Patent Publication Number: US-9847668-B2

Title: Over-voltage protection device for resonant wireless power transmission device and method for controlling the over-voltage protection device

Description:
PRIORITY 
     This application is a Continuation Application of U.S. patent application Ser. No. 13/722,211 and claims priority under 35 U.S.C. §119(a) to a Korean Patent Application filed in the Korean Intellectual Property Office on Jan. 11, 2012, and assigned Serial No. 10-2012-0003341, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a wireless power transmission and reception technique applied to a wireless charging technique, and more particularly, to an over-voltage protection device for a transmission device in a resonant wireless power transmission and reception technique and a method for controlling the over-voltage protection device. 
     2. Description of the Related Art 
     A wireless charging (or non-contact charging) technique using wireless power transmission and reception has recently been developed and used for various electronic devices. The wireless charging technique allows a user to automatically charge a battery by simply placing it on a charging deck without connecting a device such as a cellular phone, to a separate charging connector. 
     Wireless electric toothbrushes and wireless electric shavers are commonly known devices that use the wireless charging technique. The wireless power transmission and reception technique may increase sealing force and waterproof features as it wirelessly charges electronic products and thus does not need an external charging terminal, and may also increase portability of electronic devices because it does not require wired chargers. The wireless charging related technique is expected to evolve significantly in the growing era of electric vehicles. 
     The wireless charging technique roughly includes an electromagnetic induction scheme using coils, a resonant scheme using resonance, and a Radio Frequency (RF)/microwave radiation scheme that converts electrical energy into a microwave and transfers the energy. An electromagnetic induction-based power transmission method involves transferring power between a primary coil and a secondary coil. The resonant scheme uses frequency resonance between a transmission device and a reception device that use a resonance frequency. 
     In the wireless power transmission and reception technique, when abnormal conditions occur, such as incorrect placement of a reception device on a charging deck of a transmission device, the reception device is abnormal, or a metallic substance is placed on the charging deck, excessive power beyond a normal value may be generated in the reception device. Therefore, a wireless power transmission and reception system as well as the reception device essentially require an over-voltage protection circuit. 
     For the over-voltage protection circuit, a Zener diode may be included in the reception device. However, the Zener diode needs a preparation period corresponding to a time necessary for its operation, and in that preparation period, over-voltage protection is difficult to achieve. Moreover, as the amount of power to be handled increases, the size and capacity of the required Zener diode also increases. In this case, there may be significant restrictions on a mounting size. 
     Such restrictions make it difficult to include a corresponding wireless power reception device in a portion of an electronic device for which a size limitation is vital. As such, there is a need in the art for an over-voltage protection circuit capable of achieving rapid handling and having high efficiency while reducing a mounting size in wireless power transmission and reception devices. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a resonant wireless power transmission device, which achieves rapid handling, has high efficiency, easily handles a high over-voltage, and solves a mounting size problem. 
     According to an aspect of the present invention, a wireless power transmission device is provided. The wireless power transmission device includes a resonance signal generator and a controller. The resonance signal generator is configured to transmit wireless power to a wireless power reception device. The controller is configured to adjust the wireless power transmitted to the wireless power reception device, when a predetermined condition caused by over-voltage protection operation at the wireless power reception device is detected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of an embodiment of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a resonant wireless power transmission and reception system according to an embodiment of the present invention; 
         FIG. 2  illustrates a reception device of  FIG. 1 ; 
         FIG. 3A  illustrates over-voltage protection of a controller of a reception device of  FIG. 1 ; 
         FIG. 3B  illustrates over-voltage protection of a controller of a transmission device of  FIG. 1 ; 
         FIG. 4  illustrates a voltage waveform applied to a constant-voltage generator of a reception device of  FIG. 2 ; 
         FIGS. 5A through 5C  illustrate voltage waveforms applied to a constant-voltage generator for respective set capacities of respective detuning capacitors included in an over-voltage protector of a reception device of  FIG. 2 ; 
         FIG. 6  illustrates a rate of power transferred from a transmission device to a reception device with respect to a set capacity of each detuning capacitor included in an over-voltage protector of a transmission device of  FIG. 2 ; and 
         FIGS. 7A through 7F  illustrate a rate of power transferred from a transmission device to a reception device with respect to a set capacity of each detuning capacitor included in an over-voltage protector of a reception device of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, the same or similar elements may be designated by the same reference numerals in different drawings. Further, detailed descriptions of known functions and configurations incorporated herein may be omitted for the sake of clarity and conciseness. 
