Patent Publication Number: US-2021178915-A1

Title: A power transfer system for electric vehicles and a control method thereof

Description:
The present invention relates to the field of power transfer systems for electric vehicles. In particular, the present invention regards an inductive power transfer system capable of exchanging electric power between an electric power system and a battery on board an electric vehicle, in a wireless manner. 
     Wireless inductive power transfer systems for electric vehicles are well known in the state of the art. 
     Typically, these systems are used to charge the electric battery of an electric vehicle. 
     When used as a battery charging systems, wireless inductive power transfer systems employ a transmitter coil, which is placed on or embedded in a ground surface, to inductively transmit electric power to a receiver coil mounted on board an electric vehicle through the air gap between the road surface and the vehicle itself. 
     As in traditional transformers, an AC current flowing in the winding of the transmitter coil produces a magnetic flux making an induced AC current to flow in the winding of the receiver coil. In this way, electric power may be inductively transferred from the transmitter coil to the receiver coil. 
     When used as battery charging systems, wireless power transfer systems for electric vehicles typically include a transmitter-side section, which normally includes the transmitter coil and a power supply system connectable to the mains to feed the transmitter coil, and a receiver-side section, which normally includes, on board the electric vehicle, the receiver coil and a power conversion system to feed the battery with electric power inductively received by the receiver coil. 
     Both the transmitter-side section and the receiver-side section of the power transfer system include a number of controllers to control their operation. Controllers arranged at different sections can mutually communicate through a dedicated communication channel, which is typically a wireless communication channel, e.g. of the Wi-Fi type. 
     In order to ensure a suitable charging process of the battery on board the electric vehicle, electric power transferred to the battery has to be properly controlled according to a charging profile that is typically set-up depending on the characteristics and state-of-charge of the battery and on other additional aspects, such as the reduction of the energy consumption during a charging cycle, the reduction of the time required for the charging process, and the like. 
     For this reason, controllers of a wireless power transfer system for electric vehicles typically implement control architectures configured to control the operation of the above-mentioned power supplying system as a function of the selected power transfer profile. 
     Control arrangements currently used in power transfer systems of the state of the art often show poor performances in optimizing power exchange efficiency in consideration of the on-going operative conditions. 
     As a consequence, currently available power transfer systems may operate inefficiently and require unpredictably prolonged time to carry out a desired power exchange process between the electric power system and the battery on board the electric vehicle. 
     The main aim of the present invention is to provide a wireless power transfer system for electric vehicles, which allows overcoming the above-described disadvantages. 
     Within this aim, another object of the present invention is to provide a wireless power transfer system ensuring a suitable exchange of electric power between an electric power system and a battery on board a vehicle, in accordance with a given power transfer profile. 
     Another object of the present invention is to provide a wireless power transfer system ensuring good performances in optimizing power exchange efficiency, even when sudden and unpredictable changes in the operating conditions of the power transfer system occur. 
     Another object of the present invention is to provide a wireless power transfer system relatively easy and inexpensive to arrange and produce at industrial level. 
     The above aim and objects, together with other objects that will be more apparent from the subsequent description and from the accompanying drawings, are achieved by a power transfer system for electric vehicles, according to the following claim  1  and the related dependent claims. 
     In a general definition, the power transfer system, according to the invention, comprises:
         a transmitter-side power sub-system adapted to exchange AC electric power with said electric power system, said transmitter-side power sub-system comprising a first rectifying stage electrically coupleable with said electric power system, a DC-bus stage electrically coupled with said first rectifying stage and an inverter stage electrically coupled with said DC-bus stage;   a transmitter-side coil sub-system adapted to exchange AC electric power with said transmitter-side power sub-system, said transmitter-side coil sub-system comprising a transmitter coil electrically coupled with said inverter stage;   a receiver-side coil sub-system comprising a receiver coil inductively coupleable with said transmitter coil, said transmitter-side coil sub-system and said receiver-side coil sub-system forming a resonant electric circuit for the exchange of AC electric power, when said transmitter coil and said receiver coil are inductively coupled;   a receiver-side power sub-system adapted to exchange AC electric power with said receiver-side coil sub-system and DC electric power with said battery, said receiver-side power sub-system comprising a second rectifying stage electrically coupled with said receiver coil;   control means comprising transmitter-side control means, adapted to control operation of said transmitter-side power sub-system and transmitter-side coil sub-system, and receiver-side control means adapted to control operation of said receiver-side coil sub-system and receiver-side power sub-system, said transmitter-side and receiver-side control means being capable to mutually communicate through a wireless communication channel.       

