Abstract:
Apparatus and method is disclosed for overcoming the voltage attenuation and ground shifts normally associated with providing DC power to distributed loads from a DC power supply without the need for excessively large conductors, the need to distribute the DC power supply or the need to provide converters at or near each load. Reasonably accurate voltage regulation at each load is provided according to the invention while using low but not insignificant resistance power conductors by providing similarly low resistance voltage sense conductors. The loads are connected between the sense conductors in a distributed fashion. Current approximating that drawn by each load is injected from the power conductors to the sense conductors at or near each load. Dynamic current requirements are supplied by capacitors connected across each load, usually mounted on the load circuit boards. Stability during power up is provided by capacitors connected between each power conductor and it&#39;s respective sense conductor at the power supply.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to machines that require high DC currents to be distributed over significant distances to multiple high current loads. More particularly, the present invention relates to an economical DC current distribution method and apparatus for use with electronic devices having fairly constant and well defined DC current requirements and that do not exhibit large dynamic variation or that are substantially buffered by capacitors at each load. 
     2. Description of Related Art 
     The use of an AC to DC converter or DC to DC converter at each load is a known way to provide for distributing DC power to each load without the negative effects of resistive voltage drop and ground shift as is experienced in a simple metal conductor distribution system. Each converter acts as a power supply for its&#39; load. This approach is expensive in that numerous complex power control circuits are required in the system. It becomes even more expensive when redundancy is required to be built into the distribution system since a redundant supply or converter is needed for each load. Also when the power distribution system becomes large, distributed converters must be placed at intervals of several meters along a cable of ten meters or more. Such placement often presents a problem of instability and noise in the paralleling circuits and sensing circuits. 
     U.S. Pat. No. 5,319,536 issued to Malik, is an example of paralleling in which three converters,  11 ,  13  and  15  are connected in parallel to load  23 . 
     U.S. Pat. No. 5,500,791 issued to Kheraluwala et al. teaches solving these problems by providing a dual active bridge converter generating 100,000 Hertz AC power square wave output which can be converted to DC by a converter at each load. The converters of Kheraluwala need not have such massive magnetic paths as would be required by a 60 Hertz system but there is still the need for a transformer, rectifier and possibly a voltage regulator at each load. 
     U.S. Pat. No. 5,254,877 issued to Tice et al. is another example of additional active power supply units being added along a distribution line. In Tice et al. a control panel provides power and communicates with smoke detectors and intrusion detectors. The line conductors  14   a  and  14   b  of Tice et al. serve as both signaling lines and power distribution lines and as is usual, the detectors farthest from the control panel would receive attenuated power levels. Tice et al. overcome this attenuation by providing distributed power supplies with synchronizing circuits, the added power supplies sense and respond to power pulses from the main control panel to inject supplemental amounts of power into the line during the power distribution time intervals. These added power supplies are relatively costly and they themselves require an external source of power such as from a AC power receptacle. 
     U.S. Pat. No. 5,777,276 issued to Zhu describes distribution of power on a computer motherboard using an auxiliary conductor system to reduce voltage loss due to high currents through resistive areas in the contact regions between connector posts and conductive layers buried in the motherboard. In the motherboard of Zhu, the conductors themselves are considered to have negligible resistive losses which of course is not the case in machines having larger distribution distances. The teachings of Zhu do not account for voltage loss in the original conductor or the auxiliary conductor and therefore his teachings do not concern voltage variations as a function of distance. 
     U.S. Pat. No. 4,788,449 issued to Katz describes a matrix of loads being supplied by a column of power supplies and redundantly by a row of power supplies. Although this teaching may solve a problem of the prior art with a short circuit in one load causing failure of power to all others in the same row, this teaching does not solve the problem of DC power attenuation at the farthest most load such as load  15  for example 
     The present invention overcomes these inadequacies, problems and disadvantages of the prior art by means of the apparatus and method of the invention which is summarized below. 
