JP2015050848A - Power transmission device and wireless power transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power transmission device and a wireless power transmission system having the same, capable of efficient power transmission irrespective of whether or not a power reception device includes a piezoelectric transformer.SOLUTION: A transmission-side active electrode 12 has capacitive coupling with a reception-side active electrode 16, and a transmission-side passive electrode 13 has capacitive coupling or direct contact with a reception-side passive electrode 17, to thereby transmit power from a power transmission device 101 to a power reception device 201. The power transmission device 101 sweeps an AC voltage frequency to be applied to the reception-side active electrode 16 and the reception-side passive electrode 17, to detect input impedance viewed from the power transmission device 101 to the power reception device 201 side whenever the frequency changes. Based on the detection result, the power transmission device 101 determines whether or not the power reception device 201 includes a piezoelectric transformer 18.

Description

  The present invention relates to a power transmission device that transmits power to a power reception device using a piezoelectric transformer, and a wireless power transmission system including the power transmission device.

  Patent Document 1 discloses an electric field coupling type wireless power transmission system in which electrodes of a power transmission device and a power reception device are opposed to each other and are electrically coupled to each other, and electric power is transmitted from the power transmission device to the power reception device by the electric field coupling. It is disclosed. In general, in a technique for increasing the transmission efficiency of a power transmission system, it is effective to incorporate a low-loss resonance circuit. This resonance circuit is composed of an electrostatic capacity and an inductor of a coupling portion between the power transmission device and the power reception device. And in patent document 1, a power transmission device searches a resonance frequency by performing a frequency sweep in a state where the power reception device is mounted, and uses the resonance frequency obtained by the search as a drive frequency to transmit power to the power reception device. I do.

Special table 2009-531009

  By the way, in order to incorporate the above-described resonance circuit into a device that has been reduced in size and thickness in recent years, it is a problem to realize a reduction in the size of the resonance circuit and a reduction in loss. As one method for solving the problem, it is effective to use a piezoelectric device for the inductor. However, an inductor using a piezoelectric device has a resonance frequency characteristic different from that of a winding transformer. For this reason, when the power receiving device uses a piezoelectric transformer, the method disclosed in Patent Document 1 cannot detect the resonance frequency or detects an incorrect resonance frequency, and the power transmission device Efficient power transmission to the power receiving device cannot be performed.

  Accordingly, an object of the present invention is to provide a power transmission device capable of efficiently transmitting power regardless of whether the power receiving device has a piezoelectric transformer, and a wireless power transmission system including the power transmission device.

  According to the present invention, the first electrode is capacitively coupled to the first external electrode of the external device, and the second electrode is capacitively coupled to the second external electrode of the external device or is in direct contact, so that the external device is wirelessly connected. In a power transmission device for transmitting power, a DC voltage is converted into an AC voltage, and a DC / AC conversion circuit applied to the first electrode and the second electrode, and a frequency of the AC voltage output from the DC / AC conversion circuit are swept. A frequency sweeping circuit, a detection circuit for detecting an input impedance when the first electrode and the second electrode are viewed from the DC / AC conversion circuit for each change in the frequency of the AC voltage, and a detection result of the detection circuit And a determination unit that determines whether the external device has a piezoelectric transformer or a winding transformer.

  For example, an AC voltage is applied at a constant current to the first electrode facing the first external electrode of the external device and the second electrode facing (or contacting) the second external electrode of the external device, and the AC voltage applied When the frequency is swept, if the external device includes a resonance circuit, frequency characteristics of a voltage having a peak (resonance point) can be obtained. The frequency characteristics obtained differ depending on whether the inductance component of the resonance circuit is an inductance component due to a piezoelectric transformer or an inductance component due to a winding transformer. That is, a frequency characteristic can be obtained by detecting a DC voltage every time the frequency of the AC voltage changes, and it is determined from the frequency characteristic whether the transformer of the external device is a piezoelectric transformer but a winding transformer. be able to.

  Since the setting method of the driving frequency of power transmission differs depending on whether the transformer included in the external device is a piezoelectric transformer or a winding transformer, the power transmission device can determine the optimal driving frequency setting method because the determination can be made. It can be selected as appropriate. Thereby, efficient electric power transmission is realizable.

