Abstract:
A power supply includes signaling circuitry coupled to output terminals for engaging in bidirectional communications with a load. A controller is capable of conducting the bidirectional communications and selecting among different operating modes for the power supply based on the communications. The modes may include a constant-current mode suitable for applications such as battery charging and power LED lamps, and a constant-voltage mode suitable for a variety of conventional uses such as powering electronic circuitry. The signaling circuitry may include a power switching transistor in series with the load, which is pulsed in a binary fashion to transmit communications to the load. A signal-forming resistor in parallel with the power switching transistor develops a signaling voltage monitored by the controller to receive communications from the load.

Description:
BACKGROUND 
       [0001]    The present invention is related to the field of switching power supplies, and in particular to configurable switching power supplies capable of being configured in distinct ways for different applications. 
       SUMMARY 
       [0002]    In a first aspect, a disclosed power supply includes power circuitry configured and operative to provide DC power to a separate load via a pair of conductive terminals, the power circuitry including a power switching transistor in series with the load via one of the terminals, the power switching transistor being closed when the power supply is delivering power to the load. A signal-forming resistor in parallel with the power switching transistor provides a path for signaling current conducted by the load and generates a corresponding signaling voltage when the power switching transistor is open. A controller operates to conduct bidirectional communications with the load via the pair of conductive terminals and generate load-dependent values of configuration parameters of the power supply in response to the communications. The bidirectional communications include (i) in a transmit mode, supplying a pulsed binary control signal to the power switching transistor to convey an outgoing message to the load as a sequence of distinct signaling voltage values across the pair of conductive terminals, and (ii) in a receive mode, maintaining the power switching transistor open and monitoring the signaling voltage to receive incoming messages from the load as a sequence of distinct voltage values developed by the signal-forming resistor in response to corresponding signaling current values generated by signaling action of the load. 
         [0003]    The communications may be used for any of a variety of purposes, including for example to enable the load to control the configuration of the power supply. This functionality can enable a power supply to be used in a desired load-specific manner. 
         [0004]    In another aspect, a disclosed power supply includes power circuitry configured and operative to provide DC power to a separate load via a pair of conductive terminals, the load being of either a first type requiring a first mode of power delivery or a second type requiring a distinct second mode of power delivery. Communications signaling circuitry is coupled to the load via the pair of conductive terminals for bidirectional communications between the power supply and the load, where the bidirectional communications includes (i) transmitting messages as sequences of binary pulses of signaling voltages detected by the load, and (ii) receiving messages by detecting sequences of binary pulses of signaling current conducted by the load. A controller is configured and operative to conduct the bidirectional communications with the load via the communications signaling circuitry and the pair of conductive terminals and, in response to the bidirectional communications, to (1) select from among a set of two or more power delivery modes for the power supply including at least the first power delivery mode and second power delivery mode, the first power delivery mode being selected when the load has communicated that it is of the first type, and the second power delivery mode being selected when the load has communicated that it is of the second type, and (2) configure the power supply to operate in the selected power delivery mode. 
         [0005]    The first and second power delivery modes may be constant-current and constant-voltage modes, respectively, where the constant-current mode is selected based on the load communicating that it is of a corresponding type. In one example the load includes batteries that are charged by the power supply using the constant-current operating mode. The communications signaling circuitry may be implemented in a variety of manners, including switching of a power switching transistor in series with the load. The controller may implement protection by selectively opening the transistor when excessive load current is detected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. 
           [0007]      FIG. 1  is a block diagram of an electrical system; 
           [0008]      FIG. 2  is a schematic diagrams of a power supply; 
           [0009]      FIG. 3  is a part-block, part-schematic diagram of a power supply; 
           [0010]      FIG. 4  is a state diagram; 
           [0011]      FIG. 5  is a waveform diagram for voltage and current during charging. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    A power supply has features for communicating with a load and for adjusting aspects of its own operation, such as output voltage and current, based on communications with the load. The power supply is especially tailored for applications with constant-current loads, such as battery charging or powering light-emitting diode (LED) lamps. 
