Patent Publication Number: US-2019181682-A1

Title: Active rectifier having maximum power transfer and maximum efficiency over distance

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Korean Patent Application No. 10-2017-0170394 filed on Dec. 12, 2017 and all the benefits accruing therefrom under 35 U.S.C. § 119, the contents of which are incorporated by reference in their entirety. 
     BACKGROUND 
     The present invention relates to an active rectifier capable of maximizing power transfer efficiency and maximizing output power by changing the impedance of the active rectifier included in a wireless power reception device. 
       FIG. 1  shows a wireless charging system using a conventional passive rectifier or active rectifier. 
     The wireless charging reception system  200  may include a transmission unit  201 , a transmission side antenna  261 , a reception side antenna  262 , an external matching element  203 , a passive rectifier or active rectifier  204 , and a voltage regulator  205 . Here, the transmission unit  201  and the transmission side antenna  261  may be included in the transmission device  240 . Then, the reception side antenna  262 , the external matching element  203 , the passive rectifier or active rectifier  204 , and the voltage regulator  205  may be included in the reception device  250 . The reception device  250  is distinguished from the transmission device  240 . Each of the pair of antennas  261  and  262  may include a coil, for example. 
     The wireless charging reception system  200  may refer to the transmission device side as a primary side and the reception device side as a secondary side with respect to the antennas  261  and  262 . In a wireless charging reception system using a passive rectifier or the conventional active rectifier  204 , the impedance seen in the transmission device changes according to the distance d between the transmission device and the reception device. As a result, there is a problem that the power transfer efficiency is lowered as the distance between the transmission device and the reception device increases. 
       FIG. 2  shows an impedance equivalent circuit of the wireless charging system of  FIG. 1 . The impedance equivalent circuit may include an impedance  210  of the transmission device and an impedance  211  of the reception device. In the wireless charging reception system, the impedance  210  of the transmission device may vary depending on the distance d between the transmission device and the reception device. 
       FIG. 3  is a graph showing the relation between the power transfer efficiency and the output power with respect to the impedance, in order to facilitate understanding of the present invention. The horizontal axis represents the impedance (RL/RS), the left vertical axis represents the efficiency, and the right vertical axis represents the output power. 
     Conventional wireless power transmission systems are designed in such a way that the impedance of the reception device can not be adjusted, so that the variation of the impedance according to the distance between the transmission device and the reception device can not be compensated. Therefore, the output power and efficiency of the reception device are not optimized according to the distance. In order to complement this, another technique for compensating the impedance of the reception device by using an external capacitance matrix or the like has been proposed according to the distance. However, this technique requires additional external components, and as the external device is added, the cost and area increase so that the amount of compensation is limited depending on the external capacitance. 
     A rectifier including an impedance conversion used in another conventional wireless power transmission system includes a structure for converting a load impedance. Since the conversion of the load impedance is to limit the load current or the voltage, there is a problem that the operation may be restricted depending on the application. 
     SUMMARY 
     As described above, in the wireless power transmission system, the output power and the power transfer efficiency of the reception device are affected by the operation of the circuit, the loss consumed by the circuit, the loss consumed by the antenna, and the loss due to the coupling coefficient of the antenna, which occurs while power is transferred. 
     In order to minimize the power loss of the circuit, it is necessary to design a small resistance element such that an impedance matching between transmission and reception devices minimizes reactance components and minimizes conduction losses. In relation to this, the distance between the transmission device and the reception device affects the impedance change and greatly affects the power transfer efficiency and output power. If the impedance can be maintained constant according to the distance between transmission device and reception device, high power transfer efficiency and output power can be obtained. 
     The present invention is to provide a method of varying the impedance of an active rectifier by adjusting a turn-on time of a switch used in an active rectifier, a gate voltage level, and a switch resistance in a rectifier. The present invention provides a wireless power reception device that adjusts the impedance of the active rectifier so as to receive maximum power transfer efficiency or maximum output power by adjusting the impedance according to the distance using the above method. 
