Rem output stage topology

• US 5,695,494 A
• Filed: 12/22/1994
• Issued: 12/09/1997
• Est. Priority Date: 12/22/1994
• Status: Expired due to Term

• Current Assignee: Covidien AG (Medtronic Plc)
• Original Assignee: Valleylab Inc. (Medtronic Plc)
• Inventors: Becker, Daniel J.
• Primary Examiner(s): Shay, David M.
• Application Number:
  US08/362,091
• Time in Patent Office: 1,083 Days
• Field of Search: 606/32-52
• US Class Current: 606/33
• CPC Class Codes: A61B 18/1233 with circuits for assuring patient safety
• CPC Class Codes: A61B 18/16 Indifferent or passive electrodes for grounding

Abstract

Apparatus monitors RF return current to maximize the AC signal of impedance at two return electrodes. A transformer with driving and driven windings isolates ESU and patient. At ends of the driving winding are signal and ground terminals joined to the return electrodes with capacitors returning current. An AC coupling capacitor at the signal terminal has a timing circuit in sync to the voltage wave and relative to impedance of the return electrodes. Microprocessing the voltage at the signal terminal of the driving winding watches impedance and determines if the RF return current path is adequate. Voltage detection within the timing circuit has a voltage shaping circuit. A voltage comparator after the voltage detection forms a square wave. A current detection circuit and a coupling capacitor allow AC flow to the driving winding. Current shaping circuit in the current detection circuit has a voltage comparator at the output to form a square wave. Phase detection at the voltage and current detection circuits outputs filters the phase difference that is sampled and held as DC input to a switch, with an output and a few inputs to DC voltages. Phase locking an oscillating voltage source directly and/or through the sample and hold or DC switch tunes oscillation frequency and maximizes the voltage detection circuit output. Monitoring the return current with a signal from the voltage detection circuit connected to an oscillating voltage that is phase locked to the current phase therein shows that no phase difference and maximum signal voltage occur simultaneously.

Claims

1. An electrosurgical return electrode circuit for developing an impedance measurement between two contact pads, comprising:

an isolation transformer in the electrosurgical return electrode circuit, the isolation transformer having a driving winding and a driven winding having two driven ends and, the driving winding having two driving ends and having a signal terminal and a ground terminal each connected to one driving end thereof;

a first electrical conductor and a second electrical conductor each attached to one driven end of the driven winding;

a first patient contact pad and a second patient contact pad electrically coupled to the first and second electrical conductors, respectively, and the driven winding for resonating at a monitoring frequency;

a voltage detection circuit connected to the signal terminal of the driving winding for producing an electrical phase signal of the voltage across the driven winding;

a voltage shaping circuit included in the voltage detection circuit and in electrical communication therewith;

a current detection circuit connected to the signal terminal of the driving winding for producing an electrical phase signal of the current flowing in the driving winding;

a current shaping circuit included in the current detection circuit and in electrical communication therewith;

a phase detector connected to the voltage shaping circuit and the current shaping circuit for providing an electrical signal indicating a phase difference signal between the voltage across the signal terminal and current at the signal terminal;

a phase filter connected to the phase detector for providing a response characteristic of the phase difference signal, and

a phase locked oscillating voltage source for operating at the monitoring frequency of oscillation, the phase locked oscillating voltage source connected between the phase filter and the signal terminal of the driving winding for producing an electrical driving signal upon receipt of the response characteristic of the phase difference signal so that the monitoring frequency of oscillation is continuously adjusted to keep the phase difference between the voltage and the current at the signal terminal substantially zero thereby delivering a maximum voltage signal to the voltage detection circuit, the electrical driving signal used to develop the impedance measurement.

2. The electrosurgical return electrode circuit of claim 1, wherein the first and the second patient contact pads are carried on a common substrate which is an electrical insulator.

3. The electrosurgical return electrode circuit of claim 1, wherein the first and the second patient contact pads are carried on a common substrate with an electrically conductive gel.

