Abstract:
A contactless power transfer system, including a coil configured to supply or receive power contactlessly via magnetic coupling, a bridge circuit having two direct current (DC) terminals and two alternating current (AC) terminals, and a smoothing capacitor connected between the DC terminals. A load is connectable to either end of the smoothing capacitor. One of the AC terminals is connected to one end of the coil via a first capacitor. The other of the AC terminals is connected to the other end of the coil. The bridge circuit includes two serially-connected circuits each having upper and lower arms, each arm having a semiconductor switch and a diode in reverse parallel connection. A second capacitor is connected in parallel to the semiconductor switch of an upper arm, or of a lower arm, or to two semiconductor switches respectively of an upper arm and of a lower arms, of the bridge circuit.

Description:
[0001]    This application is based on and claims the benefit of priority under 35 U.S.C. §119 from Japanese Patent Application Nos. 2010-257809 and 2011-203097, respectively filed on Nov. 18, 2010 and Sep. 16, 2011, the contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a contactless power transfer system and control method thereof that supply power mainly across a space, utilizing magnetic coupling in a contactless condition between coils. 
         [0004]    2. Related Art 
         [0005]    A contactless power transfer system supplies power to a load utilizing magnetic coupling between coils caused by electromagnetic induction. The principle thereof is that a sort of transformer is formed by magnetically coupling plural coils across a space, and power is supplied and received utilizing the electromagnetic induction between the coils. 
         [0006]    For example, by disposing a primary side coil corresponding to a power supply source in a rail form as a feeder wire, configuring a moving body by integrating a secondary side coil and power receiving circuit, and causing the primary side coil and secondary side coil to oppose each other, it is possible to contactlessly transfer power to the moving body moving along the feeder wire. 
         [0007]    Herein,  FIG. 27  shows a heretofore known technology of a contactless power transfer system described in JP-A-2002-354711 (especially, paragraphs [0028] to [0031] and to [0045], FIGS. 1, 6, and the like). In  FIG. 27 , a primary side feeder wire  110  acting as a coil is connected to either end of a high frequency power source  100 . A power receiving coil  120  is magnetically coupled to the primary side feeder wire  110 , and the primary side feeder wire  110  and power receiving coil  120  configure one kind of transformer. 
         [0008]    Both ends of the power receiving coil  120  are connected to alternating current terminals of a full-wave rectifier circuit  10  via a resonant capacitor C. Herein, the power receiving coil  120  and resonant capacitor C configure a series resonance circuit. 
         [0009]    The full-wave rectifier circuit  10  is configured by bridge connecting diodes D u , D v , D x , and D y . 
         [0010]    A constant voltage control circuit  20 , which controls in such a way that the direct current output voltage of the full-wave rectifier circuit  10  is of a reference voltage value, is connected to direct current terminals of the full-wave rectifier circuit  10 . The constant voltage control circuit  20  is configured of a boost chopper circuit formed from, for example, a reactor L 1 , a diode D 1 , a smoothing capacitor C 0 , and a semiconductor switch SW 1 . Also, a load R is connected to either end of the smoothing capacitor C 0 . 
         [0011]    A control device for switching the semiconductor switch SW 1  is omitted from  FIG. 27 . 
         [0012]    With the heretofore known technology of  FIG. 27 , a high frequency current is caused to flow along the primary side feeder wire  110  by the high frequency power source  100 , and the high frequency power supplied is input into the full-wave rectifier circuit  10  via the power receiving coil  120 , and converted into direct current power. 
         [0013]    Generally, with this kind of contactless power transfer system, the voltage induced in the power receiving coil  120  changes due to a change in length of the gap between the primary side feeder wire  110  and power receiving coil  120 , and due to positional deviations of the two, because of which the direct current output voltage of the full-wave rectifier circuit  10  fluctuates. The characteristics of the load R are also a cause of the direct current output voltage of the full-wave rectifier circuit  10  fluctuating. 
         [0014]    For this reason, with the heretofore known technology of  FIG. 27 , the direct current output voltage of the full-wave rectifier circuit  10  is controlled to a constant value by the constant voltage control circuit  20 . 
         [0015]    For a contactless power transfer system, the higher the frequency of the current supplied via the coil, the lower the exciting inductance needed for carrying out a power transmission, and it is possible to miniaturize the coil and a core disposed in the periphery thereof. However, for a power converter configuring a high frequency power source device or a power receiving circuit, as the frequency of the current flowing through the circuit increases, the switching loss of the semiconductor switch increases, and the power transfer efficiency decreases, meaning that it is common to set the frequency of the contactlessly fed power between a few kilohertz and a few tens of kilohertz. 
         [0016]    The contactless power transfer system shown in  FIG. 27 , and in particular the power receiving circuit after the resonant capacitor C, has the following problems. 
         [0017]    1. As the power receiving circuit is configured of the full-wave rectifier circuit  10  and constant voltage control circuit  20 , the circuit as a whole increases in size, which leads to an increase in installation space and an increase in cost. 
         [0018]    2. As loss also occurs in the reactor L 1 , semiconductor switch SW 1 , and diode D 1 , in addition to in the diodes D u , D v , D x , and D y  of the full-wave rectifier circuit  10 , these losses cause a decrease in power transfer efficiency. 
       SUMMARY OF THE INVENTION 
       [0019]    Therefore, an object of the invention is to provide a contactless power transfer system that enables a miniaturization and the reduction in cost of the circuit. 
         [0020]    Also, another object of the invention is to provide a contactless power transfer system and control method thereof that reduce loss caused by the circuit elements, and carry out a highly efficient, stable power transfer. 
         [0021]    In order to achieve the objects, a contactless power transfer system according to an aspect of the invention includes a power receiving coil that supplies and receives power contactlessly via magnetic coupling with a primary side feeder wire connected to an alternating current power source, and a power receiving circuit connected to the power receiving coil via a capacitor, wherein direct current voltage is supplied to a load from the power receiving circuit. 
         [0022]    Herein, the power receiving circuit includes a bridge circuit including plural series circuits including upper and lower arms and having a reverse parallel connection circuit of a semiconductor switch and a diode in each of the upper and lower arms, and a smoothing capacitor, and in an aspect of the invention, a capacitor is connected in parallel to the semiconductor switch of either the upper arms or the lower arms, or to the semiconductor switches of both the upper and lower arms, of the bridge circuit. 
         [0023]    As a control method of the contactless power transfer system, all of the semiconductor switches are put into an off condition during a period for which the power transfer to the power receiving coil is stopped due to a power outage, or the like, and a switching action of each semiconductor switch is carried out after a zero-crossing of the current of the power receiving coil is detected when the power transfer is started. 
