RF surgical generator and method for driving an RF surgical generator

[0001] Embodiments of the invention relate to an RF (radio frequency) surgical generator as well as a method for driving such a generator.

[0002] Increasingly, RF surgical generators are used for tissue cutting and coagulating. Compared with mechanical incisions, haemostasis of the cutting edges is considered one of the advantages of RF surgical incisions, this haemostasis being due to thermal coagulation, in particular. In this case, the depth of the coagulation zone is largely dependent on the perfusion of the treated tissue, resulting in the requirement that the depth of the coagulation zone used by the RF surgical generators can be adjusted in as reproducible a manner as possible. In conjunction with this, the shape of the RF output signals of the RF surgical generator is of importance and, in particular, the ratio of peak value to effective value. Consequently, amplitude-modified RF voltages (currents) are selected, where the modulation is sufficient for the RF surgical generator to potentially also generate only a single cycle followed by a longer pause.

[0003] Another requirement is that the RF surgical generator be as highly efficient as possible because, if the efficiency is too low, any heat loss must be removed by elaborate cooling measures, which is not desirable in the operating room.

[0004] German Publication DE 102 18 895 A1 discloses an RF surgical generator to achieve greater efficiency in that the DC voltage power supply usually provided for such RF surgical generators can work in two modes of operation, i.e., on one hand, it can work as a power supply for the power oscillator (in a conventional manner) and, on the other hand, it can work in a mode in which an energy transfer from the power supply of the power oscillator back to the input of the DC voltage power supply takes place. In this case, substantial improvements in shaping short pulses are not possible.

[0005] German Publication DE 100 46 592 A1 discloses an RF surgical generator of the type addressed herein. In this generator, the ohmic resistance is withdrawn when the RF output signal is to be completed, said ohmic resistance being connected parallel to the consumer, as it were. The efficiency of this device, however, is minimal.

[0006] An object of the embodiments of the invention is to provide an RF surgical generator of the aforementioned type, as well as a method for driving such a generator such that great efficiency and an exact and rapid shaping of the RF output signal can still be achieved.

[0007] This object is achieved by an RF surgical generator and method of operating said generator, wherein said RF surgical generator comprises a power supply with at least one storage capacitor, and comprising a controllable switching device with at least one energy storage, e.g., an output transformer, by which an RF output signal that can be delivered to an RF surgical instrument is generated. A regenerative device is connected between the energy storage and the storage capacitor and a control means is provided, said control means controlling the switching device and the regenerative device such that, if the RF output signal is to be completed, the regenerative device is energized for at least part of the time, and the energy storage is at least partially discharged in the storage capacitor.

[0008] Consequently, it is an essential aspect of the embodiments of the invention that the electrical, or also magnetic, energy that is stored in the usually provided dummy elements, e.g., the usually provided output transformer, is withdrawn from this energy storage, thus quickly resetting the RF output signal to zero. This energy that is delivered to the storage capacitor of the power supply is then available for a subsequent RF output signal. This increases the efficiency of the arrangement.

[0009] An improvement of signal controllability with—at the same time—minimal circuitry is given whenever a discharge device that drains residual energy from the energy storage is provided. To accomplish this, a transformation into thermal energy due to an ohmic load can also be used, ensuring that the oscillator will die out reliably. However, there remains a considerable increase of efficiency due to regeneration. Preferably, the discharge device is controlled by the control means such that the residual energy is drained following a partial discharge of the energy storage.

[0010] In a preferred embodiment, the energy storage comprises an output transformer, and the regenerative device comprises a separate winding on the output transformer. This way, it is possible to easily adjust the desired voltage conditions.

[0011] It is possible to accomplish regeneration via an electronic switch that is controlled by the control means. Alternatively, regeneration is possible via a diode line via which the energy storage is discharged upon the completion of the RF output signal.

[0012] Preferably, the storage capacitor of the RF surgical generator is arranged such that it—by the interposition of controlling elements—is able to absorb the excess energy of the output transformer.

