Abstract:
A power converter (such as a battery charger) includes a cable configured to deliver a source voltage and current to a load, where the cable is anticipated to drop some voltage as the load current increases. The power converter also includes a regulator having a feedback-adjusting transistor configured to gradually compensate for the dropped cable voltage as the load current increases. The transistor has a gate capacitance and a resistance forming an integrator configured to filter a volt-second product of an output waveshape of the converter to derive an average voltage correlated to the load current as the load current increases. The regulator is configured to increase a gate voltage of the transistor through a threshold region of the transistor and gradually turn the transistor on. The transistor is configured to apply an adjusting resistance coupled to a feedback sensing node of the regulator to increase the source voltage to compensate for the cable voltage drop and improve the load voltage regulation.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority of U.S. Patent Application Ser. No. 61/745,303 filed Dec. 21, 2012 entitled Volt-second Integration Cable Compensation Circuit, the teachings of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure is generally directed to battery chargers and AC-line adapters that have a cable between a voltage source and a load, such as chargers for portable electronic devices including, but not limited to mobile telephones and smartphones. 
       BACKGROUND 
       [0003]    Flyback-derived chargers and similar adapters (generically termed “converters” hereafter) often have a cable between a voltage source and a load. This cable drops some voltage as the load current increases, unless there is compensation by a source controller integrated circuit (IC). Primary-side controllers using optically-coupled feedback from the secondary side of a regulator cannot incorporate cable compensation, so this function is usually accomplished external to the regulator. Opto-coupled feedback is commonly known as galvanic isolation and prevents a continuous electrically conductive path between output and input. Although the optical feedback path is used to regulate the voltage delivered to the regulator output, detecting a voltage drop across the cable due to its wire resistance cannot be accomplished using common feedback loop mechanisms. Moreover, a primary-side controller is generally incapable of over-riding the reference voltage of the secondary-side regulator without some elaborate additional circuitry with its own isolation. 
         [0004]    Controllers located on the primary side often employ Primary-Side Regulation (PSR) techniques for adjusting the converter&#39;s regulation reference based on primary-side current information indicative of the load current. However, PSR is subject to transient response limitations that may be inadequate for certain application performance requirements. Instead, a low-cost secondary-side shunt regulator is used to generate an error signal which is optically coupled to the primary-side controller which controls the power conversion based on the feedback signal level. 
       SUMMARY 
       [0005]    Embodiments of this disclosure include a cable compensation circuit. A converter has a regulator configured to compensate for a voltage drop across a cable as load current increases to increase and regulate a load voltage. The regulator has a feedback-adjusting transistor configured to gradually compensate for the cable&#39;s voltage drop as the load current increases. The transistor has a parasitic capacitance and a gate resistance forming an integrator configured to filter a volt-second product of a flyback output waveshape to derive an average voltage correlated to the load current as the load current increases. The gate voltage of the transistor is configured to increase through a threshold region of the transistor and gradually turn the transistor on, and the transistor is configured to adjust a resistance coupled to a feedback sensing node of the regulator to increase the source voltage so as to compensate for the cable voltage drop as the load current increases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    For a more complete understanding of this disclosure and its features, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
           [0007]      FIG. 1  is a partial schematic of a converter with an opto-coupled flyback regulator and no cable compensation; 
           [0008]      FIG. 2  is a partial schematic of a converter with an opto-coupled flyback regulator and remote-sense cable compensation; 
           [0009]      FIG. 3  is a partial schematic of a converter with an opto-coupled flyback regulator and local sense cable compensation; 
           [0010]      FIG. 4  is a partial schematic of a converter with an opto-coupled flyback regulator and amplified-sense cable compensation; 
           [0011]      FIG. 5  is a partial schematic of a converter with an opto-coupled flyback regulator and V-s integration cable compensation; 
           [0012]      FIG. 6A  and  FIG. 6B  are charts of parameters of a 5 W converter board without load before active cable compensation; 
           [0013]      FIG. 7A  and  FIG. 7B  are charts of parameters of a 5 W converter board output voltage with load after V-s integration cable compensation; 
           [0014]      FIG. 8  is a chart of parameters of a 5 W load board input voltage with a load before and after active cable compensation; 
           [0015]      FIGS. 9 and 10  are waveform diagrams depicting voltages at V out  and V load ; 
           [0016]      FIG. 11  is a waveform diagram depicting V out  and V load  in response to a load-step with no compensation; 
           [0017]      FIG. 12  is a waveform diagram depicting V out  and V load  in response to an unload-step with no compensation; 
           [0018]      FIG. 13  is a waveform diagram depicting V out  and V load  in response to a load-step with V-s integration compensation; and 
           [0019]      FIG. 14  is a waveform diagram depicting V out  and V load  in response to an unload-step with V-s integration compensation. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIGS. 1 through 14 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitable manner and in any type of suitably arranged device or system. 
