Patent Publication Number: US-2023163617-A1

Title: Reducing corruption of communication in a wireless power transmission system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of prior application Ser. No. 16/540,964, filed Aug. 14, 2019, currently pending; 
     Which was a divisional of prior application Ser. No. 15/959,443, filed Apr. 23, 2018, now abandoned; 
     Which was a continuation of prior application Ser. No. 15/131,732, filed Apr. 18, 2016, now U.S. Pat. No. 9,954,402, granted Apr. 24, 2018; 
     Which was a continuation of prior application Ser. No. 12/838,298, filed Jul. 16, 2010, now U.S. Pat. No. 9,318,897, granted Apr. 19, 2016; 
     Which claims priority to U.S. Provisional Application No. 61/227,332, filed Jul. 21, 2009, the entirety of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     Inductive coupling, often called magnetic coupling, couples magnetic energy from one coil to another coil. Inductive coupling may be used in transformers to change a voltage on a primary coil (also called a winding) to a higher or lower voltage on a secondary coil. For example, high voltage power lines use transformers to “step down” high voltages on power lines to lower voltages (e.g. 120 volts) that may be used in homes. Transformers may be used to isolate one electrical system from another electrical system. 
     The transfer of energy can be more efficient if a magnetic medium such as iron is used. However, energy may be transferred through a medium such as air from one coil to another coil. Because inductive coupling does not need an electrical conductor to transfer energy from one coil to another coil, batteries in electronic devices may be charged without requiring an electrical cord to be attached to the electronic device. For example, many electric toothbrushes use inductive coupling to recharge batteries in the electrical toothbrushes. These electric toothbrushes are charged by simply placing them on a stand that contains a source of magnetic energy. 
     Inductive coupling may be used to transfer electrical power as in the case where high voltage transformers transfer power from high voltage power lines to homes. In addition to transferring power, inductive coupling may be used for communication. For example, a transformer may be used to transfer an analog signal (e.g. an audio signal) from an amplifier to a loudspeaker in order to produce sound. In addition, digital signals may be transmitted from one coil to another coil in order to facilitate communication. 
     Inductive coupling may be used to transfer power and transmit information concurrently. For example, backscatter modulation is a method of communicating in an inductively coupled system where power is transferred from the power transmitter to the power receiver and information is communicated from the power receiver back to the power transmitter. When inductive coupling is used to transfer power and transmit information concurrently, the transfer of power may corrupt the transmission of information. For example, in a system using backscatter modulation, the charging of a battery can create digital pulses that are transmitted from a receiver to the transmitter corrupting data that is communicated from the receiver to the transmitter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an embodiment of a wireless power transmission system. 
         FIG.  2    is a schematic diagram of an embodiment of a wireless power transmission system with a charge control circuit. 
         FIG.  3    is a schematic diagram of a first embodiment of a charge control circuit. 
         FIG.  4    is a schematic diagram of a second embodiment of a charge control circuit. 
         FIG.  5    is a plot of an embodiment of a charging profile for a lithium-ion battery as a function of time and current. 
         FIG.  6    is a flow chart of an embodiment of a method for reducing corruption of communication in a wireless power transmission system. 
     
    
    
     DETAILED DESCRIPTION 
     The drawings and description, in general, disclose embodiments of a method and apparatus for reducing corruption of communication in a wireless power transmission system. In summary, a primary coil in a transmitter generates a magnetic field which is magnetically coupled to a secondary coil of a receiver. The magnetic field coupled from the primary coil to the secondary coil induces an AC (alternating current) voltage on the secondary coil, which is then rectified to generate a DC (direct current) voltage at the rectifier node. This DC rectifier voltage will cause a current to flow into an output load. A charge control circuit placed between the rectifier circuit and the load limits the change in current to the load. 
     When a charge control circuit is not placed between the rectifier circuit and the load, the current supplied to the load may change significantly. The current supplied to the load may change significantly due to the dynamic load requirements associated with power transmission in a cellular phone. Change in the current supplied to the load induces a change in current on the secondary coil in the receiver. Change in current on the secondary coil induces a signal to be magnetically coupled from the receiver to the transmitter. The signal magnetically coupled from the receiver to the transmitter may corrupt communication being sent concurrently from the receiver to the transmitter. 
