Patent ID: 12230977

DESCRIPTION OF THE EMBODIMENTS

An aspect of the invention provides a contactless power feed apparatus including a power transmission device and a power reception device contactlessly receiving power from the power transmission device. In the contactless power feed apparatus, the power reception device includes: a resonant circuit, having a reception coil receiving power from the power transmission device; an estimation circuit, estimating a load resistance of an entirety of the contactless power feed apparatus; and a reception-side communicator, transmitting an estimated value of the load resistance of the entirety of the contactless power feed apparatus to the power transmission device.

In addition, the power transmission device includes: a transmission coil, supplying power to the power reception device; a power supply circuit, including: a transmission-side DC-DC converter converting a voltage of DC power supplied from a DC power source; and a DC-AC converter, converting the DC power output from the transmission-side DC-DC converter into AC power, the power supply circuit supplying the converted AC power to the transmission coil; a transmission-side communicator, receiving the estimated value of the load resistance of the entirety of the contactless power feed apparatus from the reception-side communicator, and a control circuit, controlling the transmission-side DC-DC converter, so that, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is less than a predetermined allowed lower limit, an input/output gain of the transmission-side DC-DC converter is decreased by a first predetermined amount and, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is equal to or greater than the predetermined allowed lower limit, the input/output gain of the transmission-side DC-DC converter is maintained or increased by a second predetermined amount.
According to such configuration, the contactless power feed apparatus can suppress the deterioration of the power transmission efficiency even in the case where the load connected with the device on the power reception side changes.

In the contactless power feed apparatus, it may also be that the power transmission device further includes: an additional resistor connected in series with the transmission coil; and an additional resistor switch connected in parallel with the additional resistor, and the control circuit of the power transmission device short-circuits the additional resistor switch when power transmission from the power transmission device to the power reception device is started and, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is less than the predetermined allowed lower limit even though the input/output gain of the transmission-side DC-DC converter is decreased to a lower limit of an operation range of the transmission-side DC-DC converter, the control circuit cuts off the additional resistor switch.

With such configuration, in the contactless power feed apparatus, even if the load resistance of the power reception device increases, the load resistance of the entire contactless power feed apparatus can be equal to or greater than the allowed lower limit. As a result, even if the load resistance of the power reception device increases, the contactless power feed apparatus can suppress the deterioration of the power transmission efficiency.

In the contactless power feed apparatus, it may also be that the control circuit of the power transmission device transmits information representing a voltage of the DC power output from the transmission-side DC-DC converter to the power reception device via the transmission-side communicator. In addition, it may also be that the power reception device further includes: a rectifying and smoothing circuit, rectifying and converting AC power output from the resonant circuit into DC power, and outputting the DC power; and a voltmeter measuring a voltage of the DC power output from the rectifying and smoothing circuit. In addition, it may also be that the estimation circuit of the power reception device estimates a load resistance of the power reception device based on the voltage of the DC power measured by the voltmeter, and calculates the estimated value of the load resistance of the entirety of the contactless power feed apparatus based on an estimated value of the load resistance of the power reception device and the information representing the voltage of the DC power output from the transmission-side DC-DC converter, transmitted via the transmission-side communicator, and received via the reception-side communicator.

With the above procedure, the contactless power feed apparatus can accurately estimate the load resistance of the entire contactless power feed apparatus.

In such case, it may also be that the power reception device further includes an ammeter measuring a current of the DC power output from the rectifying and smoothing circuit. In addition, it may also be that the estimation circuit of the power reception device estimates a mutual inductance between the transmission coil and the reception coil based on the current of the DC power measured by the ammeter and the information representing the voltage of the DC power output from the transmission-side DC-DC converter, and calculates the estimated value of the load resistance of the entirety of the contactless power feed apparatus based on the mutual inductance that is estimated and the estimated value of the load resistance of the power reception device.

With the above procedure, the contactless power feed apparatus can accurately estimate the load resistance of the entire contactless power feed apparatus.

In the contactless power feed apparatus, it may also be that, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is less than the predetermined allowed lower limit, the control circuit of the power transmission device sets the first predetermined amount in accordance with the estimated value of the load resistance of the entirety of the contactless power feed apparatus and an input/output gain currently applied to the transmission-side DC-DC converter.

With such configuration, the contactless power feed apparatus can reduce the time required for the load resistance of the entire contactless power feed apparatus to reach or exceed the predetermined allowed lower limit.

Another aspect of the invention provides a contactless power feed apparatus including a power transmission device and a power reception device contactlessly receiving power from the power transmission device. In the contactless power feed apparatus, the power reception device includes: a resonant circuit, having a reception coil receiving power from the power transmission device; a rectifying and smoothing circuit, rectifying and converting AC power output from the resonant circuit into DC power, and outputting the DC power; a voltmeter, measuring a voltage of the DC power output from the rectifying and smoothing circuit; and a reception-side communicator, transmitting output information to the transmission device, the output information representing an output of the DC power and including a measurement value of the voltage of the DC power output from the rectifying and smoothing circuit.