       FIG. 1  illustrates a resonant wireless power transmission and reception system according to an embodiment of the present invention, and  FIG. 2  illustrates a reception device of  FIG. 1 . Referring to  FIGS. 1 and 2 , the resonant wireless power transmission and reception system includes a wireless power transmission device  1 , such as a charging device, and a wireless power reception device  2 , such as one provided in a portable terminal. 
     The wireless power transmission device  1  may include a signal generator  10  which includes a Voltage Control Oscillator (VCO), to generate a signal of a preset frequency such as a 6.78 MHz resonance frequency; an amplifier  12  which includes an amplification unit that amplifies the signal generated by the signal generator  10  to a high-power signal; a resonance signal generator  14  which includes a resonator that generates a wireless resonance signal, for example, of 6.78 MHz according to the high-power signal generated by the amplifier  12 ; a voltage/current detector  16  that senses (a peak level of) voltage and current of the wireless resonance signal generated by the resonance signal generator  14 ; and a controller  18  that collectively controls a wireless power transmission operation of the wireless power transmission device  1 , and controls operations of the signal generator  10  and the amplifier  12  to maintain a value in a normal range by monitoring a current and a voltage of a wirelessly transmitted resonance signal based on the current and voltage detected by the voltage/current detector  16 . 
     The wireless power transmission device  1  may further include a sensor  17  which includes an object sensor that provides a sensing signal to the controller  18  by sensing placement of the wireless power reception device  2 , and a wireless communication unit  19  configured to use one selected from among various wireless near-field communication schemes for communication with the wireless power reception device  2  in relation to the wireless power transmission operation under control of the controller  18 . 
     The controller  18  of the wireless power transmission device  1  may include a Micro Control Unit (MCU) (not shown), and may be configured to monitor a value detected by the voltage/current detector  16  through an Analog/Digital Conversion (ADC) port (not shown). In particular, the controller  18  determines a periodic impedance change, which is induced by an over-voltage protection circuit of the wireless power reception device  2  and is detected by the voltage/current detector  16  for an over-voltage protection operation, and adjusts a power supply level of the wireless power transmission device  1 . The over-voltage protection operation of the controller  18  of the wireless power transmission device  1  according to the present invention will be described later in detail with reference to  FIG. 3B . 
     The wireless power reception device  2  may include a resonance signal receiver  24  which includes a resonator, to receive a wireless resonance signal transmitted from the resonance signal generator  14  of the wireless power transmission device  1 , a rectifier  22  that rectifies Alternating Current (AC) power received in the resonance signal receiver  24  into Direct Current (DC) power, a smoother  21  that smoothes the DC power rectified by the rectifier  22 , a constant-voltage generator  20  that converts the power output from the smoother  21  into operating power (e.g., +5V) desired by the portable terminal to which the wireless power reception device is applied, an over-voltage protector  25  which is driven by a driver  27  to detune a resonance frequency of the resonator of the resonance signal receiver  24  during an over-voltage protection operation, thereby reducing reception power, a voltage detector  26  that detects an input voltage of the constant-voltage generator  20 , and a controller  28  which is implemented with an MCU or the like for collectively controlling a wireless power reception operation of the wireless power reception device  2 , determining whether an over-voltage occurs according to the signal detected by the voltage detector  26 , and outputting a control signal for driving the over-voltage protector  25  to the driver  27  for the over-voltage protection operation if determining that the over-voltage occurs. 
     The wireless power reception device  2  may further include a wireless communication unit  29  which uses one selected from among various wireless near-field communication schemes to communicate with the wireless power transmission device  1  in relation to the wireless power reception operation under control of the controller  28 , and a waveform stabilizer (not shown) which includes an LC circuit to stabilize and output a DC waveform output from the constant-voltage generator  20 . 
     Referring to  FIG. 2 , the rectifier  22  may have a full-bridge diode structure using four diodes D 1 , D 2 , D 3 , and D 4 , in which for example, a serial connection circuit of the first and second diodes D 1  and D 2  and a serial connection circuit of the third and fourth diodes D 3  and D 4  are connected in parallel, and out of two nodes connected in parallel, a connection node of the first and third diodes D 1  and D 3  is set to be an output node of a DC power Input Voltage signal (VIN), and a connection node of the second and fourth diodes D 2  and D 4  is connected with a ground terminal. 
     A terminal VIN_P out of two connection terminals VIN_P and VIN_N of the resonator of the resonance signal receiver  24  is connected with a connection point of the first and second diodes D 1  and D 2 , and the other terminal VIN_N is connected with a connection point of the third and fourth diodes D 3  and D 4 . 
