2. The driver unit itself consists
of two driver switch units which are identical. Since the switch units are
identical only one will be described in detail.
Each driver switch unit of driver 140 is connected to one end of
center-tapped primary winding 250 of transformer 260. The secondary of
transformer 260 drives the semiconductor switches. In the quiescent or
"off" state, input 200 of the upper unit is normally held at a "high"
voltage by logic circuitry in the timing circuitry. In addition, resistor
210 which is connected to positive voltage source 215 helps to hold input
200 "high". The positive voltage on terminal 200 is applied via the
resistive divider, consisting of resistors 217 and 230, to the base of
transistor 235, turning it "on". In its "on" state, transistor 235 places
a low signal (near ground) on its collector. The gate electrode of FET
switch 240, which is connected to the collector of transistor 235, is thus
held at ground and FET 240 is therefore held "off". In addition, the high
signal appearing on terminal 200 is applied via the resistive divider,
consisting of resistors 216 and 218, to the base of transistor 220 holding
it in its "off" state.
The driver unit becomes active when the control circuitry places a
negative-going pulse on terminal 200. The negative-going pulse is applied,
via the resistive divider consisting of resistors 217 and 230, to the base
of transistor 235, turning it "off". In addition, the negative-going pulse
is applied via the resistive divider, consisting of resistors 216 and 218,
to the base of transistor 220, turning it "on". Current thus flows from
the t supply through diode 219 and transistor 220 and resistor 236 to
ground, raising the potential on the gate lead of FET switch 240. In
response thereto, FET switch 240 turns "on" and current flows from
positive voltage source 251 through one half of the primary windings 250
of transformer 260 through FET switch 240 to ground. Transformer 260, in
turn, controls the semiconductor switches by means of two secondary
windings 270 and 271 (in the diagram, for clarity, only winding 270 is
shown connected to a semiconductor switch. Winding 271 is connected in an
analogous manner to another switch). The output of winding 270 is
connected to the gate and source electrodes of the semiconductor FET
switches 285 and 287 through leads 288 and 289 respectively. In addition,
a pair of Zener diodes 275, 280 (having a breakdown voltage rating of
approximately 12 volts) in series with resistor 290 are connected across
winding 270. These three components prevent an accidental high-voltage
spike occurring across secondary 270 from damaging the semiconductor
switch transistors.
At the end of the "on" time interval, the voltage on terminal 200 provided
by the timing circuit again returns to its quiescent "high" state,
transistor 235 turns "on" and transistor 220 turns "off", thereby allowing
resistor 236 to return the potential on the gate electrode of FET 240 to
ground. Responsive thereto, FET 240 ceases conducting and current ceases
flowing from source 251 through the upper half of primary winding 250 of
transformer 260.
However, in order to insure quick turnoff of the semiconductor switches,
the current in the primary winding 250 of transformer 260 is effectively
"reversed" to "dump" the flux in the transformer windings insuring that
the voltage in the secondary falls quickly to zero. In particular, as
hereinafter described, a short time after terminal 200 returns to its
quiescent "high" state, the control circuitry places a short
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