Patent Publication Number: US-2021184501-A1

Title: Wireless power transmission with modular output

Description:
FIELD OF INVENTION 
     The present invention relates to a wireless power transmission system, and in particular to the provision of a modular output structure at a mobile side of a wireless power transmission system, to the measurement of an output current of the modular output structure, and to a control method for a synchronous rectifier operated at the mobile side of the wireless power transfer system. 
     BACKGROUND ART 
     Generally, wireless power transfer systems have a number of advantages over conductive power transmission systems. E.g., for electric vehicles it means that a plug in of a cable is no longer required. 
       FIG. 1  shows a schematic diagram of a wireless power transmission system  100  as known in the art. 
     As shown in  FIG. 1 , a wireless power transmission system  100  comprises at a stationary side a DC/AC converter  102 , a stationary side controller  104 , a stationary side compensation capacitor  106 , and a transmitter coil  108  connected in series to the stationary side compensation capacitor  106 . The series connection of the stationary side compensation capacitor  106  and the transmitter coil  108  is connected to the output side of the DC/AC converter  102 . 
     As shown in  FIG. 1 , the wireless power transmission system  100  comprises at a mobile side a receiver coil  110  connected in series to a mobile side compensation capacitor  112 . The series connection of the receiver coil  110  and the mobile side compensation capacitor  112  is connected to an input side of an AC/DC converter  114  which is operated under control of a mobile side controller  116 . Parallel to the series connection of the receiver coil  110  and the mobile side compensation capacitor  112  there may connected a transformer  118  to improve galvanic decoupling. At the output of the AC/DC converter  114  there is connected a load  118 . For the connection of the load  118  there may be provided a DC/DC converter for control of the power level delivered to the load  118  (not shown in  FIG. 1 ). 
     As shown in  FIG. 1 , a wireless communication link  122  may be established from the mobile side to the stationary side for exchange of control data and/or measurement data between from the mobile side to the stationary side. 
     Operatively, the DC/AC converter  102  is adapted to receive a DC input signal and adapted to convert it into a stationary side AC signal. The stationary side AC signal is output to the series connection of the stationary side compensation capacitor  106  and the transmitter coil  108  for generation of an oscillating magnetic field. The stationary side controller  104  is adapted to measure the characteristics of the stationary side AC signal and optionally the DC input signal for control of the DC/AC converter  102 . In more detail, the stationary side controller  104  is adapted to control the DC/AC converter  102  such that the generated magnetic field oscillates at resonant frequency of the series connection of the stationary side compensation capacitor  106  and the transmitter coil  108 . 
     Operatively, the receiver coil  110 , when placed in the magnetic field produced by the transmitter coil  108 , receives energy transmitted by the transmitter coil  108  through inductive coupling. The inductive coupling leads to the generation of a mobile side AC signal. Under control of the mobile side controller  116  the AC/DC converter  114  is adapted to convert the mobile side AC signal into a load side DC signal which is then forwarded to the load  118 . 
     Operatively, the mobile side controller  116  is adapted to measure the mobile side AC signal and optionally the load side DC signal for control of a power delivered to the load  118 . Operatively, measurement data and control data may be sent over the wireless communication link  120  to improve the control and to inform the stationary side on fault conditions at the mobile side. 
     Generally, the cables to coils in the wireless power transmission system  100  as described above are operated at the power transfer frequency and carry significant voltage and power. Further, at the mobile side of the wireless power transmission system  100  the output current flows through the mobile side compensation capacitor  112  and rectifier elements of the AC/DC converter  114 . Thus, as the current increases also losses increase leading to a degrading electrical performance. 
     SUMMARY OF INVENTION 
     In view of the above, the object of the present invention is to provide efficient ways to improve electrical performance in a mobile side output circuitry of a wireless power transmission system. 
     According to a first aspect of the present invention this object is achieved by a mobile side circuitry of a wireless power transmission system. The mobile side circuitry of the wireless power transmission system comprises a mobile side resonant circuit adapted to inductively couple the mobile side circuitry to a stationary side circuitry of the wireless power transmission system, a mobile side rectifier stage adapted to rectify an input signal for supply of power to a mobile side load, and a mobile side transformer stage connected at its input side to the mobile side resonant circuit and connected at its output side to the mobile side rectifier stage. Further, the mobile side transformer stage comprises at least one primary side winding and a plurality of secondary side windings and the mobile side rectifier stage comprises a plurality of mobile side AC/DC converters each connected to one of the plurality secondary side windings. According to the first aspect of the present invention output terminal pairs of the plurality of mobile side AC/DC converters are connected in series or output terminal pairs of the plurality of mobile side AC/DC converters are connected in parallel or mobile side AC/DC converters are grouped into a plurality of mobile side output groups such that output terminal pairs within each mobile side output group are connected in series and output terminal pairs of different mobile side output groups are connected in parallel. 