[0013] The regenerative device comprises a DC/DC converter to perform an appropriate voltage adaptation. Said converter may be configured as a step-up chopper.

[0014] In an alternative embodiment, the regenerative device comprises a step-down chopper whose input voltage is adjusted to a constant voltage using an electronic switch. This embodiment of the circuit is particularly simple.

[0015] The method for driving an RF surgical generator comprising a power supply with at least one storage capacitor, and a controllable switching device with an output transformer, said device generating an RF output signal that can be delivered to an RF surgical instrument is characterized in that, for switching off the RF output signal, energy stored in the output transformer is delivered to the storage capacitor and stored in said storage capacitor. The advantages of this method have already been explained above. This applies, analogously, to corresponding developments of the method, which are apparent in view of the above-described circuit features.

[0023] In the description hereinafter, the same reference signs are used for components that are the same or that have the same function.

[0024] FIG. 1 shows a circuit comprising a power supply 10, where only the DC/DC converter is identified with a capacitor C located at its output. Of course, additional converter devices are provided in order to supply power to the DC/DC converter from an AC main supply.

[0025] The generator comprises an oscillating circuit C_R and a (primary) winding W₁ of an output transformer T_R and is connected to the capacitor C via transistor T₂. A control input “b” of transistor T₂ is connected to a control mechanism 20 that, consequently, can generate an oscillation due to an appropriate actuation of transistor T₂ in the oscillating circuit C_R-W₁, the oscillation can be delivered—via a secondary winding W_A and a series oscillating circuit L_A, C_A—as the output voltage U_a to an RF surgical instrument (not shown).

[0026] An additional winding W₂ of the transformer T_R is also connected to the capacitor C via diode D₁ and transistor T₁, on one side, and via a direct line, on the other. The control input “a” of transistor T₁ is connected to the control mechanism 20. A series circuit comprising a resistor R and transistor T₃ is provided parallel to the additional winding W₂. A control input “c” of transistor T₃ is also connected to the control mechanism 20.

[0027] Hereinafter, the function of the circuit in accordance with FIG. 1 will be explained with reference to FIG. 5.

[0028] The control mechanism 20 generates the activation pulses n and n+1 that are delivered to the control input “b” of transistor T₂. By activating transistor T₂, the oscillating circuit C_R, W₁ is excited and generates an output voltage U_a.

[0029] To complete the oscillation of the oscillating circuit C_R, W₁ the control mechanism 20 generates a signal “a” that activates transistor T₁. As a result of this, only the additional winding W₂ is connected to the capacitor C via diode D₁ and transistor T₁ so that current I₁ flows, said current moving the energy stored in the transformer T_R into the capacitor C.

[0030] Following the relatively short control pulse “a” (see FIG. 5), transistor T₃ is turned on by the control mechanism 20 via control input “c” of said transistor, so that only current I₂ flows through the resistor R, thereby converting a residual discharge of energy from the transformer T_R into thermal energy. Consequently, the output voltage U_a is completely set to zero; in which case a substantial quantity of the energy stored in the output transformer T_R is again available on the capacitor C to be used during a subsequent activation of transistor T₂ via a control pulse (pulse n+1).

[0031] The circuit in accordance with FIG. 2 has a similar layout as the circuit in accordance with FIG. 1 considering the power supply 10 and the output circuit. Referring to this circuit, the primary winding W₁ of the transformer T_R is connected to an upper pole of the capacitor C via transistor T₂₁ and to the lower pole of the capacitor C via transistor T₂₂. The collector/emitter lines of the two transistors T₂₁ and T₂₂ are bridged by free-wheeling diodes.

[0032] The collector of the lower transistor T₂₂ is connected to the upper pole of the capacitor C via diode D₃. The emitter of transistor T₂₁ is connected to the lower pole of the capacitor C via diode D₂, said diode D₂ being bridged via a series circuit comprising a resistor R and transistor T₃.

[0033] The current paths are indicated for explanation of the function of this circuit in accordance with FIG. 2.

