Patent Publication Number: US-2020287375-A1

Title: Over voltage protection for a wireless power transfer system

Description:
RELATED APPLICATIONS 
     This application is a Continuation of and claims the priority benefit of U.S. application Ser. No. 15/717,855 filed Sep. 27, 2017 which claims the priority benefit of India Application No. 201641033404 filed Sep. 30, 2016, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Embodiments of the present disclosure relate generally to wireless power transfer systems and more particularly relate to over voltage protection for a wireless power transfer system. 
     In one or more industries, an electric vehicle or a hybrid vehicle includes one or more batteries that supply electrical power to drive the vehicle. In one example, the batteries supply energy to an electric motor to drive a shaft in the vehicle, which in turn drives the vehicle. The batteries are used for supplying the power and hence may be drained and need to be charged from an external power source. 
     In general, power transfer systems are widely used to transfer power from a power source to one or more electric loads, such as for example, the batteries in the vehicle. Typically, the power transfer systems may be contact based power transfer systems or contactless power transfer systems. In the contact based power transfer systems, components, such as plug, socket connectors, and wires are physically coupled to the batteries for charging the batteries. However, due to environmental impact, such connectors and wires may be damaged or corroded. Also, high currents and voltages are used for charging the batteries. Hence, establishing a physical connection between the power source and the batteries in the vehicle may involve cumbersome safety measures. Also, this power transfer system may become bulkier and heavier compared to the contactless power transfer system. 
     In the contactless power transfer systems, power converters are used to convert an input power to a transferrable power, which is further transmitted to the electric load, such as the batteries in the vehicle. The power converter includes switches which are operated at a particular switching frequency to convert the input power to the transferrable power. Typically, depending upon the load, the switching frequency of the power converter is changed to regulate or control an output voltage of the power transfer system. However, if the electric load is disconnected or varied, the output voltage of the power transfer system may attain a very high value in a very short time period. Such a sudden increase in the output voltage may lead to failure of operation and may also damage one or more components in the power transfer system. 
     Therefore, there is a need for an improved system and method for protecting the power transfer system. 
     SUMMARY 
     In accordance with one embodiment of the present disclosure, a wireless power transfer system is disclosed. The wireless power transfer system includes a first converting unit configured to convert a first DC voltage of an input power to an AC voltage. Further, the wireless power transfer system includes a contactless power transfer unit communicatively coupled to the first converting unit and configured to receive the input power having the AC voltage from the first converting unit and transmit the input power having the AC voltage. Also, the wireless power transfer system includes a second converting unit communicatively coupled to the contactless power transfer unit and configured to receive the input power having the AC voltage from the contactless power transfer unit, convert the AC voltage of the input power to a second DC voltage, and transmit the input power having the second DC voltage to an electric load. In addition, the wireless power transfer system includes a switching unit including a first switch electrically coupled in parallel to a first diode in a first branch of the second converting unit, a second switch electrically coupled in parallel to a second diode in a second branch of the second converting unit, and a controller electrically coupled to the first switch and the second switch and configured to activate the first switch and the second switch to decouple the electric load from the contactless power transfer unit if the second DC voltage across the electric load is greater than a first threshold value. 
     In accordance with another embodiment of the present disclosure, a method for operating a wireless power transfer system is disclosed. The method includes converting, by a first converting unit, a first DC voltage of an input power to an AC voltage. Further, the method includes receiving from the first converting unit and transmitting, by a contactless power transfer unit, the input power having the AC voltage. Also, the method includes converting, by a second converting unit, the AC voltage of the input power to a second DC voltage. In addition, the method includes transmitting the input power having the second DC voltage from the second converting unit to an electric load. Furthermore, the method includes decoupling, by a switching unit, the electric load from the contactless power transfer unit if the second DC voltage across the electric load is greater than a first threshold value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings. 
         FIG. 1  is a block diagram representation of a wireless power transfer system having a switching unit in accordance with an embodiment. 
         FIG. 2  is a schematic representation of a wireless power transfer system in accordance with an embodiment. 
         FIG. 3  is a schematic representation of a wireless power transfer system in accordance with another embodiment. 
         FIG. 4  is a flow chart illustrating a method for protecting a wireless power transfer system in accordance with an embodiment. 
         FIG. 5  is a flow chart illustrating a method for decoupling and coupling a converting unit in a wireless power transfer system in accordance with an embodiment. 
         FIG. 6  is a schematic representation of a wireless power transfer system in accordance with another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As will be described in detail hereinafter, various embodiments of a system and method for protecting a wireless power transfer system are disclosed. Also, various embodiments of a system and method for regulating an output voltage of the wireless power transfer system are disclosed. In particular, the system and method disclosed herein employ a switching unit to protect one or more components of the wireless power transfer system. More specifically, the switching unit decouples the one or more components in the system if an output voltage of the wireless power transfer system increases to an undesirable value. Further, the switching unit is configured to control the output voltage of the wireless power transfer system even if magnitude of an electric load coupled to the wireless power transfer system changes substantially. 