     Said control means are adapted to control a DC electric power exchanged with said battery and are adapted to control an operating frequency of a first AC current circulating along said transmitter coil to track a resonant frequency of the resonant electric circuit formed by said transmitted-side coil sub-system and said receiver-side coil sub-system. 
     Preferably, said control means are adapted to provide frequency variations of an operating frequency of said first AC current and observe variations of one or more electric quantities of said power transfer system in response to said frequency variations to track said resonant frequency. 
     Preferably, said control means are adapted to:
         control operation of said inverter stage to obtain said first AC current with a varied frequency obtained by imposing a frequency variation of an operating frequency of said first AC current;   calculate variations of one or more electric quantities of said power transfer system in response to the obtaining of said first AC current having said varied frequency;   determine whether said varied frequency is closer to or farther from said resonant frequency depending on the calculated variations of said one or more electric quantities;   control operation of said inverter stage to obtain said first AC current with an operating frequency set depending on the results of said determination.       

     Preferably, said DC-bus stage includes a DC/DC converter and has a first port electrically coupled with said first rectifying stage and a second port electrically coupled with said inverter stage. 
     Preferably, said control means are adapted to:
         control operation of said first rectifying stage to obtain a first DC voltage having said minimum value;   control operation of said DC/DC converter to obtain a second DC voltage having said requested value;
 
if a value requested for a second DC voltage at said second port is lower or equal to a possible minimum value for a first DC voltage at said first port.
       

     Preferably, said control means are adapted to:
         control operation of said first rectifying stage to obtain a first DC voltage having a value equal to the value requested for said second DC voltage;   control operation of said DC/DC converter to obtain a second DC voltage equal to said first DC voltage;   if a value requested for a second DC voltage at said second port is higher than a possible minimum value for a first DC voltage at said first port.       

     In a further aspect, the present invention related to a method for controlling a power transfer system, according to the following claim  10  and the related dependent claims. 
     The power transfer system comprises:
         a transmitter-side power sub-system adapted to exchange AC electric power with said electric power system, said transmitter-side power sub-system comprising a first rectifying stage electrically coupleable with said electric power system, a DC-bus stage electrically coupled with said first rectifying stage and an inverter stage electrically coupled with said DC-bus stage;   a transmitter-side coil sub-system adapted to exchange AC electric power with said transmitter-side power sub-system, said transmitter-side coil sub-system comprising a transmitter coil electrically coupled with said inverter stage;   a receiver-side coil sub-system comprising a receiver coil inductively coupleable with said transmitter coil, said transmitter-side coil sub-system and said receiver-side coil sub-system forming a resonant electric circuit for the exchange of AC electric power, when said transmitter coil and said receiver coil are inductively coupled;   a receiver-side power sub-system adapted to exchange AC electric power with said receiver-side coil sub-system and DC electric power with said battery, said receiver-side power sub-system comprising a second rectifying stage electrically coupled with said receiver coil.       

     The method, according to the invention, comprises controlling a DC electric power exchanged with said battery and controlling an operating frequency of a first AC current circulating along said transmitter coil to track a resonant frequency of the resonant electric circuit formed by said transmitted-side coil sub-system and said receiver-side coil sub-system. 
     Preferably, the method comprises providing frequency variations of an operating frequency of said first AC current and observing variations of one or more electric quantities of said power transfer system in response to said frequency variations to track said resonant frequency. 
     Preferably, the method comprises the following steps:
         controlling operation of said inverter stage to obtain said first AC current with a varied frequency obtained by imposing a frequency variation of an operating frequency of said first AC current;   calculating variations of one or more electric quantities of said power transfer system in response to the obtaining of said first AC current having said varied frequency;   determining whether said varied frequency is closer to or farther from said resonant frequency depending on the calculated variations of said one or more electric quantities;   controlling operation of said inverter stage to obtain said first AC current with an operating frequency set depending on the results of said determination.       

     Preferably, said DC-bus stage includes a DC/DC converter and has a first port electrically coupled with said first rectifying stage and a second port electrically coupled with said inverter stage. 
     Preferably, the method comprises the following steps: 
     if a value requested for a second DC voltage at said second port is lower or equal to a possible minimum value for a first DC voltage at said first port:
         controlling operation of said first rectifying stage to obtain a first DC voltage having said minimum value;   controlling operation of said DC/DC converter to obtain a second DC voltage having said requested value;
 
if a value requested for a second DC voltage at said second port is higher than a possible minimum value for a first DC voltage at said first port:
   controlling operation of said first rectifying stage to obtain a first DC voltage having a value equal to the value requested for said second DC voltage;   controlling operation of said DC/DC converter to obtain a second DC voltage equal to said first DC voltage.       

    
    
     
       Further characteristics and advantages of the present invention will be more apparent with reference to the description given below and the accompanying figures, provided purely for explanatory and non-limiting purposes, wherein: 
         FIG. 1  schematically illustrates the power transfer system, according to the present invention; 
         FIG. 2  schematically illustrates a portion the power transfer system, according to the present invention, in a possible embodiment; 
         FIG. 3  schematically illustrates a frequency control procedure carried out by the power transfer system, according to the present invention. 
     
