Abstract:
An apparatus and method for load sharing among N current supplies, where N&gt;1. N current supply paths are coupled to corresponding N independent power sources, respectively. A system load is coupled to the outputs of the N current supply paths to receive N current supplies. There is a common current share bus configured to connect to the N current supply paths to provide a common current share signal, used to indicate the current contribution needed from each of the N current supply paths. In this configuration, each of the N current supply paths adjusts an adjustable voltage drop between its power source and the current supply it provides to the system load in accordance with the common current share signal so that the current supplied from each current supply path is consistent with the common current share signal.

Description:
RELATED APPLICATION 
       [0001]    The present invention claims priority of provisional patent application No. 61/141,766 filed Dec. 31, 2008, the contents of which are incorporated herein in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present teaching relates to method and system for analog circuits. More specifically, the present teaching relates to method and system for power supply load sharing and systems incorporating the same. 
         [0004]    2. Discussion of Technical Background 
         [0005]    Connecting the output of two or more power supplies together allows them to share a common load current. Load sharing has various advantages. Load sharing puts less thermal stress on each individual power supply&#39;s components, thus increasing the overall power system&#39;s reliability and lifetime. It allows smaller power supplies to be used in parallel to supply a larger load. In systems with time-varying load currents supplied by dynamically-managed parallel supplies, load sharing enables each supply to be operated at its peak power-conversion efficiency point. High-availability electronic systems usually employ an N+n configuration of power supplies, where N is the number of supplies that are required to supply the load current and n is the number of extra or redundant supplies. Load sharing is often an essential feature of such systems. 
         [0006]    The division, or sharing, of the load current between the supplies depends on their individual output voltages and the connection resistance to the common load. This is called droop sharing. To prevent reverse current into a supply, which is called back-feeding, a diode can be added in series with each supply output. In this case, the diode voltage drop also influences the current sharing between supplies. 
         [0007]    An active method of load sharing is implemented in a load sharing controller from Linear Technology Corporation. In this load sharing controller, the current from each power supply is monitored by sensing the voltage drop across a series current sense resistor. The load sharing controller compares this current sense signal against a share bus signal. The share bus signal indicates the load current needed per supply to maintain regulation of the load voltage. The load sharing controller then adjusts the output voltage of the supply via the supply&#39;s feedback network or trim input to match its current to the share bus, thus achieving load sharing. 
         [0008]    Texas Instruments offers device UCC39002, designed to achieve load sharing by adjusting the power supply voltage. The share bus signal indicates the highest of all the supply currents. National Semiconductor Corporation provides a device that uses a share bus that is the average of all the supply currents. 
         [0009]    Those traditional approaches to load sharing have some disadvantages. Although droop sharing is simple, sharing accuracy can be hard to control. While the back-feeding problem is solved with one or more diodes connected in series, the diode itself wastes power. Although the load sharing controller from Linear Technology Corp. and other existing controller devices solve these problems, designs based on such conventional technologies can be complicated because one has to accommodate the power supply loop dynamics into the load sharing loop dynamics, with each supply requiring custom loop stability compensation. Additionally, these controllers can only manipulate supplies with a trim/adjust pin or an accessible feedback network. This may not be readily available, or there may be noise injection concerns about routing this signal on a circuit board. Furthermore, the need to route the share bus signal to all the supplies also introduces a potential single point of failure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The inventions claimed and/or described herein are further described in terms of exemplary embodiments. These exemplary embodiments are described in detail with reference to the drawings. These embodiments are non-limiting exemplary embodiments, in which like reference numerals represent similar structures throughout the several views of the drawings, and wherein: 
