Abstract:
A power suppoly for a capacitive-resistive load includes plural paralleled phase correcting modules together with current sharing controllers for tending to equalize their currents. Each module is provided with a diode, poled to prevent forward current from flowing in the return current path, for aiding in equalizing module currents. Surge currents are reduced by a single saturable reactor coupled to the combined outputs of current sharing controllers, thereby avoiding the need for soft-start in each controller. A precharging path extends from a source of pulsating direct voltage to the load, for precharging the load capacitance at turn-on.

Description:
GOVERNMENTAL INTEREST  
       [0001]     This invention was prepared under government contract N00014-99-2-0002 (HBMRS). The United States Government has a non-exclusive, non-transferable, paid-up license in this invention. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to electrical power supplies, and more particularly to paralleled power supplies in which, variously, turn-on surge currents are controlled, unidirectional return current equalization is assured, and a capacitive load is precharged.  
       BACKGROUND OF THE INVENTION  
       [0003]     It is often necessary to parallel power supplies in order to achieve a desired level of power. Such paralleling allows the use of standardized or commercial-off-the-shelf (COTS) modules or units to achieve a level of power which might otherwise require a costly custom-designed power supply. For example, most single-phase power-factor-corrected (PFC) boost AC-DC power supplies available as COTS modules offer no more than 1 KW of power capacity, and must be paralleled in order to provide, say, 10 KW.  FIG. 1  is a simplified diagram in block and schematic form illustrating a prior-art paralleled power supply  10  for providing direct voltage to a capacitive load  12 . In  FIG. 1 , the capacitive load  12  includes a load resistor  14  which represents the real or energy-absorbing portion of the load, and a paralleled capacitor  16  which represents the quadrature or out-of-phase (imaginary) portion of the load. The capacitor  16  may be an actual discrete capacitor or capacitor bank, and it may also include the stray capacitance of various components andor connections. One end of resistor  14  and capacitor  16  is connected to a load reference or ground conductor LG, and the other ends are connected to a load hot terminal LH.  
         [0004]     In  FIG. 1 , a source of alternating current, such as power-line mains, is illustrated as  18 . The source of alternating voltage drives a full-wave rectifier represented as a block  20 , which as known produces pulsating direct voltage (also known as pulsating direct current) represented by a symbol  22 . Pulsating direct voltage is characterized by unidirectional half-sinusoids of voltage, with the voltage value between voltage peaks going to approximately zero volts. The pulsating direct voltage may be viewed as being established or generated between a first common conductor  24  relative to a common second or reference conductor  26 . In  FIG. 1 , a plurality  28  of standardized single-phase switching phase correcting power-supply boost modules  28   a ,  28   b , . . . ,  28   n  are connected to conductors  24  and  26  for receiving pulsating direct voltage from rectifier  20 , and for generating direct voltage for ultimate application to the load  12 . Each power supply module of set  28  includes first and second power input terminals. More particularly, power supply  28   a  includes first and second power input terminals or ports  28   ai   1  and  28   ai   2 , respectively, which are connected to common power conductors  24  and  26 , respectively. Similarly, power supply  28   b  includes first and second power input terminals or ports  28   bi   1  and  28   bi   2 , respectively, which are connected to common power conductors  24  and  26 , respectively, and power supply  28   n  includes first and second power input ports  28   ni   1  and  28   ni   2 , respectively, which are connected to common power conductors  24  and  26 , respectively. It should be noted that the term “port” formally includes a pair of terminals or electrodes, but common usage extends the definition. Each power supply of set  28  also includes first and second power output terminals, and more particularly power supply  28   a  includes first and second output terminals  28   ao   1  and  28   ao   2 , power supply  28   b  includes first and second output terminals  28   bo   1  and  28   bo   2 , and power supply  28   n  includes first and second output terminals  28   no   1  and  28   no   2 . One example of such single-phase power-factor correcting boost power supply modules is model PFC-1000 manufactured by RO Associates, Inc. of 246 Caspian Drive, P.O. Box 61419, Sunnyvale, Calif. 94088.  
