Abstract:
The invention relates to a power supply device having several switch-mode power supplies connected in parallel to supply at least one consuming unit, each switch-mode power supply generating an output current I 0  and an output voltage U 0 (I 0 , R L ) that is a function of the output current I 0  and a load resistance R L , and having a control device for each switch-mode power supply, the control device having a first stage with a P element ( 54 ) that receives a P element input voltage which is derived from the output voltage U 0 (I 0 , R L ), and generates a P element control voltage U VS , that is used to control the respective switch-mode power supply, the first stage being active when 0≦I 0 ≦I 0P , a second stage having a current reproduction circuit which reproduces the output current I 0  of the respective switch-mode power supply and generates an output current control voltage U P  which is used to control the respective switch-mode power supply, the second stage being active when I 0P ≦I 0 ≦I 0S , and a third stage having an amplifier circuit which amplifies a signal proportional to the output current I 0  and generates an amplified output current control voltage m·U S  which is used to control the respective switch-mode power supply, the third stage being active when I 0S ≦I 0 ≦I K .

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a National Stage of International Application No. PCT/EP03/03275, filed Mar. 28, 2003, which claims priority to German patent application 102 14 190.8, filed Mar. 28, 2002, the entire contents of each of which are hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The invention relates to a power supply device having several switch-mode power supplies connected in parallel to supply at least one load, each switch-mode power supply generating an output current and an output voltage, and having a control device for each switch-mode power supply. The control device controls the output voltage of the switch-mode power supply which is a dependent on the output current and a load resistance. 
   BACKGROUND OF THE INVENTION 
   The basic principles of switch-mode power supplies connected in parallel are described, for example, in Elektronik, Volume 13, 2000, pages 114–118 “Schaltnetzteile parallel geschaltet-technische Details zur passiven Stromaufteilung” by Martin Rosenbaum. The aim of connecting switch-mode power supplies in parallel is to increase the power by increasing the output current and to reduce the failure rate by providing redundant switch-mode power supplies in such a way that a defective power supply unit can be exchanged during the operation of a device which is supplied by the switch-mode power supplies. Connection in parallel can be realized by means of an active current division or a passive current division. 
   Active current division measures the output current of each power supply and controls the output voltages as a function of the output current of all switch-mode power supplies resulting in a uniform division of current to one or more loads. This method has the advantage that an exact division of current and a uniform load on the switch-mode power supplies connected in parallel can be achieved. The disadvantages can be seen in the greater complexity of the circuitry and the higher costs thus incurred. 
   In the case of passive current division, the current division is made as uniform as possible by setting a “softer output characteristic” for the switch-mode power supply as shown, for example, in  FIG. 1 . The advantages include a less complex circuitry and the almost limitless number of switch-mode power supplies that can be connected in parallel. A disadvantage is the somewhat less exact division of current in some applications. 
     FIG. 2  shows a block diagram of an example for N switch-mode power supplies  10 ,  11 ,  12  connected in parallel which supply a load  13 . Further details on the circuit illustrated in FIG.  2  are illustrated and explained in the above-mentioned article in Elektronik 13/2000. Reference is made to this publication. 
   In order to set the output characteristics of the individual switch-mode power supplies connected in parallel as required, where passive current division is concerned the prior art provides for one or more shunt resistors to be added to the output line of the respective switch-mode power supplies so that the output voltage of the respective switch-mode power supplies is established as a function of the load, within certain tolerances, according to predetermined characteristics.  FIG. 3  shows an example of such an output characteristic for a single switch-mode power supply which has three ranges that can be established by the provision of three shunt resistors connected in the output line of the switch-mode power supply. 
   The output characteristic shown in  FIG. 3  occurs in a first range I, which characterizes the normal operation of the switch-mode power supply, being relatively flat with only a slight voltage drop following an increase in output load and thus an increase in output current. A first shunt resistor R VS  is active in this range I, which could also be formed by the line resistance. When the output current I 0  exceeds a first threshold value I 0P , a second shunt resistor R P  is connected which causes the voltage at the output of the switch-mode power supply to fall more strongly. This second range, indicated by II, can, for example, be a charging range in which the power supplies not only supply a load but also charge batteries or other energy storage units, which are intended as an emergency power supply to supply the load during a power failure. 