         [0021]      FIG. 1  is a partial schematic of a flyback converter  10  with an opto-coupled shunt-regulator circuit  12  and no cable compensation circuit. Without cable compensation, the load voltage V load  falls directly as the load current I load  increases through the cable resistance R wire . While this circuit may be suitable for low current designs, it is insufficient for higher-current applications as V load  drops to unacceptable voltages. 
         [0022]    Flyback converter  10  is seen to include an input transformer  14 , which receives a pulsing source voltage Vs and couples a transformed, rectified, and filtered voltage V out  to a cable  16 . The wire resistance of the cable  16  is denoted as R wire  on each cable wire  18 , and the associated voltage drop for each wire is denoted as V wire . The delivered load voltage at the termination of the cable  16  is V load , which is V out −2V wire . Wires  18  provide a continuous electrical path, where one is denoted the positive (+) wire and the other is denoted the negative wire (−). 
         [0023]    The shunt-regulator circuit  12  provides an opto-coupled feedback error signal FB based on V out  at the proximal end of the cable  16 . The optical coupler T 1  provides galvanic isolation for the feedback signal FB. The shunt-regulator circuit  12  does not account for the resistance R wire  of each of the cable wires  18 , even though those resistances R wire  create the voltage drop V wire  proportional to the load current I load . The shunt-regulator circuit  12  taps a feedback signal from V out  controlled by resistive divide network resistors R fb1  and R fb2  with gain k, where the feedback signal is compared to an internal reference voltage V ref  established by shunt-regulator Z 1 . Shunt-regulator Z 1  generates a current proportional to the voltage difference between kV out  and V ref  to create the feedback error signal FB, which modulates the power stage (not shown) duty-cycle to regulate the output voltage V out . Since there is no cable compensation, the load voltage V load  falls directly as the load current I load  increases. The regulator  12  also includes trimming components R tl , R opt , and C fb . 
         [0024]      FIG. 2  is a partial schematic of a flyback converter  20  with an opto-coupled shunt-regulator circuit  22  which includes a remote-sense cable compensation network. Like reference numerals refer to like elements including those described with reference to  FIG. 1 . The shunt-regulator  22  is responsive to both V out  at the proximal end of the cable  16  and also to a remote sense voltage V rs  derived from the load voltage V load  of the cable  16  via wire  24 . The voltage V rs  is established by a pair of remote sense resistors R rs1  and R rs2  which form a resistive divide network with gain k between the negative terminal of V load  and proximal ground. Advantageously, the transistor T 1  responds to a voltage relationship between V out  and V rs  to create the opto-coupled feedback signal FB, where the voltage reference V rs  is a function of the cable resistance. Remote sense wire  24  carries insignificant current and so its voltage drop is negligible. This allows the converter  20  to compensate for resistance of the cable wires  18  and their resulting voltage drops. Advantageously, the compensation automatically adjusts with cable length since it accounts for the varying voltage drop from the varying resistance of the cable wires  18 . 
         [0025]      FIG. 3  is a partial schematic of a flyback converter  30  with an opto -coupled shunt-regulator  32  which includes a local sense cable compensation network. In particular, the converter  30  implements linear cable compensation with an additional local sense resistance R sense . In some embodiments, the local sense resistance R sense  is designed as a narrow section of printed circuit board (PCB) copper with a resistance proportional to the cable wire resistance R wire . This converter  30  is simpler than the remote sensing converter  20 , although it may be less accurate, does not adjust with cable length, and incurs additional loss. 
         [0026]      FIG. 4  is a partial schematic of a flyback converter  40  with an opto -coupled shunt-regulator  42  which includes an amplified-sense cable compensation network. In particular, the converter  40  implements linear cable compensation with an additional local sense resistance R sense  and an amplifier  44 . This approach uses a lower-valued local sense resistor R sense  to reduce loss, and the amplifier  44  is used to amplify a V sense  signal. In this approach, the compensation incurs higher complexity, does not adjust with cable length, and no-load losses can increase. 
         [0027]      FIG. 5  is a partial schematic of a flyback converter  50  with an opto -coupled shunt-regulator  52  which includes V-s integration cable compensation. In particular, the converter  50  uses non-linear cable compensation with volt-second (V-s) integration supported by a transistor M 1  (such as a MOSFET transistor). The transistor M 1  has a parasitic gate capacitance C iss  and resistors R INT1  and R INT2 , which form an integrator that filters the volt-second product of the flyback output waveshape to derive an average voltage V INT  roughly proportional to the output current I load . As the V-s product of V SEC  increases, the gate voltage V INT  increases through the threshold region of the transistor M 1  and gradually turns the transistor M 1  on, such that the transistor M 1  responsively applies an adjusting resistance including R FB3  to the feedback sensing network of the regulator  52 . This responsively increases the voltage V out  by a desired amount. The regulator  52  taps a voltage V sec  from the secondary side of the transformer  14  prior to rectification and feeds it to a resistive divide network formed by resistors R int1  and R int2 . The node between this resistive divide network is coupled to the gate of the transistor M 1 . The values of R int1  and R int2  can be chosen empirically to form an integrator with C iss  of the transistor M 1 , which integrates the average value of V sec  and turns on the transistor M 1  gradually. 