       FIG. 1  is a diagrammatical representation of a wireless power transfer system  100  in accordance with an embodiment of the present disclosure. The wireless power transfer system  100  is used to transmit an electrical power from a power source  102  to one or more electric loads  132  such as batteries, mobile devices such as cell phones, laptops, HVAC systems, and the like. Particularly, in an automobile industry, an electric vehicle or a hybrid vehicle includes one or more batteries that supply electrical power to drive the vehicle. Such batteries may be electrically charged from the power source  102  via the wireless power transfer system  100 . In one embodiment, the wireless power transfer system  100  may also be referred to as a contactless power transfer system. 
     In the illustrated embodiment, the wireless power transfer system  100  includes a first converting unit  104  (inverter), a control unit  106 , a contactless power transfer unit  108 , and a second converting unit  110  (rectifier). The first converting unit  104  is electrically coupled to the power source  102  and the control unit  106 . The power source  102  is configured to supply an input power having a first DC voltage  112  to the first converting unit  104 . In some embodiments, the input power may be in a range from about 100 W to about 6.6 kW. In one embodiment, the power source  102  may be a part of the wireless power transfer system  100 . In another embodiment, the power source  102  may be positioned external to the wireless power transfer system  100 . 
     The first converting unit  104  is configured to receive the input power having the first DC voltage  112  from the power source  102 . Further, the first converting unit  104  is configured to operate at a determined switching frequency to convert the first DC voltage  112  of the input power to an AC voltage  114 . Particularly, the control unit  106  may determine the switching frequency of the first converting unit  104  based on the electric load  132 . In one embodiment, the control unit  106  may include a digital circuit or a processor that performs one or more functions based on pre-stored instructions or programs. Upon converting the first DC voltage  112  of the input power to the AC voltage  114 , the first converting unit  104  is further configured to transmit the input power having the AC voltage  114  to the contactless power transfer unit  108 . 
     The contactless power transfer unit  108  includes two or more coils or an array of coils  116  that are magnetically coupled to each other. The coils  116  are used for wirelessly transmitting the input power having the AC voltage  114  from the first converting unit  104  to the second converting unit  110 . The details pertaining to transmitting the power using the coils  116  are explained in greater detail below with reference to  FIG. 2 . 
     The second converting unit  110  is electrically coupled to the contactless power transfer unit  108  via a switching unit  130 . Upon receiving the power having the AC voltage  114  from the contactless power transfer unit  108 , the second converting unit  110  is configured to convert the AC voltage  114  of the input power to a second DC voltage  118 . Further, the second converting unit  110  is configured to transmit the input power having the second DC voltage  118  to the electric load  132 . In one embodiment, the input power having the second DC voltage may be used for charging the electric load including one or more batteries that are coupled to the wireless power transfer system  100 . 
     Additionally, the wireless power transfer system  100  includes a voltage sensor  120 , a first transceiver  122 , and a second transceiver  124  that together form a feedback loop  126 . The voltage sensor  120  is used to sense the second DC voltage (output voltage)  118 . The feedback loop  126  is used to transmit a voltage signal (V o )  128  representative of the second DC voltage  118  from the voltage sensor  120  to the control unit  106  via the first transceiver  122  and the second transceiver  124 . Further, the control unit  106  is used to adjust the switching frequency of the first converting unit  104  based on the received voltage signal (V o )  128  to control or regulate the second DC voltage  118  across the electric load  132 . 
     However, since the voltage signal (V o )  128  is communicated using a wireless communication path between the first transceiver  122  and the second transceiver  124 , the control unit  106  may receive the voltage signal (V o )  128  after a certain time delay. In one embodiment, the delay may be in a range from about 1 millisecond to about 5 milliseconds. In such a scenario, the control unit  106  may not be able to timely control the second DC voltage  118  across the electric load  132  due to the delay in communicating the voltage signal (V o )  128 . As a result, the second DC voltage  118  may increase above a critical value, which in turn may affect the second converting unit  110  and/or other components in the wireless power transfer system  100 . The critical value may be a voltage value above which the components in the wireless power transfer system  100  may be affected. In one embodiment, the critical value may be in a range from about 400V to about 500V. 
     To overcome the issues related to increase of the second DC voltage  118  above a critical value, the exemplary wireless power transfer system  100  includes the switching unit  130  to protect the second converting unit  110  or other components in the wireless power transfer system  100  from being affected. Particularly, the switching unit  130  is electrically coupled to the contactless power transfer unit  108  and the second converting unit  110 . The switching unit  130  is configured to decouple the electric load  132  from the contactless power transfer unit  108  if the second DC voltage  118  is greater than a first threshold value (V o Max). The first threshold value (V o Max) is less than the critical value. In one embodiment, the first threshold value (V o Max) may be in a range from about 300V to about 400V. 
