Yeonje Cho

Thin PCB-Type Metamaterials for Improved Efficiency and Reduced EMF Leakage in Wireless Power Transfer Systems

Current wireless power transfer (WPT) technology can only allow power transfer over a limited distance because, as the distance between the transmitter (Tx) and 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 this paper, we proposed a novel metamaterial structure to realize high efficiency and low EMF leakage. Metamaterials can confine the magnetic fields between the Tx and Rx coils by negative relative permeability. We designed and fabricated a thin metamaterial using a 1.6-mm dual layer printed circuit board (PCB) with a high dielectric constant substrate and a fine pattern to achieve a negative relative permeability with low loss at 6.78 MHz. The thin PCB-type metamaterial has a wide range of applications with low fabrication cost, light weight, and a simple fabrication process. We demonstrated a 44.2% improvement in the PTE and 3.49-dBm reduction in the EMF leakage around the WPT system at 20-cm distance. Furthermore, we first analyzed metamaterials from an EMF point of view using the 3-D magnetic field scanner. Finally, we discussed a combination of metamaterials and ferrites to further improve the PTE and reduce the EMF leakage for long-distance mobile WPT systems.

Three-phase magnetic field design for low EMI and EMF automated resonant wireless power transfer charger for UAV

Wireless power transfer (WPT) technology enables convenient and automatic battery charging for unmanned aerial vehicles (UAVs). However, in WPT charging systems, strong electromagnetic fields (EMFs) are inevitably generated and the power source contains a wide range of harmonics. In this paper, we propose an automated resonant wireless power transfer charger for UAV with three-phase magnetic field for electromagnetic interference (EMI) and EMF reduction. The simulation results show that the proposed six-step three-phase resonant WPT system for UAV charging reduces the total harmonic distortion (THD) of the Tx and Rx currents by 14.21% and 7.45%, respectively. The strength of magnetic field from the UAV three-phase resonant WPT charging system also can be reduced compared to the single-phase resonant WPT system.

EMI reduction in wireless power transfer system using spread spectrum frequency dithering

This paper presents the use of spread spectrum technique to reduce electromagnetic interference caused by fast switching transitions of a resonant inverter and rectifier used for wireless power transfer (WPT) systems. We investigate the magnetic field emission from a series-resonant WPT system having the resonance frequency of 20 kHz in a frequency band from 9 kHz to 150 kHz. The averaging spectral estimate (Welchs Periodogram) of the simulated leakage magnetic field shows that a harmonic peak can be reduced up to 8.7 dB by dithering the switching frequency with a triangular modulation profile.

Electromagnetic interference reduction method from handheld resonant magnetic field charger (HH-RMFC) for electric vehicle

The proposed handheld resonant magnetic field charger (HH-RMFC) is a convenient method of charging batteries in electric vehicle (EV) without any electrical contact. With close distance between Tx and Rx windings, high mutual inductance causes large reflected impedance seen by the power source. The large reflected impedance can cause distortion of current flowing through Tx and Rx windings. In frequency-domain, the distortion of current produces large current harmonics, which may have electromagnetic compatibility (EMC) issues caused by leakage magnetic field. To quantify the distortion of current, the total harmonic distortion (THD) is used. In this paper, the design method for low EMI from the HH-RMFC for EV are proposed and verified using a circuit simulator. The circuit simulation results show that THD of Tx and Rx current is below 5% using proposed method.

Ultra-thin printed circuit board metamaterial for high efficiency wireless power transfer

Metamaterial composed of artificial periodic structure is applied to a wireless power transfer system to enhance the efficiency of long distance power transfer. Metamaterial can control the direction of magnetic fields due to its negative permeability. Previously reported metamaterials were too thick and large in size to increase the power transfer distance. In this paper, ultra-thin (0.16 cm) and small (18.6 × 18.6 cm) metamaterial structure is proposed and its enhanced efficiency (with maximum of 44.2 % improvement) is demonstrated. Furthermore, this paper shows simplified modeling method for complicated metamaterial structure simulation.

Hybrid metamaterial with zero and negative permeability to enhance efficiency in wireless power transfer system

Metamaterials with negative relative permeability can change magnetic field in the opposite direction. It can also change the magnetic field direction to straight if it has zero relative permeability. Previously reported metamaterials use only single characteristic which is a negative or a zero permeability to enhance efficiency by field confinement in wireless power transfer (WPT) systems. In this paper, we combined two kinds of structures of metamaterial cells, which have the negative and zero permeability. The hybrid metamaterial slab (HMS) is designed by using two spiral type patterns on a double layered PCB. It has the negative permeability at the edge of the slab and the zero permeability at the center of the slab. The HMS can confine greater magnetic fields around and inside transmit and receive coils. Therefore, the hybrid metamaterials can increase the power efficiency by 21.4% at 20 cm distance in 6.78 MHz WPT systems compared to the previous single permeability metamaterial slab (SMS).

