Abstract:
A power distribution system and method, including a direct-current current source which is commutated among a number of distribution circuits which function as independent and modular power sources. Each distribution circuit includes a capacitor which is connected to the direct-current current source through a switch. The load is connected in parallel with the capacitor. The capacitor is charged to a predetermined level, generally matching the required load voltage, and periodically checked and recharged as necessary. There may be a plurality of distribution circuits to provide power to separate circuits or specific portions of a larger circuit. By varying the switch timing and logic, the output voltage of each distribution circuit may be varied, allowing a centralized power supply to provide power to a variety of electrical components with varying voltage requirements.

Description:
The present invention relates to a system and method for distributing power to circuit elements, and more specifically, to a central current source which is commutated among a number of energy storage devices, such as capacitors, which function as independent power sources. 
     BACKGROUND OF THE INVENTION 
     There are electrical applications in which a number of independent and varying power supplies are necessary. For instance, in printed circuit boards, there are usually a variety of components and sub-circuits which require independent power supplies. Separate power supplies are required for a variety of reasons, such as varying voltage requirements, noise and isolation concerns, etc. Generally, the voltage requirements in integrated circuits are relatively small, ranging between 5 volts to 0.50 volts. In light of the low operational voltages of such circuits, voltage deviations on the power supply line due to phenomena such as source variations, load variations, and electrical noise, can be a significant portion of the output voltage and can cause operational problems. Also, given the trend of electronic circuits becoming smaller in physical dimensions, it is becoming increasingly impractical to use multiple power supplies which would limit the extent to which printed circuit boards can be reduced in size. 
     Therefore, there is a need for a power distribution system and method which can be utilized to provide separately regulated power to multiple loads with a variety of voltage requirements while reducing the number of required power supplies. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a novel power distribution system and method in which a single power source can distribute power to multiple loads with varying voltage requirements. 
     Another object of the present invention is to provide a novel power distribution system and method in which the reliability of the individual load output voltages is enhanced by reducing electrical noise and interference. 
     A further object of the present invention is to provide a novel power distribution system which will increase power efficiency. 
     In accordance with one embodiment of the present invention, all of these objects, as well as others not herein specifically identified, are achieved generally by the present power distribution system and method in which a centralized current source is commutated among multiple energy storage devices, such as capacitors, which are connected in parallel with their respective loads. 
     More specifically, one embodiment of the present invention includes a switching regulated current source which is sequentially connected in parallel circuit arrangement with a number of load capacitors, each capacitor being connected to the current source through a switch. Each capacitor is connected to a load and can be charged to predetermined voltages during its charging time slot, thereby providing power to each load. The capacitors can be charged to varying voltages, allowing for a single current source to provide a number of output load voltages. The frequency and duration of the switch operations can be varied to enhance efficiency of the power distribution or to accommodate the particular requirements of the electrical system. Further, additional circuit arrangements can be utilized to enhance the efficiency and enhance isolation of each distribution circuit. 
     Because a significant portion of a switching regulator&#39;s power losses are independent of load, this approach achieves higher efficiency as well as smaller size in comparison to using multiple individual power supplies for each output voltage requirement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects of the invention, taken together with additional features contributing thereto and advantages occurring therefrom, will be apparent from the following description of the invention when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1A is a schematic circuit diagram of an embodiment of a power distribution system in accordance with the present invention; 
     FIG. 1B is a schematic circuit diagram of the embodiment of a power distribution system of FIG. 1A further showing the details of an exemplary switch control circuit in accordance with the invention; 
     FIG. 2 is a chart showing various illustrative values of current and voltage over time; 
     FIG. 3 is an example of a Schmitt trigger comparator circuit arrangement for comparing the energy storage device voltage to its reference voltage; 
     FIG. 4 is a logic diagram of an example of a control logic circuit for driving the switches, including an asynchronous override feature; 
     FIG. 5 is a schematic circuit diagram of an illustrative pulse-width modulator controlled single-ended primary inductor converter; 
     FIG. 6 is a schematic circuit diagram of an embodiment of the present invention utilizing a single-ended primary inductor converter as the current source; 
     FIG. 7 is a schematic circuit diagram illustrating an example of a portion of the circuit in FIG. 6 with a return path diode serving as an energy return switch; 
     FIG. 8 is a schematic circuit diagram of another embodiment of the present invention with enhanced isolation of the distribution circuit; and 
     FIG. 9 is a logic diagram for controlling the state of an illustrative pulse-width modulator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1A, an embodiment of the present power distribution system is shown and generally designated as  10 . The power distribution system  10  includes a direct-current (“DC”) current source  12  and at least one distribution circuit  14  shown as the elements encompassed by the broken lines. Each distribution circuit  14  of the embodiment of FIG. 1A includes a switch S 1 , S 2 , S 3 , and a capacitor C 1 , C 2 , C 3 , respectively. Load resistors R 1 , R 2 , R 3  are included in the diagram for the purposes of explanation. With respect to the first distribution circuit  14 , the capacitor C 1 , the load resistor R 1 , and the DC current source  12  are in parallel circuit arrangement with each other. The switch S 1  is connected in series between the output of the current source  12  and the positive terminal of the capacitor C 1 . As can be seen in FIG. 1A, the additional distribution circuits  14  are configured similarly. 
