Abstract:
A variable gain circuit used in an in-band communication system is provided that includes a current sense pickup that is coupled to the output of a DC power source that senses current from the DC power source and provides a first output signal. A variable controlled amplifier structure, that is coupled to the DC power source, receives the first output signal and provides a specified amount of gain to the first output signal so as to produce a second output signal. A digital signal is produced using the second output having a selected frequency bandwidth.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional application Ser. No. 61/885,604 filed Oct. 2, 2013, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The invention is related to the field of charging based systems, and in particular a wireless charger communication automatic gain control. 
         [0003]    Current wireless power transfer systems (also referred to as wireless charging) in the market that employ in-band communication use a fixed receiver gain signal chain. Automatic gain control can be used to increase the dynamic range of the in-band communications system to improve communication performance. The invention is an improvement over existing wireless charging in-band communication systems that use fixed receiver gain topologies. 
       SUMMARY 
       [0004]    According to one aspect of the invention, there is provided a variable gain circuit used in an in-band communication system comprising. The variable gain circuit includes a current sense pickup that is coupled to the output of a DC power source that senses current from the DC power source and provides a first output signal. A variable controlled amplifier structure, that is coupled to the DC power source, receives the first output signal and provides a specified amount of gain to the first output signal so as to produce a second output signal. A digital signal is produced using the second output having a selected frequency bandwidth. 
         [0005]    According to another aspect of the invention, there is provided a wireless power system. The wireless power system includes a transmitter side that transmits a transmitted signal for reception, and a receiver side that receives the transmitted signal from the transmitter side. A current sense pickup is coupled to the output of a DC power source that senses current from the DC power source and provides a first output signal. A variable controlled amplifier structure, that is coupled to the DC power source, receives the first output signal and provides a specified amount of gain to the first output signal so as to produce a second output signal. A digital signal is produced using the second output having a selected frequency bandwidth. 
         [0006]    According to another aspect of the invention, there is provided a wireless power system. The wireless power system includes a transmitter side that transmits a transmitted signal for reception, and a receiver side that receives the transmitted signal from the transmitter side. A current sense pickup structure that receives a signal stream having a plurality of DC signals and senses the current for each input signal producing a second signal stream. A plurality of variable amplifier structures receive the second signal stream, each variable amplifier structure provides a specified amount of gain to their respective second signal in the second signal stream so as to produce a third output signal stream. A multiplexer selects a third signal from the third output signal stream, via a signal stream selection module, to produce a digital signal. 
         [0007]    According to another aspect of the invention, there is provided a wireless power system. The wireless power transmitter includes a detection circuit for in-band communications. The detection circuit includes one or more sense amplifiers, wherein the gain of the detection circuit is variable. 
         [0008]    According to another aspect of the invention, there is provided a method for decoding an in-band communications signal in a wireless power transmitter. The method includes providing a detection circuit for in-band communications, and providing one or more sense amplifiers, wherein the gain of the detection circuit is variable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram illustrating a wireless source/loads pad configuration used in accordance with the invention. 
           [0010]      FIG. 2  is a schematic diagram illustrating a wireless power system with load modulation used in accordance with the invention. 
           [0011]      FIG. 3  is a schematic diagram illustrating a wireless power system used in accordance with the invention. 
           [0012]      FIG. 4  is a schematic showing the equivalent circuit with N×Load devices shown on the Source side as reflected impedance. 
           [0013]      FIG. 5  is a schematic diagram illustrating a low complexity communication system used in accordance with the invention. 
           [0014]      FIG. 6  is a schematic diagram illustrating the RX communication path used in accordance with the invention. 
           [0015]      FIGS. 7A-7B  are graphs illustrating the performance of the pulse shaping filter used in accordance with the invention. 
           [0016]      FIG. 8  is schematic diagram illustrating a current sense AGC circuit used in accordance with the invention. 
           [0017]      FIG. 9  is schematic diagram illustrating another embodiment of the current sense AGC circuit used in accordance with the invention. 
           [0018]      FIG. 10  is a schematic diagram illustrating an AGC circuit having a N-path gain setting arrangement. 
           [0019]      FIG. 11  is a schematic diagram illustrating another embodiment of the AGC circuit having a N-path gain setting arrangement. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The invention presents an in-band communications system designed for a non-radiative, near field, wireless power transfer (WPT) system. Desirable attributes of a WPT system are to provide a safe and efficient wireless charging environment for single or multiple mobile consumer devices. A WPT charging system consists of a Source (Charger) and one or more Load devices where power is transferred wirelessly from the Source to one or more Loads. 