     The smoother  21  may include at least one of parallel-connected capacitors C 3  and C 4 . 
     The constant-voltage generator  20  is provided with the output of the rectifier  30  through the smoother  21  and converts the rectified DC power into the DC power of a desired level. To this end, the constant-voltage generator  20  may include a step-down converter such as a DC-DC buck converter including a Low Drop Output (LDO). 
     The over-voltage protector  25  may include resonance-detuning first and second capacitors C 1  and C 2  having ends that are connected to the two connection terminals VIN_P and VIN_N of the resonator of the resonance signal receiver  24 , respectively. 
     The driver  27  may have a switch structure in which the driver  27  performs a switching operation according to a control signal output from the controller  28  to connect the other terminals (C_OVP 1 , C_OVP 2 ) of the first and second capacitors C 1  and C 2  of the over-voltage protector  25  to the ground terminal. 
     The controller  28  in a normal state controls a switching state of the driver  27  to be an open state, such that the other terminals (C_OVP 1 , C_OVP 2 ) of the first and second capacitors C 1  and C 2  of the over-voltage protector  25  maintain the open state. In this case, the first and second capacitors C 1  and C 2  of the over-voltage protector  25  do not affect the resonator of the resonance signal receiver  24 . In the over-voltage protection operation, the controller  28  controls the switching state of the driver  27  to be a connected state, such that the other terminals (C_OVP 1 , C_OVP 2 ) of the first and second capacitors C 1  and C 2  of the over-voltage protector  25  are connected with the ground terminal, and in this case, the first and second capacitors C 1  and C 2  of the over-voltage protector  25  affect the resonator of the resonance signal receiver  24 , detuning the resonance frequency. As a result, the signal transmitted from the wireless power transmission device  1  cannot be received efficiently, which detrimentally affects the power transmission. 
     The constant-voltage generator  20 , the voltage detector  26 , the controller  28 , and the driver  27  may be configured in the form of a single Integrated Circuit (IC) chip (BUCK-IC). 
     Referring to  FIG. 2 , although not shown in  FIG. 1 , an auxiliary over-voltage protector  23  including a Zener Diode (ZD) may be further included between the rectifier  22  and the smoother  21  for additional circuit protection. The auxiliary over-voltage protector  23  is not essential, but may be further included to supplement the over-voltage protection function. The ZD may be set such that for example, a breakdown voltage is 30V to prevent an output voltage VIN of the rectifier  22  from exceeding a preset level. 
     When the over-voltage protector is implemented with only the ZD, over-voltage protection is difficult to achieve in a preparation period corresponding to a time required for the ZD to operate, and if the amount of power from the output voltage VIN of the rectifier  22  to an input voltage BUCK_IN of the constant-voltage generator  20  is large, the necessary capacity and size of the ZD have to be undesirably increased. Generally, a ZD having a size of about 1608 (16×08 mm) may protect a circuit of about 300 mW. Thus, to protect 1 W in a portable mobile device, three or more parts of a size of 1608 or larger have to be used. Since a reception stage of a cellular phone or mobile wireless charging resonance system has a small mounting space for a product, such that over-voltage protection should be performed in a small-area region, the reception stage should be protected up to 5 W for a cellular phone and up to 10 W for a tablet phone or a Personal Computer (PC). This requirement is difficult to achieve when the over-voltage protection circuit is implemented only with the ZD. 
     Therefore, in the reception device of the present invention, the over-voltage protection circuit is implemented by using resonance-detuning capacitors (two 1005 parts). 
       FIG. 3A  illustrates over-voltage protection of the controller  28  of the reception device  2  of  FIG. 1 . Referring to  FIG. 3A , the controller  28  of the reception device  2  monitors the input voltage BUCK_IN of the constant-voltage generator  20  through an output of the voltage detector  26  in step  302 . Thereafter, in step  304 , the controller  28  determines whether the input voltage BUCK_IN is greater than or equal to a preset over-voltage protection start reference voltage (e.g., 25.7V). If the input voltage BUCK_IN is not greater than or equal to the preset over-voltage protection start reference voltage, the monitor  28  continues the input voltage monitoring operation of step  302 . Unless the input voltage BUCK_IN is greater than or equal to the preset over-voltage protection start reference voltage, the monitor  28  proceeds to step  306  to perform an over-voltage operation. 