         [0013]      FIG. 1  shows a system environment. A power supply  10  receives electrical energy from an AC or DC source, such as mains power, a high-power system level supply, etc. The power supply  10  is connected to a load  12  via connections  14 . The power supply  10  provides DC power to the load  12  via the connections  14 , and as further described below the connections  14  are also used for two-way communications between the power supply  10  and load  12 . In one embodiment, the power supply  10  is capable of different modes of operating, and further capable to select a given operating mode in response to communications received from the load  12 . This functionality enables the power supply  10  to have a more general design that may be used in a variety of applications. When used in a given application, the power supply  10  receives communications via the connections  14  (from either a load such as load  12  or some other device capable of engaging in the required communications) that causes control circuitry within the power supply  10  to configure the power supply  10  to operate in a corresponding application-specific manner. Examples are described below. 
         [0014]    In one embodiment, the power supply  10  and load  12  are in distinct physical enclosures and the connections  14  are carried by a connector having specified mechanical and electrical characteristics. In particular, in one application the load  12  is a mobile electromechanical device such as a robotic appliance that operates on rechargeable batteries and is capable of roaming independently for performing a task, then docking with the power supply  10  to recharge its batteries when the appliance is not in use. In one configuration, the connections  14  are formed by conductive contacts located on outer surfaces of the power supply  10  and the robotic appliance. The contacts make physical and electrical contact with each other when the robotic appliance is docked, and charging current and communications signals are then delivered via the contacts. 
         [0015]    Operation of the power supply is now briefly described. Operation includes two specific aspects, namely communications and configuration functionality as well as protection functionality. 
         [0016]    The load  12  communicates with the power supply  10  to enable the load  12  to effectively control the charging mode voltage and current of the power supply  10 . Initially, the load  12  presents an impedance that is much larger than would normally be present during regular operation (e.g., larger than the relatively low impedance of a battery in a charging application). The power supply  10  detects this non-zero, high-impedance load and enters a communication mode. While in communication mode, the power supply  10  monitors the voltage at the VX point with respect to the secondary side ground. The load  12  transmits messages to the power supply  10  by switching off and on an internal communication resistor to represent logic 0 and 1 values. The power supply  10  interprets a sequence of these pulses as a message. Similarly, the power supply  10  asynchronously transmits messages to the load  12  by closing and opening an internal switch, providing pulses corresponding to logic 0 and 1 values. The load  12  interprets a sequence of these pulses as a message. Example circuitry and other details are provided below. 
         [0017]    Once communication is complete, the power supply  10  uses the contents of the received message(s) to configure itself accordingly. A specific example is described below in which the power supply  10  is capable of either constant-current operation or constant-voltage operation. The latter is typical of many applications in which output power is provided in the form of a predetermined constant output voltage and a generally variable current drawn by the load. In contrast, constant-current operation provides a predetermined current at a generally variable voltage. Constant-current operation is useful in certain applications including battery charging and powering LED lamps. Thus in a battery charging application such as described above (battery powered robotic device), after completion of communication the power supply  10  enters a constant-current charging mode. 
         [0018]    Another aspect of operation is protection, which may have both internal and external aspects. The internal aspect is to protect against damage to internal components such as the switch Q 1  by detecting certain fault conditions and responding accordingly, e.g., opening the switch Q 1  to interrupt current. There may also be conditions in which dangerous external arcing might occur absent suitable protection, and the protection circuitry addresses this aspect as well. Additional details regarding protection are given below. 
         [0019]      FIG. 2  is an electrical schematic diagram of the power supply  10 . It includes a power transformer  20  having DC-isolated primary and secondary sides. On the primary side is a primary power stage and controller  22  that receives the system-level input voltage (AC or DC) and generates AC in the transformer  20 . The secondary side includes a rectifying diode  24 , filter capacitors Cx and a filtering transformer  26  through which DC output current flows to the separate load via terminals shown as “+” and “−”. An error amplifier and reference circuit  28  generates a feedback signal sent from the secondary side to the primary power stage and controller  22  via an isolation device  30  such as an opto-isolator. 
