Abstract:
In an electronic system with multiple power supplies, a method and apparatus dynamically determines the number of power supplies required to power up the system without overloading any supply. The individual power supplies do not turn on until the determined number of power supplies have received AC power and become operational before attempting a complete system power on. The amount of required power is determined before power up based on the actual power load present. The actual load is determined by sensing load indicators in each load device and computing the total power load. The amount of power from supplies which have received AC power is determined by detecting when AC power has been applied to each power supply and computing the total amount of power available. System power up is delayed when the number of power supplies which have received AC power is insufficient to power the system without an overload situation occurring.

Description:
FIELD OF THE INVENTION 
     This invention relates to the power up of an electronic system in response to the application of AC power and, more particularly, to power up of a system which has multiple independent power supplies. 
     BACKGROUND OF THE INVENTION 
     In many electrical systems, such as computers, it is often desirable to have several independent power supplies operational in the system simultaneously. The term “independent” is used here to mean, among other things, that each supply has its own connection to the main AC power source and produces all, or a portion of, the total DC power which is used to run the system. In most cases, however, the DC power outputs of the power supplies are connected in parallel. There are several advantages inherent in multiple supplies. 
     For example, the ability to add power supply units to the system after the initial configuration allows the number of power supply units to be tailored in accordance with system power requirements. In a computer system with pluggable units such as CPU&#39;s, memory, I/O cards, and disk drives, the power consumption of the system may vary dramatically depending on the current set of installed units. The number of power supplies that are installed can be selected to match the actual power load. 
     Further, extra power supplies can be added for redundancy. In contemporary terminology, systems can be made “N+1” redundant with respect to the power subsystem by adding one more power supply than is actually needed to run the system. In the event of the failure of one supply, such systems can continue operating normally while the failed supply is being replaced. 
     In general, it is often required that, following a loss, and subsequent reapplication, of the main AC power, the system remain in the same state that it was in before power was removed. In other words, the system must have some facility for remembering the state of its power supply subsystem (ON or OFF) at the time of the AC removal, so that, following the reapplication of AC power, the system can resume its previous state. The removal of AC power could occur either intentionally when the system is powered up or unintentionally in the case of an AC power failure. If the system was “ON” before the power removal, it is often necessary for the system to correctly power up and resume normal operation. 
     However, when such a system has multiple power supply units and each unit has an independent connection to the main AC power, a difficulty can arise in correctly recovering from a temporary lapse of the main AC power. In particular, since each power supply has its own connection to AC power, it will recover when AC power is applied and it is possible that the separate AC power connections might not recover simultaneously. 
     For example, consider a system with “M” independent power supplies which, while in the “ON” state, loses AC power. Further, assume that the system requires at least “N” operational power supplies to power it, where “N” is less than or equal to “M”. Depending on the source and routing of AC power in the facility where the system is located, it is possible that the “M” power supplies may experience the reapplication of AC power at different times, or “steps” over several seconds or longer. This staggered power application could be due either to fault conditions in the facility, or to a controlled reapplication of AC power via circuit breakers. 
     When power is reapplied, if “L” power supplies receive AC power in the first such “step”, where “L” is less than “N”, then the power consumption requirements of the system will exceed the capabilities of the “L” supplies and the operational power supplies will be overloaded. In this situation, the operational power supplies experience over-current faults. Typically, once a power supply enters such an over-current fault state, it must be manually serviced before it will again attempt to power the system. This service can be accomplished by manually turning the supply OFF and then ON again. 
     At the next “step” in the facility&#39;s recovery of AC power, and at each subsequent “step”, if less than “N” supplies receive AC power at the same time, then they will also enter an over-current state. Thus, it is possible for all the supplies in the system to enter the over-current fault state, in two or more steps. Most system designs would require manual intervention once in this state before the system could be successfully powered up. 
     SUMMARY OF THE INVENTION 
     In accordance with the principles of the invention, a method and apparatus dynamically determines the number of power supplies required to power up a system without overloading any supply. The power subsystem is required to wait until the determined number of power supplies has received AC power before attempting a system power on. 
