Wireless power transmission apparatus and supply power control method of wireless power transmission apparatus

A wireless power transmission apparatus which is charged in a short charging time and prevents the shortening of the life of a secondary battery is provided. A wireless power transmission apparatus includes a current-voltage detector configured to measure an input impedance Zin of the wireless power transmission apparatus including the power-supplied device and a control device configured to determine whether a constant current charging period (CC) is finished by utilizing a change in the input impedance Zin measured by the current-voltage detector and terminate the charging of the lithium ion secondary battery when it is determined that the constant current charging period (CC) is finished.

TECHNICAL FIELD

The present invention relates to a wireless power transmission apparatus configured to supply power by resonance phenomenon from a power-supplying module connected to a power source to a power-receiving module connected to a power-supplied device including a secondary battery chargeable by a constant current-constant voltage charging system, and a supply power control method of the wireless power transmission apparatus.

BACKGROUND

Portable electronic devices such as laptop PCs, tablet PCs, digital cameras, mobile phones, portable gaming devices, earphone-type music players, wireless headsets, hearing aids, recorders, which are portable while being used by the user are rapidly increasing in recent years. Many of these portable electronic devices have therein a rechargeable battery, which requires periodical charging. To facilitate the work for charging the rechargeable battery of an electronic device, there are an increasing number of devices for charging rechargeable batteries by using a power supplying technology (wireless power transmission technology performing power transmission by varying the magnetic field) that performs wireless power transmission between a power-supplying device and a power-receiving device mounted in an electronic device.

For Example, as a wireless power transmission technology, there have been known, for Example, a technology that performs power transmission by means of electromagnetic induction between coils (e.g. see PTL 1), a technology that performs power transmission by means of resonance phenomenon (magnetic field resonant state) between resonators (coils) provided to the power-supplying device and the power-receiving device (e.g. see PTL 2).

As a method for charging a rechargeable battery (e.g., a lithium ion secondary battery), a constant current-constant voltage charging system has been known. In the constant current-constant voltage charging system, charging is performed by a constant current for a while after the start of the charging. When the voltage applied while the charging by the constant current is being performed increases to a predetermined upper limit voltage, the charging is performed by a constant voltage while the voltage is maintained at the upper limit voltage.

CITATION LIST

Patent Literatures

SUMMARY OF INVENTION

Technical Problem

However, when a lithium ion secondary battery is charged under the constant current-constant voltage charging system by the wireless power transmission apparatus performing power transmission by wireless, a current value supplied to the rechargeable battery is attenuated as shown inFIG. 5, at the shift from the constant current charging to the constant voltage charging. As a result, a charging amount with respect to a charging time is decreased in the constant voltage charging, and hence it takes time to perform the charging to the full amount.

Furthermore, the life of a secondary battery such as a lithium ion secondary battery is typically shortened when the secondary battery is repeatedly charged to the full amount.

An object of the present invention is therefore to provide a wireless power transmission apparatus which is charged in a short charging time and prevents the shortening of the life of a secondary battery.

Solution to Problem

According to an aspect of the invention for achieving the object above, a wireless power transmission apparatus is configured to supply power by resonance phenomenon from a power-supplying module connected with a power source to a power-receiving module connected with a power-supplied device including a secondary battery which is chargeable by a constant current-constant voltage charging system, the wireless power transmission apparatus including: an input impedance measuring apparatus configured to measure an input impedance of the wireless power transmission apparatus including the power-supplied device; and a control device configured to determine whether a constant current charging period is finished based on a change in the input impedance measured by the input impedance measuring apparatus, and terminate charging when it is determined that the constant current charging period is finished.

According to the arrangement above, when a secondary battery chargeable with the shift from the constant current charging to the constant voltage charging is charged by utilizing resonance phenomenon, it is determined that the constant current charging period is finished when there is a change in the input impedance measured by the input impedance measuring apparatus, and the charging is terminated when the constant current charging period is finished.

The life of the secondary battery is typically elongated when, instead of repeating the charging to the full amount, charging which is terminated a while before reaching the full amount is repeated. Because the charging of the secondary battery is terminated at the end of the constant current charging period as above, the charging terminated a while before reaching the full amount becomes possible, and hence the life of the secondary battery is elongated. Typically, the secondary battery chargeable by constant current-constant voltage charging is charged to about 80% of the full amount in only the constant current charging period, and hence the charging amount is sufficient.

Furthermore, because the charging is terminated at the end of the constant current charging period without performing the constant voltage charging, the charging time is shortened.

According to another aspect of the invention for achieving the object above, in the wireless power transmission apparatus, the control device determines that the constant current charging period is finished, when the input impedance measured by the input impedance measuring apparatus becomes higher than or lower than a predetermined threshold.

According to this arrangement, it is determined that the constant current charging period is finished when the input impedance measured by the input impedance measuring apparatus becomes higher than or lower than the predetermined threshold.

According to another aspect of the invention for achieving the object above, in the wireless power transmission apparatus, the control device determines that the constant current charging period is finished when a load variation characteristic becomes higher than or lower than a predetermined threshold, the load variation characteristic being an amount of change with respect to a charging time in the input impedance measured by the input impedance measuring apparatus.

According to the arrangement above, it is determined that the constant current charging period is finished when a load variation characteristic becomes higher than or lower than the predetermined threshold, the load variation characteristic being an amount of change with respect to a charging time in the input impedance measured by the input impedance measuring apparatus.

According to another aspect of the invention for achieving the object above, in the wireless power transmission apparatus, the power-supplying module and the power-receiving module include at least a power-supplying coil, a power-supplying resonator, a power-receiving resonator, and a power-receiving coil, and the load variation characteristic is adjustable by adjusting at least one of a coupling coefficient, between the power-supplying coil and the power-supplying resonator, a coupling coefficient between the power-supplying resonator and the power-receiving resonator, and a coupling coefficient between the power-receiving resonator and the power-receiving coil.

According to the arrangement above, the load variation characteristic is adjustable by adjusting at least one of a coupling coefficient, between the power-supplying coil and the power-supplying resonator, a coupling coefficient between the power-supplying resonator and the power-receiving resonator, and a coupling coefficient between the power-receiving resonator and the power-receiving coil.

With this arrangement, for example, the measurement accuracy of the input impedance measuring apparatus is improved when the load variation characteristic is increased, because a change in the load variation characteristic within a short time becomes large.

According to another aspect of the invention for achieving the object above, in the wireless power transmission apparatus, the load variation characteristic is increased by increasing the coupling coefficient between the power-supplying coil and the power-supplying resonator.

According to this arrangement, the load variation characteristic is increased by increasing the coupling coefficient between the power-supplying coil and the power-supplying resonator. With this arrangement, the measurement accuracy of the input impedance measuring apparatus is improved because a change in the load variation characteristic within a short time becomes large.

According to another aspect of the invention for achieving the object above, in the wireless power transmission apparatus, the load variation characteristic is increased by increasing the coupling coefficient between the power-receiving resonator and the power-receiving coil.

According to this arrangement, the load variation characteristic is increased by increasing the coupling coefficient between the power-receiving resonator and the power-receiving coil. With this arrangement, the measurement accuracy of the input impedance measuring apparatus is improved because a change in the load variation characteristic within a short time becomes large.