         [0021]      FIG. 1  shows an arrangement  2  having a source pad  8  charging a mobile phone  4  and laptop  6  positioned on the charging surface. To provide a good user experience it is essential to have some degree of communication at the very least from the Load to the Source. A distinction must be made between resonant wireless power (RWP) systems and inductive wireless power (IWP) systems as a resonant charging system can operate in loosely coupled conditions between the Source and the Load devices. The ratio of the area of the Source to Load coil for an RWP system can be &gt;1 whereas IWP systems tend have a coil ratio of ≈1. RWP systems allow freedom of placement encompasses charging at a distance, arbitrary orientation of the Load on the Source and multi-Load simultaneous charging. Communication from Load to Source allows a range of features from power management including: Load device detection and intelligent control of power transfer between Source and each Load and low power standby mode; and safety including: foreign object detection (FOD) and Load device over temperature and voltage. 
         [0022]      FIG. 2  shows a block diagram of a wireless power system  12  that incorporates in-band communications from Load to Source via load modulation. The amplifier  16  converts a DC power source  14  to AC at the power transmission frequency. The AC power is applied to a source inductor L 1  through a matching network  18 . Voltage is induced on the Load device inductor L 2  through the mutual inductance M. 
         [0023]    The receiver matching network  20  conveys this power to the rectifier  22 , which converts the power back to a DC power source  24  for use in the target device. Load modulation is applied to the rectifier  22  input voltage in this example, through switching of impedance Zmod. For resonant systems this impedance is typically resistive. Switching the impedance changes the loading seen by the Source, which causes the power delivered across the air interface to be modulated. This modulation can be detected by monitoring the input current to the amplifier, Isrc. 
         [0024]      FIG. 3  shows an example of a fixed gain circuit  28  that can be used sense an in-band communication received signal Isrc. The current sense pickup  32  determines the current being received from the DC-DC converter  30 . The output from the current sense pickup  32  is provided to the variable control amplifier  34 . The output of the amplifier  34  is sent to bandpass filter  38 . The output of the bandpass filter  38  is provided to a ADC  36  and converted in to a digital signal. 
         [0025]    A low complexity communication system is shown in  FIG. 5  and  FIG. 6 . The in-band transmitter  40  in  FIG. 5  uses an encoding module  42  to format a message by attaching a CRC to the message for error detection and then channel encodes the message for error correction. A BCH (15, 7) double error correcting code may be used for error correction. A switch  46  is used to perform biphase modulation causing changes in reflected impedance seen by the Source. A Golay complementary code (preamble)  44  may be prefixed to the message. 
         [0026]      FIG. 6  shows the in-band receiver  52  using an impedance sensing circuit  54  to detect changes in the reflected impedance followed by an analog anti-aliasing filter  56  and the analog-to-digital converter (ADC)  58 . The front end digital filter  60  does pulse shaping and noise rejection. The receiver preamble detection block  62  is used for message detection and synchronization/timing adjustment. A message decoding block  64  performs biphase demodulation with error correction as well as BCH error correction channel decoding, and error detection (CRC). 
         [0027]    The received communication signal at Isrc can have modulated on a large DC offset depending on the total power delivered to all the Load devices. The design includes a course DC offset removal in the analog domain and a fine DC offset removal in the digital decoder. Any modulation scheme used needs to be able to pass through the DC removal without appreciable distortion. 
         [0028]    In-band communication in a wireless charging system can result in some challenging channel conditions as shown in PSD of  FIG. 6 . It should be noted that only the main lobe of the biphase signal is required to decode the received signal. The pulse shaping filter  60  can be designed to attenuate everything but the main lobe as shown in  FIGS. 7A-7B  show the improvement in performance after filtering. 
         [0029]    In low coupling conditions or when one or multiple devices are charging the change in load impedance seen at the Source due to switching the modulation impedance at the Load can be small. It is possible that in these cases the dynamic range or sensitivity of a fixed gain load impedance circuit is not enough to allow reliable reception of messages from the Load. Table 1 shows the voltage measured at the output of the impedance sense circuit versus distance and Vsource for a single Load taken from a two Load device charging pad.  FIG. 2  shows the location of the measurements Vsdc and Vrect. 
         [0030]    The current sense circuit ( FIG. 3   32 ) used a resistor of 48 mΩ as the current sense pickup and an amplifier gain of 100 ( FIG. 3   34 ) giving an overall gain of 4.8. If a 54 dB (9 bit) dynamic range ADC was used in the receiver circuit with a full range of 0-3.3V then the V Isense  signal level shown in Table 1 is approaching the limit of the signal level that can be decoded successfully. In this case with a 54 dB dynamic range ADC the smallest ADC step is 6.44 mV. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Measured data of output of impedance sense circuit versus 
               