     In accordance with an exemplary embodiment, a wireless power reception device includes: a gate driver  110  for generating a gate signal S_g 1  for switching between a turn-on voltage and a turn-off voltage; a rectifier  120  connected to both ends of an inductor  22  and including FETs M 1  to M 4  whose on/off states are controlled by the gate signal; a rectifier output detection unit  130  for sensing an output value of the rectifier; and an impedance control unit  140  for controlling an impedance of the rectifier by controlling at least one of a duty ratio of the gate signal and a turn-on voltage for turning on the FET based on the output value. 
     The output value may include an output voltage and an output current of the rectifier, wherein the impedance control unit may maximize a power transfer efficiency between the wireless power reception device and a wireless power transmission device that transmits power to the wireless power reception device or maximize an output power of the wireless power reception device based on the output value. 
     The gate signal may be a PWM signal provided directly to a gate of the FET, wherein the PWM signal may have one of the turn-off voltage and the turn-on voltage, and a magnitude of the turn-on voltage may be controlled by the impedance control unit. 
     The impedance control unit may include: a gate voltage adjustment unit  141 ; a power/efficiency calculation unit  142  for calculating a power transfer efficiency between the wireless power reception device and a wireless power transmission device transmitting power to the wireless power reception device or an output power of the wireless power reception device; and a parameter setting unit  143  for, based on the calculated power transfer efficiency or output power, determining a duty ratio of the gate signal to provide the determined duty ratio to the gate driver and determining a gate voltage level for turning on the FET to provide the determined gate voltage level to the gate voltage adjustment unit, wherein the gate voltage adjustment unit may be configured to adjust the turn-on voltage of the gate signal according to the provided gate voltage level, and the gate driver may output the gate signal so that the duty ratio of the gate signal may have the determined duty ratio. 
     The rectifier may include four FETs connected in bridge form by the inductor. 
     An operation power of the gate voltage adjustment unit may be supplied from the rectifier. 
     The wireless power reception device may further include a voltage regulator for regulating the output of the rectifier. 
     The rectifier output detection unit may include a voltage detection unit  132  for sensing an output voltage of the rectifier and a current detection unit  131  for sensing an output current of the rectifier, wherein the power/efficiency calculation unit may calculate the power transfer efficiency or calculates the output power using the output voltage sensed by the voltage detection unit and the output current sensed by the current detection unit. 
     In accordance with another exemplary embodiment, there is a wireless power transfer control method for controlling a transfer efficiency of a wireless power and a maximum power value of a wireless power in an active rectifier including □a gate driver for generating a gate signal for switching between a turn-on voltage and a turn-off voltage, □a rectifier connected to both ends of an inductor and including FETs whose on/off states are controlled by the gate signal, □a rectifier output detection unit for sensing an output value of the rectifier, and □an impedance control unit for controlling an impedance of the rectifier. The method includes: sensing, by the output detection unit, an output voltage and an output current of the rectifier; varying, by the impedance control unit, at least one of the duty ratio of the gate signal and the turn-on voltage that turns the FET on, based on the output voltage and the output current; and determining, by the impedance control unit, the duty ratio and the turn-on voltage to maximize the power transfer efficiency between the active rectifier and a wireless power transmission device transmitting power to the active rectifier or the output power of the active rectifier and maintaining it with the determined value. 
     The rectifier may include four FETs connected in bridge form by the inductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments can be understood in more detail from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a wireless charging system using a conventional passive rectifier or active rectifier; 
         FIG. 2  shows an impedance equivalent circuit of the wireless charging system of  FIG. 1 ; 
         FIG. 3  is a graph showing the relation between the power transfer efficiency and the output power with respect to the impedance, in order to facilitate understanding of the present invention; 
         FIG. 4  illustrates a structure for changing the input impedance of a wireless charging reception device without additional external elements according to an embodiment of the present invention; 
         FIG. 5  is a detailed configuration diagram of the active rectifier of  FIG. 4  according to an embodiment of the present invention; 
         FIG. 6  illustrates an active rectifier for impedance compensation according to an embodiment of the present invention; 
         FIG. 7  is a timing diagram illustrating an impedance in response to a switching signal of an active device according to an embodiment of the present invention; 
         FIG. 8  is a graph showing an impedance change amount for a gate-source voltage and an input impedance change amount for a turn-on time/period according to an embodiment of the present invention; 
         FIG. 9  shows an example of compensating for output power or efficiency according to an embodiment of the present invention; 
