Abstract: A circuit capable of keeping input impedance constant is provided. Further, a circuit which can contribute to improvement in power feeding efficiency in power feeding by a magnetic resonance method is provided. A voltage (a former voltage) proportional to a direct-current voltage input to a DC-DC converter from the outside and a voltage (a latter voltage) proportional to a current input from the outside are detected, and the ratio of the former voltage and the latter voltage are held constant. Accordingly, input impedance can be kept constant. Further, impedance conversion is performed in the DC-DC converter. Thus, even when the battery in which power feeding is performed exists on an output side of the DC-DC converter, input impedance can be kept constant. Consequently, power can be supplied to a power receiving device including the DC-DC converter and the battery with high power feeding efficiency by a magnetic resonance method.

Abstract: A contactless power feeding system includes a power transmitting device and a power receiving device. The power transmitting device includes a first AC power source configured to generate an AC power with a first frequency, a second AC power source configured to generate an AC power with a second frequency which is different from the first frequency, a first electromagnetic induction coil, and a first resonant coil. The power receiving device includes a second resonant coil, a second electromagnetic induction coil, and a power storage unit. Power is wirelessly supplied to the power storage unit at the second frequency by a magnetic resonance phenomenon which occurs between the first resonant coil and the second resonant coil.

Abstract: Disclosed is a semiconductor device having a memory cell which comprises a transistor having a control gate and a storage gate. The storage gate comprises an oxide semiconductor and is able to be a conductor and an insulator depending on the potential of the storage gate and the potential of the control gate. Data is written by setting the potential of the control gate to allow the storage gate to be a conductor, supplying a potential of data to be stored to the storage gate, and setting the potential of the control gate to allow the storage gate to be an insulator. Data is read by supplying a potential for reading to a read signal line connected to one of a source and a drain of the transistor and detecting the change in potential of a bit line connected to the other of the source and the drain.

Abstract: A novel power receiving device and a novel power transmission device are provided. Power feeding and communication are performed using a magnetic resonance method. Specifically, in one embodiment of the present invention, power feeding is performed by generating a second high-frequency voltage based on a first high-frequency voltage induced in a resonant coil and communication is performed by modulating amplitude of the first high-frequency voltage induced in the resonant coil. Thus, it is possible to perform communication and power feeding based on data obtained by the communication in pseudo-parallel.

Abstract: A modulation circuit includes a load and a transistor serving as a switch. The transistor has an oxide semiconductor layer in which hydrogen concentration is 5×1019/cm3 or less. The off-state current of the transistor is 1×10?13 A or less. A modulation circuit includes a load, a transistor serving as a switch, and a diode. The load, the transistor, and the diode are connected in series between the terminals of an antenna. The transistor has an oxide semiconductor layer in which hydrogen concentration is 5×1019/cm3 or less. An off-state current of the transistor is 1×10?13 A or less. On/off of the transistor is controlled in accordance with a signal inputted to a gate of the transistor. The load is a resistor, a capacitor, or a combination of a resistor and a capacitor.

Abstract: An electric field coupling type wireless power feed system including a power transmitting device and a power receiving device which is unlikely to be affected by a noise and which has a reduced size is provided. A capacitive coupling type wireless power feed system includes a power receiving device including a first electrode using an oxide semiconductor film, and a battery and a power transmitting device including a second electrode, wherein the battery is charged by a voltage generated based on an electric field generated between the first electrode and the second electrode. The charging of the battery may be stopped by applying a positive direct-current voltage from a charge control circuit to the first electrode.

Abstract: A circuit capable of keeping input impedance constant is provided. Further, a circuit which can contribute to improvement in power feeding efficiency in power feeding by a magnetic resonance method is provided. A voltage (a former voltage) proportional to a direct-current voltage input to a DC-DC converter from the outside and a voltage (a latter voltage) proportional to a current input from the outside are detected, and the ratio of the former voltage and the latter voltage are held constant. Accordingly, input impedance can be kept constant. Further, impedance conversion is performed in the DC-DC converter. Thus, even when the battery in which power feeding is performed exists on an output side of the DC-DC converter, input impedance can be kept constant. Consequently, power can be supplied to a power receiving device including the DC-DC converter and the battery with high power feeding efficiency by a magnetic resonance method.

Abstract: An object is to provide a power feeding system and a power feeding method which are more convenient for a power feeding user at the power receiving end. An object is to provide a power feeding system and a power feeding method which also allow a power feeding provider (a company) which feeds power (at the power transmitting end) to supply power without waste. A power feeding device which wirelessly supplies power to a power receiver detects the position and the resonant frequency of the power receiver to be supplied with power, and controls the frequency of a power signal to be transmitted to the power receiver on the basis of the information. An efficient power feeding service can be offered by transmitting a power signal to the power receiver at an optimum frequency for high power transmission efficiency.

Abstract: An object is to provide a power feeding device, a power feeding system, and a power feeding method which are more convenient for a power feeding user at the power receiving end. The power feeding device includes a means of controlling a frequency of a power signal transmitted to a power receiver, based on a proportion of signals, among power signals output to an antenna circuit, that return from the power receiver to the antenna circuit without feeding power to the power receiver.

Abstract: Not a structure in which a resonance frequency of a power transmission device is set after a resonance frequency of a power receiving device is directly measured but a structure in which the resonance frequencies of the power receiving device and the power transmission device are estimated after reflection of an electromagnetic field for transmitting electric power to the power receiving device is monitored by the power transmission device is employed. After a capacitance value of a variable capacitor in a resonance coil of the power receiving device is once set to 0, an S11 parameter is detected while a frequency of an electromagnetic wave is changed, and the resonance frequency of the power transmission device is estimated on the basis of the S11 parameter.