     In accordance with one embodiment, the amount of required power is determined before power up based on the actual power load present. The actual load is determined by sensing load indicators in each load device and computing the total power load. 
     In accordance with another embodiment, the number of power supplies which have received AC power is determined by detecting when AC power has been applied to each power supply and computing the total amount of power available for power up. 
     In accordance with a further embodiment, power up is delayed when the number of power supplies that have received AC power is insufficient to power the system without an overload situation occurring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
     FIG. 1 is a block schematic diagram of a prior art electronic system with multiple power supplies. 
     FIG. 2 is a block diagram of a power control system constructed in accordance with the principles of the present invention. 
     FIG. 3 is a circuit diagram of an illustrative circuit that computes the amount of available power. 
     FIG. 4 is a circuit diagram of an illustrative circuit that computes the load present upon power up. 
     FIG. 5 is a circuit diagram of an illustrative circuit that determines whether an overload situation is present. 
     FIG. 6 is a circuit diagram of an illustrative circuit that controls power up to prevent an overload situation from occurring. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is an illustrative diagram of a prior electronic system  100  that has multiple power supplies of which supplies  108 ,  110 ,  112  and  114  are shown. Such a it system  100  typically has multiple load devices, such as load devices  102 ,  104  and  106 . In a computer system, such load devices might consist of CPUs, memory or I/O devices. In other electronic circuits, the load devices might be DSPs, data processing devices or other electronic devices. Each load device is connected to a ground bus  116  and to one or more power busses or “rails”  118  and  120 . In a simple electronic system, a single power bus with a single voltage may be used, whereas in more complicated electronic systems, such as that shown in FIG. 1, several power busses  118  and  120  may be present with different voltages. For example, in a typical computer system, the power supplies might produce 5 volts, 12 volts, 3.3 volts and other voltages. 
     Depending on the power drain at each voltage, some busses, such as bus  118  in FIG. 1, can have two or more power supplies connected in parallel to the bus  118  in order to provide for sufficient total current output to prevent any of the power supplies from overloading due to an overcurrent condition. In FIG. 1, multiple power supplies are connected to bus  118  of which three,  108 ,  110  and  112  are shown. Other busses may have only a single supply, such as bus  120  in FIG. 1 that has only supply  114  connected to it. 
     Each of supplies  108 , 114  is connected to AC power illustrated as AC power sources  122 ,  124 ,  126  and  128 . Generally, each power supply would have its own direct connection to an AC power main (via circuit breakers, not shown.) In this manner, a failure in one supply will still leave other supplies operational as long as AC power is present. 
     FIG. 2 illustrates an electronic system with a power application or failure recovery system constructed in accordance with the principles of the present invention. In the electronic system  200  illustrated in FIG. 2, the actual power distribution busses and AC power supplies have been omitted for clarity, but these elements would be constructed as shown in FIG.  1 . The inventive system operates by dynamically calculating before power up both the power potentially available and the power required and delaying power up until sufficient power is available so that no power supply will overload. 
     Specifically, each of power supplies  202 - 206  generates a power available signal when it is operational and available to provide power. Generally, the supply will be available to provide power when AC power is applied to it. The power available signal generated by each power supply is representative of the total amount of power the supply can provide to a specified power rail without going into an overload condition. The power available signals generated by each power supply are provided to a power available calculator  208  that generates an indication of the total available power. The total available power may change over time as more and more power supplies receive AC power and turn on. 
     In a similar manner, each of power loads  210 ,  212  and  214  generates a power-required signal that indicates the amount of power required by that load. The power use of the system may vary as loads are added or removed from the system. The power required signals are provided to a load calculator  216  which computes the total amount of power required by the current system configuration. 