         [0024]    As another control method, the semiconductor switch of the upper armor lower arm may be put into an on condition during a period for which the power transfer to the power receiving coil is stopped, and a switching action of each semiconductor switch carried out after a zero-crossing of the current of the power receiving coil is detected when the power transfer is started. 
         [0025]    Also, as another control method, all of the semiconductor switches may be maintained in the switching condition immediately before the current of the power receiving coil becomes zero due to the stopping of the power transfer to the power receiving coil during a period for which the power transfer to the power receiving coil is stopped, and a switching action of each semiconductor switch carried out after a zero-crossing of the current of the power receiving coil is detected when the power transfer to the power receiving coil is started. 
         [0026]    Also, as another example of the contactless power transfer system according to an aspect of the invention, the bridge circuit may be configured of a switching arm series circuit wherein two switching arms formed from a reverse parallel connection circuit of a semiconductor switch and diode are connected in series, and a diode series circuit wherein two diodes are connected in series. In this case, a connection point of the switching arms and a connection point of the diodes form alternating current terminals of the bridge circuit, and a connection point of the switching arm series circuit and diode series circuit forms direct current terminals of the bridge circuit. 
         [0027]    In this contactless power transfer system, a capacitor may be connected in parallel to at least one of the semiconductor switches. 
         [0028]    As a control method of the contactless power transfer system, all of the semiconductor switches are put into an off condition during a period for which the power transfer to the power receiving coil is stopped, and a switching action of each semiconductor switch is carried out after a zero-crossing of the current of the power receiving coil is detected when the power transfer is started. 
         [0029]    As another control method, all of the semiconductor switches may be maintained in the switching condition immediately before the current of the power receiving coil becomes zero due to the stopping of the power transfer to the power receiving coil during a period for which the power transfer to the power receiving coil is stopped, and a switching action of each semiconductor switch may be carried out after a zero-crossing of the current of the power receiving coil is detected when the power transfer is started. 
         [0030]    Also, as another example of the contactless power transfer system according to an aspect of the invention, resonant capacitors may be embedded in the bridge circuit in the power receiving circuit configured of a switching arm series circuit wherein two switching arms formed from a reverse parallel connection circuit of a semiconductor switch and a diode are connected in series, and a capacitor series circuit wherein two resonant capacitors are connected in series, connected in parallel. In this case, a connection point of the switching arms and a connection point of the resonant capacitors form alternating current terminals of the bridge circuit, and a connection point of the switching arm series circuit and capacitor series circuit form direct current terminals of the bridge circuit. 
         [0031]    In this contactless power transfer system, a capacitor may be connected in parallel to at least one of the semiconductor switches. 
         [0032]    As a control method of the contactless power transfer system, all the semiconductor switches are put into an off condition during a period for which the power transfer to the power receiving coil is stopped, and a switching action of each semiconductor switch is carried out after a zero-crossing of the current of the power receiving coil is detected when the power transfer is started. 
         [0033]    As another control method, all of the semiconductor switches may be maintained in the switching condition immediately before the current of the power receiving coil becomes zero due to the stopping of the power transfer to the power receiving coil during a period for which the power transfer to the power receiving coil is stopped, and a switching action of each semiconductor switch may be carried out after a zero-crossing of the current of the power receiving coil is detected when the power transfer is started. 
         [0034]    According to the invention, it is possible to control the direct current output voltage to a constant by a phase control of drive signals of the semiconductor switches configuring the bridge circuit inside the power receiving circuit, without using a constant voltage control circuit as with the heretofore known technology. That is, as the power receiving circuit can be configured of only the bridge circuit and a smoothing capacitor, it is possible to achieve a simplification, miniaturization, and reduction in cost of the circuit configuration. At the same time, it is possible to reduce loss by reducing the number of circuit components, enabling a highly efficient, stable contactless power transfer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]      FIG. 1  is a circuit diagram showing a first embodiment of a contactless power transfer system according to the invention; 
           [0036]      FIG. 2  is an operation illustration of  FIG. 1 ; 
           [0037]      FIG. 3  is another operation illustration of  FIG. 1 ; 
           [0038]      FIG. 4  is an operation illustration of a soft switching in a predetermined period of  FIG. 2 ; 
           [0039]      FIG. 5  is a circuit diagram showing a second embodiment of the contactless power transfer system according to the invention; 
           [0040]      FIG. 6  is a circuit diagram showing a third embodiment of the contactless power transfer system according to the invention; 
           [0041]      FIG. 7  is an operation illustration showing a first embodiment of a control method according to the invention; 
           [0042]      FIG. 8  is an operation illustration showing a second embodiment of the control method according to the invention; 
           [0043]      FIG. 9  is an operation illustration showing a third embodiment of the control method according to the invention; 
           [0044]      FIG. 10  is a circuit diagram showing a fourth embodiment of the contactless power transfer system according to the invention; 
           [0045]      FIG. 11  is an operation illustration of  FIG. 10 ; 
           [0046]      FIG. 12  is a circuit diagram showing a fifth embodiment of the contactless power transfer system according to the invention; 
           [0047]      FIG. 13  is an operation illustration of a soft switching in a predetermined period of  FIG. 11 ; 
           [0048]      FIG. 14  is a circuit diagram showing a sixth embodiment of the contactless power transfer system according to the invention; 
           [0049]      FIG. 15  is a circuit diagram showing a seventh embodiment of the contactless power transfer system according to the invention; 
           [0050]      FIG. 16  is an operation illustration showing a fourth embodiment of the control method according to the invention; 
           [0051]      FIG. 17  is an operation illustration showing a fifth embodiment of the control method according to the invention; 
           [0052]      FIG. 18  is a circuit diagram showing an eighth embodiment of the contactless power transfer system according to the invention; 
           [0053]      FIG. 19  is an operation illustration of  FIG. 18 ; 
           [0054]      FIG. 20  is another operation illustration of  FIG. 18 ; 
           [0055]      FIG. 21  is a circuit diagram showing a ninth embodiment of the contactless power transfer system according to the invention; 
           [0056]      FIG. 22  is an operation illustration of a soft switching in a predetermined period of  FIG. 19 ; 
           [0057]      FIG. 23  is a circuit diagram showing a tenth embodiment of the contactless power transfer system according to the invention; 
           [0058]      FIG. 24  is a circuit diagram showing an eleventh embodiment of the contactless power transfer system according to the invention; 
           [0059]      FIG. 25  is an operation illustration showing the fourth embodiment of the control method according to the invention; 
           [0060]      FIG. 26  is an operation illustration showing the fifth embodiment of the control method according to the invention; and 
           [0061]      FIG. 27  is a circuit diagram of heretofore known technology described in JP-A-2002-354711 (paragraphs [0028] to [0031] and [0041] to [0045], FIGS. 1, 6, and the like). 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0062]    Hereafter, a description will be given, based on the drawings, of embodiments of the invention. As it is mainly the configuration of a power receiving circuit connected to a stage subsequent to a power receiving coil  120  that differs from  FIG. 27  in each embodiment, a description of each embodiment will be given hereafter centered on this point. Also, in each embodiment, circuit components having the same function as in  FIG. 27  will be given the same reference numerals and characters. 