  The determination unit is configured so that the frequency of the AC voltage output from the DC / AC conversion circuit is a predetermined value, and the value of the input impedance detected by the detection circuit is equal to or less than a predetermined threshold value. Is preferably determined to have a piezoelectric transformer.

  In this configuration, a reliable determination can be made by determining based on the characteristics of the frequency characteristics of the piezoelectric transformer.

  In the determination unit, the external device has a piezoelectric transformer because the input impedance detected by the detection circuit has a plurality of resonance points and anti-resonance points in a frequency band swept by the frequency sweep circuit. Then, it is preferable to determine.

  In this configuration, a reliable determination can be made by determining based on the characteristics of the frequency characteristics of the piezoelectric transformer.

  According to the present invention, since it is possible to determine whether the transformer included in the external device is a winding transformer or a winding transformer, the power transmission device can appropriately select an optimal driving frequency setting method. Thereby, efficient electric power transmission is realizable.

1 is a circuit diagram of a wireless power transmission system according to a first embodiment. Block diagram showing the configuration of the high-frequency high-voltage generation circuit of the power transmission device 1 is an equivalent circuit diagram of the wireless power transmission system shown in FIG. The figure which shows the frequency characteristic and transmission efficiency characteristic of the wireless power transmission system which concern on embodiment Circuit diagram of wireless power transmission system with power receiving device equipped with winding transformer The figure which shows the frequency characteristic and transmission efficiency characteristic of the wireless power transmission system shown in FIG. 2 is a flowchart showing the operation of the control circuit shown in FIG. The figure which shows the frequency characteristic of the wireless power transmission system when the piezoelectric transformer is driven in the higher order mode

(Embodiment 1)
FIG. 1 is a circuit diagram of a wireless power transmission system 1 according to the present embodiment.

  The wireless power transmission system 1 includes a power transmission device 101 and a power reception device 201. The power receiving apparatus 201 includes a load circuit RL. The load circuit RL includes a secondary battery and a charging circuit that charges the secondary battery. And the power receiving apparatus 201 is a portable electronic device provided with the load circuit RL, for example. Examples of the portable electronic device include a mobile phone, a PDA (Personal Digital Assistant), a portable music player, a notebook PC, and a digital camera. The power receiving apparatus 201 is placed on the power transmitting apparatus 101. Then, the secondary battery of the placed power receiving apparatus 201 is charged.

  The power transmission apparatus 101 includes a high frequency high voltage generation circuit 11, a power transmission side active electrode 12, a power transmission side passive electrode 13, and an ACV detection circuit 14. The power transmission side active electrode 12 corresponds to the “first electrode” and “power transmission side first electrode” of the present invention, and the power transmission side passive electrode 13 corresponds to the “second electrode” and “power transmission side second electrode” of the present invention. It corresponds to. The capacitance element CG shown in FIG. 1 is a capacitance mainly composed of the power transmission side active electrode 12 and the power transmission side passive electrode 13. The ACV detection circuit 14 capacitively divides the voltage V1 between the electrodes 12 and 13, and generates a DC voltage obtained by rectifying the divided AC voltage as a detection signal V (ACV).

  FIG. 2 is a block diagram illustrating a configuration of the high-frequency high-voltage generation circuit 11 of the power transmission apparatus 101.

  The high frequency high voltage generation circuit 11 generates a high frequency voltage of 100 kHz to several tens of MHz, for example, and applies the voltage between the passive electrode 12 and the active electrode 13. The high frequency high voltage generation circuit 11 includes a control circuit 111, a drive power supply circuit 112, a DCV / DCI detection circuit 113, a drive control circuit 114, a switching circuit 115, and a booster circuit 116.

  The control circuit 111 corresponds to a “determination unit” according to the present invention. The drive control circuit 114 corresponds to a “frequency sweep circuit” according to the present invention. The switching circuit 115 corresponds to a “DC / AC converter circuit” according to the invention.

