Abstract:
Load sharing apparatus for load sharing between a plurality of power supplies ( 1, 2 ). The apparatus comprises one or more load sharing modules (Share 1 , Share 2 ), each for association with a respective power supply. The or each load sharing module comprises load determining means for generating a signal which represents a load value corresponding to the power or current supplied by the respective power supply, and voltage control means for controlling the output voltage of the respective power supply to vary inversely with said load value at a first rate when the load value is below a threshold value, and for controlling the output voltage of the respective power supply to vary inversely with said load value at a second rate when the load value equals or exceeds said (first) threshold value. Said second rate is greater than said first rate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to load sharing apparatus for load sharing between a plurality of power supplies. 
         [0002]    In systems which are required to operate with high reliability, it is often necessary to operate multiple power supplies in parallel, such that if one power supply should fail, a sufficient number of power supplies remain in operation for the system to continue to operate. This is known as “n+1 redundancy”. 
         [0003]    For example, 4 power supplies rated at 300 W would be required to supply a 900 W load in a 3+1 redundant system, assuming that the power supplies share the load equally. 
         [0004]    In practice, however, power sharing accuracies of +/−10% of full load are not uncommon, due to variations in output voltage setting accuracy and load regulation characteristics between power supplies. 
         [0005]    Thus, in the above example, at least one additional power supply is required, to ensure that no one power supply is caused to operate above its  300 W rating. 
         [0006]    There are two established techniques used for improving the sharing accuracy between parallel connected power supplies. 
         [0007]    One method is to use a share bus. A share bus is an analogue or digital interface between the parallel connected power supplies, which forces the power supplies to output the same current. However, this solution is not appropriate for high reliability applications because failure of the share bus can cause the whole system to fail. 
         [0008]    The second method is to use a technique known as droop sharing. In this technique, the load regulation of a power supply, i.e. the variation of output voltage with current, is deliberately made higher. This forces the output voltage to reduce as the power supply is loaded. 
         [0009]    Where power supplies are parallel connected, the power supply with the highest output voltage will deliver current to the load. However, if droop sharing is applied, the output voltage of that power supply will reduce as it is loaded, thereby forcing the other supplies to deliver some current as well. The amount of droop, over the load range (i.e. the total reduction in voltage over the load range), needs to exceed the variation in output voltage setting accuracy if all supplies are to deliver some current to the load. However, if good sharing accuracy is required, the amount of droop must be substantially higher. 
         [0010]    A known technique which allows good sharing accuracy at full load, is to apply droop only when the output current or power of each power supply exceeds a threshold load value close to 100% of full load, by applying a large dynamic impedance. In this way, the output voltage of the respective power supply falls significantly for only a small change in current above this threshold. This ensures that load sharing takes place before any one power supply exceeds is load rating. 
         [0011]    However, this technique does not allow for load sharing at lighter loads, with the result that only one or some of the power supplies deliver power to the load during normal operation. This results in a temperature difference between the power supplies and increases the spread in their mean time to failure (MTTF). 
         [0012]    Moreover, the above technique does not allow peak current to be drawn from the power supply, because the output voltage will become low above 100% load. 
       SUMMARY OF THE INVENTION 
       [0013]    According to one aspect of the present invention, there is provided load sharing apparatus for load sharing between a plurality of power supplies, the apparatus comprising one or more load sharing modules, each for association with a respective power supply, the or each load sharing module comprising: 
         [0014]    load determining means for generating a signal which represents a load value corresponding to the power or current supplied by the respective power supply; and 
         [0015]    voltage control means for controlling the output voltage of the respective power supply to vary inversely with said load value at a first rate when the load value is below a (first) threshold value, and for controlling the output voltage of the respective power supply to vary inversely with said load value at a second rate when the load value equals or exceeds said (first) threshold value; 
         [0016]    wherein said second rate is greater than said first rate. 
         [0017]    Thus, with the present invention, droop can be applied over the full load range of each power supply, but with a two-stage profile, which is less steep at lighter loads than at heavier loads. 
         [0018]    As a result, high sharing accuracy can be achieved at high loads, whilst still allowing for some load sharing at lighter loads. 