According to another aspect of the invention for achieving the object above, in the wireless power transmission apparatus, the load variation characteristic is increased by increasing the coupling coefficient between the power-supplying coil and the power-supplying resonator and the coupling coefficient between the power-receiving resonator and the power-receiving coil.

With this arrangement, the coupling coefficient between the power-supplying coil and the power-supplying resonator and the coupling coefficient between the power-receiving resonator and the power-receiving coil are increased. With this arrangement, the measurement accuracy of the input impedance measuring apparatus is improved because a change in the load variation characteristic within a short time becomes large.

Another aspect of the invention for achieving the object above is a supply power control method of a wireless power transmission apparatus for supplying, by changing a magnetic field, power from a power-supplying module connected with a power source to a power-receiving module connected with a power-supplied device including a secondary battery which is chargeable by a constant current-constant voltage charging system, the wireless power transmission apparatus including: an input impedance measuring apparatus configured to measure an input impedance of the power transmission apparatus; and a control device, and the control device being configured to execute the steps of: determining whether a constant current charging period is finished, by utilizing a change in the input impedance measured by the input impedance measuring apparatus; and terminating charging when it is determined that the constant current charging period is finished.

According to the method above, when a secondary battery chargeable with the shift from the constant current charging to the constant voltage charging is charged by utilizing resonance phenomenon, it is determined that the constant current charging period is finished when there is a change in the input impedance measured by the input impedance measuring apparatus, and the charging is terminated when the constant current charging period is finished.

The life of the secondary battery is typically elongated when, instead of repeating the charging to the full amount, charging which is terminated a while before reaching the full amount is repeated. Because the charging of the secondary battery is terminated at the end of the constant current charging period as above, the charging terminated a while before reaching the full amount becomes possible, and hence the life of the secondary battery is elongated. Typically, the secondary battery chargeable by constant current-constant voltage charging is charged to about 80% of the full amount in only the constant current charging period, and hence the charging amount is sufficient.

Furthermore, because the charging is terminated at the end of the constant current charging period without performing the constant voltage charging, the charging time is shortened.

Advantageous Effects

A wireless power transmission apparatus which is charged in a short charging time and prevents the shortening of the life of a secondary battery is provided.

DESCRIPTION OF EMBODIMENTS

The following will describe a wireless power transmission apparatus and a supply power control method of the wireless power transmission apparatus of an embodiment of the present invention.

Embodiment

To begin with, as shown inFIG. 1, the present embodiment will describe, as an example of a wireless power transmission apparatus1, a charger101including a power-supplying module2and a wireless headset102including a power-receiving module3.

(Structure of Wireless Power Transmission Apparatus1)

As shown inFIG. 1, the wireless power transmission apparatus1is formed of a charger101and a wireless headset102. As shown inFIG. 2, the charger101includes a power-supplying module2including a power-supplying coil21and a power-supplying resonator22, a current-voltage detector4(equivalent to an input impedance measuring apparatus), and a control device5. In the meanwhile, the wireless headset102includes an earphone speaker portion102a, a power-receiving module3including a power-receiving coil31and a power-receiving resonator32, a stabilizer circuit7configured to rectify received AC power, a charging circuit8configured to prevent overcharge, and a lithium ion secondary battery. (It is noted that devices providing functions as an audio apparatus are omitted). The power-supplying coil21of the power-supplying module2is, via the control device5, connected with an AC power source6(a power supplier61and an oscillation circuit62on the outside) configured to supply power to the power-supplying module2, whereas the power-receiving coil31of the power-receiving module3is, via the stabilizer circuit7and the charging circuit8, connected with the lithium ion secondary battery9. In the figures, for the sake of convenience, the stabilizer circuit7, the charging circuit8, and the lithium ion secondary battery are illustrated outside the power-receiving module3. However, these members are provided on the coil inner circumference side of the power-receiving coil31and the power-receiving resonator32which are solenoids. It should be noted that, as shown inFIG. 1andFIG. 2, the stabilizer circuit7, the charging circuit8, and the lithium ion secondary battery9of the present embodiment are power-supplied devices10each of which is the final destination of the supplied power. The power-supplied device10is a generic term for the entire device to which the supplied power is destined, which is connected to the power-receiving module3.

Although not illustrated, the charger101is provided with a housing groove which is provided for housing the wireless headset102and is shaped in accordance with the shape of the wireless headset102. As the wireless headset102is housed in this housing groove of the charger101, the wireless headset102is positioned so that the power-supplying module2of the charger101and the power-receiving module3of the wireless headset102oppose each other.

The power-supplying coil21plays a role of supplying power obtained from the AC power source6to the power-supplying resonator22by means of electromagnetic induction. As shown inFIG. 4, the power-supplying coil21is constituted by an RLC circuit whose elements include a resistor R1, a coil L1, and a capacitor C1. The coil L1part is made of a copper wire material (coated by an insulation film) and is 15 mmϕ in coil diameter. The total impedance of a circuit element constituting the power-supplying coil21is Z1. In the present embodiment, Z1is the total impedance of the RLC circuit (circuit element) constituting the power-supplying coil21, which includes the resistor R1, the coil L1, and the capacitor C1. Further, the current that flows in the power-supplying coil21is I1.

The power-receiving coil31plays roles of receiving the power having been transmitted as a magnetic field energy from the power-supplying resonator22to the power-receiving resonator32, by means of electromagnetic induction, and supplying the power to the lithium ion secondary battery9via the stabilizer circuit7and the charging circuit8. As shown inFIG. 4, the power-receiving coil31, similarly to the power-supplying coil21, is constituted by an RLC circuit whose elements include a resistor R4, a coil L4, and a capacitor C4. The coil L4part is made of a copper wire material (coated by an insulation film) and is 11 mmϕ in coil diameter. The total impedance of a circuit element constituting the power-receiving coil31is Z4. In the present embodiment, Z4is the total impedance of the RLC circuit (circuit element) constituting the power-receiving coil31, which includes the resistor R4, the coil L4, and the capacitor C4. The total impedance of the power-supplied device10connected with the power-receiving coil31is ZL. Further, the current that flows in the power-receiving coil31is I4. While the total impedance of the power-supplied device10is ZL, this may be replaced with RLfor the sake of convenience.

As shown inFIG. 4, the power-supplying resonator22is constituted by an RLC circuit whose elements include a resistor R2, a coil L2, and a capacitor C2. Further, as shown inFIG. 4, the power-receiving resonator32is constituted by an RLC circuit whose elements include a resistor R3, a coil L3, and a capacitor C3. The power-supplying resonator22and the power-receiving resonator32each serves as a resonance circuit and plays a role of creating a magnetic field resonant state. The magnetic field resonant state (resonance phenomenon) here is a phenomenon in which two or more coils resonate with each other in a resonance frequency band. The total impedance of a circuit element constituting the power-supplying resonator22is Z2. In the present embodiment, Z2is the total impedance of the RLC circuit (circuit element) constituting the power-supplying resonator22, which includes the resistor R2, the coil L2, and the capacitor C2. The total impedance of a circuit element constituting the power-receiving resonator32is Z3. In the present embodiment, Z3is the total impedance of the RLC circuit (circuit element) constituting the power-receiving resonator32, which includes the resistor R3, the coil L3, and the capacitor C3. Further, the current that flows in the power-supplying resonator22is I2, and the current that flows in the power-receiving resonator32is I3.

