Pulse-Density Modulation of a High-Frequency Voltage-Source Inverter for Atmospheric-Pressure Plasma Generation

This paper deals with a new control method for an atmospheric-pressure discharge plasma generator. The plasma generator consists of a voltage-source inverter, a step-up transformer, a resonant inductor and two parallel electrode plates. The high-frequency high voltage induced by series resonance between the equivalent capacitance of the plates and the resonant inductor generates glow plasma between the plates. However, a continuously-applied voltage may cause streamer or arc discharges instead of glow plasma, when the plates are under atmospheric pressure. This paper discusses an application of pulse density modulation (PDM) to the plasma generator, and proposes a new PDM method suitable for atmospheric-pressure plasma. A conventional PDM method repeats alternate run and stop to adjust the output power. The switching pattern is designed to minimize power variations. In contrast, the switching pattern of the proposed PDM method is designed to achieve a quick decrease in discharge current and electrode voltage during the inverter is stopped. As a result, the proposed method makes it possible to eliminate the streamer or arc discharges, leading to stable glow plasma. Experimental results are shown to verify the viability of the proposed PDM method.