         [0019]    By improving the sharing accuracy at high loads, the number of power supplies needed to achieve n+1 redundancy is reduced, which in turn reduces both the overall cost and size of the power supply system. 
         [0020]    By also allowing for load sharing at lighter loads, a reduced spread in MTTF of the power supplies is achieved. 
         [0021]    The (first) threshold load value is preferably between 95% and 100% of the maximum load rating of the power supply. 
         [0022]    In a preferred embodiment, the voltage control means is configured for controlling the output voltage of the respective power supply to vary inversely with said load value at a third rate, which is lower than the second rate, when the load value equals or exceeds a second threshold value which is higher than the first threshold value. 
         [0023]    More specifically, the load sharing means may comprise limiting means for limiting the total voltage reduction at said second rate, such that the output voltage of the respective power supply varies at the second rate between the first threshold value and a second threshold value. 
         [0024]    Thus, droop is applied with a three-stage profile, which has regions corresponding to light load, heavy load and overload. The profile is steepest in the heavy load region, and less steep in the light load and overload regions. 
         [0025]    This allows peak loading to be applied for limited duration. 
         [0026]    The third rate is preferably substantially equal to the first rate. Alternatively, it may be higher than the first rate, or it may be lower than the first rate. 
         [0027]    The second threshold load value is preferably between 100% and 105% of the maximum load rating of the power supply. 
         [0028]    Preferably, the first threshold load value is below 100% of the maximum load rating of the power supply. In this case, the second threshold load value is preferably at least 95% of the maximum load rating of the power supply. 
         [0029]    Preferably, the second threshold load value is above 100% of the maximum load rating of the power supply. In this case, the second threshold load value is preferably at least 95% of the maximum load rating of the power supply. 
         [0030]    According to a further aspect of the present invention, there is provided load sharing apparatus for load sharing between a plurality of power supplies, the apparatus comprising one or more load sharing modules, each for association with a respective power supply, the or each load sharing module comprising: 
         [0031]    a load determining module for generating a signal which represents a load value corresponding to the power or current supplied by the respective power supply; 
         [0032]    a mode selecting module for selecting a first mode of operation when said load value is below a (first) threshold value and a second mode of operation when said load value equals or exceeds said (first) threshold value; and 
         [0033]    a voltage control module for controlling the output voltage of the respective power supply to vary inversely with said load value at a first rate when the first mode is selected, and for controlling the output voltage of the respective power supply to vary inversely with said load value at a second rate when the second mode is selected; 
         [0034]    wherein said second rate is greater than said first rate. 
         [0035]    The mode selecting module is preferably further configured for selecting a third mode of operation when said load value equals or exceeds a second threshold value which is higher than the first threshold value, in which case the voltage control means is preferably configured for controlling the output voltage of the respective power supply to vary inversely with said load value at a third rate, lower than the second rate, when the third mode is selected. 
         [0036]    The load sharing module may comprise a limiting module for limiting the total voltage reduction at said second rate, such that the output voltage of the respective power supply varies at the second rate between the first threshold value and a second threshold value. 
         [0037]    The third rate preferably equals the first rate, but may alternatively be lower or higher than the first rate. 
         [0038]    The load sharing apparatus may further comprise a load limiting means for monitoring the load value, and reducing the output voltage of the respective power supply when the load exceeds a (third) threshold value for a predetermined duration. 
         [0039]    The third threshold value is preferably between 120% and 140% of the maximum load rating of the power supply. 
         [0040]    This prevents any one power supply operating in overload for too long. 
         [0041]    The or each load sharing module may be implemented as an analogue circuit. In this case, the load determining module, voltage control means/module, and/or mode selecting modules may comprise a combination of suitably connected electronic components. 
         [0042]    Alternatively, the or each load sharing module may be implemented on a suitably programmed micro-processor. Where there are a plurality of load sharing modules, these are preferably implemented on separate microprocessors. 
         [0043]    In the following, the term “droop” refers to the rate at which voltage decreases with increasing load, and is thus equivalent to load regulation, i.e. the rate of change of voltage with load. 
