Abstract:
A redundant power supply connected to a common load is provided. Each power supply is connected to the common load through a series of MOSFET pairs. Each MOSFET in a MOSFET pair is individually controlled to reduce power consumption as well as the need for heat sinks on discrete diodes. Moreover, by providing individually controllable MOSFETs the present invention is capable of switching between power supplies without shorting the power supplies or having a significant drop in bus voltage.

Description:
FIELD 
     The present invention is generally directed to redundant power supplies and more particularly to mechanisms for connecting redundant power supplies to a common load. 
     BACKGROUND 
     Critical power consuming systems which require an uninterrupted power supply are commonly connected to at least two redundant power supply sources. It is typically the case that one of the power supplies is a primary power supply and all other power supplies are backup power supplies provided in case the primary power supply fails or otherwise becomes incapable of providing the necessary power to the power consuming system. Redundant power supplies are very common in many medical applications, such as ventilators, since breath delivery may depend upon power being continuously provided to the power consuming system. Redundant power supplies are also common in many other mission critical applications such as computing applications, database applications, refrigeration systems, and so on. 
     Prior and existing redundant power supply designs utilize discrete diodes to isolate different power sources that are connected to a common power bus. As can be seen in  FIG. 1 , each power source  104   a ,  104   b  in a redundant power supply system  100  generally has its own discrete diode  108   a ,  108   b , respectively, that controls the amount of power provided by each source  104  to the power consuming load  116 . A capacitor  112  may also be connected in parallel across the load  116  to provide temporary power to the bus and hence to the load  116 . 
     The discrete diodes  108  serve to avoid short circuits between power supplies  104   a ,  104   b  and also protect the load  116  from reversed polarity. If one of the power supplies  104   a  fails, the load  116  can continue to operate with power supplied from the other power supply  104   b  without a power interruption. 
     While this solution is popular and has been used in many power consuming systems, there are several disadvantages to utilizing the discrete diodes  108 . First of all, the diodes  108  dissipate a non-trivial amount of power. This causes the overall efficiency of the system  100  to decrease due to the losses realized in the diodes  108 . Additionally, since the diodes  108  dissipate so much power they also generate heat. This creates a need for attaching heat sink devices to the diodes  108  to help cool the diode  108  in systems that deliver significant power. Without the heat sink devices, the diode  108  could overheat and become inoperable, thereby jeopardizing the entire system  100 . 
     SUMMARY 
     There have been some attempts to address the shortcomings associated with utilizing discrete diodes to isolate power supplies. One such example is provided in U.S. Pat. No. 7,038,522 to Fauh et ale, the entire contents of which are hereby incorporated herein by reference. Fauh provides a redundant power supply that utilizes MOSFETs to isolate power supplies. While Fauh has recognized the shortcomings of utilizing discrete diodes to isolate power supplies, Fauh still has disadvantages. For example, each pair of MOSFETs in Fauh are controlled with a single control signal. Hence, all MOSFETs associated with a particular power supply are either active or inactive together depending upon the single control signal received. 
     Thus, in Fauh, a first power supply must be disconnected from the load before a second power supply can be connected to the load, otherwise there may be a risk of shorting the power supplies. The amount of time between when the first power supply is disconnected and the second power supply is connected can cause the bus voltage at the load to drop significantly. The significant potential drop in bus voltage necessitates a relatively larger capacitor to support the bus voltage during the transition. Since a storage capacitor&#39;s cost increases as the capacitor&#39;s capacity increases, the cost of implementing such a system also increases. Other disadvantages exist with large storage capacitors such as time to charge and larger inrush transients. A larger capacitor also requires more volume, a disadvantage in a space constrained product. 
     Some embodiments of the present invention provide a more cost effective and efficient redundant power supply system and methods of operating such a system. In accordance with at least some embodiments of the present invention, a redundant power supply system is provided that generally includes a first power supply operable to provide power to a load via a first isolation switch comprising at least two switching devices, a second power supply operable to provide power to the load via a second isolation switch comprising at least two switching devices, and a controller operable to independently control each switching device in the first and second isolation switches. 
