Pulse Density Modulated Power Control of a Voltage-Source Inverter with a Series-Resonant Circuit Suitable for Corona Discharge Treaters.

The purpose of corona discharge treaters using high frequency inverters is to render the affinity of polyethylene films to inks, glues and coatings. A frequency control scheme for such a high frequency inverter has been proposed to achieve the power control of a corona discharge treater. It is, however, impossible to realize zero current switching except for full-power operation. Partial discharge appears in the frequency control scheme under the condition of a low power because the applied voltage between electrodes is decreased as the output power is reduced.This paper presents a 30kHz 5kW voltage-source inverter using IGBTs for corona discharge treaters, which is characterized by the power control based on pulse density modulation (PDM). Although the voltage-source inverter operates at a constant dc voltage and a constant output frequency, the average output power can be controlled by adjusting a period ratio of the square wave pulse having a constant amplitude to zero voltage. The PDM inverter can always operate with zero current switching, so that switching losses and electromagnetic noises are reduced. The prototype system developed in this paper shows good corona discharge characteristics in a wide output power range from 0.5% to 100%, which makes it possible to realize both strong and weak surface treatment.