         [0005]     Each switching power supply module or element of set  28  of power supplies of  FIG. 1  includes internal circuitry, the nature of which may or may not be known to the user. Such power supplies almost always include an input inductor, which is represented in  FIG. 1  by inductors  28   a I,  28   b I, . . . ,  28   n I connected to the first input ports  28   ai   1 ,  28   bi   1 , . . . ,  28   ni   1  of power supplies  28   a ,  28   b , . . . ,  28   n , respectively. The power supplies also often include a unidirectional current conducting device, illustrated as a diode or rectifier  28   a D,  28   b D, . . . ,  28   n D, through which an output or integrating capacitor is charged. In power supply  28   a  of  FIG. 1 , these capacitors are represented by a capacitor designated  28   a C, and capacitors  28   b C and  28   n C of power supplies  28   b  and  28   n  correspond. The integrating capacitor  28   a C,  28   b C, . . . ,  28   n C of each of the power supply modules  28   a ,  28   b , . . . ,  28   n  is connected across the output terminals  28   ao   1 ,  28   ao   2 ;  28   bo   1 ,  28   bo   2 ; . . . ;  28   no   1 ,  28   no   2  of the module, for providing a low output impedance. Each switching power supply of set  28  also includes a current sensing resistor for sensing the current flow in the return path. In  FIG. 1 , power supply  28   a  has a return current sensing resistor  28   a R, power supply  28   b  has a return current sensing resistor  28   b R, and power supply  28   n  has a return current sensing resistor  28   n R. The purpose of these return current sensing resistors in the various switching power supply module or element of set  28  is to provide a signal representing the return current at the second input terminal; this return current signal is compared by a comparator (not illustrated) with a scaled version of the full-wave rectified voltage  22  to produce an error signal, which error signal forces the return current to follow or track the full-wave voltage, thereby forcing the current to be in-phase with the applied voltage, which is the essence of phase correction. Each power supply  28   a ,  28   b , . . . ,  28   n  of set  28  is also associated with a further return current equalizing resistor R 1 , R 2 , . . . , Rn of a set  29  of return current equalizing resistors. More particularly, each power supply  28   a ,  28   b , . . . ,  28   n  of set  28  is also associated with a further return current equalizing resistor R 1 , R 2 , . . . , Rn, respectively, which is connected between the return current output terminal and the load ground LG. Thus, resistor R 1  is connected to return current output terminal  28   ao   2  of power supply  28   a  and to LG, resistor R 2  is connected to return current output terminal  28   bo   2  of power supply  28   b  and to LG, and resistor Rn is connected to LG and to the return current output terminal  28   no   2  of power supply  28   n.    
         [0006]     Within each switching power supply module or element of set  28  of power supplies of  FIG. 1 , a “line current shaping controller LCSC and associated power FET perform the boost power conversion. When the FET of a module of set  28  is ON or conducting, energy is stored in the associated input inductor ( 28   a I,  28   b I, . . .  28   n I) associated with the input port of the module. When the FET goes OFF or becomes nonconductive, the inductor produces a reaction voltage which adds to the input voltage to produce the boosted output voltage. At the same time, the average input port current follows the shape of the full-wave rectified or pulsating direct input voltage  22 .  