         [0063]      FIG. 1  is a circuit diagram showing a first embodiment of a contactless power transfer system according to the invention, and corresponds to a first aspect of the invention. 
         [0064]    In  FIG. 1 , reference numeral  310  is a power receiving circuit. The power receiving circuit  310  includes bridge connected semiconductor switches Q u , Q x , Q v , and Q y , diodes D u , D x , D v , and D y  connected in reverse parallel to the switches Q u , Q x , Q v , and Q y  respectively, capacitors C x  and C y  connected in parallel to the lower arm switches Q x  and Q y  respectively, and a smoothing capacitor C 0  connected between direct current terminals of a bridge circuit (bridge inverter) formed from these elements. A series circuit of a resonant capacitor C and the power receiving coil  120  is connected between alternating current terminals of the bridge circuit, and a load R is connected to either end of the smoothing capacitor C 0 . 
         [0065]    Also, reference numeral  200  is a control device that generates a drive signal for switching the semiconductor switches Q u , Q x , Q v , and Q y . The control device  200  generates the drive signal based on current i of the power receiving coil  120  detected by a current detector unit CT and on a direct current output voltage Vo of the power receiving circuit  310 . 
         [0066]    Next, a description will be given of actions of the contactless power transfer system shown in  FIG. 1  at a normal time. 
         [0067]    The circuit shown in  FIG. 1  is such that a bidirectional power supply is possible between the power receiving coil  120  and the load R. Hereafter, a description will be given of two kinds of circuit action, a case of supplying power from the power receiving coil  120  to the load R and a case of supplying power from the load R to the power receiving coil  120 . 
         [0068]    Firstly, a description will be given of the actions in the case of supplying power from the power receiving coil  120  to the load R. 
         [0069]      FIG. 2  shows an operation waveform of the current i flowing through the power receiving coil  120  of  FIG. 1  and of an alternating current voltage v of the bridge circuit, and shows drive signals of the semiconductor switches Q u , Q x , Q v , and Q y . 
         [0070]    As shown in  FIG. 2 , the semiconductor switches Q u , Q x , Q v , and Q y  switch in a constant frequency in synchronization with the current i. Hereafter, a description will be given of an action in each period I to VI of  FIG. 2 . 
         [0071]    1. Period I (switches Q x  and Q y  on): the current i of the power receiving coil  120  flows along a path from the resonant capacitor C through the switch Q x  and diode D y  to the power receiving coil  120 , and the voltage v of the bridge circuit is at a zero voltage level, as shown in the drawing. 
         [0072]    2. Period II (switches Q u  and Q y  on): the current i flows along a path from the resonant capacitor C through the diode D u , smoothing capacitor C 0 , and diode D y  to the power receiving coil  120 , and the voltage v is at a positive voltage level corresponding to the direct current output voltage V o , as shown in  FIG. 2 . In this period, the smoothing capacitor C 0  is charged by the current i. 
         [0073]    3. Period III (switches Q u  and Q y  on): in this period, the polarity of the current i is inverted, and the current i flows along a path from the resonant capacitor C through the power receiving coil  120 , switch Q y , and smoothing capacitor C 0  to the switch Q u , and the smoothing capacitor C 0  is discharged. 
         [0074]    4. Period IV (switches Q u  and Q v  on): the current i flows along a path from the resonant capacitor C through the power receiving coil  120  and diode D v  to the switch Q u , and the voltage v is at a zero voltage level, as shown in the drawing. 
         [0075]    5. Period V (switches Q x  and Q v  on): the current i flows along a path from the resonant capacitor C through the power receiving coil  120 , diode D v , and smoothing capacitor C 0  to the diode D x , and the voltage v is at a negative voltage level corresponding to the direct current output voltage V o  as shown in the drawing. In this period, the smoothing capacitor C 0  is charged by the current i. 
         [0076]    6. Period VI (switches Q x  and Q v  on): in this period, the polarity of the current i is inverted, and the current i flows along a path from the resonant capacitor C through the switch Q x , smoothing capacitor C 0 , and switch Q v  to the power receiving coil  120 , and the smoothing capacitor C 0  is discharged. 
         [0077]    Subsequently, there is a transition to the switching mode of period I, and the same actions are repeated. 
         [0078]    Next, a description will be given of the case of supplying power from the load R to the power receiving coil  120 . 
         [0079]      FIG. 3 , in the same way as  FIG. 2 , shows an operation waveform of the current i flowing through the power receiving coil  120  and of the alternating current voltage v of the bridge circuit, and drive signals of the semiconductor switches Q u , Q x , Q v , and Q y . 
         [0080]    As shown in  FIG. 3 , the semiconductor switches Q u , Q x , Q v , and Q y  switch at a constant frequency in synchronization with the current i. The drive signals of the semiconductor switches Q u , Q x , Q v , and Q y  in  FIG. 3  are signals deviating by a half cycle of the current i from the drive signals of the semiconductor switches Q u , Q x , Q v , and Q y  shown in  FIG. 2 . Hereafter, a description will be given of an action in each period I′ to VI′ of  FIG. 3 . 
         [0081]    1. Period I′ (switches Q x  and Q y  on): the current i of the power receiving coil  120  flows along a path from the resonant capacitor C through the power receiving coil  120  and switch Q y  to the diode D x , and the alternating current voltage v of the bridge circuit is at a zero voltage level, as shown in the drawing. 
         [0082]    2. Period II′ (switches Q u  and Q y  on): the current i flows along a path from the resonant capacitor C through the power receiving coil  120 , switch Q y , and smoothing capacitor C 0  to the switch Q u , and the smoothing capacitor C 0  is discharged. 
         [0083]    3. Period III′ (switches Q u  and Q y  on): in this period, the polarity of the current i is inverted, and the current i flows along a path from the resonant capacitor C through the diode D u , smoothing capacitor C 0 , and diode D y  to the power receiving coil  120 , and the voltage v, continuing from period II′, is at a positive voltage level corresponding to the direct current output voltage V o . In this period, the smoothing capacitor C 0  is charged by the current i. 
         [0084]    4. Period IV′ (switches Q u  and Q v  on): the current i flows along a path from the resonant capacitor C through the diode D u  and switch Q v  to the power receiving coil  120 , and the alternating current voltage v is at a zero voltage level, as shown in the drawing. 