     SUMMARY OF THE INVENTION 
     An advantage of the present invention is that the size of the DC power distribution cables can be a smaller gauge without excessively sacrificing voltage regulation at each distributed load. Another way of stating this advantage is that by dividing a power distribution conductor into a power distribution conductor and a similarly sized sense conductor, significantly better voltage regulation may be obtained at each load to which power is being distributed. The improved regulation at each load is accomplished without requiring an increase in the combined conductivity of the power and sense conductors over that needed in a single power distribution conductor. 
     Another advantage of the instant invention is that multiple redundant power supplies, each with remote sensing, may be provided at a base module, redundantly supplying power to multiple modules without the problem of instability that often accompanies such redundant systems. 
     A further advantage of this invention is that a module may be removed from the system or a module may be added to the system without excessively changing the regulation of voltage provided to other modules. 
     Another advantage of the invention is provided by permitting implementation of current controlling resistors to be in the form of cable wire. Such implementation serves multiple purposes. It simplifies connections while balancing currents to loads and at the same time provides parallel paths in a power cable, which has the effect of lowering overall resistance power losses. It also has the benefit of distributing the heat over a wider area so that extra cooling or heat sinks are not needed. 
     These and other advantages of the invention, which will become apparent to the reader, are obtained by a novel arrangement of current carrying conductors which tailor and balance current delivered to each load. Tailoring of current to a load is accomplished by a current controlling resistance at each load. Balance of voltages between loads is accomplished by allowing a sense conductor to carry excess currents from one load to another. Current substantially equal to that drawn by each load is injected from current supply conductors to the sense conductors at or near each load. Current controlling resistances are embodied in lengths of wire in certain embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a high level diagram of an example machine in which the invention finds utility. 
     FIG. 2 is a circuit diagram showing the parts of the invention. 
     FIG. 3 is a circuit diagram of an alternate embodiment of the invention. 
     FIG. 4 is a diagram showing how current controlling resistors are embodied as lengths of wire. 
     FIG. 5 is a circuit diagram of an alternate embodiment of the invention in a star configuration. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 portrays an overall environment of a document processing machine wherein the invention finds utility. The machine comprises a base module  12  having a document input station  10  and a power supply not shown. Base module  12  may for example also contain the main computer for the machine as well as read stations to capture the intelligence in the various media on the document. Postal letter or other sorting machines, check processing machines, telephony equipment, and rack mounted equipment are example machines that benefit from the instant invention. 
     Module  16  is an image capture device such as a microfilm unit, scanner or other electronic image capture equipment. Modules  18 ,  20 ,  22 ,  24  and  26  are document stacker modules. The number of stacker modules may be different from one machine to another due to the differing number of sorts required. It is desirable that the stacker modules all be identical so that their order of placement in the machine is not critical to the quality of voltage regulation. 
     FIG. 2 is a circuit diagram of the preferred embodiment of the invention. FIG. 2 shows the base module  12  and two of the stacker modules  16  and  26 . The base module  12  has a power supply  211 . Power supply  211  has positive regulated voltage and current supplying output VS+ and negative regulated voltage and current supplying output VS−. Power supply  211  is of a remote sensing design where the voltage between VS− and VS+ is adjusted by the supply so that the voltage between a positive voltage sensing input SENSE+ and a negative voltage sensing input SENSE− attains a desired value such as for example, a nominal five volts. Five volts with a plus or minus five percent tolerance is used for driving digital circuit loads  220 ,  240  and  280  such as computers and other digital logic circuit boards. The sense inputs are usually connected to the VS outputs at the load being supplied with power so that the voltage drop in conductors between the VS outputs and the load is accounted for by a higher voltage setting which is controlled by the sense inputs. 
     This remote sensing design works well when a single load is driven remotely by the power supply. When several remote loads are being driven, other solutions must be found such as use of a very low resistance conductor to minimize voltage drops between loads or providing a regulated power supply at each load. Both of these solutions are more expensive and have other drawbacks as well. Since zero resistance can not be obtained at room temperatures, there will always be a difference between the voltages supplied to each load when power is supplied by a simple low resistance conductor. 