  The control circuit 111 controls each unit 112 to 116 included in the high frequency high voltage generation circuit 11. The drive power supply circuit 112 is a power supply circuit that inputs a constant DC voltage (for example, DC12V or 5V) converted from a commercial power supply. The drive power supply circuit 112 includes a power supply impedance switching circuit 112A. The power supply impedance switching circuit 112 </ b> A is a circuit that switches the output impedance of the drive power supply circuit 112, and switches between applying a constant voltage or supplying a constant current to the switching circuit 115.

  The drive control circuit 114 switches the switch element of the switching circuit 115 according to the signal output from the control circuit 111. The switching circuit 115 performs switching control to alternately drive the input unit of the booster circuit 116. The step-up circuit 116 includes a step-up transformer having a primary coil and a secondary coil.

  The DCV / DCI detection circuit 113 detects the voltage DCV applied to the switching circuit 115 and the drive current DCI flowing through the switching circuit 115. The control circuit 111 reads the detection signals V (DCV) and V (DCI) detected by the DCV and DCI detection circuit 113. In the constant voltage mode, the control circuit 111 reads the detection signal V (ACV) detected by the ACV detection circuit 14 and feedback-controls the voltage generated by the high frequency high voltage generation circuit 11 so that the AC voltage V1 is constant. To do.

  Returning to FIG. 1, the power receiving apparatus 201 includes a power receiving side active electrode 16, a power receiving side passive electrode 17, and a step-down circuit including a piezoelectric transformer 18 and an inductance element L <b> 2. The capacitance element CL shown in FIG. 1 is a capacitance mainly composed of the power receiving side active electrode 16 and the power receiving side passive electrode 17. The coupling between the power transmission side active electrode 12 and the power transmission side passive electrode 13 of the power transmission apparatus 101 and the coupling electrode of the power reception apparatus 201 and the power reception side active electrode 16 and the power reception side passive electrode 17 is performed via the mutual capacitance Cm. Can be expressed as being connected.

  The piezoelectric transformer 18 steps down the voltage applied between the reference potential terminal E12 and the input terminal E20 and outputs it to the output terminal E11. When the high frequency high voltage generation circuit 11 generates a high frequency voltage, the piezoelectric transformer 18 steps down the voltage 100 to 3 kV induced in the capacitance element CL to 5 to 24 V and outputs it to the load circuit RL.

  FIG. 3 is an equivalent circuit diagram of the wireless power transmission system 1 shown in FIG.

  As shown in FIG. 3, the piezoelectric transformer 18 is represented by capacitance elements C1 and C2, an inductance element Lp, a capacitance element Cp, a resistor Rp, and an ideal transformer Tp. The capacitance element C2 is an equivalent output capacity of the piezoelectric transformer, and the capacitance element Cp and the inductance element Lp are electromechanical parameters.

  The resonance frequency of the piezoelectric transformer 18 is determined mainly by the resonance of the series resonance circuit formed by the capacitance element Cp and the inductance element Lp. Since electrical energy conversion is via elastic vibration, it has a natural resonance frequency determined by the elastic wave propagation velocity and dimensions of the piezoelectric ceramic. A first resonance circuit RC1 is configured by the capacitance of the capacitance element CL of the coupling electrode portion of the piezoelectric transformer 18 and the power receiving device 201 and the capacitance element CG of the coupling electrode portion of the power transmission device 101. The resonance frequency of the first resonance circuit RC1 is determined by the circuit constant of the first resonance circuit RC1.

  In addition, a second resonance circuit RC2 is configured by the capacitance element C2 and the inductance element L2, which are equivalent output capacities of the piezoelectric transformer 18. The resonance frequency of the second resonance circuit RC2 is determined by the capacitance of the capacitance element C2 and the inductance of the inductance element L2.

  In the present embodiment, the first resonance circuit RC1 and the second resonance circuit RC2 are set to constants so that the respective resonance frequencies are substantially the same, and perform complex resonance (coupled resonance). By making the resonance frequencies of the first resonance circuit RC1 and the second resonance circuit RC2 the same, it is possible to efficiently transmit power. As will be described in detail later, the high-frequency high-voltage generation circuit 11 determines the frequency of the generated high-frequency voltage from two resonance frequencies based on the complex resonance (coupled resonance) of the first resonance circuit RC1 and the second resonance circuit RC2. Between.