     For example, in a wireless power transmission system that uses backscatter modulation, the intended communication from the receiver to the transmitter may be corrupted by a signal induced on the secondary coil by a change in current to the load being charged. To reduce corruption of communication while a load is being charged, a charge control circuit is placed between a rectifier and a load. Because the change in current to the load is reduced when the charge control circuit is placed between the rectifier and the load, corruption of communication from the receiver to the transmitter is reduced. The function of the charge control circuit within a wireless power transmission system will be discussed later in more detail. 
     Power may be transferred from a transmitter to a receiver concurrently with communication being sent from the receiver to the transmitter. Because power is being transferred concurrently with communication being sent, power induced in the receiver can create a signal that is sent from the receiver to the transmitter that may corrupt the intended communication sent from the receiver to the transmitter.  FIG.  1    illustrates how communication may be corrupted in a wireless power transmission system  100  that uses backscatter modulation. 
     A control circuit  102  controls a driver circuit  104  through electrical connections  112  and  114 . The driver circuit  104  drives a current I 1  through the resonant circuit  106 . In this example, the resonant circuit is an LC (inductor-capacitor) circuit containing a capacitor C 1  and a coil L 1  in series. Other resonant circuits may be used such as an LLC (inductor-inductor-capacitor) circuit. In this example, the current I 1  is approximately sinusoidal. 
     When current I 1  is drawn through coil L 1 , an approximately sinusoidal current I 2  is induced on coil L 2  in the rectifier circuit  108 . Rectifier circuit  108  contains a center-tapped coil, two diodes D 1  and D 2 , and a capacitor C 2 . The rectifier circuit  108  in this example is a full-wave center-tapped rectification circuit; however, other types of rectification may be used such as full-wave non-center-tapped rectification or half-wave rectification. The rectifier circuit  108  provides a voltage on node  124 . The voltage on node  124  causes a current I 3  to charge the load  110 . The load  110 , for example, may be a battery, a battery charger or a power supply. 
     In this example, the current I 3  that charges the load  110  also causes a change in current I 4  in the center-tapped coil L 2 . The change in current I 4  in the center-tapped coil L 2  induces a change in current I 5  in the coil L 1 . The change in current I 5  in the coil L 1  may corrupt communication from the receiver  130  to the transmitter  128 . Modulating the voltage on node COMM, creates communication from the receiver  130  to the transmitter  128  by backscatter modulation. The change in current I 5  due to the change in current I 3  into the load  110  may corrupt the communication sent by the modulation of node COMM. 
     In an embodiment of the invention, the corruption of communication from the receiver  130  to the transmitter  128  may be reduced by limiting the change in current I 3  charging the load  110 . In an embodiment of the invention, the change in current I 3  for charging the load  110  is limited by inserting a charge control circuit between the rectifier circuit  108  and the load  110 . Several embodiments of the charge control circuit and how they are used in a wireless power transmission system that uses backscatter modulation will now be discussed in order to best explain the applicable principles and their practical application. 
       FIG.  2    is a schematic drawing of an embodiment of a wireless power transmission system  200  that uses backscatter modulation wherein corruption of communication is limited. A control circuit  102  controls a driver circuit  104  through electrical connections  208  and  210 . The driver circuit  104  drives a current I 1  through the resonant circuit  106 . The driver circuit  104 , in this example, contains a PFET (p-type field-effect transistor) PFET 1  and an NFET (n-type field-effect transistor) NFET 1 ; however, other transistors may be used. An input  208  to the driver circuit  104  is connected to the gate of PFET 1  and an input  210  of the driver circuit  104  is connected to the gate of NFET 1 . The source of PFET 1  is connected to VDD and the source of NFET is connected to ground. The drains of PFET 1  and NFET 2  are connected to the output  212  of the driver circuit  104 . 