In the RLC circuit which is the resonance circuit in each of the power-supplying resonator22and the power-receiving resonator32, the resonance frequency is f which is derived from (Formula 1) below, where the inductance is L and the capacity of capacitor is C. In the present embodiment, the resonance frequency of the power-supplying coil21, the power-supplying resonator22, the power-receiving resonator32, and the power-receiving coil31is set to 970 kMHz.

The power-supplying resonator22is a solenoid coil made of a copper wire material (coated by an insulation film) and is 15 mmϕ in coil diameter. The power-receiving resonator32is a solenoid coil made of a copper wire material (coated by an insulation film) and is 11 mmϕ in coil diameter. The resonance frequency of the power-supplying resonator22and that of the power-receiving resonator32are matched with each other. Each of the power-supplying resonator22and the power-receiving resonator32may be a spiral coil or a solenoid coil as long as it is a resonator using a coil.

In regard to the above, the distance between the power-supplying coil21and the power-supplying resonator22is denoted as d12, the distance between the power-supplying resonator22and the power-receiving resonator32is denoted as d23, and the distance between the power-receiving resonator32and the power-receiving coil31is denoted as d34(seeFIG. 1).

Further, as shown inFIG. 4, a mutual inductance between the coil L1of the power-supplying coil21and the coil L2of the power-supplying resonator22is M12, a mutual inductance between the coil L2of the power-supplying resonator22and the coil L3of the power-receiving resonator32is M23, and a mutual inductance between the coil L3of the power-receiving resonator32and the coil L4of the power-receiving coil31is M34. In the wireless power transmission apparatus1, the coupling coefficient between the coil L1and the coil L2is represented as k12, the coupling coefficient between the coil L2and the coil L3is represented as k23, and the coupling coefficient between the coil L3and the coil L4is k34.

The wireless power transmission apparatus1(including the stabilizer circuit7, the charging circuit8, and the lithium ion secondary battery9) is represented by a circuit diagram shown in the lower drawing inFIG. 3. In this circuit diagram, the entirety of the wireless power transmission apparatus1(including the stabilizer circuit7, the charging circuit8, and the lithium ion secondary battery9) is replaced with one input impedance Zin, and a voltage applied to the wireless power transmission apparatus1is represented as a voltage Vinand a current input to the wireless power transmission apparatus1is represented as Iin.

To represent the input impedance Zinof the wireless power transmission apparatus1in a more detailed manner, the structure of the wireless power transmission apparatus1is represented by an equivalent circuit as shown inFIG. 4. Based on the equivalent circuit shown inFIG. 4, the input impedance Zinof the wireless power transmission apparatus1can be represented as shown in (Formula 2).

The impedances Z1, Z2, Z3, Z4, and ZLof the power-supplying coil21, the power-supplying resonator22, the power-receiving resonator32, and the power-receiving coil31of the wireless power transmission apparatus1of the present embodiment are represented as shown in (Formula 3).

The resistance values, inductances, and capacities of capacitors, of R1, L1, and C1of the RLC circuit of the power-supplying coil21, R2, L2, and C2of the RLC circuit of the power-supplying resonator22, R3, L3, and C3of the RLC circuit of the power-receiving resonator32, and R4, L4, and C4of the RLC circuit of the power-receiving coil31and the coupling coefficients k12, k23, and k34are preferably set to satisfy the relational expression (Formula 4) as parameters variable at the stage of designing and manufacturing.

The current-voltage detector4provided in the charger101includes a current detector and a voltage detector. These detectors are configured to detect the voltage Vinapplied to the wireless power transmission apparatus1and the current Iininput to the wireless power transmission apparatus1, respectively.

As detailed later, the control device5has functions of calculating an input impedance Zinbased on the voltage Vinand the current Iindetected by the current-voltage detector4(see Formula 5), and determining whether to supply power from the AC power source6to the power-supplying module2in accordance with a change in the calculated input impedance Zin, and further has a function of blocking the power supply from the AC power source6to the power-supplying module2when it is determined that no power is supplied. The control device5is, for example, formed of a microcomputer, a storage device, or the like. The current-voltage detector4configured to detect the voltage Vinand the current Iinis equivalent to an input impedance measuring apparatus.

Zi⁢⁢n=Vi⁢⁢nIi⁢⁢n(Formula⁢⁢5)
(Charging Characteristic in Charging of Lithium Ion Secondary Battery9)

Now, the following will describe a solution based on the charging characteristic in the charging of the lithium ion secondary battery9which is a target of power supply when the wireless power transmission apparatus1of the present embodiment is used.

In the present embodiment, the lithium ion secondary battery9is used as a power-supplied device10to which power is supplied. The lithium ion secondary battery9is typically charged based on a constant current-constant voltage charging system. In the charging of the lithium ion secondary battery9based on this constant current-constant voltage charging system, as shown in the graph of the charging characteristic of the lithium ion secondary battery9inFIG. 5, the lithium ion secondary battery9is charged by a constant current (Ich) for a while after the start of the charging (CC: constant current). While the charging by the constant current is being performed, the voltage (Vch) applied to the lithium ion secondary battery9is increased to a predetermined upper limit voltage (4.2V in the present embodiment). When the voltage (Vch) is increased to the upper limit voltage, shift to the charging by a constant voltage occurs while the voltage is maintained at the upper limit voltage (CV: constant voltage). As the charging by the constant voltage (CV) is performed, the current value (Ich) input to the lithium ion secondary battery9is attenuated, and the charging to the full amount is achieved when the current value reaches a predetermined current value or when a predetermined time elapses.

However, at the shift from the constant current charging (CC) to the constant voltage charging (CV), as shownFIG. 5, the current value (Ich) supplied to the lithium ion secondary battery9is attenuated. As a result, the charging amount with respect to the charging time is decreased in the constant voltage charging (CV), and hence it takes time to perform the charging to the full amount.

In consideration of this charging characteristic, it is understood that the charging time is shortened if the charging of the lithium ion secondary battery9is terminated when the constant current charging (CC) is finished. In the lithium ion secondary battery9chargeable based on the constant current-constant voltage charging, because the charging to about 80% of the full amount is possible in the constant current charging period (CC) only, a sufficient charging amount is achieved even if the charging of the lithium ion secondary battery9is terminated when the constant current charging (CC) is finished. In addition to the above, because it has been known that the life of the lithium ion secondary battery9is typically shortened when the charging to the full amount is repeated, the life of the lithium ion secondary battery9is elongated when the charging of the lithium ion secondary battery9is terminated when the constant current charging (CC) is finished, instead of the charging to the full amount.

The object above is achieved in such a way that, whether the shift from the constant current charging (CC) to the constant voltage charging (CV) is performed is determined, and when the shift from the constant current charging (CC) to the constant voltage charging (CV) is detected, the charging of the lithium ion secondary battery9is terminated because the constant current charging (CC) is finished.

To be more specific, at the shift from the constant current charging (CC) to the constant voltage charging (CV), because the current value (Ich) input to the lithium ion secondary battery9is attenuated, the load impedance of the power-supplied device10including the lithium ion secondary battery9is increased.