         FIG. 10  illustrates a voltage of a gate signal over time according to another embodiment of the present invention; and 
         FIG. 11  shows a detailed configuration diagram of an active rectifier according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described herein, but may be implemented in various other forms. The terminology used herein is for the purpose of understanding the embodiments and is not intended to limit the scope of the present invention. In addition, the singular forms used below include plural forms unless the phrases expressly have the opposite meaning. 
       FIG. 4  illustrates a wireless charging reception system  150  according to an embodiment of the present invention. 
     That is,  FIG. 4  includes the structure of a wireless charging reception device  50  according to one embodiment of the present invention, which is designed to change the input impedance of a wireless power reception device without additional external elements. 
     The wireless charging reception system  150  may include a transmission device  1 , an antenna  2 , and a wireless charging reception device  50 . 
     The wireless power reception device  50  may include a reception device side antenna of the antenna  2 , an external matching element  3 , an active rectifier  4 , a voltage regulator  5 , and an impedance reception unit  6 . 
     The transmission device  1 , the antenna  2 , and the external matching element  3  may be identical to conventional components. 
     An active rectifier  4  may be provided in accordance with an embodiment of the present invention and may be one that enables impedance compensation. 
     The impedance reception unit  6  may include a maximum power calculation unit  161  and a maximum efficiency calculation unit  162 . 
     The power received through the antenna  2  can be rectified by using an active rectifier  4  for impedance compensation. That is, the voltage induced through the antenna  2  can be rectified using the active rectifier  4 . Then, finally, an output voltage can be provided through the voltage regulator  5 . The impedance reception unit  6  can calculate the maximum output power and the maximum efficiency by sensing the output voltage and current of the voltage regulator  5  or the output voltage and current of the active rectifier  4 . The input impedance of the active rectifier  4  can be changed according to the calculated maximum output power and maximum efficiency value. In order to have maximum power or maximum efficiency through the method described above, the input impedance of the active rectifier  4  can be compensated. 
     As a method of obtaining maximum power, a method may be used in which the current calculated output power is compared with the previously calculated output power to determine whether the current or previous maximum power transfer is performed. 
       FIG. 5  is a detailed configuration diagram of the wireless power reception device of  FIG. 4  according to an embodiment of the present invention. 
     The wireless power reception device may include an external matching element  3 , an active rectifier  4 , a gate driver  41 , a current detection unit  51 , a voltage detection unit  52 , a gate voltage adjustment unit  61 , a power/efficiency calculation unit  62 , and a parameter setting unit  63 . The parameter setting unit  63  may perform generation of a gate signal, adjustment of a duty ratio, and setting of a gate voltage level. 
     The active rectifier  4  may include switch resistors M 1 , M 2 , M 3 , and M 4 . The switch resistances M 1 , M 2 , M 3 , and M 4  may refer to a turn-on resistance. 
     In one embodiment, the active rectifier  4  may include four FET switches. In this case, the four switch resistors M 1 , M 2 , M 3 , and M 4  may be provided by the four FET switches, respectively. 
     In another embodiment, the active rectifier  4  may include two FET switches and two passive elements. In this case, two of the four switch resistances M 1 , M 2 , M 3  and M 4  may be provided by the two FET switches, and the other two switch resistors may be provided by the two passive elements, respectively. 
     The gate driver  41  can control on/off of the switch resistance of the rectifier  4  according to the output value of the gate voltage adjustment unit  61 . The operation of the gate voltage adjustment unit  61  can be controlled by the parameter setting unit  63 . 
     The load current IRECT and voltage VRECT rectified by the active rectifier  4  can be sensed using the current detection unit  51  and the voltage detection unit  52 . The current and voltage sensed by the current detection unit  51  and the voltage detection unit  52  may be referred to as a detection current and a detection voltage, respectively. 
     The power/efficiency calculation unit  62  can obtain the output power of the active rectifier  4  using the detection voltage and the detection current. 
     The parameter setting unit  63  may vary the input impedance of the active rectifier  4  by adjusting the turn-on time, gate voltage and/or switch resistance of each of the FET switches included in the active rectifier  4 . At this time, by comparing the ‘Current output power’ outputted from the active rectifier  4  according to the changed input impedance with the ‘previous output power’ outputted by the active rectifier  4 , the maximum power or maximum efficiency of the active rectifier  4  can be obtained. And, by controlling the gate voltage, it is possible to adjust the impedance as shown below. The turn-on resistance R on  of the FET switch is inversely proportional to the gate-source voltage V GS  of the FET switch as shown in Equation 1 below. 
     