       SUMMARY OF THE DRAWINGS 
       [0044]    An embodiment of the present invention will now be described with reference to the accompanying drawings in which: 
         [0045]      FIG. 1  shows a power supply system which incorporates an analogue load sharing module in accordance with a first embodiment of the present invention; 
         [0046]      FIG. 2  illustrates the relationship between output voltage and output load for one of the power supplies in  FIGS. 1 ,  5  or  6 ; 
         [0047]      FIG. 3  shows a power supply system which incorporates an analogue load sharing module in accordance with second and third embodiments of the present invention; 
         [0048]      FIG. 4  illustrates the relationship between output voltage and output load for one of the power supplies in  FIG. 3 ; 
         [0049]      FIG. 5  shows a power supply system which incorporates an analogue load sharing module in accordance with a fourth embodiment of the present invention; 
         [0050]      FIG. 6  shows a digitally controlled power supply system, which incorporates a load sharing module in accordance with a sixth embodiment of the present invention; 
         [0051]      FIG. 7  illustrates an implementation of the droop calculating module of the load sharing module in  FIG. 6 ; 
         [0052]      FIG. 8  illustrates output voltage plotted against output power for three power supplies rated at 300 W; 
         [0053]      FIG. 9  illustrates output voltage plotted against system output power for three power supplies, and for the system overall, when the power supplies are connected together in parallel; and 
         [0054]      FIG. 10  illustrates power supply output power plotted against the overall system output voltage when the three power supplies are connected in parallel. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0055]      FIG. 1  shows a power supply system which incorporates an analogue circuit in accordance with a first embodiment of the present invention. 
         [0056]    Two DC:DC power supplies  1 ,  2  are connected in parallel to nodes N 1 , N 2 , via lines L 11 , L 12  and lines L 21 , L 22  respectively. Nodes N 1 , N 2  are respectively connected to output terminals T 1 , T 2 , for connection to a load (not shown). A capacitor C 1  is connected between nodes N 1 , N 2 . 
         [0057]    With reference to the first power supply  1 , two capacitors C 11 , C 12  are connected in parallel between lines L 11 , L 12 . Two lines L 13 , L 14  branch from line L 12  at nodes N 11 , N 12  respectively. A shunt resistor Rs 1  is connected along line L 12  between nodes N 11 , N 12 . Lines L 13 , L 14  carry input to a load sharing block Share 1 , which is described in more detail below. Output from Share 1  is carried on line L 15  to an input terminal of the first power supply  1 . 
         [0058]    A similar arrangement applies to the second power supply  2 . That is to say, two capacitors C 21 , C 22  are connected in parallel between lines L 21 , L 22 . Two lines L 23 , L 24  branch from line L 22  at nodes N 21 , N 22  respectively. A resistor Rs 2  is connected on line L 22  between nodes N 21 , N 22 . Lines L 23 , L 24  carry input to a load sharing block Share 2 , which is described in more detail below. Output from Share 2  is carried on line L 25  to an input terminal of the second power supply  2 . 
         [0059]    For the purposes of illustration, only the second load sharing block Share 2  is shown in detail in  FIG. 1 . However, the first load sharing module Share 1  comprises corresponding components and circuitry. Thus, the following description applies to both load sharing modules. 
         [0060]    Each load sharing module comprises two circuit blocks  10 ,  20 . 
         [0061]    Block  10  comprises a first operational amplifier A 10 , which is configured as an inverting amplifier. Line L 23  (L 13 ) is connected to the non-inverting input of amplifier A 10 . Line L 24  (L 14 ) is connected to resistor R 101 , which is turn connected to the inverting input of amplifier A 10 . The output of amplifier A 10  is connected to parallel connected resistor R 102  and capacitor C 102 , which are in turn connected to the inverting input of amplifier A 10 . 
         [0062]    The output of amplifier A 10  is further connected to series connected resistors Rref 2 , Rref 1 , which are in turn connected to first voltage reference VrefDC, which is connected to ground. Line L 25  (L 15 ), which carries input from the load sharing module to the power supply, branches from a node Nref between resistors Rref 1  and Rref 2 . 