In addition, the power transmission includes: a transmission coil, supplying power to the power reception device; a power supply circuit, including: a transmission-side DC-DC converter converting a voltage of DC power supplied from a DC power source; and a DC-AC converter, converting the DC power output from the transmission-side DC-DC converter into AC power, the power supply circuit supplying the converted AC power to the transmission coil; and a transmission-side communicator, receiving the output information from the reception-side communicator; and a control circuit, estimating a load resistance of an entirety of the contactless power feed apparatus based on the output information, and controlling the transmission-side DC-DC converter, so that, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is less than a predetermined allowed lower limit, an input/output gain of the transmission-side DC-DC converter is decreased by a first predetermined amount and, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is equal to or greater than the predetermined allowed lower limit, the input/output gain of the transmission-side DC-DC converter is maintained or increased by a second predetermined amount.
According to such configuration, the contactless power feed apparatus can suppress the deterioration of the power transmission efficiency even in the case where the load connected with the device on the power reception side changes.

Yet another aspect of the invention provides a power transmission method for a contactless power feed apparatus including a power transmission device and a power reception device contactlessly receiving power from the power transmission device. According to the power transmission method, the power reception device includes: a resonant circuit, having a reception coil receiving power from the power transmission device; a rectifying and smoothing circuit, rectifying and converting AC power output from the resonant circuit into DC power, and outputting the DC power; an estimation circuit, estimating a load resistance of an entirety of the contactless power feed apparatus; and a reception-side communicator, transmitting an estimated value of the load resistance of the entirety of the contactless power feed apparatus to the power transmission device. In addition, the power reception device includes: a transmission coil, supplying power to the power reception device; a power supply circuit, including: a transmission-side DC-DC converter converting a voltage of DC power supplied from a DC power source; and a DC-AC converter, converting the DC power output from the transmission-side DC-DC converter into AC power, the power supply circuit supplying the converted AC power to the transmission coil; and a transmission-side communicator, receiving the estimated value of the load resistance of the entirety of the contactless power feed apparatus from the reception-side communicator.

In addition, the power transmission method includes: controlling the transmission-side DC-DC converter, so that, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is less than a predetermined allowed lower limit, an input/output gain of the transmission-side DC-DC converter is decreased by a first predetermined amount; and controlling the transmission-side DC-DC converter, so that, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is greater than or equal to the predetermined allowed lower limit, the input/output gain of the transmission-side DC-DC converter is maintained.
According to such procedures, the power transmission method can suppress the deterioration of the power transmission efficiency even in the case where the load connected with the device on the power reception side changes.

Still another aspect of the invention provides a power transmission method for a contactless power feed apparatus including a power transmission device and a power reception device contactlessly receiving power from the power transmission device. According to the power transmission method, the power reception device includes: a resonant circuit, having a reception coil receiving power from the power transmission device; a rectifying and smoothing circuit, rectifying and converting AC power output from the resonant circuit into DC power, and outputting the DC power; a voltmeter, measuring a voltage of the DC power output from the rectifying and smoothing circuit; and a reception-side communicator, transmitting output information to the transmission device, the output information representing an output of the DC power and including a measurement value of the voltage of the DC power output from the rectifying and smoothing circuit. In addition, the power reception device includes: a transmission coil, supplying power to the power reception device; a power supply circuit, including: a transmission-side DC-DC converter converting a voltage of DC power supplied from a DC power source; and a DC-AC converter, converting the DC power output from the transmission-side DC-DC converter into AC power, the power supply circuit supplying the converted AC power to the transmission coil; and a transmission-side communicator, receiving the output information from the reception-side communicator.

In addition, the power transmission method includes: estimating a load resistance of an entirety of the contactless power feed apparatus based on the output information; controlling the transmission-side DC-DC converter, so that, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is less than a predetermined allowed lower limit, an input/output gain of the transmission-side DC-DC converter is decreased by a first predetermined amount; and controlling the transmission-side DC-DC converter, so that, in a case where the estimated value of the load resistance of the entirety of the contactless power feed apparatus is greater than or equal to the predetermined allowed lower limit, the input/output gain of the transmission-side DC-DC converter is maintained or increased by a second predetermined amount.

According to such procedures, the power transmission method can suppress the deterioration of the power transmission efficiency even in the case where the load connected with the device on the power reception side changes.

In the following, a contactless power feed apparatus according to an embodiment of the invention and a power transmission method executed by the contactless power feed apparatus are described with reference to the drawings. The contactless power feed apparatus drives a switching element of a power supply circuit supplying AC power to a coil on a power transmission side by using a switching frequency included in an ISM band with few usage limitations. In addition, the inventors have found that, regarding the switching element of the power supply circuit, even if ZDS is not achieved, if ZVS is achieved, and the difference between the timing at which the switching element is changed from OFF to ON and the timing of becoming the voltage applied to the switching element is sufficiently small, even if the switching element is driven by using a switching frequency included in the ISM band, the switching loss of the switching element can be reduced to an extent that has no problem in practical use.

Therefore, the contactless power feed apparatus estimates the load resistance of the entire contactless power feed apparatus, and controls the voltage of the AC power supplied from the power supply circuit on the power transmission side to the coil on the power transmission side in accordance with the estimated load resistance. Accordingly, it is possible for the contactless power feed apparatus to achieve ZVS and make the difference between the timing at which the switching element changes from OFF to ON and the timing of becoming the voltage applied to the switching element sufficiently small, the switching loss in the switching element can be reduced, and the deterioration of the power transmission efficiency can be suppressed.