[0034] When transistor T₂₁ and transistor T₂2 are turned on, the result is current path 4, said path 4 generating a (pulse-shaped) output voltage U_a. To quickly complete this output pulse (after closing transistors T₂₁, T₂₂) the energy contained as current I₁ in the transformer T_R is returned to the capacitor C via diodes D₂ and D₃. To decrease any residual energy the control mechanism 20 activates transistor T₃, so that the residual energy contained as current I₂ in the transformer T_R is converted into thermal energy via current path 6.

[0035] The layout shown in FIG. 3 is similar to the circuit in accordance with FIG. 1 considering the generation of oscillations and considering the transformer T_R. However, in this arrangement, the additional winding W₂ is connected to the capacitor C via a step-up chopper comprising diode D₁, capacitor C_H, inductor L_H and another diode D_H. More precisely, a series circuit comprised of diode D₁ and capacitor C_H is provided parallel to the additional winding W₂. One end of the coil L_H is connected to the coupling point between the diode and the capacitor, and the other end is connected to the capacitor C via diode D_H. The coupling point between diode D_H and inductor L_H is connected to the other pole of the capacitor. C via transistor T₁.

[0036] Furthermore, a series circuit comprising an ohmic resistor R and a transistor T₃ is connected in parallel to the additional winding W₂.

[0037] Energization takes place in accordance with the diagram of FIG. 5; however, it is also possible to turn on the regenerative transistor T₁—different from the illustration of FIG. 5—as follows:

[0038] The circuit shown in FIG. 3 comprising L_H, T₁ and D_H represents a step-up chopper. This step-up chopper is able to reload energy from the storage capacitor C_H to the input capacitor C. Transistor T₁ can be turned on in pulse width modulation (PWM) mode, current regulation mode or a similar quasi-continuous mode of operation. The intent of the regulation of the step-up chopper is to maintain the voltage across the storage capacitor C_H constant and, accordingly, vary the choke current by means of L_H.

[0039] The circuit shown in FIG. 4 is different from that shown in FIG. 3 in that capacitor C provided in the actual generator circuit is not used, but instead capacitor C_Z provided in an intermediate circuit for regenerating the energy stored in transformer T_R via the current I₁ is used. This circuit also shows that, in the embodiment of the invention, the element that absorbs the energy stored in the output transformer T_R and makes said energy available for repeated excitation and generation of an output voltage U_A is not important.

[0040] Considering the circuit shown in FIG. 6, again transistor T₂ is provided for excitation and generation of an output voltage U_a, said transistor T₂ closing an electric circuit between the output capacitor C of the power supply 10 and the primary winding W₁ of the transformer T_R when turned on by signal “b” from the control mechanism 20.

[0041] This arrangement comprises a series circuit that is connected to the capacitor C for regenerating the energy contained in the transformer T_R on the capacitor C, said circuit comprising a first diode D₁₁, second diode D₁₂, transistor T₁ and a coil L_T. The connecting point between transistor T₁ and the coil L_T is connected to the lower pole of the capacitor C via diode D_T. The coupling point between diode D₁₂ and transistor T₁ is connected to the lower pole of the capacitor C via capacitor C_T. The connecting point between the diodes D₁₂ and D₁₁ is connected to the upper pole of the capacitor C via a series circuit comprising a resistor R and transistor T₃.

[0042] Considering this circuit, transistor T₁ is responsible for regeneration and is driven via its control input “a” by the control mechanism 20 in PWM mode (or in another controlled mode, e.g., current mode or two-point regulation) such that the input voltage of the step-down chopper set up here is regulated to a constant voltage for charging the capacitor C. Other than that, the circuit is driven like the previously driven circuits, in which case—also in this circuit (as in FIG. 2)—the generator is not a sinewave generator, but is designed for single pulse excitation. Consequently, a capacitor parallel to the primary winding W₁ is not necessary in this embodiment. Here, the regenerated energy is restricted to the magnetizing energy stored in the transformer.