         [0085]    5. Period V′ (switches Q x  and Q v  on): the current i flows along a path from the resonant capacitor C through the switch Q x , smoothing capacitor C 0 , and switch Q v  to the power receiving coil  120 , and the smoothing capacitor C 0  is discharged. 
         [0086]    6. Period VI′ (switches Q x  and Q v  on): in this period, the polarity of the current i is inverted, and the current i flows along a path from the resonant capacitor C through the power receiving coil  120 , diode D v  and smoothing capacitor C 0  to the diode D x , and the voltage v, continuing from period V′, is at a negative voltage level corresponding to the direct current output voltage V o . In this period, the smoothing capacitor C 0  is charged by the current i. 
         [0087]    Subsequently, there is a transition to the switching mode of period I′, and the same actions are repeated. 
         [0088]    By controlling the semiconductor switches Q u , Q x , Q v , and Q y  as heretofore described, the alternating current voltage v of the bridge circuit is controlled by positive and negative voltages with the direct current output voltage V o  as a crest value. Power fed from a primary side feeder wire  110  to the power receiving circuit  310  is the product of the current i of the power receiving coil  120  and voltage v of the bridge circuit shown in  FIG. 2 , and control of the power fed, that is, a constant control of the direct current output voltage V o  is enabled by the control device  200  adjusting the phases of the drive signals of the semiconductor switches Q u , Q x , Q v , and Q y  based on the detected value of the direct current output voltage V o . Also, by configuring the power receiving circuit  310  with a bridge circuit, actions keeping the power constant are possible even when the load R is a regenerative load. 
         [0089]    Furthermore, on/off actions of the semiconductor switches when there is a switching between the periods shown in  FIGS. 2 and 3  are such that it is possible to carry out a so-called soft switching owing to the action of the capacitors C x  and C y  connected in parallel to the lower arm side semiconductor switches Q x  and Q y . 
         [0090]      FIG. 4  shows operation waveforms of the semiconductor switches Q u  and Q x  when there is a shift from period I (switches Q x  and Q y  on) to period II (switches Q u  and Q v  on) of  FIG. 2 . In period I, the current i of the power receiving coil  120  flows through the switch Q x  and, as a switching action switching to period II, the switch Q x  turns off based on a drive signal output from the control device  200 . At this time, the current i flows as a charging current of the capacitor C x  connected in parallel to the switch Q x , and the kind of delay shown in the drawing occurs in the rise of a voltage V Qx  applied to the switch Q x . 
         [0091]    Because of this, the switch Q x  is switched to zero voltage, and it is possible to reduce a loss accompanying the switching action. Also, after charging the capacitor C x , the current is commutated to the upper arm side diode D u . By providing a delay time of an off signal of the switch Q x  so that a drive on signal to the switch Q u  is input after the action of commutation to the diode D u , no switching loss accompanying an on action of the switch Q u  occurs. 
         [0092]    A description has been given here with a time of switching from period I to period II as an example, but on/off actions of the semiconductor switches when there is a switching between other periods are also such that, in the same way, it is possible to carry out a zero voltage switching owing to the charging and discharging actions of the capacitors C x  and C y  connected in parallel to the switches Q x  and Q y . 
         [0093]    Also, as examples of capacitors being connected in parallel to semiconductor switches, capacitors may be connected to the upper arm side switches Q u , and Q v , as shown in a second embodiment of  FIG. 5 , or capacitors may be connected to all the upper and lower arm semiconductor switches Q u , Q v , Q x , and Q v , as shown in a third embodiment of  FIG. 6 . In these cases too, it is possible to carry out a zero voltage switching. 
         [0094]    Next, a description will be given of a first embodiment of a control method according to the invention. Each embodiment of the control method described hereafter is an embodiment in a case in which the power transfer to the power receiving coil  120  is temporarily stopped because of a power outage or the like, and the power transfer subsequently restarted. 
         [0095]      FIG. 7  shows an operation waveform of the current i of the power receiving coil  120  and alternating current voltage v of the bridge circuit, and drive signals of the semiconductor switches Q u , Q x , Q v , and Q y , from a stoppage to a restart of the power transfer from the primary side feeder wire  110 , with the circuit of  FIG. 1  as a subject. 
         [0096]    When the power transfer is stopped from a normal power transfer condition at a timing (a) of  FIG. 7 , a loss of the current i is detected by the current detection unit CT in  FIG. 1 , all the semiconductor switches Q u , Q x , Q v , and Q y  are put into an off condition, and that condition is maintained. 
         [0097]    Next, on the power transfer being restarted at a timing (b) of  FIG. 7 , a voltage in accordance with a high frequency current of the primary side feeder wire  110  is induced in the power receiving coil  120 . At this time, as all the semiconductor switches Q u , Q x , Q v , and Q y  are in an off condition as heretofore described, the bridge circuit inside the power receiving circuit  310  is equivalent to a diode full-wave rectifier circuit. 
         [0098]    Because of this, a resonant current flows along a path from the power receiving coil  120  through the diode D v , smoothing capacitor C 0 , and diode D x  to the resonant capacitor C in  FIG. 1 . The polarity of the current inverts at a timing (c) of  FIG. 7 , and the current flows along a path from the power receiving coil  120  through the resonant capacitor C, diode D u , and smoothing capacitor C o  to the diode D y . 
         [0099]    A zero-crossing of the current i at the timing (c) is detected by the current detector unit CT, and the control device  200  controls in such a way as to restart the switching action of each semiconductor switch. 
         [0100]    Because of this, in the embodiment, the path of the resonant current flowing through the power receiving coil  120  when the power transfer is restarted is secured by temporarily carrying out a full-wave rectifying action with the diodes, and it is possible to restart normally by subsequently starting a desired switching action after the zero-crossing of the current i is detected. 
         [0101]    The heretofore described circuit action is established under a condition whereby the power receiving coil induced voltage when the power transfer is restarted is greater than the direct current output voltage V o  (the smoothing capacitor C 0  voltage). When the power receiving coil induced voltage when the power transfer is restarted is smaller than the direct current output voltage V o  because of the characteristics of the connected load R, it is possible to carry out a restarting action using the following second embodiment and third embodiment. 
         [0102]      FIG. 8  is an operation illustration showing a second embodiment of the control method according to the invention and, in the same way as  FIG. 7 , shows an operation waveform of the current i and voltage v, and drive signals of the semiconductor switches Q u , Q x , Q v , and Q y , from a stoppage to a restart of the power transfer from the primary side feeder wire  110 . 
         [0103]    When the power transfer is stopped from a normal power transfer condition at a timing (a) of  FIG. 8 , a loss of the current i is detected by the current detection unit CT in  FIG. 1 , and the individual semiconductor switches Q u , Q x , Q v , and Q y  are controlled into “Q u : off, Q x : on, Q v : off, and Q y : on” conditions. 