         [0020]    The secondary side contains additional circuitry including a protection switch Q 1  in the form of a power field-effect transistor (FET), a current sense resistor Rsns in series with the load, and a multi-purpose resistor Rchk. A secondary side controller  32  performs a variety of control functions which include controlling operation of the switch Q 1  and modifying operation of the primary side via the error amplifier and reference circuit  28 . The controller  32  receives a signal lout from the sense resistor Rsns as an indication of a level of output current. It also receives the output voltages Vout (+) and Vx (−). 
         [0021]      FIG. 3  is a schematic diagram of a power supply  10 ′ at a somewhat higher level and also having additional secondary side circuitry. The basic power supply circuitry such as primary power stage, transformers, filter capacitors, etc. are represented by a component labelled Supply  40 . The controller  32  is shown as a separate component, as well as the circuitry at the output such as switch Q 1  and transistors Rchk and Rsns. As shown, this output section also includes a second circuit made up of switches Q 2  and Q 3 , diode D 1 , and resistors including a limit resistor Rlim. The above description of operation of the supply  10  is also applicable to the supply  10 ′. 
         [0022]    The power supply  10 ′ has an analog trim function (controlled via an external connection TRIM) that sets the power supply output voltage over a specific range by application of a trim voltage. This trim voltage is established by filtering a digital PWM signal generated by the controller  32 . The controller  32  also monitors Vout, Vx, and Isns via respective analog-to-digital (ADC) inputs, and controls Q 1  and Q 2  using signals Q 1 _ON and Q 2 _ON. 
         [0023]    Regarding the trim function, the controller  32  executing firmware can control the output voltage of the supply by calculating a difference between Vout and Vx and adjusting the duty cycle of the PWM signal to drive the difference to a desired value. This is a constant voltage mode of operation. The controller firmware can alternatively control output current by monitoring the Isns signal (proportional to output current) and adjusting the PWM signal so that the supply generates an output voltage that results in a desired output current through Rsns. This is a constant current mode of operation. The controller firmware can decide based on a desired algorithm whether to control current or voltage in described manner. These algorithms can be used to establish different charging conditions for various battery chemistries, for example. Algorithms can also be implemented to control the current and voltage for a load  12  in the form of a string of LED lamps. The controller  32  can decide when to turn on/off Q 1  and/or Q 2  based on these algorithms. 
         [0024]    Prior to the application of load, Q 1  and Q 2  are off, and the output voltage rises to a high open-circuit value. When a load is applied, outrush current is limited by Rchk, which can be sized to limit arcing across the output terminals. In one embodiment Rchk has a value of 3.9 Kohms. It will be appreciated that having such a suitable value, Rchk can play two distinct roles—limiting outrush current upon initial application of the load  12 , and developing appropriate levels of the voltage Vx in response to the signaling switching of the load to enable the controller  32  to receive communications from the load  12 . 
         [0025]    When Q 1  is on and the voltage across Rsns is very low, indicating a light load, it may not be possible to determine load presence due to insufficient resolution of the ADC within the controller  32  for the Isns signal. In this case, the controller  32  can turn Q 2  on and Q 1  off. If a light load is present, current will flow through D 1  and Q 2 , and the voltage drop across these devices can be monitored at the Vx node. If load is completely removed the voltage Vx drops to near zero because there is no current to bias D 1  and Q 2 . If this condition is detected the controller  32  can then turn off Q 2  and Q 1  in preparation of establishing a future non-arcing connection. The controller  32  can also use this condition as a signal to wait for external communication. 
         [0026]    The controller may implement protection against short circuits when Q 1  is on. It should be noted that it is important to distinguish between the fault condition of a short circuit and the normal condition of providing charging current to a heavily discharged battery. This can be done by monitoring the output voltage Vx. As long as it stays below a certain threshold, it is inferred that operation is normal. Once Vx rises above that threshold, then it can be inferred that a short circuit is occurring. The value for the threshold may vary in different embodiment; in one example it is 2.5 V. Also, it may be desirable to include a brief waiting period after first detecting the above-threshold Vx and then a second sampling. If Vx remains too high, then the short circuit fault condition can be declared, whereas otherwise it is not. In this way, very brief transitory spikes of output current are tolerated and do not trigger a fault. 
         [0027]      FIG. 4  and Table 1 below provide a specific example of control operation of the controller  32 .  FIG. 4  shows operating states or modes as Idle  50 , Constant Current  52 , Constant Voltage  54 , Determine Load  56 , and Communication  58 . Table 1 summarizes state transitions as well as operating conditions in each state. The table presents values for various operating parameters for an embodiment providing a nominal output voltage Vout of 12 VDC and a power rating of 40 W. Those skilled in the art will appreciate how the parameters can be adjusted based on other characteristics of the power supply  10  and load  12 . 
         [0000]    
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                 Output  
                   