On this account, the input impedance Zinof the entire wireless power transmission apparatus1including the power-supplied device10is varied (seeFIG. 8).

In consideration of the above, in the charging operation for the wireless power transmission apparatus1of the present embodiment, a change in the input impedance Zinof the entire wireless power transmission apparatus1including the power-supplied device10is measured at the shift from the constant current charging (CC) to the constant voltage charging (CV), and the charging of the lithium ion secondary battery9is because the constant current charging (CC) is finished, when a change in the input impedance Zinis observed.

(Change in Input Impedance Zin)

In the present embodiment, whether the constant current charging (CC) period is finished is determined based on measurement of a change in the input impedance Zin. In regard to such a change in the input impedance Zin, it is determined that the constant current charging period is finished, when the input impedance Zinbecomes greater than or smaller than a predetermined threshold, or it is determined that the constant current charging period is finished, when a load variation characteristic which indicates an amount of change in the input impedance Zinwith respect to a charging time becomes greater than or smaller than a predetermined threshold. Either way, it is necessary to set a predetermined threshold with prior knowledge in how the input impedance Zinis changed at the shift from the constant current charging (CC) to the constant voltage charging (CV). The following will therefore describe setting of a change in the input impedance Zinwith reference to Measurement Experiments.

The load variation characteristic is an amount of change in the input impedance Zinof the wireless power transmission apparatus1with respect to a charging time at the shift from the constant current charging to the constant voltage charging. This load variation characteristic is a predetermined amount of change (ΔY) in the Y axis relative to a predetermined amount of change (ΔX) in the X axis, where the X axis indicates a charging time whereas the Y axis indicates the input impedance Zin(see the input impedance ZininFIG. 8), and the load variation characteristic indicates an inclination. On this account, when the load variation characteristic increases, an amount of change in the input impedance Zinof the wireless power transmission apparatus1with respect to the charging time increases, and the inclination becomes steep.

(Setting of Variation Tendency of Input Impedance Zinat Shifting to Constant Voltage Charging)

In the present embodiment, when the lithium ion secondary battery9is charged by the wireless power transmission apparatus1based on the constant current-constant voltage charging, to increase the input impedance Zinat the shift to the constant voltage charging (CV), variable parameters of the power-supplying module2and the power-receiving module3such as resistance values, inductances, and capacities of capacitors R1, L1, and C1of the RLC circuit of the power-supplying coil21, R2, L2, and C2of the RLC circuit of the power-supplying resonator22, R3, L3, and C3of the RLC circuit of the power-receiving resonator32, and R4, L4, and C4of the RLC circuit of the power-receiving coil31, and coupling coefficients k12, k23, and k34are set to cause the transmission characteristic S21of the wireless power transmission apparatus1relative to a later-described power-source frequency of the power supplied to the wireless power transmission apparatus1to have a double-hump characteristic. As the power-source frequency of the power supplied to the wireless power transmission apparatus1is adjusted after the transmission characteristic S21of the wireless power transmission apparatus1with respect to the power-source frequency of the power supplied to the wireless power transmission apparatus1is arranged to have the double-hump characteristic, a variation tendency of the input impedance value of the wireless power transmission apparatus1in the constant voltage charging is set.

When the transmission characteristic S21of the wireless power transmission apparatus1relative to the power-source frequency of the power supplied to the wireless power transmission apparatus1is arranged to have the double-hump characteristic, what variation tendency is shown by the input impedance value of the wireless power transmission apparatus1at the shift to the constant voltage charging, when the power-source frequency of the power supplied to the wireless power transmission apparatus1is adjusted, will be explained with reference to Measurement Experiments 1-1 to 1-3.

In the wireless power transmission apparatus1used in Measurement Experiments 1-1 to 1-3, the power-supplying coil21is constituted by an RLC circuit including a resistor R1, a coil L1, and a capacitor C1, and the coil L1part is set at 15 mmϕ in coil diameter. Similarly, the power-receiving coil31is constituted by an RLC circuit including a resistor R4, a coil L4, and a capacitor C4. The coil L4is 11 mmϕ in coil diameter. The power-supplying resonator22is constituted by an RLC circuit including a resistor R2, a coil L2, and a capacitor C2, and the coil L2part is a solenoid coil with the coil diameter of 15 mmϕ. Furthermore, the power-receiving resonator32is constituted by an RLC circuit including a resistor R3, a coil L3, and a capacitor C3, and the coil L3is a solenoid coil with the coil diameter of 11 mmϕ. The values of R1, R2, R3, and R4in the wireless power transmission apparatus1used in the Measurement Experiment 1-1 to 1-3 were set at 0.65Ω, 0.65Ω, 2.47Ω, and 2.0Ω, respectively. Furthermore, the values of L1, L2, L3, and L4were set at 3.1 μH, 3.1 μH, 18.4 μH, and 12.5 μH, respectively. The coupling coefficients k12, k23, and k34were set at 0.46, 0.20, and 0.52, respectively. The resonance frequency of the power-supplying resonator22and the power-receiving resonator32was 970 kHz.

In Measurement Experiments 1-1 to 1-3, after the wireless power transmission apparatus1was set to have the double-hump characteristic by the arrangements above, the current Iinand the input impedance Zinwhen the charging (power supply) of the lithium ion secondary battery9was performed were measured while the power-source frequency of the AC power supplied to the power-supplying module2was changed to three states (seeFIG. 6), i.e., an in-phase resonance mode (fL), an antiphase resonance mode (fH), and a resonance frequency (f0) described below. In Measurement Experiment 1-1 to 1-3, the current Iinand the input impedance Zinwith respect to a charging time (Charging Time (min)) when an input voltage Vinfrom the AC power source6to the wireless power transmission apparatus1was 5V were measured.

In Measurement Experiments, the transmission characteristic S21of the wireless power transmission apparatus1with respect to the power-source frequency of the power supplied to the wireless power transmission apparatus1has the double-hump characteristic. The transmission characteristic “S21” is signals measured by a network analyzer (e.g., E5061B made by Agilent Technologies, Inc.) connected to the wireless power transmission apparatus1, and is indicated in decibel. The greater the value, it means the power transmission efficiency is high. The transmission characteristic “S21” of the wireless power transmission apparatus1relative to the power-source frequency of the power supplied to the wireless power transmission apparatus1may have either single-hump or double-hump characteristic, depending on the strength of coupling (magnetic coupling) by the magnetic field between the power-supplying module2and the power-receiving module3. The single-hump characteristic means the transmission characteristic “S21” relative to the power-source frequency has a single peak which occurs in the resonance frequency band (f0) (See dotted line51FIG. 6). The double-hump characteristic on the other hand means the transmission characteristic S21relative to the driving frequency has two peaks, one of the peaks occurring in a power-source frequency band lower than the resonance frequency (fL), and the other occurring in a power-source frequency band higher than the resonance frequency (fH) (See full line52inFIG. 6). The double-hump characteristic, to be more specific, means that the reflection characteristic “S11” measured with the network analyzer connected to the wireless power transmission apparatus1has two peaks. Therefore, even if the transmission characteristic S21relative to the power-source frequency appears to have a single peak, the transmission characteristic “S21” has a double-hump characteristic if the reflection characteristic S11measured has two peaks. The power transmission efficiency indicates a ratio of the power supplied to the power-supplying module2to the power received by the power-receiving module3.