     According to a second aspect of the present invention the object outlined above is achieved by use of a current transformer for evaluation of an output current of a wireless power transfer system. According to the second aspect the current transformer has a primary side winding connected to an input of a mobile side circuitry of a wireless power transmission system and a secondary side winding connected to a monitoring circuit adapted to evaluate the output current of a wireless power transfer system. 
     According to a third aspect of the present invention the object outlined above is achieved by a monitoring circuit for determining an output current of a wireless power transfer system. The monitoring circuit comprises a current transformer having a primary side winding and a secondary side winding, wherein the primary side winding is connected to an input of a mobile side circuitry of a wireless power transmission system. The monitoring circuit further comprises a rectifying circuit connected to the secondary side winding. According to the third aspect the monitoring circuit further comprises an averaging circuit connected to the rectifying circuit and adapted to determine an average of the output of the rectifying circuit as equivalent to the output current of the wireless power transmission system. 
     According to a fourth aspect of the present invention the object outlined above is achieved by a controller for controlling at least one synchronous AC/DC converter operated in a mobile side circuitry of a wireless power transmission system comprising a signal processing unit and a control processing unit. According to the fourth aspect of the present invention the signal processing unit is adapted to receive an output signal of a current transformer having a primary side winding connected to an input of the mobile side circuitry of the wireless power transmission system, to classify a polarity of the output signal with respect to a reference potential as positive polarity or negative polarity, and to compare the output signal with a threshold value. Further, according to the fourth aspect of the present invention the control processing unit is adapted to turn on at least one first switching circuit of the at least one synchronous AC/DC converter when the output signal has positive polarity and the absolute value of the output signal is larger than the threshold value. Otherwise, the control processing unit is adapted to turn on at least one second switching circuit of the at least one synchronous AC/DC converter being different from the at least one first circuit when the output signal has negative polarity and the absolute value of the output signal is larger than the threshold value. 
     According to a fifth aspect of the present invention the object outlined above is achieved by a method of controlling operation of at least one synchronous AC/DC converter operated in a mobile side circuitry of a wireless power transmission system. The method comprises the steps of receiving an output signal of a current transformer having a primary side winding connected to an input of the mobile side circuitry a wireless power transmission system, of classifying a polarity of the output signal with respect to a reference potential as positive polarity or negative polarity, and of comparing the output signal with a threshold value. The method of controlling operation of at least one synchronous AC/DC converter further comprises the steps of turning on at least one first switching circuit of the at least one synchronous AC/DC converter when the output signal has positive polarity and an absolute value of the output signal is larger than the threshold value and of turning on at least one second switching circuit of the at least one synchronous AC/DC converter being different from the at least one first switching circuit when the output signal has negative polarity and the absolute value of the output signal is larger than threshold value. 
    
    
     
       DESCRIPTION OF DRAWING 
       In the following different aspects and examples of the present invention will be explained with reference to the drawing in which: 
         FIG. 1  shows a schematic circuit diagram of a wireless power transmission system as known in the art; 
         FIG. 2  shows a schematic circuit diagram of a wireless power transmission system explained for an understanding of the present invention; 
         FIG. 3  shows a basic concept of the modular output circuit at the mobile side of the wireless power transfer system; 
         FIG. 4  shows a modification of the basic concept of the modular output circuit as shown in  FIG. 3 ; 
         FIG. 5  shows a first series configuration of the modular output circuit shown in  FIG. 3 ; 
         FIG. 6  shows the first series configuration as shown in  FIG. 3  with balancing modules connected in parallel being added; 
         FIG. 7  shows an example of the first series configuration shown in  FIG. 3  with four output terminal pairs connected in series; 
         FIG. 8  shows a second series configuration of the modular output circuit shown in  FIG. 3 ; 
         FIG. 9  shows a group configuration of the modular output circuit shown in  FIG. 3  where rectifier circuits and related secondary side windings are grouped into mobile side output groups, output terminals of rectifier circuits within each mobile side output group are connected in series, and output terminal pairs of different mobile side output groups are connected in parallel; 
         FIG. 10  shows an example of the group configuration shown in  FIG. 9  where each mobile side output group has two transformer units as well as two mobile side AC/DC converters, respectively; 
         FIG. 11  shows a parallel configuration of the modular output circuit shown in  FIG. 3 ; 
         FIG. 12  shows an example of the parallel configuration shown in  FIG. 11  with four output terminal pairs connected in parallel; 
         FIG. 13  shows a schematic diagram illustrating different current forms at different stages of a power train as motivation for indirect DC output current measurement according to the present invention; 
         FIG. 14  shows a schematic diagram of a mobile side circuitry of a wireless power transfer system using a current transformer for indirect measurement of a DC output current and subsequent use of the measurement result for control of mobile side rectifier circuits; 
         FIG. 15  shows a schematic circuit diagram of a controller apparatus for an inductive power transfer system according to present invention; and 
         FIG. 16  shows a flowchart of operation for the controller apparatus shown in  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     In the following the present invention will be explained in detail with reference to the drawing. Here, it should be understood that such explanation is related to examples of the present invention only and not binding to the scope of the present invention as defined by the claims. As far as reference is made to specific circuit components this is to be considered as example for the underlying functionality such the circuit components are clearly exchangeable as long as the same functionality is achieved. 