                   
               
               
                   
                   
                 Switch  
                 Switch  
                 Voltage/  
                 Transition  
                   
               
               
                 State 
                 Description 
                 Q2 
                 Q1 
                 Current 
                 Condition 
                 Next State 
               
               
                   
               
             
             
               
                 Idle 
                 Wait for load  
                 Open 
                 Open 
                  19 V  
                 Detect load  
                 Default Mode 
               
               
                   
                 to connect 
                   
                   
                 (open ckt.) 
                 (&lt;50 k) 
                   
               
               
                 Constant  
                 Maintain constant  
                 Closed 
                 Open 
                 Constant  
                 Drop in output  
                 Determine Load 
               
               
                 Current Mode 
                 output current 
                   
                   
                 specified  
                 current  
                   
               
               
                   
                   
                   
                   
                 current 
                 (min. dwell) 
                   
               
               
                 Constant  
                 Maintain constant  
                 Closed 
                 Open 
                 Constant  
                 Drop in output  
                 Determine Load 
               
               
                 Voltage Mode 
                 output voltage 
                   
                   
                 specified  
                 current  
                   
               
               
                   
                   
                   
                   
                 voltage 
                 (min. dwell) 
                   
               
               
                 Determine  
                 Sense if  
                 Open 
                 Closed 
                 8 V to 18 V 
                 Load &gt;100 mA and  
                 Constant Current 
               
               
                 Load 
                 accidental or  
                   
                   
                   
                 was in Constant  
                   
               
               
                   
                 transitory  
                   
                   
                   
                 Current Mode 
                   
               
               
                   
                 disconnect or if  
                   
                   
                   
                 Load &gt;100 mA and  
                 Constant Voltage 
               
               
                   
                 load wants to  
                   
                   
                   
                 was in Constant  
                   
               
               
                   
                 enter Comm.  
                   
                   
                   
                 Voltage Mode 
                   
               
               
                   
                 Mode 
                   
                   
                   
                 Load &lt;100 mA 
                 Communication 
               
               
                   
                   
                   
                   
                   
                 (non-transient) 
                   
               
               
                 Communication 
                 Receive and  
                 Open 
                 Open 
                 7.5 V 
                 No load  
                 Idle 
               
               
                   
                 respond to  
                   
                   
                   
                 (&gt;100 k) or Idle  
                   
               
               
                   
                 messages from  
                   
                   
                   
                 Command 
                   
               
               
                   
                 load 
                   
                   
                   
                 (a) Constant  
                 Constant Current  
               
               
                   
                   
                   
                   
                   
                 Current Request 
                 Mode 
               
               
                   
                   
                   
                   
                   
                 Command or  
                   
               
               
                   
                   
                   
                   
                   
                 (b) No  
                   
               
               
                   
                   
                   
                   
                   
                 communication 
                   
               
               
                   
                   
                   
                   
                   
                 within timeout 
                   
               
               
                   
                   
                   
                   
                   
                 Constant  
                 Constant Voltage  
               
               
                   
                   
                   
                   
                   