         [0063]    Block  20  comprises a second operational amplifier A 20 . Line L 6  branches from line L 22  (L 21 ) between capacitor C 22  (C 21 ) and node N 2 . Line L 6  is connected to resistor R 205  which is in turn connected to the non-inverting input of amplifier A 20 . The non-inverting input of amplifier A 20  is further connected to a capacitor C 204 , which is in turn connected to ground. The inverting input of amplifier A 20  is connected to resistor R 201 , which is in turn connected to second voltage reference Vref 2  is in turn connected to ground. The output of amplifier A 20  is connected to parallel connected resistor R 202  and capacitor C 202 , which are in turn connected to the inverting input of amplifier A 20 . 
         [0064]    The output of amplifier A 20  is further connected to the anode of a diode D 20 , which is in turn connected to series connected resistors R 203 , R 204 . Resistor R 204  is further connected to the inverting input terminal of the first amplifier A 10  in block  10 . The cathode of a Zenor Diode DZ 20  is connected at to a node N 20  located between series connected resistors R 203 , R 204 . The anode of Zenor diode DZ 20  is connected to ground. 
         [0065]    In use, power supplies  1 ,  2  supply power to a load (not shown) connected to output terminals T 1 , T 2 . The load sharing modules Share 1 , Share 2  ensure that power/current is drawn from both power supplies by varying the reference voltage applied to the respective power supply according to its output load. 
         [0066]    The reference voltage applied to the power supply by the respective load sharing module is set by VrefDC, divided down by resistors Rref 1 , Rref 2 , and is thus dependent on the output voltage of amplifier A 10 . 
         [0067]      FIG. 2  illustrates the relationship between output voltage and output load (i.e. current or power) for one of the power supplies  1 ,  2 . 
         [0068]    When the output load of the power supply is zero, amplifier A 10  has an output voltage of zero. 
         [0069]    Above zero load, amplifier A 10  has a negative output voltage whose magnitude is proportional to the load value. Below a first threshold value Th 1 , just below the load rating LR of the power supply, the variation of output voltage of amplifier A 10  is defined by the voltage across the shunt resistor Rs 2  (Rs 1 ) and its own gain, which is determined by resistors R 101 , R 102 . Capacitor C 102  and associated resistor R 102  provide low pass filtering. 
         [0070]    Accordingly, in a first region of the graph in  FIG. 2 , which corresponds to a range of load value from 0-Th 1 , the reference voltage applied to the power supply, and thus the output voltage of the power supply, decreases with increasing load at a substantially constant first rate. 
         [0071]    When the voltage across the shunt resistor Rs 1 , Rs 2  exceeds a second reference voltage set by Vref 2 , the output voltage of the second amplifier A 20  varies proportionally with load. This provides a varying input voltage to the inverting input of the first amplifier A 10  via resistors R 203 , R 204 , which increases the rate at which the output voltage of A 10  varies with increasing load. The range over which this increased rate is applied is limited by diode DZ 20 , which prevents the amplifier A 20  having an effect on the output voltage of amplifier A 10  above a second threshold load value Th 2 , just above the load rating of the power supply. Capacitor C 202  and associated resistor R 202  provide low pass filtering. 
         [0072]    Accordingly, in a second region of the graph in  FIG. 2 , which corresponds to a range of load value from Th 1 -Th 2 , the reference voltage applied to the power supply, and thus the output voltage of the power supply, decreases with increasing load at a substantially constant second rate, which is greater than the first rate. 
         [0073]    However, in a third region of the graph, which corresponds to load values above Th 2 , the reference voltage applied to the power supply, and thus the output voltage of the power supply, again decreases with increasing load at the substantially constant first rate. 
         [0074]    With the above described three-stage profile, load regulation (or droop) is applied over the whole load range. Below Th 1 , load regulation is applied at a first, lower, rate, which is sufficient to ensure some load sharing between the power supplies. This reduces the spread of MTTF between the devices. Above Th 1 , load regulation is applied at a second, higher rate, to achieve more accurate load sharing when the power supplies are operating at or around their maximum load rating. This reduces the number of power supplies required to achieve n+1 redundancy. Above Th 2 , load regulation is again applied at the first, lower rate. This makes it possible for the power supplies to operate above their maximum power rating, which is desirable in systems which operate at lower power for most of the time, but require the ability to draw higher power over short time periods. 