    
    
     With reference to  FIG. 1 , the present invention refers to a power transfer system  1  for electric vehicles capable of exchanging electric power between an electric power system  100  and a battery  200  on board an electric vehicle, in a wireless manner. 
     The power transfer system  1  is particularly adapted to be used as a battery charging system capable of transferring electric power harvested from an electric power system  100  (e.g. the mains) to a battery  200  on board an electric vehicle and it will be described with specific reference to this application for the sake of simplicity. 
     However, the power transfer system  1  may be suitable to implement bidirectional power transfer functionalities and therefore it may be employed to transfer electric power from the battery  200  to the electric system  100 . 
     Referring to its application as battery charging system, the power transfer system  1  comprises a transmitter-side section and a receiver-side section, which respectively include a number of power sub-systems and components arranged off-board and on-board the electric vehicle. 
     At the transmitter-side section, the power transfer system  1  comprises a transmitter-side power sub-system  2  electrically coupleable with the electric power system  100  and adapted to exchange AC electric power P AC  with this latter. 
     The transmitter-side power sub-system  2  comprises a first AC/DC rectifying stage  21  electrically coupleable with the electric power system  100  and conveniently adapted to provide a first DC current I 1   DC  and a first DC voltage V 1   DC . 
     Preferably, the first rectifying stage  21  includes a filter and a switching converter (not shown) electrically coupled in cascade. 
     Preferably, the first rectifying stage  21  includes power switches controllable by means of suitable control signals. 
     Preferably, the first rectifying stage  21  includes a first sensing arrangement  210  adapted to detect the first DC current I 1   DC  and the first DC voltage V 1   DC  and provide detection signals indicative of these electric quantities. 
     The transmitter-side power sub-system  2  further comprises a DC-bus stage  22  electrically coupled with the first rectifying stage  21  and adapted to provide a second DC current I 2   DC  and a second DC voltage V 2   DC . 
     In some embodiments of the invention, the bus-stage  22  may comprise a capacitive circuit (e.g. a so-called DC-link circuit. In this case, the second DC voltage V 2   DC  provided by the first rectifying stage  21  substantially coincides with the first DC voltage V 1   DC  provided by the first rectifying stage  21 . Additionally, the amplitude of the second DC current I 2   DC  and the second DC voltage V 2   DC  provided by the DC bus-stage  22  may be controlled by suitably tuning the duty-cycle of the power switches of the first rectifying stage  21 . 
     According to alternative embodiments ( FIG. 2 ), the bus-stage  22  may include a DC-DC switching converter  220  (e.g. a buck switching converter) that conveniently includes power switches controllable by means of suitable control signals. 
     The switching converter  220  has a first port  221  electrically coupled with the first rectifying stage  21  and a second port  222  electrically coupled with the inverter stage  23 . 
     At the first port  221 , the switching converter  220  receives the first DC voltage V 1   DC  provided by the first rectifying stage  21 , whereas, at the second port  222 , the switching converter  220  provides the second DC voltage V 2   DC , which is lower than or equal to the first DC voltage V 1   DC . 
     The amplitude of the second DC current I 2   DC  and the second DC voltage V 2   DC  provided by the DC-bus stage  22  may be controlled by suitably tuning the duty-cycle of the power switches of the DC-DC switching converter  220  and the duty-cycle of the power switches of the first rectifying stage  21 . 
     Preferably, the DC-bus stage  22  comprises a second sensing arrangement  220 A adapted to detect the second DC current I 2   DC  and the second DC voltage V 2   DC  and provide detection signals indicative of said electric quantities. 
     The transmitter-side power sub-system  2  further comprises a DC/AC inverter stage  23  electrically coupled with the first DC-bus stage  22 . 
     The inverter stage  23  is adapted to receive the second DC current I 2   DC  and the second DC voltage V 2   DC , provided by the DC-bus stage  22 , and provide a first AC current I 1   AC  and a first AC voltage V 1   AC . 
     Preferably, the inverter stage  23  comprises a DC/AC switching converter including power switches controllable by means of suitable control signals. 
     The frequency of the first AC current I 1   AC  and the first AC voltage V 1   AC  provided by the inverter stage  23  may be controlled by suitably tuning the switching frequency of the power switches of such an electronic stage. 
     Preferably, the inverter stage  23  comprises a third sensing arrangement  230  adapted to detect the first AC current I 1   AC  and the first AC voltage V 1   AC  and provide detection signals indicative of said electric quantities. 
     At the transmitter-side section, the power transfer system  1  comprises a transmitter-side coil sub-system  3  adapted to exchange AC electric power with the transmitter-side power system  2 . 
     The transmitter-side coil sub-system  3  comprises a transmitter coil  31  adapted to receive the first AC current I 1   AC  provided by the inverter stage  23 . 
     Preferably, the transmitter-side coil sub-system  3  comprises also a first resonant capacitor  32  electrically coupled (e.g. in series as shown in  FIG. 1 ) with the transmitter coil  31 . 
     Preferably, the transmitter-side coil sub-system  3  comprises auxiliary circuits (not shown) operatively associated with the transmitter coil  31 , e.g. electronic circuits including temperature sensors, and the like. 
     At the receiver-side section, the power transfer system  1  comprises a receiver-side coil sub-system  4  comprising a receiver coil  41  inductively coupleable with the transmitter coil  31 . 
     When the transmitter coil  31  and the receiver coil  41  are inductively coupled (obviously with an air gap in therebetween), a first AC current I 1   AC  flowing along the transmitter coil  31  produces a magnetic flux making an induced second AC current I 2   AC  to flow along the receiver coil  41  (and vice-versa). In this way, electric power may be inductively exchanged between the transmitter coil  31  and the receiver coil  41 . 