       FIG. 2  shows a schematic diagram of a wireless power transmission system  10  explained for an understanding of the present invention. 
     As shown in  FIG. 2 , the wireless power transfer system  10  has a transmission unit  12  at the stationary side. The transmission unit  12  comprises a stationary side DC/AC converter  14 , a stationary side controller  16 , a stationary side compensation capacitor  18 , and a transmitter coil  20  connected in series to the stationary side compensation capacitor  18 . The series connection of the stationary side compensation capacitor  18  and the transmitter coil  20  is connected to the output side of the stationary side DC/AC converter  14 . 
     Operatively, the stationary side DC/AC converter  14  is adapted to receive a stationary side DC voltage U 1,dc  and a stationary side direct current I 1,dc  as input and to convert it into a stationary side high frequency voltage U 1,hf  and a stationary side high frequency current I 1,hf . The stationary side high frequency voltage U 1,hf  and the stationary side high frequency current l 1,hf  are then supplied to the series connection of the stationary side compensation capacitor  18  and transmitter coil  20  for generation of an oscillating magnetic field. 
     Operatively, the stationary side controller  16  is adapted to measure the stationary side high frequency current l 1,hf  and optionally the stationary side high frequency voltage U 1,hf , the stationary side direct current I 1,dc  and/or the stationary side dc voltage U 1,dc . The stationary side controller  16  is adapted to process a measurement result for control of the stationary side DC/AC converter  14 . The stationary side DC/AC converter  14  is adapted to supply the stationary side high frequency voltage U 1,hf  and the stationary side high frequency current l 1,hf  to the series connection of the stationary side compensation capacitor  18  and the transmitter coil  20 . The control of the stationary side controller  16  is such that the magnetic field generated by the transmitter coil  20  oscillates at resonant frequency of the series connection of the stationary side compensation capacitor  18  and the transmitter coil  20 . 
     As shown in  FIG. 2 , the wireless power transmission system  10  also has at least one receiving unit  22  separated from the transmission unit  12 . 
     As shown in  FIG. 2 , the receiving unit  22  comprises a receiver coil  24  connected in series to a mobile side compensation capacitor  26 . The receiving unit  22  further comprises a mobile side AC/DC converter  28 . At an input side of the mobile side AC/DC converter  28  there is connected a mobile side transformer stage  30  which at the input side is connected to the series connection of the receiver coil  24  and the mobile side compensation capacitor  26 . At it the output side the transformer stage  30  is connected to the mobile side converter  28 . Further, at the output side of the mobile side converter  28  there is connected a load  32 . The receiving unit  22  comprises a mobile side controller  34  adapted to control the mobile side AC/DC converter  28 . 
     Operatively, the receiver coil  24 , when placed in the magnetic field produced by the transmitter coil  20 , receives energy transmitted by the transmitter coil  20  through inductive coupling. The inductive coupling leads to generation of a mobile side high frequency voltage U 2,hf  and a mobile side high frequency current  1   2,hf . 
     Operatively, the mobile side AC/DC converter  28  is adapted to convert the mobile side high frequency voltage U 2,hf  and the mobile side high frequency current I 2,hf  after transformation through the mobile side transformer  30  into a mobile side DC voltage U 2,dc  and a mobile side direct current I 2,dc  under control of the mobile side controller  34 . 
     Operatively, the output transformer stage  30  is adapted to reduce the currents flowing in the receiver coil  24  and the mobile side compensation capacitor  26  while the current in the mobile side AC/DC converter  28  remains the output current o the load  32 . 
     Operatively, the mobile side controller  34  is adapted to optionally measure the mobile side high frequency current I 2,hf  and to optionally measure the mobile side high frequency voltage U 2,hf,  the mobile side direct current I 2,dc  and/or the mobile side DC voltage U 2,dc . The mobile side controller  34  is adapted to process a measurement result for controlling the mobile side AC/DC converter  28 . The mobile side AC/DC converter  28  is adapted to supply of the mobile side DC voltage U 2,dc  and the mobile side direct current I 2,dc  to the load  32 , e.g., either directly or via a DC/DC converter (not shown in  FIG. 2 ). 
       FIG. 3  shows a basic concept of the modular output circuit at the mobile side of the wireless power transfer system. 