         [0007]     In theory, it should be possible to simply connect the output terminals of the various power supplies of  FIG. 1  to the load  12 . However, some problems arise when the power supplies are paralleled in this manner and connected to the load. A first problem is that the internal impedances of the various power supplies  28   a ,  28   b , . . . ,  28   n  may not be equal, with the result that the current provided by each module may differ from the current provided by the other modules. Such differences in internal impedance may be the result of differences in the gain of the feedback circuits, which as known tends to change the impedance. It may also arise as a result of stray differences in connection resistances. Such current-sharing problems are controlled in the prior art by a set  30  of forward current sharing controllers, including current-sharing controllers  30   a ,  30   b , . . . ,  30   n , which tend to maintain the same forward current to the load from each power supply module of set  28 . Current-sharing controller  30   a  has an input port  30   ai  connected to output terminal  28   ao   1  of power supply module  28   a  and an output terminal  30   ao  connected to load conductor LH, and further includes a connection  30   ar  to ground conductor LG. Current-sharing controller  30   b  has an input port  30   bi  connected to output terminal  28   bo   1  of power supply module  28   b , an output terminal  30   bo , which is connected to load conductor LH, and a reference terminal  30   br , which is connected to ground conductor LG. Current-sharing controller  30   n  has an input port  30   ni  connected to output terminal  28   no   1  of power supply module  28   n , an output terminal  30   no  connected to load conductor LH, and a reference terminal  30   nr  connected to ground conductor LG. Thus, the output ports of the current sharing controllers of set  30  are connected in common to load supply conductor LH. Each of the current sharing controllers of set  30  is also connected by a reference terminal to ground conductor LG. The current sharing controllers of set  30  are of the soft ramp-up variety, to thereby prevent surge currents from occurring when the initially uncharged load capacitor  16  is connected to the charged output capacitor  28   a C,  28   b C, . . . ,  28   n C of any one of the power supply modules of set  28 . Such surge currents, as known, may be large enough to cause failure of a capacitor or the interconnections, or to reduce their life expectancy.  FIG. 7  is a simplified diagram in schematic form of a prior-art current sharing controller  30  with soft start.  
         [0008]      FIG. 7  is a simplified schematic diagram of a prior-art soft-start current sharing controller, together with some ancillary circuits. For definiteness, the controller of  FIG. 7  is designated as  30   a . In  FIG. 7 , current sharing controller  30   a  includes a power FET (PFET) having its power current controlling path connected to input terminal or port  30   ai  and, by way of a series current sensing resistor  710 , to output terminal or port  30   ao . Output port  30   ao  of current sharing controller  30   a  is connected by way of a terminal  724   a  to a common node  726 . Other current sharing controllers (not illustrated in  FIG. 7 ) are connected to common node  726  by way of terminals  724   b , . . . ,  724   n . A current sensor  728  senses the total current supplied by all the current sharing controllers, and generates a current sense signal on a path  730 . Path  730  carries information about the total current to a current share input terminal  732   a.    
         [0009]     In  FIG. 7 , the gate of the PFET is connected to input port  30   ai  by way of a resistor  712 , which provides the PFET with gate voltage more positive than the voltage at output port  30   ao  to tend to hold the PFET conductive or ON. The gate of the PFET is also coupled to the collector of an NPN bipolar transistor  714 . The emitter of transistor  714  is connected to ground by way of an emitter resistor  716 . When transistor  714  is ON, collector current flows through resistor  712 , and turns OFF the PFET by reducing its gate current toward zero volts. The base of transistor  714  is driven by way of a resistor  718  from the output of a comparator (a high-gain amplifier)  720 . When comparator  720  tends to higher output, transistor  714  conducts more and the PFET conducts less. A current regulating arrangement includes resistor  710  and a bipolar PNP transistor  722 . When the output current of current sharing controller  30   a  becomes large enough, the base-emitter junction of transistor  722  becomes forward biased, and the transistor becomes active. When active, transistor  722  adjusts the voltage at the positive (+) input terminal of comparator  720 , to tend to drive its output positive and thereby turn OFF the PFET. The inverting (−) input terminal of comparator  720  is connected to A “startup” signal is generated by an external logic circuit (not illustrated) which uses a variety of logic schemes to determine the existence of a start-up condition, and a start-up signal is applied to the noninverting input terminal of comparator  720  by way of an intermediary FET  736 .  
         [0010]     Improved paralleled power supply arrangements are desired.  
       SUMMARY OF THE INVENTION  
       [0011]     An electrical apparatus according to an aspect of the invention is for powering a load, where the load includes a resistive and a parallel capacitive component. The electrical apparatus comprises a source of pulsating direct voltage, and a first plurality of power factor correction units coupled to the source of pulsating direct voltage, each for converting the pulsating direct voltage into a direct voltage at an output port, and for tending to maintain the current through the source of pulsating direct voltage in-phase with the pulsating direct voltage. The apparatus also includes a plurality, equal to the first plurality, of current sharing controllers, each of which includes a port coupled to the output port of one of the power factor correction units, and each of which also includes an output port in common with all output ports of the current sharing controllers, the current sharing controllers being subject to surge current when the direct voltage at the output port of the associated one of the power factor correction units is coupled to the capacitive component of the load at turn-on. The apparatus also includes a saturable reactor coupled between the common output port of the current sharing controllers and the load, for tending to oppose the surge current.  