Abstract: An object is to provide a power feeding device, a power feeding system, and a power feeding method which are more convenient for a power feeding user at the power receiving end. The power feeding device includes a means of controlling a frequency of a power signal transmitted to a power receiver, based on a proportion of signals, among power signals output to an antenna circuit, that return from the power receiver to the antenna circuit without feeding power to the power receiver.

Abstract: To provide a power feeding system and the like with which charging can be performed without a decrease in the power supply efficiency. To provide a power feeding system and the like with which can offer a power feeding service which is efficient to both a power feeding user and a power feeding provider. The power transmission state in each of power transmitting portions is monitored, the power transmitting portion having the highest power transmission efficiency is selected based on positional advantage, and the power transmitting resonance coil included in the selected power transmitting portion is kept at a first resonance frequency, whereby power transmission continues. The resonance frequency of the power transmitting resonance coil included in the non-selected power transmitting portion (the number of the non-selected power transmitting portions may be plural) is set to a second resonance frequency, whereby power transmission is stopped.

Abstract: A modulation circuit includes a load and a transistor serving as a switch. The transistor has an oxide semiconductor layer in which hydrogen concentration is 5×1019/cm3 or less. The off-state current of the transistor is 1×10?13 A or less. A modulation circuit includes a load, a transistor serving as a switch, and a diode. The load, the transistor, and the diode are connected in series between the terminals of an antenna. The transistor has an oxide semiconductor layer in which hydrogen concentration is 5×1019/cm3 or less. An off-state current of the transistor is 1×10?13 A or less. On/off of the transistor is controlled in accordance with a signal inputted to a gate of the transistor. The load is a resistor, a capacitor, or a combination of a resistor and a capacitor.

Abstract: A contactless power feeding system includes a power transmitting device and a power receiving device. The power transmitting device includes a first AC power source configured to generate an AC power with a first frequency, a second AC power source configured to generate an AC power with a second frequency which is different from the first frequency, a first electromagnetic induction coil, and a first resonant coil. The power receiving device includes a second resonant coil, a second electromagnetic induction coil, and a power storage unit. Power is wirelessly supplied to the power storage unit at the second frequency by a magnetic resonance phenomenon which occurs between the first resonant coil and the second resonant coil.

Abstract: The frequency of high-frequency voltage which is output by a variable high frequency power source included in a power transmitting device is controlled in accordance with the value of electric power received by a power receiving device. That is to say, the frequency of the high-frequency voltage is controlled in accordance with data directly relating to power supply. Thus, electric power is accurately supplied with high transmission efficiency in the power supply system.

Abstract: Provided is a power receiving device in which supply of power from a power supply device can be stopped while a reduction in Q-value is suppressed. The power receiving device includes a first antenna which forms resonant coupling with an antenna of the power supply device; a second antenna which forms electromagnetic induction coupling with the first antenna; a rectifier circuit including a plurality of switches and performing a first operation or a second operation depending on whether the plurality of switches is ON or OFF, the first operation being an operation in which voltage applied from the second antenna is rectified to be outputted, and the second operation being an operation in which a pair of power supply points is short-circuited; a load to which the voltage is applied; and a control circuit which generates a signal for selecting ON or OFF of the plurality of switches.

Abstract: A contactless power supply device capable of appropriately supplying power to a plurality of power supplied devices at the same time without contact is provided. A contactless power supply device capable of supplying power without contact even when the positional flexibility of a plurality of power supplied devices is increased is provided. The contactless power supply device determines a power supply condition that power can be most appropriately supplied to a plurality of power supplied devices that need power supply in a short time. The contactless power supply device can appropriately supply power to a plurality of power supplied devices at the same time without contact by controlling the resonant capacitance of a power transmission coil in an optimized power supply condition. The positional flexibility of the plurality of power supplied devices with respect to the contactless power supply device can be increased.

Abstract: A contactless power feeding system includes a power transmitting device and a power receiving device. The power transmitting device includes a first AC power source configured to generate an AC power with a first frequency, a second AC power source configured to generate an AC power with a second frequency which is different from the first frequency, a first electromagnetic induction coil, and a first resonant coil. The power receiving device includes a second resonant coil, a second electromagnetic induction coil, and a power storage unit. Power is wirelessly supplied to the power storage unit at the second frequency by a magnetic resonance phenomenon which occurs between the first resonant coil and the second resonant coil.

Abstract: Provided is a power receiving device in which supply of power from a power supply device can be stopped while a reduction in Q-value is suppressed. The power receiving device includes a first antenna which forms resonant coupling with an antenna of the power supply device; a second antenna which forms electromagnetic induction coupling with the first antenna; a rectifier circuit including a plurality of switches and performing a first operation or a second operation depending on whether the plurality of switches is ON or OFF, the first operation being an operation in which voltage applied from the second antenna is rectified to be outputted, and the second operation being an operation in which a pair of power supply points is short-circuited; a load to which the voltage is applied; and a control circuit which generates a signal for selecting ON or OFF of the plurality of switches.

Abstract: The frequency of high-frequency voltage which is output by a variable high frequency power source included in a power transmitting device is controlled in accordance with the value of electric power received by a power receiving device. That is to say, the frequency of the high-frequency voltage is controlled in accordance with data directly relating to power supply. Thus, electric power is accurately supplied with high transmission efficiency in the power supply system.