Abstract:
A system is provided that effectively obviates shortcomings of conventional, diode-ORed, redundant power supply systems by forcing a primary power source to provide power to the system load most the time, even when the primary source provides a lower voltage than that of a secondary power source. This preferential selection of the lower-voltage primary is achieved by interposing a power switching unit between the secondary power source and the ORing diode in front of the load. The power switching unit of the illustrative embodiment comprises a voltage regulator that is regulated at a first voltage level, which is lower than the nominal output voltage level provided by the primary power source. Meanwhile, the primary power source is able to charge the secondary power source, which is a battery in the illustrative embodiment, so that the secondary power source can provide power to the load when the primary power source is either interrupted or falls below the first voltage level.

Description:
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH 
     This invention was made with Government support under W31P4Q-04-C-0059 awarded by the Department of the Army. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to power supplies in general, and, more particularly, to switchover circuits for redundant power supplies. 
     BACKGROUND OF THE INVENTION 
     Power supplies for electronic equipment often need to provide more than one possible source of power to a system load, for reasons of redundancy. The multiple power sources might have the same power characteristics or they might be different from each other. For example, different power sources might be configured to provide power at different voltages. Where the power sources are different, or at least where one is typically preferred over another, the preferred power source is typically designated as the primary power source and the other is designated as the secondary power source. 
     When both a primary and secondary power source are required, a power supply should be designed so that when the primary source fails, the secondary source will immediately take over without an interruption in the operation of the equipment being powered. This is especially true in military applications, in which the equipment is required to conform to a demanding set of performance specifications. 
       FIG. 1  in the prior art depicts a block diagram of a redundant power supply, in which primary power source  101  and secondary power source  102  provide power to system load  110 . Load  110  comprises the equipment that is being powered. Sources  101  and  102  are connected to load  110  by means of a relatively common technique called “diode-ORing.” The two power supplies are connected to load  110  through associated ORing-diodes  103  and  104 , respectively, wherein source  101  provides voltage V 1  and source  102  provides voltage V 2 . With the two power sources diode-ORed together, the power source with the larger output voltage will establish the voltage that is delivered to load  110  and which is present at node  105 . By connecting the power sources in this way, if one power source fails, the other source will take up load  110  with little or no interruption in the power that is provided. 
     As is the case with the prior-art system depicted in  FIG. 1 , the primary power source, in addition to providing power to load  110  whenever possible, can also be used to charge the secondary power source, which is a battery in this case. When the primary power source is interrupted, because the battery-based secondary power source is kept charged, it can take over for the primary source. 
     There are operational scenarios, however, in which the secondary source&#39;s output voltage, V 2 , is higher than the primary source&#39;s output voltage, V 1 . For example, the primary power source might be designed to deliver 28 Volts DC at V 1 , and the secondary power source might be designed to deliver 33.6 Volts DC at V 2 , as is the case in certain military applications. In this case, the ORing diodes will select V 2  as the voltage to be delivered to the load, as provided by the secondary power source. Ordinarily, this might be acceptable, especially if the battery of the secondary source is a more reliable source of power than the primary source. However, a mode of operation in which the secondary power source is normally selected might not be either desirable or sustainable—particularly, for example, if the lower-voltage primary source is being used to charge the higher-voltage secondary source. 
     It is, therefore, desirable to have the secondary power source charging and available as a backup to the primary power source during periods of low-power demand and also to have the primary power source available as a backup for the secondary power source during periods of high-power demand. To achieve this, what is needed is a power supply system without some of the disadvantages in the prior art. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, shortcomings of conventional, diode-ORed, redundant power supply systems, such as those described above, are effectively obviated by a new and improved control mechanism. The mechanism forces a primary power source to provide power to the system load most the time, even when the primary source provides a lower voltage than that of a secondary power source. This preferential selection of the lower-voltage primary is achieved by interposing a power switching unit between the secondary power source and the ORing diode in front of the load. The power switching unit of the illustrative embodiment comprises a controller, secondary-power relay, and voltage regulator. The voltage regulator of the illustrative embodiment is regulated at a first voltage level, which is lower than the nominal output voltage level provided by the primary power source. Meanwhile, the primary power source is able to charge the secondary power source, which is a battery in the illustrative embodiment, so that the secondary power source can provide power to the load when the primary power source is either interrupted or falls below the first voltage level. 
     The normal operational mode of the power switching unit is summarized here. The controller initially i) opens the secondary-power relay, thereby cutting off the ORing diode from the battery of the secondary power source, and ii) directs the battery to be charged, such as by the primary power source. In the event that the primary power source falls below a threshold voltage, which is related to the first output voltage level of the regulator, the voltage regulator begins to conduct current to supply the system load. The control circuit of the voltage regulator then transitions quickly from a linear control of the regulator&#39;s MOSFET transistors to fully saturating control. If this condition exists for less than a predetermined time interval, such as a few seconds, the voltage regulator then provides power to the system load for the remainder of the time interval. Once the time interval has elapsed, the controller then closes the secondary-power relay, thereby bypassing the voltage regulator entirely and completing a seamless power changeover to the secondary power source. 
