Patent Abstract:
The specification discloses a system and related method for ensuring that power supply units operated in a parallel fashion evenly distribute the load among them. More particularly, the specification discloses a system and related method for correcting current sharing signals produced by each power supply unit for offsets induced by imperfections in the electronic circuitry that produces the current sharing signals.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The preferred embodiments of the present invention are directed generally to systems having multiple power supply units, with each power supply unit responsible for supplying some portion of the electrical current to the load. More particularly, the preferred embodiments are directed to increasing the accuracy of the current sharing signal that extends between the power supply units to ensure that each power supply unit carries a proportionate amount of the load. 
     2. Background of the Invention 
     It is common in the design of electronic systems, especially systems that need redundancy, to provide multiple power supply units. In normal operation, the load is split between two or more power supply units; however, in the event that one power supply unit fails, the remaining power supply unit (or units) provides the additional power to keep the system operational. FIG. 1 shows, in block diagram form, a related art system having two power supply units  10  and  12  supplying power to an exemplary load  14 . In particular, FIG. 1 shows that each of the power supply units  10 ,  12  has a power supply  16  and  18  respectively. This power supply converts the incoming power, which may be an alternating current (AC) or direct current (DC) signal, into an appropriate, typically DC, source for use by the load  14 . Each power supply unit has a diode  20 ,  22  to prevent electrical current from one power supply unit from flowing in reverse fashion into a second power supply unit. 
     In order to balance the load carried between multiple power supply units in a system, it is necessary to measure the output current of each individual power supply unit  10 ,  12 . In the related art, this is done by a low value series resistor  24 ,  26  in series with the output of the power supply unit  10 ,  12 . Current flowing through the series resistor  24 ,  26  develops a differential voltage, which is indicative of the overall power supplied by each power supply unit  10 ,  12 . The differential voltage is fed to a load control unit  28 ,  30 , which converts the differential voltage into a current sharing signal that is coupled to other power supply units by way of the load control line  32 . Each power supply unit  10 ,  12  monitors the current sharing signal on the load control line  32 , and adjusts its output voltage (and therefore its output current) in an attempt to level the load between the various power supplies. 
     FIG. 2 shows a related art load control unit. In particular, the differential voltage from the series resistor feeds an amplifier  34 , which takes the differential signal and creates a voltage proportional to the differential voltage. The Intelligent Platform Management Interface (IPMI) standard, a specification created by a conglomerate of computer manufacturers, requires that the current sharing signal range from zero to eight volts as the power supply unit power output ranges from zero to full load. The output of the amplifier  34  feeds through an isolation amplifier  36  to the load control line  32 . Consider, for purposes of explanation, that another power supply unit is supplying more of the total required power than the power supply unit in which the load control circuit of FIG. 2 resides. In this case, the voltage at the output of the amplifier  34  is lower than the voltage on the voltage control line  32 . The difference is sensed by the combination of isolation amplifier  38  and circuit  40 , and is fed to a power supply voltage control unit (not shown). 
     Because of electrical noise, resistive shot-noise, imperfections in amplifier manufacturing, and the like, the internal current sharing signal  42  may not accurately reflect the load carried by the power supply unit, especially DC offsets in the signal not attributable to current flow. There are two related art approaches to addressing, though not solving, this problem. The first is to adjust the gain of the load control unit such that at fall load the current sharing signal  42  accurately reflects the output power. This, however, may create substantial errors at the low supplied-power end, e.g. at the 50% load, where power supply units operate in a two-power supply unit configuration. The second approach is to adjust the internal current sharing signal  42  to accurately match actual supplied power at the mid-range (or some other relevant loading), but this method too induces error both at the higher and lower ends of the supplied power spectrum. The IPMI specification allows for as much as a 10% difference in load carried among parallel supply units. 
     Thus, what is needed in the art is a system and related method for accurately detecting and generating the current sharing signal for more precise load sharing in multiple power supply situations. 
     BRIEF DESCRIPTION OF SOME OF THE PREFERRED EMBODIMENTS 
     The problems noted above are solved in large part by a system and related method that compensates for the errors in creation of the current sharing signal. More particular, the preferred embodiments are directed to a circuit which reads the differential signal created by the series resistor, and creates a current sharing signal, which is adjustable not only with respect to its gain, but also with respect to its offset. In a second aspect, the preferred embodiments are directed to a method of calibrating the load control circuits of power supply units with a computer that involves first setting the gain of the load control circuit, followed by setting the offset of the load control circuit. 
