Note: Descriptions are shown in the official language in which they were submitted.
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.u.=
Cl CUIT AND METHOD FOR CONTROLLING AN ELECTRO3URGICAL
GENERATOR USING A FULL BRtDGE TOPOLOGY
BACKGROUND
1_ Technical Field
The present disclosure is directed to electrosurgicai systems, and, in
particular,
to a circuit and method for cQntrolGng an electrosurgical generator using a
fuli bridge
topology to control the radio frequency (RF) energy ouiput_
2. Descriation Qf the Related Art
An electrosurgical generator is used in surgical procedures to deliver
electrical
energy to the tissue of a patient. When an electrode is connected to the
generator, the
electrode can be used for cutting, coagulating or sealing the tissue of a
patient with high
frequency electrical energy. During normal operation, alternating electrical
current from
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the generator flows between an active electrode and a return electrode by
passing
through the tissue and bodily fluids of a patient.
The electrical energy usually has its waveform shaped to enhance its ability
to
out, coagulate or seal tissue, Different waveforms correspond to different
modes of
operation of the generator, and each mode gives the surgeon various operating
advantages. Modes may include out, coagulate, a blend thereof, desiccate, or
spray. A
surgeon can easily select and change the different modes of operation as the
surgical
procedure progresses.
In each mode of operation, it is important to regulate the electrflsurgicaf
power
delivered to the patient to achieve the desired surgical effect. Applying more
electrosurgical power than necessary results in tissue destruction and
prolongs healing.
Applying less than the desired amount of alcfctrosurgical powrer inhibits the
surgical
procedure. Thus, it is desirable to control the output energy from the
electrosurgical
generator for the type of tissue being treated.
Dlfferent types of tissues will be encountered as the surgical procedure
progn:sses and each unique tissue requires more or less power as a function of
frequently changing tissue 9mpedance. As different types of tissue and bodily
fluids are
encountered, the impedance changes and the response time of the
electrosurgical
control of output power must be rapid enough to seamlessly permit the surgeon
to treat
the tissue. Moreover, the same tissue type can be desiccated during
electrosurgica!
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treatment and thus its impedance will change dramatically in the space of a
very brtef
time. The electrosurgical output power control has to respond to that
impedance change
as weq.
Two conventional types of power regulation are used in commercial
electrosurgical generators. The most common type controls the DC power supply
of the
generator by limiting the amount of power provided from the AC mains to which
the
generator is connected. A feedback control loop regulates dutput voltage by
oomparing
a desired voltage with the output voltage supplied by the power supply.
Another type of
power regulation in commercial electrosurgical generators controls the gain of
the high-
frequency or radio frequency amplifier. A feedback control loop compares the
output
power supplied from the RF amplifier for adjustment to a desired power level.
Generators that have feedback control are typically designed to haki a
constant output
voltage, and not to hold a constant output power.
U.S, Pat. Nos. 3,964,487; 3,980,085; 4,168,927 and 4,092,986 have circultry to
reduce the output current in accordance with increasing load impedance. In
those
patents, constant vottage output is maintained and the current is decreased
with
increasing load impedance.
U.S. pat. No. 4,126,137 controls the power amplifier of the electrosurgical
unit in
accord with a non-linear compensation circuit applied to a feedback signal
derived from
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a comparison of the power level reference signal and the mathematical product
of two
signals including sensed current and voltage in the unit.
U.S. Pat. No. 4,668,819 has an electrosurgicai generator which has a
microprocessor controller based means for decreasing the output power as a
func6on of
changes in tissue impedance.
U.S. Pat. No. 4,727,874 includes an eiectrosurgicai generator with a high
frequency pulse width modulated feedback power control wherein each cycle of
the
generator is regulated in power content by modulating the width of the driving
energy
pulses.
U.S. Pat. No. 3,601,126 has an etectrosurgicaf generator having a feedback
circuit that atfempts to maintain the output current at a constant amplitude
over a wide
range of tissue impedances.
SUMMARY
An eleotrosurgical generator that uses a full bridge topology as the RF output
stage to control the output RF energy and a control method therefore are
provided.
Most electrosurgical generators have a closed loop control signal that is
derived from
the difference of what is measured and what is desired. This control signal is
feed back
to the DC converter, which varies the DC voltage that feeds into the RF stage.
The
closed loop of the full bridge topology of the present disclosure uses the
control signal
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._ ~
that is derived from the RF energy that is measured and what is desired, to
drive the RF
output stage. The full bridge topology can be either pulse width driven or
phase shift
driven, in either case, as the control signal changes so will the amount of
dme the full
btidge will place voltage across the primary of the RF transformer. Varying
the amount
of time the voltage is imposed across the transformer's primary winding
controls the
amount of energy on the RF transformer. This voltage is a square wave whose
pulse
length varies in relation to the control signal. The corresponding energy on
the
seGondary side of the RF transformer is filtered, usually at the operating
frequency, to
produce a sinusoidal waveform. Using a full bridge topology, the RF energy can
be
controlled directly at the RF output stage instead of using a varying DC input
This
allows the DC input to be at a steady level or an unregulated level and,
therefore. the
DC converter can be removed from the closed loop of the generator.