   When the output current I 0  of the respective switch-mode power supply exceeds a further threshold value I 0S , a third shunt resistor is activated which is dimensioned in such a way that the voltage output characteristic of the switch-mode power supply declines abruptly. This range, indicated by III, can be considered a safety cut-off range in which the switch-mode power supply is short circuited and turned off when a specific threshold current I 0S  is exceeded. The third shunt resistor is indicated by R S . 
   Although the above-described prior art solution for establishing the output characteristic of the switch-mode power supplies as a function of the output current has a simple circuitry and allows for different operating ranges of the output characteristic, the shunt resistors, which can be located in the power supply unit or outside it in the output line of the switch-mode power supply (s.R L  in  FIG. 2 ), generate considerable losses and thus reduce the overall efficiency of the power supply and of the system. 
   The object of the invention is to provide a power supply device having several switch-mode power supplies connected in parallel to supply at least one load which operates with a passive current division and allows the output characteristic of each switch-mode power supply to be adjusted for different operating ranges. This object is solved by a power supply device having the characteristics outlined in claim  1 . 
   SUMMARY OF THE INVENTION 
   The invention proposes a power supply device having several switch-mode power supplies connected in parallel to supply at least one load in which each switch-mode power supply generates an output current I 0  and an output voltage U 0 (I 0 , R L ), which is a function of the output current I 0  and an associated load resistance R L . A control device is provided for each switch-mode power supply. In accordance with the invention, the control device is divided into three stages for the purpose of creating an output voltage characteristic of the respective switch-mode power supply having three operating ranges. These three operating ranges are preferably characterized by an output characteristic of the switch-mode power supply which declines more steeply as the load, and thus the output current, increases, as shown for example in  FIG. 3 . 
   The first stage has a P (proportional) element that receives a P element input voltage which is derived from the output voltage U 0 (I 0 , R L ), and generates a P element control voltage U VS  that is used to control the respective switch-mode power supply. The P element generates a slightly declining output characteristic whose height can preferably be adjusted. The first stage is active in a normal operating range up to a first threshold value I 0P  of the output current I 0  and can be deactivated on exceeding the threshold value I 0P . 
   The second stage has a current imaging circuit which reproduces the output current I 0  of the respective switch-mode power supply and generates an output current control voltage U P  which is used to control the respective switch-mode power supply. The output current control voltage U P  is directly proportional to the output current I 0  and is adjusted in such a way that a more strongly declining output characteristic of the switch-mode power supply is produced. The second stage is active when the output current I 0  exceeds the first threshold value I 0P  which is selected in such a way that it characterizes, for example, a departure from the normal operation and a transition to a charging operation of the switch-mode power supply, as described above with reference to  FIG. 3 . 
   The third stage has an amplifier circuit which amplifies a signal proportional to the output current I 0  and generates an amplified output current control voltage U S  which is used to control the respective switch-mode power supply. The third stage is preferably connected down-stream from the second stage and uses the output current control voltage U P  as its input signal. The amplification of the third stage is preferably adjusted in such a way that a steeply declining output characteristic of the switch-mode power supply is produced. The third stage is active when the output current I 0  exceeds a second threshold value I 0S  which, for example, indicates an overload condition making it necessary to turn off the switch-mode power supply. 
   The power supply device according to the invention makes it possible to adjust the output characteristic of a switch-mode power supply according to specifications in several different operating ranges, the adjustment being largely loss-free due to the use of the P element, the current imaging circuit and the amplifier circuit, but nevertheless requiring a less complex circuitry than is the case with an active current division which is based on measuring the output currents of all the switch-mode power supplies and effecting control depending on the measurement of all the switch-mode power supplies. 