     The outputs of calculators  208  and  216  representing the total power available and the total power required are provided to an overload detector  218  which compares the total power available to the total power required. The overload detector  218 , in turn, generates an input to a power supply control  220 . If the result of the overload calculation indicates that there is sufficient power to turn on the system, then the power control circuitry  220  allows the system to come up normally. If, however, the overload computation determines that there is insufficient power available to safely power on the machine, the power control circuitry  220  will block any “power on” command coming from other circuitry (not shown) in the system  200 . This operation prevents system power up until the available power meets or exceeds the power required. 
     The power available signals and the power required signals could be generated in a number of ways. At the beginning of the power on procedure, the power supplies  202 - 206  are not yet powered on so that the voltage normally provided to the system  200  by the power supplies  202 ,  204  and  206  is not present even though AC power may be provided to one or more of the power supplies. However, in accordance with a preferred embodiment, the power supplies in system  200  are designed to include a standby power source, which generates a reduced level output whenever any of the power supplies receives AC power. Many conventional personal computers have such standby power sources and these sources are also present in several larger computer systems, such as those designed and manufactured by Sun Microsystems, Palo Alto, California. Power supply designs in these systems have migrated toward the use of “soft” power on/off controls. In such designs, instead of physical switches that turn the AC power completely on or completely off, AC power is always present. The power supplies have circuitry that is powered and functional even when the unit appears to be “off.” For example, most modern televisions have circuitry that is powered and functional at any time the television is receiving AC power; this circuitry detects a “power-on” command from a remote control and turns on the main power supplies. 
     In accordance with the preferred embodiment, the mechanisms which generate the power available and power required signals and the calculators and detectors are powered from one or more of these standby power sources. The mechanisms are designed to have very modest power requirements. As such, they require that AC power be provided to only a single power supply in order to generate all needed standby power. The standby power can be used to perform the power and load calculations and postpone an attempted power-on until enough power supplies are receiving AC power such that the electrical load presented to the power supplies by the system will not overload the power supplies. 
     More particularly, the available power signal generated by each power supply and the power-required signal generated by each load are generated by a single resistor in each power supply or load. This resistor is connected to the standby power and its value is selected to indicate the power that the supply is able to supply or the power that the load consumes. The physical connector which connects each power supply or load unit to the main system must dedicate a single “power configuration” pin over to which this resistor is connected to allow the power information to pass to the power computation mechanism. The system itself may also contain a single resistor to indicate to the power consumption circuitry the power consumption of the non-removable parts of the system. 
     An example of an analog circuit that can be used to compute the available power it is shown in FIG.  3 . In FIG. 3, three power supplies are shown, however, more or fewer power supplies can be used without departing from the spirit and scope of the invention. The standby power source is a 5 volt source denoted as “5V_STBY”. In the case of power supplies, the standby source for each power supply is the standby source associated with that power supply so that standby power for a power supply becomes available when AC power is applied to that power supply. Although not shown in FIG. 3, these individual standby sources are also connected together to form a common standby source 5V_STCOM which can provide power when any of the power supplies receives AC power. 
     More specifically, power supplies  300 ,  302  and  304  each contain a single supply indicator resistor  312 ,  314  and  316 , respectively, which is connected on one end to a 5V_STBY standby source for the respective power supply,  306 ,  308  and  310 . The other end of each supply indicator resistor  312 ,  314  and  316  is connected to the available power calculator via the aforementioned power configuration pin. The power configuration pins of the three power supplies are connected together, creating a net, labeled as “P SOURCE ” which is connected to the negative input  326  of amplifier  318 . When each source receives AC power, a current runs through the corresponding supply indicator resistor. Since the resistors are selected to represent the power source available power, with the particular summing amplifier configuration illustrated in FIG. 3, the current will also be proportional to the available power. In FIG. 3, each power supply has a supply indicator resistor with the same value, R S , but this need not necessarily be the case. The circuitry works equally well for supplies of varying capacity, with a corresponding change in the supply indicator resistor of each supply. Although not shown in FIG. 3, it may be necessary with some power supplies to place a diode in series with the supply indicator resistor for that supply to prevent a supply that does not have AC power from leaking current away from the P SOURCE  net, as this would corrupt the information on the available power, which is encoded as current. 