     This invention solves the problem of conductor resistance. Higher current densities are conducted over substantial distances by allowing the voltage between VS+ and VS− supplied by power supply  211  to rise adequately to provide a substantially correct voltage to a most difficult to supply load, while employing current controlling resistors to limit current and thereby voltage at other loads. The use of current control to achieve voltage regulation is usually considered to be a less than optimum approach because of it&#39;s sensitivity to load current changes. This invention overcomes regulation sensitivity to load changes and to load current changes by connecting a low resistance sense conductor from load to load. The sense conductor tends to distribute excess current at one load to other loads, changing the sense conductor voltage, which is then detected by the power supply and used to modify the voltage between outputs VS. 
     Power is distributed to each module of the machine according to a preferred embodiment of the invention through segmented current supply conductors  213  and  219  which are connected at one end to the outputs VS+ and VS− respectively of supply  211 . A segment of the current supply conductor is contained within each module. As the modules are connected together in a daisy chain fashion, the segments are connected together, end to end, to form each conductor. A redundant supply  212  may be connected in parallel to conductors  213  and  219  to provide power in the event that supply  211  fails. Supplies  211  and  212  will be connected to conductors  213  and  219  through isolating diodes to isolate an operating supply from a failed supply that failed due to a short circuit in it&#39;s output. 
     Conductors  213  and  219  have low but not zero resistance. Conductor resistance is a function of the conductor length. The resistance of conductors  213 ,  215 ,  217  and  219  is shown in FIG. 2 as being distributed discretely for convenience in explanation of the invention. Resistor  221  is the resistance of the length of a two gauge wire from each power supply output terminal VS+ to connection node  222 . Resistor  223  is the resistance of a length of an eight gauge wire from connection node  222  to a connector  224  between base module  12  and a stacker module  16  for providing current to stacker  16 . 
     Sense conductors  215  and  217  also have low but not zero resistance. In prior designs, negligible current flowed in a sense line since its only function was to supply a voltage reading from a single remote load back to the sense input of the power supply. In this invention, a sense conductor also serves to balance voltages among loads when a load is removed or when a load changes its current consumption. Thus resistor  225  is the resistance of the length of a wire from the power supply sense terminal SENSE+ to connection node  226 . Resistor  227 , at times carries load balancing current and it is the resistance of parallel lengths of two eight gauge wires from connection node  226  to a connector  228  between base module  12  and stacker module  16  for providing sense voltage and balancing current to and from stacker  16 . 
     Capacitors  205  and  206  are provided to control voltage swings during power on. Capacitors  205  and  206  prevent large output voltage over shoot at the VS terminals by providing a low impedance path from the current supply conductors to the sense conductors during power on transients. Capacitor  205  is connected between VS+ and SENSE+. Capacitor  206  is connected between VS− and SENSE−. 
     Power supply  211  has an over voltage shut down feature which shuts off the power supply  211  if the voltage between VS+ and VS− exceeds a value such as 6.7 volts. This feature is advantageously used to improve safety in this invention by shutting down the supply of power if a short circuit occurs some distance from the supply. The resistances of this invention may otherwise continue to generate more than normal heat during the short circuit condition and that could be a safety concern. 
     The current provided to load  220  in the base unit  12  is controlled by discrete current supply resistor  229 . Load  220  includes a main computer and other digital logic circuits. Resistor  229  is connected between connection node  222  and connection node  226 . The positive nominal five volt power input to the circuit boards of load  220  in the base unit is also connected to node  226 . Unlike later described resistors, resistor  229  will usually be implemented as a discrete power resistor due the large voltage drop that it must create in order not to over drive load  220 . If resistor  229  were implemented as a length of wire, it would usually be too long and/or it may also generate too much heat in a power cable to be practical. 
     A mirror image of the resistors and connections just described are provided at the negative side of the power supply  211  and load  220 . Resistor  231  is the resistance of a length of two gauge wire from each power supply output terminal VS− to connection node  232 . Resistor  233  is the resistance of a length of eight gauge wire from connection node  232  to a connector  234  between base module  12  and stacker module  16  for providing a current path from stacker  16 . 