In the wireless power transmission apparatus1having the single-hump characteristic, the transmission characteristic “S21” is maximized (power transmission efficiency is maximized) when the power-source frequency is at the resonance frequency f0, as indicated by the dotted line51ofFIG. 6.

On the other hand, in the wireless power transmission apparatus1having the double-hump characteristic, the transmission characteristic “S21” is maximized in a power-source frequency band (fL) lower than the resonance frequency f0and in a power-source frequency band (fH) higher than the resonance frequency f0, as indicated by the full line52ofFIG. 6.

It should be noted that, in general, if the distance between the power-supplying resonator and the power-receiving resonator is the same, the maximum value of the transmission characteristic “S21” having the double-hump characteristic (the value of the transmission characteristic “S21” at fL or fH) is lower than the value of the maximum value of the transmission characteristic “S21” having the single-hump characteristic (value of the transmission characteristic “S21” at f0) (See graph inFIG. 6).

To be more specific, if the power-source frequency of the AC power supplied to the power-supplying module2is set at a frequency fL around a peak on the low frequency side in the double-hump characteristic (in-phase resonance mode), the power-supplying resonator22and the power-receiving resonator32are in phase and resonated, with the result that the direction of the current flowing in the power-supplying resonator22is identical with the direction of the current flowing in the power-receiving resonator32. As the result, as shown in the graph ofFIG. 6, the value of the transmission characteristic S21is made relatively high, even if the power-source frequency does not match with the resonance frequency of the power-supplying resonator22of the power-supplying module2and the power-receiving resonator32of the power-receiving module3, although the value still may not be as high as that of the transmission characteristic S21in wireless power transmission apparatuses in general aiming at maximizing the power transmission efficiency (see dotted line51). In this regard, the resonance state in which the direction of the current flowing in the coil (power-supplying resonator22) in the power-supplying module2is identical with the direction of the current flowing in the coil (power-receiving resonator32) in the power-receiving module3are identical will be referred to as an in-phase resonance mode.

Further, in the in-phase resonance mode, because the magnetic field generated on the outer circumference side of the power-supplying resonator22and the magnetic field generated on the outer circumference side of the power-receiving resonator32cancel each other out, the magnetic field spaces each having a lower magnetic field strength than the magnetic field strengths in positions not on the outer circumference sides of the power-supplying resonator22and the power-receiving resonator32(e.g., the magnetic field strengths on the inner circumference sides of the power-supplying resonator22and the power-receiving resonator32) are formed on the outer circumference sides of the power-supplying resonator22and the power-receiving resonator32, as the influence of the magnetic fields is lowered. When a stabilizer circuit7, a charging circuit8, a lithium ion secondary battery9, and the like desired to have less influence of the magnetic field are placed in this magnetic field space, occurrence of Eddy Current attributed to the magnetic field is restrained or prevented. This restrains negative effects due to generation of heat.

In the meanwhile, when, for example, the power-source frequency of the AC power supplied to the power-supplying module2is set at a frequency fH around a peak on the high frequency side in the double-hump characteristic (antiphase resonance mode), the power-supplying resonator22and the power-receiving resonator32are in antiphase and resonated, and hence the direction of the current flowing in the power-supplying resonator22is opposite to the direction of the current flowing in the power-receiving resonator32. As the result, as shown in the graph ofFIG. 6, the value of the transmission characteristic S21is made relatively high, even if the power-source frequency does not match with the resonance frequency of the power-supplying resonator22of the power-supplying module2and the power-receiving resonator32of the power-receiving module3, although the value still may not be as high as that of the transmission characteristic S21in wireless power transmission apparatuses in general aiming at maximizing the power transmission efficiency (see dotted line51). The resonance state in which the current in the coil (power-supplying resonator22) in the power-supplying module2and the current in the coil (power-receiving resonator32) of the power-receiving module3flow in directions opposite to each other is referred to as antiphase resonance mode.

Further, in the antiphase resonance mode, because the magnetic field generated on the inner circumference side of the power-supplying resonator22and the magnetic field generated on the inner circumference side of the power-receiving resonator32cancel each other out, the magnetic field spaces each having a lower magnetic field strength than the magnetic field strengths in positions not on the inner circumference side of the power-supplying resonator22and the power-receiving resonator32(e.g., the magnetic field strengths on the outer circumference side of the power-supplying resonator22and the power-receiving resonator32) are formed on the inner circumference sides of the power-supplying resonator22and the power-receiving resonator32, as the influence of the magnetic fields is lowered. When a stabilizer circuit7, a charging circuit8, a lithium ion secondary battery9, and the like desired to have less influence of the magnetic field is placed in this magnetic field space, occurrence of Eddy Current attributed to the magnetic field is restrained or prevented. This restrains negative effects due to generation of heat. Further, since the magnetic field space formed in this antiphase resonance mode is formed on the inner circumference side of the power-supplying resonator22and the power-receiving resonator32, assembling the electronic components such as the stabilizer circuit7, the charging circuit8, the lithium ion secondary battery9, and the like within this space makes the wireless power transmission apparatus1itself more compact, and improves the freedom in designing.

When the transmission characteristic S21of the wireless power transmission apparatus1relative to the power-source frequency of the power supplied to the wireless power transmission apparatus1has the double-hump characteristic as described above, the input impedance Zinof the wireless power transmission apparatus1is maximized while the power transmission efficiency is maintained at a high value as shown inFIG. 7(see the full line55) when the power-source frequency of the AC power supplied to the power-supplying module2is set in the in-phase resonance mode (fL) or in the antiphase resonance mode (fH). Furthermore, when the power-source frequency of the AC power supplied to the power-supplying module2is set at the resonance frequency (f0), the input impedance Zinof the wireless power transmission apparatus1is minimized as shown inFIG. 7(see the full line55). In Measurement Experiments 1-1 to 1-3, the current Iinand the input impedance Zinwhen the charging (power supply) of the lithium ion secondary battery9was performed were measured while the power-source frequency of the AC power supplied to the power-supplying module2was changed to three states, i.e., the in-phase resonance mode (fL), the antiphase resonance mode (fH), and the resonance frequency (f0).

In the present embodiment, on condition that the transmission characteristic S21of the wireless power transmission apparatus1relative to the power-source frequency of the power supplied to the wireless power transmission apparatus1has the double-hump characteristic, settings and combinations of the variable parameters of the power-supplying module2and the power-receiving module3such as resistance values, inductances, and capacities of capacitors of R1, L1, and C1of the RLC circuit of the power-supplying coil21, R2, L2, and C2of the RLC circuit of the power-supplying resonator22, R3, L3, and C3of the RLC circuit of the power-receiving resonator32, and R4, L4, and C4of the RLC circuit of the power-receiving coil31, and coupling coefficients k12, k23, and k34are design choices and can be optionally set.

In Measurement Experiment 1-1, the input current Iinand the input impedance Zinrelative to a charging time (Charging Time(min)) were measured when the power-source frequency of the AC power supplied to the power-supplying module2was set at a frequency fL around a peak on the low frequency side in the double-hump (in-phase resonance mode: fL=870 kHz). The measurement result is shown inFIG. 8. The input voltage Vinwas 5V (constant).