     The over-voltage protection operation of step  306  is performed after a reaction time elapses (of about 10 microseconds (μs) or less) after the input voltage BUCK_IN increases, for example, to 25.7V. The controller  28  outputs a control signal to the driver  27  to connect the pins C_OVP 1  and C_OVP 2  of the over-voltage protector  25  to the ground terminal, such that the over-voltage level decreases. In this state, the controller  28  may transmit a signal indicating that the current state of the reception device  2  is an over-voltage protection operation state, and according to this signal, the wireless communication unit  29  transmits information indicating that the reception device  2  is currently in the over-voltage protection operation state to the transmission device  1 . 
     In step  308 , the controller  28  monitors the input voltage BUCK_IN of the constant-voltage generator  20 . In step  310 , the controller  28  determines whether the input voltage BUCK_IN is less than or equal to a preset over-voltage protection-release reference voltage (e.g., 24.8V. If the input voltage BUCK_IN is not less than or equal to the preset over-voltage protection-release reference voltage, the controller  28  continues the input voltage monitoring operation of step  308 . Unless the input voltage BUCK_IN is less than or equal to the preset over-voltage protection-release reference voltage, the monitor  28  proceeds to step  312  to release the over-voltage operation. 
     As to a procedure for releasing the over-voltage protection operation in step  312 , if the over-voltage protection operation is performed in step  306  and thus the input voltage BUCK_IN decreases to below 24.8V, an over-voltage recovery operation is performed, such as 700-1000 μs, and then the controller  28  outputs a control signal to the driver  27  to control the switch structure of the driver  27  to have an open state, such that the pins C_OVP 1  and C_OVP 2  of the over-voltage protector  25  are opened. Thus, the over-voltage protector  25  is deactivated. 
     After the over-voltage protection operation is released in step  312 , the controller  28  returns to step  302  to repeat the foregoing process. If over-voltage occurrence conditions in the reception device  2  are not solved, the over-voltage protection operation and the over-voltage protection-release operation may be repetitively performed. 
       FIG. 4  illustrates a voltage waveform applied to the constant-voltage generator  20  of the reception device  2  of  FIG. 2 , showing a waveform of a continuance of the over-voltage protection operation and the over-voltage protection-release operation continue when the over-voltage occurrence conditions are not solved. Referring to  FIG. 4 , after an initial wireless power reception operation, the internal 3V Low DropOut (LDO) of the constant-voltage generator  20  operates when the output voltage VIN of the rectifier  22  is about 4V. Then the waveform of the output voltage VIN increases and at 4V, the waveform becomes level for about 250 μS. Thereafter, the output voltage VIN continuously increases. 
     The 3V LDO supplies power (3V, 40 mA max) to the external controller  28  (e.g., the MCU). A boot-up time that can be controlled by a General Purpose Input/Output (GPIO), which is the external controller  28 , is 7.2 mS. The constant-voltage generator  20  operates, e.g., at the input voltage BUCK_IN of 5.5V, and outputs a constant voltage such as 5V. 
     Thereafter, an over-voltage protection operation starts after a response time at the output voltage VIN of, for example, 25.7V. The pins C_OVP 1  and C_OVP 2  of the over-voltage protector  25  are connected to the ground terminal by means of the switch structure of the driver  27 . Then, a signal OVP which indicates the over-voltage protection operation state is activated by the controller  28  (signal OVP L=&gt;H: over-voltage protection operation state). 
     At the output voltage VIN of 24.8V or less, for example, the over-voltage protection operation is released after an over-voltage recovery time. In this state, the pins C_OVP 1  and C_OVP 2  are opened and the signal OVP is deactivated by the controller  28  (signal OVP H=&gt;L: normal state). 
     As the over-voltage protection operation and the over-voltage protection-release operation are continued when the over-voltage occurrence conditions are not solved in the reception device  2 , the output voltage VIN periodically shows a waveform which rises over the over-voltage protection start reference voltage (e.g., 25.7V) and falls below the over-voltage protection-release reference voltage (e.g., 24.8V). 
       FIGS. 5A through 5C  illustrate voltage waveforms applied to the constant-voltage generator  20  for respective set capacities of the respective detuning capacitors C 1  and C 2  included in the over-voltage protector  25  of the reception device  2  of  FIG. 2 , in which a periodic waveform of the input voltage BUCK_IN applied to the constant-voltage generator  20  when the over-voltage occurrence conditions are not solved is shown. 
