Leakage current cancelling circuit for use with electrosurgical instrument

• US 3,946,738 A
• Filed: 10/24/1974
• Issued: 03/30/1976
• Est. Priority Date: 10/24/1974
• Status: Expired due to Term

• Current Assignee: David W. Newton, John M. Adan
• Original Assignee: David W. Newton, John M. Adan
• Inventors: Newton, David W., Adan, John M.
• Primary Examiner(s): Frinks, Ronald L.
• Assistant Examiner(s): Cohen, Lee S.
• Application Number:
  US05/517,492
• Time in Patent Office: 523 Days
• Field of Search: 128/303.13, 128/303.14, 128/303.17, 128/422, 128/2.1 P, 317/18 D, 317/9 A, 317/9 AC
• US Class Current: 606/34
• CPC Class Codes: A61B 18/12 by passing a current through the tissue to be heated, e.g. high-frequency current
• CPC Class Codes: A61B 18/1206 Generators therefor
• CPC Class Codes: A61B 18/1233 with circuits for assuring patient safety
• CPC Class Codes: A61B 2018/0066 without feedback, i.e. open loop control
• CPC Class Codes: H02H 9/08 Limitation or suppression of earth fault currents, e.g. Petersen coil

Abstract

Circuitry for cancelling leakage current is disclosed. The leakage cancelling circuit is utilized in conjunction wih the active or patient electrodes of an electrosurgical instrument used in electrosurgery. In electrosurgery, a radio frequency current is passed through a patient between an active electrode and a patient electrode with the active electrode determining the point where surgery is performed and the patient electrode being made surgically inactive by large patient contact area. Current leakage at the electrodes is caused by stray capacity to ground in an isolated system and excessive leakage from the active electrode to ground can cause surgery to be performed without a patient connection while excessive leakage from the patient electrode to ground can cause RF burns to the patient and perhaps to operating room personnel having contact with the patient. In one embodiment, current leakage is cancelled by addition of current to the patient connection in correct phase and magnitude to cancel the existing patient leakage currents, the added current being supplied through circuitry that includes a capacitor in series with the tertiary winding on the output power transformer supplying current to the active and patient electrodes. In a second embodiment, a series LC circuit is placed in parallel with the stray capacitance to cancel the same.

Claims

1. The combination comprising:

an electrosurgical instrument having an active electrode and a patient electrode;

a power transformer having an isolated output circuit connected to said active and patient electrodes for supplying current thereto;

means for supplying power to said power transformer; and

,

a leakage current cancelling circuit for use in cancelling leakage current at said isolated output circuit, said cancelling circuit comprising means for offsetting leakage current caused by stray capacitance to ground at said output circuit, said offsetting means including inductive and capacitive elements connected with one side of said output circuit and ground and having values selected to accomplish said leakage current cancellation.

2. The combination as claimed in claim 1 wherein said offsetting means includes a tertiary winding on said transformer and a capacitor connected in series with said tertiary winding, said capacitor being connected to said one side of said output circuit.

3. The combination as claimed in claim 2 wherein said one side of said output circuit includes the patient electrode of said electrosurgical instrument.

4. The combination as claimed in claim 1 wherein said offsetting means includes a series connected inductor and capacitor forming a series LC circuit connected between ground and said one side of said output circuit.

5. The combination as claimed in claim 1 wherein said one side of said output circuit includes the active electrode of said electrosurgical instrument.

6. In an electrosurgical unit having an electrosurgical instrument, an R.F. power source, and a transformer responsive to said R.F. power source, with said electrosurgical instrument having an active electrode and a patient electrode connected to said transformer by means of an isolated output circuit, the improvement comprising:

a leakage current cancelling circuit for use in cancelling leakage current on the patient electrode side of said isolated output circuit, said leakage current cancelling circuit comprising;

a first capacitor connected between said transformer and said patient electrode,

a tertiary winding on said transformer one side of which is connected with ground, and

a second capacitor connected at one side to the other side of said tertiary winding and at the other side between said first capacitor and said patient electrode whereby current of correct magnitude and phase are coupled to the patient side of said output circuit to cancel the existing leakage currents thereon.

7. In an electrosurgical unit having an electrosurgical instrument, an R.F. power source, and a transformer responsive to said R.F. power source, with said electrosurgical instrument having an active electrode and a patient electrode connected to said transformer by means of an isolated output circuit, the improvement comprising:

a leakage current cancelling circuit for use in cancelling leakage current on one side of said isolated output circuit, said leakage current cancelling circuit comprising;

a first capacitor connected between said transformer and one said electrodes;

a second capacitor one side of which is connected with ground; and

an inductor one side of which is connected with the other side of said second capacitor and the other side of which is connected between said first capacitor and said one of said electrodes whereby existing leakage currents on said one side of said output circuit are cancelled.

8. The leakage current cancelling circuit of claim 7 wherein the value of said inductor is such that it has as high a Q as is practical and in conjunction with said first capacitor such that said inductor and said first and second capacitors form a pole of impedance at the output fundamental frequency, the line frequency sink current is minimal, so that the resulting circuit admittance pole is spaced from any significant frequency component of the output waveform.