High-Resolution Synthesized Magnetic Field Focusing for RF Barcode Applications

A prototype of a high-resolution synthesized magnetic field focusing (SMF) system for a possible application to radio frequency (RF) barcodes is proposed in this paper. It was recently found that coordinated control of array coil currents makes it possible to focus magnetic field, which is requisite for medical imaging, wireless power, and RF identification. Different from conventional phased array RF antennas, SMF is independent of frequency, which is quite promising for various applications. In this paper, an array of rectangular air coils is proposed, and various implementation issues, such as accurate control of coil currents and compact RF barcode design, are newly suggested. A prototype of an SMF system composed of a transmitter (Tx), an RF barcode, and a receiver (Rx) was implemented at below 1 MHz operating frequency. The Tx includes modularized control drivers, full-bridge switching converters, and 16 arrayed air coils, where the RF barcodes proposed in this paper include a self-resonating circuit at 20 mm × 20 mm and a tunable resonating circuit at 15 mm × 15 mm. In this way, a 5 mm resolution at a distance of 20 mm was achieved for the Tx arrayed air coils with spacing of 5 mm, which is about four times sharper magnetic field focusing compared to a nonfocusing conventional coil. Furthermore, the proposed SMF system allows the designed 5-bit RF barcode array to be fully distinguishable at a 15 mm distance.

Thin Hybrid Metamaterial Slab With Negative and Zero Permeability for High Efficiency and Low Electromagnetic Field in Wireless Power Transfer Systems

Current wireless power transfer (WPT) systems have limited charging distance and high induced electromagnetic field (EMF) leakage. Thus, we first proposed a thin printed circuit board (PCB) type hybrid metamaterial slab (HMS) combining two kinds of metamaterial cell structures. The metamaterial cells in the center area of the HMS have zero relative permeability and straighten the magnetic field direction. The metamaterial cells located at the edges of the HMS have negative relative permeability and change the outgoing magnetic fields to opposite direction by magnetic boundary condition. Therefore, the magnetic field can be more confined between transmitter and receiver coils, enhancing the power transfer efficiency, while decreasing the EMF leakage in a WPT system. In this paper, we demonstrated that increased power transfer efficiency from 34.5% to 41.7% and reduced EMF leakage from −19.21 to −26.03 dBm in 6.78-MHz WPT system. Furthermore, we proposed new analysis method for relative permeability measurement of the metamaterial using a novel cubic structure with perfect electrical conductor and perfect magnetic conductor boundary.

Low EMI high-k tightly-coupled resonant magnetic field (TCR-HMF) charger with impedance design for a 3-wheeler vehicle

In this paper, we propose low electromagnetic interference (EMI) high-k tightly-coupled resonant magnetic field (TCR-HMF) charger. The high coupling coefficient, k, between Tx and Rx coils is essential for increasing power transfer efficiency (PTE) and reducing size of proposed TCR-HMF charger. As the k increase, the mutual inductance also increases. Moreover, the larger mutual inductance results in frequency splitting phenomena. Because the current harmonics are determined by the ratio of the magnitude of the input impedance at the resonant frequency to the magnitude in its harmonic frequencies, the current harmonics are increased by the frequency splitting phenomena. In this paper, a reduction method of the harmonic currents in TCR-HMF charger with impedance design method is proposed. The circuit simulation results show that the harmonic currents flowing through Tx and Rx coils can significantly be reduced by applying the proposed method. The circuit simulation results of TCR-HMF charger show that proposed method can reduces THD of Tx current and Rx current by 14.55 % and 57.13 %, respectively.

Design and analysis of hybrid loop-array for high efficiency and low EMF level in wireless high power transfer system

For wireless power transfer (WPT) systems transferring high power, both high efficiency and low magnetic field leakage have been regarded as the main design objectives to be achieved. For the first time, in this paper, we propose a new concept of hybrid loop-array structure and its design procedure to achieve high efficiency and low magnetic field leakage simultaneously in high-power WPT system. The simulation results show that the proposed hybrid loop-array design helps increase the efficiency while reducing the leakage magnetic field level of the WPT system transferring 1 kW load power at 60 kHz resonance frequency.