       
         
           
             
               
                 
                   
                     R 
                     on 
                   
                   ∝ 
                   
                     1 
                     
                       
                         V 
                         GS 
                       
                       - 
                       
                         V 
                         TH 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
       FIG. 6  illustrates an active rectifier for impedance compensation according to another embodiment of the present invention. 
     In one embodiment, in relation to the active rectifier  4 , only the switch resistance M 1  and the switch resistance M 2  can be active elements. That is, the switch resistance M 3  and the switch resistance M 4  may be passive elements. At this time, the switch resistance M 1  and the switch resistance M 2  may be provided by FET switches, respectively. 
     On the other hand,  FIG. 5  shows an embodiment using a switch resistor as a Complementary Metal Oxide Semiconductor (CMOS), and includes all the active elements usable as a switch. 
     In another embodiment, the active rectifier  4  can be mixed with an active element and a passive element. 
       FIG. 7  is a timing diagram illustrating an input impedance in response to a switching signal of an active device according to an embodiment of the present invention. 
       FIG. 7( a )  shows the impedance value with time, and  FIG. 7( b )  shows the voltage of the gate signal with time. 
     In  FIG. 7( a ) , the horizontal axis represents time and the vertical axis represents the input impedance of the active rectifier  4 . In  FIG. 7( b ) , the horizontal axis represents time and the vertical axis represents the voltage of the gate signal supplied to the gate of the FET device included in the active rectifier  4 . 
     When the gate signal S_g 0  is turned on, impedance appears to be small (Z Low ), and when the gate signal S_g 0  is turned off, since the switch is off, the impedance looks very large (Z High ). 
     In Equation 2 below, referring to  FIG. 5 , the turn-on resistance of the switch is Ron, and when assuming that the turn-off resistance is infinite, it is the value of the input impedance of the active rectifier  4 . 
     
       
         
           
             
               
                 
                   
                     
                       Z 
                       Low 
                     
                     = 
                     
                       
                         ( 
                         
                           
                             R 
                             on 
                           
                           + 
                           
                             R 
                             RECT 
                           
                         
                         ) 
                       
                        
                       PVER 
                        
                       
                         1 
                         
                           sC 
                           RECT 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       Z 
                       High 
                     
                     = 
                     ∞ 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     Therefore, the input impedance Z 1  during one period of the active rectifier  4  is expressed by Equation 3. 
         Z   in =( Z   Low   ×D )+( Z   High ×(1− D ))  [Equation 3]
 