         [0012]     In a preferred embodiment of this aspect of the invention, the power factor correction units are boost power factor correction converters which produce an output voltage generally greater than the input voltage. In a more preferred embodiment of this aspect of the invention, the apparatus further comprises a plurality, equal to the first plurality, of ground current equalizing impedances coupled between a common reference terminal and a current return port of each of the power factor correction units. The ground current equalizing impedances may comprise unidirectional current conducting means poled to prevent the flow of forward current from the return current terminal of the associated one of said power factor correction units. In a more preferred apparatus, a controllable path is coupled to the source of pulsating direct voltage and to the load, for tending to charge the capacitive component of the load at turn-on, and for ceasing charging after turn-on. The controllable path may include a controllable switch. The controllable switch may include a unidirectional current conducting device such as a diode or rectifier which conducts when the pulsating direct voltage is greater than the voltage on the capacitive component and which ceases conduction when the pulsating direct voltage is less than the voltage on the capacitive component. In this last most preferred embodiment, when using diodes or rectifiers, the power factor correction units are voltage-boosting units which produce a direct voltage greater than the peak value of the pulsating direct voltage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]      FIG. 1  is a simplified diagram of a prior-art paralleled power supply, showing unwanted paths along which forward current may flow;  
         [0014]      FIG. 2  is a simplified diagram of a paralleled power supply according to an aspect of the invention, in which unidirectional current conducting devices are placed so as to prevent flow of forward current among the paralleled units, and showing paths by which current flows;  
         [0015]      FIG. 3  is a simplified diagram similar to that of  FIG. 1 , showing a saturable reactor which tends to suppress surge currents at turn-on;  
         [0016]      FIG. 4  is a simplified magnetization curve of a saturable material;  
         [0017]      FIG. 5  is a simplified diagram similar to that of  FIG. 1 , showing the use of a controlled current path for precharging a load capacitance at turn-on;  
         [0018]      FIG. 6  is a simplified diagram illustrating a combination of the arrangements of  FIGS. 2, 3 , and  5 ;  
         [0019]      FIG. 7  is a simplified diagram in schematic form illustrating a prior-art soft-start current sharing controller which may be used in the arrangements of  FIGS. 1, 2 , and  5 ; and  
         [0020]      FIG. 8  is a simplified diagram in schematic form illustrating a current sharing controller arrangement which may be used in conjunction with the saturable reactor embodiment of  FIG. 3 . 
     
    
     DESCRIPTION OF THE INVENTION  
       [0021]     It has been discovered that the arrangement of  FIG. 1  may not be as stable or consistent in performance as desired. More particularly, it has been discovered that a forward cross circulation current, represented in  FIG. 1  as a dash line  40 , can flow from one PFC module to another, as for example from PFC module  28   a  to PFC module  28   b , returning to conductor  26 . This cross circulation current tends to disrupt the current sensing mechanism of the affected module, and eventually the AC line current shaping. In addition, the uncontrolled circulation may easily exceed the rating of the current-balancing resistors of the PFC modules, such as resistor  28   b R of module  28   b , for example, and lead to component destruction. Further, the cross circulation current also causes signal ground drift (reference shift) and erroneous signal processing.  