       FIGS. 5A through 5C  illustrate the waveforms of the input voltage BUCK_IN when the capacities of the detuning first and second capacitors C 1  and C 2  of the over-voltage protector  25  are 2.2 nanoFarads (nF), 4.7 nF, and 22 nF, respectively. It can be seen that when the capacities of the detuning capacitors are different (as will be described below, for the capacity of 2.2 nF or more), the respective waveforms show periodicity without any significant difference therebetween. 
     However, for the capacity of 2.2 nF or less, the over-voltage protection operation is not performed effectively, as will be described below in detail with reference to  FIG. 6 . 
       FIG. 6  illustrates a rate of power transferred from the transmission device  1  to the reception device  2  with respect to a set capacity of each detuning capacitor included in the over-voltage protector  25  of the transmission device  2  of  FIG. 2 , and  FIGS. 7A through 7F  illustrate a rate of power transferred from the transmission device  1  to the reception device  2  with respect to a set capacity of each detuning capacitor included in the over-voltage protector  25  of the reception device  2  of  FIG. 2 .  FIG. 7A  illustrates when the detuning capacitors are open (or a normal state when there is no capacitor), and  FIGS. 7B through 7F  illustrate when the capacities of the detuning capacitors are 2.2 nF, 5.0 nF, 7.0 nF, 10.0 nF, and 22.0 nF, respectively. 
     Referring to  FIGS. 6 and 7A through 7F , once the pins C_OVP 1  and C_OVP 2  for the detuning capacitors are opened, a rate of power transferred from the transmission device (resonator) to the reception device (resonator), S 21 , is about 80-90%, indicating that power transmission from the transmission device to the reception device has been successfully performed. 
     As shown in  FIG. 7B , if the capacities of the detuning capacitors are 2.2 nF, upon connection of the pins C_OVP 1  and C_OVP 2  to the ground terminal, about 60% of the transmission power is transferred to the reception device. Likewise, as shown in FIGS.  7 C and  7 F, when the capacities of the detuning capacitors are 5.0 nF, 7.0 nF, 10.0 nF, and 22.0 nF, respectively, about 38.5%, 26.7%, 13.2%, and 4.1% of the transmission power are transferred, respectively. In  FIG. 7F , for the capacity of 22 nF, S 21  is 4.1%, such that little or no power transmission is performed. 
     In the above description, if the capacity of the detuning capacitor is too small (e.g., 2 nF or less), S 21  is 61%, such that even if the over-voltage protection operation is performed, over-voltage protection may not be achieved and thus the voltage may continuously increase. This is because the detuning effect is not large even if the over-voltage protection operation is performed and thus the pins C_OVP 1  and C_OVP 2  for the detuning capacitors are connected with the ground terminal. In this case, the output voltage VIN continuously increases, which damages internal components of the reception device. When the detuning capacitor of 2.2 nF is used in which S 21  of the power transferred from the transmission device to the reception device is experimentally reduced by 30%, S 21  is about 60%. 
     Thus, in the present invention, the capacities of the detuning capacitors are set to a value in which ΔS 21  is 30% or more, as given in Equation (1) by:
 
Δ S 21=( S 21)−( S 21 in over-voltage protection state)   (1)
 
     It can be seen that S 21 , which is a rate of the power transmitted from the transmission device to the reception device, after execution of the over-voltage protection operation is 30% reduced from S 21  before execution of the over-voltage protection operation. 
     In view of a Voltage Standing Wave Ratio (VSWR, reflection coefficient), transmission of power at 60% or less indicates that a reflection coefficient is greater than 4. That is, when the over-voltage protection operation is executed, VSWR≧4. 
     In view of a Q value, a change of the Q value is more than 0.1. That is, Q may be expressed in Equation (2) as follows:
 
 Q =(Δ f /13.56)*1.5 (Δ f  indicates a frequency variation)   (2)
 
     Thus, for example, for Δf=1 MHz, Q=0.11. As such, for the Q value of 0.1 or more, the frequency may be regarded as detuned. 
     As described above, the over-voltage protection operation is performed by detuning the resonance frequency at the reception device  2  using the detuning capacitors. The reception device has to continuously repeat the over-voltage protection operation when the over-voltage occurrence conditions are not solved, unless other measures are taken. 
     Moreover, the controller  28  of the reception device  2  may be configured to recognize the over-voltage state and transmit a message requesting the transmission device  1  to solve the over-voltage problem to the transmission device  1  through the wireless communication unit  29 . Until over-voltage protection is performed, however, this process is very time-consuming. Therefore, it may be preferable that the transmission device  1  directly determines and then cancels the over-voltage state. In the present invention, therefore, the transmission device  1  instantly determines the over-voltage state of the reception device  2  and reduces the power transmitted from the transmission device  1 , thereby protecting the reception device  1 . 