     As shown in  FIG. 3 , according to the present invention the mobile side transformer stage  30  comprises at least one primary side winding  36 _ 1 , . . . ,  32 _n and a plurality of secondary side windings  38 _ 1 , . . . ,  38 _n. The mobile side rectifier stage  30  further comprises a plurality of mobile side AC/DC converters  40 _ 1 , . . . ,  40 _n each connected to one of the plurality secondary side windings  38 _ 1 , . . . ,  38 _n. 
     According to a first configuration alternative of the present invention the output terminal pairs  42 _ 1 , . . . ,  42 _n of the plurality of the mobile side AC/DC converters may be connected in series. 
     According to a second configuration alternative of the present invention output terminal pairs  42 _ 1 , . . . ,  42 _n of the plurality of mobile side AC/DC converters  40 _ 1 , . . . ,  40 _n may be connected in parallel. 
     According to a third configuration alternative of the present invention mobile side AC/DC converters  40 _ 1 , . . . ,  40 _n are grouped into a plurality of mobile side output groups such that output terminal pairs within each mobile side output group are connected in series and output terminal pairs of different mobile side output groups are connected in parallel. 
     In the most general sense and as will be explained in more detail in the following according to the present invention:
         the number of mobile side AC/DC converters  40 _ 1 , . . . ,  40 _n is n&gt;1;   the number of mobile side output group(s) is g≥1;   the number of mobile side AC/DC converter(s) per mobile side output group is 1≤r ≤n;   output terminal pairs of mobile side AC/DC converters in each mobile side output group are connected in series if r &gt;1; and   output terminal pairs of the mobile side output groups are connected in parallel if g&gt;1.       

     As shown in  FIG. 3 , according to a first realization concept the modular output circuit the mobile side transformer stage  30  comprises a same number n of primary side windings  36 _ 1 , . . . ,  36 _n and secondary side windings  38 _ 1 , . . . ,  38 _n such that corresponding pairs of primary side windings  36 _ 1 , . . . ,  36 _n and secondary side windings  36 _ 1 , . . . ,  36 _n form transformer modules  44 _  1 , . . . ,  44 _n. It should be noted that transformer modules  44 _ 1 , . . . ,  44 _n share a common transformer core (not shown in  FIG. 3 ). 
       FIG. 4  shows a modification of the basic concept of the modular output circuit as shown in  FIG. 3 . 
     As shown in  FIG. 4 , according to a second realization concept the mobile side transformer stage  30  the mobile side circuit comprises one primary winding  36  being common to the plurality of secondary side windings  38 _ 1 , . . . ,  38 _n. It should be noted that the one primary side winding  36  and the plurality of secondary side windings  38 _ 1 , . . . ,  38 _n share a common transformer core (not shown in  FIG. 4 ). 
     Operatively, an advantage of the second realization concept the mobile side transformer stage  30  shown in  FIG. 4  is that occurrence of an unbalance between different primary side winding  36 _ 1 , . . . ,  36 _n may be avoided. 
       FIG. 5  shows a first series configuration of the modular output circuit  30  shown in  FIG. 3 . 
     As shown in  FIG. 5 , the input terminals of the plurality of transformer modules  44 _ 1 , . . . ,  44 _n are connected in series. 
     As shown in  FIG. 5 , the output terminal pairs  42 _ 1 , . . . ,  42 _n of the plurality of mobile side AC/DC converters  40 _ 1 , . . . ,  40 _n are connected in series leading to an all series circuit configuration having one single output terminal group, g=1, which accommodates all rectifier circuits  40 _ 1 , . . . ,  40 _n with n=r&gt;1. 
     Operationally, each transformer module  44 _ 1 , . . . ,  44 _n may be realized as ideal transformer having a transformer ratio ü:1. Further, assuming that the input voltage U 1  is equally divided across the primary windings  36 _ 1 , . . . ,  36 _n, at each primary winding  36 _ 1 , . . . ,  36 _n there is applied a voltage U 1 /n which is transformed to U 1 /nü=U 1 /ü. at the secondary side of each ideal transformer. 
     Assuming that also at the secondary side after rectification the related voltages are added due to series connection, then U 2  may be approximately, n* U1/nü=U1/ü. In conclusion the series configuration of the modular output circuit  30  leads to a voltage level at the secondary side being modified according to the transformer ratio ü of the ideal transformer. 
     Further, operationally the current at the secondary side of each transformer module  44 _ 1 , . . . ,  44 _n is l2=ü*l1. Due to the series connection a similar current will flow at the output side of ach mobile side AC/DC converters  40 _ 1 , . . . ,  40 _n, e.g., a current of approximately ü*l1, however, being rectified. In conclusion the series configuration of the modular output circuit  30  leads to a current level in line with the transformer ratio of the ideal transformer. 
     Further, it should be noted that while operatively the overall power input into the series configuration of the modular output circuit  30  is transferred to the output side, nevertheless, the power to be handled by each combination of transformer module  44 _ 1 , . . . ,  44 _n and mobile side AC/DC converters  40 _ 1 , . . . ,  40 _n is reduced approximately by a factor of n. This is a significant advantage when higher levels of power have to be transferred to the load  32 . 