FIG.1is a schematic diagram illustrating a configuration of a contactless power feed apparatus according to an embodiment of the invention. As shown inFIG.1, the contactless power feed apparatus1has a power transmission device2and a power reception device3to which power is transmitted contactlessly via space from the power transmission device2. The power transmission device2has a power supply circuit11, a transmission coil12, a capacitor13, an additional resistor14, an additional resistor switch15, a thermometer16, a communicator17, and a control circuit18. Meanwhile, the power reception device3has a resonant circuit20including a reception coil21and a resonant capacitor22, a rectifying and smoothing circuit23, a reception-side DC-DC converter24, a voltmeter25, an ammeter26, a communicator27, and an estimation circuit28. In addition, the power reception device3is connected with the load circuit4. The load circuit4is, for example, a secondary battery, a charging circuit of a secondary battery, or a circuit operating by using DC power. The power reception device3receives power transmitted via the transmission coil12of the power transmission device2by using the reception coil21, and coverts the received power into DC power by using the rectifying and smoothing circuit23. The power converted into DC is output to the load circuit4via the reception-side DC-DC converter24.

Firstly, the power transmission device2is described.

The power supply circuit11supplies the AC power having an adjustable voltage to the transmission coil12. Therefore, the power supply circuit11has a transmission-side DC-DC converter31and a DC-AC converter32.

The transmission-side DC-DC converter31is a DC-DC converter able to adjust an input/output gain in accordance with the control of the control circuit18, such as a DC-DC converter of a step-up type, a step-down type, or a step-up/down type DC-DC converter. In addition, the transmission-side DC-DC converter31is connected with a DC power source10on the input side thereof, converts the voltage of the DC power supplied from the DC power source10into a voltage in accordance with the input/output gain set in accordance with the control of the control circuit18, and outputs the converted voltage.

The DC-AC converter32converts the DC power output from the transmission-side DC-DC converter31into AC power, and supplies the converted AC power to the transmission coil12. Therefore, the DC-AC converter32has a coil33, a capacitor34, and a switching element35.

The coil33is connected between the output terminal on the positive electrode side of the transmission-side DC-DC converter31and an end of the transmission coil12. In addition, an end of the capacitor34is connected between the coil33and an end of the transmission coil12, the other end of the capacitor34is connected with the output terminal of the negative electrode side of the transmission-side DC-DC converter31. In addition, the coil33and the capacitor34convert the DC power output from the transmission-side DC-DC converter31into AC power having a switching frequency at which the switching element35is switched ON and OFF.

The switching element35is connected between the capacitor34and the coil33so as to be parallel with the capacitor34. That is, an end of the switching element35is connected with the output terminal on the positive electrode side of the transmission-side DC-DC converter31via the coil33, and the other end of the switching element35is connected with the output terminal on the negative electrode side of the transmission-side DC-DC converter31.

In addition, the switching element35is an element able to switch ON and OFF by using a switching frequency included in the ISM band. For example, the switching element17can be a field effect transistor formed of gallium nitride (GaN FET). The switching terminal (e.g., the gate terminal of the GaN FET) of the switching element35is connected with the control circuit18, and is switched ON and OFF in accordance with a control signal from the control circuit18. As described above, with the switching element35being switched ON and OFF by using the switching frequency, the DC power output from the transmission-side DC-DC converter31is converted into the AC power having the switching frequency by the coil33and the capacitor34. The converted AC power is then output to the transmission coil12.

The transmission coil12generates a magnetic field that changes periodically around the transmission coil12in accordance with the AC power supplied from the power supply circuit11. In addition, the transmission coil12transmits the AC power to the resonant circuit20of the power reception device3via the magnetic field.

The capacitor13is connected in series with the transmission coil12between an output terminal on the positive electrode side of the power supply circuit11and an end of the transmission coil12. In addition, the AC power output from the power supply circuit11is supplied to the transmission coil12via the capacitor13.

The additional resistor14is connected in series with the transmission coil12between the other end of the transmission coil12and the output terminal on the negative electrode side of the power supply circuit11. In addition, the additional resistor14is used for adjusting the load resistance of the entire contactless power feed apparatus1. The resistance value of the additional resistor14may be equal to or greater than a allowed lower limit R1to be described afterwards.

The additional resistor switch15is connected in parallel with the additional resistor14between the other end of the transmission coil12and the output terminal on the negative electrode side of the power supply circuit11. That is, when the additional resistor switch15is turned ON and short-circuited, the DC power flows without going through the additional resistor14. Meanwhile, when the additional resistor switch15is turned OFF and cut off, the DC power flows via the additional resistor14. The switching of ON/OFF of the additional resistor switch15is controlled by the control circuit18. The additional resistor switch15can be arranged as a relay switch. In addition, the additional resistor14and the additional resistor switch15may also be connected in parallel between the transmission coil12and the capacitor13.

The thermometer16is provided near the switching element35of the power supply circuit11and measures the temperature of the switching element35. In addition, the thermometer16outputs the measurement value of the temperature of the switching element35to the control circuit18.

The communicator17is an example of the transmission-side communicator, and is configured to be able to perform radio communication with the communicator27of the power reception device3. Therefore, the communicator17, for example, has an antenna receiving a radio signal under a predetermined radio communication standard and a communication circuit demodulating such radio signal. Example of the predetermined radio communication standard can include, for example, ISO/IEC 15693, ZigBee (registered trademark) or Bluetooth (registered trademark). When it is possible to communicate with the communicator27of the power reception device3, the communicator17receives, from the control circuit18, voltage information representing the voltage of the AC power output from the power supply circuit11to the transmission coil12. In addition, the communicator17generates a radio signal including such voltage information, and transmits the voltage information to the power reception device3by outputting such radio signal. In addition, when receiving the radio signal from the communicator27of the power reception device3, the communicator17passes the estimated value of the load resistance of the entire contactless power feed apparatus1included in the radio signal to the control circuit18. Details of the load resistance of the entire contactless power feed apparatus1will be described afterwards.