The measurement result inFIG. 8shows that the input impedance Zinhas a tendency to rise after the shift from the charging by the constant current (CC) to the charging by the constant voltage (CV).

According to Measurement Experiment 1-1 above, when the power-source frequency of the AC power supplied to the power-supplying module2is set at the frequency fL around a peak on the low frequency side in the double-hump after the transmission characteristic S21of the wireless power transmission apparatus1relative to the power-source frequency of the power supplied to the wireless power transmission apparatus1is arranged to have the double-hump characteristic, the input impedance Zinafter the shift from the constant current charging (CC) to the constant voltage charging (CV) has a tendency to rise. With this, by setting the predetermined threshold for determining whether the constant current charging period is finished at a value higher than the input impedance Zinin the constant current charging, such determination is enabled. Similarly, by setting the predetermined threshold for determining whether the constant current charging period is finished at a value higher than the load variation characteristic in the constant current charging, such determination is enabled.

In Measurement Experiment 1-2, the input current Iinand the input impedance Zinrelative to a charging time (Charging Time(min)) were measured when the power-source frequency of the AC power supplied to the power-supplying module2was set at a frequency fH around a peak on the high frequency side in the double-hump (antiphase resonance mode: fH=1070 kHz). The measurement result is shown inFIG. 9. The input voltage Vinwas 5V (constant).

The measurement result inFIG. 9shows that the input impedance Zinhas a tendency to rise after the shift from the constant current charging (CC) to the constant voltage charging (CV). According to Measurement Experiment 1-2 above, when the power-source frequency of the AC power supplied to the power-supplying module2is set at the frequency fH around a peak on the high frequency side in the double-hump after the transmission characteristic S21of the wireless power transmission apparatus1relative to the power-source frequency of the power supplied to the wireless power transmission apparatus1is arranged to have the double-hump characteristic, the input impedance Zinafter the shift from the constant current charging (CC) to the constant voltage charging (CV) has a tendency to rise. With this, by setting the predetermined threshold for determining whether the constant current charging period is finished at a value higher than the input impedance Zinin the constant current charging, such determination is enabled. Similarly, by setting the predetermined threshold for determining whether the constant current charging period is finished at a value higher than the load variation characteristic in the constant current charging, such determination is enabled.

(Measurement Experiment 1-3: Setting of Power-Source Frequency at Resonance Frequency)

In Measurement Experiment 1-3, the input current Iinand the input impedance Zinrelative to a charging time (Charging Time(min)) were measured when the power-source frequency of the AC power supplied to the power-supplying module2was set at a resonance frequency f0 in the double-hump (resonance frequency: f0=970 kHz). The measurement result is shown inFIG. 10. The input voltage Vinwas 5V (constant).

The measurement result inFIG. 10shows that the input impedance Zinhas a tendency to fall after the shift from the constant current charging (CC) to the constant voltage charging (CV).

According to Measurement Experiment 1-3 above, when the power-source frequency of the AC power supplied to the power-supplying module2is set at the resonance frequency f0 in the double-hump after the transmission characteristic S21of the wireless power transmission apparatus1relative to the power-source frequency of the power supplied to the wireless power transmission apparatus1is arranged to have the double-hump characteristic, the input impedance Zinafter the shift from the constant current charging (CC) to the constant voltage charging (CV) has a tendency to fall. With this, by setting the predetermined threshold for determining whether the constant current charging period is finished at a value lower than the input impedance Zinin the constant current charging, such determination is enabled. Similarly, by setting the predetermined threshold for determining whether the constant current charging period is finished at a value lower than the load variation characteristic in the constant current charging, such determination is enabled.

(Setting of Load Variation Characteristic)

Now, for example, when the variation tendency of the input impedance value of the wireless power transmission apparatus1in the constant voltage charging (CV) is set to the tendency to rise as in Measurement Experiment 1-1 above, the measurement accuracy of the voltage detector4(including the control device5) is improved when the load variation characteristic which indicates an amount of change in the input impedance Zinof the wireless power transmission apparatus1to a charging time is increased, because a change in the load variation characteristic within a short time becomes large. The measurement accuracy indicates, for example, that the shift from the constant current charging (CC) to the constant voltage charging (CV) can be determined within a short time.

(Adjustment of Load Variation Characteristic by Coupling Coefficients)

In the present embodiment, the load variation characteristic is adjusted by changing the coupling coefficients k12, k23, and k34. The following will explain in what manner the load variation characteristic is changed by changing the coupling coefficients k12, k23, and k34, with reference to Measurement Experiments 2-1 to 2-5.

The values of R1, R2, R3, and R4in the wireless power transmission apparatus1used in the Measurement Experiment 2-1 were set at 0.65Ω, 0.65Ω, 2.47Ω, and 2.0Ω, respectively. Furthermore, the values of L1, L2, L3, and L4were set at 3.1 ρH, 3.1 μH, 18.4 μH, and 12.5 μH, respectively. Furthermore, the resonance frequency of the power-supplying resonator22and the power-receiving resonator32was 970 kHz.

In Measurement Experiment 2-1, after the wireless power transmission apparatus1was set to have the double-hump characteristic by the arrangements above, the power-source frequency of the AC power supplied to the power-supplying module2is set in the antiphase resonance mode (fH). Furthermore, after the coupling coefficients k23and k34were fixed to 0.20 and 0.52, respectively, the input impedance Zinwhen the charging (power supply) of the lithium ion secondary battery9was performed was measured while the coupling coefficient k12was set at 0.3 and while the coupling coefficient k12was set at 0.46. In Measurement Experiment 2-1, the input impedance Zinwith respect to a charging time (Charging Time(min)) when the input voltage Vinfrom the AC power source6to the wireless power transmission apparatus1is 5V is measured.

According to the measurement result of Measurement Experiment 2-1 shown inFIG. 11, while an amount of change in the input impedance Zinin the constant voltage charging (CV) with respect to the charging time is about 10Ω when the coupling coefficient k12is set at 0.3, an amount of change of the input impedance Zinin the constant voltage charging (CV) with respect to the charging time is about 20Ω when the coupling coefficient k12is set at 0.46. In this way, the load variation characteristic is larger when the coupling coefficient k12is set at 0.46 than when the coupling coefficient k12is set at 0.3.

The wireless power transmission apparatus1used in Measurement Experiment 2-2 is identical with the wireless power transmission apparatus1used in Measurement Experiment 2-1. In Measurement Experiment 2-2, after the wireless power transmission apparatus1is arranged to have the double-hump characteristic, the power-source frequency of the AC power supplied to the power-supplying module2is set at the resonance frequency (f0) of the power-supplying resonator22and the power-receiving resonator32. Furthermore, after the coupling coefficients k23and k34were fixed to 0.20 and 0.52, respectively, the input impedance Zinwhen the charging (power supply) of the lithium ion secondary battery9was performed was measured while the coupling coefficient k12was set at 0.3 and while the coupling coefficient k12was set at 0.46. In Measurement Experiment 2-2, as the power-source frequency of the AC power supplied to the power-supplying module2is set at the resonance frequency (f0) of the power-supplying resonator22and the power-receiving resonator32, the input impedance Zinafter the shift from the constant current charging (CC) to the constant voltage charging (CV) has a tendency to fall.