     The use of a controller that is operable to independently control each switching device (e.g., each Field Effect Transistor (FET) in a FET pair associated with a power supply) provides a number of advantages over the prior art. For example, in some embodiments the load is provided continuously with power from a power supply, since the falling power supply will not compromise the other power supply. In some aspects, this occurs because the backup source is diode-OR&#39;d into the bus and the bus voltage will not fall more than a diode drop (e.g., 0.7V) below the backup supply level. This may be particularly advantageous in healthcare applications, such as ventilator applications, where a temporary power loss can be extremely detrimental. 
     In some embodiments, the FET&#39;s body diode and the proper sequencing of FETs in the power switching network help save power, and the discrete diodes and heat sinks may not be needed. At the end of the switching sequence, when the bus is solely connected to the desired power supply, in some embodiments the isolation feature of the body diode is not needed and the FET having the isolation diode is activated. As a result, the final FET activation reduces or eliminates the power loss in the body diode. 
     Some embodiments of the present invention benefit from the discrete isolation-diode topology, but also take advantage of the parasitic body diode in the FET, thus eliminating parts, and shunts the power dissipating diode when the switching is complete. 
     In accordance with at least some embodiments of the present invention, a method of operating a redundant power supply system is also provided that generally includes: 
     determining that a first power supply connected to a load via a first isolation switch and providing power to the load has a decreasing voltage, wherein the first isolation switch comprises at least two switching devices; and 
     independently controlling each switching device in the first isolation switch to cause power to be supplied to the load by a second power supply. 
     In accordance with at least some embodiments of the present invention, the power supplies may or may not provide an identical or even relatively similar voltage. For example, one power supply may be an AC-to-DC converter that conditions AC power from a wall into a 24V de source. The other power supply (i.e., a backup power supply) may be a 24V battery. Alternatively, the other power supply may be a 28V de battery (e.g., a fully charged battery). These are merely exemplary power supply operating voltages that can be supported with embodiments of the present invention. One skilled in the art will appreciate that embodiments of the present invention are not limited to such exemplary voltages discussed herein and that other power source may be accommodated with the appropriate use of other circuit devices. 
     Additional features and advantages of embodiments of the present invention will become more readily apparent from the following description, particularly when taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram depicting a redundant power supply connected to a common load in accordance with embodiments of the prior art; 
         FIG. 2  is a schematic diagram depicting a Source A supplying power to a load with a Source B power supply in a standby state in at least some embodiments of the present invention; 
         FIG. 3  is a schematic diagram depicting a first sequence step of switching from a first power supply source to a second power supply source in accordance with at least some embodiments of the present invention; 
         FIG. 4  is a schematic diagram depicting a second sequence step of switching from a first power supply source to a second power supply source in accordance with at least some embodiments of the present invention; 
         FIG. 5  is a schematic diagram depicting a third sequence step of switching from a first power supply source to a second power supply source in accordance with at least some embodiments of the present invention; and 
         FIG. 6  is a schematic diagram depicting a fourth sequence step of switching from a first power supply source to a second power supply source in accordance with at least some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to  FIG. 2 , an exemplary redundant power supply system  200  will be described in accordance with at least some embodiments of the present invention. The power supply system  200  may comprise a first power supply  204   a  and a second power supply  204   b  operable to provide power to a load  216  along with a capacitor  212  to control a bus voltage at the load  216 . In one embodiment, first power supply  204   a  may be in communication with or connected in circuit with the load  216  via a pair of switching devices  208   a ,  208   b . Similarly, the second power supply  204   b  may be in communication with or connected in circuit with the load  216  via a pair of switching devices  208   c ,  208   d.    