     The input power is not transmitted to the electric load  132  by decoupling the electric load  132  from the contactless power transfer unit  108 . As a result, the second DC voltage  118  across the electric load  132  is reduced to less than the first threshold value (V o Max). The protection of the second converting unit  110  is described in greater detail with reference to  FIG. 2 . 
     Furthermore, in one embodiment, the switching unit  130  may be used to regulate or control the second DC voltage  118  across the electric load  132 . If a difference between the second DC voltage  118  and a voltage reference value (V o ref) is above a predefined value, the switching unit  130  is configured to regulate or control the second DC voltage  118  without decoupling the electric load  132  from the contactless power transfer unit  108 . Such a regulation of the second DC voltage  118  is described in greater detail with reference to  FIG. 3 . 
     Referring to  FIG. 2 , a schematic representation of the wireless power transfer system  100  in accordance with an embodiment of the present disclosure is depicted. The first converting unit  104  includes a plurality of switches  220  and diodes  222  that are electrically coupled between an input terminal  219  and an output terminal  221 . In one embodiment, the switches  220  may include electronic switches, such as MOSFETs or IGBTs. The switches  220  are activated and deactivated based on a switching frequency of the first converting unit  104  to convert the first DC voltage  112  of the input power to the AC voltage  114 . Particularly, the control unit  106  is configured to determine the switching frequency of the first converting unit  104  based on the electric load  132 . Further, the control unit  106  is configured to send one or more gate signals  226  that are representative of the switching frequency to the plurality of switches  220  to convert the first DC voltage  112  of the input power to the AC voltage  114 . The input power having the AC voltage  114  is transmitted from the first converting unit  104  to the contactless power transfer unit  108 . 
     The contactless power transfer unit  108  includes a primary coil  116   a  and a secondary coil  116   b . The primary coil  116   a  is electrically coupled to the first converting unit  104 . The secondary coil  116   b  is electrically coupled to the second converting unit  110 . The primary coil  116   a  and the secondary coil  116   b  are magnetically coupled to each other. 
     In addition to the primary coil  116   a  and the secondary coil  116   b , the contactless power transfer unit  108  includes a field focusing coil  116   c  and a compensation coil  116   d . The field focusing coil  116   c  is positioned between the primary coil  116   a  and the secondary coil  116   b . The field focusing coil  116   c  is magnetically coupled to the primary coil  116   a  and the secondary coil  116   b . The compensation coil  116   d  is magnetically coupled to the secondary coil  116   b.    
     The input power having the AC voltage  114  is used to excite the primary coil  116   a  and the field focusing coil  116   c  simultaneously. A primary current corresponding to the AC voltage  114  flows through the primary coil  116   a  resulting in excitation of the primary coil  116   a , which in turn generates a magnetic field that excites the field focusing coil  116   c . Further, the magnetic field generated by the primary coil  116   a  is focused towards the secondary coil  116   b  via the field focusing coil  116   c . The secondary coil  116   b  is configured to receive the magnetic field and convert the magnetic field to the input power having the AC voltage  114 . When the secondary coil  116   b  receives the magnetic field, a secondary current corresponding to the AC voltage  114  flows through the secondary coil  116   b . Further, the input power having the AC voltage  114  is then transmitted from the secondary coil  116   b  to the second converting unit  110 . 
     In one embodiment, the field focusing coil  116   c  is electrically coupled to one or more resonators which are excited simultaneously by the input power to enhance the magnetic coupling between the primary coil  116   a  and the secondary coil  116   b . The compensation coil  116   d  is configured to match an impedance of the contactless power transfer unit  108  with the second converting unit  110 . 
     The second converting unit  110  includes a first branch  268  and a second branch  270  that are electrically coupled between the input terminal  271  and the output terminal  273 . Particularly, the first branch  268  includes a pair of first diodes  272 ,  274  and the second branch  270  includes a pair of second diodes  276 ,  278 . In some embodiments, the second converting unit  110  may be configured using electronic switches, such as MOSFETs, IGBTs along with diodes or without using diodes. The voltage sensor  120  is electrically coupled to the output terminal of the second converting unit  110  to determine the second DC voltage  118  across the electric load  132 . 
     The first transceiver  122  includes an antenna  248  configured to transmit the voltage signal (V o )  128  to an antenna  250  of the second transceiver  124 . In one embodiment, the first transceiver  122  is positioned proximate to the electric load  132  and the second transceiver  124  is positioned proximate to the first converting unit  104  or the power source  102 . 