  FIG. 4 is a diagram illustrating frequency characteristics and transmission efficiency characteristics of the wireless power transmission system 1 according to the present embodiment.

  The solid line shown in FIG. 4 shows the frequency characteristics of the input impedance Zin when the power transmission side active electrode 12 and the power transmission side passive electrode 13 are viewed from the high frequency high voltage generation circuit 11 of the power transmission device 101. Specifically, the input impedance Zin is an impedance when the power receiving device 201 is viewed from the primary side of the booster circuit 116, and is detected by reading the detection signal V (DCV) from the DCV and DCI detection circuit 113 ( (See FIG. 2).

  When the frequency of the high-frequency voltage generated by the high-frequency high-voltage generation circuit 11 is swept while the power receiving device 201 is mounted on the power transmission device 101, the input impedance Zin of the input impedance Zin is shown in FIG. The frequency characteristic has two local minimum values. These two minimum values are called peak frequencies f1 and f2. The peak frequencies f1 and f2 are resonance frequencies due to complex resonance (coupled resonance) of the first resonance circuit RC1 and the second resonance circuit RC2.

  A broken line illustrated in FIG. 4 indicates frequency characteristics of power transmission efficiency from the power transmitting apparatus 101 to the power receiving apparatus 201. Looking at the frequency characteristics of the power transmission efficiency, the frequency f0, which is approximately the midpoint between the peak frequencies f1 and f2, is the most efficient. This frequency f0 is an operating point of the wireless power transmission system 1, that is, a frequency for efficiently performing power transmission. Hereinafter, this frequency f0 is referred to as drive frequency f0.

  Note that the peak shown in the vicinity of about 380 kHz in FIG. 4 is a peak caused by the influence of the series resonance circuit on the power transmission device 101 side.

  Next, in order to compare with the case where the power receiving apparatus 201 includes the piezoelectric transformer 18, a wireless power transmission system in the case where the step-down circuit of the power receiving apparatus is formed of a winding transformer will be described.

  FIG. 5 is a circuit diagram of a wireless power transmission system in which the power receiving device includes a winding transformer.

  The wireless power transmission system 2 includes the power transmission device 101 having the same configuration as the wireless power transmission system 1 described with reference to FIGS. 1 and 2. In the power receiving device 202 provided in the wireless power transmission system 2, the step-down circuit 21 includes a winding transformer TL and an inductance element LL. The step-down circuit 21 forms a resonance circuit RC3 with the capacitance element CL.

  FIG. 6 is a diagram showing frequency characteristics and transmission efficiency characteristics of the wireless power transmission system 2 shown in FIG.

  The solid line in FIG. 6 shows the frequency characteristics of the voltage applied to the switching circuit 115 in a state where a constant current is supplied to the switching circuit 115 (see FIG. 2). When the high-frequency high-voltage generation circuit 11 is driven with a constant current while the power-receiving device 202 is mounted on the power transmission device 101, and further, the frequency of the high-frequency voltage generated by the high-frequency high-voltage generation circuit 11 is swept. As shown by the solid line in FIG. 6, the frequency characteristic of the voltage has one maximum value. This maximum value is referred to as a peak frequency f3. The peak frequency f3 is the resonance frequency of the resonance circuit RC3. Note that the frequency characteristic of the voltage has substantially the same waveform as the frequency characteristic of the input impedance when the power receiving device 202 is viewed from the primary side of the booster circuit 116 (see FIG. 2).

  The broken line in FIG. 6 indicates the frequency characteristic of power transmission efficiency. Looking at the frequency characteristics of the power transmission efficiency, the peak frequency f3 is the most efficient. This peak frequency f3 is an operating point of the wireless power transmission system 2, that is, a driving frequency for efficiently performing power transmission.