     In this example, the resonant circuit  106  is an LC (inductor-capacitor) circuit containing a capacitor C 1  and a coil L 1  in series. Other resonant circuits may be used such as an LLC (inductor-inductor-capacitor) circuit. In this example, the current I 1  is approximately sinusoidal. When current I 1  is drawn through coil L 1 , an approximately sinusoidal current I 2  is induced on coil L 2  in the rectifier circuit  108 . 
     Rectifier circuit  108  contains a center-tapped coil L 2 , two diodes D 1  and D 2 , and a capacitor C 2 . The rectifier circuit  108 , in this example, is a full-wave center-tapped rectification circuit; however, other types of rectification may be used such as full-wave non-center-tapped rectification or half-wave rectification. The anode of diode D 1  is connected to a terminal of center-tapped coil L 2  and the anode of diode D 2  is connected to another terminal of center-tapped coil L 2 . The center of center-tapped coil L 2  is grounded. The cathode of diode D 1 , the cathode of diode D 2  and a terminal of C 2  are connected to the output of the rectifier circuit  108  at node  218 . The rectifier circuit  108  provides a voltage on node  218 . The voltage on node  218  causes a current I 3  to flow. In an embodiment of the invention, the charge control circuit  206  limits the change in current I 3 . 
     Because the change in current I 3  is limited, the change in current I 4  in the center-tapped coil L 2  is substantially reduced. Reducing the change in current I 4  in the center-tapped coil L 2  also reduces the change in current I 5  in the coil L 1 . Because the change in current I 5  in the coil L 1  is reduced, the probability of corruption of communication from the receiver  202  to the transmitter  128  is also reduced. Modulating the voltage on node COMM of the communication switch  204 , creates communication from the receiver  202  to the transmitter  128  by backscatter modulation. 
     A communication switch  204  includes resistor R 1  and NFET NFET 2  in series. When the change in current I 3  is reduced by the charge control circuit  206 , the communication created by the communication switch  204  has a lower probability of being corrupted. 
     In a first embodiment of the charge control circuit  206 , the charge control circuit  206  measures current I 3  being supplied to the load  110 . After measuring the current I 3 , the control circuit  206  sets a current regulation threshold below the present value of current I 3 . Setting the current regulation threshold below the present value of current I 3  forces the charge control circuit  206  to operate in current limit. Operating in current limit reduces the change in current I 3 . As a result, the probability of communication being corrupted to the transmitter  128  is reduced. In this first embodiment of a charge control circuit  206 , the load  110  that is charged may be a battery, a battery charger or a power supply. However, the load is not limited to a battery, a battery charger or a power supply. 
     In a second embodiment of the charge control circuit  206 , the charge control circuit  206  measures current I 3  being supplied to the load  110 . After measuring the current I 3 , the control circuit  206  sets a current regulation threshold slightly above the present value of current I 3 . Typically, a 200 ma current pulse is used for communication. Therefore, if the change in current increases by 100 ma or less, communication will not be corrupted. However, if the current pulse used for communication is increased, the current regulation threshold may also be raised without corrupting communication. In this second embodiment of a charge control circuit  206 , the load  110  that is charged may be a battery, a battery charger or a power supply. However, the load is not limited to a battery, a battery charger or a power supply. 
     In a third embodiment of the charge control circuit  206 , the charge control circuit  206  measures current I 3  being supplied to the load  110 . When the measured current is 200 ma or less, the current regulation threshold is set at 200 ma. One reason for setting the current regulation limit to 200 ma when the measured current I 3  is 200 ma or less is to protect against the wireless power transmission system shutting down based on an under-voltage or an under-current condition. In this third embodiment of a charge control circuit  206 , the load  110  that is charged may be a battery, a battery charger or a power supply. However, the load is not limited to a battery, a battery charger or a power supply. 
     In a fourth embodiment of the charge control circuit  206 , when there is no communication between the receiver  202  and the transmitter  128 , no current regulation threshold is set. Instead, the change in current I 3  may be increased to the load  110 . A full charge current may be used during this condition because there is no communication between the receiver  202  and the transmitter  128 . Because there is no communication between the receiver  202  and the transmitter  128 , there is no communication to corrupt. In this fourth embodiment of a charge control circuit  206 , the load  110  that is charged may be a battery, a battery charger or a power supply. However, the load is not limited to a battery, a battery charger or a power supply. 