                 Voltage Request  
                 Mode 
               
               
                   
                   
                   
                   
                   
                 Command 
               
               
                   
               
             
          
         
       
     
         [0028]    The transition out of Idle is shown in Table 1 as entering a default mode. In the example of  FIG. 4 , the default operating mode is the constant current mode  52 . In other embodiments some other default operating mode may be used. 
         [0029]    The transitions from either mode  52  or  54  into Determine Load  56  are shown as “load disconnect” in  FIG. 4  and with the term “minimum dwell” in Table 1. Disconnect is inferred from a correspondingly low value of load current; specific examples are given below. The “minimum dwell” signifies that a predetermined period (e.g., 1.5 seconds) is allowed to pass after entering the state before sampling the output current. The transitions in the other direction (from Determine Load  56  to either mode  52  or  54 ) occur when load current is reestablished and it is inferred that the disconnect was accidental or transitory. The transition to Communication  58  is based on the load current being less than 100 mA for some predetermined period such as 2 seconds. The transition from Communication back to Idle occurs when load current drops below 0.5 mA. The transition from Communication to the default mode (Constant Current in this case) occurs when a timeout (e.g., 5 seconds) elapses without receiving any communication from the load. 
         [0030]    Table 2 presents values for various operating parameters according to one embodiment. These may be suitable for the above-described 12 V, 40 W power supply. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Parameter 
                 Value 
                 Notes 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Load detection 
                 0.5-2.5 
                 mA 
                 8 k to 40 k impedance 
               
               
                 Input logic 0 
                 2.5 
                 mA 
                 3 k nominal impedance 
               
               
                 Input logic 1 
                 0.5 
                 mA 
                 15 k nominal impedance 
               
               
                 Output logic 0 
                 12 
                 V 
               
               
                 Output logic 1 
                 7.5 
                 V 
               
               
                 Bit rate 
                 75 
                 Baud 
               
               
                 Message timeout 
                 0.25 
                 S 
                 Timeout for gap between stop 
               
               
                   
                   
                   
                 bit and next start bit (error) 
               
               
                 Mode Timeout 
                 5.0 
                 S 
                 Timeout for awaiting load 
               
               
                   
                   
                   
                 communication before 
               
               
                   
                   
                   
                 entering default mode 
               
               
                   
               
             
          
         
       
     
         [0031]    As mentioned above, the ability of the power supply  10  to self-configure based on communications it receives via the output terminals enables the power supply  10  to have a more general design that may be used in a variety of applications. When used in a given application, the power supply  10  receives communications via the connections  14  (from either a load such as load  12  or some other device capable of engaging in the required communications) that causes control circuitry within the power supply  10  to configure the power supply  10  to operate in a corresponding application-specific manner. This type of operation is usable even if the power supply is implemented in a completely sealed manner, such as by encapsulation, as is required for certain applications. It is not necessary to set mechanical switches or jumpers to achieve a desired configuration. The power supply  10  can be stocked in quantity, and for a given application a manufacturer or distributor may employ a programming device to connect to the supply via the output terminals and send application-specific communications to the supply to enable it to self-configure as necessary. 
         [0032]      FIG. 5  illustrates a use of the disclosed communications technique. In this example, a single charging operation includes 4 sequential modes of operating, shown as A, B, C and D. The load  12  has communicated specific charging requirements specific to its battery chemistry and capacity, and the power supply  10  sets up parameters for the various charging modes accordingly. In this example, mode A provides a first constant current value until the output voltage reaches a defined voltage level. In mode B a different constant current value is provided until the output reaches a second defined voltage level. Mode C is a constant-voltage mode providing a constant voltage level until the output current tapers to a defined value, and mode D is a final constant voltage level. The charge parameters may be communicated to include some or all of these modes, and it is also is possible that in Mode D the power switch (Q 1 ) is shut off to remove charging voltage. With configuration by way of communication, the power supply  10  can accommodate charging curves for potentially a large number of distinct battery chemistries. 
         [0033]    While various embodiments of the invention have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.