     As discussed earlier, the control unit  106  is configured to determine a change in the electric load  132  based on the voltage signal (V o )  128  representative of the second DC voltage  118 . In response to receiving the voltage signal (V o )  128 , the control unit  106  is configured to determine or adjust the switching frequency of the first converting unit  104 . Further, the control unit  106  is configured to send gate signals  226  that are representative of the switching frequency to the first converting unit  104  to control the AC voltage  114  of the first converting unit  104 , which in turn controls the second DC voltage  118  across the electric load  132 . In other words, the control unit  106  is configured to control or regulate the second DC voltage  118  of the wireless power transfer system  100  based on the voltage signal (V o )  128  received via the feedback loop  126 . 
     The switching unit  130  includes a first switch  280 , a second switch  282 , and a controller  254 . The controller  254  is electrically coupled to the first switch  280 , the second switch  282 , and the sensor  120 . In one embodiment, the first switch  280  and the second switch  282  may include one or more electronic switches such as MOSFETs, IGBTs, and the like. 
     In the illustrated embodiment, the first switch  280  is electrically coupled across the first diode  272  in the first branch  268  of the second converting unit  110 . In a similar manner, the second switch  282  is electrically coupled across the second diode  276  in the second branch  270  of the second converting unit  110 . If the second DC voltage  118  across the electric load  132  is greater than a first threshold value, the first switch  280  and the second switch  282  are operated in a protection mode to decouple the electric load  132  from other components in the wireless power transfer system  100 . Particularly, the first switch  280  and the second switch  282  are activated to short-circuit the secondary coil  116   b  of the contactless power transfer unit  108 . As a result, the electric load  132  is decoupled from the contactless power transfer unit  108 . 
     If the second DC voltage  118  across the electric load  132  is less than a second threshold value, the first switch  280  and the second switch  282  are operated in a normal mode to couple the electric load  132  to the components of the wireless power transfer system  100 . Particularly, the first switch  280  and the second switch  282  are activated and deactivated based on switching signals received from the controller  254  to couple the electric load  132  to the contactless power transfer unit  108 . The first switch  280  and the second switch  282  are activated and deactivated to convert the AC voltage  114  to the second DC voltage  118 . The switching signals include switching pulses corresponding to the secondary current of the contactless power transfer unit  108 . 
     The controller  254  includes a first voltage comparator  256 , a second voltage comparator  258 , a flip-flop unit  260 , a current comparator  284 , a NOT gate  286 , a first OR gate  288 , and a second OR gate  290 . The first comparator  256  and the second comparator  258  are electrically coupled to an input terminal of the flip-flop unit  260 . Further, an output terminal of the flip-flop unit  260  is coupled to input terminals of the first OR gate  288  and the second OR gate  290 . 
     An input terminal of the current comparator  284  is electrically coupled to a current sensor  292  that is coupled to the secondary coil  116   b  of the contactless power transfer unit  108 . The current sensor  292  is used to sense the secondary current of the contactless power transfer unit  108  and transmit a current signal to the current comparator  284 . Further, an output terminal of the current comparator  284  is directly coupled to the input terminal of the first OR gate  288 . The output terminal of the current comparator  284  is coupled to the input terminal of the second OR gate  290  via the NOT gate  286 . Further, an output terminal of the first OR gate  288  is coupled to the first switch  280  and an output terminal of the second OR gate  290  is coupled to the second switch  282 . 
     Specifically, the controller  254  is configured to receive the voltage signal (V o )  128  that is representative of the second DC voltage  118  from the sensor  120 . Further, the received voltage signal (V o )  128  is transmitted to the first comparator  256  and the second comparator  258 . The first comparator  256  is configured to compare the second DC voltage  118  with a first threshold value (V o Max). If the second DC voltage  118  is greater than the first threshold value (V o Max), the first comparator  256  is configured to trigger the flip-flop unit  260  to generate a first control signal  261  at the output terminal of the flip-flop unit  260 . In one embodiment, the first control signal  261  is representative of binary ‘1’. Further, the first control signal  261  is transmitted to the first OR gate  288  to generate a first switching signal  265 . The first control signal  261  is transmitted to the second OR gate  290  to generate a second switching signal  267 . In this scenario, the first OR gate  288  and the second OR gate  290  generates the first and second switching signals  265 ,  267  to operate the first and second switches  280 ,  282  in the protection mode. Particularly, if the flip-flop unit  260  generates the first control signal  261 , the first OR gate  288  generates the first switching signal  265  that is same as the first control signal  261 . Concurrently, the second OR gate  290  generates the second switching signal  267  that is same as the first control signal  261 . In one embodiment, the first and second switching signals  265 ,  267  are representative of binary ‘1’. Further, the first switching signal  265  is transmitted to the first switch  280  to activate the first switch  280 , and the second switching signal  267  is transmitted to the second switch  282  to activate the second switch  282 . By activating the first switch  280  and the second switch  282 , the secondary coil  116   b  of the contactless power transfer unit  108  is short-circuited. As a result, the electric load  132  is decoupled from the contactless power transfer unit  108 . Further, second DC voltage  118  across the electric load  132  is reduced to below the first threshold value. 