       FIG.  3    is a schematic drawing of an embodiment of a charge control circuit  300 . The charge control circuit  300  in  FIG.  3    acts as a current limiter whose threshold tracks the present value of current I 3  through device NFET 3 . In this charge control circuit  300 , the element  322  acts as a current sensor  322 . The current sensor  322  allows charge current I 3  to pass from the input  326  of the charge control circuit  300  to node  308  substantially unchanged. An output from current sensor  322  is connected to node  324 . The current I 6  drawn through resistor R 2  is a scaled version of the current I 3  at node  308 . The current I 6  is approximately equal to I 3 /k, where the value of k is typically between 100 and 1000. Current I 6  flows through the resistor R 2  creating a voltage at node  324  that is proportional to the current I 3 . 
     In  FIG.  3   , op-amp  306  and associated resistors R 3  and R 4  create a non-inverting gain function that amplifies the voltage at node  324  by a factor, typically 1.05. However, other factors are also possible. The resistor R 5  and the capacitor C 3  form a low-pass filter that acts to hold the amplified voltage at node  316 . The charge pump  302  acts as current source that will turn on the gate of NFET 3  and provide a bias voltage for the gate of NFET 3 . 
     An embodiment of the current limit function of the charge control circuit  300  operates as follows: when the load current I 7  pulled from node  310  changes quickly (quickly in this embodiment means approximately 10 times faster than the time constant T 4 , where T 4  equals (R 5 ×C 3 ) seconds), then the voltage at node  324  will increase proportionally to the change in load current I 7  while the voltage at node  316  does not substantially change at the same time. Therefore, a negative differential voltage is created at the input of op-amp  304  that will cause current to flow into op-amp  304 . The current flowing into op-amp  304  will lower the voltage on the gate  312  of NFET 3  and inhibit the change in load current I 7 . 
     A negative feedback loop formed in the charge control circuit  300  limits the rate at which the load current I 7  can change by the time constant T 4 . The time constant T 4  can be changed by varying the value of the external capacitor C 3 . When the time constant T 4  is slower than the communication time, the circuit  300  will improve communication by shielding fast current changes from a transmitter. 
     Other embodiments of charge control circuit  300  are also possible. For example, an enabling function may be added that would turn on charge control circuit  300  during the communication period and then turn it off when communication is complete. In this way, the charger can quickly deliver its full current when communication is not active. 
     In the embodiment of the charge control circuit  300  shown in  FIG.  3   , the charge control circuit  300  sets a tracking current limit which is used during voltage regulation. However, another amplifier (not shown) may be added to the charge control circuit  300  to set a maximum current limit independent of the voltage on capacitor C 3 . The charge current into node  308  will not run away because the tracking current limit will be active for relatively short periods of time when power is applied. 
       FIG.  4    is a schematic drawing of an embodiment of a charge control circuit  400 . The charge control circuit  400  in  FIG.  4    acts as a current limiter whose threshold tracks the present value of current I 3  through device NFET 4 . In this charge control circuit  400 , the element  322  acts as a current sensor  322 . The current sensor  322  allows charge current I 3  to pass from the input  420  of the charge control circuit  400  to node  408  substantially unchanged. An output from current sensor  322  is connected to node  422 . The current I 6  drawn through resistor R 6  and R 7  is a scaled version of the current I 3  at node  408 . The current I 6  is approximately equal to I 3 /k, where the value of k is typically between 100 and 1000. Current I 6  flows through the resistors R 6  and R 7  creating voltages at nodes  422  and  410  that are proportional to the current I 3 . 
     In this embodiment, the input of the charge control circuit  400  is connected to node  420 . A voltage proportional to the change in current I 3  on node  410  is presented to a first input of the op-amp  406 . In this embodiment, a voltage divider is created by resistor R 6  in series with resistor R 7 . In this example, the value of R 6  is equal to (0.05)*R 7 . Because the value of R 6  is equal to (0.05)*R 7 , the voltage on node  410  is approximately (0.95) times the voltage on node  422 . 