     Then, D is the duty ratio of the gate signal input to the gate of the FET included in the active rectifier  4  and is a value smaller than 1. 
       FIG. 8  is a graph showing an impedance change amount Z Low  for the gate-source voltage V GS  and an input impedance change amount Z in  of the active rectifier  4  for the turn-on time/period (T 1 /T) according to an embodiment of the present invention. 
       FIG. 8( a )  shows an impedance change amount Z Low  for the gate-source voltage V GS  and  FIG. 8( b )  shows an input impedance change amount Z in  for the turn-on time/period (T 1 /T). 
     The input impedance range that can be changed according to the turn-on time T 1  is from Z High  to Z Low . More precisely, the driver&#39;s gate-source voltage V GS  can be used to adjust the input impedance or change the turn-on resistance of the FET switch. Therefore, the input impedance of the active rectifier can be adjusted without using an external device. 
       FIG. 9  shows an example of a result of compensating for output power or efficiency according to an embodiment of the present invention. 
     Reference numeral  811  in  FIG. 9  represents power transfer efficiency from a transmission device to a reception device according to a distance between a transmission device and a reception device. Reference numeral  812  in  FIG. 9  represents the output power outputted from the reception device according to the distance between the transmission device and the reception device. 
     That is,  FIG. 9  illustrates an effect obtained by the configuration according to an embodiment of the present invention, and illustrates an example of compensating efficiency according to the distance of a transmission/reception device and compensating for output power. 
     Since the input impedance of the active rectifier  4  varies depending on the distance d between the transmission device and the reception device, a phenomenon that efficiency is rapidly reduced and increased according to the distance is repeated (see reference numeral  811 ). However, by changing the input impedance of the active rectifier  4 , it is possible to find the optimum efficiency. 
     On the other hand, in situations where the output power of the active rectifier  4  is more important than efficiency, impedance can be varied to obtain maximum power and enable wireless charging at a greater distance (see reference numeral  812 ). 
     In the area A of  FIG. 9 , there is no relatively large change in the output power, but the efficiency greatly changes. Therefore, it may be desirable to control the efficiency in the area A of  FIG. 9  rather than optimize the output power. In the area B of  FIG. 9 , the efficiency decreases sharply with distance. Therefore, it may be desirable to control the output power to be optimized rather than to optimize efficiency in the area B of  FIG. 9 . However, this control method is according to a preferred embodiment, and the present invention is not necessarily limited to such a method. 
       FIG. 10  shows the voltage of the gate signal S_g 1  based on time according to an embodiment of the present invention. The gate signal S_g 1  may have a waveform in the form of a pulse train, and may be a PWM waveform in particular. The gate signal S_g 1  may be a signal provided to the gate of the FET switch included in the active current device  4 . 
       FIG. 11  shows a detailed configuration diagram of a wireless power reception device  100  according to another embodiment of the present invention. 
     Hereinafter, this will be described with reference to  FIGS. 10 and 11 . 
     The wireless power reception device  100  may include a gate driver  110 , an active rectifier  120 , a rectifier output detection unit  130 , and an impedance control unit  140 . 
     The gate driver  110  may generate a gate signal S_g 1  that switches between a turn-on voltage and a turn-off voltage. 
     The active rectifier  120  is connected to both ends of the inductor  22  and may include FETs whose on/off states are controlled by the gate signal S_g 1 . That is, the active rectifier  120  may include four FETs M 1 , M 2 , M 3 , and M 4  connected in the form of a bridge by the inductor  22 . At this time, the external matching element  3  may be further connected to both ends of the inductor  22 . 
     The rectifier output detection unit  130  may detect an output value of the active rectifier  120 . The output value may include an output voltage and an output current of the active rectifier  120 . 
     The impedance control unit  140  may include a gate voltage adjustment unit  141 , a power/efficiency calculation unit  142 , and a parameter setting unit  143 . 
     The impedance control unit  140  may control the impedance of the active rectifier  120  by controlling at least one of the duty ratio of the gate signal S_g 1  and the turn-on voltage that turns on the FET, based on the output value. At this time, the gate signal S_g 1  may be a PWM signal directly provided to the gate of the FET. Then, the PWM signal may have one of the turn-off voltage and the turn-on voltage, and the magnitude of the turn-on voltage may be controlled by the impedance control unit  140 . 
     The power/efficiency calculation unit  142  of the impedance control unit  140  may calculate the power transfer efficiency between the wireless power reception device  100  and the wireless power transmission device transmitting power to the wireless power reception device  100 , and the output power of the wireless power reception device. That is, the impedance control unit  140  may maximize the power transfer efficiency between the wireless power reception device  100  and the wireless power transmission device transmitting power to the wireless power reception device  100 , and the output power of the wireless power reception device based on the output value of the active rectifier  120 . 
     The output power of the wireless power reception device may be the output power of the active rectifier  120 . 
     The parameter setting unit  143  of the impedance control unit  140  determines the duty ratio of the gate signal S_g 1  based on the calculated power transfer efficiency or output power to provide the determined duty ratio to the gate driver  110 , and determines the gate voltage level at which the FET is turned on to provide the determined gate voltage level to the gate voltage adjustment unit  141 . 
     The gate voltage adjustment unit  141  may be configured to adjust the turn-on voltage of the gate signal S_g 1  according to the provided gate voltage level. 
     The gate driver  110  may output the gate signal S_g 1  such that the duty ratio of the gate signal S_g 1  has the determined duty ratio. 
     The ‘active rectifier’ described herein may be referred to simply as a ‘rectifier’. 
     According to the present invention, depending on the distance between the wireless charging reception device and the transmission device, the input impedance of the wireless charging reception device can be changed to allow maximum efficiency or maximum power transfer without the addition of external devices. In addition, according to the present invention, since no external element is used, it is advantageous in cost and area reduction, and the impedance compensation can be made finer. In addition, according to the present invention, the power transmission distance can be increased compared with a conventional wireless charging reception device. 
     It will be apparent to those skilled in the art that various modifications and variations may be made in the present invention without departing from the spirit or essential characteristics thereof. The contents of each claim may be combined with other claims without departing from the scope of the claims.