Fault tolerant active current sharing

[0002] 1. Field of the Invention

[0003] The present invention relates in general to regulating power supplies, and in particular to a system and method for load-balancing within a parallel power supply. Still more particularly, the present invention relates to an open-loop system in which an external voltage source provides an independent reference for balancing a current load within a parallel power supply.

[0004] 2. Description of Related Art

[0005] Conventional power supply systems often utilize multiple power supply circuits connected in parallel for improving load variation adaptability and increased reliability. Examples of such parallel power supply systems can be found in Malik, U.S. Pat. No. 5,319,536, Gerner, U.S. Pat. No. 5,266,838, and Line; U.S. Pat. No. 5,745,670.

[0006] Referring now to FIGS. 1, 2, and 3, conventional parallel power supply systems 101, 201, and 301 are illustrated. As shown in FIG. 1, parallel power supply system 101 includes two parallel power supply modules, PS1102 and PS2104 that provide independent sources of power to a system load 106. PS1102 and PS2104 generate output currents I1108 and I2110, respectively. Included among the advantages provided by the parallel configuration of power supply system 101 over a unitary power supply system are reduced power supply component costs, enhanced operational flexibility, and improved system reliability.

[0007] In the interest of uniformity of device stress and corresponding reduced device failure rates, it is desirable to maintain a balanced power output provided by parallel power sources such as PS1102 and PS2104. FIGS. 2 and 3 illustrate conventional systems for achieving such a balance under varying electrical loads.

[0008] As depicted in FIG. 2, power supply system 201 includes parallel power supply modules PS1202 and PS2204 that operate in a “droop” current sharing mode in supplying power to a system load 206. PS1202 and PS2204 maintain a required system voltage level, V_OUTat a common output node 215. Decoupling diodes D1212 and D2214 serve as decoupling barriers between supply-side nodes 217 and 219 and common output node 215. In order to achieve the desired operating balance between PS1202 and PS2204, the current, I1, through node 217 should be approximately equal to the current, I2, flowing through node 219 (one half of the I_Tflowing through common output node 215).

[0009] As further illustrated in FIG. 2, power supply system 201 includes circuitry within each of the power supply modules 202 and 204 for maintaining a balance in the relative current load borne by each supply. The technique by which such current sharing is achieved in power supply system 201 is commonly referred to in the art as droop current sharing. As depicted in FIG. 2, PS1202 and PS2204 each include a variable resistor (R1220 and R2222 respectively) which serves as a setting and adjustment mechanism for the respective output voltages at nodes 217 and 219. Error amplifiers U1224 and U2226 are configured as negative feedback devices such that as the current drawn by system load 206 from PS1202 and PS2204 increases, the output voltage level at output node 215 decreases.

[0010] The current balancing achieved by the closed-loop voltage “droop” effect relies on very precise and symmetric device mirroring between the individual power supply modules PS1202 and PS2204. Such precision is costly and difficult to achieve. Dynamic variations in system load 206 further magnify imbalances between I1 and I2 and result in a wider tolerance required for current imbalance within power supply system 201.

[0011] FIG. 3 depicts an alternate closed-loop current sharing technique known in the art as “active current sharing.” As illustrated in FIG. 3, power supply system 301 includes a pair of parallel power supply modules PS1302 and PS2304 each having a current sense resistor (R1306 and R2308). Active current sharing at current sharing feedback node 310 is achieved by utilizing the voltage references developed across R1306 and R2308. For example, if the voltage output of PS2304 at supply-side node 314 is higher than that at supply-side node 312 within PS1302, the voltage differential across current sense resistors R1306 and R2308 indicates that PS2304 is providing the majority of the current to system load 306.

[0012] The imbalance causes the voltage drop across current sense resistor 308 to be higher than the voltage drop across current sense resistor 306. A higher voltage drop across R2308 results in an increase in the voltage level at current share node 310. This increase in current share node 310 voltage will increase the driving voltages or amplifiers 316 and 317 and will force a higher current to flow through resistor 320 and transistor 318. An increased voltage drop across resistor 320 will result in a higher voltage at node 312. This process continues until both the power supplies PS1302 and PS2304 start current sharing. In this manner, whichever of PS1302 or PS2304 has a higher voltage becomes the master and the other become a slave.

[0013] The load balancing within power supply system 301 thus relies on closed-loop feedback provided between PS1302 and PS2304 at current sharing feedback node 310. A varying current through activated transistor 318 or transistor 319 develops an offset voltage across resistors 320 or 321 which causes the output voltage of PS1302 or PS2304 to increase until both PS1302 and PS2304 have equal output voltage levels and thus provide equal current to system load 306.

[0014] By providing feedback between each of its constituent power supply modules, the active current sharing system depicted in FIG. 3 provides a more accurately balanced current load between its respective power supply modules than the droop current sharing system depicted in FIG. 2. However, this active sharing method requires expensive overhead components and devices such as very precise and low resistance current sense resistors (R1306 and R2308), and low-offset amplifiers (U1322 and U2324) within each power supply module. This closed-loop feedback approach relies on interdependencies among parallel power supplies to ensure accurate load balancing. Low tolerance devices, such as amplifiers with very low offsets are required to achieve a reliable interdependence.

[0015] It would therefore be desirable to provide an system and method for maintaining an accurate load balance for parallel power supplies without relying on interdependent operating parameters among individual power supply modules.