     Node  408  is connected to the current sensor  322  and the drain of NFET NFET 4 . Node  422  is connected to the input of ADC  402  and resistor R 6 . The output  416  of the ADC  402 , an 8-bit bus, is connected to the input of the DAC (digital-to-analog converter)  404 . An 8-bit bus is used in this example; however, the output  416  of the ADC  402  may be any size bus. For example, the bus may be a 16 bit bus. The DAC  404  converts the digital signal from the output  416  of the ADC  402  to an analog signal on node  412 . The voltage on node  412  is presented to a second input of the op-amp  406 . 
     The op-amp  406  compares the voltages on nodes  412  and  410 . When the voltage on node  412  is higher than the voltage on node  410 , the op-amp  406  increases the voltage on node  414 . The node  414  is connected to the gate of NFET 4 . When the voltage on node  412  is lower than the voltage on node  410 , the op-amp  406  decreases the voltage on node  414 . Changing the voltage on node  412  based on the current I 6  drawn through resistors R 6  and R 7  regulates the current drawn on node  408 . By regulating the current through NFET 4 , the current drawn on node  408  is substantially constant. Keeping the current approximately constant on node  408  reduces corruption in communication from the receiver  202  to the transmitter  128 . 
     The use of the ADC  402  and the DAC  404  between nodes  408  and  412  allows for more precise control of the tracking current limit. In addition, the charge control circuit  400  does not need an external capacitor as in the embodiment of the charge control circuit  300  shown in  FIG.  3   . 
     An embodiment of the invention may be used to charge, for example, lithium-ion batteries. A typical charging profile for lithium-ion batteries is shown in  FIG.  5   . The charging of a lithium-ion battery begins with a preconditioning period T 1 . During this time, the charge current CC is relatively low and held at a constant value. Because the charge current CC is a constant value, the charge voltage CV increases linearly until a minimum charge voltage is reached. 
     When the minimum charge voltage is reached, the precondition phase T 1  ends and the current regulation phase or “fast charge” phase T 2  begins. During the current regulation phase T 2 , the charge current CC is increased and held at a constant value. Because the charge current CC is a constant value, the charge voltage CV increases linearly until the lithium-ion battery is nearly charged. 
     When the lithium-ion battery is nearly charged, the current regulation phase T 2  ends and the voltage regulation phase T 3  begins. During the voltage regulation phase T 3 , the charge current CC tapers until the charge is complete. During the voltage regulation phase T 3 , the charge current CC is no longer constant. Because the charge current CC is changing during the voltage regulation phase T 3 , a change in current will also occur in a coil L 2  of a receiver  202 . When a change in current occurs in the coil L 2 , communication sent from the receiver  202  to the transmitter  128  may be corrupted. 
     An embodiment of the invention may be used to lower the probability of communication being corrupted during the voltage regulation phase T 3  of charging lithium-ion batteries in a wireless power transmission system. The voltage regulation phase T 3  of charging lithium-ion batteries is shown in  FIG.  5    where the charging current CC is reduced over a time period. The use of a charge control circuit, for example the charge control circuit  300  shown in  FIG.  3   , during the voltage regulation phase T 3  of charging lithium-ion batteries, will reduce the change in charging current CC. Because the change in charging current CC is reduced, the probability of corruption of communication from the receiver  202  to the transmitter  128  will be reduced as well. 
       FIG.  6    is a flow diagram of an embodiment of a method of reducing corruption of communication in a wireless power transmission system  200 . In step  602 , power is transmitted from a primary coil L 1  to a secondary coil L 2  by induction. In step  604 , the voltage induced on the secondary coil L 2  by the transmission of power from the primary coil L 1  to the secondary coil L 2  is rectified. In step  606 , the change in current supplied to a load  110  while power is induced on the secondary coil L 2  is limited. Because the change in current is limited, corruption of communication from a receiver  202  to a transmitter  128  in a wireless transmission system  200  is reduced. 
     The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiments were chosen and described in order to best explain the applicable principles and their practical application to thereby enable others skilled in the art to best utilize various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments except insofar as limited by the prior art.