         [0104]    This on/off control corresponds to period I shown in  FIG. 2 , wherein only the lower arm side semiconductor switches Q x  and Q y  are put into an on condition. 
         [0105]    Next, on the power transfer being restarted at a timing (b) of  FIG. 8 , a voltage in accordance with a high frequency current of the primary side feeder wire  110  is induced in the power receiving coil  120 . At this time, the semiconductor switches of the bridge circuit of the power receiving circuit  310  are in the heretofore described on and off conditions, and a resonant current flows along a path from the power receiving coil  120  through the switch Q y  and diode D x  to the resonant capacitor C. The polarity of the current inverts at a timing (c) of  FIG. 8 , and the current flows along a path from the power receiving coil  120  through the resonant capacitor C and switch Q x  to the diode D y . 
         [0106]    A zero-crossing of the current i at a timing (c) is detected by the current detector unit CT, and the control device  200  controls in such a way as to restart the switching action of each semiconductor switch. That is, by maintaining the lower arm side semiconductor switches Q x  and Q y  in the on condition during the period from the timing (a) to the timing (b) for which the power transfer is stopped, it is possible to secure the path of the resonant current flowing through the power receiving coil  120  when the power transfer is restarted, and to restart normally. 
         [0107]    In  FIG. 8 , an example is shown of a condition wherein only the lower arm side semiconductor switches Q x  and Q y  are in the on condition during the period for which the power transfer is stopped, but by putting only the upper arm side semiconductor switches Q u  and Q v  into the on condition too, in the same way as heretofore described, it is possible to secure the path of the resonant current flowing through the power receiving coil  120  when the power transfer is restarted, and it is possible to restart a desired switching action after the zero-crossing of the current i is detected. 
         [0108]    Next,  FIG. 9  is an operation illustration showing a third embodiment of the control method according to the invention and, in the same way as  FIGS. 7 and 8 , shows an operation waveform of the current i and voltage v, and drive signals of the semiconductor switches Q u , Q x , Q v , and Q y , from a stoppage to a restart of the power transfer from the primary side feeder wire  110 . 
         [0109]    In the embodiment, when the power transfer is stopped from a normal power transfer condition at a timing (a) of  FIG. 9 , a loss of the current i of the power receiving coil  120  is detected by the current detection unit CT, and the condition of each semiconductor switch Q u , Q x , Q v , and Q y  is maintained at the same control condition as immediately before the current i is lost. 
         [0110]    The on/off control of the semiconductor switches Q u , Q x , Q v , and Q y  at this time corresponds to period II or period V in  FIG. 2 . In  FIG. 9 , as the current i is negative, a case is shown wherein the semiconductor switches Q u , Q x , Q v , and Q y  are maintained in the same switching conditions as in period V of  FIG. 2 . 
         [0111]    Next, on the power transfer being restarted at a timing (b) of  FIG. 9 , a voltage in accordance with a high frequency current of the primary side feeder wire  110  is induced in the power receiving coil  120 . At this time, the semiconductor switches of the bridge inverter circuit of the power receiving circuit  310  are in the heretofore described on and off conditions, that is, “Q u : off, Q x : on, Q v : on, and Q y : off”. 
         [0112]    Because of this, a resonant current flows along a path from the power receiving coil  120  through the diode D v , smoothing capacitor C 0 , and diode D x  to the resonant capacitor C, but when the power receiving coil  120  induced voltage when the power transfer is restarted is smaller than the direct current output voltage V o  it is not possible that the current i flows along this path. 
         [0113]    Next, as the current i starts to flow along a path from the resonant capacitor C through the switch Q x , smoothing capacitor C 0 , and switch Q v  to the power receiving coil  120  on the polarity of the current i inverting at a timing (c) of  FIG. 9 , a smoothing capacitor C 0  discharge period is entered. Furthermore, the polarity of the current i inverts at a timing (d) of  FIG. 9 , the current i flows along a path from the power receiving coil  120  through the diode D v , smoothing capacitor C 0 , and diode D x  to the resonant capacitor C, and a smoothing capacitor C 0  charge period is entered. 
         [0114]    As a zero-crossing of the current i at a timing (d) of  FIG. 9  is detected by the current detector unit CT, and the control device  200  controls in such a way as to restart the same kinds of switching action as the normal actions shown in  FIG. 2 , the semiconductor switches shift to the “Q u : on, Q x : off, Q v : on, and Q y : off” conditions, and the current i flows along the same path as in period IV shown in  FIG. 2 . 
         [0115]    That is, by maintaining the semiconductor switches in the switching conditions immediately before the timing (the timing (a) of  FIG. 9 ) at which the current i of the power receiving coil  120  is lost, the path of the resonant current flowing through the power receiving coil  120  when the power transfer is restarted is secured, and it is possible to restart normally by detecting the zero-crossing of the current i, and restarting the switching actions. 
         [0116]    Next,  FIG. 10  is a circuit diagram showing a fourth embodiment of the contactless power transfer system according to the invention. 
         [0117]    A characteristic of the contactless power transfer systems shown in  FIGS. 1 ,  5 , and  6  is that, by configuring the power receiving circuit by bridge connecting the semiconductor switches Q u , Q x , Q v , and Q y , it is possible to control the direct current output voltage V o  to a constant, regardless of whether a motoring load or regenerative load is connected to the subsequent stage. However, as four semiconductor switches are necessary, there is a danger of an increase in size and increase in cost of the device when factoring in a cooling unit, or the like. 
         [0118]    Therefore, the contactless power transfer system of the fourth embodiment seeks to achieve a reduction in size and reduction in cost of the device by being compatible only with a motoring load, and not with a regenerative load. 
         [0119]    In  FIG. 10 , a power receiving circuit  340  has a switching arm series circuit wherein an arm in which the diode D u  is connected in reverse parallel to the semiconductor switch Q u  and an arm in which the diode D x  is connected in reverse parallel to the semiconductor switch Q x  are connected in series, and has a diode series circuit wherein the diodes D v  and D y  are connected in series. Then, the switching arm series circuit and diode series circuit are connected in parallel, and the smoothing capacitor C o  is connected to either end of the diode series circuit. An internal connection point of the switching arm series circuit and an internal connection point of the diode series circuit form alternating current terminals of the bridge circuit, and both ends of the diode series circuit form direct current terminals. Other than the power receiving circuit  340 , the configuration is the same as in each previously described embodiment. 
         [0120]    The control device  200  generates drive signals of the semiconductor switches Q u  and Q x  based on the direct current output voltage Vo of the power receiving circuit  340  and the detection signal of the current i of the power receiving coil  120 . 