The control circuit18has a processor, a memory, a gate driver, and a communication interface. In addition, the control circuit18switches ON and OFF of the switching element35of the power supply circuit11via the gate driver by using the switching frequency (e.g., 6.78 MHz or 13.56 MHz) included in the ISM band and the predetermined duty ratio (e.g., 0.5). In addition, the control circuit18controls the transmission-side DC-DC converter31and the switching of ON/OFF of the additional resistor switch15based on the estimated value of the load resistance of the entire contactless power feed apparatus1received from the power reception device3via the communicator17. In addition, the control circuit18switches ON/OFF of the additional resistor switch15based on the measurement value of the temperature of the switching element35measured by the thermometer16. Moreover, the control circuit18outputs the voltage information representing the voltage of the DC power output from the transmission-side DC-DC converter31. The control circuit18may obtain the voltage of the DC power output from the transmission-side DC-DC converter31based on the voltage of the DC power source10and a constant of each circuit element of the power supply circuit11stored in advance in the memory and the input/output gain currently applied to the transmission-side DC-DC converter31. Alternatively, a voltmeter (not shown) for measuring the voltage of the DC power output from the transmission-side DC-DC converter31may also be provided between the transmission-side DC-DC converter31and the DC-AC converter32. In addition, the control circuit18may also include the measurement value obtained by such voltmeter in the voltage information. Details about the control of the control circuit18will be described afterwards.

Then, the power reception device3is described.

The resonant circuit20is an LC resonant circuit in which the reception coil21and the resonant capacitor22are connected in series. In addition, an end of the reception coil21provided in the resonant circuit20is connected with an input terminal of the rectifying and smoothing circuit23via the resonant capacitor22. In addition, the other end of the reception coil21is connected with the other input end of the rectifying and smoothing circuit23. However, the resonant circuit20is not limited to the example, and may also be an LC parallel resonant circuit in which the reception coil21and the resonant capacitor22are connected in parallel.

By resonating with the AC current flowing in the transmission coil12of the power transmission device2together with the resonant capacitor22, the reception coil21receives power from the transmission coil12. In addition, the reception coil21outputs the received power to the rectifying and smoothing circuit23via the resonant capacitor22. That is, the inductance of the reception coil21and the capacitance of the resonant capacitor22are set so that the resonant frequency of the resonant circuit20is substantially equal to the switching frequency. The number of turns of the reception coil21may be the same as or different from the number of turns of the transmission coil12of the power transmission device2.

The resonant capacitor22is connected in series with the reception coil21. That is, an end of the resonant capacitor22is connected with an end of the reception coil21, and the other end of the resonant capacitor22is connected with the rectifying and smoothing circuit23. In addition, the resonant capacitor22outputs the AC power received by resonating with the reception coil21to the rectifying and smoothing circuit23.

The rectifying and smoothing circuit23, for example, is formed by a full-wave rectifying circuit having four bridge-connected diodes and a smoothing capacitor. One of the two terminals on the input side of the full-wave rectifying circuit is connected with the resonant capacitor22, and the other of the two terminals on the input side are connected with the reception coil21. In addition, one of the two terminals on the output side of the full-wave rectifying circuit is connected with an end of the smoothing capacitor, and the other of the two terminals on the output side is connected with the other end of the smoothing capacitor. In addition, the rectifying and smoothing circuit23rectifies and converts the AC power output from the resonant circuit20into DC power. It is noted that, in place of a full-wave rectifying circuit, the rectifying circuit23may also include a half-wave rectifying circuit.

The reception-side DC-DC converter24is connected with the output side of the rectifying and smoothing circuit23, and steps up or down the voltage of the DC power output from the reception-side DC-DC converter24. In addition, the reception-side DC-DC converter24outputs the DC voltage that is stepped up or down to the load circuit4. The reception-side DC-DC converter24may be a step-up type DC-DC converter. By arranging the reception-side DC-DC converter24as a step-up type DC-DC converter, it is easy to increase the voltage output from the power reception device3to the load circuit4. Therefore, it is easy to increase the load resistance of the entire contactless power feed apparatus1. As a result, it is easy to achieve ZVS in the power supply circuit11of the power transmission device2. Details of the estimation of the load resistance of the entire contactless power feed apparatus1and the control for achieving ZVS will be described afterwards. In addition, in the case that the load circuit4can sufficiently operate at the voltage in accordance with the AC power received from the power reception device3, the reception-side DC-DC converter24may be omitted.

The voltmeter25is connected between the rectifying and smoothing circuit23and the reception-side DC-DC converter24and measures a voltage (referred to as output voltage in the following) of the DC power output from the rectifying and smoothing circuit23. In addition, the voltmeter25outputs the measurement value of the output voltage to the estimation circuit28.

The ammeter26is connected between the rectifying and smoothing circuit23and the reception-side DC-DC converter24or between the reception-side DC-DC converter24and the load circuit4, and measures a current (referred to as output current in the following) of the DC power output from the rectifying and smoothing circuit23. In addition, the ammeter26outputs the measurement value of the output current to the estimation circuit28.