The advantage of using a full bridge topology for the RF stage is that the DC
voltage no longer controls the output RF energy. This means the DC canverter
can be
removed from the electrosurgical generator, or the DC level can be preset such
as the
elosed loop of the electrosurgicat generator will not control it during
operation. By
removing the DC converter from the closed loop has the advantage of a much
faster
closed loop response of the electrosurgical generator. The other advantage of
using the
full bridge topology is the RF transformer is - driven symmetrically which
makes the
output stable to all loads.
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According to one embodiment of the present disclosure, a system for
controlling
an electrosurgical generatvr using a fui( bridge topology is disclosed. The
system
includes a high voltage direct current power source which supplies power and a
radio
frequency output stage which receives power from the high voltage direct
current power
source and outputs radio frequency energy at a predetermined radio frequency
set
point, The system also includes one or more sensors which determine one or
more
parameters of radia frequency energy being applied to tissue and a
micmprooessor
configured to receive one or more parameters and outputs the predetermined
radio
frequency set point to the radio frequency output stage as a function of one
or more of
the parameters of radio frequency energy.
According to another embodiment of the present disclosure, a method for
controlling an electrosurgical generator using a full bridge topology is
disclosed. The
method includes the steps of supplymg power from a high voltage direct current
power
source and receiving power from the high voltage direct current power source
at a radio
frequency output stage. The method also includes the steps of outputting radio
frequency energy at a predetermined radio frequency set point and determining
one or
more parameters of radio frequency energy being applied to a load. The method
further
includes the step of receiving the one or more parameters at a microprocessor
and
outputting the predetermined radio frequency set point to the radio frequency
output
stage as a function of the one or more of the parameters.
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BRIEF DESCRIPTIaN OF TTIEDRAWIN0.5
The above and other aspects, features, and advantages of the present
disclosure
will become more apparent in light of the foNowing detailed description when
taken in
conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram of a radio frequency (RF) output control circuit of
a
conventional electrosurgicaf generator;
FIG. 2 is a block diagram of RF output control circuit of an electrosurgical
generator according to the teachings of the present disclosure;
FIG. 3 is a schematic diagram of an RF output stage according to an
embodiment of the present disciosure;
FIG. 4 is a schematic diagram of an over-current protection circuit;
FIG. 6 is a schematic diagram of a first over-voltage protection circuit;
FIG_ 6 is a schematic diagram of a se pnd over-voltage protection circuit;
FIG. 7 is a schematic diagram of a conventional RF output stage oircuit;
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FIG. 8 illustrates several views of how the RF output stage can be modeled
over
time; and
FIGS. 9A-B illustrate several views of output curves after experiencing
arcing,
DETAILF-D DESCRIPTION
Embodiments of the present disclosure will be described herein below with
reference to the accompanying drawings. In the foilowing description, well-
known
functions or constructions are not described in detail to avoid obscurirtig
the invention in
unnecessary detail. In the figures, like reference numerals represent like
elements.
The speed of any electrosurgical generator is governed by the slowest block or
component in the control Ioop_ In conventional electrosurgical generator
topology as
shown in FIG. 1, the high voltage DC power source (HVDC) response time is 10
times
slower than the intended microprocessQr (uP) loop speed, e.g., 2.5ms vs.
250us. This
results in the microprocessor waiting for the HVDC to respond thereby not
utilizing
faster response times offered by electronic circuitry.
Referring to FIG. 2, an electrosurgicat generator according to the present
disclosure is illustrated. The electrosurgical generator 10 includes a high
voltage DC
power source (HVDC) 12 for supplying power to the generator, an RF output
stage 14
for receiving power from the HVDC 12 and outputting RF power at a
predetermined set
point, sensors 16 for determining parameters, e.g., current and voltage, of
power being
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'v T
applied to a load, e.g., tissue, and a microprocessor 18 for controlling the
overall
operations of the eiectrosurgical generator 10 and for receiving the measured
current
and volfiage and outputting a set point to the RF output stage 14,
ltt embodiments, the HVDC 12 is set to a fixed voltage level, The response of
the
HVDC 12 becomes a concern only at initialization of the generator 10. The RF
output
stage's response time with a phase shift controlled topology is now 10 times
faster than
the microprocessor control loop speed, as wiil be described below. The
limiting factor is
no longer the HVDC, but the speed of the microprocessor control loop, which Is
10
times faster than the intended microprocessor (uP) loop speed, e.g.. 2.5ms vs.