         [0121]      FIG. 11  is an operation illustration of the circuit of  FIG. 10 , and shows an operation waveform of the current i and voltage v, and drive signals of the semiconductor switches Q u  and Q x . 
         [0122]    As shown in  FIG. 11 , the semiconductor switches Q u  and Q x  carry out a switching action at a constant frequency in synchronization with the current i of the power receiving coil  120 . Hereafter, a description will be given of an action in each period i to iv of  FIG. 11 . 
         [0123]    1. Period i (switch Q x  on, diode D y  has continuity): the current i flows along a path from the resonant capacitor C through the switch Q x  and diode D y  to the power receiving coil  120 , and the voltage v is at a zero voltage level, as shown in the drawing. 
         [0124]    2. Period ii (switch Q u  on, diode D y  has continuity): the current i flows along a path from the resonant capacitor C through the diode D u , smoothing capacitor C 0 , and diode D y  to the power receiving coil  120 , and the voltage v is at a positive voltage level corresponding to the direct current output voltage V o , as shown in the drawing. In this period, the smoothing capacitor C 0  is charged by the current i. 
         [0125]    3. Period iii (switch Q u  on, diode D v  has continuity): the current i flows along a path from the resonant capacitor C through the power receiving coil  120  and diode D v  to the switch Q u , and the voltage v is at a zero voltage level, as shown in the drawing. 
         [0126]    4. Period iv (switch Q x  on, diode D v  has continuity): the current i flows along a path from the resonant capacitor C through the power receiving coil  120 , diode D v , and smoothing capacitor C 0  to the diode D x , and the voltage v is at a negative voltage level corresponding to the direct current output voltage V o , as shown in the drawing. In this period, the smoothing capacitor C 0  is charged by the current i. 
         [0127]    Subsequently, there is a transition to the switching mode of period i, and the same actions are repeated. 
         [0128]    By controlling the semiconductor switches Q u  and Q x  as heretofore described, the alternating current voltage v of the bridge circuit is controlled by positive and negative voltages with the direct current output voltage V o  as a crest value. Power fed from the primary side feeder wire  110  to the power receiving circuit  340  is the product of the current i and voltage v shown in  FIG. 11 , and control of the power fed, that is, a constant control of the direct current output voltage V o , is enabled by the control device  200  adjusting the phases of the drive signals of the semiconductor switches Q u  and Q x  based on the detected value of the direct current output voltage V o . 
         [0129]      FIG. 12  is a circuit diagram showing a fifth embodiment of the contactless power transfer system according to the invention. In this embodiment, the capacitor C x  is connected to the lower arm side semiconductor switch Q x , with the same idea as in the first embodiment of  FIG. 1 . According to the fifth embodiment, it is possible to carry out a so-called soft switching at a time of on/off actions of the semiconductor switches when there is a switching between the periods shown in  FIG. 11 . 
         [0130]      FIG. 13  is an operation waveform drawing of the semiconductor switches Q u  and Q x  in period i to period ii shown in  FIG. 11 . As the operation waveform drawing is the same as the operation waveform drawing of the semiconductor switches Q u  and Q x  in period I to period II shown in  FIG. 4 , a description will be omitted here. 
         [0131]    It is possible to carry out a zero voltage switching owing to the charging and discharging actions of the capacitor C x connected in parallel to the switch Q   x  for the on/off actions of the semiconductor switches not only when switching from period i to period ii, but also when switching between other periods. 
         [0132]    Also, as examples of capacitors being connected in parallel to semiconductor switches, a capacitor may be connected to the upper arm side switch Q u , as shown in a sixth embodiment of  FIG. 14 , or capacitors may be connected to the upper and lower arm semiconductor switches Q u  and Q x , as shown in a seventh embodiment of  FIG. 15 . In these cases too, a zero voltage switching is possible. 
         [0133]    Next,  FIG. 16  is an operation illustration showing a fourth embodiment of the control method according to the invention, and shows an operation waveform of the current i and voltage v, and drive signals of the semiconductor switches Q u  and Q x , from a stoppage to a restart of the power transfer from the primary side feeder wire  110 . 
         [0134]    When the power transfer from the power receiving coil  120  is stopped from a normal power transfer condition at a timing (a) of  FIG. 16 , a loss of the current i is detected by the current detection unit CT in  FIG. 10 , both switches Q u  and Q x  are put into an off condition, and that condition is maintained. 
         [0135]    Subsequently, on the power transfer being restarted at a timing (b) of  FIG. 16 , a voltage in accordance with a high frequency current of the primary side feeder wire  110  is induced in the power receiving coil  120 . At this time, as the switches Q u  and Q x  are in an off condition as heretofore described, the bridge circuit of the power receiving circuit  340  is equivalent to a diode full-wave rectifier circuit. Because of this, a resonant current flows along a path from the power receiving coil  120  through the diode D v , smoothing capacitor C 0 , and diode D x  to the resonant capacitor C. The polarity of the current inverts at a timing (c) of  FIG. 16 , and the current flows along a path from the power receiving coil  120  through the resonant capacitor C, diode D u , and smoothing capacitor C 0  to the diode D y . 
         [0136]    A zero-crossing of the current i at the timing (c) is detected by the current detector unit CT, and the control device  200  controls in such a way as to restart the switching action of each semiconductor switch Q u  and Q x . Because of this, the path of the resonant current flowing through the power receiving coil  120  when the power transfer is restarted is secured by temporarily carrying out a full-wave rectifying action with the diodes, and it is possible to restart normally by starting a desired switching action after the zero-crossing of the current i is detected. 
         [0137]    The heretofore described circuit action is established under a condition whereby the power receiving coil induced voltage when the power transfer is restarted is greater than the direct current output voltage V o  (the smoothing capacitor C 0  voltage). Because of this, when the power receiving coil induced voltage when the power transfer is restarted is smaller than the direct current output voltage V o  because of the characteristics of the connected load, it is possible to carry out a restarting action using a control method according to the following fifth embodiment. 
         [0138]      FIG. 17  is an operation illustration showing a fifth embodiment of the control method according to the invention, and shows an operation waveform of the current i and voltage v, and drive signals of the semiconductor switches Q u  and Q x , from a stoppage to a restart of the power transfer from the primary side feeder wire  110 . 
         [0139]    When the power transfer from the power receiving coil  120  is stopped from a normal power transfer condition at a timing (a) of  FIG. 17 , in the embodiment, a loss of the current is detected by the current detection unit CT, and the condition of each switch Q u  and Q x  is maintained at the same control condition as immediately before the current i is lost. 
         [0140]    The on/off control at this time corresponds to period ii or period iv shown in  FIG. 11 . In  FIG. 17 , as the current i is negative, a case is shown wherein the semiconductor switches Q u  and Q x  are maintained in the same switching conditions as in period iv of  FIG. 11 . 