           [0011]      FIG. 1  depicts an exemplary circuit to control load sharing in accordance with an embodiment of the present teaching; 
           [0012]      FIG. 2  depicts a more detailed circuit implemented to control load sharing in accordance with an embodiment of the present teaching; 
           [0013]      FIG. 3(   a ) shows plots of voltage levels of different command voltage sources in controlling load sharing, in accordance with an embodiment of the present teaching; 
           [0014]      FIG. 3(   b ) show plots of the supply currents normalized to the load current, in accordance with an embodiment of the present teaching; 
           [0015]      FIG. 4  provides an exemplary implementation of a circuit for controlling load sharing, in accordance with an embodiment of the present teaching; 
           [0016]      FIGS. 5(   a ) and  5 ( b ) show alternative implementations of a controllable series voltage drop using power MOSFETs, in accordance with embodiments of the present teaching; 
           [0017]      FIG. 6  depicts another exemplary circuit to control load sharing in accordance with an embodiment of the present teaching; 
           [0018]      FIG. 7  depicts yet another exemplary circuit to control load sharing in accordance with an embodiment of the present teaching; and 
           [0019]      FIG. 8  depicts an exemplary circuit for N-supply load sharing, in accordance with an embodiment of the present teaching. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present teaching relates to load sharing control schemes and implementations thereof to enable two or more power supplies to share a single load current. The load sharing control scheme as described herein is voltage independent in the sense that the common mode of the supply voltages does not affect load sharing. In addition, the load sharing control scheme, as disclosed herein, does not require a trim/adjust pin on the power supply. The load sharing control scheme of the present teaching does not need to be physically close to the power supply and the power supply loop dynamics do not need to be figured into the design, allowing it to work with a wide variety of input supplies. The load sharing method and system as disclosed herein also blocks reverse currents to prevent potential damage caused by back-feeding. All of these features achieved by the present teaching lead to a simpler and faster design of a load sharing power supply system. 
         [0021]      FIG. 1  depicts an exemplary circuit  100  designed to control load sharing, in accordance with an embodiment of the present teaching. For illustration purposes, the circuit  100  is used to show the concept of the present teaching based on a two supply system. As will be seen later ( FIG. 8 ) and understood by a person skilled in the art, the present teaching is not limited to a two supply system and can be applied for load sharing in an N-supply system. 
         [0022]    In  FIG. 1 , circuit  100  includes power supplies,  110  and  104 , that are connected in parallel to supply load  107 . As shown, power supply  110  provides V IN1 , which goes through a path which comprises an adjustable voltage drop  101  that produces an intermediate voltage level V OUT1 , and a current sense resistor  102 . Similarly, power supply  104  provides V IN2 , which goes through another parallel path which comprises an adjustable voltage drop  105  that produces an intermediate voltage level V OUT2 , and a current sense resistor  106 . It is noted that a voltage drop as described herein can be either a positive or a negative value. Both the adjustable voltage drops ( 101  and  105 ) and current sense resistors ( 102  and  106 ) are inserted in the parallel power supply paths between each supply and the common load  107  with voltage V LOAD . The voltage drop of each of the adjustable voltage drops  101  and  105  can be dynamically controlled. The current sense resistors  102  and  106  can either be explicit sense resistors or the resistance inherent in the circuit board traces. 
         [0023]    Along each of the power supply paths, the original voltage provided from its power source may differ. This may subsequently cause the intermediate voltages to vary, i.e., V OUT1  does not equal V OUT2 . This will further lead to unequal currents through the current sense resistors  102  and  106  and, hence, the unequal supply of current from each power supply path to the common load. To dynamically produce an equal contribution of current to the common load from each power supply path, the sensed difference between V OUT1  and V OUT2  is fed to a current balancing control element. In some embodiments, such a current balancing control element can be implemented based on an error amplifier device shown in  FIG. 1  ( 103 ), which is deployed to adjust the voltage drop on  101  and/or  105  based on the sensed difference in current through the current sense resistors. 