         [0075]    It will be appreciated that limiter DZ 20  can be omitted, in which case there will be a two-stage profile, in which load regulation is applied at a lower rate in the range 0-Th 1  and at a higher rate at or above Th 1 . 
         [0076]      FIG. 1  shows two power supplies  1 ,  2  connected to output terminals T 1 , T 2 . However, it will be appreciated that one or more additional power supplies can be connected to the same output terminals in a similar manner. In this case, each additional power supply may have an associated load sharing module similar to Share 1 , Share 2  to ensure load sharing between all the power supplies. 
         [0077]      FIG. 3  shows two variations of the power supply system shown in  FIG. 1 , in which each load sharing module Share 1 , Share 2  comprises an additional circuit block  30 , in accordance with third and fourth embodiments of the present invention. 
         [0078]    Block  30  comprises a third operational amplifier A 30 . Line L 7  branches from node N 30  between resistor R 205  and capacitor C 204  in block  20 , and is connected to the non-inverting input of amplifier A 30 . The inverting input of amplifier A 30  is connected to resistor R 301 , which is in turn connected to voltage reference Vref 3 , which is in turn connected to ground. The output of amplifier A 30  is connected to parallel connected resistor R 302  and capacitor C 202 , which are in turn connected to the inverting input of amplifier A 30 . The output of amplifier A 30  is further connected the anode of diode D 30 . In the first variation, the cathode of diode D 30  is connected to resistor R 303 , which is in turn connected to the inverting input of the first amplifier A 10  in block  10 . In the second variation, the cathode of diode D 30  is connected to resistor R 304 , which is in turn connected to node Nref between resistors Rref 1  and Rref 2 . Capacitor C 302  and associated resistor R 302  provide low pass filtering. 
         [0079]    When the voltage across the shunt resistor Rs 1 , Rs 2  exceeds a third reference voltage set by Vref 3 , the output voltage of the amplifier A 30  varies proportionally with load, at a lower rate than the second amplifier A 20 . 
         [0080]    In the first variation, where the output from diode D 30  is connected to the input of the first amplifier A 10  via resistor R 303 , this provides a varying input voltage to the first amplifier A 10  to increase the rate at which its output voltage varies with load. 
         [0081]    In the second variation, where the output from diode D 30  is connected node Nref via resistor R 304 , the output voltage of amplifier A 30  reduces the effect of the output voltage of amplifier A 10  at node Nref. 
         [0082]    In both cases, the system is configured such that Vref 3  substantially coincides with the second threshold load value Th 2 , at which the effect of the second amplifier A 20  is limited. 
         [0083]    Accordingly, in the first variation, block  30  causes the reference voltage for the power supply, and thus its output voltage, to decrease with load at a third rate which is higher than the first rate, but less than the second rate. 
         [0084]    Whereas, in the second variation block  30  causes the reference voltage for the power supply, and thus its output voltage, to decrease with load at a third rate which is less than the first rate. 
         [0085]      FIG. 4  illustrates the relationship between output voltage and output load for one of the power supplies  1 ,  2  in the power supply system of  FIG. 3 . The dotted line represents the first variation and the solid line represents the second variation. 
         [0086]      FIG. 5  shows a further variation of the power supply system illustrated in  FIG. 1 . 
         [0087]    Two power supplies,  1 ,  2  are connected output terminals T 1 , T 2 , as described above in relation to  FIG. 1 . Each power supply has an associated load sharing module Share 1 ″, Share 2 ″. Only the load sharing module Share 2 ″ is illustrated in detail in  FIG. 5 . However, the load sharing module Share 1 ″ comprises corresponding components and circuitry. Thus, the following description applies to both Share 1 ″ and Share 2 ″. 
         [0088]    The load sharing module Share 1 ″, Share 2 ″ comprises two circuit blocks  10 ′,  20 ′. 