     The load  216  may correspond to any type of circuit adapted to receive and utilize electrical power. For example, the load  216  may be circuitry associated with a ventilator system. Alternatively, the load  216  may be any kind of circuitry, including for example a server, communications gear, a computer, an IV pump, security electronics, etc. 
     In accordance with at least some embodiments of the present invention, the switching devices  208  may be any type of Field Effect Transistor (FET) such as a p-channel MOSFET, n-channel MOSFET, or any other type of current/voltage control mechanism. In accordance with at least some embodiments of the present invention, a switching device  208  used to control whether current flows from a power supply  204  to the load  216  may be variably adjustable. More specifically, while embodiments of the present invention discuss operating the switching devices  208  in either an active or inactive state, the switching devices  208  do not necessarily need to operate in only two binary states. 
     Instead, one or both switching devices  208  associated with a particular power supply  204  may be capable of variably controlling the amount of current flowing there through (i.e., the amount of current provided to the load  216 ). This may occur, in some embodiments, by pulsing the gate and controlling the on-time duty cycle of one or both switching devices  208 . Each pair of switching devices (e.g., the first and second MOSFETs  208   a  and  208   b  collectively) may be referred to herein as an isolation switch. In one embodiment, the MOSFETs  208  each comprise an intrinsic body diode. 
     In the configuration depicted in  FIG. 2 , the first and second transistors  208   a ,  208   b  associated with the first power supply  204   a  are in an active state and substantially no current flows through the body diodes of the transistors  208   a ,  208   b . Rather, a current (depicted by a dashed arrow  232 ) flows through the transistors  208   a ,  208   b  without encountering any substantial resistance and without incurring any significant amount of losses. This current  232  is used to provide electrical power to the load  216 . Meanwhile, third and fourth transistors  208   c ,  208   d  associated with the second power supply  204   b  are in an inactive state and the body diode of the fourth transistor  208   d  is substantially inhibiting current from flowing between the second power supply  204   b  and the load  216 . 
     Although the current inhibiting switching devices  208   b ,  208   d  (e.g., the second and fourth transistors  208   b ,  208   d  comprising the backward biased body diode) are depicted as being behind the first and third switching devices  208   a ,  208   c  (i.e., the second and fourth switching devices  208   b ,  208   d  are down circuit from the first and third switching devices  208   a ,  208   c ), one skilled in the art will appreciate that the order of the switching devices  208  is not limited to the depicted embodiments. As an example, the second and fourth switching devices  208   b ,  208   d  (i.e., the current inhibiting switching devices  208   b ,  208   d  having the backward biased diode) may be up circuit from the first and third switching devices  208   a ,  208   c  and the overall operation of the system  200  will remain substantially the same. 
     The operational states of each switching device  208  may be independently controlled by a controller  224 . The controller  224  may comprise a control output  228   a - d  for each of the switching devices  208   a - d , respectively. Furthermore, the controller  224  may receive input from a monitor circuit  220  that is capable of monitoring the relative potential of each power supply  204   a ,  204   b . In accordance with at least one embodiment of the present invention, the monitor circuit  220  is operable to monitor the supply voltages of each power supply  204   a ,  204   b  and compare them to each other. 
     In accordance with at least some embodiments of the present invention, the monitor circuit  220  may comprise a collection of discrete, linear devices, i.e. comparators and operational amplifiers that are adapted to receive voltage inputs from each of the power supplies  204   a ,  204   b  and compare said voltages. The output of the monitor circuit  220  may correspond to the output of an operational amplifier and may be provided as an input to the controller  224 . The controller  224  may comprise a digital signal processor, a firmware, or other component that is or can be adapted to receive and understand the comparison information provided by the monitor circuit  220  and then control the switching devices  208  based on the comparison information. In an alternative embodiment, the monitor circuit  220  and controller  224  may be combined into a single element such as a digital signal processor with an analog-to-digital converter that is capable of monitoring the power supply voltages and then making a control decision based on that comparison. 