         [0024]    The error amplifier device  103  controls the series voltage drops  101  and  105  to force V OUT1  to equal V OUT2 . For example, when the output voltage of supply  110  rises, it temporarily causes an increase of V OUT1  above V OUT2 . When the error amplifier device  103  receives input (via the current variation sensed by the current sense resistors  102  and  106 ) indicating such, as a corrective measure, the error amplifier device  103  raises the voltage drop of  101  to equalize V OUT1  and V OUT2 . In some embodiments, the adjustment of voltage drop can be performed in both paths, i.e., it raises the voltage drop of  101  while lowering the voltage drop of  105  to again equalize V OUT1  and V OUT2 . By making the adjustment on the voltage drops to ensure that V OUT1 =V OUT2  in different power supply paths, the current from supply  110  is the same as the current in current sense resistor  102 , which is (V OUT1 −V LOAD )/R 102 . Similarly, the current from supply  104  is the same, i.e., (V OUT2 −V LOAD )/R 106 , assuming R 102 =R 106 . In this way, the two supply currents turn out to be equal. Since currents from both power supply paths add up to the load current I 107 , each of the paths provides half of the load current i.e., I 110 =I 104 =I 107 /2. In this manner, the two supplies share the load current equally. In some applications, a ratiometric current sharing may be needed. In those situations, the resistances of the two current sense resistors  102  and  106  can be appropriately set in accordance with a ratio so that I 110 /I 104 =R 106 /R 102 . 
         [0025]    In operation, reverse current may occur in a power supply path. For example, reverse currents may flow when a power supply is at a lower potential than the common load bus. It is commonly known that a large reverse current can damage the power supply. In addition, reverse currents waste power. Hence, it is often necessary to block reverse currents.  FIG. 2  depicts a more detailed circuit  200  implemented to control load sharing in accordance with an embodiment of the present teaching. The circuit  200  as illustrated is capable of preventing reverse current and protecting the power supply from any damage caused by reverse current. 
         [0026]    Circuit  200  is constructed similarly to circuit  100  except that the voltage drop is implemented in a certain way, according to some embodiment of the present teaching. Specifically, circuit  200  comprises two power supply paths, each of which has a power supply source ( 201  and  209 ), an adjustable voltage drop, and a current sense resistor ( 203  and  211 ). The currents from both power supply paths flow to a common load  212 . Each of the adjustable voltage drops is implemented based on an N-channel MOSFET ( 202  and  210 ), a command voltage source ( 205  and  207 ), and a servo amplifier ( 204  and  208 ). In operation, the error amplifier device  206  equalizes V OUT1  and V OUT2  by controlling the voltage sources  205  and  207 . In the top power supply path, the N-channel MOSFET  202 , the command voltage source  205 , and the servo amplifier  204  achieve the function of an adjustable voltage drop in the following manner. The servo amplifier  204  controls the gate voltage of the N-channel MOSFET  202  based on the intermediate voltage V OUT1  and the command voltage source  205  (which is controlled by the error amplifier device  206 ). Specifically, the servo amplifier  204  is used to keep the forward voltage drop (the source to drain voltage) across the N-channel MOSFET equal to the voltage source  205 . The other power supply path is similarly controlled via servo amplifier  208 , N-channel MOSFET  210 , and voltage source  207 , which is controlled by the error amplifier device  206 . 
         [0027]    There are some practical considerations in terms of limiting the range of the MOSFET voltage drop. To prevent reverse currents flowing through the MOSFET, the voltage drop should not be allowed to go below zero. In general, it is preferred to limit the voltage drop to a small positive value (V F(MIN) ), such as 30 mV. The maximum voltage drop (V F(MAX) ) across a single MOSFET may also be limited, e.g., to a diode drop by an intrinsic body diode between the source and drain of the MOSFET. To allow a larger voltage drop, a series of MOSFETs may be employed in place of a single MOSFET. This is shown in  FIG. 5(   a ), where two or more MOSFET ( 510 ,  520 , and  530 , as illustrated) can be connected in series to replace each single MOSFET  202  and  210 . Alternatively, two MOSFETs can also be connected back to back, as shown in  FIG. 5(   b ) ( 540  and  550 ) to achieve the same. Although the back to back connection in  FIG. 5(   b ) is shown to be source to source connection, drain to drain connection is also feasible to achieve the same functionality (not shown). 