         [0022]     Circulation of cross currents from one module to the others is prevented by the use of unidirectional current conducting devices such as rectifiers or diodes (diode). In  FIG. 2 , a diode or rectifier of a set  210  of unidirectional current conducting devices is connected in series with a return current equalizing resistor of set  29 . More particularly, a diode  210   a  is connected in series with resistor R 1 , a diode  210   b  is connected in series with resistor R 2 , and a diode  210   n  is connected in series with resistor Rn. The diodes of set  210  are poled to allow the flow of return current to the module in question, but prevent the flow of forward current from the second output port of each power-supply module. More particularly, diode  210   a  is poled with its cathode adjacent second output port  28   ao  of power supply module  28   a , diode  210   b  is poled with its cathode adjacent second output port  28   bo  of power supply module  28   b , and diode  210   n  is poled with its cathode adjacent second output port  28   no  of power supply module  28   bn . With the cathodes adjacent the output return current ports, forward current cannot flow from an output return current port, and therefore cannot flow into the return current port of another power supply module. Instead, the forward current in each power-supply module of set  28  flows in a path, illustrated in conjunction with power-supply module  28   a , extending from conductor  24 , through the first input port  28   ai   1  of the power-supply module, through at least the internal capacitor  28   a C, through the internal current sensing resistor  28   a R, and out to conductor  26 .  
         [0023]     The cost of providing soft-start current ramp-up in each of the current-sharing controllers of set  30  of  FIG. 1  may be excessive. According to an aspect of the invention, the need for soft-start current ramp-up in each current-sharing controller is avoided by the addition of a single saturable reactor between the paralleled power supply modules and the load. More particularly, referring to  FIG. 3 , a saturable reactor  50  is connected in series between conductor portions LH′ and LH″, between load  12  and the paralleled output terminals  30   ao ,  30   bo , . . . ,  30   no  of set  30  of current sharing controllers. A saturable reactor has a magnetic core which is characterized by a BH curve  55  such as that illustrated in  FIG. 4 , where B is the magnetic induction and H is the magnetizing force. The incremental induction, represented by the slope of curve  55 , is maximum near the center of the curve, and is much less at the ends of the curve. The regions of large slope represent operating regions in which the inductor has a large reactive impedance, and the zero-slope regions at the ends of the curve represent regions in which the inductor has little or no reactive impedance. The magnetic core of the saturable inductor is selected in conjunction with the number and layout of turns in order to provide maximum induction and inductance at high rate of lead current changes, and low or zero induction and inductance at low rate of current of load resistor  14 . The relatively large inductance presented by the saturable inductor  50  of  FIG. 3  to rapidly changing or surge currents tends to suppress surges. Thus, any of the phase correcting power-supply modules or units of set  28  which may tend to produce a surge current finds that such a surge is opposed by a reaction of saturable reactor  50 . The opposition to the surge essentially suppresses the surge. Since the presence of saturable reactor  50  tends to suppress any surge currents flowing to the capacitive component  16  of load  12 , the set  30  of current sharing controllers need not have soft-start characteristics. In general, the use of a single saturable reactor, such as reactor  50 , will be cheaper and more reliable than the use of a soft-start controller.  FIG. 8  is a simplified diagram illustrating a current sharing controller similar to that of  FIG. 7 , but in which the soft-start feature is absent in accordance with an aspect of the invention, and the signal paths required for distributing startup signals to the various controllers are also absent.  
         [0024]     Circuit arrangement  500  of  FIG. 5  is similar to circuit arrangement  10  of  FIG. 1 , and corresponding elements are designated by the same alphanumerics. Circuit arrangement  500  differs from circuit arrangement  10  by the addition of a precharging current path including a diode (D)  60 . The precharging current path extends from conductor  24  at the output of full-wave bridge rectifier  20  to conductor LH adjacent the load  12 . In operation at turn-on, the pulsating direct voltage  22  produced by rectifier  20  is immediately applied to the anode of diode  60 , and current flows through diode  60  and load capacitance  16 , thereby charging capacitance  16  even in the absence of significant voltage at the output terminals  28   ao   1 ,  28   bo   1 , . . . ,  28   no   1  of the set  28  of power-factor correcting modules. Thus, by the time the set  28  of power-factor correcting modules reaches a nominal output voltage and the set  30  of current sharing controllers couples the set  28  of power-factor correcting modules to load  12  by way of conductor LH, the load capacitance  16  is already at least partially charged. The precharge applied to load capacitance  16  tends to reduce the magnitude of surge currents which might occur when the current sharing controllers couple the power-factor correcting modules to the load.  