     The currents from the three supplies add to create a current “I SOURCE  ”, which flows through the feedback resistor  320  labeled as R F . The output voltage, V SOURCE , of the summing amplifier  318 , is given as a function of R F , V BIAS , and the supply indicator resistors, R S . It should be noted that, since the summing amplifier  318  configuration is “inverting” in nature, the output voltage, V SOURCE , will decrease in value with an increase in P SOURCE  and a corresponding increase in I SOURCE . For this reason, the circuit is biased to a positive voltage chosen high enough to give the circuit a reasonable dynamic range, and low enough to meet the DC specifications of the amplifier  318 . Although not shown in FIG. 3, it is assumed that the amplifier  318  is powered from the common 5V_STCOM standby source  322 . For that reason, amplifier  318  is held at a bias voltage V BIAS  on the “+” input  332  of the amplifier  318  by a voltage divider comprising resistors  324  and  328  connected between supply  322  and ground  330 . With the values shown, V BIAS  is approximately 3 volts. 
     The relationship between R F  and R S  is computed, assuming three identical power supplies  300 ,  302  and  304 . First, V SOURCE  is chosen to be zero volts when all three supplies have received AC power and have turned on. This gives a ratio of R F /R S =½. Next, a value for R F  is chosen, for example, 1K ohms, which dictates a value for R S  of 2K ohms, and the resulting V SOURCE  computed for 0, 1, 2, and 3 supplies present giving values of 3, 2, 1 and 0 volts, respectively. The mathematics for power supplies of dissimilar power capabilities is straightforward and not shown. 
     FIG. 4 shows an illustrative implementation of the “load calculator” mechanism, which computes the total load present. It is very similar in topology to the circuit that creates the V SOURCE  signal. The load indicator resistors  418 - 424  contained within the removable loads  402 - 408  are designated with a value of R L1 , and are connected to the common standby source 5V_STCOM indicated as sources  410 - 416 . Although the load indicator resistors  402 - 408  are shown with equal values, they could also be selected to have values that differ when the load values differ. 
     The load indicator resistors are connected together to form the P LOAD  net. Consequently, with the particular summing amplifier configuration illustrated in FIG. 4, current running into the P LOAD  net is proportional to the power required by the removable load boards. The load indicator resistor  430  with a value R L2  may also be provided which is connected to the common standby source  428  and provides current to the P LOAD  net which current is proportional to the power consumed by the non-removable portion of the main system, i.e., the part that is always present. Thus, the total current on the P LOAD  net, represented here by I LOAD  is proportional to the total load of the system. Note that, since the load indicator resistors are connected to the common standby source, the current I LOAD  representing the entire load in the system will be available as soon as any power supply receives AC power. 
     Although not shown in FIG. 4, it is assumed that the amplifier  442  is powered from the common 5V_STCOM standby source  432 . For that reason, amplifier  442  is held at a bias voltage V BIAS  on the “+” input  440  of the amplifier  442  by a voltage divider comprising resistors  434  and  438  connected between supply  432  and ground  436 . With the values shown, V BIAS  is approximately 3 volts. 
     Note that the R L1 , and R F  resistor values in FIG. 4 must be chosen so that the voltage on V LOAD  represents “watts of load per volt” with the same scale that the voltage on V SOURCE  generated by the power available calculator illustrated in FIG. 3 represents “watts of power supply per volt.” Only in this manner can the two voltages be compared in a meaningful way. The required resistor values can be chosen in the following manner: 
     1. Divide the dynamic range of the V LOAD  output, which turns out to be V BIAS  volts (in this configuration, the output of amplifier  442  can vary only between V BIAS  and 0 volts), by the maximum power provided when all supplies have received AC power and are turned on. This result, denoted Av, relates the output V LOAD  in volts to consumed power in watts. 