       FIG. 6  shows the first series configuration of the modular output circuit as shown in FIG. 
       3  with a plurality of balancing modules  46 _ 1 , . . . ,  46 _n connected in parallel being added. 
     As shown in  FIG. 6 , the plurality of balancing modules  46 _ 1 , . . . ,  46 _n is respectively configured as series connection of a balancing winding Lb_ 1 , Lb_n and a related balancing resistor Rb_ 1 , Rb_n. Each balancing module  46 _ 1 , . . . ,  46 _n is added to one transformer module  44 _ 1 , . . . ,  44 _n and the balancing modules  46 _ 1 , . . . ,  46 _n are connected in parallel. 
     Operatively, all transformer modules  44 _ 1 , . . . ,  44 _n have the same primary current and therefore also the same output current but without the balancing modules  46 _ 1 , . . . ,  46 _n nothing is defining the voltage across each transformer module  44 _ 1 , . . . ,  44 _n. Thus, a small leakage current may create large voltage differences between transformer modules  44 _ 1 , . . . ,  44 _n. 
     In view of this, balancing modules  46 _ 1 , . . . ,  46 _n serve to keep similar voltages across different transformer modules  44 _ 1 , . . . ,  44 _n. An extra balancing winding Lb_ 1 , . . . , Lb_n is added to each transformer module  44 _ 1 , . . . ,  44 _n and the balancing windings Lb_ 1 , Lb_n are connected in parallel through a related balancing resistor Rb_ 1 , . . . , Rb_n. If each balancing winding Lb_ 1 , . . . , Lb_n has a same voltage then no current will flow. However, if one voltage is different a balancing current will flow to keep voltages on a same level. The balancing resistors Rb_ 1 , . . . , Rb_n reduce circulating current in particular during switching transitions. 
       FIG. 7  shows an example of the first series configuration shown in  FIG. 3  with four output terminal pairs  42 _ 1 , . . . ,  42 _ 4  connected in series. 
     As shown in  FIG. 7 , four output terminal pairs  42 _ 1 , . . . ,  42 _ 4  of four mobile side AC/DC converters  40 _ 1 , . . . ,  40 _ 4  are connected in series leading to an all series circuit configuration having one single output terminal group, g=1, which accommodates four mobile side AC/DC converters  40 _ 1 , . . . ,  40 _ 4  with n=r=4. 
     Operatively, according to the explanations outlined above with respect to  FIG. 5  in general, the series circuit configuration allows to reduce the thermal load to each combination of transformer module  44 _ 1 , . . . ,  44 _ 4  and mobile side AC/DC converter  40 _ 1 , . . . ,  40 _ 4  by a factor of four. 
       FIG. 8  shows an example of a second series configuration of the modular output circuit shown in  FIG. 4 . 
     As shown in  FIG. 8 , according to a second realization concept the mobile side transformer stage  30  of the mobile side circuit comprises one primary winding  36  being common to the plurality of secondary side windings  44 _ 1 , . . . ,  44 _n. The output terminal pairs  42 _ 1 , . . . ,  42 _n of the secondary side windings  44 _ 1 , . . . ,  44 _n are connected in series. 
     It should be noted that the one primary side winding  36  and the plurality of secondary side windings  38 _ 1 , . . . ,  38 _n may share a common transformer core (not shown in  FIG. 8 ). 
     Operatively, the considerations outlined above with respect to  FIG. 5  also apply to the second realization concept the mobile side transformer stage  30 . An advantage of the second realization concept the mobile side transformer stage  30  over the first realization concept the mobile side transformer stage  30  shown in  FIG. 5  and  FIG. 6  is that there is no need to provide a balancing mechanism between the primary windings  36 _ 1 , . . . ,  36 _n at the primary side. 
       FIG. 9  shows a group configuration of the modular output circuit shown in  FIG. 3  where rectifier circuits and related mobile side windings are grouped, output terminals of rectifier circuits within each group are connected in series, and output terminal pairs of different groups are connected in parallel. 
     As shown in  FIG. 9 , according to the group configuration of the modular output circuit  30  generally there are set up g mobile side output groups  48 _ 1 , . . . ,  48 _g, each comprising r transformer modules  44 _ 11 , . . . ,  44 _ 1 r, . . . ,  44 _g 1 , . . . ,  44 _gr and related mobile side AC/DC converters  40 _ 11 , . . . ,  40 _ 1 r, . . . ,  40 _g 1 , . . . ,  40 _gr. 
     As shown in  FIG. 9 , output terminal pairs within each mobile side output group  48 _ 1 , . . . ,  48 _g are connected in series and output terminal pairs of different mobile side output groups  48 _ 1 , . . . ,  48 _g are connected in parallel. 