According to the measurement result of Measurement Experiment 2-2 shown inFIG. 11, while an amount of change in the input impedance Zinin the constant voltage charging (CV) with respect to the charging time is about 3Ω when the coupling coefficient k12is set at 0.3, an amount of change of the input impedance Zinin the constant voltage charging (CV) with respect to the charging time is about 6Ω when the coupling coefficient k12is set at 0.46. In this way, the load variation characteristic is larger when the coupling coefficient k12is set at 0.46 than when the coupling coefficient k12is set at 0.3. In this case, the inclination which is the amount of change in the input impedance Zinof the wireless power transmission apparatus1with respect to the charging time in the constant voltage charging is negative. However, the load variation characteristic in Measurement Experiment 2-2 is evaluated to be large, as the load variation characteristic is evaluated by the absolute value.

The wireless power transmission apparatus1used in Measurement Experiment 2-3 is identical with the wireless power transmission apparatus1used in Measurement Experiment 2-1. In Measurement Experiment 2-2, after the wireless power transmission apparatus1is arranged to have the double-hump characteristic, the power-source frequency of the AC power supplied to the power-supplying module2is set in the antiphase resonance mode (fH). Furthermore, after the coupling coefficients k12and k23were fixed to 0.46 and 0.20, respectively, the input impedance Zinwhen the charging (power supply) of the lithium ion secondary battery9was performed was measured while the coupling coefficient k34was set at 0.25 and while the coupling coefficient k34was set at 0.52.

According to the measurement result of Measurement Experiment 2-3 shown inFIG. 12, while an amount of change in the input impedance Zinin the constant voltage charging (CV) with respect to the charging time is about 15Ω when the coupling coefficient k34is set at 0.25, an amount of change of the input impedance Zinin the constant voltage charging (CV) with respect to the charging time is about 20Ω when the coupling coefficient k34is set at 0.52. In this way, the load variation characteristic is larger when the coupling coefficient k34is set at 0.52 than when the coupling coefficient k34is set at 0.25.

The wireless power transmission apparatus1used in Measurement Experiment 2-4 is identical with the wireless power transmission apparatus1used in Measurement Experiment 2-1. In Measurement Experiment 2-4, after the wireless power transmission apparatus1is arranged to have the double-hump characteristic, the power-source frequency of the AC power supplied to the power-supplying module2is set at the resonance frequency (f0) of the power-supplying resonator22and the power-receiving resonator32. Furthermore, after the coupling coefficients k12and k23were fixed to 0.46 and 0.20, respectively, the input impedance Zinwhen the charging (power supply) of the lithium ion secondary battery9was performed was measured while the coupling coefficient k34was set at 0.25 and while the coupling coefficient k34was set at 0.52. In Measurement Experiment 2-4, in a manner similar to Measurement Experiment 2-2, as the power-source frequency of the AC power supplied to the power-supplying module2is set at the resonance frequency (f0) of the power-supplying resonator22and the power-receiving resonator32, the input impedance Zinafter the shift from the constant current charging (CC) to the constant voltage charging (CV) has a tendency to fall.

According to the measurement result of Measurement Experiment 2-4 shown inFIG. 12, while an amount of change in the input impedance Zinin the constant voltage charging (CV) with respect to the charging time is about 1.5Ω when the coupling coefficient k34is set at 0.25, an amount of change of the input impedance Zinin the constant voltage charging (CV) with respect to the charging time is about 6Ω when the coupling coefficient k34is set at 0.52. In this way, the load variation characteristic is larger when the coupling coefficient k34is set at 0.52 than when the coupling coefficient k34is set at 0.25.

With the arrangement above, in Measurement Experiment 2-5, after the wireless power transmission apparatus1is arranged to have the double-hump characteristic, the power-source frequency of the AC power supplied to the power-supplying module2is set in the antiphase resonance mode (fH). Furthermore, after the coupling coefficient k23was fixed to 0.20, the input impedance Zinand the input current Iin(I1) when the charging (power supply) of the lithium ion secondary battery9was performed were measured while the coupling coefficient k12was set at 0.38 and the coupling coefficient k34was set at 0.37 and while the coupling coefficient k12was set at 0.46 and the coupling coefficient k34was set at 0.52. In Measurement Experiment 2-5, the input impedance Zinand the input current Iinwith respect to the charging time (Charging Time(min)) when the input voltage Vinfrom the AC power source6to the wireless power transmission apparatus1is 5V are measured.

According to the measurement result of the input impedance Zinin Measurement Experiment 2-5 shown inFIG. 13, when the coupling coefficient k12is 0.38 and the coupling coefficient k34is 0.37, an amount of change in the input impedance Zinwith respect to the charging time at the shift from the constant current charging (CC) to the constant voltage charging (CV) is about 12Ω (41Ω−29Ω), whereas, when the coupling coefficient k12is 0.46 and the coupling coefficient k34is 0.52, an amount of change in the input impedance Zinwith respect to the charging time in the constant voltage charging (CV) is about 17Ω (47Ω−30Ω). In this way, the load variation characteristic is larger when the coupling coefficient k12is set at 0.46 and the coupling coefficient k34is set at 0.52 than when the coupling coefficient k12is set at 0.38 and the coupling coefficient k34is set at 0.37.

As described above, the measurement accuracy of the voltage detector4(including the control device5) is improved when the load variation characteristic is increased, because a change in the load variation characteristic within a short time becomes large.

(Method of Adjusting Coupling Coefficient)

Now, the following will describe a method of adjusting a coupling coefficient which is a parameter for adjusting the load variation characteristic above.

As shown inFIG. 14, in the wireless power transmission, the relationship between the distance between coils and the coupling coefficient k is such that the coupling coefficient k tends to increase as the distance between the coils is reduced (shortened). When this relationship is applied to the wireless power transmission apparatus1of the present embodiment, the coupling coefficient k12between the power-supplying coil21(coil L1) and the power-supplying resonator22(coil L2), the coupling coefficient k23between the power-supplying resonator22(coil L2) and the power-receiving resonator32(coil L3), and the coupling coefficient k34between the power-receiving resonator32(coil L3) and the power-receiving coil31(coil L4) are increased by reducing the distance d12between the power-supplying coil21and the power-supplying resonator22, the distance d23between the power-supplying resonator22and the power-receiving resonator32, and the distance d34between the power-receiving resonator32and the power-receiving coil31, respectively. In the meanwhile, the coupling coefficient k12between the power-supplying coil21(coil L1) and the power-supplying resonator22(coil L2), the coupling coefficient k23between the power-supplying resonator22(coil L2) and the power-receiving resonator32(coil L3), and the coupling coefficient k34between the power-receiving resonator (coil L3) and the power-receiving coil31(coil L4) are decreased by increasing the distance d12between the power-supplying coil21and the power-supplying resonator22, the distance d23between the power-supplying resonator22and the power-receiving resonator32, and the distance d34between the power-receiving resonator32and the power-receiving coil31, respectively.

(Charging Operation of Wireless Power Transmission Apparatus1: Charging Operation Flow)

Based on the structure of the wireless power transmission apparatus1or the like, the charging operation of the lithium ion secondary battery9by utilizing the wireless power transmission apparatus1(supply power control method) will be described. To be more specific, a charging operation flow (process) executed mainly by the control device5in the wireless power transmission apparatus1will be described with reference toFIG. 15.