     Preferably, the receiver-side coil sub-system  4  comprises a second resonant capacitor  42  electrically coupled (e.g. in series as shown in  FIG. 1 ) with the receiver coil  41 . 
     Preferably, the receiver-side coil sub-system  4  comprises auxiliary circuits (not shown) operatively associated with the receiver coil  41 , e.g. electronic circuits including temperature sensors, and the like. 
     As it is evident from the above, the transmitter-side coil sub-system  3  and the receiver-side coil sub-system  4  are adapted to exchange an AC electric power, when the transmitter coil  31  and said receiver coil  41  are inductively coupled. 
     Conveniently, the receiver-side coil sub-system  4  and the transmitter-side coil sub-system  3  form a resonant electric circuit  340  for the exchange of electric power, when the transmitter coil  31  and said receiver coil  41  are inductively coupled. 
     In particular, the resonant capacitors  32 ,  42  are conveniently designed to form a resonant RLC circuit  340  together with the inductance of the transmitter and receiver coils  31 ,  41  and the equivalent impedance seen at the output terminals of the receiver-side coil sub-system  4 . 
     By operating the inverter stage  23  in such a way that the first AC current I 1   AC  flowing along the transmitter coil  31  has a fundamental frequency close or corresponding to a resonant frequency f R  of the resonant electric circuit  340 , AC electric power may be exchanged between the transmitter-side coil sub-system  3  and the receiver-side coil sub-system  4  with high efficiency values despite of the necessarily large air gap between the transmitter coil  31  and the receiver coil  41 . 
     Additionally, the amplitude of the first AC current I 1   AC  flowing along the transmitter coil  31  can be reduced or minimized due to nearly-zero phase shift between said current and the first AC voltage V 1   AC . 
     At the receiver-side section, the power transfer system  1  comprises a receiver-side power sub-system  5  adapted to exchange AC electric power with the receiver-side coil sub-system  4  and DC electric power Pic with the battery  200 . 
     The receiver-side power sub-system  5  comprises a second rectifying stage  51  electrically coupled with the receiver-side coil sub-system  4  and adapted to receive a second AC current I 2   AC  and second AC voltage V 2   AC  from this latter. 
     Preferably, the second rectifying stage  51  includes a full-wave diode bridge electrically coupled in cascade with a filter. 
     As an alternative embodiment, the second rectifying stage  51  may include a switching converter and a filter (not shown) electrically coupled in cascade to provide a suitable rectification and filtering of the currents and voltages received from the receiver-side coil sub-system  4 . In this case, the second rectifying stage  51  may include power switches controllable by means of suitable control signals. 
     The second rectifying stage  51  is electrically coupleable with the battery  200  and is adapted to provide a third DC current I 3   DC  and a third DC voltage V 3   DC  to this latter. 
     Preferably, the second rectifying stage  51  comprises suitable sensing arrangement  510  adapted to detect the third DC current I 3   DC  and the third DC voltage V 3   DC  and provide detection signals indicative of these electric quantities. 
     According to the invention, the power transfer system  1  comprises control means  10  to control its operation. 
     At the transmitter-side section, the control means  10  comprise one or more transmitter-side controllers (collectively indicated with the reference number  6 ) to control the operation of the transmitter-side power sub-system  2  and the transmitter-side coil sub-system  3 . 
     As an example, the transmitter-side controllers  6  may include a controller to control the operation of the rectifying stage  21 , a controller to control the operation of the bus stage  22  (when including a DC-DC switching converter), a controller to control the operation of the inverter stage  23  and, possibly, a controller to control the operation of possible auxiliary circuits included in the transmitter-side coil sub-system  3 . 
     At the receiver-side section, the control means  10  comprise one or more receiver-side controllers (collectively indicated with the reference number  7 ) to control operation of the receiver-side power sub-system  5  and the receiver-side coil sub-system  4 . 
     As an example, receiver-side controllers  7  may include a controller to control the operation of the rectifying stage  51  and a controller to control the operation of the auxiliary circuits included in the receiver-side coil sub-system  4 . 
     The control means  10  comprise at least a wireless communication channel  8 , through which the transmitter-side and receiver-side controllers  6 ,  7  are capable to mutually communicate. As an example, a Wi-Fi™ communication protocol may be adopted for the communication channel  8 . 
     According to the invention, the control means  10  are capable of suitably controlling the exchange of electric power with the battery  200 . 
     According to some embodiments, the control means  10  are adapted to control a DC electric power P DC  exchanged with the battery  200  by controlling the operation of at least one of the rectifying stage  21  and the DC-bus stage  22 , namely by controlling the amplitude of the voltages and currents provided by at least one of the rectifying stage  21  and the DC-bus stage  22 . 
     According to other embodiments, the control means  10  are adapted to control a DC electric power P DC  exchanged with the battery  200  by controlling the operation of the inverter stage  23 , namely by controlling the duty-cycle of the AC electric quantities (e.g. the first AC voltage V 1   AC ) provided by said electronic stage. 
     Preferably, the control means  10  implement a closed-loop control arrangement adapted to process power reference signals P REF , which are indicative of desired values for the DC electric power P DC  to be exchanged with the battery  200 , and suitable detection signals P D  (which are conveniently provided by the sensing arrangement  510 ) indicative of measured values of the DC electric power P DC  actually exchanged with the battery  200 . 