     Furthermore, the second comparator  258  is configured to receive the voltage signal (V o )  128  that is representative of the second DC voltage  118 . The second comparator  258  is configured to compare the received second DC voltage  118  with a second threshold value (V o Min). It should be noted herein that the second threshold value (V o Min) is less than the first threshold value (V o Max). If the second DC voltage  118  is less than the second threshold value (V o Min), the second comparator  258  is configured to trigger the flip-flop unit  260  to generate a second control signal  263  at the output terminal of the flip-flop unit  260 . In one embodiment, the second control signal  263  is representative of binary ‘0’. Further, the second control signal  263  is transmitted to the first OR gate  288  and the second OR gate  290 . The first OR gate  288  and the second OR gate  290  generates switching signals to operate the first and second switches  280 ,  282  in the normal mode. In one embodiment, the switching signals are generated based on the secondary current of the contactless power transfer unit  108 . More specifically, the current comparator  284  compares the secondary current of the contactless power transfer unit  108  with a zero value to trigger the first OR gate  288  to generate a third switching signal  269  and the second OR gate  290  to generate a fourth switching signal  275 . The third switching signal  269  and the fourth switching signal  275  are complimentary to each other. The third switching signal  269  and the fourth switching signal  275  include switching pulses that correspond to the secondary current of the contactless power transfer unit  108 . Further, the third switching signal  269  is transmitted to the first switch  280  and the fourth switching signal  275  is transmitted to the second switch  282  to convert the AC voltage  114  of the input power to the second DC voltage  118 . 
     During normal operation of the wireless power transfer system  100 , the first switch  280  and the second switch  282  are activated and deactivated based on the third and fourth switching signals  269 ,  275  to couple the electric load  132  to the contactless power transfer unit  208  and convert the AC voltage  114  of the input power to the second DC voltage  118 . At this stage, the controller  254  sends the third and fourth switching signals  269 ,  275  to operate the first and second switches  280 ,  282  in the normal mode. 
     In certain situations, if the full load  132  or a portion of the load  132  is disconnected or decoupled suddenly from the second converting unit  110 , the second DC voltage  118  across the load  132  may increase above the first threshold value (V o Max). The sensor  120  determines and sends the voltage signal (V o )  128  that is representative of this second DC voltage  118  to the controller  254  and the first transceiver  122 . The first comparator  256  compares the second DC voltage  118  with the first threshold value (V o Max). If the second DC voltage  118  is greater than the first threshold value (V o Max), the first comparator  256  triggers the flip-flop unit  260  to generate the first control signal  261  which is transmitted to the first OR gate  288  and the second OR gate  290  to generate the first switching signal  265  and the second switching signal  267 . Further, the first and second switching signals  265 ,  267  are transmitted to activate the first and second switches  280 ,  282 . As a result, the electric load  132  is decoupled from the contactless power transfer unit  108 . 
     Concurrently, the first control signal  261  is transmitted from the controller  254  to the first transceiver  122 . Further, the first transceiver  122  transmits the voltage signal (V o )  128  received from the sensor  120  and the first control signal  261  received from the controller  254  to the second transceiver  124 . The voltage signal (V o )  128  and the first control signal  261  are further transmitted to the control unit  106 . 
     Upon receiving the voltage signal (V o )  128  and the first control signal  261 , the control unit  106  determines that the first and second switches  280 ,  282  are activated. As a result, the control unit  106  deactivates the first converting unit  104 . Specifically, the control unit  106  sends the gate signals  226  to the switches  220  to deactivate or open the switches  220 . As a result, the first converting unit  104  is deactivated from transmitting the power to the contactless power transfer unit  108  and the second converting unit  110 . 
     Furthermore, after a predetermined time period, the control unit  106  sends a reset signal  262  to the second transceiver  124 , which is further transmitted to the first transceiver  122 . The first transceiver  122  sends the reset signal  262  to the flip-flop unit  260 . In response to receiving the reset signal  262 , the flip-flop unit  260  resets and generates the second control signal  263  at the output terminal of the controller  254 . The generated second control signal  263  is further transmitted to the first OR gate  288  and the second OR gate  290  to generate the third switching signal  269  and the fourth switching signal  275  that include switching pulses corresponding to the secondary current of the contactless power transfer unit  108 . Further, the third and fourth switching signals  269 ,  275  are transmitted to the first and second switches  280 ,  282  to convert the AC voltage  114  of the input power to the second DC voltage  118 . The electric load  132  is coupled to the contactless power transfer unit  108  via the second converting unit  110  to receive the converted second DC voltage  118 . 