     2. Choose the value of R F  in load circuit  400  at some arbitrary convenient value. Define the term A I , which is defined as Av divided by R F . This term relates the output voltage V LOAD  in volts to the current I LOAD  in amperes. Next, the load indicator resistance value (R L1 ) in ohms is given by the following formula in which the load of the load device in watts is denoted by P UNIT :          R   L1     =         5      V_STCOM     -     V   BIAS           P   UNIT     ×     A   1                                
     FIG. 5 shows an illustrative comparator circuit that compares the V LOAD  and V SOURCE  signals generated by the circuits described above. The comparator circuit  500  generates at its output  508  a low-true OVERLOAD_L signal, which when asserted LOW indicates that not enough power supplies are on-line with AC power. In particular, the V SOURCE  signal is applied to the negative input  502  of operational amplifier  506  and the V LOAD  signal is applied, via input resistor  504  to the positive input of amplifier  506 . The hysteresis resistor  510 , denoted as R F , works with the input resistor  504  to prevent the output OVERLOAD_L from oscillating when V LOAD  and V SOURCE  are nearly equal in value. 
     FIG. 6 illustrates a simple asynchronous state machine (ASM) which uses the OVERLOAD_L signal generated by the comparator illustrated in FIG. 5 to pass or block an ONCTL_L signal generated by the system. In the absence of this state machine the ONCTL_L signal would normally connect directly to the power supplies, turning them ON when ONCTL_L is LOW. The purpose of using an ASM rather than simple combinatorial logic is to prevent the power control circuit from turning the power supplies OFF, once it has been turned them ON. This choice has been made because power supplies typically provide their own overcurrent protection, and, as such, will power-off on their own in the event of a fault. This might be the case, for example, if a supply fails in a system configuration that is not “N+1” redundant. In this case, it is deemed better to let the overcurrent fault be recorded in the power supply, rather than to have the system powered off by the OVERLOAD logic. Either way, the system will power off. Also, there is the chance that the system will stay up even with the supplies operating in a slightly overcurrent condition. Consequently, the circuit illustrated in FIG. 6 prevents a system from transitioning from OFF to ON while OVERLOAD_L is TRUE (LOW). 
     Specifically, the ONCTL_L signal is provided to the upper input  602  of gate  606 . The OVERLOAD_L signal is provided to the input  604  of inverter  608  and the resulting inverted signal is applied to the lower input  610  of gate  606 . If the ONCTL_L signal is LOW indicating that the supplies should turn on and the OVERLOAD_L signal is HIGH, indicating no overload condition, then gate  606  will be enabled. The HIGH output produced by gate  606  is applied to the upper input  620  of NOR gate  622 , which thereupon produces a LOW output  624  designated the PS_ONCTL_L output which turns on the supplies. 
     The LOW PS_ONCTL_L output is applied to the input  616  of gate  612 . As long as gate  612  receives a LOW ONCTL_L signal on its upper input  614 , indicating that the power supplies should remain ON, gate  612  will generate a HIGH output  618 . When this HIGH output is applied to the lower input of NOR gate  622 , it will keep the output PS_ONCTL_L LOW regardless of the state of the OVERLOAD_L signal. 
     The exemplary embodiment has the added feature that the linear proportional nature of the V SOURCE  AND V LOAD  signals allows a calculation of the amount of excess power available. If this latter amount of power exceeds the power available from the largest supply, then the system has N+1 redundancy. This latter power calculation is conventional and so is not described in detail herein. 
     Although an exemplary embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. For example, it will be obvious to those reasonably skilled in the art that, in some implementations, the power source and load calculations may need to be performed separately for each power bus or rail, and thus the power source calculator and load calculator described above must be replicated for each power rail, depending on the power budget of the system. In addition, other mechanisms in addition to the circuitry shown can be used to perform the power and load calculations. For example, it would be possible to digitize the power available signals and utilize software to perform the power available, load and overload calculations. With some of these other configurations, the proportional nature of the signals might not be maintained. Other aspects, such as the specific circuitry utilized to achieve a particular function, as well as other modifications to the inventive concept are intended to be covered by the appended claims