         [0141]    Next, on the power transfer being restarted at a timing (b) of  FIG. 17 , a voltage in accordance with a high frequency current of the primary side feeder wire  110  is induced in the power receiving coil  120 . At this time, the bridge circuit of the power receiving circuit  340  is in the heretofore described on and off condition (switch Q u  is off, switch Q x  is on), and a resonant current flows along a path from the power receiving coil  120  through the diode D v , smoothing capacitor C 0 , and diode D x  to the resonant capacitor C, but when the power receiving coil induced voltage when the power transfer is restarted is smaller than the direct current output voltage V o  it is not possible that the current flows along this path. 
         [0142]    Next, as the current i starts to flow along a path from the resonant capacitor C through the switch Q x  and diode D y  to the power receiving coil  120  on the polarity of the current inverting at a timing (c) of  FIG. 17 , the voltage v is at a zero voltage level, as shown in the drawing. Furthermore, on the polarity of the current i inverting at a timing (d) of  FIG. 17 , a zero-crossing of the current i is detected in the control device  200  from the output of the current detector unit CT. 
         [0143]    Then, as the control device  200  controls in such a way as to restart the same kinds of switching action as the normal actions shown in  FIG. 11 , the switch Q u  switches to an on condition, and the switch Q x  to an off condition, and the current flows along the same path as in period iii shown in  FIG. 11 . 
         [0144]    That is, even when putting the switches Q u  and Q x  into the same control conditions as immediately before the current i is lost, the path of the resonant current flowing through the power receiving coil  120  when the power transfer is restarted is secured, and it is possible to restart normally by detecting the zero-crossing of the current i, and restarting the switching actions. 
         [0145]    Next,  FIG. 18  is a circuit diagram showing an eighth embodiment of the contactless power transfer system according to the invention. With the contactless power transfer systems shown in  FIGS. 10 ,  12 ,  14 , and  15 , the number of semiconductor switches is reduced, and a reduction in size and cost of the device is sought, by configuring the bridge circuit with a switching arm series circuit and diode series circuit. However, these contactless power transfer systems are compatible only when a load connected at a subsequent stage is a motoring load, and are not compatible when the load is a regenerative load. 
         [0146]    Therefore, the contactless power transfer system of the eighth embodiment has a configuration compatible with both a motoring load and a regenerative load, while seeking a reduction in size and reduction in cost of the device. 
         [0147]    In  FIG. 18 , a power receiving circuit  380  has a switching arm series circuit wherein an arm in which the diode D u  is connected in reverse parallel to the semiconductor switch Q u  and an arm in which the diode D x  is connected in reverse parallel to the semiconductor switch Q x  are connected in series, and has a capacitor series circuit wherein resonant capacitors C v  and C y  are connected in series. As the resonant capacitors C v  and C y  are connected in the same positions as the capacitors C v  and C y  in  FIG. 6  and the like, the same reference numerals and characters are used. 
         [0148]    Then, the switching arm series circuit and capacitor series circuit are connected in parallel, and the smoothing capacitor C o  is connected to either end of the capacitor series circuit. An internal connection point of the switching arm series circuit and an internal connection point of the capacitor series circuit form alternating current terminals of the bridge circuit, and both ends of the capacitor series circuit form direct current terminals. The power receiving coil  120  is connected to the alternating current terminals of the bridge circuit, and the load R is connected to the direct current terminals. 
         [0149]    The control device  200  generates drive signals of the semiconductor switches Q u  and Q x  based on the direct current output voltage V o  of the power receiving circuit  380  and the detection signal of the current i of the power receiving coil  120 . 
         [0150]    Next, a description will be given of actions when the contactless power transfer system shown in  FIG. 18  is normal. 
         [0151]    The circuit shown in  FIG. 18  is such that a bidirectional power supply is possible between the power receiving coil  120  and the load R. Hereafter, a description will be given of two kinds of circuit action, a case of supplying power from the power receiving coil  120  to the load R and a case of supplying power from the load R to the power receiving coil  120 . 
         [0152]    Firstly, a description will be given of the actions in the case of supplying power from the power receiving coil  120  to the load R. 
         [0153]      FIG. 19  shows an operation waveform of the current i flowing through the power receiving coil  120  of  FIG. 18  and of the alternating current voltage v of the bridge circuit, and drive signals of the semiconductor switches Q u  and Q x . As shown in  FIG. 19 , the semiconductor switches Q u  and Q x  carry out a switching action at a constant frequency in synchronization with the current i of the power receiving coil  120 . Hereafter, a description will be given of an action in each period i to iv of  FIG. 19 . 
         [0154]    1. Period i (switch Q x  on): the current i flows along a path from the power receiving coil  120  through the switch Q x  to the capacitor C y , and the capacitor C y  is discharged. At this time, the voltage v is at a negative voltage level corresponding to the voltage of the capacitor C y . 
         [0155]    2. Period ii (switch Q u  on): the current i flows along a path from the power receiving coil  120  through the diode D u  and smoothing capacitor C 0  to the capacitor C y , the smoothing capacitor C 0  is charged, and the capacitor C y  is discharged. At this time, the voltage v is at a voltage level corresponding to the difference between the direct current output voltage V o  and the voltage of the capacitor C y . 
         [0156]    3. Period iii (switch Q u  on): the current i flows along a path from the power receiving coil  120  through the capacitor C v  to the switch Q u , and the capacitor C v  is discharged. At this time, the voltage v is at a positive voltage level corresponding to the voltage of the capacitor C v . 
         [0157]    4. Period iv (switch Q x  on): the current i flows along a path from the power receiving coil  120  through the capacitor C v  and smoothing capacitor C 0  to the diode D x , the smoothing capacitor C 0  is charged, and the capacitor C v  is discharged. At this time, the voltage v is at a voltage level corresponding to the difference between the voltage of the capacitor C v  and the direct current output voltage V o . 
         [0158]    Subsequently, there is a transition to the switching mode of period i, and the same actions are repeated. 
         [0159]    Next, a description will be given of the case of supplying power from the load R to the power receiving coil  120 . 
         [0160]      FIG. 20 , in the same way as  FIG. 19 , shows an operation waveform of the current i flowing through the power receiving coil  120  and of the alternating current voltage v of the bridge circuit, and drive signals of the semiconductor switches Q u  and Q x . 
         [0161]    As shown in  FIG. 20 , the semiconductor switches Q u  and Q x  carry out a switching action at a constant frequency in synchronization with the current i of the power receiving coil  120 . Hereafter, a description will be given of an action in each period i′ to iv′ of  FIG. 20 . 
         [0162]    1. Period i′ (switch Q x  on): the current i flows along a path from the power receiving coil  120  through the capacitor C y  to the diode D x , and the capacitor C y  is charged. At this time, the voltage v is at a negative voltage level corresponding to the voltage of the capacitor C y . 