     Concurrently, the generated second control signal  263  is transmitted from the controller  254  to the first transceiver  122 . In addition to the second control signal  263 , the first transceiver  122  receives the voltage signal (V o )  128  representative of the second DC voltage  118  across the load  132 . Further, the first transceiver  122  transmits the voltage signal (V o )  128  and the second control signal  263  to the second transceiver  124 , which is further transmitted to the control unit  106 . 
     Upon receiving the voltage signal (V o )  128  and the second control signal  263  from the second transceiver  124 , the control unit  106  determines whether the second DC voltage  118  is less than or equal to the first threshold value (V o Max). If the second DC voltage  118  is less than or equal to the first threshold value (V o Max), the control unit  106  sends the gate signals  226  to the switches  220  to activate the first converting unit  104 . Further, the control unit  206  adjusts the switching frequency of the first converting unit  104  based on the second DC voltage  118  across the electric load  132 . In one embodiment, the control unit  106  adjusts the switching frequency of the first converting unit  104  to regulate the second DC voltage  118  across the electric load  132 . If the second DC voltage  118  is greater than the first threshold value (V o Max), the control unit  106  waits for the predetermined time period to send another reset signal to the controller  254 . If the second DC voltage  118  continues to be greater than the first threshold value (V o Max) after transmitting a determined number of reset signals, the control unit  106  deactivates the electric load  132  from the contactless power transfer unit  108 . In one embodiment, the control unit  106  may shut down or deactivate the power transfer system. 
     Accordingly, by employing the switching unit  130  and the control unit  106 , the second DC voltage  118  is prevented from increasing to greater than the critical value. As a result, the second converting unit  110  is protected from damage even if the electric load  132  is decoupled from the contactless power transfer unit  108 . 
     Referring to  FIG. 3 , a schematic representation of a wireless power transfer system  300  in accordance with another embodiment of the present disclosure is depicted. The wireless power transfer system  300  is similar to the wireless power transfer system  100  of  FIG. 2  except that a controller  302  of a switching unit  312  is configured to regulate or control the second DC voltage  118  (output voltage) of the second converting unit  110 . 
     During operation, the second DC voltage  118  across the electric load  132  may vary substantially from a voltage reference value (V o ref). In one example, the second DC voltage  118  may increase or decrease below the voltage reference value (V o ref). In certain scenario, due to change in the electric load  132 , difference between the second DC voltage  118  and the voltage reference value (V o ref) may increase above a predefined value. As a consequence, one or more components in the second converting unit  110  may be affected or damaged. Thus, it is required to control or regulate the second DC voltage  118  so that the second DC voltage  118  is maintained below a critical value. In one example, the voltage reference value (V o ref) is less than the critical value. 
     The sensor  120  transmits the voltage signal (V o )  128  representative of the second DC voltage  118  to the controller  302 . A voltage comparator  306  of the controller  302  is configured to compare the second DC voltage  118  with the voltage reference value (V o ref). If a difference between the second DC voltage  118  and the voltage reference value (V o ref) is above the predefined value, the voltage comparator  306  generates a third control signal  304  having a determined duty cycle at an output terminal. In one embodiment, the controller  302  may determine or select the duty cycle using a look-up table. For example, if the second DC voltage  118  is 90 volts, a duty cycle of 0.75 is selected from the look-up table. In another example, if the second DC voltage  118  is 170 volts, a duty cycle of 0.5 is selected from the look-up table. In yet another example, if the second DC voltage  118  is 250 volts, a duty cycle of 0.25 is selected from the look-up table. The third control signal  304  includes switching pulses corresponding to the determined duty cycle. 
     The voltage comparator  306  transmits the third control signal  304  having the determined duty cycle to the first OR gate  288  and the second OR gate  290 . Concurrently, the current comparator  284  transmits signals corresponding to the secondary current to the first OR gate  288  and the second OR gate  290 . The first OR gate  288  generates the fifth switching signal  277  and the second OR gate  290  generates the sixth switching signal  279  based on the third control signal  304  and the secondary current of the contactless power transmitting unit  108 . The fifth switching signal  277  and the sixth switching signal  279  are complimentary to each other. The fifth switching signal  277  and the sixth switching signal  279  include switching pulses corresponding to the third control signal  304  and the secondary current of the contactless power transfer unit  108 . Further, the fifth switching signal  277  is transmitted to the first switch  280  and the sixth switching signal  279  is transmitted to the second switch  282  to regulate the second DC voltage  118  across the load  132 . 
     More particularly, if the difference between the second DC voltage  118  and the voltage reference value (V o ref) is greater than the predefined value, the fifth switching signal  277  and the sixth switching signal  279  are transmitted to first switch  280  and the second switch  282  respectively, to activate and deactivate the first and second switches  280 ,  282 . The fifth and sixth switching signals  277 ,  279  include switching pulses corresponding to the switching pulses of the third control signal  304 . As a result, the difference between the second DC voltage  118  and the voltage reference value (V o ref) is reduced below the predefined value. 