   According to the invention, the operating ranges in which the first, second or third stage are active are controlled as a function of the output current I 0 . The ranges are indicated in  FIG. 3  by I, II, III. If I 0  is located in range I, only the first stage is active; if I 0  is located in range II, the second stage is active, the P element of the first stage draws the output voltage of this first stage to zero, as described in more detail below, so that the first stage no longer exerts any influence. If the output current I 0  is located in range III, although the second stage remains active, the third stage dominates due to the considerably higher amplification factor so that the contribution of the second stage to the control voltage can be largely disregarded, as is explained below in more detail. 
   The control device preferably comprises a pulse width modulator component with an integrated coupling amplifier which receives the P element control voltage U VS , the output current control voltage U P  and the amplified output current control voltage mU S  and generates a control signal V T  for the respective switch-mode power supply as a function of these control voltages. Depending on which stage is activated, the pulse width modulator component receives the P element control voltage U VS , the output current control voltage U P  and/or the amplified output current control voltage mU S . 
   In a preferred embodiment, the first stage includes a voltage divider that determines the size of the output voltage U 0  and generates a P element input voltage that is proportional to the output voltage U 0 . In addition, the output characteristic of the switch-mode power supply can be shifted using a controlled current source that is connected to the voltage divider, as described in more detail below. 
   The P element of the first stage preferably has an operational amplifier, one of whose inputs receives the P element input voltage and whose other input receives a first reference voltage U ref1  and whose output emits the P element control voltage U VS . The operational amplifier is preferably connected to the pulse width modulation component via a blocking diode. 
   The current imaging circuit of the second stage preferably has a transformer element that is connected in parallel to the main transformer element of the respective switch-mode power supply and generates an output signal that is proportional to the output current I 0  of the switch-mode power supply. This is explained in more detail with reference to the figures. 
   The amplifier circuit of the third stage preferably takes the form of an operational amplifier, one of whose inputs being connected to the current imaging circuit of the second stage and whose other input is connected to the reference voltage and whose output emits the amplified output current control voltage. 
   From DE 100 19 329, a power supply having several switch-mode power supplies connected in parallel is known which is regulated to generate an output characteristic in several sections. A first segment of the characteristic has a constant output voltage, a second segment corresponds to a straight line with a declining gradient and a third segment provides a short-circuit current limiter. In the first stage, DE 100 19 329 uses an I (integrating) element to generate a constant output voltage to regulate the current—in contrast to operating range I according to the invention. The prior art circuit, however, would not function with a pure P (proportional) element. One reason is that in DE 100 19 329 the current is mainly measured by using an optocoupler. The optocoupler amplification or attenuation influences the open-loop gain which determines the gradient of the characteristic. The optocoupler amplification, however, is not linear and is heavily dependent on the temperature, it is also dependent on the respective tolerances of the actual components used. Thus the solution revealed in DE 100 19 329 is not suitable for the generation of identical, reproducible characteristics for several switch-mode power supplies connected in parallel since by using different optocouplers, each switch-mode power supply would generate a deviating, non-predictable characteristic making a defined current division impossible. Moreover, the optocoupler generates non-linearities which have to be compensated by correspondingly high amplification. 
   An advantage of using a P element according to the invention is that current-dependent characteristics can be generated, in contrast to segment 1 of DE 100 19 329, and that the equations of the straight lines can be set more easily. This is important for the load division when several power supplies are operated in parallel. 
   Another very important difference between the invention and DE 100 19 329 is that DE 100 19 329, like the prior art described above, operates with a resistance shunt in the output branch. The output characteristic is mainly established by the resistance shunt and the entire output current passes through this shunt. As a result, considerable losses are incurred. Moreover, the characteristic can be less flexibly adjusted than in the invention. 
   In contrast, the invention does not need an ohmic load to establish the characteristic in the output branch, it being possible to set any required curve equations. The invention can nevertheless realize short circuit current limitation. 

