Chan-Hun Yu

Plasma-activated water (PAW) is a subfield of plasma nitrogen fixation, and various plasma sources have been explored for its production. Active research is underway for its application in agriculture and biotechnology. In this study, we demonstrated the feasibility of an eco-friendly, NO _x emission-free PAW production system by developing a closed-loop system that recycles unreacted gases generated during PAW production. The system was designed to allow natural inflow of gas from the outside in proportion to the volume of gas dissolved into the water, thereby maintaining equilibrium between the internal pressure and atmospheric pressure. A comparative experiment was conducted on the nitrogen-to-oxygen mixing ratio between the internal gas and the inflowing gas. It was confirmed that the system operated most stably at a nitrogen-to-oxygen ratio of 1:2. Although a higher oxygen ratio increased power consumption and could enhance the PAW production capacity, it was observed that when the oxygen ratio exceeded 50%, the nitrate production efficiency plateaued at approximately 55 g/kWh. These findings suggest that the amount of gas drawn into the closed-loop PAW production system can serve as a real-time indicator of the system’s performance.

This article proposes a solid-state circuit breaker (SSCB) for current commutation in hybrid dc circuit breaker. The proposed SSCB can achieve dynamic voltage balance through a gate driver which has fast rising time and synchronously drives several solid-state switches (SSs) by applying a multicore transformer. In addition, the transformer size in the gate driver can be reduced by a state-hold mode, which maintains on and off states of switches by applying pulse voltage with short pulse to the transformer. Moreover, the proposed SSCB is designed with a folded structure making the upper and lower current paths in opposite directions to reduce parasitic inductance which causes voltage ringing. Experimental results from the proposed SSCB with conduction test (peak current of 9 kA and base pulse width of 2 ms) and blocking test (9 kA) are presented to verify the proposed works.

This article proposes a solid-state pulsed power modulator (PPM) with asymmetric bipolar pulses and high repetition rates for semiconductor manufacturing processes. Since the proposed PPM is based on high voltage solid-state switches (HVSSes) consisting of series connected silicon carbide (SiC) metal-oxide silicon field effect transistors (MOSFETs), it is simple to obtain variable asymmetric bipolar pulses as well as unipolar pulses. To solve the voltage unbalance problem, which is a critical challenge when the SiC MOSFETs are connected in series, a simple voltage balancing circuit is proposed. This balancing circuit shows superior features such as low loss and high reliability because it is composed of only passive components such as transformers and capacitors. In addition, the proposed PPM can achieve the high repetition rate since the soft switching of all SiC MOSFETs can be accomplished by the balancing transformer. Furthermore, the soft switching characteristic and the low loss of balancing circuit can contribute to the miniaturization of PPM by reducing the sizes of components and cooling systems. The design and implementation of 5 kV PPM with high repetition rate of 500 kHz are presented. Experiment results from the developed PPM are shown to verify the proposed works.

This article describes designs for power supplies for driving a high-power thyratron switch. To improve the delay time jitter of the thyratron, three driving power supplies (trigger, cathode heater, and reservoir heater) are designed. Based on inductive energy storage (IES) with an opening switch, the trigger power supply (TPS) is designed to generate controllable output pulse voltage ( $1$ –10 kV) as well as pulse energy ( $1$ –5 mJ) for investigating thyratron switching features. Depending on the specifications of the cathode heater ( $8$ V, 100 A) and reservoir heater (8 V, 25 A) power supply, ${LCC}$ resonant converters are designed to use their soft-switching properties. Since the output ripples affect the thyratron switching characteristics, the high-frequency operation of the converter allows for reducing ripples with a minimized output filter. Designs of three power supplies were verified from individual rated operations as well as experiments with L4888 thyratron. Switching features of the L4888 thyratron were analyzed based on operating conditions of power supplies and 152 ps of rms delay time jitter was achieved.

This article describes a design of dc power supply (DCPS) for a gliding arc discharge (GAD) for plasma-activated water (PAW) production. The GAD reactor requires both high-and low-voltage condition depending on the existence of plasma. This dynamic load characteristic depends on airflow and discharge current conditions. Therefore, the design of DCPS less susceptible to the dynamic load condition is necessary. As a topology for stable current supply, discontinuous conduction mode series resonant converter (DCMSRC) is applied. Owing to zero current switching (ZCS) characteristic, DCMSRC can operate with high switching frequency. The output current is proportional to the operating frequency regardless of the load resistance. This feature contributes to removing a ballast resistor, which is used to maintain unstable arc current. The designs of resonant parameters with explanation of operating principle are introduced. An additional analysis considering parasitic resonant capacitor caused by transformer (TX) and output rectifier circuit is described for predicting the ZCS region. Using the developed DCPS, experiments with a resistor load and the GAD reactor are carried out. Experimental results under various airflow and discharge current conditions are introduced. Finally, the maximum efficiency of the PAW production is achieved as 500 L/kWh.