[0016] A system and method are disclosed for maintaining a balanced current level among multiple power supply modules within a parallel power supply. An external voltage is applied to an internal reference translator circuit within each of the parallel power supply modules, such that an internal current-sharing voltage reference is generated within each power supply module. An external reference translator circuit is biased by an independent voltage level, such that the external reference translator circuit generates an external current-sharing voltage reference that is maintained at a higher voltage level than the internal current-sharing voltage references generated within the parallel power supply modules. The external current-sharing voltage reference acts as a master current share reference with respect to which each of the internal current-sharing voltage references. A current adjustment circuit within each of the power supply modules compares the internal current-sharing voltage reference with the master current share reference and generates a current adjustment signal in response to detecting a voltage differential between the master current share reference and the internal current-sharing voltage reference, such that load-sharing is provided among the parallel power supply modules.

[0017] FIG. 1 is a block diagram depicting a load-sharing parallel power supply system;

[0018] FIG. 2 is a schematic diagram illustrating a conventional droop load-sharing parallel power supply;

[0019] FIG. 3 is a schematic diagram depicting a conventional active load-sharing parallel power supply; and

[0020] FIG. 4 is a schematic diagram illustrating an active load-sharing parallel power supply in accordance with a preferred embodiment of the present invention.

[0021] A current sharing controller system and method are disclosed herein for maintaining a balanced current load within a load sharing parallel power supply. As described in further detail with reference to the figures, the load sharing power supply includes multiple power supply modules, each providing an output current level to the total load current. An external voltage is applied to each of the power supply modules to generate an internal voltage reference level against which a higher voltage master reference is compared in order to achieve load balancing among the parallel power supply modules.

[0022] With reference now to FIG. 4, there is illustrated a power supply system 400 in accordance with a preferred embodiment of the present invention. As shown in FIG. 4, power supply system 400 includes a load-sharing parallel power supply 401, a master reference translator circuit 407, and a system load 405. Included within parallel power supply 401 are power supply modules PS1402 and PS2404. A system load bus 412 provides a total load current from PS1402 and PS2404 to system load 405.

[0023] As illustrated in FIG. 4, PS1402 and PS2404 include slave reference translator circuits 406 and 408 respectively. In accordance with the embodiment depicted in FIG. 1, slave reference translator circuits 406 and 408 generate internal voltage references at nodes 441 and 443 within PS1402 and PS2404 respectively. As shown in FIG. 4, slave reference translator circuits 406 and 408 include amplifiers 436 and 438, decoupling diodes 440 and 442, representative reference networks 444 and 448, and 446 and 450, and a common voltage source (V_REF) 432.

[0024] Slave reference translator circuits 406 and 408 are utilized to generate reference outputs for PS1402 and PS2404 in case the common node reference 413 is not available due to a common fault condition. The dynamic voltage reference at common node 413 is controlled by master reference translator circuit 407. In the depicted embodiment, master reference translator circuit 407 is located within a power backplane 410 and is thus physically, as well as electrically, independent of any and all parallel power supply modules within load-sharing parallel power supply 401.

[0025] In the illustrated embodiment, master reference translator circuit 407 is similar in its essential construction to slave reference translator circuits 406 and 408. A master voltage reference 430 is connected to the non-inverting terminal of an operational amplifier (op-amp) 452, and a negative feedback network connected to its inverting terminal (R2456 and R3458) to generate a master reference equal to the output voltage of the power supply. Master reference translator circuit 407 translates an external voltage from external voltage source 430 into a master voltage reference 453.

[0026] In an important feature of the present invention, master voltage reference 453 is maintained at a slightly higher level (tenths of a volt) than the voltage at internal references at nodes 441 and 443, such master voltage reference 453 (applied across decoupling diode 454) controls the voltage level at common node 413. Master voltage reference 453 therefore ensures that master reference translator circuit 407 serves as an independent and controlling reference for parallel power supply modules PS1402 and PS2404.

[0027] In another important feature of the depicted embodiment, a current sharing controller within parallel power supply 400 includes internal voltage reference nodes 441 and 443 (within slave reference translator circuits 406 and 480 respectively) which are connected in parallel to the independent voltage reference as set by master reference translator circuit 407 at common node 413. Slave reference translator circuits 406 and 408 may independently generate output voltage levels in accordance with independent comparisons of the internal voltage references at nodes 441 and 443 to the independent voltage reference at common node 413.

[0028] The offset tolerance for amplifiers 436 and 438 are not a critical attribute of the load-sharing capability of parallel power supply 400. The output voltage of the power system is equal to the node voltage 413, which is generated by a combination of master voltage reference 430, offset voltage of offset amplifier 452, and the ratio of the resistance values of R2456 and R3458. As common reference 413 acts as a master for all power supplies in parallel, the output voltage of all sources in parallel is equal to this master voltage. The offset in error amplifier 452 and the error ratio of R2456 and R3458 is applicable to all the sources in parallel.

[0029] As illustrated in FIG. 4, PS1402 and PS2404 further include error amplifiers 426 and 428 which regulate pulse-width modulators (PWMS) 414 and 416 in accordance with the voltage level at common node 413. In this manner the output voltage level applied to system load bus 412 is maintained at a relative constant under varying load conditions.

[0030] Similar to the active load-sharing configuration depicted in FIG. 3, parallel power supply modules PS1402 and PS2404 include sense resistors R1418 and R1420. It should be noted, however, that the fine tolerance requirements and linear characteristics required for R1306 and R1308 are not as critical for R1418 and R1420, which are utilized only for overcurrent protection rather than for load-sharing as in FIG. 3.

[0031] While the invention has been particularly shown as described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined in the appended claims.