4. The electrosurgical return electrode circuit of claim 1, wherein a sample and hold circuit is connected between the phase filter and the phase locked oscillating voltage source for receiving the response characteristic of the phase difference signal, the sample and hold circuit passing substantially DC voltages and not passing radio frequency voltages to the phase locked oscillating voltage source.

5. The electrosurgical return electrode circuit of claim 1, wherein the voltage detection circuit is a timing circuit synchronous with the phase locked oscillating voltage source for selecting specific portions of the monitoring waveform of the electrical signal proportional to the impedance between the first and second patient contact pads.

6. The electrosurgical return electrode circuit of claim 1, wherein a microprocessor is in electrical communication with the signal terminal of the driving winding for monitoring the monitoring waveform of the electrical signal proportional to the impedance between the first and second patient contact pads to determine if the electrosurgical current path is adequate.

7. The electrosurgical return electrode circuit of claim 1, wherein a switch with a plurality of inputs, one input is connected to the phase locked oscillating voltage source and the other inputs are each connected to a fixed frequency oscillating voltage source, an output connected to the driving winding so that either the phase locked oscillating voltage source signal or a plurality of fixed frequency oscillating voltages may be selected.

8. The electrosurgical return electrode circuit of claim 1, wherein a duty cycle correcting device is connected between the phase locked oscillating voltage source and the driving winding for correcting the phase locked oscillating voltage source waveform to a duty cycle of substantially one half.

9. The electrosurgical return electrode circuit of claim 1, wherein a switch with a plurality of input positions is connected between the phase filter and the phase locked oscillating voltage source, the first input connected to the phase filter and the other inputs each connected to a specific voltage, for causing the phase locked oscillating voltage source to provide either the frequency determined by the phase filter output or a specific oscillating frequency.

10. The electrosurgical return electrode circuit of claim 1, wherein a logic flip flop device or microprocessor communicates with both the voltage shaping circuit and the current shaping circuit for providing the phase difference signal.

11. The electrosurgical return electrode circuit of claim 10, wherein the microprocessor with a first input, a second input and at least one output the first connected to the single output of the first voltage comparator, the second input connected to the single output of the second voltage comparator so the output of the microprocessor is a phase difference signal.

12. The electrosurgical return electrode circuit of claim 1, wherein a voltage comparator input connects to the voltage detection circuit and the comparator output connects to the phase detector to form a waveform that is generally square in shape.

13. The electrosurgical return electrode circuit of claim 1, wherein a voltage comparator input connects to the current detection circuit and the comparator output is connected to the phase detector to form a waveform that is generally square in shape.

14. The electrosurgical return electrode circuit of claim 1, wherein a capacitor is connected between the driving winding and the voltage detection circuit for coupling only alternating voltage signals therebetween.

15. The electrosurgical return electrode circuit of claim 1, wherein a capacitor is connected between the driving winding and the current detection circuit for coupling only alternating current signals therebetween.

16. The electrosurgical return electrode circuit of claim 1, wherein a resistor is connected in series with a capacitor between the driving winding and the phase locked oscillating voltage source, the series combination providing a voltage representation of current and preventing DC current flow in the driving winding.

17. The electrosurgical return electrode circuit of claim 1, wherein a voltage controlled oscillator is included in the phase locked oscillating voltage source.

18. The electrosurgical return electrode circuit of claim 1, wherein an adjustable timer is included in the phase locked oscillating voltage source.

19. A method of using an electrosurgical return electrode circuit to measure impedance between two contact pads, the circuit having an isolation transformer, the isolation transformer having a driving winding and a driven winding, the driving winding having a signal terminal and a ground terminal each connected to an end thereof, a first electrical conductor and a second electrical conductor attached to the first and second end, respectively, of the driven winding, an electrosurgical return current connection connected to the driven winding to provide a return path for electrosurgical current so the electrosurgical return current connection is in series with each end of the driven winding of the isolation transformer, a first and a second patient contact pad electrically coupled to the first and second electrical conductors, respectively, and the driven winding, the method of using comprising the steps of:

producing an electrical phase signal proportional to the voltage across the driving winding with a voltage detection circuit connected to the signal terminal of the driving winding;