       FIG. 3B  illustrates over-voltage protection of the controller  18  of the transmission device  1  of  FIG. 1 . Referring to  FIG. 3B , the controller  18  monitors a resonance-stage voltage of the resonance signal generator  14  through an output of the voltage/current detector  16  in step  320 . As the reception device  2  periodically executes the over-voltage protection operation and release operation as mentioned previously, the waveforms as shown in  FIG. 4  or  FIGS. 5A through 5C  are generated. Likewise, (a peak value of) the voltage detected in the resonance stage of the transmission device  1  also exhibits a similar waveform. That is, even if the transmission device  1  transmits a constant output through the amplifier  12 , the voltage of the resonator of the resonance signal generator  14  changes according to the frequency resonance. 
     In step  322 , the controller  18  of the transmission device  1  determines whether rise and fall are repeated at particular intervals, such as 500 μS-2 mS when the resonance-stage voltage is sampled and a value thereof is larger than a preset over-voltage reference voltage (e.g., 15V) during a period. If corresponding determination conditions are not satisfied, the controller  18  returns to step  320  to continue monitoring the resonance-stage voltage, whereas if the determination conditions are satisfied, the controller  18  proceeds to step  324 . 
     In step  324 , the controller  18  of the transmission device  1  regards that the reception device  2  is currently in the over-voltage state, and thus controls the output of the transmission device  1  to be reduced by a value, such as 30% or higher, during a period. The controller  18  performs a control operation such that the output of the amplifier  12  is reduced. Herein, decreasing the output by 30% or more is an important factor because if the output is decreased by 30% or less, in spite of execution of the over-voltage protection operation, over-voltage protection is not achieved in the reception device  2  such that the voltage may continuously increase. 
     After execution of the operation of step  324 , the controller  18  returns to step  320  to repeat the foregoing operation. 
     By executing the foregoing operation, even if the reception device  2  does not inform the transmission device  1  of the over-voltage situation, the transmission device  1  may automatically sense the over-voltage situation of the reception device  2  and reduce the transmission power, thereby handling the over-voltage situation. By reducing the transmission power, the over-voltage situation in the reception device  2  is released. 
     In addition, by monitoring the magnitude of the voltage transmitted from the transmission device  1 , the transmission power for over-voltage protection is controlled, which conserves the time taken for the reception device  2  to receive a signal indicating the over-voltage situation, thereby rapidly protecting a circuit from the over-voltage state. 
     As such, the over-voltage protection scheme for the resonant wireless power transmission device according to the present invention may be realized. Embodiments of the present invention have been described in the foregoing description, but other embodiments or modifications or changes thereto may be made. 
     For example, while the reception device  2  corresponding to the transmission device  1  according to the present invention adopts a configuration using a resonance frequency detuning scheme in the foregoing description, the voltage waveform of the resonance stage sensed by the transmission device  1  may experience periodicity when the reception device  2  repeats the over-voltage protection operation and release operation even if the reception device  2  adopts an over-voltage protection circuit having other configurations. It can also be understood that the transmission device  1  according to the present invention effectively handles the over-voltage situation by performing the disclosed over-voltage protection operation. 
     In addition, while the transmission power is decreased by 30% for over-voltage protection in the transmission device  1  in the foregoing description, the transmission device  1  may shut down the transmission power for over-voltage protection. 
     In the foregoing description, the detuning capacitors in the reception device  2  are selectively connected with the ground terminal by the driver when they are connected to the resonance stage, but they may also be selectively connected with the resonance stage by the driver when being connected with the ground terminal. 
     The controller  28  of the reception device  2  may provide information regarding execution of the over-voltage protection operation to a portable terminal to which the reception device  2  is applied, such that the information is displayed on a display device of the portable terminal or may be output as an alarming sound through a speaker provided in the portable terminal. 
     In the reception device  2 , the controller  28  may further decrease the over-voltage protection-release reference voltage or may transmit a message requesting stop of power transmission to the transmission device  1 , if the over-voltage protection operation and release operation are continuously repeated at intervals. 
     As is apparent from the foregoing description, the over-voltage protection scheme for the resonant wireless power transmission device according to the present invention solves a size increase problem while offering rapid handling and high efficiency. 
     While embodiments of the present invention has been described, it will be obvious to those of ordinary skill in the art that various modifications can be made without departing from the scope of the present invention.