     Here, assuming that the number of mobile side output groups  48 _ 1 , . . . ,  48 _g is g, that the number of mobile side AC/DC converters is n, that the number of mobile side AC/DC converters per mobile side output group is r, and that each mobile side output group  48 _ 1 , . . . ,  48 _g comprises a same number r of mobile side AC/DC converters, then 1&lt;g&lt;n, n mod g=0, r &gt;1, and g*r=n applies. 
     Operationally, each transformer module  44 _ 1 , . . . ,  44 _n may be realized by an ideal transformer having a transformer ration ü:1. Further, assuming that the input voltage U 1  is equally divided across the g mobile side output groups  48 _ 1 , . . . ,  48 _g and related primary windings, at each primary winding there is applied a voltage U 1 /n which is transformed to U 1 /nü at the secondary side of each ideal transformer. 
     Assuming that also at the secondary side the related voltages have to be added due to series connection, then the output voltage at each mobile side output group  48 _ 1 , . . . ,  48 _g may be approximately, r* U 1 /nü=r*U 1 /g*r*ü=U 1 /g*ü. In conclusion the group configuration of the modular output circuit  30  leads to a voltage level at the secondary side decreased by r/n=r/g*r=1/g when being compared to the series configuration of the modular output circuit  30  shown in  FIG. 5 . 
     Further, operationally the current at the secondary side of each transformer module  44 _ 1 , . . . ,  44 _n is l 2 =ü*I 1 . Due to the parallel connection of the different mobile side output groups a superimposed current of g*ü*l 1  will flow at the output side of the group configuration of the modular output circuit  30 , however, being rectified. In conclusion the group configuration of the modular output circuit  30  leads to a current level at the secondary side increased by a factor of g when being compared to the series configuration of the modular output circuit  30  shown in  FIG. 5 . 
     Further, it should be noted that while operatively the overall power input into the series configuration of the modular output circuit  30  is transferred to the output side, nevertheless, the power handed by each mobile side output group  48 _ 1 , . . . ,  48 _g is a factor of r/n=r/g*r=1/g of the input power. This again reduces the load for each mobile side output group  48 _ 1 , to  48 _g. 
       FIG. 10  shows an example of the group configuration shown in  FIG. 9  where each mobile side group has two transformer units  44 _ 1 ,  44 _ 2  and  44 _ 3 ,  44 _ 4  as well as two mobile side AC/DC converters  40 _ 1 ,  40 _ 2 , ad  40 _ 3 ,  40 _ 4 , respectively. 
     Generally, assuming that the number n of mobile side AC/DC converters  40 _ 1 , . . . ,  40 _ 4  is a power of two n=2 i , i=1, 2, 3, . . . , and that also the number of mobile side output groups is a power of two, then for the possible number of mobile side output groups g=2 j , 0≤j≤i−1 applies. 
       FIG. 11  shows a parallel configuration of the modular output circuit shown in  FIG. 3 . 
     As shown in  FIG. 11 , according to the group configuration of the modular output circuit  30  generally there are set up g mobile side output groups  48 _ 1 , . . . ,  48 _g, each comprising one transformer module and one related mobile side AC/DC converter. 
     As shown in  FIG. 11 , output terminal pairs of the different mobile side output groups  48 _ 1 , . . . ,  48 _g are connected in parallel. 
     Here, assuming that the number of mobile side AC/DC converters is n, that the number of mobile side AC/DC converters per mobile side output group is r=1, g=n&gt;1 and r=1. 
     Operationally, each transformer module  44 _ 1 , . . . ,  44 _n may be realized by an ideal transformer having a transformer ration ü:1. Further, assuming that the input voltage U 1  is equally divided across the g mobile side output groups  48 _ 1 , . . . ,  48 _g and related primary windings, at each primary winding there is applied a voltage U 1 /n which is transformed to U 1 /nü at the secondary side of each ideal transformer. 
     Also at the secondary side the related voltages are directly mapped to the output due to parallel connection and the output voltage is U 1 /nü. In conclusion the parallel configuration of the modular output circuit  30  leads to a voltage level at the secondary side decreased by 1/n when being compared to the series configuration of the modular output circuit  30  shown in  FIG. 5 . 
     Further, operationally the current at the secondary side of each transformer module  44 _ 1 , . . . ,  44 _n is l 2 =ü*l 1 . Due to the parallel connection of the different mobile side AC/DC converters  40 _ 1 , . . . ,  40 _n a superimposed current of n*ü*l 1  will flow at the output side of the parallel configuration of the modular output circuit  30 , however, being rectified. In conclusion the parallel configuration of the modular output circuit  30  leads to a current level at the secondary side increased by a factor of n when being compared to the series configuration of the modular output circuit  30  shown in  FIG. 5 . 