     Further, if the difference between the second DC voltage  118  and the voltage reference value (V o ref) is less than the predefined value, the fifth switching signal  277  and the sixth switching signal  279  are transmitted to the first and second switches  280 ,  282  to activate and deactivate the first and second switches  280 ,  282 . The fifth and sixth switching signals  277 ,  279  include switching pulses corresponding to the secondary current of the contactless power transfer unit  108 . As a result, the second converting unit  210  converts the AC voltage  114  of the input power to the second DC voltage  118  and supplies the converted second DC voltage  118  to the electric load  132 . 
     Concurrently, the voltage signal (V o )  128  is transmitted from the sensor  120  to the first transceiver  122 . Further, the first transceiver  122  transmits the voltage signal (V o )  128  to the second transceiver  124 , which in turn is transmitted to the control unit  206 . 
     The control unit  106  generates gate signals  226  based on the second DC voltage  118 . Further, the control unit  106  transmits the gate signals  226  to the switches  220  to adjust the switching frequency of the first converting unit  104 . As a result, the AC voltage  114  is regulated, which in turn controls the second DC voltage  118  across the load  132 . However, the regulation of the second DC voltage  118 , using the control unit  106 , occurs after the regulation of the second DC voltage, using the controller  302 . Hence, the controller  302  performs faster regulation of the second DC voltage  118  compared to the regulation of the second DC voltage  118  by the control unit  106 . 
     Referring to  FIG. 4 , a flow chart illustrating a method  400  for protecting the wireless power transfer system in accordance with an embodiment of the present disclosure is depicted. At step  402 , a first DC voltage of an input power is converted to an AC voltage by a first converting unit. The first converting unit is operated at a determined switching frequency to convert the first DC voltage of the input power to the AC voltage. 
     Subsequently, at step  404 , the method includes receiving and transmitting the input power having the AC voltage by a contactless power transfer unit. The contactless power transfer unit transmits the input power having the first AC voltage to a second converting unit. Further, at step  406 , the AC voltage of the input power is converted to a second DC voltage by a second converting unit. At step  408 , the input power having the second DC voltage is transmitted from the second converting unit to an electric load. In one embodiment, the electric load may be one or more batteries that are electrically charged using the input power having the second DC voltage received from the second converting unit. 
     At step  410 , the electric load is decoupled from the contactless power transfer unit if the second DC voltage across the electric load is greater than a first threshold value (V o Max). Specifically, a switching unit is used to decouple the electric load from the contactless power transfer unit. As a result, the second DC voltage across the electric load is reduced to less than the first threshold value (V o Max), thereby protecting the second converting unit from being affected due to over voltage. If the determined second DC voltage is less than a second threshold value (V o Min), the switching unit couples the electric load to the contactless power transfer unit to continue supplying power having the second DC voltage to the electric load. 
     Referring to  FIG. 5 , a flow chart illustration a method for decoupling and coupling a second converting unit in a wireless power transfer system in accordance with an embodiment of the present disclosure is depicted. At step  502 , a voltage signal (V o ) representative of a second DC voltage across an electric load is transmitted by a sensor. More specifically, the sensor transmits the voltage signal (V o ) to a controller of a switching unit. Further, the sensor transmits the voltage signal (V o ) to a control unit via a first transceiver and a second transceiver. 
     At step  504 , the controller determines whether the voltage signal (V o ) representative of the second DC voltage is greater than a first threshold value (V o Max). If the voltage signal (V o ) representative of the second DC voltage is greater than the first threshold value (V o Max), the controller transmits a first switching signal to a first switch and a second switching signal to a second switch to activate or close the first and second switches as depicted in step  506 . As a result, the electric load is decoupled from the contactless power transfer unit and thereby the second DC voltage across the load is reduced to less than the first threshold value (V o Max). More specifically, the second DC voltage is prevented from attaining a critical value that is greater than the first threshold value (V o Max). The critical value is a voltage value greater than which the second converting unit may be affected. Concurrently, the controller sends a first control signal to the control unit via the first transceiver and the second transceiver. 
     At step  508 , the controller determines whether the voltage signal (V o ) representative of the second DC voltage is less than a second threshold value (V o Min). If the voltage signal (V o ) representative of the second DC voltage is less than the second threshold value (V o Min), the controller sends a third switching signal to the first switch and a fourth switching signal to the second switch to activate and deactivate the first and second switches. As a result, the electric load is coupled to the contactless power transfer unit and the AC voltage is converted to the second DC voltage which is supplied to the electric load as depicted in step  510 . 
     At step  512 , the control unit receives the voltage signal (V o ) and the first control signal. Specifically, the control unit receives the voltage signal (V o ) from the sensor via the first transceiver and the second transceiver. The control unit receives the first control signal from the controller via the first transceiver and the second transceiver. 