  As described above, the waveform of the frequency characteristic differs between the case where the step-down circuit of the power receiving apparatus is formed of the piezoelectric transformer 18 and the case of the winding transformer TL. This is because, when the step-down circuit is composed of the piezoelectric transformer 18, the first resonance circuit RC1 and the second resonance circuit RC2 are formed, and these two resonance circuits resonate (combined resonance). When the step-down circuit is composed of the winding transformer TL, this is because only one resonant circuit is resonating.

  That is, the control circuit 111 of the power transmission apparatus 101 can determine whether the step-down circuit of the power receiving apparatus is composed of the piezoelectric transformer 18 or the winding transformer TL from the waveform (peak frequency) of the frequency characteristics of the input impedance. And in each case, the drive frequency for performing the most efficient power transmission can be determined. By determining whether or not the step-down circuit of the power receiving device is composed of the piezoelectric transformer 18, it is possible to improve the reliability of whether or not the determined drive frequency can perform the most efficient power transmission.

  As the simplest discrimination method, when the switching circuit 115 is driven at the driving frequency in the case of the winding transformer and the absolute value of the input impedance at that time is higher than a predetermined threshold, the power receiving device 201 If it is determined that the step-down circuit is formed of the winding transformer TL and is lower than a predetermined threshold value, a method of determining that the step-down circuit of the power receiving apparatus 201 is formed of the piezoelectric transformer 18 can be considered.

  Further, when the step-down circuit of the power receiving apparatus 201 is composed of the piezoelectric transformer 18, a low-order or high-order resonance waveform appears in the frequency band swept by the drive control circuit 114, and therefore, within the frequency band swept by the drive control circuit 114. It is also possible to determine whether there are a plurality of resonance points and anti-resonance points.

  FIG. 7 is a flowchart showing the operation of the control circuit 111 shown in FIG.

  When the power receiving device is mounted on the power transmitting device 101, the control circuit 111 sets the constant current to be supplied to the switching circuit 115 by switching the power source impedance switching circuit 112A (S11). Note that the placement detection of the power receiving device may be detection by a sensor, or may be detection by fluctuation of a transmission current (DCI) to the power receiving device. Next, the control circuit 111 sets an initial value of the frequency range to be swept (swept), and drives the drive control circuit 114 at that frequency (S12). In this state, V (DCV) is read from the detection signal of the DCV / DCI detection circuit 113 (S13). More specifically, the control circuit 111 repeatedly performs a process of reading V (DCV) by shifting the frequency by Δf, and repeats this process until the frequency reaches the final value of the setting range.

  The control circuit 111 detects the minimum value or the maximum value of the frequency characteristic (frequency characteristic of the input impedance) of the voltage DCV obtained by the frequency sweep (S14). Then, from the detected minimum value or maximum value, the control circuit 111 determines whether or not the frequency characteristic is that of the piezoelectric transformer 18 (S15).

  For example, when two minimum values are detected within the frequency sweep range, the control circuit 111 determines that the frequency characteristic is the characteristic of the piezoelectric transformer 18 (S15, YES), and determines two minimum values (FIG. 4 is determined as a drive frequency for performing power transmission (drive frequency f0) between the peak frequencies f1 and f2) shown in FIG. 4 (S16). On the other hand, when one maximum value (the peak frequency f3 shown in FIG. 6) is detected within the frequency sweep range, the control circuit 111 does not have the frequency characteristic of the piezoelectric transformer 18, that is, the winding It is determined that the transformer has a characteristic of TL (S15, NO), and the detected maximum value (peak frequency f3 shown in FIG. 6) is determined as a drive frequency for power transmission (S17). Then, the control circuit 111 performs power transmission at the determined drive frequency (S18).

  As described above, in the present embodiment, the power transmission device 101 can determine whether or not the placed power receiving device has the piezoelectric transformer 18, and thus is optimal according to the characteristics of the piezoelectric transformer 18 or the winding transformer TL. It is possible to set a proper driving frequency. Thereby, efficient electric power transmission is realizable.

(Modification)
In the present embodiment, the control circuit 111 detects two local minimum values in the frequency characteristics when the power receiving apparatus 201 includes the piezoelectric transformer 18, but detects the local maximum value and determines the frequency between them. The driving frequency may be determined.