     Further, it should be noted that while operatively the overall power input into the series configuration of the modular output circuit  30  is transferred to the output side, nevertheless, the power handed by each mobile side output group  48 _ 1 , . . . ,  48 _g is a factor of 1/n of the input power. This again reduces the load for each mobile side output group  48 _ 1 , to  48 _g. 
       FIG. 12  shows an example of the parallel configuration shown in  FIG. 11  with four output terminal pairs connected in parallel. 
     For the specific example shown in  FIG. 12 , assuming that, e.g., ü=1 then U2=U1/4 and l2=4*l1 applies. Further, each pair of transformer module  44 _ 1 , . . . ,  44 _ 4  and mobile side AC/DC converters  40 _ 1 , . . . ,  40 _ 4  handles ¼ of the input power. 
       FIG. 13  shows a schematic diagram illustrating different current forms at different stages of a power train for motivation of indirect DC output current measurement according to the present invention. 
     As shown in  FIG. 13 , along the power train the current has different waveforms and related current values. According to  FIG. 13  it is assumed that the mobile side AC/DC converter is realized as ideal transformer having a transformer ratio of 4:1. 
     As shown in  FIG. 13 , at the input side of the ideal transformer the current  11  is sinusoidal and easy to measure. Also, at the output side of the ideal transformer the current is sinusoidal, however, at a higher current level due to the transformer ratio of  4 : 1  of the ideal transformer. 
     As shown in  FIG. 13 , at the output side of the diode bridge realizing the mobile side AC/DC converter a current l 2  is of a rectified sinusoidal form. The smoothing capacitor at the output of the diode bridge acts as a low pass so that finally a DC current  13  is supplied to the load  32 . Generally, l3=l1(2√2)/π applies. 
     In conclusion, according to the present invention it is suggested to measure the sinusoidal current l 1  at a comparatively low current level instead of the DC current  13  at a much higher current level. The is also advantageous in that AC current sensors are cheaper as DC current sensors. 
     Further, should there be several mobile side output groups where the current needs to be measured a state of the art solution would measure the current at each mobile side output module thus leading to the use of a plurality of current sensors. Optionally, another state of the art solution would be to measure a combined output current leading to the disadvantage hat bulky output cables have to routed through the current sensor. Also, the accuracy would be compromised as the dynamic range of the current sensor would be larger than required for a single output current. 
     Contrary to that, the approach according to the present invention and as illustrated in  FIG. 13  allows to measure the input current  11  and to calculate the DC output current  13  therefrom. 
       FIG. 14  shows a schematic diagram of a mobile side circuitry of a wireless power transfer system using a current transformer for indirect measurement of a DC output current and subsequent use of the measurement result for control of mobile side rectifier circuits. 
     As shown in  FIG. 14 , a current transformer  48  having a primary side winding  50  connected between the mobile side resonant circuit and the input of the mobile side transformer stage  30  and a secondary side winding  52  connected to a circuit  54  is adapted to evaluate an output current of a wireless power transfer system. 
     It should be noted that the concept underlying indirect current measurement according to the present invention is counter intuitive as normally it would be expected that the magnetizing currents of the transformers  44 _ 1 , . . . ,  46 _n and  48  as well as losses in the rectifier would make the measurement barely usable. However, these magnetizing currents merely add phase shift but do not have any effect on the accuracy of the output current measurement. This means that in practice that the use of the current transformer  48  allows for a more accurate current measurement than would be possible through use of normal DC current sensors. 
     Further, it should be noted that the present invention also covers the use of the current transformer  48  for evaluation of an output current of a wireless power transfer system, wherein the current transformer  48  has a primary side winding  50  connected to an input of a mobile side circuitry a wireless power transmission system and a secondary side winding  52  connected to a monitoring circuit  54  adapted to evaluate the output current of a wireless power transfer system. 
     As shown in  FIG. 14 , the monitoring circuit  54  for determining an output current of a wireless power transfer system comprises a rectifying circuit  56  connected to the secondary side winding  52  of the current transformer  48 . Further, the monitoring circuit  54  comprises an averaging circuit  58  connected to the rectifying circuit  56  which is adapted to determine an average of the output of the rectifying circuit  56  as equivalent to the output current of the wireless power transmission system. 
     Optionally, the averaging circuit  58  comprises a smoothing capacitor connected to the output terminals of the rectifying circuit  56  and a resistor connected in parallel to the smoothing capacitor. 
     Operatively and as will be explained in the following, the output of the averaging circuit  58  may be used to control the mobile side AC/DC converters  40 _ 1 , . . . ,  40 _n if these are synchronous rectifiers. 
       FIG. 15  shows a schematic circuit diagram of a controller apparatus  60  for an inductive power transfer system according to present invention. 
     As shown in  FIG. 15 , the controller apparatus  60  comprises at least one interface  62 , e.g., a radio interface. The interface  62  is suitable for wireless information exchange, e.g., with a remote controller in the inductive power transfer system  10  or with an external control station of the inductive power transfer system  10 . In some scenarios, the interface  62  may also be used for exchange of information with external systems, e.g., a maintenance system. 
