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A digital approach to provide phase surveillance in resonant converters is proposed. The approach provides a mechanism for controllers in resonant inverters to maintain zero-voltage-switching (ZVS) mode operation despite large and abrupt changes in load behavior. Applications are focused on but not limited to generation and control of electrical discharges. The phase surveillance provides effective arc ignition, prevents the eventual arc extinction and assures ZVS in MOSFET turn-on transients. The proposed circuit and control approach makes the converter robust to distorted waveforms associated with either low Q operation, the use of nonlinear soft-saturation core inductors or any other cause that jeopardizes ZVS in high-frequency resonant inverters, including component tolerance, aging, and temperature effects. Two practical circuits are used to demonstrate the phase surveillance operation. A two-phase resonant inverter controlling a welding arc shows how the phase surveillance assures ZVS operation and a single-phase resonant electronic ballast for high-intensity discharge lamps shows how the phase surveillance solves the problem of arc generation. Phase surveillance maintains the frequency dependent characteristics of the resonant inverter, such as the high output impedance that stabilizes the arc beyond the control loop bandwidth, while achieving robust operation for any combination of load, component tolerances, and driver dead times.
Date of Publication: June 2012