     Resistor  235  is the resistance of a length of a wire from the power supply sense terminal SENSE− to connection node  236 . The power distribution network of the invention is grounded at connection node  236  so the wire of resistor  235  need not carry ground currents. Resistor  237  is implemented in the resistance of two parallel lengths of eight gauge wire from connection node  236  to a connector  238  between base module  12  and stacker module  16  for providing sense voltage and balancing current from and to stacker  16 . 
     The current provided from load  220  in the base unit  12  is controlled by discrete current supply resistor  239 . Resistor  239  is connected between connection node  232  and connection node  236 . The nominal five volt power return connection to the circuit boards of load  220  in the base unit is also connected to node  236 . By including resistors  229  and  239  in the power path of load  220  in the base unit, the voltage between VS+ and VS− can be made large enough to drive adequate current at specified voltage to a most remote load  280  while still not over driving load  220 . The connections of SENSE+ and SENSE− to sense conductors  215  and  217  respectively, and not to the current supply lines  213  and  219  as is common in the prior art, provide the negative feedback necessary for controlling the voltage between VS+ and VS− to be larger than five volts in order to drive adequate current at a nominal five volts nominal to a most remote load. The resistance of the first current supply resistor  229  is directly proportional to current supplied to the second DC current consuming device  240 , and to the resistance of the current supply conductor between the first DC current consuming device and the second DC current consuming device. 
     Referring now to the resistors within stacker module  16 , resistor  241  is the resistance of a length of an eight gauge wire from the connector  224  to connection node  242 . Likewise resistor  243  is the resistance of a length of an eight gauge wire from connection node  242  to a connector  244  between module  16  and a next module not shown, for providing current to this next module and other further downstream modules including module  26 . 
     Sense conductor  215  is implemented within module  16  by resistor  245  and is the resistance of two parallel lengths of eight gauge wire from connector  228  to connection node  246 . Resistor  247 , is the resistance of two parallel lengths of eight gauge wire from connection node  246  to a connector  248  between module  16  and the next module for providing sense voltage and balancing current to and from the next module. 
     The current provided to load  240  in the module  16  is controlled by discrete current supply resistor  249 . Resistor  249  is connected between connection node  242  and connection node  246 . The positive nominal five volt power input to the circuit boards of load  240  is also connected to node  246 . 
     A mirror image of the resistors and connections just described are provided at the negative side of load  240 . Resistor  251  is the resistance of a length of an eight gauge wire from the connector  234  to connection node  252 . Likewise resistor  253  is the resistance of a length of an eight gauge wire from connection node  252  to a connector  254  between module  16  and the next module for providing a current path from the next module. 
     Resistor  255  is the resistance of two parallel lengths of a wire from connector  238  to connection node  256 . Resistor  257  is the resistance of two parallel lengths of eight gauge wire from connection node  256  to a connector  258  between module  16  and the next module for providing sense voltage and balancing current from and to the next module. 
     The current provided from load  240  in the module  16  is controlled by discrete current supply resistor  259 . Resistor  259  is connected between connection node  252  and connection node  256 . The nominal five volt power return connection to the circuit boards of load  240  in module  16  is also connected to node  256 . By including resistors  249  and  259  in the power path of load  240 , the voltage between connection nodes  242  and  252  remains large enough to drive adequate current at specified voltage to a most remote load while still not over driving load  240 . 
     Referring now to the resistors within the most remote module  26 , resistor  281  is the resistance of a length of an eight gauge wire from the connector  264  to connection node  282 . Sense conductor  215  is implemented within module  26  by resistor  285  and is the resistance of two parallel lengths of eight gauge wire from connector  268  to connection node  286 . The current provided to load  280  in the module  26  is controlled by discrete current supply resistor  289 . Resistor  289  is connected between connection node  282  and connection node  286 . The positive nominal five volt power input to the circuit boards in load  280  in the module  26  is also connected to node  286 . 