         [0089]    Block  10 ′ comprises a first operational amplifier A 10 ′, which is configured as a non-inverting amplifier. Line L 23  (L 13 ) is connected to resistor R 101 ′, which is turn connected to the inverting input of amplifier A 10 ′. Line L 24  (L 14 ) is connected to resistor R 103 , which is turn connected to the non-inverting input of amplifier A 10 ′. The non-inverting input of amplifier A 10 ′ is further connected to a capacitor C 101 , which is in turn connected to ground. The output of amplifier A 10 ′ is connected to parallel connected resistor R 102  and capacitor C 102 , which are in turn connected to the inverting input of amplifier A 10 ′. 
         [0090]    The output of amplifier A 10 ′ is further connected to series connected resistors Rref 2 , Rref 1 , which are in turn connected to capacitor C 21  (C 11 ). Line L 25 ′ (L 15 ′), which carries input from the load sharing module to the power supply, branches from a node FB between resistors Rref 1  and Rref 2  which is the feedback input to the dc:dc converter. 
         [0091]    Block  20 ′ is similar to block  20  in  FIG. 1 . However, line L 6 , resistor R 205  and capacitor C 204  are omitted. Instead, the non-inverting input of amplifier A 20 ′ is connected to resistor R 103  and capacitor C 101  in Block  10 ′. Further, resistor R 204  is connected to node Nref, instead of an input terminal of the first amplifier A 10 ′. 
         [0092]    In the embodiment of  FIG. 5 , the first amplifier A 10 ′ controls the output voltage of the respective power supply by modifying the feedback signal to the respective power supply, to give a defined output impedance. 
         [0093]    At zero load, the output voltage of both amplifiers A 10 ′, A 20  is zero (assuming the use of rail-to-rail amplifiers operating off a single positive supply). The feedback signal supplied to the power supply is then the output voltage divided down by the network of Rref 1 , Rref 2 , R 203  and R 204 . 
         [0094]    Above zero load, the output voltage of amplifier A 10 ′ increases at a substantially constant rate with increasing load. Below a threshold value Th 1 , just below the load rating of the power supply, the output voltage of amplifier A 20  is zero. However, above threshold value Th 2 , which is defined by Vref 2 , the output voltage of the second amplifier A 20 ′ increases at a substantially constant rate with increasing load. 
         [0095]    Zener diode DZ 20 , limits the effect of second amplifier A 20  above a certain voltage, which corresponds to a second threshold load value Th 2 . 
         [0096]    Thus, in the range 0-Th 1 , the effect of the first amplifier A 10 ′ causes the output voltage of the power supply to vary with load at a first substantially constant rate. In the range Th 1 -Th 2 , the additional contribution of the second amplifier A 20  causes the output voltage of the power supply to vary with load at a second substantially constant rate, which is higher than the first rate. Above Th 2 , the second amplifier has no additional effect, and the rate at which the output voltage of the power supply varies with load reverts to the first rate. 
         [0097]    As with the embodiment of  FIG. 1 , it would be possible to incorporate additional circuitry similar to block in  FIG. 3 , to control the rate at which the output voltage of the power supply varies with load above Th 2 . 
         [0098]      FIG. 6  shows a block diagram for a digitally controlled power supply, within a multiple power supply system, which incorporates a load sharing module in accordance with a fifth embodiment of the present invention. 
         [0099]    The power supply comprises a DC:DC converter  60  and a microprocessor  62 , which incorporates a load sharing software module  68 . The power supply is connected to an ORING circuit  64 , which allows one or more additional power supplies (not shown), to be connected together in parallel. The combined output voltage from all power supplies is output by the ORING circuit, and made available to external applications by a voltage output interface  66 . 
         [0100]    The converter  60  comprises a power converting module  60   a  for converting a DC input voltage to a DC output voltage, a current sensing module  60   b  for sensing the converter current, a feedback module (winding)  60   c  and a control input  60   d  for receiving a control signal from the microprocessor. The microprocessor  62  comprises a load sharing module  68 . The load sharing module  68  comprises a power calculating module  68   a , a droop calculating module  68   b , a reference module  68   c , a control module  68   d  and a power limiting module  68   e.    
         [0101]    In use, a supply voltage (for example, a 380V DC input voltage) is input to the converter, and converted by the power converting module  60   a  to provide an isolated DC output voltage. The isolated DC output voltage is input to an ORING circuit  64 , to be combined with the output voltage from other power supplies in the system. 