  Further, in order to increase the power transmission, the piezoelectric transformer 18 of the power receiving apparatus 201 is driven in a higher-order mode, and there is a resistive load that consumes power on the subsequent stage (load circuit RL) side of the piezoelectric transformer 18. When connected, the waveform of the frequency characteristic shown by the solid line in FIG. 4 may be rounded. In this case, since the Q value of the resonance circuits RC1 and RC2 becomes low due to the coupling between the resistive load and the resonance circuits RC1 and RC2, the control circuit 111 cannot detect the extreme value (resonance peak) of the frequency characteristics. For this reason, the control circuit 111 cannot determine the drive frequency for performing power transmission efficiently.

  Hereinafter, a method for determining the drive frequency when the piezoelectric transformer 18 is driven in the nλ / 2 (n is an integer of 2 or more) mode will be described.

  FIG. 8 is a diagram illustrating frequency characteristics of the wireless power transmission system when the piezoelectric transformer 18 is driven in a higher-order mode. In FIG. 8, frequency characteristics (broken line) when the power receiving apparatus 201 is not placed on the power transmitting apparatus 101 are also shown.

  When the piezoelectric transformer 18 is driven in the secondary mode (n = 2), the Q value is low in the high-order mode as described above, and the peak frequency does not appear in the region A in FIG. 8 (small). For this reason, the control circuit 111 cannot detect the frequencies corresponding to the peak frequencies f1 and f2 described in FIG. Therefore, the control circuit 111 detects the resonance peak of the piezoelectric transformer 18 in the low-order mode. Specifically, the control circuit 111 detects a resonance peak of the m-th order mode (1 ≦ m ≦ n−1). In this case, as shown in a region B in FIG. 8, a resonance peak (resonance frequency and antiresonance frequency) of the primary mode appears.

  The control circuit 111 determines a frequency obtained by multiplying the detected frequency between the resonance frequency f4 and the anti-resonance frequency f5 by n / m, that is, a frequency twice as the drive frequency. In the case of FIG. 8, 540 [kHz] obtained by doubling the frequency 270 [kHz] between the resonance frequency f4 and the antiresonance frequency f5 is determined as the drive frequency.

  As a method for determining the drive frequency, the drive frequency is determined to be twice the frequency between the resonance frequency f4 and the antiresonance frequency f5 detected in the primary mode by driving in the secondary mode. It is not limited to that. For example, a method of driving in the λ / 2 mode and determining a frequency that is ½ times the frequency between the resonance frequency and the anti-resonance frequency detected in the secondary mode may be used.

  As described above, in the case where the piezoelectric transformer 18 is driven in the high-order mode, if the resonance peak cannot be detected, the control circuit 111 can determine that the power receiving device 201 includes the piezoelectric transformer 18 and the low-order mode. By detecting the resonance peak in the mode, it is possible to determine the drive frequency for efficiently performing power transmission. Since this setting can be dealt with by software setting of the control circuit 111, it is not necessary to add a hardware configuration separately, so that the complexity of the structure can be suppressed.

  As described above, even when the piezoelectric transformer 18 is driven in the high-order mode, the power transmission apparatus 101 can determine that the power reception apparatus 201 uses the piezoelectric transformer 18 and perform power transmission. The driving frequency for determining can be determined.

C1, C2-capacitance element CG-capacitance element CL-capacitance element Cm-mutual capacitance Cp-capacitance element E11-output terminal E12-reference potential terminal E20-input terminal L2-inductance element LL-inductance element Lp-inductance element RC1-first 1 resonance circuit RC2-second resonance circuit RC3-resonance circuit RL-load circuit Rp-resistance TL-winding transformer Tp-ideal transformers 1, 2-wireless power transmission system 11-high frequency high voltage generation circuit 12-power transmission Side active electrode (power transmission side first electrode)
13-Passive electrode on power transmission side (second electrode on power transmission side)
14-ACV detection circuit 16-power-receiving-side active electrode (first external electrode, power-receiving-side first electrode)
17-Power-receiving-side passive electrode (second external electrode, power-receiving-side second electrode)
18-piezoelectric transformer 21-step-down circuit 101-power transmission device 111-control circuit (determination unit, frequency determination circuit)
112-Drive power supply circuit 112A-Power supply impedance switching circuit 113-DCI detection circuit (detection circuit)
114-drive control circuit (frequency sweep circuit)
115-switching circuit (DC-AC converter circuit)
116—Boost circuit 201, 202—Power receiving device (external device)