   
     SHORT DESCRIPTION OF THE DRAWINGS 
     The invention is described in more detail below on the basis of preferred embodiments with reference to the drawings. The figures show: 
       FIG. 1  a diagram with three output characteristics of three switch-mode power supplies connected in parallel according to the prior art; 
       FIG. 2  shows in block diagram form the connection in parallel of several switch-mode power supplies to supply a load according to the prior art; 
       FIG. 3  shows the diagram of an output characteristic of a switch-mode power supply which is to be established according to the invention; 
       FIG. 4  shows a circuit diagram of a power supply device having a control device according to the invention, with only the output stage of a switch-mode power supply being schematically represented in  FIG. 4 ; 
       FIG. 5  shows a circuit diagram of the input stage of a switch-mode power supply according to the prior art which could be used in conjunction with the invention; 
       FIG. 6  shows a schematic block diagram of a power supply device according to the invention for the supply of several loads. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 , which has been described above, shows a diagram with three so-called “soft” characteristics of three switch-mode power supplies connected in parallel with a passive current division according to the prior art. An example of three or n switch-mode power supplies connected in parallel according to the prior art is shown in  FIG. 2 .  FIG. 2  shows a first switch-mode power supply  10 , a second switch-mode power supply  11 , and an nth switch-mode power supply  12 , which are connected in parallel and wired symmetrically to a load  13 . The line resistances of the wiring are schematically represented by the resistors R L . In the arrangement shown in  FIG. 2 , the line resistors R L  correspond to the first shunt resistor to set the output characteristic of each switch-mode power supply  10 ,  11 ,  12 , it being necessary in the prior art to provide an additional shunt resistor in order to achieve an exact current division. This solution is inflexible, however, and results in additional losses at the shunt resistor. 
     FIG. 3 , which has been described above, shows an output characteristic of a switch-mode power supply having three operating ranges which are to be created by the power generating device according to the invention. Applied to the schematic representation in  FIG. 2 , it is the object of the invention that each of the switch-mode power supplies  10 ,  11 ,  12  generates an adjustable output characteristic, such as an output characteristic in accordance with  FIG. 3  or any other, without the need for loss-related shunt resistors.  FIG. 3  shows a first operating range I which characterizes the normal operation of the power supply device and ends at a first threshold current I 0P , a second operating range II, which characterizes the charging operation of the power supply device according to the invention and ends at a second threshold current I 0S , and a third operating range III, which characterizes the cut-off range of the power supply device according to the invention, the power supply device cutting out completely when there is a short-circuit current I K . 
     FIG. 4  shows a schematic circuit diagram of the power supply device according to the invention with only the output stage of a switch-mode power supply and the associated control device being schematically illustrated in  FIG. 4 . 
     FIG. 4  schematically illustrates the output stage of a switch-mode power supply  20  having a controlled electronic switch  22 , which is a MOS-FET in the illustrated embodiment but can be realized as an IGBT or any other suitable transistor switch, and a storage capacitor  24  as well as an output transformer  26 . Downstream from the output transformer  26  are an output/free-wheeling diode  28  and an LC circuit  30  which rectify the chopped output voltage of the transistor switch  22  and transformer  26 . The output current of the switch-mode power supply  20 , that is illustrated only schematically in  FIG. 4 , is indicated by I 0 , and the output voltage is indicated by U 0 . At the output of the switch-mode power supply  20 , a load resistor R L    32  is illustrated in  FIG. 4  representing one or more loads. In the embodiment illustrated in  FIG. 4 , the output stage of the switch-mode power supply  20  further comprises a second controlled electronic switch  61  which is controlled in common mode with the first switch  22 . 
   For explanatory purposes,  FIG. 5  shows an example of an input stage of the switch-mode power supply according to the prior art which can be connected upstream of the output stage shown in  FIG. 4 . However, this input stage of a switch-mode power supply serves only by way of example since the invention can be realized using all kinds of switch-mode power supplies. In particular, the switch-mode power supply of  FIG. 5  comprises an input rectifier consisting of four rectifier diodes  34 ,  35 ,  36 ,  37 , which are arranged in the form of a bridge circuit. The rectifier bridge receives its input AC voltage, in particular a mains voltage, at the connections X 1 , X 2  and sends its rectified output voltage via a storage and smoothing inductor  38 , through which a current passes in one direction only, to a controlled electronic switch  40  which is connected via the output of the bridge rectifier. The transistor switch  40  receives a control voltage U i , which is not specified in more detail in  FIG. 5  and determines the output voltage of the switch-mode power supply. Associated with the transistor switch  40  is an output/free-wheeling diode  42  which rectifies the chopped output voltage of the transistor switch. At the output of the switch-mode power supply, a unipolar storage capacitor  44  is connected to store and smooth the output voltage. 
   According to a well-known control method, the controlled electronic switch  40 , or  22 ,  61  in  FIG. 4 , is operated at a high switching frequency U T  compared to the mains frequency of the AC voltage supply (at the connections X 1 , X 2 ). By changing the relative switch-on duration of the electronic switch  40  or  22 ,  61 , it is possible to adjust the output voltage U C  at the capacitor  44  or  24  and thus the output voltage of the switch-mode power supply U 0 . 
   Referring again to  FIG. 4 , we will now describe how the control voltage U T  is determined by means of the three-stage control device according to the invention. The first stage of the control circuit according to the invention is illustrated in  FIG. 4  in a box indicated by  50 , the second stage is in a box indicated by  60  and the third stage is in a box indicated by  70 . 
   The first stage  50  of the control circuit comprises a voltage divider consisting of resistors  51 ,  52 ,  53 , a P element, that takes the form of an operational amplifier  54 , and a blocking diode  56 . These components are connected to each other as shown in  FIG. 4 . The output voltage U 0MAX  can be adjusted via the voltage divider  51 ,  52 ,  53  and a selectable, constant first reference voltage U REF1 . 
   The voltage divider  51 ,  52 ,  53  is dimensioned in such a way that for the required output voltage U 0 , a voltage is generated at the connection between the resistors  52  and  53 , this voltage essentially corresponding to the first reference voltage U REF1 . Accordingly, the P element  54  generates a P element control voltage U VS , which is applied via the diode  56  to a pulse width modulation component  80  in order to control the switch-mode power supply  20  in such a way that produces the slightly declining output characteristic (due to the effect of the P element) in range I of  FIG. 3 . 
   In a preferred embodiment of the invention as shown in  FIG. 4 , a current reduction device  58  can be provided to adjust U 0MAX  in order to generate a characteristic field according to requirements. A second characteristic with a slightly downwards shifted U 0MAX  is shown in  FIG. 3 , for example, by the broken line. For this purpose, the voltage divider is divided into the resistors  51  (R T ) and  52  and a controlled current source or a current reduction device  58  is connected to the connecting point between the resistors  51  and  52 . The current reduction device  58  draws a constant current I T  through the resistor  51  (R T ), so that at the resistor  51 , an additional constant, adjustable voltage drop occurs which shifts the output characteristic of the switch-mode power supply as required. 
   The output signal U VS  of the P element  54  is applied to the pulse width modulation component  80 , having an integrated coupler amplifier, which generates the control signal U T  for the switch-mode power supply  20 . 
   As long as the output current I 0  of the switch-mode power supply  20  remains under a predetermined first threshold value I 0P  that characterizes the end of a normal operating range I, the second stage  60  and the third stage  70  do not emit any output signals. The output voltage of the switch-mode power supply  20  is then given as:
 