     A mirror image of the resistors and connections described immediately above are provided at the negative side of load  280 . Resistor  291  is the resistance of a length of an eight gauge wire connected from connector  274  to connection node  292 . Resistor  295  is the resistance of two parallel lengths of eight gauge wire from connector  278  to connection node  296 . 
     The current provided from load  280  in the module  26  is controlled by discrete current supply resistor  299 . Resistor  299  is connected between connection node  292  and connection node  296 . The nominal five volt power return connection to the load  280  in module  26  is also connected to node  296 . By including resistors  289  and  299  in the power path of load  280 , the voltage between connection nodes  242  and  252  remains more stable when currents through other loads change in the machine. On the other hand, resistors  289  and  299  require that supply voltage VS be larger overall and therefore greater resistive heat losses are the tradeoff for greater DC stability with changing DC loads. 
     FIG.  3 . is another embodiment of the invention which decreases sensitivity to load changes and also decreases over all power loss. In this embodiment, a second current supply conductor  313  is provided in parallel with conductor  213 . Conductor  313  runs all the way through the modules and drives current into the most remote connection node  282 . In the case where the invention is embodied in a loop configuration, conductor  313  will be connected at the last or end load in the loop as it is completed back to the power supply. Conductor  313  comprises a resistor  321  in the form of a length of wire inside base unit  12  connected from connection node  222  to a connector  324  between modules  12  and  16 . Conductor  313  is further made up of resistor  341  in the form of a length of eight gauge wire connected between connectors  324  and  344  and resistor  381  in the form of a length of eight gauge wire connected between connectors  364  and connection node  282 . In FIG. 3, the resistance of current supply resistor  289  has been reduced to substantially zero in order to minimize over all resistance power loss. The embodiment of FIG. 3 also shows another variety of the invention that is possible to implement. In some applications of the invention, the modules are mounted in highly conductive frames that are bolted together as one conductive unit as in a motor vehicle, watercraft, aircraft or other such craft. In such an application it may be possible to use the frame of the machine as a very low resistance return current path, and such an implementation removes the need for the mirror image resistances  239 ,  259  and  299 . In this implementation, the VS−, and SENSE− terminals are connected together and become the ground connection for the machine. The frame of the machine becomes a very low resistance return path that does not introduce significant ground shifts. Accordingly logic control signals to the loads remain reliable. 
     FIG. 4 is a circuit diagram showing how current controlling resistors are embodied as lengths of wire. Module  16  of previous Figures is used as the example. Module  16  has power input connector  224  and power pass through connector  244 . Instead of providing a separate connection node  242  as used in FIG. 2, when the required current controlling resistance is low enough, a resistor is implemented as a three and one half foot length of fourteen gauge wire  449  connected at one end to connector  224  and connected at another end to connection node  246 . A single length of eight gauge wire  441  is then connected between connector  224  and connector  244  to implement both  241  and  243  of FIG.  2 . On the negative side of the load  240 , module  16  has power input connector  234  and power pass through connector  254 . Instead of providing a separate connection node  252  as used in FIG. 2, a resistor is implemented as a length of fourteen gauge wire  459  connected at one end to connector  234  and connected at another end to connection node  256 . A single length of eight gauge wire  451  is then connected between connector  234  and connector  254  to implement both  251  and  253  of FIG.  2 . The load  240  is connected between nodes  246  and  256  as was done in FIG.  2  and the ground is also still connected to the sense line  217 . 