     The disclosed devices and methods comprise a combination of features and advantages which enable it to overcome the deficiencies of the prior art devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
     FIG. 1 shows a related art system having two power supply units supplying a single load; 
     FIG. 2 shows a related art load control circuit; 
     FIG. 3 shows an embodiment of a system having three power supply units; 
     FIG. 4 shows, in block diagram form, a power supply unit of the preferred embodiments; 
     FIG. 5 shows, in greater detail than FIG. 4, the load control circuit  64  of the preferred embodiment; 
     FIG. 6 shows an electrical schematic of various components of the load control circuit  64  of the preferred embodiment; 
     FIG. 7 shows a system for automatically adjusting the gain and offset of a power supply unit of the preferred embodiment; 
     FIG. 8 shows a flow diagram of adjusting the gain and offset of a power supply unit of the preferred embodiments; 
     FIG. 9 shows an exemplary plot of output current versus the current sharing signal prior to gain and offset adjustment; and 
     FIG. 10 shows an exemplary plot of output current versus current sharing signals after gain adjustment, but prior to offset adjustment. 
    
    
     NOTATION AND NOMENCLATURE 
     Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, computer and electronics companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows an embodiment having multiple power supply units feeding one or more power consuming devices, collectively referred to as load  52 . In particular, FIG. 3 shows three power supply units  50 A-C, each feeding the load  52 . While FIG. 3 shows three such power supply units  50 , any number of power supply units are within the contemplation of this invention. In the preferred embodiments, each of the power supply units  50  supplies direct current (DC) power at its output, and thus the load  52  utilizes DC power for its operation. FIG. 3 further shows that each of the power supply units  50  couples to a power source. The power source could be either a DC source, meaning that the power supply units  50  would each contain a DC—DC switching power supply, or the power source may be an alternating current (AC) signal, meaning that the power supply units  50  each contain a AC-DC power supply. The preferred embodiments of the present invention may be used regardless of the type of input power source, and thus all such sources, and combinations, fall within the contemplation of this invention. 
     Situations where multiple power supply units  50  supply a load  52 , such as that shown in FIG. 3, typically implement the redundancy because of the importance of the load  52 . That is, the load  52  may be a critical system such as an on-line retail server, a radar system for air traffic control, a control system for operation of a large industrial plant, or the like, and thus may need to be able to withstand the failure of any particular power supply unit. Further, the various power supply units  50  of the system may be coupled to different sources, thus protecting the load  52  from the loss of any individual source. Regardless of the precise reasons or the individual setup, in normal operation it is desired that the total power supplied to the load  52  be evenly divided among the various power supply units  50 . As discussed in the Background section, related art systems adhering to the IPMI standard could only assure load balancing within approximately 10%. However, as the number of power supply units increase, or the total load decreases, a 10% load difference between power supply units may be significant. The preferred embodiments of the present invention decrease the amount of differential between the power supply units, and thus increase the load distribution among the various power supply units  50 . Using the embodiments described below, distribution errors as low as 2% or less are achieved. 
     FIG. 4 shows, in block diagram form, a more detailed power supply unit  50  of the preferred embodiment. In particular, each power supply unit  50  preferably comprises a power supply  60 , which converts the source power  62  into DC power which is fed to the load  52 . The power supply unit  50  also comprises a load control unit  64  which couples to a resistor  66 . The resistor  66  is placed in series with current supplied to the load  52 , and the resistor  66  preferably has a very small value, on the order of 0.01 ohm or less. As shown in the exemplary drawing of FIG. 4, the load control circuit  64  couples to both the upstream  68  and downstream  70  side of the resistor  66 . In this way, the load control of circuit  64  is able to read a differential voltage developed across the resistor  66 , which is proportional to the current flow from the individual power supply unit  50 . The power supply unit  50  also comprises a diode  72  which ensures that direct current amperage only flows out of each individual power supply unit. The power supply  60  within the power supply unit  50  preferably has the capability of supplying current as required by the particular load, but also adjusting its output voltage so that the amount of power supplied to the load  52  may be controlled. The power supply  60  preferably makes this adjustment based on a voltage adjust signal  74  coupled between the load control unit  64  and the power supply  60 . 
     FIG. 5 shows a more detailed block diagram of the load control circuit  64  of the preferred embodiments. In particular, the differential voltage created by current flow across the resistor  66  is sensed by the differential sense circuit  76 . The differential sense circuit  76  has an output  78  that feeds the gain adjust circuit  80 . The offset circuit  82  of the preferred embodiments produces a signal which is effectively summed with the output of a differential sense circuit  76  in the gain adjust circuit  80 . Thus, the gain adjust circuit  80  performs at least two functions: 1) sums the signal created by the offset circuit  82  with the signal created by the differential sense circuit  76 ; and 2) provides an adjustable gain. Finally, the internal current sharing signal  84 , which represents the current, and therefore the power, produced by the particular power supply unit  50 , is applied to a unity gain amplifier  86 , which simply acts to isolate the load control circuit  64  from the load control line  56 . As was discussed with respect to FIG. 3, the voltage on load control line  56  couples to each of the power supply units  50 A-C, and it is by monitoring this line that each power supply unit adjusts its output voltage (and therefore power) to evenly distribute the load. 