shaping the voltage in the voltage detection circuit with a voltage shaping circuit connected to the voltage detection circuit;

monitoring the electrical signal proportional to the impedance between the first and the second contact pads by connecting a voltage detection circuit across the signal terminal of the driving winding;

producing an electrical phase signal of the current flowing in the driving winding by connecting a current detection circuit to the signal terminal of the driving winding;

shaping the electrical phase signal of the current in the current detection circuit with a current shaping circuit connected to the current detection circuit;

providing an electrical signal indicating the phase relationship between the voltage across the signal terminal and current at the signal terminal with a phase detector connected to the voltage shaping circuit and the current shaping circuit;

providing a response characteristic of the phase difference signal with a phase filter connected to the phase detector for providing the response characteristic of the phase difference signal, and

producing an electrical driving signal upon receipt of the response characteristic of the phase difference signal so that a monitoring frequency of oscillation is continuously adjusted to keep the phase difference between the voltage across the signal terminal and the current at the signal terminal substantially zero for delivering a maximum voltage signal to the voltage detection circuit with a phase locked oscillating voltage source connected between the phase filter and the signal terminal of the driving winding;

measuring the impedance between the first and second patient contact pads using the driving signal as a source for developing the measurement.

20. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of applying the first and the second patient contact pads, which are carried on an electrically insulating common substrate, to a patient.

21. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of attaching the first and the second patient contact pads, on a common substrate with an electrically conductive gel, to a patient.

22. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of receiving a response characteristic of the phase difference signal with a sample and hold circuit connected between the phase filter and the phase locked oscillating voltage source, the sample and hold circuit passing substantially DC voltages and not passing radio frequency voltages to the phase locked oscillating voltage source.

23. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of selecting specific portions of the electrical signal proportional to the impedance between the first and the second patient contact pads waveform with a voltage detection circuit which is a timing circuit that is synchronous with the phase locked oscillating voltage source.

24. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of monitoring whether the electrosurgical current connection is adequate with a microprocessor in electrical communication with the signal terminal of the driving winding to measure the electrical signal proportional to the impedance between the first and the second patient contact pads.

25. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of selecting either the phase locked oscillating voltage signal or a plurality of fixed frequency oscillating voltages with a switch with a plurality of inputs, one input connected to the phase locked oscillating voltage source and the other inputs each connected to a fixed frequency oscillating voltage source, the output connected to the driving winding.

26. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of correcting the phase locked oscillating voltage source waveform to a duty cycle of substantially one half the waveform period with a duty cycle correcting circuit connected between the phase locked oscillating voltage source and the signal terminal of the driving winding.

27. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of providing specific oscillating frequencies with a switch with a plurality of input positions connected between the phase filter and the phase locked oscillating voltage source, the first input connected to the phase filter and the other inputs each connected to a specific voltage, for causing the phase locked oscillating voltage source to provide either the frequency determined by the phase filter output or a specific, and fixed, oscillating frequency.

28. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of providing the phase difference signal with a logic flip flop device to communicate with both the voltage shaping circuit and the current shaping circuit.

29. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of forming a waveform that is generally square in shape with a voltage comparator whose input connects to the voltage detection circuit and whose output connects to the phase detector.

30. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of forming a waveform that is generally square in shape with a voltage comparator whose input connects to the current detection circuit and whose output connects to the phase detector.

31. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of coupling only alternating voltage signals to the voltage detection circuit with a capacitor connecting the driving winding and the voltage detection circuit.

32. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of coupling only alternating current signals to the current detection circuit with a capacitor connecting the driving winding and the current detection circuit.

33. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of providing a voltage representation of current and preventing DC current flow in the driving winding with a resistor connected in series to a capacitor between the driving winding and the phase locked oscillating voltage source.

34. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of signalling the driving winding with a voltage controlled oscillation according to the phase difference signal between the current and voltage waveforms at the driving winding.

35. The method of using an electrosurgical return electrode circuit for monitoring electrosurgical current of claim 19, having the step of oscillating a voltage signal to the driving winding with an adjustable timer to connect between the phase difference signal and the driving winding.