The communicator27is an example of the reception-side communicator, and is configured to be able to perform radio communication with the communicator17of the power transmission device2. Therefore, the communicator27has an antenna receiving a radio signal under the radio communication standard followed by the communicator17and a communication circuit demodulating the radio signal. When it is possible to communicate with the communicator17of the power transmission device2, the communicator27passes the voltage information included in the radio signal received from the communicator17to the estimation circuit28. In addition, when receiving the estimated value of the load resistance of the entire contactless power feed apparatus1from the estimation circuit28, the communicator27generates a radio signal including the estimated value and outputs the radio signal, thereby transmitting the estimated value of the load resistance of the entire contactless power feed apparatus1to the power transmission device2.

The estimation circuit28has a processor, a memory, and a communication interface. In addition, the estimation circuit28estimates the load resistance of the entire contactless power feed apparatus1based on the measurement value obtained from the voltmeter25via the communication interface, the measurement value of the output current obtained from the ammeter26via the communication interface, and the voltage information obtained from the communicator27via the communication interface. In addition, the estimation circuit28notifies the power transmission device of the estimated value of such load resistance by outputting the estimated value of the load resistance of the entire contactless power feed apparatus1to the communicator27.

In the following, the operation of the contactless power feed apparatus1is described in detail.

FIG.2is an equivalent circuit diagram of the contactless power feed apparatus1. In an equivalent circuit100shown inFIG.2, the transmission-side DC-DC converter31and the reception-side DC-DC converter24of the power reception device3are respectively omitted. In addition, the equivalent circuit100is an equivalent circuit of the contactless power feed apparatus1when the additional resistor switch15is short-circuited. Here, Lf, Ltx, and Lrx are respectively the inductances of the coil33, the transmission coil12, and the reception coil21, and Csh, Cs, and C2are respectively the capacitances of the capacitor34, the capacitor13, and the resonant capacitor22. In addition, Rac is a load resistance of the power reception device3. In addition, M is the mutual inductance between the transmission coil12and the reception coil21.

In the case where the resonant circuit20of the power reception device3is resonating, the equivalent circuit100can be rewritten as an equivalent circuit200or an equivalent circuit300. At this time, an impedance Z of the entire contactless power feed apparatus1, that is, the impedance observed from the side of the power transmission device2during power transmission is represented as follows:

[Equation⁢1]1/Z=1/j⁢ω⁢M+1/(Rac-j⁢ω⁢M)=Rac/(ω2⁢M2+j⁢ω⁢MRac)⁢Z=Ro+j⁢ω⁢M⁢Ro=(ω·M)2Rac⁢M=k·LTX·LRX(1)
ω(=2πf) is an angular frequency corresponding to a frequency f (that is, the switching frequency) of the AC power supplied to the transmission coil12. In addition, k is a coupling degree between the transmission coil12and the reception coil21. In addition, Ro is an equivalent load resistance connected with respect to the entire contactless power feed apparatus1, that is, a load resistance observed from the side of the power transmission device2(simply referred to as the load resistance of the entire contactless power feed apparatus1) during power transmission.

As can be shown from Equation (1), when the load resistance Rac of the power reception device3decreases, the load resistance Ro of the entire contactless power feed apparatus1increases. Comparatively, when the load of the power reception device3decreases and the load resistance Rac increases, the load resistance Ro of the entire contactless power feed apparatus1decreases. Moreover, when the load resistance Ro becomes excessively small, ZVS is no longer achieved, or, compared with the timing at which the switching element35becomes ON, the timing at which the voltage applied to the switching element35becomes zero is too early. When the timing at which the voltage applied to the switching element35becomes zero is too early compared with the timing at which the switching element35becomes ON, a loss due to the current flowing in the body diode of the switching element35occurs. Details in this regard may be referred to, for example, Chapter 2 of “Load-Independent Class-E Power Conversion”, available at https://vtechworks.lib.vt.edu/handle/10919/97601 and published in 2020.

Therefore, the estimation circuit28of the power reception device3estimates the load resistance Ro of the entire contactless power feed apparatus1and notifies the power transmission device2of the estimated value via the communicator27. The control circuit18of the power transmission device2controls the transmission-side DC-DC converter31and the additional resistor switch15so that ZVS can be achieved and the difference (simply referred to as timing difference in the following) of the timing at which the voltage applied to the switching element35becomes zero with respect to the timing at which the switching element35becomes ON is equal to or greater than the allowed lower limit R1, which is a level that has no problem in practical use.

Firstly, the process performed by the estimation circuit28of the power reception device3is described. The estimation circuit28estimates the load resistance Rac of the power reception device3for estimating the load resistance Ro of the entire contactless power feed apparatus1. In addition, the estimation circuit28estimates the coupling degree between the transmission coil12and the reception coil21based on the load resistance Rac of the power reception device3, the voltage information received from the power transmission device, and the output voltage measured by the voltmeter25. In addition, the estimation circuit28estimates the load resistance Ro of the entire contactless power feed apparatus1based on the estimated coupling degree, the inductances of the transmission coil12and the reception coil21, and the switching frequency.

Firstly, regarding the estimation of the load resistance Rac of the power reception device3, the estimation circuit28estimates the load resistance Rac in accordance with the following equation:

[Equation⁢2]Rac=VoIo·8π2(2)
Here, Vo is a measurement value of the output voltage obtained by the voltmeter25, and Io is a measurement value of the current obtained by the ammeter26. Since the load resistance Rac is an AC resistance value, Vo/Io is multiplied by 8/π2. Also, when the load circuit4includes a charging circuit of a secondary battery, in order to manage the charging state of the secondary battery, the power output by the charging circuit from the power reception device3to the load circuit4is monitored. In such case, the estimation circuit28may also estimate the load resistance Rac in accordance with the equation as follows, in place of Equation (2).

