Abstract:
The monitoring of regulated redundant power supplies is enhanced by returning to each power supply a feedback (sensed) voltage signal that is indicative of the voltage level that the respective power supply is outputting and the voltage level supplied to a system load. In one embodiment, a feedback signal that is supplied to a power supply as the sensed signal is derived using a voltage divider network across the output of the power supply and a common connection at which the outputs of the power supplies are “Ored” for delivery to the system load. In this way, each power supply regulates its output voltage as a function of the level of the feedback signal and the level of a respective preset signal.

Description:
FIELD OF THE INVENTION 
     The invention relates to redundant power supply systems and more particularly relates to monitoring concurrently the active and backup power supplies. 
     BACKGROUND OF THE INVENTION 
     Critical circuits within a complex electronic system require a highly reliable source of regulated power. Such systems typically employ several power supplies, active and backup power supplies, to provide the required reliability. A conventional method of combining redundant power supplies uses diodes to “Or” the outputs of the power supplies. To regulate the level of the voltage that is being supplied to the drain (load), the voltage at the drain is sensed and fed back to each of the redundant power supplies. As will be explained below in detail, each such power supply compares the value of the sensed signal with the value of a respective preset signal and changes its output as a function of the difference between the sensed and preset signal. Such regulation may have the effect of causing the output voltage level of one of the power supplies (typically the backup power supply) to decrease significantly, while the other power supply outputs an acceptable voltage level. Apparatus that monitors the backup power supply would thus be unable to determine if that power supply is operating properly, and, more likely, would incorrectly conclude that the backup supply has failed. 
     SUMMARY OF THE INVENTION 
     We have recognized that the foregoing problem may be dealt with, in accordance with an aspect of the invention, by using as the sense signal a feedback signal that is derived as a function of both the voltage signal outputted by a power supply and the voltage signal (sensed signal) delivered to the load. Accordingly, then, the regulation of the outputted voltage signal is based on both the outputted voltage signal and sensed voltage signal. More specifically, in accordance with various aspects of the invention, a feedback signal that is supplied to a power supply as the sensed signal is derived using a voltage divider network across the output of the power supply and a common connection at which the sensed voltage is derived. A power supply then regulates its output voltage as a function of the level of the feedback signal and the level of the preset signal. 
     These and other aspects of the invention will become more apparent from the following detailed description read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING: 
     In the drawing: 
     FIG. 1 is broad block diagram of a conventional regulated power supply; 
     FIG. 2 is broad block diagram of a redundant power supply system regulated in the manner shown in FIG. 1; 
     FIG. 3 is broad block diagram of redundant power supply system regulated in accordance with the principles of the invention. 
    