Claims (8)

The first electrode is capacitively coupled to the first external electrode of the external device, and the second electrode is capacitively coupled to the second external electrode of the external device or directly contacts, thereby transmitting power to the external device wirelessly. In power transmission equipment,
A DC / AC conversion circuit for converting a DC voltage into an AC voltage and applying the DC voltage to the first electrode and the second electrode;
A frequency sweep circuit for sweeping the frequency of the AC voltage output from the DC / AC converter circuit;
A detection circuit for detecting an input impedance when the first electrode and the second electrode are viewed from the DC / AC converter circuit for each change in the frequency of the AC voltage;
A determination unit that determines whether the external device has a piezoelectric transformer based on a detection result of the detection circuit;
A power transmission device comprising: The determination unit
The external device has a piezoelectric transformer because the frequency of the AC voltage output from the DC / AC converter circuit is a predetermined value, and the value of the input impedance detected by the detection circuit is not more than a predetermined threshold. The power transmission device according to claim 1, wherein the power transmission device is determined. The determination unit
Determining that the external device has a piezoelectric transformer by having an input impedance detected by the detection circuit having a plurality of resonance points and anti-resonance points in a frequency band swept by the frequency sweep circuit. The power transmission device according to claim 1. The power transmission side first electrode of the power transmission device is capacitively coupled with the power reception side first electrode of the power reception device, and the power transmission side second electrode of the power transmission device is capacitively coupled with or directly in contact with the power reception side second electrode of the power reception device. Thus, in a wireless power transmission system that wirelessly transmits power from the power transmission device to the power reception device,
The power transmission device is:
A DC / AC conversion circuit for converting a DC voltage into an AC voltage and applying the DC voltage to the first electrode and the second electrode;
A frequency sweep circuit for sweeping the frequency of the AC voltage output from the DC / AC converter circuit;
A detection circuit for detecting an input impedance when the first electrode and the second electrode are viewed from the DC / AC converter circuit for each change in the frequency of the AC voltage;
A determination unit for determining whether the power receiving device has a piezoelectric transformer based on a detection result of the detection circuit;
Having a wireless power transmission system. The determination unit
When the frequency of the AC voltage output from the DC / AC converter circuit is a predetermined value and the value of the input impedance detected by the detection circuit is equal to or less than a predetermined threshold value, the power receiving device operates the piezoelectric transformer. The wireless power transmission system according to claim 4, wherein the wireless power transmission system is determined to have. The power receiving device is:
A piezoelectric resonance transformer having an input electrode connected to each of the power receiving side first electrode and the power receiving side second electrode, an output capacitance of the piezoelectric transformer, and an inductor connected between the output electrodes of the piezoelectric transformer; A parallel resonant circuit,
Have
The determination unit has a plurality of resonance points and anti-resonance points in the frequency band swept by the frequency sweep circuit, so that the power receiving device has a piezoelectric transformer. The wireless power transmission system according to claim 4, wherein the wireless power transmission system is determined to be. The power transmission device is:
When the determination unit determines that the power receiving device has a piezoelectric transformer, the frequency of the AC voltage output from the DC / AC converter circuit is set to two frequencies generated by the complex resonance of the natural resonance circuit and the parallel resonance circuit. The frequency between the resonance point or anti-resonance point,
The wireless power transmission system according to claim 6. The piezoelectric transformer is driven in an nλ / 2 (n is an integer of 1 or more) mode, where λ is a wavelength at a resonance frequency of the natural resonance circuit,
The frequency of the AC voltage output from the DC / AC converter circuit is multiplied by n / m times the frequency set from the resonance point and antiresonance point of the mth order (m is an integer of 1 or more and m ≠ n). To
The wireless power transmission system according to claim 7.

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