250us.
It is envisioned that the difference will be even larger as more advanced
microprocessors are developed and implemented_
Referring to FIG. 3, the RF output stage is itlustrated in more detail. The RF
output stage 14 includes a transformer 20 and a plurality of transistors 22,
24, 28, 28 for
adjusting the RF output power of the transformer 20 configured in a#ull bridge
topotogy_
The RF output stage 14 further includes a pulse-width modulator 30 for driving
the
plurality of transistors 22, 24, 26, 28. The pulse width modulator (PWM) 30
has at least
two drive outputs, e.g., Drive A and Drive B. Drive A is coupled to transistor
22 and
Drive A shifted 180 degrees is coupled to transistor 24. Similarly. prive B is
coupled to
transistor 28 and Drive B shifted 180 degrees is cpupled to transistor 28. A
push-pull
typology is used to accomplish voltage from DC to RF conversion. Two gate
drive
signals that are 180 out of phase are used to drive the plurality of
transistors 22, 24,
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26, 28, caiied T ON and T oN_1$0. The gate drive signals turn on each of the
FET's at
opposite times to deliver a wavefoml at the specified pawer. The pulse width
modulator
30 Includes an input for receiving the RF output set point from microprocessor
18.
Aitemativeiy, the transistors may be phase shift drlven from a single control.
In embodiments, the RF set point is not proportional to the output level, Le,,
the
RF set point runs "open loop" control. For example, in the HVDC, which is a
closed
loop, 1V set point = 25V out, independent of load. In the generator employing
the full
bridge topology of the present disctosure, the RF output stage will have a set
point that
will range preferably from about 0.7V to about 3.7V. This is a range that is
set up by the
ramp and slope of the PWM 30. This also means that 1V set point into 100 ohms
won't
be the same vutput as 1 V set point into 1 k Ohm.
Since, the RF output stage 14 responds to a dynamic load ten times faster than
the microprocessor, over-voltage and over-current conditions may occur at the
load.
Therefore, three hardware solutions have been implemented to help minimize
these
conditions.
Over-current protection -s implemented by an over-current proteetion circuit
15
shown in FIG. 4. Referring ta F1G_ 4, current sense transformer TX1 located on
the
sense board 16 measures the output current to the load_ The sensed current is
input to
comparator 40 which is then compared to a reference voltage Vref. If a fault
condition
occurs, such as arcing, the comparator 40 will determine that the sensed
current is
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greater than the reference voltage Vref and will trigger the RF set point to
be pulled low
through transistor M1 via connection point TP26. A timer 42, e.g., a 555
timer, will keep
the RF set point at OV for a predetermined peciod of tirne, e.g., 150uS, to
effectively
reduce the RF output from the RF output stage 14. Altematively, the timer
could be
replaced by a flip flop that will pull down the RF set point by halt and send
an alarm to
the microprocessor 18. Once the microprocessor 18 handles the alarm, the
microprocessor 18 would reset the flip flop.
Over-voltage condibons may occur If low impedance is sensed, e.g., when a
surgical Instrument is removed from the operative site, and the generator
attempts to
increase the output voltage. Referring to FIG_ S. over-voltagQ proteetlon is
achieved by
adding an inductor Lp to the primary side of the RF transformer 20. As the
voltage on
the output fioreases so will the current flowing through the output capacitor
Vout. The
prirrtary current (Ip) equals the secondary current because it is dominated by
the
capac'itor current (lc). Aa Ip increases, the voltage that Is across the
primary Inductor Lp
will also increase. The voltage across the inductor will subtract from the
voltage across
the transformcir which witl mduce the amount of voltage to the secondary and
as that is
reduced, so is the amount of output voltage. This protection works cycle by
cycle, will
prevent a runaway condition of the output voltage and prevant a full-bore
output from
occurring at the Idad.
Additionally, over-voltage protection will be implemented to IImit the amount
of
overshoot while an instrument goes from a low to high impedance. This over-
voltage
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protection scheme is implemented in an over-voltage protection circuit 17
shvwn in FIG.
6, Transformer TXZ monitors the current on the primary winding of the RF
transformer
20, or alternatively, will monitor the output vottage from the sense board. As
described
above, when the primary current Increases so'wili the voltage at the output.
Since the
capacitance is a constant, the primary current can be calculated such that a
certain
voitage will be across the output capacitance. Once this level goes above a
specific
value, e.g_, Vref, comparator 60 will start to pull down the RF set point
through dlode Dl
via connection point TP26. This protection has the capability of changing the
level of
overshoot by changing the value of variable resistor R2, which can be switched
in. The
speed of this protection is about 10Khz, or 10Dus. This is limited to about
10% over the
voltage limit which will allow the microprocessor to see the error and adjust
the set point
accordingiy.