    
     DETAILED DESCRIPTION 
     The prior art system illustrated in FIG. 1 includes power supply  10  and load  30  represented by resistor RL. Power supply  10  outputs to path  11  a voltage level derived as a function of an externally supplied preset voltage level, V set , which may be established via one of a number of different conventional ways, e.g., programming, zener diode, precision bridge, etc. Path  11  may include distributed resistance (represented in the FIG. by resistor  20  also designated Rd) which decrease the level of the voltage that supply  10  provides to load  30 . The actual level of the voltage that is supplied to load  30  is sensed in a conventional way and fed via path  12  to one input of instrumentation amplifier  40 , whose gain is set by the value of resistor Rg. Amplifier  40  compares the sensed voltage level with the preset voltage level, V set , that is supplied to another input of amplifier  40 . Amplifier  40  then outputs to voltage generator  50  an error signal that is indicative of the difference between the level of the sensed voltage and preset voltage. Voltage generator  50 , in a conventional manner, changes the level of the voltage signal that it is supplying to path  11  with respect to ground. For example, if the sensed voltage is lower (higher) than the preset voltage, then generator  50  increases (decreases) the level of the voltage signal that it is outputting to path  11 . An equilibrium is reached when the level of the sensed voltage equals the level of the preset voltage, V set . 
     This voltage regulation scheme may be applied in a conventional manner to a system having redundant power supplies, as shown in FIG. 2, in which each of the power supplies  100  and  200  are similar to power supply  10  of FIG.  1 . Also, voltage supply paths  211  and  212  extending to load  130  (represented by RL 2 ) may similarly include some distributed resistance respectively represented by resistors Rd 21  and Rd 22 . Diodes D 21  and D 22  provide an Or function as well as isolating power supplies  100  and  200  from one another. The level of voltage supplied to load  130  via path  121  with respect to ground is sensed by sense paths  213  and  214  respectively connected to supplies  100  and  200  in the manner shown in FIG.  1 . As discussed above, power supply  100  adjusts the level of the voltage that it is outputting across path  211  and ground as a function of the difference between the levels of the sensed voltage level supplied via path  21  (sense 21 ) and preset voltage Vset 21 . Power supply  200  operates similarly with respect to the sensed voltage level supplied via path  214  (sense 22 ) and preset voltage Vset 22 . 
     We have recognized that a problem arises when the values of the preset voltages, Vset 21  and Vset 22  have approximately the same nominal value, but do not actually equal one another. Specifically, the power supply connected to the higher preset voltage level continues to increase the level of the voltage signal that it is supplying to load  130  until the level of the voltage at path  121  equals the higher preset voltage level. The power supply that is connected to the lower preset voltage level, on the other hand, continues to decrease the level of the voltage signal that it is supplying to load  130  as a way of attempting to match its preset voltage level with the voltage level supplied via sense path  214 . For example, assume that preset voltage Vset 21  equals +5.3 volts and preset voltage Vset 22  equals +5.2 volts. Although the preset voltage levels nominally equal one another and are within a specified limit, power supply  100  will, nevertheless, increase its output to a point where the voltage supplied to load  130  equals +5.3. Power supply  200  “seeing” that the sensed voltage of +5.3 volts is greater than its preset voltage of +5.2 volts decreases the level of its output voltage to drive the voltage that is being supplied to load  130  toward a value of +5.2 volts. Each time power supply  200  decreases the level of its output voltage the difference between the sensed voltage level and Vset 22  increases. Disadvantageously, power supply  200  continues to operate in this manner until the level of the voltage signal that it is outputting effectively reaches zero. At that point, monitor  400  may conclude that power supply  200  is not operating properly and may output an alarm message indicating that the power supply failed. A craftsperson responding to the message may then replace the supposedly failed power supply  200 . 
     We have further recognized that the foregoing problem may be dealt with by forcing both power supplies to operate as expected. We do this, in accordance with an aspect of the invention, by “tailoring” the sensed voltage level that is supplied to a power supply to the preset voltage level that is connected to that power supply. Such tailoring may be achieved, in accordance with another aspect of the invention, by sensing the voltage level at the load and at the output of a power supply using, for example, a voltage divider across a respective “Oring” diode as is shown in FIG.  3 . In this way, a voltage level with respect to ground will appear at the junction of resistors R 102  and R 103  (R 202  and R 203 ) between the voltage level at  311  ( 312 ) and the voltage level at  321 . In effect, the difference between the voltage level at  311  ( 312 ) and voltage level at  321  will be the voltage drop across diode D 31  (D 32 ). In an illustrative embodiment of the invention, diodes D 31  and D 32  are Shottky diodes having a voltage drop in the range of, for example, 0.2 volts. Thus, the sensed voltage is a value that is weighted in accordance with the values of the resistors forming the voltage divider, which, in accordance with an illustrative embodiment of the invention, equal one another and each may have a value of, e.g., 100,000 ohms. 
     With reference to FIG. 3, the sensed voltage, Vs 1 , supplied to supply  3100  may be expressed as follows:              Vs1   =       (       R103   *   V100     +     R102   *   Vout       )       R102   +   R103               (   1   )                                
     where V 100  is the output voltage at power supply  3100  and Vout is the voltage level at  321 . The sensed voltage supplied to power supply  3200  may be similarly expressed as follows:              Vs2   =       (       R203   *   V200     +     R202   *   Vout       )       R202   +   R203               (   2   )                                
     where V 200  is the output voltage at power supply  3200  and, as mentioned above, Vout is the voltage level at  321 . 
     Based on the foregoing, power supplies  3100  and  3200  may now independently adjust their respective output voltages, V 100  and V 200 , such that the sensed voltages supplied to those power may be adjusted to equal the respective preset voltage level, as will be shown below. Thus, assume that the level of Vset 100  is greater than the level of Vset 200 . For that case we may express Vset 31  as follows:                  Vset   100     =       (       R103   *   V100     +     R102   *   Vout       )       R102   +   R103                         Solving                 for                 Vout                 yields                 the                 following                 expression        :               (   3   )               Vout   =         (     R103        (       Vset   100     -   v100     )         R102     +     Vset   100               (   4   )                                
     In general, Vout and V 100  differ by an amount related to the voltage drop through Oring diode D 31 , which may be, for example, a Shottky diode. As is well known, the voltage drop, δ(I) across a Shottky diode is approximately 200 mv. If we letting V 100 =δ(I)+Vout, then the equation for Vout may be re-written as follows:              Vout   =       Vset   100     -         δ        (   I   )       *   R103       R102   +   R103                 (   5   )                                
     It is noted that V out  will be somewhat less than the setpoint V set100 . For the case where R 102 =R 103 , the difference should be approximately 100 mv. Increasing the resistance of R 102  will minimize this difference. 
     The output voltage V 200  in terms of V 100  may be determined starting with the following expression:                      (     R102   +   R103     )     *     Vset   100       -     R102   *   V100       R102     =           (     R202   +   R203     )     *     Vset   200       -     R202   *   V200       R202             (   6   )                                
     If the values of all of the resistors are equal, then (6) may be expressed as follows: 
     
       
         2* V set 100   −V   100 =2* V set 200   −V   200    (7)  
       
     
     If Vset 100 =Vset 200 +Δ, then V 200  in (7) may be expressed as follows: 
     
       
           V   200 = V   100 −2*Δ  (8)  
       
     
     Thus, as shown by equation (8), when the monitor of FIG. 2 observes the outputs of power supplies  3100  and  3200  it will find that the voltage level V 200  will be slightly less than voltage level V 100 —differing by merely the twice the difference between the setpoints—, thereby confirming that power supply  3200  is operating properly, all in accordance with an aspect of the invention. 
     It will thus be appreciated that, although the invention illustrated herein is described in the context of a specific illustrative embodiment, those skilled in the art will be able to devise numerous alternative arrangement which, although, not explicitly shown or described herein, nevertheless, embody the principles of the invention and are within its spirit and scope.