         [0028]      FIG. 6A  and  FIG. 6B  are charts of parameters of a 5 W source converter without load before implementing active cable compensation. Referring to  FIG. 6A , there is depicted a chart showing test data using a 5 W (5V out @1 A capability) source converter board with no load using the converter  10  having the regulator  12  without cable compensation as shown in  FIG. 1 .  FIG. 6B  shows test data with no load using the converter  50  having the regulator  52  including cable compensation as shown in  FIG. 5 . As can be seen here, the cable compensation network of regulator  52  of  FIG. 5  could essentially make no difference in no-load operating performance. 
         [0029]      FIG. 7A  and  FIG. 7B  are charts of parameters of a 5 W source board with load before and after V-s integration cable compensation is implemented. Referring to  FIG. 7A  and  FIG. 7B , there are depicted charts showing source voltage test data using the same test board having a load, comparing circuit parameters using the regulator  12  without cable compensation as shown in  FIG. 1  and using the regulator  52  including cable compensation as shown in  FIG. 5 . These charts illustrate the advantageous compensation for the cable wire resistance as the load current V load  increases. 
         [0030]      FIG. 8  is a chart of parameters of a 5 W test board with a load before and after active V-s cable compensation. The 5 W test board is coupled to the 5 W source converter board by a cable with total resistance of approximately 0.3 ohms. In particular, the chart illustrates the load voltage V load  at currents corresponding to those of  FIGS. 7A and 7B  with and without V-s cable compensation for a couple of operating parameters.  FIG. 8  illustrates the effective cable wire compensation achieved using the compensation regulator  52 . 
         [0031]      FIGS. 9 and 10  are diagrams depicting voltages at V out  and V load  from  FIGS. 7A ,  7 B and  8  plotted with respect to load current for two different input parameters. In particular,  FIGS. 9 and 10  illustrate graphs depicting V out  and V load  of the converter  10  without cable compensation and V out  and V load  of the converter  50  with the V-s cable compensation regulator  52 . For the converter  10  including the regulator  12  without cable compensation, the voltages V out  and V load  are shown to linearly decrease as the load current I load  increases, where lines A reflect V out  and lines B reflect V load  without cable compensation. Although V out  declines slightly due to some source impedance, V load  is seen to decline significantly due to the cable resistance. For the converter  50  with the V-s cable compensation regulator  52 , lines C show V out  including the voltage compensation increase as the transistor M 1  turns on for currents over 0.5 A. Similarly, lines D show V load  including the voltage compensation increase based on V out . Lines D show that V load  more closely follows the original source voltage V out  lines A even when the load current increases. 
         [0032]      FIGS. 11-14  are waveform diagrams depicting V out  in response to a load-step with no compensation, in response to an unload-step with no compensation, in response to a load-step with compensation, and in response to an unload-step with compensation, respectivley. In particular, there is shown the load-step on the 5 W board without and with active cable compensation using the V-s integration technique detailed with respect to the converter  50  having the regulator  52  as shown in  FIG. 5 . In all cases, the top waveform depicts V out ac-coupled at 100 mV per division to show the transient response details, and the bottom waveform depicts V load  at 5 V per division at the input to an electronic load through a cable with 0.3-ohm total resistance. 
         [0033]      FIG. 11  shows ac-coupled V out  at 100 mV/div as signal E without compensation due to a positive 1-A load step.  FIG. 13  shows V out  as signal F with compensation due to the same positive 1-A load step.  FIG. 12  shows V out  as signal G without compensation due to a negative 1-A load step, and  FIG. 14  shows V out  as signal H with compensation due to the same negative 1-A load step. The delay shown is an electronic load response time after connection to the source is made, and it is noted that active cable compensation does affect transient response. DC levels of V out  are not shown due to the ac-coupling of the signal. 
         [0034]    Although the above description has described specific embodiments of active cable compensation using V-s integration, various changes may be made to the active cable compensation mechanism. For example, the active cable compensation mechanism is not limited to use with the circuit of  FIG. 5 . Also, the operational characteristics shown in  FIGS. 6A through 14  are examples only and do not limit the active cable compensation mechanism to any particular set of operational characteristics. 
         [0035]    It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more elements, whether or not those elements are in physical contact with one another. The terms “transmit,” “receive,” and “communicate,” as well as derivatives thereof, encompass both direct and indirect communication of information. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrase “associated with,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. 
         [0036]    While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.