 U   0 ( I   0 )= U   0MAX   −R   VS   *I   0   −R   T   *I   T ,
 
where
 
   
     
       
         
           
             R 
             VS 
           
           = 
           
             
               R 
               Powerloss 
             
             Closedloopgain 
           
         
       
     
   
   When the output current I 0  exceeds the first threshold value lop, the second stage  60  is activated in the illustrated embodiment. The second stage  60  is active in range II, the output voltage U 0  in this range being smaller than the first reference voltage U REF1 , so that the P element  54  of the first stage has a high-ohmic output and the first stage  50  thus makes no further contribution to the adjustment of the control signal U T . 
   The second stage  60  of the control device consists of a transformer  62 , a zener diode  63 , a capacitor  64  and a resistor R P    65 , which are connected to each other as shown in  FIG. 4 . The transformer  62  is controlled by the electronic switch  61 . 
   The control signal U T  is applied in parallel to the two electronic switches  22 ,  61  so that they are switched in common mode. The primary current of the switch-mode power supply, which flows through the switches  22 ,  61  and the transformer  62 , corresponds exactly with the secondary current through the diodes  28 , multiplied by ü 1 . The transformer  62  divides the primary current by ü 2 . Thus the output current through the second transformer  62  is an exact replica of the output current I 0  of the main transformer  26  divided by (ü 1 ·ü 2 ). The voltage drop via the resistor R P    65  is thus a measure for the output current I P , according to the following equation: 
   
     
       
         
           