         [0025]     It should be noted that if the power factor correction modules of set  28  of  FIG. 5  are voltage boost modules producing a direct output voltage which exceeds the peak value of the pulsating direct voltage  22  produced by rectifier  20 , the precharging path including diode or rectifier  60  will be turned OFF or become open-circuited, because the greater positive value of the direct voltage applied to the cathode of device  60  by comparison with the lesser positive value of the pulsating direct voltage  22  will result in reverse bias of the diode or rectifier. This arrangement avoids the need for a separate switch and timing circuit to disconnect the precharging path.  
         [0026]      FIG. 6  illustrates a circuit arrangement similar to that of  FIG. 1 , with the inclusion of a set  210  of unidirectional current conducting devices connected in a manner similar to that described in conjunction with  FIG. 2 , and also including a saturable reactor  50  as described in conjunction with  FIGS. 3 and 4 . In addition, the arrangement of  FIG. 6  also includes a precharging device or path  60  corresponding to that of  FIG. 5 . These changes to the arrangement of  FIG. 1  tend to improve the performance of the parallel supply.  
         [0027]     Thus, an electrical apparatus ( 300 ) according to an aspect of the invention is for powering a load ( 12 ), where the load ( 12 ) includes a resistive ( 14 ) and a parallel capacitive ( 16 ) component. The electrical apparatus ( 300 ) comprises a source ( 20 ) of pulsating direct voltage, and a first plurality (n) of power factor correction units ( 28 ) coupled to the source ( 20 ) of pulsating direct voltage, each of the power factor correction units ( 28 ) being for converting the pulsating direct voltage ( 22 ) into a direct voltage at an output port (such as  28   ao   1 ,  28   ao   2 ), and for tending to maintain the current through the source of pulsating direct voltage in-phase with the pulsating direct voltage ( 22 ). The apparatus ( 300 ) also includes a plurality (n), equal to the first plurality, of current sharing controllers ( 30 ), each of which includes a port (such as  30   ai , LG) coupled to the output port (such as  28   ao   1 ,  28   ao   2 )of one of the power factor correction units ( 28 ), and each of which also includes an output port (such as  30   ao ) in common with all output ports of the current sharing controllers ( 30 ), the current sharing controllers ( 30 ) being subject to surge current when the direct voltage at the output port (such as  28   ao   1 ,  28   ao   2 ) of the associated one (such as  28   a ) of the power factor correction units ( 28 ) is coupled to the capacitive component ( 16 ) of the load ( 12 ) at turn-on. The apparatus ( 300 ) also includes a saturable reactor ( 50 ) coupled between the common output port ( 30   ao ,  30   bo , .  30   no ) of the current sharing controllers ( 30 ) and the load ( 12 ), for tending to oppose the surge current.  
         [0028]     In a preferred embodiment of this aspect of the invention, the power factor correction units ( 28 ) are boost power factor correction converters which produce an output voltage generally greater than the input voltage. In a more preferred embodiment of this aspect of the invention, the apparatus ( 300 ) further comprises a plurality (n), equal to the first plurality, of ground current equalizing impedances ( 29 ) coupled between a common reference terminal (LG) and a current return port (such as  28   ao   2 ) of each of the power factor correction units ( 28 ). The ground current equalizing impedances ( 29 ) may comprise unidirectional current conducting means ( 210 ) poled to prevent the flow of forward current from the return current terminal (such as  28   ao   2 ) of the associated one of said power factor correction units ( 28   a ). In a more preferred apparatus, a controllable path ( 60 ) is coupled to the source ( 20 ) of pulsating direct voltage ( 22 ) and to the load ( 12 ), for tending to charge the capacitive component ( 16 ) of the load ( 12 ) beginning at turn-on, and for ceasing charging after turn-on. The controllable path ( 60 ) may include a controllable switch. The controllable switch may include a unidirectional current conducting device such as a diode or rectifier which conducts when the pulsating direct voltage is greater than the voltage on the capacitive component and which ceases conduction when the pulsating direct voltage is less than the voltage on the capacitive component. In this last most preferred embodiment, when using diodes or rectifiers, the power factor correction units are voltage-boosting units which produce a direct voltage greater than the peak value of the pulsating direct voltage.