This article describes the development of a 15-kV and 1.8-kJ/s capacitor charging power supply (CCPS) for a kicker modulator system, which requires a high reproducibility of the charging voltage. The developed CCPS can achieve the high reproducibility by reducing a bucket size through the high-frequency operation of an LCC resonant converter. For the high-frequency switching, a gate driver circuit of silicon carbide (SiC) metal–oxide–semiconductor field effect transistors (MOSFETs) is proposed. To obtain a small parallel resonant capacitor, which is essential for the high-frequency operation, a resonant tank is implemented by applying multicore transformer structure. In addition, to reduce a voltage unbalance of the rectifier diodes, a complementary winding method is proposed. Furthermore, a burst mode control for flat-top voltage is presented. Experimental results of the developed CCPS are presented to verify the proposed works.

This article describes the development of a 120 kV and 60 kW cathode power supply (CPS) for an electron beam welding (EBW) system. Since the developed CPS is based on a three phase LCC resonant converter, it shows many superior features such as parasitic component utilization and low output voltage ripple. In addition, by adopting a symmetrical bipolar voltage multiplier (SBVM) as the CPS rectifier, the voltage imbalance among SBVM stages can be reduced even at the high output current. Therefore, the CPS can effectively achieve the output voltage of 120 kV and output power of 60 kW with low voltage stress on SBVM components. Furthermore, by using phase shift modulation (PSM) for load regulation, the conduction and switching losses are reduced in the light load operation due to the fixed operating frequency and small circulating current compared with the conventional pulse frequency modulation (PFM). For the practical use of the three phase LCC resonant converter with SBVM, several issues about the implementations of the transformer and interleaved PSM controller are addressed. The developed CPS is installed on the 120 kV and 60 kW EBW systems and the experimental results are presented to verify the performance of the developed CPS.

This article proposes an optimal design of an LCC resonant converter with a phase shift control for a fuel cell system that has wide input and output voltage ranges. The proposed optimal design focuses on the circulating energy minimization by adjusting the transformer turns ratio according to the maximum gain of resonant tank. This makes the root mean square value of resonant current and turn- off current stationary even though the resonant tank change. Thus, designers can select resonant tank optimized for the converter specifications without considering the conduction and switching losses. By the proposed optimal design, the efficiency degradation at worst case, which is a significant issue of the fuel cell system with wide output voltage range, can be minimized. In addition, for the fuel cell converter with wide input voltage range, a phase shift control that operates the full-bridge inverter with a constant effective duty cycle according to the fuel cell voltage is proposed. This control scheme improves the light-load efficiency without deteriorating the rated-load efficiency by reducing the circulating current and switching frequency. The validity of the proposed works is confirmed by experimental results of the 30-kW LCC resonant converter.

This article describes the design and implementation of a 10 kW, 400 kHz ac power supply for a ferrite inductively coupled plasma (ICP) reactor. The developed power supply is composed of an ac inverter and a front-end converter not only to offer stable dc input voltage for the inverter but also to respond to the dynamic load characteristic of the plasma reactor. A brief introduction of a series-parallel resonant converter (SPRC) with a trapezoidal approximation and determining a design point considering a wide load range are described. In the case of the inverter, a low-loaded quality factor design method based on the analysis of a series-parallel resonant inverter (SPRI) to obtain high efficiency is proposed. With respect to the plasma ignition, a control logic without an additional plasma ignition circuit is proposed. Finally, the performance of the developed power supply is demonstrated under various experimental conditions including an experiment with a ferrite ICP load.

This article proposes a modular pulsed power modulator (PPM) with a short pulse width and high repetition rate for pulsed lasers. The presented PPM is based on the modular structure to use semiconductor switches with low voltage ratings since these switches have fast switching speed, which is an essential requirement to generate short pulses. In addition, to make a short gate signal which is also needed to generate short pulses, a gate driver circuit that applies positive and negative voltages asymmetrically is proposed. Moreover, a heatsink structure to minimize the parasitic capacitances is presented for the high repetition rate. Furthermore, a dc–dc converter for the modular PPM is developed, which connects transformers in parallel at the primary side to achieve voltage balance without any compensation circuit. Experimental results from a 5 kV PPM with the pulse width below 20 ns and repetition rate of 1 MHz are presented to verify the proposed works.