     Each distribution circuit  14  can provide a separate and distinct voltage to an electrical load R 1 , R 2 , R 3 , and each load would be provided with the load voltages of v 1 , v 2 , or v 3 , respectively. The value of the load voltages v 1 , v 2 , and V 3  can be varied by the amount of charge provided to the capacitors C 1 , C 2 , and C 3 . 
     In the embodiment of FIG. 1A, a Switch Control  16  is used to control the operations of the switches S 1 , S 2 , and S 3 . A variety of circuit functions can be utilized in the Switch Control  16  to control the switching of switches S 1 , S 2 , and S 3 . These circuit functions can include Schmitt triggers, control logic circuits, and clock circuits. 
     Predetermined reference voltages are selected for each distribution circuit, V Ref1 , V Ref2 , and V Ref3 , which represent the desired output load voltages to which the capacitors, C 1 , C 2 , and C 3 , respectfully, are charged. The actual voltages across the capacitors, C 1 , C 2 , and C 3 , are designated as v 1 , v 2 , and V 3 , respectfully. 
     A Schmitt comparator circuit arrangement, as shown in FIG. 3, is an example of a circuit that may be used to compare the actual voltages (v 1 , v 2 , and V 3 ) across the capacitors with the predetermined voltages, V Ref1 , V Ref2 , and V Ref3 , respectfully. In FIG. 3, v k  and V Refk  are compared to produce and output cmp k , where the index “k” represents the distribution circuit number. When v k  is less than V Refk , the output cmp k  is low. If v k  is not less than V Refk , then the output is high. 
     FIG. 1B depicts the circuit of FIG. 1A illustrating an embodiment of the Switch Control  16  in more detail, incorporating a Schmitt trigger comparator and showing a more detailed view of the logic circuits. Referring to the first distribution circuit of FIG. 1B, during its clock cycle, the voltage v 1  across capacitor C 1  is compared to its reference voltage V Ref1  by the Schmitt trigger comparator having an output of cmp 1  to determine whether charging is required. If v 1  is sufficiently less than V Ref1 , switch S 1  is activated by the logic circuit to allow charging of the capacitor C 1 . During the next clock cycle, the second distribution circuit, comprised of switch S 2  and capacitor C 2  is examined in the same manner as the first distribution circuit. The frequency of the clock cycle is preferably selected to be relatively high with respect to the charge and discharge rates of the capacitors C 1 , C 2 , and C 3 . A relatively higher clock cycle frequency is chosen to eliminate the possibility of one capacitor fully discharging while another capacitor is being charged. 
     A chart representing an example of a sequential mode of operation is presented in FIG. 2, wherein CLK is the clock cycle, and the designations of clk 1 , clk 2 , and clk 3  represent sequential clock cycles in which the first, second and third distribution circuits are being examined, respectively. During the initial clock cycle, v 1  is compared with V Ref1  by the Schmitt Trigger comparator. Since v 1  is less than V Ref1 , switch S 1  is activated and v 1  rises during this cycle. In the second clock cycle, switch S 1  is deactivated and v 2  is compared with V Ref2 . Since v 2  is less than V Ref2 , switch S 2  is activated, allowing v 2  to increase. During the third clock cycle, switch S 2  is deactivated and v 3  is compared to V Ref3 . Since v 3  is already equal to V Ref3 , meaning that the capacitor C 3  is fully charged to the predetermined level, switch S 3  is not activated and no charging takes place. The cycles then repeat. The present power distribution system  10  has been described with three distribution circuits  14  for the purposes of explanation in FIGS. 1A,  1 B, and  2 . However, in principle, any number of distribution circuits  14  may be utilized in this power distribution system. 
