**1. Introduction**

Plasma systems and plasma treated materials are now commonly used. The cold, nonthermal plasma (NTP) is produced usually by high voltage (HV) electrical discharges. In nonthermal plasma, most of the electric energy is used to produce high-energy electrons, not to heat the gas. The electrons themselves do not treat the surface. The high-energy electrons (1–2 eV) excite electronic states of molecules, vibrational states, and provide molecular dissociation (oxygen at the most). Namely atomic oxygen and electronically excited molecules contribute to the surface treatment and pollution control. Cold plasma applications are very diverse. The main applications of NTP include surface modification of plastics [1–3], ozone generation [4,5], surface decontamination [6–11], sterilization of wounds and soil [12,13], toxic and harmful gas and sewage decomposition [14–20]. One of the methods of producing cold plasma is the dielectric barrier discharge (DBD). A number of studies present theoretical foundations of barrier discharges, including models of discharge chambers and their substitute schemes, analytical description of current and voltage waveforms, voltage-charge characteristics (the Lissajous figures) [21–24]. The article [21] additionally includes a review of the applications of DBD to high power CO2 lasers, excimer based ultraviolet and fluorescent lamps and flat large-area plasma displays. Another important application of barrier discharges and corona discharges (CD) is the investigation of the ionic wind generation and examination of its results for the development of propulsion [25]. The use of both barrier and corona discharges when supplying a set of several

electrodes with alternating and direct voltage enables the creation of curtains from nonthermal plasma with a relatively large width of the air gap [26].

Many articles are focused on the development of high voltage generators that are components of DBD plasma systems. Depending on the application, the power of supplies ranges from single watts to hundreds of kilowatts. The choice of power and feed method is important for the operation of the DBD reactor and for the intensity of the reaction. The most common voltage waveform used to power DBD plasma reactors is the high voltage alternating current (AC) wave. In DBD reactors and in pulsed corona discharge reactors (PCD, without a dielectric layer) unipolar and bipolar voltage waves are used, sometimes a discontinuous wave with a prepolarization.

Already in 1857, Werner von Siemens reported on first experimental investigations with DBD. He applied a mechanical pulser ("Wagnerscher Hammer") interrupting the primary winding circuit of the HV transformer as the high-voltage generator [27,28]. Another simple high-voltage generator for barrier discharges can consist of an autotransformer and a high-voltage transformer fed directly from the power grid of voltage frequency 50 or 60 Hz [3]. However, the high voltage delivered to the electrodes with a frequency of 50 or 60 Hz has a much higher value than the voltage with the increased frequency generated by an inverter. Very popular solutions are voltage source inverters (VSI), with unregulated or regulated input voltage and with high voltage transformers connected to the output [22,26,29–35]. These inverters can often have a full or half-bridge structure. When supplying a DBD system from an inverter the selection of the supply frequency is associated with the resonant frequency of the transformer inductances (and additional inductances if present) and electrode set capacities. Other generator designs are also used. Low power generators can be made as fly-back converters [22,25]. Bridge converters can be equipped with snubber circuits that reduce d*u*/d*t* or d*i*/d*t* when transistors are switching (on/off). An interesting solution is the full-bridge in which only one branch uses the LDR (coil, diode, resistor) circuit for d*i*/d*t* reduction [36]. This solution may be useful if a pulse width modulation with phase shift (PS-PWM) control is used. An example of the multiresonant generator is presented in [37]. The transformer inductances and capacitances of the electrode set (together with an additional capacitor) form one resonant circuit. Another resonant circuit provides conditions for zero current switching (ZCS) off transistors. The paper [38] describes variations of a diagonal half-bridge resonant converter topology (with four diodes and two transistors), which can be used to produce a single-period AC sinusoidal waveform. The method allows power regulation within very wide limits and makes possible the precharge pulse generation for transformer magnetization and gap voltage symmetrization. The construction of a HV generator [39,40] that allows the production of voltage waves with many different shapes is also very interesting. It consists of a 24-level cascaded H-bridge inverter and works without an HV transformer. A transformerless HV generator for DBD plasma producing is described in [41]. In the abovementioned example, the HV generator uses the phenomenon of voltage increase in a circuit under serial resonance conditions. The topology of the inverters with additional AC intermediate resonant circuits has been presented in [42]. Depending on the AC intermediate circuits these generators are characterized by properties of the current or voltage sources. In case of operation as the current source, the DBD discharges were very stable and the system was insured against arc discharges in a natural way.

The basic control methods of high voltage alternating current (HVAC) generators that are used to generate DBD plasma are described in [29,30,43]. These are pulse amplitude modulation (PAM), pulse width modulation (PWM), phase shift-pulse width modulation (PS-PWM, PSC), pulse density modulation (PDM), and pulse frequency modulation (PFM). In [31,32], the hybrid control of PDM and PFM is described.

While the above articles provide a good overview of the fundamentals of DBD discharge, their applications and HV generators design, they do not discuss the impact of individual components parameters and control variables on the power of DBD discharges. The aim of the paper at hand is to fill the gap. The discussion focusses on the following parameters: inverter input voltage, inverter output voltage frequency, DBD discharge ignition voltage, resonant circuit parameters together with the discharge chamber model parameters and transformer ratio. This article presents DBD generators and plasma reactors used for surface treatment of plastics and decontamination of loose organic materials, developed under the supervision of the authors. Generators are presented as well as entire technological devices.

The HV generators, which the authors designed, manufactured, and tested consist of power electronics converters: rectifier, DC/DC converter (if any), voltage-source inverter, and HV transformer. Transformer leakage inductances (and additional inductances if used) form a series resonant circuit with the capacities of the electrode sets. The resonant circuits create the conditions for soft switching, which is almost lossless. Thanks to the soft switching, the converters have high efficiency and generate little radio interference. Soft switching may appear as zero current switching (ZCS) or zero voltage switching (ZVS). The ZVS will be preferred in the analyzed systems, which is characterized by lower losses in the frequency and power range that the authors are interested in (up to about 100 kHz by 0.5 kW and about 25 kHz by 10 kW of rated power).