         [0163]    2. Period ii′ (switch Q u  on): the current i flows along a path from the power receiving coil  120  through the capacitor C y  and smoothing capacitor C 0  to the switch Q u , the smoothing capacitor C 0  is discharged, and the capacitor C y  is charged. At this time, the voltage v is at a voltage level corresponding to the difference between the direct current output voltage V o  and the voltage of the capacitor C y . 
         [0164]    3. Period iii′ (switch Q u  on): the current i flows along a path from the power receiving coil  120  through the diode D u  to the capacitor C y , and the capacitor C y  is charged. At this time, the voltage v is at a positive voltage level corresponding to the voltage of the capacitor C v . 
         [0165]    4. Period iv′ (switch Q x  on): the current i flows along a path from the power receiving coil  120  through the switch Q x , smoothing capacitor C 0 , and capacitor C y  to the power receiving coil  120 , the smoothing capacitor C 0  is discharged, and the capacitor C v  is charged. At this time, the voltage v is at a voltage level corresponding to the difference between the voltage of the capacitor C v  and the direct current output voltage V o . 
         [0166]    Subsequently, there is a transition to the switching mode of period i′, and the same actions are repeated. 
         [0167]    By controlling the semiconductor switches Q u  and Q x  as heretofore described, the alternating current voltage v of the bridge circuit is controlled to the voltage of the capacitors C y  and C v , or to the difference between the direct current output voltage Vo and the voltage of the capacitors C y  and C v . Power fed from the primary side feeder wire  110  to the power receiving circuit  380  is the product of the current i and voltage v shown in  FIG. 19 , and control of the power fed, that is, a constant control of the direct current output voltage V o , is enabled by the control device  200  adjusting the phases of the drive signals of the semiconductor switches Q u  and Q x  based on the detected value of the direct current output voltage V o . 
         [0168]      FIG. 21  is a circuit diagram showing a ninth embodiment of the contactless power transfer system according to the invention, wherein reference numeral  390  is a power receiving circuit. In this embodiment, the capacitor C x  is connected to the lower arm side semiconductor switch Q x , with the same idea as in the first embodiment of  FIG. 1 . According to the ninth embodiment, it is possible to carry out a so-called soft switching at a time of on/off actions of the semiconductor switches when there is a switching between the periods shown in  FIGS. 19 and 20 . 
         [0169]      FIG. 22  is an operation waveform drawing of the semiconductor switches Q u  and Q x  in period i to period ii shown in  FIG. 19 . As the operation waveform drawing is the same as the operation waveform drawing of the semiconductor switches Q u  and Q x  in period I to period II of  FIG. 4 , a description will be omitted here. 
         [0170]    It is possible to carry out a zero voltage switching owing to the charging and discharging actions of the capacitor C x  connected in parallel to the switch Q x  for the on/off actions of the semiconductor switches not only when switching from period i to period ii, but also when switching between other periods. 
         [0171]    Also, as examples of capacitors being connected in parallel to semiconductor switches, the capacitor C u  may be connected in parallel to the upper arm side switch Q u , as in a contactless power transfer system  400  according to a tenth embodiment of  FIG. 23 , or the capacitors C u  and C x  may be connected in parallel to the upper and lower arm semiconductor switches Q u  and Q x  respectively, as in a contactless power transfer system  410  according to an eleventh embodiment of  FIG. 24 . In these cases too, a zero voltage switching is possible. 
         [0172]    Next,  FIG. 25 , in the same way as  FIG. 16 , is an operation illustration showing the fourth embodiment of the control method according to the invention, and shows an operation waveform of the current i and voltage v, and drive signals of the semiconductor switches Q u  and Q x , from a stoppage to a restart of the power transfer from the primary side feeder wire  110 . This control method is applied to the eighth to eleventh embodiments of the contactless power transfer system shown in  FIGS. 18 ,  21 ,  23 , and  24 , and a basic restarting method is essentially the same as the method illustrated in the operation illustration of  FIG. 16 . 
         [0173]    That is, a stoppage of the power transfer from the power receiving coil  120  and a loss of the current i are detected at a timing (a) of  FIG. 25 , the switches Q u  and Q x  are put into an off condition, and that condition is maintained. Then, on the power transfer being restarted at a timing (b), a voltage in accordance with a high frequency current of the primary side feeder wire  110  is induced in the power receiving coil  120 , and a resonant current flows along a path from the power receiving coil  120  through the resonant capacitor C v  and smoothing capacitor C o  to the diode D. The polarity of the current inverts at a timing (c), and the current flows along a path from the power receiving coil  120  through the diode D u  and smoothing capacitor C o  to the resonant capacitor C y . 
         [0174]    In the control device  200 , a restart is enabled by a zero-crossing of the current i at the timing (c) being detected by the current detector unit CT, and the switching action of each semiconductor switch Q u  and Q x  being subsequently restarted. 
         [0175]      FIG. 26 , in the same way as  FIG. 17 , is an operation illustration showing the fifth embodiment of the control method according to the invention, and shows an operation waveform of the current i and voltage v, and drive signals of the semiconductor switches Q u  and Q x , from a stoppage to a restart of the power transfer from the primary side feeder wire  110 . This control method is also applied to the eighth to eleventh embodiments of the contactless power transfer system shown in  FIGS. 18 ,  21 ,  23 , and  24 , and a basic restarting method is essentially the same as the method illustrated in the operation illustration of  FIG. 17 . 
         [0176]    That is, a stoppage of the power transfer from the power receiving coil  120  and a loss of the current i are detected at a timing (a) of  FIG. 26 , the switches Q u  and Q x  are put into the same control conditions as immediately before the current is lost, and those conditions are maintained. Next, on the power transfer being restarted at a timing (b), a resonant current attempts to flow along a path from the power receiving coil  120  through the resonant capacitor C v  and smoothing capacitor C o  to the diode D x  (shown by a broken line in  FIG. 26 ), but when the power receiving coil induced voltage when the power transfer is restarted is less than the direct current output voltage V o , it is not possible for the current to flow along this path. 
         [0177]    Next, on the polarity of the current i inverting at a timing (c), the current i starts to flow along a path from the power receiving coil  120  through the switch Q x  to the resonant capacitor C y . Then, on the polarity of the current i inverting at a timing (d), the control device  200  detects a zero-crossing of the current i, and enables a normal restarting by switching the switch Q u  to an on condition and the switch Q x  to an off condition, restarting switching actions the same as normal. 
         [0178]    The invention can be utilized in various kinds of electrical or electronic instrument, electric vehicle, and the like, to which power is supplied in a contactless condition. 
         [0179]    Finally, it is noted that while the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the present invention.