The interest in electric-powered transportation has been increased because of problems such as depletion of fossil fuel resources and environmental issues caused by internal combustion engines. The market for various kinds of small E-mobility devices (EMDs), such as electric vehicles (EVs), E-bikes, segways, and mini-boards has been growing rapidly. Of these, the distribution of the small electric vehicles called personal mobility devices (PMDs), which can address problems such as traffic issues and environmental pollution, is expected to gain momentum. However, unlike the widely used charging stations for EVs, charging systems for PMDs are insufficient. The large size of dedicated PMD charging adapters makes them inconvenient to transport. Therefore, to make PMDs more attractive, it is necessary to deploy charging stations that can charge all types of PMDs. The present study proposes a new charging system that can respond to the needs of PMDs having various voltages and implement low charging current ripple. The proposed multi-level charging system can share power by subdividing the power supply to handle the charging requirements of various PMDs. In order to verify the validity of the proposed charging system, we performed simulations and conducted experiments by building a prototype charging system. INDEX TERMS Electric vehicles (EVs), dc/dc converter, personal mobility devices (PMDs), multi-level charging system, E-mobility.

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When driving the piezoelectric transducer (PT: piezo transducer), which is a key device, it is important for the ultrasonic system (using ultrasonic waves of 20 kHz or higher) to operate at a resonant frequency that can maximize the conversion of mechanical energy (vibration) from electrical energy. The resonant frequency of the PT changes during the actual operation according to the load fluctuations and environmental conditions. Therefore, to maintain a stable output in an ultrasonic system, it is essential to track the resonant frequency in a short time. In particular, fast resonant frequency tracking (RFT: resonant frequency tracking) is an important factor in the medical ultrasonic system, i.e., the system applied in this thesis. The reason is that in the case of a medical ultrasonic system, heat-induced skin necrosis, etc., may cause the procedure to be completed within a short period of time. Therefore, tracking the RFT time for maximum power transfer is an important factor; in this thesis, we propose a new high-speed RFT method. The proposed method finds the whole system resonance frequency by using the transient phenomenon (underdamped response characteristic) that appears in an impedance system, such as an ultrasonic generator, and uses this to derive the mechanical resonance frequency of the PT. To increase the accuracy of the proposed method, parameter fluctuations of the pressure of the PT, the equivalent circuit impedance analysis of the PT, and a MATLAB simulation were performed. Through this, the correlation between the resonance frequency of the ultrasonic system, including the LC filter with nonlinear characteristics and the mechanical resonance frequency of the PT, was analyzed. Based on the analyzed results, a method for tracking the mechanical resonance frequency that can transfer the maximum output to the PT is proposed in this thesis. Experiments show that using the proposed high-speed RFT method, the ultrasonic system can track the mechanical resonance frequency of the PT with high accuracy in a short time.

Considerable efforts are being made to reduce CO2 emissions and thereby solve the problems of environmental pollution and global warming. Technologies for environmentally friendly transportation are being developed using batteries. In particular, with the increase in urbanization and one-person households, e-mobility products are drawing increasing attention as short-distance transportation devices. Among these vehicles, personal mobility devices (PMDs) are receiving attention as new transportation devices that are simple to operate. This paper proposes a new multilevel charging system that is advantageous in responding to the charging voltage specifications of various mobile devices with a single charging system while ensuring a low charging current ripple. The proposed diode-parallel multilevel converter consists of an independent Buck converter in series. The switch of the buck converter is configured at the negative terminal of the input power source so that the gate amplifier voltage is used as the power supply voltage; it can therefore be simply configured without a separate gate amplifier power supply. In addition, it is improved so as to have a wider charging voltage range in a low output voltage region and a better efficiency than the existing diode series multilevel converter. To verify the feasibility of the proposed system, simulations were performed using the software PowerSIM(PSIM), and, in order to verify the validity, a prototype charging system was fabricated to compare and analyze losses according to operating conditions.

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