To begin with, as the wireless headset102is mounted on the charger101, a magnetic field resonant state is creased as the power-supplying resonator22and the power-receiving resonator32are in resonance, and hence the power is supplied from the power-supplying resonator22to the power-receiving resonator32as magnetic field energy. As the power received by the power-receiving resonator32is supplied to the lithium ion secondary battery9via the power-receiving coil31, the stabilizer circuit7, and the charging circuit8, the constant current charging (CC) starts. In this explanation, the charging amount of the lithium ion secondary battery9when the wireless headset102is mounted on the charger101is assumed to be 0%.

Subsequently, the control device5determines whether the voltage Vinapplied to the wireless power transmission apparatus1and the current Iininput to the wireless power transmission apparatus1are detected by the current-voltage detector4(S1). The detection of the current Iinand the voltage Vinby the current-voltage detector4is performed at predetermined temporal intervals. (This predetermined temporal intervals can be optionally set.)

When the voltage Vinand the current Iinare not detected (S1: NO), a standby state is continued until the voltage Vinand the current Iinare detected.

In the meanwhile, when the voltage Vinand the current Iinare detected (S1: YES), the control device5calculates the input impedance Zinbased on the voltage Vinand the current Iindetected by the current-voltage detector4(see Formula 5) (S2).

Then the control device5determines whether the input impedance Zincalculated in S2exceeds a predetermined threshold (S3). When the input impedance Zincalculated in S2does not exceed the predetermined threshold (S3: NO), the flow shifts to S1.

In the meanwhile, when the input impedance Zincalculated in S2exceeds the predetermined threshold (S3: YES), the control device5blocks the power supply from the AC power source6to the power-supplying module2(S4). In other words, the charging of the lithium ion secondary battery9is terminated. The flow is finished at this stage.

For example, in case of the wireless power transmission apparatus1used in Measurement Experiment 1-1 shown inFIG. 8, when the threshold is set at 25Ω, while the input impedance Zinis more or less maintained at 22Ω in the constant current charging (CC), the input impedance Zinincreases after the shift to the constant voltage charging (CV). When the charging time is about 45 minutes, the input impedance Zinreaches 25Ω. At this stage, the control device5determines that the input impedance Zincalculated in S2exceeds the predetermined threshold (25Ω), and blocks the power supply from the AC power source6to the power-supplying module2so as to terminate the charging of the lithium ion secondary battery9.

The flow above is a charging operation flow when it is determined that the constant current charging period is finished when the input impedance Zinexceeds the predetermined threshold. In the meanwhile, when it is determined that the constant current charging period is finished when the load variation characteristic exceeds a predetermined threshold, the load variation characteristic is calculated in S2, and when the load variation characteristic exceeds the predetermined threshold (S3), the control device5blocks the power supply from the AC power source6to the power-supplying module2(S4) so as to terminate the charging of the lithium ion secondary battery9.

According to the configuration and method above, when the lithium ion secondary battery9chargeable with the shift from the constant current charging (CC) to the constant voltage charging (CV) is charged by using resonance phenomenon, the control device5determines that the constant current charging period (CC) is finished when there is a change in the input impedance Zincalculated based on the current Iinand the voltage Vinmeasured by the current-voltage detector4, and terminates the charging of the lithium ion secondary battery9at the finish of the constant current charging period (CC).

The life of the lithium ion secondary battery9is typically elongated when, instead of repeating the charging to the full amount, charging which is terminated a while before reaching the full amount is repeated. Because the charging of the lithium ion secondary battery9is terminated at the end of the constant current charging period (CC) as above, the charging terminated a while before reaching the full amount becomes possible, and hence the life of the lithium ion secondary battery9is elongated. Typically, the lithium ion secondary battery9chargeable by constant current-constant voltage charging is charged to about 80% of the full amount in only the constant current charging period (CC), and hence the charging amount is sufficient.

Furthermore, because the charging of the lithium ion secondary battery9is terminated at the end of the constant current charging period (CC) without performing the constant voltage charging (CV), the charging time is shortened.

In addition to the above, according to the configuration above, it is determined that the constant current charging period (CC) is finished when the input impedance Zincalculated based on the current Iinand the voltage Vinmeasured by the current-voltage detector4exceeds a predetermined threshold.

In addition to the above, according to the configuration above, it is determined that the constant current charging period (CC) is finished when the load variation characteristic calculated based on the current Iinand the voltage Vinmeasured by the current-voltage detector4exceeds a predetermined threshold.

In addition to the above, according to the configuration above, the load variation characteristic is adjustable by adjusting at least one of the coupling coefficient k12between the power-supplying coil21and the power-supplying resonator22, the coupling coefficient k23between the power-supplying resonator22and the power-receiving resonator32, and the coupling coefficient k34between the power-receiving resonator32and the power-receiving coil31. With this, because, for example, a change in the load variation characteristic within a short time becomes great when the load variation characteristic is increased, the measurement accuracy by the voltage detector4(including the control device5) is improved.

In addition to the above, according to the configuration above, the load variation characteristic is increased by increasing the coupling coefficient k12between the power-supplying coil21and the power-supplying resonator22. Because a change in the load variation characteristic within a short time becomes great by this, the measurement accuracy of the voltage detector (including the control device5) is improved.

In addition to the above, according to the configuration above, the load variation characteristic is increased by increasing the coupling coefficient k34between the power-receiving resonator32and the power-receiving coil31. Because a change in the load variation characteristic within a short time becomes great by this, the measurement accuracy of the voltage detector4(including the control device5) is improved.

In addition to the above, according to the configuration above, the coupling coefficient k12between the power-supplying coil21and the power-supplying resonator22and the coupling coefficient k34between the power-receiving resonator32and the power-receiving coil31are increased. Because a change in the load variation characteristic within a short time becomes great by this, the measurement accuracy of the voltage detector4(including the control device5) is improved.

Other Embodiments

Although the above description of the manufacturing method deals with the wireless headset102as an example, the method is applicable to any devices having a secondary battery; e.g., tablet PCs, digital cameras, mobile phone phones, earphone-type music player, hearing aids, and sound collectors.

Further, although the above description assumes that the wireless power transmission apparatus1is mounted in a portable electronic device, the use of such modules is not limited to small devices. For Example, with a modification to the specifications according to the required power amount, the wireless power transmission apparatus1is mountable to a relatively large system such as a wireless charging system in an electronic vehicle (EV), or to an even smaller device such as a wireless endoscope for medical use.

Although the above descriptions have been provided with regard to the characteristic parts so as to understand the present invention more easily, the invention is not limited to the embodiments and the Examples as described above and can be applied to the other embodiments and Examples, and the applicable scope should be construed as broadly as possible. Furthermore, the terms and phraseology used in the specification have been used to correctly illustrate the present invention, not to limit it. Further, it will be obvious for those skilled in the art that the other structures, systems, methods or the like are possible, within the spirit of the invention described in the present specification.

Accordingly, it should be considered that claims cover equivalent structures, too, without departing from the technical idea of the present invention. In addition, it is desirable to sufficiently refer to already-disclosed documents and the like, in order to fully understand the objects and effects of the present invention.

REFERENCE SIGNS LIST