               
                 distance and Vsource for a single Load taken from a  
               
               
                 two Load charging pad 
               
             
          
           
               
                   
                 Z height from 
                   
                   
                   
               
               
                   
                 SD 
                 Vsource 
                 Vrect 
                 V Isense   
               
               
                   
                 (mm) 
                 (V) 
                 (V) 
                 (mVpeak-peak) 
               
               
                   
                   
               
             
          
           
               
                   
                 15.3 
                 8.9 
                 4.6 
                 9 
               
               
                   
                 16.3 
                 10 
                 5.3 
                 14 
               
               
                   
                   
               
             
          
         
       
     
         [0031]    Similarly if multiple Loads are placed on the Source the ΔIsense, due to switching of the modulation impedance of a single Load, is decreased.  FIG. 4  shows the equivalent circuit  38  of the Source and N×Loads with the Load impedances shifted to the Source side (Primary side of the transformer). From  FIG. 4  the power amplifier must produce a high enough voltage to supply power to multiple Load devices, and as a consequence any change in reflected impedance of a single Load due to the switching of its modulation impedance, results in a smaller change in Isense than would occur with only a single Load charging. 
         [0032]    In low coupling conditions and when multiple Load are charging the in-band communication would benefit from greater dynamic range/sensitivity. 
         [0033]    The following are ways to increase the sensitivity of the current sense circuit
       1. Increase the fixed gain—does not increase the dynamic range of the circuit   2. Increase the dynamic range of the ADC—includes oversampling the input signal   3. Implement some form of variable gain or automatic gain control (AGC)       
 
         [0037]    Only 2&amp;3 listed above increase the dynamic range of the current sense circuit. Increasing the dynamic range of the ADC is a simple way of increasing the dynamic range but there is a tradeoff between increasing the dynamic range with cost and power. Increasing the dynamic range of the in-band communications receiver via variable gain is a low complexity and low cost method. 
         [0038]      FIG. 8  shows an embodiment of the AGC circuit used to sense current. The AGC circuit  66  includes current sense pickup module  68  that provides a voltage signal measuring the current and providing that a first output signal to an amplifier  70 . The current sense pickup module  68  may be implemented by a sense resistor of 48 mΩ, and the voltage across the sense resistor is dc-coupled to the sense amplifier  70 . The amplifier  70  provides variable gain of its output. The amplifier  70  produces a second output signal that is provided to a bandpass filter  72 . The bandpass filter  72  is set by the rate of communication, and produces a third output signal received by an analog digital converter (ADC)  74 . The ADC  74  converts the third output signal into a digital signal. Positioned in a feedback loop is a digital AGC control  76  that controls the variable gain parameters of the amplifier  70 . The digital part of the AGC can determine the signal level by processing the signal to give an indication of its power level. An example would be either the Root Mean Square (RMS) of the signal of just the Square of the signal. 
         [0039]      FIG. 9  shows another embodiment  80  of the AGC circuit used to sense current. The overall design is similar to the structure shown in  FIG. 8 . The key difference is the high-pass filters  82  positioned at the input of the amplifier  70 . Therefore the output of the current sense pickup module  68  is ac-coupled to the sense amplifier  70 . The DC content of the signal can be large and removing a portion of the DC component via the high-pass filter  82  allows for a greater gain value to be set by the variable amplifier  70 . 
         [0040]    The digital AGC control  76  can perform a number of functions. One of these functions is monitoring the signal level over a specified window of time and then setting the gain dynamically for each time slot depending on the expected signal level from each Load device to give the best sensitivity—if clipping is detected then reduce the gain. The digital AGC control can set the gain to maximum and when signal clipping is detected reduce the gain until clipping is eliminated. In a multiple Load charging scenario it is possible for each Load to have different coupling factors. Note that the coupling factor for a Load only changes when the device is moved and is not expected to vary quickly over time. For a synchronous communications system where it is known where each device will transmit an AGC context could be keep for each Load. A gain value specific to the Load that is about to transmit is set before each Load transmits a message. The gain setting calculated in each Load&#39;s AGC context would be based on the signal level from previous messages from that specific Load. 
         [0041]      FIG. 10  shows an arrangement  84  having a N-path with different gain settings to increase dynamic range. The arrangement includes a N-number of current paths that is coupled to amplifiers A 1 -AN and bandpass filters B 1 -BN (three amplifiers A 1 -A 3  and three bandpass filters B 1 -B 3  are shown in  FIG. 10  as an example). The current sense pickup module may be implemented by a sense resistor of 48 mΩ, and the voltage across the sense resistor is dc-coupled to the sense amplifier A 1 -A 3 . The amplifiers A 1 -A 3  provide their first output signals to bandpass filters B 1 -B 3  respectively. The bandpass filters B 1 -B 3  provide their respective outputs to a multiplexer  86 . The multiplexer  86  selects one of the outputs of the bandpass filters B 1 -B 3  to be received by an ADC  88 . In one embodiment, the multiplexer  86  selects the outputs of the bandpass filters B 1 -B 3  to be received by the ADC  88  in an interleaved fashion. Note in other embodiments of the invention more than 3 amplifiers and bandpass filters B 1 -B 3  can be used for this arrangement. 
         [0042]    This approach increases the dynamic range by providing multiple paths with different gain settings for each path. The signal stream module  90  can then select the signal with the magnitude that gives the best performance (e.g. provides the maximum signal without clipping). The signal stream selection element  90  may select the data stream dynamically for each time slot depending on the expected signal level from each target device. 
         [0043]      FIG. 11  shows another embodiment of the AGC circuit having a N-path gain setting arrangement  94 . The overall design is similar to the structure shown in  FIG. 10 . The key difference is the high pass filters  96  positioned at the input of the amplifiers A 1 -A 3 . Therefore the output of the current sense pickup module is ac-coupled to the sense amplifier A 1 -A 3 . The DC content of the signal can be large and removing a portion of the DC component via the high pass filter  96  allows for a greater gain value to be set by the variable amplifier A 1 -A 3 . 
         [0044]    Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.