     The power switching unit of the illustrative embodiment is advantageous over some systems in the prior art because it increases the flexibility of a conventional, diode-ORed power supply by enabling the source with the lower output voltage—in this case, the primary power source—to provide power to the system load for normal operation. And when the load requires the higher voltage level from the secondary power source, such as during periods of high power demand, the power switching unit is able to switch in the secondary source, wherein diode-ORing of the two sources is again achieved with the higher-voltage secondary source providing power to the load. 
     Although the voltage regulator circuit of the illustrative embodiment has been applied towards the enablement of power switching, in some alternative embodiments the voltage regulator circuit can be used for other applications, as those who are skilled in the art will appreciate. 
     The illustrative embodiment of the present invention comprises: a relay having a first terminal and a second terminal, the first terminal of the relay being electrically coupled to a secondary power source; a voltage regulator circuit having an input terminal and an output terminal, the input terminal of the voltage regulator circuit being electrically coupled to the first terminal of the relay, the output terminal of the voltage regulator circuit being electrically coupled to the second terminal of the relay, and the voltage regulator circuit being capable of selecting between a first output voltage and a second output voltage, wherein the second output voltage is based on a voltage that is present at the input terminal, and the selected output voltage is applied to the output terminal of the voltage regulator circuit; and a first diode and a second diode, each having a first terminal and a second terminal, the first terminal of the first diode being electrically coupled to a primary power source, the second terminal of the first diode being electrically coupled to a load, the first terminal of the second diode being electrically coupled to the second terminal of the relay, and the second terminal of the second diode being electrically coupled to the load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  in the prior art depicts a block diagram of a redundant power supply. 
         FIG. 2  depicts a block diagram of a redundant power supply comprising power switching unit  201 , in accordance with the illustrative embodiment of the present invention. 
         FIG. 3  depicts a block diagram of the salient components of power switching unit  201 , comprising controller  301 , relay  302 , and voltage regulator circuit  303 . 
         FIG. 4  depicts a schematic diagram of the salient components of voltage regulator circuit  303  of unit  201 . 
         FIG. 5  depicts a flowchart of the salient tasks performed by controller  301  in controlling relay  302  and voltage regulator circuit  303 . 
         FIG. 6  depicts a flowchart of the salient tasks that are part of a first saturated mode of operation. 
         FIG. 7  depicts a flowchart of the salient tasks that are part of a second saturated mode of operation. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing the embodiments of the present invention, it should be observed that the invention resides primarily, but not exclusively, in a prescribed arrangement of conventional power supply circuit components and regulation circuitry therefor, which circuitry controls the operation of such components. As a result, the configuration of such components and the manner in which they may be interfaced with other equipment, have, for the most part, been shown in the drawings by readily understandable block diagrams, which depict only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with details which will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagrams of the Figures are primarily intended to show the major components of the invention in convenient functional groupings, so that the invention may be more readily understood. Furthermore, as those who are skilled in the art will appreciate, other components may be interposed throughout the actual design without detracting from the present invention. 
       FIG. 2  depicts a block diagram of the redundant power supply of the illustrative embodiment, in which primary power source  101  and secondary power source  102  provide power to system load  110 . Sources  101  and  102  are connected to load  110  by means of a relatively common technique called “diode-ORing.” The two power supplies are connected to load  110  through associated ORing-diodes  103  and  104 , respectively, wherein source  101  provides voltage V 1  and source  102  provides voltage V 2 . With the two power sources diode-ORed together, the power source with the larger output voltage will establish the voltage delivered to load  110  and present at node  105 . By connecting the power sources in this way, if one power source fails the other source will take up load  110  with little or no interruption in the power provided. 
     Power switching unit  201  is interposed between secondary power source  102  and ORing diode  104 , and provides the switching between the primary and second power source in accordance with the illustrative embodiment. Unit  201  accepts power from source  102  via path  203  and also controls a charging function at source  102  via path  204 . In some alternative embodiments, however, a different device than unit  201  controls the charging function. Unit  201  is described below and with respect to  FIG. 3 . 
     Although the redundant power supply of the illustrative embodiment switches between a primary power source that is not a battery and a secondary power source that is a battery, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments in which the source of the power to the load is switchable between two other types of power sources. 
       FIG. 3  depicts a block diagram of the salient components of power switching unit  201 , in accordance with the illustrative embodiment of the present invention. Unit  201  comprises controller  301 , secondary-power relay  302 , and voltage regulator circuit  303 , interconnected as shown. 