     Preferably, said closed-loop control arrangement is adapted to provide first control signals C 1  to control the operation of at least one between the rectifying stage  21  and the DC-bus stage  22  or to control the operation of the inverter stage  23 . 
     As an example, when the DC-bus stage  22  does not include a DC-DC switching converter, said closed-loop control arrangement may be configured to provide first control signals C 1  to control the operation of the rectifying stage  21 . 
     As a further example, when the DC-bus stage  22  includes a DC-DC switching converter  220 , said closed-loop control arrangement may be configured to provide first control signals C 1  to control the rectifying stage  21  or the DC-bus stage  22  or both these electronic stages. Conveniently, the first control signals C 1  are adapted to control the duty-cycle of the power switches included in at least one of the rectifying stage  21  and the DC-bus stage  22 . 
     As an additional example, said closed-loop control arrangement may be configured to provide first control signals C 1  to control the duty-cycle of the power switches included in the inverter stage  23 . 
     In a practical implementation of the invention, the above-mentioned closed-loop control arrangement is mostly carried out at level of the transmitter-side controllers  6 . Conveniently, certain signals, such as the power reference signals P REF  and the detection signals P D , may be conveniently transmitted through the wireless communication channel  8  and processed by the transmitter-side controllers  6  to provide the control signals C 1 . 
     According to the invention, the control means  10  are capable of providing frequency control functionalities of the first AC current I 1   AC  circulating along the transmitter coil  31 . 
     More particularly, the control means  10  are adapted to control an operating frequency of the first AC current I 1   AC  circulating along the transmitter coil  31  to track a resonant frequency f R  of the resonant electric circuit  340  formed by the transmitted-side coil sub-system  3  and the receiver-side coil sub-system  4 , when the transmitter coil  31  and said receiver coil  41  are inductively coupled. 
     This solution finds its technical grounds in the circumstance that the resonant frequency f R  of the resonant electric circuit  340  is basically subject to variations that may depend on specific operative conditions of the power transfer system  1 , such as the mutual positioning between the transmitter coil  31  and the receiver coil  41 , temperature variations of some components (e.g. the resonant capacitors  32 ,  42 ) of the transmitter-side coil sub-system  3  and the receiver-side coil sub-system  4 , arising of saturation phenomena at the transmitter coil  31  and the receiver coil  41 , and the like. 
     For a given operating frequency of the first AC current I 1   AC  flowing along the transmitter coil  31 , these variations of the resonant frequency f R  of the resonant electric circuit  340  typically determine corresponding variations of the overall power exchange efficiency η of the power transfer system. For the sake of clarity, the overall power exchange efficiency η may be conveniently calculated as η=P DC /P AC , where P AC  is the AC electric power exchanged with the electric system  100  and P DC  is the DC electric power exchanged with the battery  200 . 
     The entity of these variations of the power exchange efficiency normally depends on differences between the actual operating frequency of the first AC current I 1   AC  circulating along the transmitter coil  31  and the actual resonant frequency f R  of the above-mentioned resonant electric circuit. 
     Thanks to above-described frequency control functionalities, for any generic operating point of the power transfer system, which is conveniently set by implementing the above-described power control functionalities, possible (often unpredictable) variations of the resonant frequency f R  of the above-mentioned resonant electric circuit can be followed by corresponding variations of the operating frequency of the first AC current I 1   AC  flowing along the transmitter coil  31 . 
     In this way, for any generic operating point, it is possible to maintain the operating frequency of the first AC current I 1   AC  close or substantially coincident the resonant frequency f R  and, consequently, maintain the overall power exchange efficiency at relatively high values despite of possible variations of the operative conditions of said power transfer system. 
     Conveniently, the control means  10  may control the operating frequency of the first AC current I 1   AC  flowing along the transmitter coil  31  by providing second control signals C 2  to control the operation of the inverter stage  23 . 
     Conveniently, the second control signals C 2  are adapted to control the switching frequency of the power switches included in inverter stage  23 . 
     According to a preferred embodiment of the invention, the control means  10  are adapted to carry-out a frequency control procedure  60  that provides for imposing frequency variations Δf to an operating frequency of the first AC current I 1   AC  circulating along the transmitter coil  31  and provides for observing variations of one or more electric quantities of the power transfer system in response to the imposed frequency variations Δf in order in order to track the resonant frequency f R  of the above-mentioned resonant electric circuit  340 . 
     Preferably, the above-mentioned electric quantities include at least one between the second DC voltage V 2   DC  at the DC-bus stage  22  and the overall power exchange efficiency of the power transfer system. 
     The frequency control procedure  60  comprises the step  61  of controlling the operation of the inverter stage  23  to obtain a first AC current I 1   AC , which circulates along the transmitter coil  31  with a varied frequency f 2 . This latter is obtained by imposing a frequency variation Δf to an initial operating frequency f 1  (e.g. the nominal frequency) of the first AC current I 1   AC . In practice, the imposed varied frequency f 2  is given by the following relation: f 2 =f 1 +Δf, where Δf may have positive or negative values. 
     According to some embodiments of the invention, the frequency variation Δf has a predefined value (in module). 