     As shown in  FIG. 15 , the controller apparatus  60  comprises at least one processor  64  coupled to the interface  62  and a memory  66  coupled to the at least one processor  64 . The memory  66  may include a read-only memory ROM, e.g., a flash ROM, a random access memory RAM, e.g., a dynamic RAM DRAM or a static RAM SRAM, a mass storage, e.g., a hard disc or solid state disc, or the like. The memory  66  also includes instructions, e.g., suitably configured program code to be executed by the at least one processor  64  in order to implement a later described functionality of the controller apparatus  60 . This functionality will be referred to in the following as units. It is noted that these units do not represent individual hardware elements of the controller apparatus  60 , but rather represent functionalities generated when the at least one processor  64  execute the suitably configured program code. 
     As shown in  FIG. 15 , the memory  66  may include suitably configured program code to implement a signal processing unit  68  and a control processing unit  70 . 
     Operatively, the signal processing unit  68  is adapted to receive an output signal of the current transformer  48  having the primary side winding  48  connected to an input of the mobile side circuitry of the wireless power transmission system, to classify a polarity of the output signal with respect to a reference potential as positive polarity or negative polarity, and to compare the output signal with a threshold value. 
     Further, operatively the control processing unit  70  is adapted to turn on at least one first switching circuit the at least one mobile side AC/DC converter  40 _ 1 , . . . ,  40 _n realized as synchronous rectifier circuit, respectively, when the output signal has positive polarity and the absolute value of the output signal is larger than the threshold value and to turn on at least one second switching circuit of the at least one synchronous rectifier circuit  40 _ 1 , . . . ,  40 _n being different from the at least one first circuit when the output signal has negative polarity and the absolute value of the output signal is larger than the threshold value. 
     It should be noted that according to the present invention the at least one mobile side AC/DC converter may be of any suitable type, e.g., be configured into a full-bridge configuration or a half-bridge configuration. 
     Here, in the full-bridge configuration there would be provided two first switching elements lying in a first diagonal of the full-bride and two second switching elements lying in a second diagonal of the full-bridge, wherein the second diagonal would be different from the first diagonal. 
     Alternatively, in the half-bridge configuration there would be provided one first switching element lying in an upper part of the half-bridge and one second switching element lying in a lower part of the half-bridge. 
       FIG. 16  shows a flowchart of operation for the controller apparatus  60  shown in  FIG. 15 . 
     As shown in  FIG. 16 , operatively the interface  62 , in cooperation with the processor  64 , is adapted to execute a step S 20  for receiving an output signal of a current transformer  48  having the primary side winding  50  connected to an input of the mobile side circuitry of a wireless power transmission system. 
     As shown in  FIG. 16 , operatively the signal processing unit  68 , in cooperation with the processor  74 , is adapted to execute a step S 22  for classifying a polarity of the output signal with respect to a reference potential as positive polarity or negative polarity. 
     As shown in  FIG. 16 , operatively the control processing unit  70 , in cooperation with the processor  64 , is adapted to execute a step S 24  for comparing the output signal with a threshold value. 
     As shown in  FIG. 16 , operatively the control processing unit  70 , in cooperation with the processor  64 , is adapted to execute a step S 26  for turning on at least one first switching circuit of the at least one mobile side AC/DC converter  40 _ 1 , . . . ,  40 _n being a synchronous rectifier circuit when the output signal has positive polarity and an absolute value of the output signal is larger than the threshold value and for turning on at least one second switching circuit of the at least one synchronous rectifier circuit  40 _ 1 , . . . ,  40 _n being different from the at least one first switching circuit when the output signal has negative polarity and the absolute value of the output signal is larger than threshold value. 
     It should be noted that the operation as shown in  FIG. 16  is not restricted to a realization using the controller apparatus as shown in  FIG. 15 . Alternatively, the method may be realized in an analogue manner using comparator circuits for classification and threshold comparison as outlined above. Then the output put of the comparators would be used as inputs to analogue gate driver circuits that drive the switching circuits of the at least one mobile side AC/DC converter  40 _ 1 , . . . ,  40 _n. 
     While in the above, the present invention has been described with reference to the drawings and figures of preferred embodiments or examples of the invention, it should be noted that clearly the present invention may also be implemented using variations and modifications thereof which will be apparent and can be readily made by those skilled in the art without departing from the scope and spirit of the present invention. E.g., functionalities described above may be realized in software, in hardware, or a combination thereof. 
     Accordingly, it is not intended that the scope of claims appended hereto is limited to the description as set forth herein, but rather that the claims should be construed so as to encompass all features of presentable novelty that preside in the present invention, including all features that would be treated as equivalent thereof by those skilled in the art to which the present invention pertains.