         [0028]    The maximum voltage drop is also limited by the MOSFET&#39;s power handling capacity, i.e., V F(MAX) &lt;P D(MAX) /I 212 , where V F(MAX)  is the maximum forward voltage drop, I 212  is the load current in  FIG. 2 , and P D(MAX)  is the maximum safe power dissipation in the MOSFET.  FIG. 3(   a ) illustrates how voltage sources  205  and  207  vary between these two limits with the difference between the two input supply voltages due to the action of amplifier  206 . When the two input supply voltages are equal (V IN1 −V IN2 2=0) both MOSFETs have the minimum voltage drop, V F(MIN) . When V IN1  rises above V IN2 , the drop across MOSFET  202  is increased, by amplifier  206 , to match the rise in V IN1 . The drop across MOSFET  210  is maintained at V F(MIN) . Thus, V OUT1  remains equal to V OUT2 . Therefore, the two supply currents are equal if R 203  equals R 211 . The forward drop across MOSFET  202  reaches V F(MAX)  when the difference between the two supply inputs is V F(MAX) −V F(MIN) . If V IN1  continues to rise above V IN2 , V OUT1  also rises above V OUT2 . Because of this, the current of supply  201  increases and that of supply  209  decreases. As the input difference increases further, a point is reached wherein the entire load current transfers to the higher supply. This is shown in  FIG. 3(   b ). The current sharing behavior is symmetric when V IN2  increases above V IN1 . 
         [0029]    In some embodiments, the load sharing scheme as described herein can be further enhanced by reducing the forward drop of the MOSFET back to minimum once the two supply voltages are separated enough that only one is supplying the entire load current. In this situation, there is no further need to maintain the maximum drop across the MOSFET especially because it wastes power. In some embodiments, this can be achieved by detecting when the MOSFET in the lower supply path has switched off. The gate signal of this MOSFET can be used to switch the drop of the conducting MOSFET back to V F(MIN) . 
         [0030]    In some situations, however, such a quick or sharp lowering of the drop may cause the load voltage to jump up which may be undesirable. An alternative approach, in some embodiments is to adopt a softer method which can use a low-gain difference amplifier (not shown) between V IN1  and V IN2  to reduce the drop gently back to V F(MIN)  after the difference has exceeded V F(MAX) −V F(MIN) . In some embodiments, another alternative approach is to reduce the forward drop so that the power dissipated in the MOSFET will not exceed V F(MAX) ·I 212 /2. 
         [0031]      FIG. 4  provides an exemplary implementation of a circuit  400  for controlling load sharing, in accordance with an embodiment of the present teaching. The MOSFET forward drop along both power supply paths is designed based on the servo amplifier function, as described herein, and is implemented using devices  433  and  423 . Device  433  comprises a servo amplifier  404 , which has a built-in 30 mV reference  413 , and an N-channel MOSFET  402 . Device  423  comprises the same, a servo amplifier  408  which has a built-in 30 mV reference  422 , and an N-channel MOSFET  410 . As discussed herein, the N-channel MOSFET can be replaced with a series of MOSFETs for an improved forward drop range, as shown in  FIGS. 5(   a ) and  5 ( b ). In addition, the built-in 30 mV is merely illustrative and this built-in value may change with the needs of applications. This built-in value can be dynamically changed to a forward drop command, which is then used by the servo amplifier to adjust the MOSFET forward drop to equalize the current in each path. 
         [0032]    In both power supply paths, the forward drop command can be increased above 30 mV by passing a current through a resistor  405  or  407  (both are illustrated to have a value of 200Ω). The increase can be made up to a maximum of 1 mA, which provides a V F(MAX)  of 230 mV (1 mA·200Ω+30 mV). Here, the exemplary V F(MIN)  is 30 mV. The error amplifier  406  may correspond to a device, which integrates the error amplifier  406  with a capacitor  420  and a resistor  421 . The output of the error amplifier  406  controls the current flowing through resistors  405  and  407 , which sets up the forward drop commands in combination with  413  and  422 , which are then used by the two servo amplifiers  404  and  408  to control the forward voltage drops on the MOSFETs  402  and  410 . 