             U 
             P 
           
           = 
           
             
               
                 I 
                 O 
               
               · 
               
                 R 
                 P 
               
             
             
               
                 Ü 
                 1 
               
               · 
               
                 Ü 
                 2 
               
             
           
         
       
     
   
   Thus with the aid of the second transformer  62 , a replica of the output current I 0  can be generated without any significant current load on the switch-mode power supply. The output voltage U P  of the second stage  60  is applied to the pulse width modulation component  80  via a controlled switch  68 . At a control input, the controlled switch  68 , schematically illustrated in  FIG. 4  by a comparator and a transistor switch, receives a second reference voltage U REF2  which is selected in such a way that the output signal U P  of the second stage is only imposed on the pulse width modulation component  80  when the output current I 0  exceeds the second threshold value I 0P . For this purpose, U REF2  is adjusted as follows: 
             U   REF2     =         I   OP     ·     R   P           Ü   1     ·     Ü   2               
the output voltage U P  of the second stage  60  is applied to the pulse width modulation component  80  as described above in order to control the pulse width modulation component  80  and to generate a required control signal U T  for the switch-mode power supply.
 
   On activation of the second stage  60 , the output voltage U 0  of the switch-mode power supply  20  is given as:
 
 U   0 ( I   0 )= U   0 ( I   0P )− k*R   P   *I   0  
 
where
 
   
     
       
         
           k 
           = 
           
             1 
             
               
                 Ü 
                 1 
               
               · 
               
                 Ü 
                 2 
               
             
           
         
       
     
   
   It is clear that through a suitable choice of R P , the rise in the output characteristic of the switch-mode power supply  22  can be influenced. Since the characteristic is only adjusted with the aid of the current imaging, it is not necessary to add another resistor to the actual output circuit of the switch-mode power supply so that losses can be kept to a minimum. 
   When the output current I 0  then exceeds a second threshold value I 0S , the third stage  70  of the control circuit is activated. The activation of the third stage  70  can be adjusted via a third reference voltage U REF3 , where 
   
     
       
         
           
             U 
             REF3 
           
           = 
           
             
               
                 I 
                 OS 
               
               · 
               
                 R 
                 P 
               
             
             
               
                 Ü 
                 1 
               
               · 
               
                 Ü 
                 2 
               
             
           
         
       
     
   
   The third stage  70  of the control circuit consists of an input diode  71  and a capacitor  72 , which form an input rectifier, as well as an amplification circuit, which is indicated in its entirety by  74  and, alongside other resistors and capacitors, has an input resistor R S    73 , and an output diode  75 , which are connected to each other as shown in  FIG. 4 . The third stage  70  of the control device receives as its input signal the output signal U P  of the second stage  60  which is proportional to the output current I 0  of the switch-mode power supply  20 . Up is a pulsed signal dependent on the control signals U T . This pulsed signal U P  is rectified by the rectifier part  71 ,  72  of the third stage  70  so that a rectified voltage U S  is applied at the input resistor R S    73  of the amplifier part  74  of the third stage, the amplitude of the rectified voltage corresponding to the voltage U P . The third stage  70  generates a control signal U S =U P  (rectified)=k·R P ·I 0 , that is amplified by the amplifier circuit  74 . The amplifier circuit  74  is designed in such a way that it has a relatively high amplification factor, m&gt;&gt;1. An output signal m·U S  is produced. 
   The output signal mU S  of the third stage  70  is entered into the pulse width modulation component  80  in order to generate the control signal U T  that generates a steep output characteristic U 0  of the switch-mode power supply  20  which, for a short-circuit current I K , becomes 0 (see range III in  FIG. 3 ). The output characteristic U 0  of the switch-mode power supply  20  is in the range I 0S &lt;I 0 &lt;I K :
 