[Equation⁢3]Rac=Vo2Wo·8π2(3)
Here, Wo is a current value output from the power reception device3to the load circuit4. The estimation circuit28may receive the information representing such power value from the load circuit4. In such case, the ammeter26may be omitted.

In the following, the estimation of the coupling degree between the transmission coil12and the reception coil21is described. In the memory of the estimation circuit28, a reference table is stored in advance for each of multiple representative values of the load resistance Rac. The reference table represents the coupling degree of each ratio of the voltage Vy output from the rectifying and smoothing circuit23of the power reception device3with respect to the voltage Vx output from the transmission-side DC-DC converter31of the power transmission device2. Among the representative values of the load resistance Rac, the estimation circuit28specifies a representative value closest to the value of the load resistance Rac estimated in accordance with Equation (2) or (3), and specifies the reference table corresponding to the specified represented value. In addition, the estimation circuit28obtains the voltage Vy output from the resonant circuit20based on the output voltage Vo measured by the voltmeter25. In addition, the estimation circuit28calculates the ratio of the voltage Vy output from the resonant circuit20with respect to the voltage Vx output from the transmission-side DC-DC converter31included in the voltage information received from the power transmission device2. By referring to the specified reference table, the estimation circuit28specifies the coupling degree corresponding to the calculated ratio, and estimates the specified coupling degree as an actual coupling degree k between the transmission coil12and the reception coil21.

When estimating the coupling degree k between the transmission coil12and the reception coil21, the estimation circuit28estimates the load resistance Ro of the entire contactless power feed apparatus1according to the following equation based on the estimated coupling degree k and the estimated value of the load resistance Rac.

[Equation⁢4]Ro=(ω·M)2Rac⁢M=k·LTX·LRX(4)
As in the above, ω(=2πf) is an angular frequency corresponding to the frequency f of the AC power supplied to the transmission coil12. In addition, M is a mutual inductance between the transmission coil12and the reception coil21. In addition, Ltx and Lrx are respectively the inductances of the transmission coil12and the reception coil21. The estimation circuit28can calculate the estimated value of the load resistance Ro of the entire contactless power feed apparatus1by applying the value of (Ltx×Lrx)1/2and the value of w stored in advance in the memory of the estimation circuit28as well as the estimated coupling degree k and load resistance Rac to Equation (4). With the above procedure, the estimation circuit28can accurately estimate the load resistance Ro of the entire contactless power feed apparatus1.

The mutual inductance M may be calculated according to an equation as follows. In such case, the estimation circuit28can calculate the mutual inductance M and the load resistance Ro of the entire contactless power feed apparatus1without referring to the reference table.

[Equation⁢5]M=a·π·VxIo·2·2·ω(5)
However, Vx is a voltage output from the transmission-side DC-DC converter31as described above. In addition, a is a constant of each circuit element from the DC-AC converter32to the transmission coil12and a constant determined by the duty ratio of the switching element35of the DC-AC converter32. That is, aVx represents the actual value of the voltage output from the transmission coil12in the equivalent circuit200shown inFIG.2. The constant a is stored in advance in the memory of the estimation circuit28. In the case where the control circuit18of the power transmission device2adjusts the duty ratio of the switching element35, the control circuit18of the power transmission device2may include the latest duty ratio applied to the switching element35in the voltage information. In addition, the estimation circuit28may apply the value of the constant a corresponding to the duty ratio included in the voltage information, among the constants a corresponding to the respective duty ratios stored in the memory in advance, to Equation (5).

In each predetermined period, the estimation circuit28estimates the load resistance Ro of the entire contactless power feed apparatus1and notifies the power transmission device2of the estimated value of the load resistance Ro via the communicator27.

It is noted that the estimation circuit28may turn OFF the reception-side DC-DC converter24when the value of the output power calculated from the measurement value of the output voltage Vo and the measurement value of the output current Io or notified by the load circuit4is equal to or less than a predetermined amount.

In the following, the process performed by the control circuit18of the power transmission device2is described. In an idling state until the power transmission from the power transmission device2to the power reception device3is started, the control circuit18cuts off the additional resistor switch15to allow current to flow to the additional resistor14. In addition, when the power transmission from the power transmission device2to the power reception device3is started, the control circuit18turns ON the additional resistor switch15, that is, the control circuit18short-circuits the additional resistor switch15. Accordingly, current does not flow to the additional resistor14. It is noted that, the control circuit18may determine that the power transmission from the power transmission device2to the power reception device3is started when the communication between the communicator17of the power transmission device2and the communicator27of the power reception device3is established.