     According to alternative embodiments of the invention, the frequency variation Δf may have variable values (in module) that are selectable depending on the calculated variations ΔV 2   DC , Δη of the above-mentioned electric quantities V 2   DC , η. As an example, the frequency variation Δf may be selected from a look-up table reporting the frequency variation Δf as a function of an electric quantity V 2   DC , η. 
     The frequency control procedure  60  comprises the step  62  of calculating variation values ΔV 2   DC , Δη of the above-mentioned electric quantities V 2   DC , η of the power transfer system in response to the obtaining of a first AC current I 1   AC  having the varied frequency f 2 . 
     Conveniently, the variation values ΔV 2   DC , Δη are calculated when the above-mentioned electric quantities V 2   DC , η have reached a steady-state in response to the imposed frequency perturbation of the first AC current I 1   AC . 
     Conveniently, the variation values ΔV 2   DC , Δη of the above-mentioned electric quantities V 2   DC , η may be calculated by the control means  10  basing on detection signals D 1  provided by suitable sensing arrangements, e.g. the sensing arrangements  210 ,  220 A,  510 . 
     The frequency control procedure  60  comprises the step  63  of determining whether the varied frequency f 2  of the first AC current I 1   AC  is closer to or farther from the actual resonant frequency f R  of the resonant electric circuit  340 . 
     Such a determination step is conveniently carried out basing on the calculated variation values ΔV 2   DC , Δη of the aforesaid electric quantities V 2   DC , η. 
     According to a possible determination criterion, a positive variation value ΔV 2   DC  of the second DC voltage V 2   DC  at the DC-bus stage  22  indicates that the varied frequency f 2  is farther from the actual resonant frequency f R  whereas a negative variation value ΔV 2   DC  indicates that the varied frequency f 2  is closer to the actual resonant frequency f R . 
     According to another determination criterion, a positive variation value Δη of the overall power exchange efficiency η indicates that the varied frequency f 2  is closer to the actual resonant frequency f R  whereas a negative variation value Δη indicates that the varied frequency f 2  is farther from the actual resonant frequency f R . 
     Other determination criteria based on combined calculated variation values ΔV 2   DC , Δη may be adopted according to the needs. 
     The frequency control procedure  60  comprises the step  64  of controlling the operation of the inverter stage  23  to obtain a first AC current I 1   AC  circulating along the transmitter coil  31  having an operating frequency that is set depending on the results of the determination process carried out at the previous step  63 . 
     If it is determined that the varied frequency f 2  of the first AC current I 1   AC  is closer to the actual resonant frequency f R  of the resonant electric circuit  340 , the varied frequency f 2  is set as new operating frequency for the first AC current I 1   AC  and the control means  10  control the operation of the inverter stage  23  in such a way to keep on obtaining a first AC current I 1   AC  having such a new operating frequency f 2 . 
     Additionally, in this case, the control procedure  60  will be repeated by imposing a new frequency variation Δf NEW  to perturb the new operating frequency f 2 , which has the same sign of the previous frequency variation Δf adopted to perturb the operating frequency f 1 . In practice, the following condition is adopted: sgn(Δf NEW )=sgn(Δf). 
     If it is determined that the varied frequency f 2  of the first AC current I 1   AC  is farther from the actual resonant frequency f R  of the resonant electric circuit  340 , the varied frequency f 2  is disregarded and the control means  10  control the operation of the inverter stage  23  in such a way to keep of obtaining a first AC current I 1   AC  with the initial operating frequency Additionally, in this case, the control procedure  60  will be repeated by imposing a new frequency variation Δf NEW  to perturb the restored operating frequency f 1 , which has an opposite sign with respect the previous frequency variation Δf adopted to perturb the operating frequency f 1 . In practice, the following condition is adopted: sgn(Δf NEW )≠sgn(Δf). It is evident from the above how the frequency control procedure  60  allows setting the operating frequency of the first AC current I 1   AC  in such a way to follow any possible variations of the resonance frequency f R  of the resonant electric circuit  340 . 
     As mentioned above, this allows optimizing the overall power exchange efficiency for any generic operating point of the power transfer system, even in presence of variations of the operative conditions of this latter. 
     Preferably, the aforesaid frequency control procedure  60  is cyclically repeated during the operating life of the power transfer system, e.g. with repetition periods of 10 s. In this way, the resonant condition of the resonant electric circuit  340  may be constantly tracked during the operating life of the power transfer system. 
     In a practical implementation of the invention, the frequency control procedure  60  is conveniently carried out at level of the transmitter-side controllers  6 . To this aim, the transmitter-side controllers  6  may receive suitable detection signals D 2 , D 3  from the sensing arrangements  210 ,  220 A and  230  and from the sensing arrangement  510 . As indicated above, detection signals D 3  from the sensing arrangement  510  may be suitably transmitted to the transmitter-side controllers  6  through the receiver-side controllers  7  and the communication channel  8 . 
     In the embodiments ( FIG. 2 ) in which the bus-stage  22  includes a DC-DC switching converter  220 , the control means  10  may be suitably configured to provide voltage control functionalities of the operating DC voltages V 1   DC , V 2   DC  at the first and second ports  221 ,  222  of the DC/DC converter  220  to reduce power losses at the DC-bus stage  22 . 
     According to said voltage control functionalities, a second DC voltage V 2   DC  requested to be provided at the second port  222  is obtained by means of a voltage regulation carried out alternatively by the first rectifying stage  21  or the DC/DC switching converter  220 . 