     When the monitor circuit  220  and controller  224  are provided as separate elements, the results of the comparison made by the monitor circuit  220  may be sent to the controller  224  where the decision is made as to whether or not a switch needs to be made between sources of power being supplied to the load  216 . In accordance with at least one embodiment of the present invention, the monitor circuit  220  may compare the relative voltages of the power supplies  204   a ,  204   b  and if one of the power supplies (e.g., the first power supply  204   a ) has a voltage that exceeds the voltage of the other power supply (e.g., the second power supply  204   b ) by a predetermined threshold, then the controller  224  may cause the power supply to the load  216  to switch to the preferred source of power. It should be noted, however, that in certain embodiments a particular voltage difference threshold may need to be exceeded before a switch from one power supply  204  to another occurs since the higher power supply  204  may be a fully charged battery that is desired to be maintained as the backup source for emergency situations. 
     Referring now to  FIGS. 3-6 , a sequence of switching from one power supply to another power supply will be described in accordance with at least some embodiments of the present invention. Initially, the system  200  may operate in the configuration depicted in  FIG. 2 . In this normal power supply configuration, power is supplied to the load  216  via the first power supply  204   a  and its associated switching devices  208   a ,  208   b  are in an active state (i.e., the first and second switching devices  208   a ,  208   b  are switched to a state that causes current to bypass the body diode of the switching devices  208   a ,  208   b ). Additionally, the capacitor  212  is fully charged and the bus current  232  flows through the switching devices  208   a ,  208   b  directly to the load  216 . 
     However, once the monitor circuit  220  and controller  224  have determined that a switch needs to be made from the first power supply  204   a  to the second power supply  204   b  (e.g., because a critical drop in the voltage provided by the first power supply  204   a  has been detected by the monitor circuit  220 ), then the controller  224  begins to initiate the switching sequence depicted in  FIGS. 3-6 . The following sequence of events is typically initiated when the monitor circuit  220  detects that the voltage of the first power supply  204   a  is beginning to decay or has decayed and the voltage of the second power supply  204   b  is at an adequate level. Other thresholds and events may also be used to initiate the switching sequence within the scope of the present invention, including for example the loss of AC Mains, a possible attack on a premise, an earthquake or other natural disruption of power, or the like. 
     As a first step in the sequence, the controller  224  may cause the first switching device  208   a  to go from an active state to an inactive state. As can be seen in  FIG. 3 , this may be accomplished by transmitting a high control signal on the first control line  228   a  to the first switching device  208   a  associated with the first power supply  204   a . It should be noted that while the depicted embodiments of the present invention utilize a low control signal to activate a switching device  208  and a high control signal to deactivate a switching device  208 , alternative designs can be implemented whereby a high control signal is used to activate a switching device  208  and a low control signal is used to deactivate a switching device  208 . 
     Once the first switching device  208   a  associated with the first power supply  204   a  is deactivated, current  232  flows through the body diode of the first switching device  208   a . Also during this state the third and fourth switching devices  208   c ,  208   d  remain in an inactive state. Thus, as the current  232  begins to drop due to the losses of the body diode in the first switching device  208   a , the capacitor  212  begins to discharge and holds up the bus voltage by providing a supplemental current  236  into the load  216 . 
     The next step of the sequence is depicted in  FIG. 4 . In the next step of the sequence, the voltage of the first power supply  204   a  continues to decay and the second power supply  204   b  is switched into the bus by activating the fourth switching device  208   d . This is accomplished by having the controller  224  send an activate signal (e.g., a low control signal) on the control line  228   d  to the fourth switching device  208   d . When the fourth switching device  208   d  is activated, current  240  begins to flow through the third switching device  208   c , because the reversed biased body diode of the fourth switching device  208   d  has been bypassed. While current  232  continues to flow from the first power supply  204   a , the second power supply  204   b  also begins to provide power to the load  216  via current  240 . 