     At step  514 , the control unit deactivates the first converting unit if the first control signal is received from the controller. The first converting unit is deactivated to prevent the supply of input power to the second converting unit. At step  516 , the control unit transmits a reset signal to the controller via the first transceiver and the second transceiver after a predetermined time period. In response to receiving the reset signal, the controller generates the third and fourth switching signals. Further, the controller sends the third and fourth switching signals to operate the first and second switches in the normal mode. As a result, the electric load is coupled to the contactless power transfer unit to receive the power via the second converting unit. 
     Concurrently, at step  518 , the controller transmits the second control signal to the control unit via the first transceiver and the second transceiver. Further, at step  518 , the sensor transmits the voltage signal (V o ) to the control unit via the first transceiver and the second transceiver. At step  520 , the control unit determines whether the voltage signal (V o ) representative of the second DC voltage is less than or equal to the first threshold value (V o Max). If the second DC voltage is less than or equal to the first threshold value (V o Max), the control unit transmits gate signals to activate the first converting unit. As a result, the input power is supplied to the second converting unit via the contactless power transfer unit. Further, the electric load receives the power having the second DC voltage from the second converting unit. As a result, one or more components in the wireless power transfer unit are protected from increase in the second DC voltage across the load. On the other hand, if the second DC voltage is still greater than the first threshold value (V o Max), the control unit continues to deactivate the first converting unit and sends another reset signal after the predetermined time period to verify whether the second DC voltage is less than or equal to the first threshold value (V o Max). If the second DC voltage is greater than the first threshold value (V o Max) after sending a determined number of reset signals, the control unit may shut down or deactivate the power transfer system. In accordance with the exemplary embodiments discussed herein, the exemplary system and method facilitate to protect one or more components in the wireless power transfer system when the load is disconnected. Further, the exemplary system and method facilitate to control or regulate the output voltage when the load is disconnected. As a result, one or more components in the system are protected without decoupling the components from each other in the system. 
     Referring to  FIG. 6 , a schematic representation of a wireless power transfer system  300  in accordance with one embodiment of the present disclosure is depicted. The wireless power transfer system  600  is similar to the wireless power transfer system  300  of  FIG. 3  except that a controller  622  of a switching unit  612  is configured to regulate or control the second DC voltage  118  (output voltage) and electrical current  604  (output current) of the second converting unit  110 . Also, in this embodiment, a battery  602  may be coupled across the second converting unit  110  as an electric load. The battery  602  may be charged by the second DC voltage  118  provided by the second converting unit  110 . 
     In this embodiment, the controller  622  may control or regulate output current  604  provided to the battery  602  along with the second DC voltage  118  while charging the battery  602 . In one example, if the battery  602  is a lead acid battery, the controller  622  may first regulate the output current  604  until the battery is charged to a predefined value. Further, the controller  622  may regulate the second DC voltage  118  until the battery is fully charged. For ease of understanding, the battery  602  is considered as the lead acid battery in the below description. 
     During operation, the controller  622  may monitor the output current  604  along with the second DC voltage  118  while charging the battery  602 . If the electrical charge in the battery  602  is below or equal to a predetermined value, the controller  622  may control or regulate the electrical current so that a constant output current  604  is provided to the battery  602  for charging the battery  602 . In one example, the predetermined value may be 80% of battery&#39;s full charge capacity. Particularly, the controller  622  may receive a signal representative of the output current  604  from a current sensor  606 . Further, the controller  622  may compare the output current  604  with a current reference value (Io ref) to generate switching signals S 1  and S 2    608 .  610  having corresponding switching pulses. These switching signals S 1  and S 2    608 ,  610  are provided to the switches  280 ,  282  to regulate the output current  604  that is used for charging the battery  602 . 
     Furthermore, if the electrical charge in the battery  602  is above the predetermined value. The controller  622  may control or regulate the second DC voltage  118  so that a constant second DC voltage is provided for charging the battery  602 . Particularly, the controller  622  may receive a signal representative of the second DC voltage  118  from the voltage sensor  120 . Further, the controller  622  may compare the second DC voltage  118  with a voltage reference value (V o ref) to generate switching signals S 1  and S 2    608 ,  610  having corresponding switching pulses. These switching signals S 1  and S 2    608 ,  610  are provided to the switches  280 ,  282  to regulate the second DC voltage  118  that is provided for charging the battery  602 . It may be noted that the controller  622  may regulate voltage and/or current based on a type of the battery that is coupled to the second converting unit  110 . In one embodiment, the controller  622  may first regulate the second DC voltage  118  until the battery is charged to the predetermined value. Further, the controller  622  may regulate the output current  604  until the battery is fully charged. 
     While only certain features of the present disclosure have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the present disclosure. 
     This written description uses examples to disclose the invention, including the example embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.