     The electronic stage  21  or  220  to be employed for providing such a voltage regulation is selected depending on the requested values V 2   DC_REQ  for the second DC voltage V 2   DC  (which has to be provided at the second port  222  of the DC/DC converter  220 ) with respect to a minimum value V MIN  of a first DC voltage V 1   DC  that can be made available by the first rectifying stage  21  at the first port  221  of the DC/DC converter  220 . 
     The minimum value V MIN  may be set depending on the design of the electronic stage  21  and/or depending on the power control functionalities carried out by the control means  10 . 
     The requested values V 2   DC_REQ  for the second DC voltage V 2   DC  mostly depend on the operating set points of the power transfer system (conveniently controlled by implementing the above-described power control functionalities). 
     If a requested value for the second DC voltage V 2   DC  to be provided at the second port  222  of the DC/DC converter  220  is lower than or equal to said possible minimum value V MIN  (in practice if V 2   DC_REQ &lt;=V MIN ) the regulation of the second DC voltage V 2   DC  can be carried out by the DC/DC converter  220 . 
     In this case, the control means  10  control the operation of the first rectifying stage  21  to obtain a first DC voltage V 1   DC  set at the minimum value V MIN  and control the operation of the DC/DC converter  220  to obtain a second DC voltage V 2   DC  having the requested value V 2   DC_REQ . 
     In this way, power losses at the DC/DC converter  220  are reduced as power switches are subject to lower direct voltages (i.e. lower drain-source or collector-emitter voltages depending on the type of said power switches). 
     If a requested value for the second DC voltage V 2   DC  is higher than the minimum value V MIN  (in practice if V 2   DC_REQ &gt;V MIN ), the regulation of the second DC voltage V 2   DC  has to be carried out by the first rectifying stage  21 . 
     In this case, the control means  10  control the operation of the first rectifying stage  21  to obtain a first DC voltage V 1   DC  having a value equal to the requested value V 2   DC_REQ  for said second DC voltage V 2   DC  and control the operation of the DC/DC converter  220  to obtain a second DC voltage V 2   DC  equal to said first DC voltage V 1   DC . 
     Again, power losses at the DC/DC converter  220  are reduced as overall commutations of power switches are reduced (i.e. power switches have a duty-cycle equal to 1). 
     The above described solution is quite advantageous as it remarkably facilitates the obtaining of a given DC electric power P DC  exchanged with the battery  200  at a given overall power transfer efficiency η. 
     In a practical implementation of the invention, the above mentioned voltage control functionalities are conveniently carried out at level of the transmitter-side controllers  6 . To this aim, the transmitter-side controllers  6  may receive suitable detection signals D 4  from the sensing arrangements  210 ,  220 A. 
     The control means  10  may provide the above mentioned voltage control functionalities by providing third control signals C 3  to control the operation of at least one between the rectifying stage  21  and the DC/DC converter stage  220 . 
     Conveniently, the third control signals C 3  are adapted to control the duty-cycle of the power switches included in at least one of the rectifying stage  21  and in the DC/DC converter stage  220 . 
     In a practical implementation of the invention, the transmitter-side power sub-system  2  may be arranged in a wall-box device for an electric vehicle charging facility, e.g. for residential purposes. Such a wall-box device may conveniently include the one or more transmitter-side controllers  6  operatively associated with the electronic stages of the transmitter-side power sub-system  2 . 
     The transmitter-side coil sub-system  3  may be arranged or embedded in a ground pad device for an electric vehicle charging facility, e.g. for residential purposes. Such a ground pad device may conveniently include possible one or more transmitter-side controllers  6  operatively associated to transmitter-side coil sub-system  3 . 
     The receiver-side coil sub-system  4 , the receiver-side power sub-system  5  and the receiver-side controllers  7  are arranged (together with the battery  200 ) on board an electric vehicle. 
     In a further aspect, the present invention relates to a control method for controlling the operation of a power transfer system as described above. 
     The method, according to the invention, comprises controlling a DC electric power P DC  exchanged with the battery  200  by controlling operation of at least one between the first rectifying stage  21  and the DC-bus stage  22  and controlling an operating frequency of a first AC current I 1   AC  circulating along the transmitter coil  31  to track a resonant frequency f R  of the resonant electric circuit  340  formed by the transmitted-side coil sub-system  3  and said receiver-side coil sub-system  4 , when the transmitter coil  31  and said receiver coil  41  are inductively coupled. 
     Preferably, the method, according to the invention, comprises providing frequency variations Δf of an operating frequency f 1  of an first AC current I 1   AC  circulating along the transmitter coil  31  and observing variations ΔV 2   DC , Δη of one or more electric quantities V 2   DC , η of the power transfer system in response to said frequency variations Δf to track the above-mentioned resonant frequency f R . 
     Preferably, the method, according to the invention, comprises the following steps:
         controlling the operation of the inverter stage  23  to obtain a first AC current I 1   AC  circulating along the transmitter coil  31  with a varied frequency f 2  obtained by imposing a frequency variation Δf of an operating frequency f 1  of said first AC current;   calculating variations ΔV 2   DC , Δη of one or more electric quantities V 2   DC , η of the power transfer system in response to the obtaining of said first AC current I 1   AC  having said varied frequency f 2 ;   determining whether said varied frequency f 2  is closer to or farther from said resonant frequency f R  depending on the calculated variations ΔV 2   DC , Δη of said one or more electric quantities (V 2   DC , η);   controlling the operation of the inverter stage  23  to obtain said first AC current I 1   AC  with an operating frequency set depending on the results of said determination.       