 U   0 ( I   0 )= U   0 ( I   0S )− k*m*R   P   *I   0  
 
   The power supply device according to the invention is used in all systems in which redundant switch-mode power supplies are needed for purposes of safety during a power failure or such-like. The invention can particularly be employed in telecommunications systems, computer systems and all other kinds of control and communications systems which need a failure-proof energy supply. In addition to the loads connected to the power supplies, batteries can also be connected which take over the supply of energy during a power failure. In its output characteristic, the power supply device according to the invention thus provides an operating range for normal operation, an operating range for charging operation under higher load and an operating range for a cut-off when there is an overload. 
     FIG. 6  shows an example of an environment in which the power supply device according to the invention can be employed. In  FIG. 6 , a mains supply is indicated in general by  90 , the mains supply  90  providing an AC voltage in the range of 90 to 230 volts and having a device for the distribution of the AC voltage to several switch-mode power supplies and the necessary interference filters on the mains side and other necessary filter devices. The mains  90  supply  n  switch-mode power supplies  92 ,  94 ,  96 ,  98  that are indicated in  FIG. 6  by Rectifier Module. In the illustrated embodiment, the switch-mode power supplies  92 – 98  should be able to provide an output power POUT of between 300 W and 2 kW. A control device, as described in reference to  FIG. 4 , is associated with each switch-mode power supply of  FIG. 6  in order to establish a desired output characteristic, the control devices not being illustrated in  FIG. 6 . The switch-mode power supplies  92 – 98  are connected via a common line to several loads  100 – 112  as well as to batteries  114 , all of which operate with a voltage in the range of 48 volts DC to 56 volts DC and which can have different power requirements, power ranges POUT from 10 watt to 100 watt and from 100 W to 300 kW being given by way of example. The loads  100 – 112  can include micro processor cards, telecommunications cards, DC converters on cards in electronic data processing systems, 19 inch DC converters for server cabinets or suchlike, all kinds of electric and electronic systems, ventilators and air conditioning units and suchlike. One example of the invention&#39;s application is in telecommunications systems which have all these components. In normal operation, i.e. in range I of the characteristic shown in  FIG. 3 , the power supplies  92 – 98  supply the loads  100 – 112  with an essentially uniform current flow and maintain the voltage of the batteries  114  at a required level, e.g. 48–56 V. When the voltage level of the batteries  114  falls during start-up or due to a disruption, during maintenance or such like, the switch-mode power supplies  92 – 98  have to recharge the batteries  114  in addition to supplying the loads  100 – 112  so that the output current of the switch-mode power supplies  92 – 98  increases due to the heavier load which means that the output characteristic of the switch-mode power supplies moves into operating range II. Once the batteries  114  have been fully charged, the current drain generally decreases again so that normal operation in operating range I can once more be assumed. In the event of a malfunction or failure in which an excessively large current I 0 &gt;I 0 S is drawn, the output characteristic of the switch-mode power supplies  92 – 98  moves into the third operating range III, which, after another increase in the output current I 0  results in the switch-mode power supplies  92 – 98  being short-circuited and not delivering any more voltage. The system illustrated in  FIG. 6  can then be supplied for a limited period by the batteries  114  before it cuts out completely unless the failure or malfunction is remedied. 
   The characteristics revealed in the above description, the claims and the figures can be important for the realization of the invention in its various embodiments both individually and in any combination whatsoever. 
   IDENTIFICATION REFERENCE LIST 
   
       
         10 ,  11 ,  12  Switch-mode power supplies 
         13  Consuming unit 
         20  Switch-mode power supply 
         22  Switch 
         24  Storage capacitor 
         26  Output transformer 
         28  Output/free-wheeling diode 
         30  LC circuit 
         32  Load resistor 
         34 ,  35 ,  36 ,  37  Rectifier diodes 
         38  Storage and smoothing inductor 
         40  Switch 
         42  Output/free-wheeling diode 
         44  Storage capacitor 
         50  First stage 
         51 ,  52 ,  53  Resistors 
         54  Operational amplifier 
         56  Blocking diode 
         58  Current reduction device 
         60  Second stage 
         61  Switch 
         62  Transformer 
         63  Zener diode 
         64  Capacitor 
         65  Resistor 
         68  Controlled switch 
         70  Third stage 
         71  Input diode 
         72  Capacitor 
         73  Resistor 
         74  Amplification circuit 
         75  Output diode 
         80  Pulse width modulation circuit 
         90  Mains supply 
         92 ,  94 ,  96 ,  98  Switch-mode power supplies 
         100 – 112  Consuming unit 
         114  Battery