     Controller  301  is a processing-capable component, such as a Field-Programmable Gate Array (FPGA), which is capable of controlling relay  302  and voltage regulator circuit  303  in well-known fashion, via respective paths  313  and  312 . Controller  301  is capable of monitoring the output voltage delivered by primary source  101  and as measured at node  202 , in well-known fashion. Controller  301  is also capable of monitoring whether a predetermined conducting current is present, by monitoring the signal level on path  311 . Additionally, controller  301  is capable of controlling the charging of secondary source  102  via path  204 , in well-known fashion; however, in some alternative embodiments, a component different from controller  301 , and which is not necessarily part of power switching unit  201 , controls the charging function. The salient tasks performed by controller  301  are described below and with respect to  FIGS. 5 through 7 . 
     Secondary-power relay  302  is a component that is used to switch in or out, in well-known fashion, the output voltage delivered from secondary source  102  through path  203  to ORing diode  104 . Relay  302  accepts, via path  313 , a control signal that originates at controller  301 . 
     Voltage-regulator circuit  303  comprises circuitry that is used to switch, when relay  302  is open, between providing a first output voltage and a second output voltage to ORing diode  104 , in accordance with the illustrative embodiment. Circuit  303  is also capable of measuring whether a predetermined conducting current is present and provides that indication to controller  301  via path  311 . In order to operate, circuit  303  accepts one or more reference voltages. Circuit  303  is described below and with respect to  FIG. 4 . 
       FIG. 4  depicts the salient components of voltage-regulator circuit  303 , in accordance with the illustrative embodiment of the present invention. In addition to providing an output voltage while operating in a linear mode, circuit  303  comprises the means to sense whether a predetermined conducting current is present in the output path to load  110 , at node  401 , and also the means to switch between the linear mode of operation and a saturated mode of operation, based in part on the presence of the conducting current and as controlled by controller  301 . 
     Circuit  303  comprises linear voltage regulator U 2  for the purpose of operating in linear mode, in well-known fashion. The particular operating voltage is based on first reference-voltage source  402 , which is related to the first output voltage of circuit  303 . In accordance with the illustrative embodiment, circuit  303  outputs a first output voltage of 21.6 Volts DC when it operates in linear mode, dissipating only a couple of watts while operating in this mode. While in linear mode, regulator U 2  provides a suitable signal that keeps MOSFET transistors Q 1  through Q N  (described below) from going into saturation, while the first output voltage is maintained. 
     In accordance with the illustrative embodiment, circuit  303  also comprises a plurality of input-resistor-and-transistor pairs, each pair comprising namely resistor R 1,n  that is electrically connected to MOSFET transistor Q n , wherein the value of n is between 1 and N, and wherein N is equal to the number of resistor-transistor pairs in the design. Circuit  303  also comprises summing-amplifier resistor R 2,n  which is electrically connected to each input-resistor-transistor pair as shown. Multiple summing resistors are used in order to lessen the effect of the variation of any single resistor from its stated value (i.e., its tolerance). The summing resistors feed into comparator component U 1 , comprising one or more operational amplifiers, which compares the summed signal from the summing resistors against a second reference voltage, and provides an output signal to controller  301  that indicates whether the predetermined conducting current is present or not. In short, summing-amplifier resistors R 2,1  through R 2,N  and comparator U 1  constitute a circuit portion that is used to sense the conducting current. 
     Additionally, circuit  303  comprises a circuit portion that is used to switch between a linear mode of operation and a saturated mode of operation, in accordance with the illustrative embodiment of the present invention. As already mentioned, circuit  303  outputs 21.6 Volts DC when operating in linear mode. In accordance with the illustrative embodiment, circuit  303  outputs a second output voltage of 33.6 Volts DC when it is switched to operate in saturated mode and dissipates around 10 Watts or so while in this second mode. As those who are skilled in the art will appreciate, voltage regulator circuit  303  can be designed to provide a different set of first and second output voltages in some alternative embodiments. 
     The switching of circuit  303  from linear mode to saturated mode is accomplished by controller  301  providing a control signal via path  312  to the gate of each MOSFET transistor Qn. Specifically, when the voltage that that corresponds to the control signal is applied, each transistor Qn is driven into saturation. In saturated mode, circuit  303  itself provides the power to load  110 , albeit for a relatively short amount of time as described later. Otherwise, circuit  303  operates in linear mode for most of the time, thereby dissipating a relatively small amount of power. 
       FIG. 5  depicts a flowchart of the salient tasks performed by controller  301 , in accordance with the illustrative embodiment of the present invention. Controller  301  performs the described tasks, in order to control how relay  302  and voltage regulator circuit  303  operate, so that the source of power for load  110  can be selected between primary power source  101  and secondary power source  102 . It will be clear to those skilled in the art which tasks depicted in  FIG. 5  can be performed simultaneously or in a different order from that depicted. Additionally, it will be clear to those skilled in the art how to condition the signals that are to be received by or that are transmitted from controller  301 , in order to make those signals suitable for use. 