Every time when the control circuit18receives the estimated value of the load resistance Ro of the entire contactless power feed apparatus1from the power reception device3, the control circuit18compares such estimated value with the allowed lower limit R1. In addition, if the estimated value of the load resistance Ro is less than the allowed lower limit R1, the control circuit18controls the transmission-side DC-DC converter31, so that the input/output gain of the transmission-side DC-DC converter31is decreased by a predetermined amount (first predetermined amount), that is, the voltage output from the transmission-side DC-DC converter31is decreased. Accordingly, the voltage of the AC power supplied to the transmission coil12is lowered, so the output voltage Vo and the output current Io in the power reception device3also change. Specifically, by decreasing the output voltage Vo together with the decrease of the voltage of the AC power supplied to the transmission coil12, the load resistance Rac on the power reception side decreases. As a result, the load resistance of the entire contactless power supply apparatus1increases. If the estimated value of the load resistance Ro is equal to or greater than the allowed lower limit R1, ZVS is achieved, and the timing difference becomes sufficiently small. Therefore, the control circuit18controls the transmission-side DC-DC converter31, so that the input/output gain of the transmission-side DC-DC converter31is maintained without change.

For example, the inductances of the coil33and the transmission coil12are respectively 1 μH and 3 μH, and the capacitances of the capacitor34and the capacitor13are respective 330 pF, 202 pF. In addition, the switching element35is driven at a switching frequency of 6.78 MHz and a duty ratio of 0.5. In this case, the minimum value of the load resistance Ro of the entire contactless power feed apparatus1at which ZVS is achieved and the difference between the timing at which the switching element is changed from OFF to ON and the timing of becoming the voltage applied to the switching element is 41.4Ω. Therefore, the allowed lower limit R1is set at a value obtained by adding a predetermined offset to 41.4Ω, such as 50Ω.

However, in the case where the load circuit4includes a secondary battery or a charging circuit of a secondary battery, since the output current Io is decreased as the secondary battery is approaching fully charged, the load resistance Rac of the power reception device3increases. As a result, even if the input/output gain of the transmission-side DC-DC converter31is decreased to the lower limit of the operation range, the load resistance Rac is not sufficiently decreased. As a result, the estimated value of the load resistance Ro of the entire contactless power feed apparatus1may be unable to reach or exceed the allowed lower limit R1. Therefore, in the case where the estimated value of the load resistance Ro is less than the allowed lower limit R1even if the control circuit18controls the transmission-side DC-DC converter31so that the input/output gain of the transmission-side DC-DC converter31reaches the lower limit of the operation range of the transmission-side DC-DC converter31, the additional resistor switch15is turned OFF. Accordingly, the current flows to the additional resistor14and, as a result, the voltage of the AC power applied to the transmission coil12is further decreased. Therefore, it is possible to further decrease the load resistance Rac of the power reception device3, that is, it is possible to further increase the load resistance Ro of the entire contactless power feed apparatus1.

In addition, when the measurement value of the temperature of the switching element35obtained by the thermometer16is equal to or greater than a predetermined upper temperature limit, the control circuit18turns OFF the additional resistor switch15. Accordingly, the current flows to the switching element35is reduced, and the switching element35is prevented from being excessively heated. In the case where the measurement value of the temperature of the switching element35repetitively reaches or exceeds the predetermined upper temperature limit, it is possible that a conductor is present in the vicinity of the transmission coil12, which may lead to hard switching in the switching element35. Therefore, it may also be that, in the case where the measurement value of the temperature of the switching element35repetitively reaches or exceeds the predetermined upper temperature limit for a predetermined number of times or more within a prescribed period, the transmission-side DC-DC converter31is turned OFF, and power transmission is stopped.

FIG.3is a flowchart illustrating an operation of a control process for power transmission executed by the control circuit18of the power transmission device2. It is noted that, during the period in which the control process for power transmission is executed, the control circuit18switches the ON/OFF of the switching element35of the power supply circuit11at a predetermined switching frequency included in the ISM band.

Before the power transmission from the power transmission device2to the power reception device3is started, the control circuit18turns OFF the additional resistor switch15and sets the input/output gain of the transmission-side DC-DC converter31at an initial value (Step S101).

The control circuit18determines whether the communication between the communicator17of the power transmission device2and the communicator27of the power reception device3is established (Step S102). In the case where the communication is not established (Step S102: NO), it is assumed that the power transmission has not started. Therefore, the control circuit18repeats the processes since Step S101after a predetermined time has passed.

When the communication between the communicator17of the power transmission device2and the communicator27of the power reception device3is established (Step S102: YES), the control circuit18turns ON the additional resistor switch15, so that the current does not flow to the additional resistor14(Step S103).

The control circuit18determines whether the communication between the communicator17of the power transmission device2and the communicator27of the power reception device3is continued (Step S104). If the communication is lost (Step5104: NO), the control circuit18executes the processes since Step S101again. Meanwhile, if the communication is continued (Step S104: YES), the control circuit18receives the estimated value of the load resistance Ro of the entire contactless power feed apparatus1from the communicator27of the power reception device3via the communicator17(Step S105). In addition, the control circuit18determines whether the estimated value of the load resistance Ro is equal to or greater than the allowed lower limit (Step S106). If the estimated value of the load resistance Ro is equal to or greater than the allowed lower limit (Step S106:YES), the control circuit18controls the transmission-side DC-DC converter31so as to maintain the input/output gain of the transmission-side DC-DC converter31(Step S107). In addition, the control circuit18repeats the processes since Step S104.

Meanwhile, if the estimated value of the load resistance Ro is less than the allowed lower limit (Step S106:NO), the control circuit18determines whether the input/output gain of the transmission-side DC-DC converter31reaches the lower limit of the operation range (Step S108). If the input/output gain of the transmission-side DC-DC converter31is greater than the lower limit of the operation range (Step S108: NO), the control circuit18controls the transmission-side DC-DC converter31, so as to decrease the input/output gain of the transmission-side DC-DC converter31by a predetermined amount (Step S109). In addition, the control circuit18repeats the processes since Step S104.