         [0102]    The operation of the power converting module  60   a  is controlled by the microprocessor  62 , and in particular, the load sharing module  68 . The current sensing module  60   b  in the converter  60  senses the converter current and generates a signal representative of the converter current. This signal is input to the power calculating module  68   a  of the load sharing module via an ADC input terminal. A signal representative of the converter voltage is also input to the power calculating module  68   a  via another ADC input terminal. The power calculating module uses these signals to calculate a load value representative of the output power (and thus the output current) of the power supply. 
         [0103]    The load value is input to the droop calculating module  68   b , together with a feedback signal from the feedback module  60   c  of the converter  60 . The droop calculating module uses the load value and the feedback signal value to calculate an adjusted reference value to be applied to the converter, in accordance with a pre-set droop profile, as discussed in more detail below. The adjusted reference value is registered as Vref by reference voltage module  68   c , and input to the voltage control module  68   d . The feedback signal is also input to the voltage control module  68   d.    
         [0104]    The voltage control  68   d  module generates a control signal in accordance with the reference voltage Vref and the feedback signal. This signal is input to the control input  60   d  of the converter, to regulate the output voltage of the power supply to follow the reference voltage, by varying the operating frequency. 
         [0105]    In addition, the output load value generated by the power calculating module  68   a  is input to the power limiting module  68   e , which monitors the output load value. The power limiting module outputs a signal to the voltage control module, to reduce the output voltage of the power supply, if the load value exceeds a pre-determined value for longer than a pre-determined duration. This ensures the power supply does not operate in overload for too long. 
         [0106]    The droop calculating module  68   b  adjusts the reference voltage Vref in accordance with the load value, such that the reference voltage varies inversely in proportion to the load value at a first rate in the range 0-Th 1 , where Th 1  is a first threshold load value just below the load rating of the power supply. Above Th 1 , and below Th 2 , which is a second threshold load value just above the load rating of the power supply, the droop calculating module adjusts the reference voltage Vref to vary inversely in proportion to the load value at a second rate which is higher than the first rate. Above Th 2 , the droop calculating module adjusts the reference voltage Vref to vary inversely in proportion to the load value at the first rate, or at a third rate which is lower than the second rate, but may be higher or lower than the first rate. Thus, the droop calculating module is configured to apply one of the droop profiles illustrated in  FIGS. 2 and 4 . 
         [0107]      FIG. 7  illustrates a possible implementation of the droop calculating module  68   b . The load value calculated by the power calculating module  68   a  is input to a mode selecting module  70 . The mode selecting module selects mode  1 ,  2 , or  3  based on whether the load value falls into a first range 0-Th 1 , a second range, Th 1 -Th 2  or a third range &gt;Th 2 , respectively. In mode  1  a first droop rate is selected, in mode  2  a second droop rate is selected, and in mode  3  a third droop rate is selected. The selected rate The result of this determination is then input to a result calculating module  72 , which calculates the reference voltage according to the formula: 
         [0000]      Voltage reference=−(selected rate)×load value+constant
 
         [0108]      FIG. 8  shows a graph of output voltage against output power for three power supplies rated at 300 W, and each incorporating a load sharing module which embodies the present invention. For the purposes of illustration, the output voltages are intentionally set to be different, to represent the worst case situation. The three droop stages are clearly apparent. 
         [0109]      FIG. 9  shows a graph of output voltage against system output power for each power supply, and for the system overall, when the power supplies are connected together in parallel. 
         [0110]      FIG. 10  shows a graph of power supply output power against the overall system output voltage when the three power supplies are connected in parallel. 
         [0111]    As can be seen, there is some load sharing at lighter loads, but more accurate sharing at higher loads, corresponding to the region Th 1 -Th 2  for one or more of the power supplies. 
         [0112]    The present invention has been described in terms of DC:DC power supplies. It will be appreciate that the invention may also apply to any power supplies with a DC output. E.g., AC:DC power supplies. 
         [0113]    The circuits illustrated herein are representative circuits. It will be appreciated that other implementations that perform the functions of the present invention are possible.