     At task  501 , controller  301  opens secondary-power relay  302 . This has the effect of powering load  110  via primary power source  101 , not secondary source  102 . 
     At task  502 , controller  301  provides a signal to switch voltage regulator circuit  303  to linear mode. In accordance with the illustrative embodiment, circuit  303  as a result outputs 21.6 Volts DC while in linear mode. 
     At task  503 , controller  301  resets counter i to zero. 
     At task  504 , controller  301  monitors for a signal that indicates that a conducting current is present at the output of voltage regulator circuit  303 . If such a signal is received, task execution proceeds to task  505 . Otherwise, no conducting current is present and task execution consequently proceeds back to task  503  with circuit  303  essentially continuing to operate in linear mode until the conducting current is detected to be present. 
     As seen in the flowcharts, controller  301  will conceivably go on to check for the conducting current a total of I max  times during a given period of time that relay  302  is open. The current is checked for, only after the first voltage has been applied to the output terminal of circuit  303  during a predetermined time interval, which is equal in this case to (I max *T), wherein T is described below and with respect to task  602 . In accordance with the illustrative embodiment, I max  has a value of three, and T has a value of one second, but alternative embodiments of I max  and T can have different values, as those who are skilled in the art will appreciate. 
     At task  505 , controller  301  configures power switching unit  201  to operate in a first saturated control mode for a specified wait time, which mode is described below and with respect to  FIG. 6 . 
     At task  506 , after the wait time has passed, controller  301  increments counter i. 
     At task  507 , controller  301  determines whether a check for a conducting current has been made I max  times, wherein I max  is a positive integer. If this is not the case, task execution proceeds to task  508 . Otherwise, controller  301  has checked for the presence of the conducting current I max  times, and task execution consequently proceeds to task  509 . 
     At task  508 , controller  301  switches voltage regulator circuit  303  to operate in linear mode (21.6 Volts DC). The purpose of this is to quickly check whether a conducting current is present. Task execution then proceeds back to task  504 . 
     At task  509 , controller  301  configures power switching unit  201  to operate in a second saturated control mode, which is described below and with respect to  FIG. 7 . This occurs when a predetermined conducting current is still present after a predetermined time terminal has elapsed since the monitoring of the current initially occurred (at task  504 ) and results in relay  302  being closed and power being provided by secondary source  102  until primary source  101  is able to provide the power instead. Once primary source  101  is able, task execution proceeds back to task  503 . 
       FIG. 6  depicts a flowchart of the salient tasks performed when power switching unit  201  operates in accordance with a first saturated control mode. As those who are skilled in the art will appreciate, some of the tasks depicted in  FIG. 6  can be performed simultaneously or in a different order from that depicted. 
     At task  601 , controller  301  switches voltage regulator circuit  303  to operate in a saturated mode. In accordance with the illustrative embodiment, circuit  303  outputs 33.6 Volts DC while in saturated mode. 
     At task  602 , controller  301  waits time T before proceeding to the next task, essentially causing circuit  303  to operate in saturated mode for time T. Task execution then proceeds to task  506 . 
       FIG. 7  depicts a flowchart of the salient tasks performed when power switching unit  201  operates in accordance with a second saturated control mode. As those who are skilled in the art will appreciate, some of the tasks depicted in  FIG. 7  can be performed simultaneously or in a different order from that depicted. 
     At task  701 , controller  301  switches voltage regulator circuit  303  to operate in a saturated mode. In accordance with the illustrative embodiment, in saturated mode the output is 33.6 Volts DC. 
     At task  702 , controller  301  closes secondary-power relay  302 . At this point, power switching unit  201  is providing power from secondary power source  102  to load  110 , not from primary source  101 . 
     At task  703 , controller  301  checks whether primary power source  101  is providing at least a minimally-sufficient output voltage V T , which in accordance with the illustrative embodiment is equal to 22 Volts DC. If not, task execution proceeds back to task  701 , thereby maintaining secondary power source  102  as the source to load  110 . Otherwise, primary power source  101  is now at sufficient voltage, and task execution proceeds to task  704 . 
     At task  704 , controller  301  opens secondary-power relay  302 . At this point, power switching unit  201  is providing power from primary power source  101  to load  110 . 
     At task  705 , controller  301  provides a signal in order to switch voltage regulator circuit  303  to operate in linear mode. In accordance with the illustrative embodiment, circuit  303  as a result outputs 21.6 Volts DC. Task execution then proceeds back to task  503 . 
     In some embodiments, controller  301  configures secondary power source  102  to accept power from primary power source  101  for charging purposes during at least some of the time while voltage regulator circuit  303  is operating in linear mode. 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.