High-Voltage LC-Parallel Resonant Converter with Current Control to Detect Metal Pollutants in Water through Glow-Discharge Plasma

Abstract

This paper presents a high-voltage power source to produce glow-discharge plasma in the frame of a specific application. The load has two well-differentiated types of behavior. To start the system, it is necessary to apply a high voltage, up to 15 kV, to produce air-dielectric breakdown. Before that, the output current is zero. Contrarily, under steady state, the output voltage is smaller (a few hundred volts) while the load requires current-source behavior to maintain a constant glow in the plasma. The amount of current must be selectable by the operator in the range 50–180 mA. Therefore, very different voltage gains are required, and they cannot be easily attained by a single power stage. This work describes why the LC-parallel resonant topology is a good single stage alternative to solve the problem, and shows how to make the design. The step-up transformer is the key component of the converter. It provides galvanic isolation and adapts the voltage gain to the most favorable region of the LC topology, but it also introduces non-avoidable reactive components for the resonant net, determining their shape and, to some extent, their magnitude. In the paper, the transformer’s constructive details receive special attention, with discussion of its model. The experimental dynamic tests, carried out to design the control, show load behavior that resembles negative resistance. This fact makes any control loop prone to instability. To compensate this effect, a resistive ballast is proposed, eliminating its impact on efficiency with a novel filter design, based on an inductor, connected in series with the load beyond the voltage-clamping capacitor. The analysis includes a mathematical model of the filtering capacitor discharge through the inductor during the breakdown transient. The model provides insight into the dimensions of the inductor, to limit the discharge current peak and to analyze the overall performance on steady state. Another detail addressed is the balance among total weight, efficiency and autonomy, which appears if the filter inductor is substituted for a larger battery in autonomous operation. Finally, a comprehensive set of experimental results on the real load illustrate the performance of the power source, showing waveforms at breakdown and at steady state (for different output currents). Additionally, the detector’s constructive principles are described and its experimental performance is explored, showing results with two different types of metallic pollutants in water.