Meanwhile, if the input/output gain of the transmission-side DC-DC converter31reaches the lower limit of the operation range (Step S108: YES), the control circuit18turns OFF the additional resistor switch15, so that the current flows to the additional resistor14(Step S110). In addition, the control circuit18repeats the processes since Step S104.

As described above, the contactless power feed apparatus drives a switching element of a power supply circuit supplying AC power to a coil on a power transmission side by using a switching frequency included in an ISM band with few usage limitations. In addition, the contactless power feed apparatus estimates the load resistance of the entire contactless power feed apparatus. Therefore, the contactless power feed apparatus controls the voltage of the AC power supplied from the power supply circuit on the power transmission side to the coil on the power transmission side, so that the load resistance of the entire contactless power feed apparatus satisfies the condition that ZVS can be achieved and that the difference between the timing at which the switching element is changed from OFF to ON and the timing of becoming the voltage applied to the switching element can be sufficiently small. Accordingly, the contactless power supply apparatus can alleviate the switching loss in the switching element to suppress the deterioration of the power transmission efficiency.

According to a modified example, the control circuit18of the power transmission device2may also determine the adjustment amount of the input/output gain based on the input/output gain currently applied to the transmission-side DC-DC converter31and the estimated value of the load resistance Ro of the entire contactless power feed apparatus1received from the power reception device3. In such case, the memory of the control circuit18stores the reference table representing the adjustment amount (the predetermined amount in the above flowchart) for making the load resistance Ro of the entire contactless power feed apparatus1equal to or greater than the predetermined allowed lower limited value R1for each combination of the input/output gain of the transmission-side DC-DC converter31and the load resistance Ro of the entire contactless power feed apparatus1. In addition, the control circuit18may specify the adjustment amount corresponding to the combination of the input/output gain currently applied to the transmission-side DC-DC converter31and the estimated value of the load resistance Ro of the entire contactless power feed apparatus1received from the power reception device3by referring to such reference table. In addition, the control circuit18may decrease the input/output gain of the transmission-side DC-DC converter31by the specified adjustment amount. Accordingly, the control circuit18can reduce the time required until the load resistance Ro of the entire contactless power feed apparatus1reaches or exceeds the predetermined allowed lower limit R1. It is noted that, in the reference table, the time when the additional resistor switch15is short-circuited and the time when the additional resistor switch15is cut off may be prepared separately. In such case, the control circuit18may switch the reference table to be referred to in accordance with whether the additional resistor switch15is short-circuited or cut off.

In addition, it may also be that when the estimated value of the load resistance Ro of the entire contactless power feed apparatus1exceeds a predetermined threshold, the control circuit18increases the input/output gain of the transmission-side DC-DC converter31by a predetermined amount (second predetermined amount). Accordingly, the control circuit18can appropriately suppress the switching loss of the switching element35. The predetermined threshold is set at a value greater than the allowed lower limit by a predetermined maintaining offset value. In addition, the predetermined amount at the time of increasing the input/output gain may be the same as or different from the predetermined amount at the time of decreasing the input/output gain.

It is noted that the load resistance Rac of the power reception device3becomes constant through the configuration of the load circuit4. In such case, the load resistance Rac that is a constant value is stored in advance in the memory of the estimation circuit28. In addition, the estimation circuit28may also estimate the load resistance Ro of the entire contactless power feed apparatus1based on the load resistance Rac that is a constant value. In addition, in the case where the load resistance Rac of the power reception device3is constant, the estimation circuit28may also assume that the coupling degree between the transmission coil12and the reception coil21is the assumed minimum value when the power transmission from the power transmission device2to the power reception device3is executed. In addition, the estimation circuit28may also estimate the load resistance Ro of the entire contactless power feed apparatus1based on the assumed minimum value of the coupling degree.

According to another modified example, the control circuit18of the power transmission device2may also estimate the load resistance of the entire contactless power feed apparatus1. In such case, the inductance of each coil and the reference table used for estimating the load resistance of the entire contactless power feed apparatus1are stored in advance in the memory of the control circuit18. Moreover, the communicator27of the power reception device3generates a radio signal including output information and transmits the radio signal to the power transmission device2. The output information includes the measurement value of the output voltage obtained by the voltmeter25and the measurement value of the output current obtained by the ammeter26or the power value received from the load circuit4. The output information represents the output of the DC power from the rectifying and smoothing circuit23. In addition, the communicator17of the power transmission device2passes the output information included in the received radio signal to the control circuit18. The control circuit18may refer to the output information and estimate the load resistance of the entire contactless power feed apparatus1by executing the same processes as the processes of the estimation circuit28in the above embodiment. In addition, the control circuit18may exert control on the transmission-side DC-DC converter33and perform switching between short-circuiting and cutting off the additional resistor switch15based on the estimated load resistance of the entire contactless power feed apparatus1. In the modified example as well, the effects same as the effects of the above embodiment can be attained.

In addition, in the embodiment or each modified example, in the case where the communicator of the power transmission device and the communicator of the power reception device can be connected in a wired manner, a communication circuit able to perform wired communication of a signal including the voltage information and a signal including the estimated value of the load resistance of the entire contactless power feed apparatus may be provided.

Accordingly, a person skilled in the art can make various modifications within the scope of the invention to suit the embodiment.