Toshio Ishizaki

3-D free-access WPT system for charging movable terminals

There are strong demands for providing small power to movable handy terminals by a resonance-type wireless power transfer system. Continuous charging for cellular phones in a room is the best example. Conventional WPT system has a limited capability to transfer power for the movable terminals with arbitrary positions and angles. We do not enjoy the merits of the wireless system. In this paper, a novel WPT system, which uses larger transmitting resonators embedded in a floor and in walls and a smaller receiving resonator installed in a handy terminal, is proposed. The larger transmitting resonator can transfer power for full space in the room, and the smaller receiving resonator is suitable for compact terminals. We evaluate the transmission efficiency in terms of coupling coefficient for any positions and any angles of the movable terminal in a room. To realize 3-D free-access, multiple transmission resonators are employed. The coupling coefficients are measured experimentally. Then, the possibility is confirmed.

Transferred power and efficiency of a coupled-resonator WPT system

A coupled-resonator WPT system is analyzed in terms of transferred power and transfer efficiency. Quality factor (fm) of the coupled resonators is defined by the product of the coupling coefficient (k) and unloaded Q (Q1, Q2) of the resonators. Though it is well known that the maximum value of transfer efficiency is determined only by fm, its condition has been unrealistic. The present paper gives a new expression that is not only far more realistic but also physically acceptable. In addition, the operating frequency selection method for the case of double peak response due to the over-coupling is proposed. A widely accepted method of the peak preference is criticized.

A 2.4 GHz-Band 100 W GaN-HEMT High-Efficiency Power Amplifier for Microwave Heating

The magnetron, a vacuum tube, is currently the usual high-power microwave power source used for microwave heating. However, the oscillating frequency and output power are unstable and noisy due to the low quality of the high-voltage power supply and low Q of the oscillation circuit. A heating system with enhanced reliability and the capability for control of chemical reactions is desired, because microwave absorption efficiency differs greatly depending on the object being heated. Recent studies on microwave high-efficiency power amplifiers have used harmonic processing techniques, such as class-F and inverse class-F. The present study describes a high-efficiency 100 W GaN-HEMT amplifier that uses a harmonic processing technique that shapes the current and voltage waveforms to improve efficiency. The fabricated GaN power amplifier obtained an output power of 50.4 dBm, a drain efficiency of 72.9%, and a power added efficiency (PAE) of 64.0% at 2.45 GHz for continuous wave operation. A prototype microwave heating system was also developed using this GaN power amplifier. Microwaves totaling 400 W are fed from patch antennas mounted on the top and bottom of the microwave chamber. Preliminary heating experiments with this system have just been initiated.

A novel concept for 2-dimensional free-access wireless power transfer system using asymmetric coupling resonators with different sizes

A novel wireless power transfer system which transfers small electric power for handy electronic terminals is proposed. In order to transfer power to handy terminals in a desired region of a room, asymmetric coupling resonators with different sizes are used. This means that a large transmitting coil and a small receiving coil are installed in a transmitter and a receiver, respectively. The possibility of 2-dimensional free-access power transfer is demonstrated by experiments. Design procedure for the system is illustrated and preferable coil structures for Tx coil and Rx coil are discussed. The possible transmission distance and 2-D free-access area are shown. Then, an illustrative system is designed and the performance is confirmed.

Design and experiment of multi-stage resonator-coupled WPT system

Resonator-coupled WPT (Wireless Power Transfer) systems are designed using the BPF theory for multi-stage configuration. Not only the transmission property versus frequency, but transmission loss property versus number of stages are studied, showing the usefulness of the multi-stage system and the theory itself at the same time. Before the design example is shown, application of the BPF theory to the WPT system is critically studied, resulting in clarification of the superiority of the theory.

Study on a purely electric-field coupled resonator for WPT systems

We investigate a toroidal coil as a purely electric-field coupled resonator in the present paper, since we have studied a two-layered spiral coils as a purely magnetic-field coupled resonator before. We will explain the difference of structure and property of each resonator first, and then check the parametric characteristic of the toroidal coil resonator through measurement and analysis. Then, we compare it with the conventional electric field coupling system. In consequence, the benefits and features of the toroidal coil resonator could be clarified for use in WPT systems.

Design of a resonator-coupled WPT system with multiple loads based on measurement with a VNA instead of an oscilloscope

Resonator-coupled wireless power transfer (WPT) systems are designed for an rf power source with constant current and multiple loads with arbitrary resistance. As long as the system does not include a nonlinear element, the system response can be conveniently measured with a vector network analyzer (VNA) that has 50 ohm input/output impedance. The S parameters thus measured are converted to Z parameters, which are invariant to the external circuits. In the next step, the transferred power and transfer efficiency are calculated for infinite ohm power source using the Z parameters of the circuit obtained above. Using this scheme, the response of a multiple load system is calculated for constant current source. The optimum system condition is also studied indicating the filter design concept is effective.

Choice of resonators for a WPT system in lossy materials

In order to cope with the necessity of transferring electric power through lossy dielectric materials such as house walls, human body or water, use of magnetically coupled resonator WPT system is recommended. A simple spiral resonator with series capacitance is the first candidate, followed by the second candidate, a little more sophisticated two layered spiral resonator with series capacitance. Since they confine electric energy in a narrow space effectively, the coupling becomes magnetic, which reduces dielectric loss appreciably.

Fabrication of a new high-performance WPT system by electric energy confinement

A WPT system based on coupled resonators makes use of the electric and magnetic couplings. Considering one can control the ratio of those couplings, it should be useful to reduce the electric coupling, since it is affected by the dielectric material that intrudes between the two resonators, resulting in the shift of operating frequency, deterioration of matching and increase of transfer loss. The present paper studies two methods to avoid them, that is, electric energy confinement by external lumped capacitor and by double layered coils. Both try to confine the electric energy of the resonators into a limited space and remove from the coupling space. Carrying out a comparative study, it concludes the latter is more desirable by the overall rating.

Applicaions of a novel disk repeater

New applications of a proposed disk repeater are described. Taking advantages of its low loss, simple structure and easy handling, it could be used for power tunneling, power catering and power mendicanting. The basic properties of the disk repeater is clarified first, being followed by parametric properties of three possible applications.