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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. [ABSTRACT FROM AUTHOR]

This paper proposes a digital controller for a low-frequency square-wave (LFSW) electronic ballast that includes the ignition sequence, a double control loop, and the selection of the positive and negative operation modes. The whole ballast is a two-stage circuit, where the first part is a power factor correction (PFC) stage and the second is a full-bridge (FB) converter used for both ignition and square-wave drive. Ignition is achieved by approaching the resonant frequency of the LC filter when the lamp is in the off state and the FB is working as a resonant inverter. After ignition, the converter operates as an LFSW inverter by controlling the FB to act alternately as a buck converter supplying positive or negative current. While ignition occurs at the LC filter resonance frequency (fo = 20\ \kHz), the buck converter switching frequency (fsw = 200\ \kHz) is selected significantly higher than fo to attenuate high-frequency harmonics and avoid exciting acoustic resonance. Lamp stability is achieved by controlling the inductor current of the LC filter, and power mode control is achieved by adjusting the average current and voltage supplied by the PFC stage. The solution is experimentally validated for different types of 150-W high-intensity discharge lamps. A coupled-inductor single-ended primary inductance converter operating in continuous conduction mode is used for the PFC stage. [ABSTRACT FROM AUTHOR]