     FIG. 5 which shows a star configuration of the invention. In FIG. 5, the connectors such as connectors  224  and  244  between modules are not shown because they may be implemented in a different way in a star configuration. The voltage and current supplying output VS+ is connected to current supply node  513 . The voltage sensing input SENSE+ is connected to sense node  515 . Both the voltage and current supplying output VS− and the voltage sensing input SENSE− is connected to node  519  where the system is grounded as it was in FIG.  3 . Power to load  520  is provided by a star configured current supply conductor, a first segment being a length of wire having a resistance  521  which is connected between node  513  and connection node  522 . The first segment of the sense conductor in this embodiment is a length of wire having a resistance  527  connected between sense node  515  and connection node  526 . The current provided to load  520  in the base unit  12  is controlled by discrete current supply resistor  529 . As before, load  520  includes a main computer and other digital logic circuits. Resistor  529  is connected between connection node  522  and connection node  526 . The positive nominal five volt power input to the circuit boards of load  520  in the base unit is also connected to node  526 . The nominal five volt power return connection to the circuit boards of load  520  in the base unit is connected to node  519  which is the ground return path through the machine frame as in FIG.  3 . 
     Power to load  540  is provided by a second segment of the star configured current supply conductor, as a length of wire having a resistance  524  which is connected between node  513  and connection node  542 . The second segment of the sense conductor in this embodiment is a length of wire having a resistance  545  connected between node  515  and connection node  546 . The current provided to load  540  in the module  16  is controlled by discrete current supply resistor  549 . Resistor  549  is connected between connection node  542  and connection node  546 . The positive nominal five volt power input to the circuit boards of load  540  is also connected to node  546 . The nominal five volt power return connection to the circuit boards of load  540  is connected to node  519  which is the very low resistance ground return path through the machine frame as in FIG.  3 . 
     Power to load  580  is provided by a third segment of the star configured current supply conductor, as a length of wire having a resistance  581  which is connected between node  513  and connection node  582 . The third segment of the sense conductor in this embodiment is a length of wire having a resistance  585  connected between node  515  and connection node  586 . The current provided to load  580  in the module  26  is controlled by discrete current supply resistor  589 . Resistor  589  is connected between connection node  582  and connection node  586 . The positive nominal five volt power input to the circuit boards of load  580  is also connected to node  586 . The nominal five volt power return connection to the circuit boards of load  580  is connected to node  519  which is the ground return path through the machine. 
     OPERATION OF THE INVENTION 
     Referring again to FIG. 3, operation of the invention will now be described, using the resistances obtained from the wire gauges described with respect to FIG.  2 . Each of the current supply resistors, except resistor  229 , is implemented as a three and one half foot length of fourteen gauge wire which yields a resistance of approximately ten milli-ohms. Resistor  229  is implemented as an eight point eight milli-ohm power resistor. Again, the nominal voltage desired across each load  220 ,  240  and  280  is five volts. In this example load  220  draws fifty five amperes through resistor  229 . Load  240  draws twelve and one half amperes and load  280  draws four amperes. Between modules  16  and  26  are five additional modules, each having loads drawing nine and one half amperes. 
     In this FIG. 3, the voltage at node  286  is 5.00 volts. The voltage at node  246  is 5.05 volts and the voltage at node  226  is 5.07 volts. The power supply  211  was set at 5.07 volts in order to center the voltage variation at the loads. Variation in voltage from load to load is due to variations in load currents and non-ideal resistances. By setting the supply internal reference voltage at 5.07, power supply  211  provides 5.66 volts at VS+ in order to drive the loads described above. 
     The two modules between module  16  and  26  that are closest to module  16  have nodes at 5.01 volts and 4.98 volts respectively. The remaining three modules have nodes at 4.95 volts. These results when added to expected load variations and parasitic machine resistances are better than plus or minus three percent regulation. The use of the same amount of conductive wire in direct connection as in the prior art without the current supply resistance of the invention yields approximately plus or minus ten percent regulation in this same application. Digital circuit loads  220 ,  240  and  280  require five volts with a plus or minus five percent tolerance as was described earlier. 
     Of course, many additional modifications and adaptations to the present invention could be made in both embodiment and application without departing from the spirit of this invention. For example, although the invention has been described with respect to five volts, currents in tens of amperes and distances in meters, other voltages, distances and currents may be used to advantage with the invention. Also, the invention has been described with the loads connected together in a daisy chain fashion, however the invention can also be applied to a loop configuration or to a star configuration. Accordingly, this description should be considered as merely illustrative of the principles of the present invention and not in limitation thereof.