The generator employing a full bridge topology of the present disclosure has
several advantages over conventional generators which control the RF output by
varying the high voltage power supply (HVDC)_ For exampie, the stored energy
on the
HVDC does not effect the output of the generator. Currently, the generator
output is left
on for a few milliseconds after the end of an activation to help bleed off the
voltage to
prevent excess energy storage in the genergtor_ The higher the impedance
present at
the output, the longer it takes to dissipate the voltage on the HVDC. The
danger here is
that the RF board may get turned off but the HVDC may still have stored
energy,
causing inadvertent errors to be reported. Additionally, as RF power
requirements
increase so will the capacitor off of the HVDC. Currently, this stored voltage
in the
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capacitor needs to be discharged after each activation. Because the set point
turns the
RF off at the PWM instead of the HVDC, residual voltage on the HVDC isn't an
Issue at
the end of an activation.
Furthermore, an RF gmplifier of the electrosurgical generator will have no
tuned
elements. The advantage of having no tuned elements on the RF amplifier is
that the
operating frequency of the output stage module will be completely independent
of the
operating frequency of the RF ampiifter, Frequency, instead, becomes
dapendertit on
the resonance of each of the separate output modes of the generator, e.g.,
coagulate,
eut blend, etc. The advantage of this eonflauration is that the output
frequency can be
changed without re-tuning the RF output stage only re-tuning the output module
relating
to a particular mode, facilitating any modifications or troubleshooting. This
delegation of
resonating components places much tess stress on the FETs durtng arcing and
the RF
output stage will be able to force the output to be more stable as wili be
describad
below.
The current topology is a class E, wh re the FET resonates with the tank,
capacitor C6, capacitor C3, and inductor L3, as shown in FIG. 7. When an
arcing
condition hits the tank, this can short out the capacitor C3 thereby causing
the
resonance of this system to change dramatically. Any stored energy in inductor
L6, the
RF choke, will be released causing the voFtage across the FET to increase.
This
voltage will eventually be clamped by the avalanche mode of the FET, for
example, in
conventional topology this is around 570V for a 500V rated FET. In the
topology of the
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present disclosure, since there Is no tank or RF choke, the voltage will not
increase.
Because of this, the generator is able to use 200V rated FETs. The lower
voltage rated
FETs have an advantaige with lower Rds on and gate charge, which have less
dissipatation and are less expensive than higher voltage rated FETs.
Additionally, the current conventional class E topology is efficient only in a
narrow
range of load impedances. As the load changes so will the waveform across the
FET.
This will' cause symmetry issues of the waveform at the output The topology of
the
present disclosure has an output fffter which is optimized for certain
voltages and
currents. Because the Fl*Ts themsetves are not tuned, the RF ampfifier to the
filter is
unaffected by load changes.
FIG. d illustrates how the transformor voltage, or output of the RF, is
induced on
to the output filter. During time A, the voltage positively charges the
inductor Lf and
capacitor Cf. During time B, the inductor Lf and capacitor Cf are of the
opposite polarity.
Tme C shows that when there is no output voltage, the inductor Lf is in
parallel with the
capacitor Cf which will keep the same resonant frequency. If the load is
shorted, the
max current = (1l"t)/L.
As mentioned above, an arcing condition in the existing class E topology
causes
the resonant frequency to change, subsequently changing ttle output frequency
as
illustrated in FIG. 9A_ Because the topology of the present disclosure is
using a
transformer that is syrnmetrlcal on the primary side, it forces the secondary
to be
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11.
symrnetrical even during arcing, as shown in 1=IG. 9B. When. the output
capacitor is
shorted on the new topology, the series inductance acts as a resistor and it
limits the
current to the output.
Furthermore, the generator of the present disclosure reduces leakage current.
Leakage current is dependarit on several variables, The component that
contributes
most to leakage current is aapacitor coupling from the primary to secondary
winding of
the RF transfonner. As the voltage increases across the transformer, the
capacitance
will start to conduct current. The advantage of the full bridge RF topology is
that lt
employs a 1 to-1 transformer and only has a maximum voltage of 15DV, compared
to
550 VRMS or 778 Vpeak of the transformers used in conventional generators.
Because
of the reduced voltage, the number of tums is reduced to about 5 turns thereby
reducing
the coupling capacitance.
While several rnbodiments of the disclosure have been shown in the drawings,
it is not intended that the disclosure be limited thereto, as it is intended
that the
disclosures be as broad in scope as the art will allow and that the
specification be read
I'ikewise, Therefore, the above description should not be construed as
limiting, but
merely as exemptifiestions of preferred embodiments.
1S