         [0033]    Circuit  400  also comprises four PNP transistors,  416 ,  417 ,  418 , and  419 , and the two reference voltages, 19V and 20V. These additional components in circuit  400  are deployed to make certain that when one forward drop command is being raised the other one stays at the minimum 30 mV as shown in  FIG. 3(   a ). In operation, when V OUT1  rises above V OUT2 , the output of error amplifier  406  rises accordingly in voltage. This drives up the base of two PNP devices,  417  and  418 . When the output of amplifier  406  reaches 20V, current from source  414  is diverted from PNP  417  towards PNP  416 . Such a current flows through resistor  405  raising the forward drop command voltage to servo amplifier  404 . The amplifier  404  then pulls down the gate of MOSFET  402  to increase its on resistance. This brings down V OUT1  and the loop eventually makes it equal V OUT2 . When the base of PNP  418  reaches about 20V, all the current from source  415  flows through PNP  419  and, hence, no longer flows through  418 . Thus the command voltage for servo amplifier  408  stays at the minimum 30 mV. 
         [0034]    In some embodiments, circuitry can be added to implement a fast turn-on and fast turn-off of the MOSFETs  402  and  410 . Such circuitry may comprise a forward and a reverse comparator that monitor the drop across the MOSFET. If the forward drop (the source to drain voltage) exceeds a threshold, then the forward comparator would trip triggering a fast turn on of the MOSFET. This limits the load voltage droop. The reverse comparator monitors the reverse voltage (drain to source voltage), and when the reverse voltage exceeds a threshold, the reverse comparator then quickly turns off the MOSFET. This limits the amount of reverse current that can flow into the power supply. 
         [0035]    In the above illustrative embodiments (circuit  200  and  400  and what is shown in  FIGS. 5(   a ) and  5 ( b )), specific implementations are provided. For example, MOSFETs are used to implement the adjustable voltage drop and current sense resistors are used for sensing the current. As a person skilled in the art would understand, those functionalities may be realized using other implementations. For instance, a series voltage drop may also be implemented based on other devices such as JFETs. In addition, to infer the current contribution of a supply, other methods such as a current sense amplifier, hall effect sensor, flux gate, transformer, or current monitor output from the supply may be used. Furthermore, the functionality achieved by PNP devices  416 - 419 , as shown in  FIG. 4 , can also be implemented using other devices such as P-channel MOSFETs. 
         [0036]      FIG. 6  depicts another exemplary circuit  600  as an alternative implementation of circuit  100  to control load sharing in accordance with an embodiment of the present teaching. In this illustrated embodiment, all elements are similarly configured as what is shown in  FIG. 1  except the current sensing parts of the circuit. In  FIG. 1 , current sense resistor  102  and  106  are employed to sense the current flowing from V OUT1  and V OUT2  to V LOAD , respectively. In  FIG. 6 , for sensing the current flowing from V OUT1  to V LOAD , current sensing amplifier  608  and resistor  610  are employed with current sense resistor  602  in the top power supply path. Similarly, for sensing the current flowing from V OUT2  to V LOAD , current sensing amplifier  609  and resistor  611  are employed with current sense resistor  606  in the bottom power supply path. In this circuit  600 , an amplifier ( 608  or  609 ) can translate a high common-mode differential current-sense signal from a current sense resistor to a single-ended ground referenced signal. The voltage-drop adjusting error amplifier  603  can take these ground referenced signals from the two supply paths as its inputs. With this sensing scheme, the circuit  600  is immune to parasitic trace resistance between sense resistors  602  and  606  and the load  607 . It is noted that a current sensing element as described herein can be located anywhere along a current supply path. 
