Wireless power transfer (WPT) technology has received voluminous attention as an alternate method to charge batteries in electric devices. WPT technology is a safe and convenient method for charging batteries because it is unaffected by the external conditions. Due to the development of many different types of drones, there are numerous uses for commercial business such as delivery, agriculture, mapping, oil and gas pipeline monitoring, cinematic filming security monitoring and, firefighting. The critical problem is that the power consumption of a drone with a limited battery size restricts its operating time, which consequently limits its range of action; therefore, drones need to be recharged frequently. WPT technology enables drone with automatic landing and takeoff technology to charge battery conveniently and automatically.

We propose a new tightly-coupled three-phase resonant magnetic field (TC-TPRMF) charger for drone operating at 60 kHz. In order to reduce the selective EMI, the conduction angle control is proposed for WPT charging system. Furthermore, the multiple reactive loop (MRRL) shielding is proposed to reduce EMF and EMI from proposed TC-TPRMF charger. The single-phase WPT charging system have been popularly applied to generate magnetic fields for WPT in many applications. However, Three-phase WPT charging system approach is an encouraging alternative to the single-phase WPT charging systems due to great benefits of EMI reduction. This is possible because output voltage of three-phase inverter is a six-step wave, which eliminates 3rd harmonic and all of its integer multiples of output phase voltage of inverter. Proposed TC-TPRMF charger with three-phase resonant magnetic field has higher VA rating and higher smoothness of power flow than single-phase WPT charging system. It results in the enhancement of power transfer efficiency and output power capability of proposed TC-TPRMF charging system. Moreover, Rx input voltage waveforms of ac-dc converter has lower current harmonics with six-step wave compared to single phase system with rectangular waveform of Rx input voltage.

<Implemented of Three-phase WPT System on Drone>


Recently, the worldwide smart wearable market is rapidly growing with the largest portion of the Smartwatch market due to its unique functions such as health monitoring, child protection and alarm functions by interlocking with Smartphones. A wristwatch has become an essential fashion item across people of all ages and both sexes. However, conventional analog watches have the limited functions such as time check, calendar and stopwatches. On the other hands, Smartwatches have variety superior functions such as health care monitoring and child protection by implementing the electronic circuits in the device. In addition, by interlocking with the Smartphone, the important event or data can be transferred and displayed on the Smartwatches, so people cannot miss the event. However, due to the large power consumption of the integrated circuits, sensors and display, the wireless charging technology is applied in the conventional Smartwatches. Sadly, the implementation of the wireless charging system in the main body of the Smartwatch increase the size of the device, limits the system performance and vulnerable to EMI and thermal problems. To solve the existing problems, we proposed the Smartwatch strap wireless charging scheme by implementing the flexible wireless charging coil and shielding in the strap. Since the Smartwatch strap is flexible and curved as the curvature of the human wrist, the wireless power transfer coil and shielding material need to be analyzed with the curvature for stable wireless charging operation.

In our laboratory, we focus on the modeling of the wireless power transfer PCB coils and we verify the proposed modeling methods using HFSS 3D EM simulation and actual measurement of the fabicated PCB coils using a vector network analyzer. Furthermore, we actually implemented Smartwatch strap wireless charging system to demonstrate the wireless power transfer shceme in the flexible enviroment and to see the feasibility of the future wireless power transfer technology.

<Conceptual view of Smartwatch Strap Wireless Charging System>


Current wireless power transfer (WPT) technology can only allow power transfer over a limited distance because, as the distance between the transmitter (Tx) and the receiver (Rx) coils increases, the power transfer efficiency (PTE) decreases with a steep slope, while the electromagnetic field (EMF) leakage increases. In order to increase the PTE and decrease the EMF leakage simultaneously, we need to develop a method to concentrate the magnetic fields between the Tx and Rx coils.

In our laboratory, we propose and design metamaterial to focus the magnetic fields between the Tx and Rx coils. Metamaterial is an artificial material that has the property not found in nature and not observed in the constituent material. We are particularly interested in negative permeability metamaterial to confine magnetic field for high efficiency and low EMF leakage in wireless power transfer system. Modeling, simulation and experimental verification of metamaterial in WPT system has been done in our lab, and we can research on that topic for better performance with a novel idea. The WPT applications using metamaterial can be mobile, drone, electric vehicle, and electric train system.

(a)The unit cell of the proposed thin PCB-type metamaterial structure for negative real permeability

(b) The simulation results of the magnetic field distribution at a 20 cm distance in WPT system.


Recently, wireless power transfer (WPT) technology has drawn attention as the next-generation charging solution for batteries in electric vehicles (EVs). Leading global automobile suppliers have been making continuous efforts to commercialize the automotive wireless charging system. Although WPT technology is very attractive to the EV charging, strong electromagnetic fields (EMFs) are inevitably generated via large air gap. The time-varying EMFs can interfere with nearby electrical devices, such as analog-to-digital converter, which is an essential device for converting analog signal from the automotive sensor to digital signal. Moreover, long-term exposure to time-varying EMFs can be harmful to humans. The harmfulness of such exposure to human health has been well documented by the International Commission on Non-Ionizing Radiation Protection (ICNIRP), the World Health Organization, and national expert groups. Moreover, the inverter with high operating frequency is typically adopted in a high-power system. The inverter contains a wide range of harmonics, which causes electromagnetic interference (EMI) issues on the operation of other sensitive electronic devices in the EV. Therefore, it is important to minimize the EMF and EMI in the WPT system.

We first propose and demonstrate a new automotive tightly coupled handheld resonant magnetic field (HH-RMF) charger operating at 20 kHz with low EMF and high efficiency. The implemented HH-RMF charger is designed to have a similar structure to that of a gas fuel handle; thus, the charger is user friendly. Using a guided magnetic flux in resonance (GMFIR) structure, the coil losses are reduced. The proposed tightly coupled HH-RMF charger is enclosed in a ferrite structure to confine and guide the magnetic field and lower the EMF. We propose the isolation inductor scheme for EMI reduction in the automotive tightly coupled HH-RMF charging system.

<Implemented of WPT System on Golf Cart>