     Amplifier  88 , a unity gain amplifier, couples to and monitors the load control line  56 , and applies the signal to the output adjust circuit  90 . More particularly, the internal current sharing signal  84  couples (though amplifier  86 , diode  87  and resistor  89 ) to the load control  56 . Amplifier  88  preferably has its inverting input coupled to the output signal of amplifier  86 , and has its non-inverting input coupled downstream of the diode  87  and resistor  89 . The signal created by amplifier  88  is thus proportional to the difference between the internal current sharing signal  84  and the voltage on the load control line  56 . The output adjust circuit  90  couples to and observes the output signal of the amplifier  88  to determine if the power supply unit at issue is supplying more or less of the overall power than other power supply units. If the power supply unit under consideration is supplying more power (and therefore diode  87  is forward biased), the output adjust circuit  90  commands the power supply  60  to decrease voltage by applying an appropriate signal across line  92 . Likewise, if the power supply unit  50  under consideration is supplying less power than remaining units (and therefore diode  87  is reversed biased), then the output adjust circuit  90  commands the power supply  60  to increase voltage. 
     FIG. 6 shows a more detailed electrical diagram of the differential sense circuit  76 , offset circuit  82  and the gain adjust circuit  80 . The heart of each of these circuits is an operational amplifier (op-amp), preferably an LM324A; however, any suitable amplifier system may be used. The current through the resistor  66  creates a differential voltage which is sensed by the op-amp  100 . In the preferred configuration shown for the differential sense circuit  76 , the op-amp  100  and related circuitry provide most of the amplification of the differential voltage required of the load control circuit  64 . Thus, the output  102  of the op-amp  100  is proportional to the voltage developed across the resistor  66  multiplied by the gain. The various resistances shown within the differential sense circuit  76 , as well as all the resistances shown in FIG. 6, are given in ohms. Likewise, the capacitances shown in FIG. 6 are all given in micro-farads. If the power supply unit  50  within which the differential sense circuit  76  resides is producing a full load of 50 amps of current (thus 50 amps are flowing through the resistor  66 ), then the output  102  of the op-amp  100  in the configuration shown is approximately 8.0 volts plus an offset voltage. 
     Still referring to FIG. 6, the preferred embodiment of the offset circuit  82  is shown. Op-amp  104 , in combination with the remaining circuit elements, is configured to be a voltage follower. In other words, the voltage present at the non-inverting input of the op-amp  104  is the same as the output voltage driven to line  106 . The offset circuit  82  is designed to produce a variable voltage, which is adjustable by way of the resistor  108 . While a standard potentiometer may be suitable for this application, the preferred embodiments use a Maxim DS1846 digitally controlled potentiometer made by Dallas Semiconductor. In this way, adjustments to power supply units  50  may be done by computer in an automated system, discussed more fully below. By adjusting the digitally controlled potentiometer  108 , the offset circuit  82  is capable of producing a voltage at its output  106 . 
     The output signal of the differential sense circuit  76  (the power sense signal), and the output signal of the offset circuit  104  are each applied to the gain adjust circuit  80 . The gain adjust circuit  80  provides an adjustable gain for the sensed current flow from the power supply in which the gain adjust circuit  80  resides, and also sums the output signal  106  from the offset circuit  82 . Much like the digitally controlled resistor  108 , the adjustable resistance  112  is likewise a Maxim DS1846 digitally controlled potentiometer. The output  114  of the op-amp  110  becomes the offset adjusted current sharing signal (or alternatively load control signal) supplied by the power supply unit in which the gain circuit  80  resides. The remaining elements of each of the differential sense circuit  76 , offset circuit  82  and gain circuit  80  are familiar to one of ordinary skill in the art, and for brevity of this discussion will not be discussed in great detail. For completeness, the diodes in each of the gain control circuit  80  and offset circuit  82  have a part number BAS16T. Implementing the diodes in each of these circuits in this manner is for over-voltage protection for the digitally controlled resistances  108  and  112 . 