         [0037]      FIG. 7  depicts yet another alternative implementation of circuit  100  to control load sharing in accordance with an embodiment of the present teaching. In operation, when the top supply voltage  701  (V IN1 ) is higher than that of  711  (V IN2 ), V OUT1  becomes higher temporarily as compared with V OUT2 . This leads to a situation where the current flowing through the current sense resistor  703  is higher than that flowing through the current sense resistor  713 . This further causes PNP device  704  to take a greater share of the 100 uA current source  709  than PNP device  714 . Such additional current is then mirrored in PNP device  705  and in NPN device  707 . This additional current thus pulls down on the gate of the N-channel MOSFET  702 , lowering its gate voltage and making it more resistive. As a consequence, this brings V OUT1  back down to make it closer to V OUT2  and thus restores balanced current sharing. 
         [0038]    In this illustrated scheme, when the MOSFET&#39;s resistance on a higher supply path is being adjusted, the other MOSFET (on the lower supply path) is turned completely on to a low resistance state. One potential issue associated with this implementation is that reverse current may occur and back-feeding of the lower supply may happen when the two supplies diverge. To solve this problem, exemplary solutions to block reverse current such as what is shown in  FIG. 4 , may be employed in conjunction with the circuit as shown in  FIG. 7 . 
         [0039]    Although what is disclosed so far involves a two power supply system, as discussed herein, the present teaching can be extended to an N-supply system.  FIG. 8  depicts an exemplary circuit for N-supply load sharing, in accordance with an embodiment of the present teaching. In this embodiment, there are N supplies  800 , . . . ,  807  providing V IN1 , . . . , V INN  to a common load  814  with voltage V LOAD . There are N power supply paths, each of which comprises a current sensing sub-circuit, an error amplifier, and an adjustable voltage drop. For example, in the first supply path, the current sensing sub-circuit comprises a sensing element around a current sensing resistor  802 , where the current sensing element includes an amplifier  803  that takes inputs from two sides of the current sensing resistor  802  and converts the two signals into one, as disclosed herein. The current sensing sub-circuit also includes two resistors  804  and  805 . The single signal converted by the amplifier  803  is sent, as an input, to the error amplifier- 1   806 , which adjusts the voltage drop across element  801 . The other input of the error amplifier- 1   806  is a specified level of current expected to be contributed from the first supply path. Circuit for each of the other supply paths can be similarly constructed, as shown in  FIG. 8 . 
         [0040]    With this implementation of an N-supply system, a common current share signal is needed to indicate the current contribution needed from each supply. As shown in  FIG. 8 , every supply path includes an error amplifier ( 806 , . . . ,  813 ) that has this common current share signal as input and compares with the signal from the current sensing circuit. Based on the difference between the two input signals, the error amplifier in each supply path tries to make its supply current equal to the common current share signal by adjusting the MOSFET forward drop. There are different methods to determine the common current share signal and different circuits to implement these methods. For example, the common current share signal can be generated by an average of all the supply currents. It may also be generated by dividing the common load current or I LOAD  by N. The common current share signal may also be determined as the highest of all the supply currents. A voltage share bus may also be added to minimize the common mode of all the voltage drops. 
         [0041]    If there is a need to utilize an error signal to indicate a break in load sharing, it can be easily achieved via different approaches. In some embodiments, it can be achieved by monitoring the output of each of the error amplifiers in each individual supply path. Such monitoring can be dynamically performed on-the-fly based on the behavior of the error amplifiers. In a two-supply system, when one of the error amplifiers is railed, the load is no longer being shared but is now flowing from only one supply. In an N-supply system, a railed error amplifier implies that a supply from that path is no longer contributing its required share of current. 
         [0042]    While the inventions have been described with reference to the certain illustrated embodiments, the words that have been used herein are words of description, rather than words of limitation. Changes may be made, within the purview of the appended claims, without departing from the scope and spirit of the invention in its aspects. Although the inventions have been described herein with reference to particular structures, acts, and materials, the invention is not to be limited to the particulars disclosed, but rather can be embodied in a wide variety of forms, some of which may be quite different from those of the disclosed embodiments, and extends to all equivalent structures, acts, and, materials, such as are within the scope of the appended claims.