     Preferably, if it is determined that the varied frequency f 2  of the first AC current I 1   AC  is closer to the actual resonant frequency f R  of the resonant electric circuit  340 , the varied frequency f 2  is set as new operating frequency of the of the first AC current I 1   AC  and the operation of the inverter stage  23  is controlled in such a way to keep on obtaining a first AC current I 1   AC  having such a new operating frequency f 2 . 
     Additionally, in this case, the control procedure  60  is repeated by imposing a new frequency variation Δf NEW  to perturb the new operating frequency f 2 , which has the same sign of the previous frequency variation Δf adopted to perturb the operating frequency f 1 . 
     Preferably, if it is determined that the varied frequency f 2  of the first AC current I 1   AC  is farther than the actual resonant frequency f R  of the resonant electric circuit  340 , the varied frequency f 2  is disregarded and the operation of the inverter stage  23  is controlled in such a way to keep of obtaining a first AC current I 1   AC  with the initial operating frequency f 1 . 
     Additionally, in this case, the control procedure  60  is repeated by imposing a new frequency variation Δf NEW  to perturb the restored operating frequency f 1 , which has an opposite sign with respect the previous frequency variation Δf adopted to perturb the operating frequency f 1    
     Preferably, the aforesaid control steps of the method, according to the invention, are cyclically repeated during the operating life of the power transfer system. 
     In the embodiments ( FIG. 2 ) in which the bus-stage  22  includes a DC-DC switching converter  220 , the method, according to the invention, preferably comprises the following steps: 
     if a value requested for a second DC voltage V 2   DC  at the second port  222  of the DC/DC converter  220  is lower or equal to a possible minimum value V MIN  for a first DC voltage V 1   DC  at the first port  221  of the DC/DC converter  220 :
         controlling operation of the first rectifying stage  21  to obtain said first DC voltage V 1   DC  set at said minimum value V MIN ;   controlling operation of the DC/DC converter  220  to obtain said second DC voltage V 2   DC  having said requested value;
 
if a value requested for the second DC voltage V 2   DC  at the second port  222  is higher than a possible minimum value V MIN  for the first DC voltage V 1   DC  at the first port  221 :
   controlling operation of the first rectifying stage  21  to obtain a first DC voltage V 1   DC  having a value equal to the value requested for said second DC voltage V 2   DC ;   controlling operation of the DC/DC converter  220  to obtain a second DC voltage V 2   DC  equal to said first DC voltage V 1   DC .       

     The power transfer system, according to the invention, allows achieving the intended aims and objects. 
     The power transfer system, according to the invention, includes a control architecture ensuring a suitable transmission of electric power to the battery on board a vehicle, in accordance with a given power transfer profile, and, at the same time, capable of providing frequency control functionalities directed to optimize the overall power exchange efficiency of said power transfer system. 
     The power transfer system, according to the invention, ensures good performances in terms of reliability, even when unpredictable changes in operating conditions occur. 
     The power transfer system, according to the invention, can be easily arranged and produced at industrial level, at competitive costs with respect to similar systems of the state of the art.