Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical, Computer & Energy Engineering

First Advisor

Robert W. Erickson

Second Advisor

Dragan Maksimović

Third Advisor

Regan Zane

Abstract

Electrosurgery is used in modern surgical practice to cause various desirable clinical effects by invoking I2R heating in tissue. The ability to achieve a consistent clinical result is predicated on the ability of the electrosurgical instrument to deliver a well-regulated constant output power, capable of quickly responding to changes in tissue impedance. Yet, electrosurgical devices available today suffer from very poor transient output power regulation, with potential for improvement limited by inherent restrictions in the chosen system architectures.

This thesis presents a revolutionary new approach to the design of electrosurgical power supplies by employing current-programmed mode (“CPM”) control strategies to inherently achieve the desired output characteristic: a constant power AC source with maximum voltage and current limits and with near-deadbeat regulation. CPM control of a full-bridge boost inverter and nonlinear carrier-control of a buck converter are presented as new methods of producing constant power source AC outputs.

Analysis and small-signal models contained herein demonstrate the efficacy of the proposed constant power control schemes, showing that major sources of non-idealities include peak-to-average error, artificial ramp-induced error, and transient error. The nonlinear carrier controlled buck power source exhibits a transient error dependent on the size of the inductor, while the CPM controlled boost power source transient error is independent of the inductor size. A prototype ESG employing CPM control generates the ideal output characteristic and achieves near-deadbeat regulation of output power. Maximum voltage- and current-limits are invoked equally quickly. The standard deviation in per-cycle output power during arc cutting is less than 2%, compared to greater than 25% in the prior art. Histological analysis of ex vivo tissue samples shows a marked decrease in collateral tissue damage, proving a previously unknown correlation between high-speed output power regulation and thermally-induced tissue necrosis.

Simultaneous control of multiple converter stages in the prototype demonstrates the ability to achieve high peak output voltages with low average power using a continuous output carrier: a significant simplification over the prior art.

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