     FIG. 7 shows an exemplary system used to calibrate the power supply unit  50  of the preferred embodiment. In particular, FIG. 7 shows a computer system  116  coupled to the power supply unit  50  and a test load  118 . Preferably the computer system  116  couples to the power supply unit  50  in such a way as to be in communication with the digitally controlled resistances  108  and  112 . Further, the computer  116  communicates to the test load  118 . This communication is required so that the electrical current required of the power supply unit  50  may be varied so that the load control circuit  64  may be adjusted. 
     FIG. 8 shows a flow diagram for calibrating the load control circuit  64 . The process starts at step  120  and proceeds to setting the adjustable resistance  112  at the midrange setting (step  122 ). If the adjustable resistance is merely a standard potentiometer, this may be done by a technician moving the wiper. In the preferred embodiments, however, setting the adjustable resistance  112  at the midrange involves electronically setting the potentiometer by way of the computer  116  (FIG.  7 ). Before moving on. It should be noted that while setting the adjustable resistor at the midrange is preferred for the initial step, the initial setting may be at any location, as it is the difference in gain realized for each step change in resistance of the digitally controlled resistance  112  that is of concern. Thus, the next step is the determination of the gain with the resistance set at midrange (step  124 ) (discussed more fully below). Next, the gain is changed a predefined number of bits—that is the resistance of the digitally controlled resistance  112  is changed a predefined number of bits (step  126 ). Thereafter, the gain of the overall circuit is determined based on the new setting of the digitally controlled resistance  112  (step  128 ). Knowing the difference between the gain realized between the two settings, the next step preferably involves calculated an amount of gain change realized for each bit-wise change in the digitally controlled resistance  112  setting (step  130 ). Once the gain per bit, or the gain responsiveness, is calculated, then preferably the computer system  116  adjusts the gain (adjusts the resistance of the digitally controlled resistance  112 ). Finally, after the gain is adjusted, the offset is adjusted by means of adjusting the digitally controlled resistance  108  in the offset circuit  82  (step  134 ). The process then ends (step  136 ). Adjusting the gain prior to adjusting the offset is important because the gain adjustment in the preferred circuit shown affects the offset, but not vice versa. Referring briefly to FIG. 6, it is seen that resistor R 5  couples to signal  102  from the differential sense circuit  76 . Likewise, resistor R 13  couples to the output signal  106  of the offset circuit  82 . Thus, the resistance of resistor R 13  plays into the circuit that determines the gain. Determining the offset voltage is done on the input side of the op-amp  104 , and thus the adjustment does not have an affect on the gain. However, one of ordinary skill could, now understanding operation of the circuits, devise equivalent circuits where the gain does not affect offset. 
     In the steps exemplified in FIG. 8, steps  124  and  128  recite determining a mid-range and second gain. Before proceeding, it must be understood that the determination of the mid-range gain is not finding a median of the possible gains, but instead is determining the gain realized with the digitally controlled resistance  112  set in its mid-position. 
     Referring now to FIG. 9, the determination of the gains will be described in more detail. FIG. 9 shows an exemplary graph of output current for a power supply unit versus current sharing signal. The dashed line  138  represents the ideal relationship between the output current (output power) and the current sharing signal according to the IPMI standard. Line  140 , however, represents two possible problems. First, line  140  represents an incorrect gain (as indicated by the difference in slope between lines  140  and  138 ). Secondly, line  140  represents an unwanted offset  142  which manifests itself most clearly when the output current approaches zero. 
     The following description applies to the determination of a gain setting. The steps described are equally applicable to steps  124  and  128  of FIG.  8 . In particular, the test load  118  is directed to require the maximum output current from the power supply unit  50 . Based on the settings in the load control circuit  64 , this produces a current sharing signal  84 , as shown by point  144  of FIG.  9 . The output current and current sharing signals are noted. Next, in the preferred embodiments, the test load  118  is set to require a 50% current draw capability of the power supply unit  50 , and likewise this produces a current sharing signal  84 , as shown by point  146 . Here again, the output current and current sharing signal are noted. Using these two data points, the slope (or gain) of the circuit at a particular setting of resistance  112  may be calculated using the following equation:              Gain   =         Y   2     -     Y   1           X   2     -     X   1                 (   1   )                                
     where the X values are the output current values for the noted locations, and the Y values are the current sharing signal values for the noted locations. 
     Finally, referring to FIG. 10, there is shown an exemplary graph of an output of the load control circuit  64  as a function of the output current after the gain has been adjusted (note that the slope of line  148  is the same as the slope of line  138 ), but before adjusting the offset by change of resistance  108 . After proper adjustment of the offset, the lines  148  and  138  would lie substantially on top of one another. 
     The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.

Technology Classification (CPC): 7