     It should also be noted that during this stage of the switching sequence current is flowing through the body diodes of the first and third switching devices  208   a  and  208   c . However, since this state is maintained for only a brief moment (for example, in one embodiment for less than 100 ms), an insignificant amount of heat generated by the body diodes. Since there is only a small amount of heat generated in the body diodes before the switching sequence continues, in some embodiments there is no need to attach a heat sink to the switching devices  208   a ,  208   c . In an alternative embodiment, one or more heat sinks (not shown) may be used. In some embodiments, the supplemental current  236  may also continue to be provided to the load  216  as needed. 
     The switching sequence continues when the controller  224  deactivates the second switching device  208   b . As can be seen in  FIG. 5 , the decaying power source (i.e. the first power supply  204   a ) is electrically disconnected from the bus and, therefore, is no longer used to supply power to the load  216 . At this point in the switching sequence, the bus voltage is now supplied by the second power supply  204   b . The current  240  from the second power supply  204   b  continues to flow through the body diode of the third switching device  208   e  and also flows through the fourth switching device  208   d  bypassing its body diode. Again, while the current  240  does pass through the body diode of the third switching device  208   c , the length of time that this particular state is maintained is relatively short and the body diode doesn&#39;t have enough time to generate a significant amount of heat that would necessitate the use of a heat sink. In an alternative embodiment, a heat sink (not shown) is included to help ensure the dissipation of heat. 
     Referring now to  FIG. 6 , the controller  224  activates the third switching device  208   c . The decaying first power source  204   a  is still not enabled to provide power to the load  216 . Since the third switching device  208   c  has been activated, the current  244  through the third switching device  208   c  is shunted around its body diode. Thus, substantially no additional current is flowing through any diodes in the system  200 . This allows the bus capacitor  212  to begin charging a diode drop higher in voltage (e.g., 0.7V) with the current  244  provided by the second power supply  204   b . The capacitor  212  will eventually reach a full charge and the current  244  from the second power supply  204   b  will be the primary current used to power the load  216 . 
     In accordance with at least some embodiments of the present invention, the entire switch sequence depicted in  FIGS. 3-6  may be executed in a thermally insignificant amount of time, i.e. perhaps 50 ms. By switching through the sequence this quickly the amount of heat generated in any particular diode is minimal and does not require a heat sink. As can be appreciated by one skilled in the art, the switching sequence may be executed in a greater or lesser amount of time depending upon the amount of power required by the load  216 , the size of the power supplies  204 , the nature of the switching devices  208 , the type of controller  224  being utilized, and other factors, taken alone or in various combinations. The timing of the switching sequence discussed herein can be design dependent and is not limited to the examples discussed. One advantage of the present invention is that while it may be desirable to switch between power supplies  204  within 50 ms, embodiments of the present invention can allow many seconds of time, depending on the particular body diode parameters. As an example, MOSFETs with a larger current capability will take a longer amount of time to switch “off” Than MOSFETs with a relatively smaller current capability. 
     Additionally, since there is a point in time during the switching sequence where both power supplies  204  are providing power to the load  216 , the size of capacitor  212  required to support the necessary bus voltage can be significantly less than would be required if the switching devices  208  associated with a particular power supply  204  were switching on or off together. This reduces the overall costs of implementing the redundant power supply system  200 . There may be some applications where the internal capacitance of the device may be significant enough that a bus capacitor is not required. This can be realized with bypass capacitors on Printed Circuit Boards (PCBs) within the electronic loads. 
     Although only two power supplies  204  are depicted, one skilled in the art will appreciate that additional power supplies (e.g., an additional one, two, three, four, or more) with some or all having a corresponding pair of switching devices may be added to the redundant power supply system  200 . This may be accomplished by adding the additional power supply and pair of switching devices in parallel to the existing power supplies. Independent control lines from the controller  224  may be connected to any additional switching devices when additional power supplies are provided. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best mode presently known of practicing the invention and to enable others skilled in the art to utilize the invention in such or in other embodiments and with various modifications required by the particular application or use of the invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.