     It may also be desirable to allow for asynchronous override of the clock cycle, for example, where only one distribution circuit  14  is heavily loaded. FIG. 4 shows a logic diagram of one embodiment of a control logic circuit for achieving asynchronous override with respect to the first distribution circuit and for normal control of switch S 1 , where the operation of switch S 1  is a function of cmp 1 , clk 1 , and the values of cmp 2−k . In the illustrative embodiment of FIG. 4, S 1  will be activated (i.e. logical value 1) only when cmp 1  is low (i.e. C 1  needs charging), and either the clock cycle is at one or cmp 2−k  are all high (i.e. C 2−k  do not need charging). Similar logic is applied with respect to each switch, allowing for asynchronous override capabilities for each distribution circuit  14 , along with normal switch operation. 
     It may be desirable to use a single-ended primary inductor converter (SEPIC) to serve as the current source  12 . A single-ended primary inductor converter provides, inter alia, (1) buck or boost capability to accommodate input voltages that may be higher or lower than the output voltages; (2) inherently switchable output; (3) “smooth” input current for low power-line electromagnetic interference; and (4) optional input/output isolation for highly noise-sensitive loads, or for instances where the various outputs do not share a common ground, or where their ground is not in common with the source. 
     An example of a well-known single-ended primary inductor converter, generally designated as  18 , is shown in FIG.  5 . The basic single-ended primary inductor converter  18  includes a DC voltage supply V s , inductors L 1  and L 2 , capacitors C A  and C B , a diode D 1  for isolation of capacitor C B  a resistor R 1 , a transistor switch S A , and a pulse width modulator PWM. The PWM can be any commonly known and available chip that compares the average current through D 1  to a reference current I ref . A current transformer T 1  with appropriate reset means can be used to sense the output current through D 1 . In the circuit shown, the voltage V B  across capacitor C B  is generally equal to V S (D)/(1−D), where D is the duty factor of the pulse width modulator PWM. When S A  is off, the current output I O  of the single-ended primary inductor converter is equal to I A  (the current across L 1 )/D. When S A  is on, the current output I O  is zero. 
     The single-ended primary inductor converter  18  of FIG. 5 can be modified and utilized to serve as the current source  12  of FIGS. 1A and 1B in accordance with the present invention. FIG. 6 shows an embodiment of the present invention in which a single-ended primary inductor converter  18  is utilized as the current source  12 . The distribution circuits  14  of FIG. 1B become part of the modified single-ended primary inductor converter  18  by including switches S 1−k  and capacitors C 1−k  of the distribution circuit  14  in the single-ended primary inductor converter  18 . In the embodiment shown in FIG. 6, the switches S 1 , S 2  , and S 3  of distribution circuits  14  in FIGS. 1A and 1B become switches S 1−k . Diodes D 1−k  in the distribution circuit  14  perform the function of D 1  in FIG.  5 . Schottky diodes may be used for D 1−k  since they generally provide a lower forward drop than bipolar diodes. Metal-oxide field-effect transistors (MOSFETs) may be used for switches S 1−k . The presence of the diodes D 1−k  reduces the concerns with situations in which multiple switches S 1−k  may be on at the same time due to a switching malfunction. Even if more than one switch were on at the same time, the charge in higher voltage capacitors will not be affected since the diode for the higher voltage capacitor will prevent discharge in the reverse direction. 
     Referring to FIG. 7, the reliability of the current source may be enhanced and the possibility of damage to the power components may be reduced by providing a return path diode to the current source circuit. As shown, a diode D A  can be connected across the inductor L 1  and the capacitor C A  to provide a path for the current in the event that no output capacitors need charging or switching functions malfunction. 
     As shown in FIG. 8, isolation of the distribution circuit  14  can be achieved by utilizing an additional winding L 3 , essentially creating a transformer by which current is provided to the distribution circuit  14  and by which the distribution circuit  14  is isolated from electrical noise from the current source. 
     To further enhance power efficiency, the PWM of the single-ended primary inductor converter  18  may be turned off when the capacitors C 1−k  do not need charging, i.e. when cmp 1−k  are all at relative high states. One embodiment of a simple logic circuit for achieving this feature is shown in FIG. 9, where the PWM is on when its ON/OFF input is high. 
     While particular embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes and modifications may be made thereto without departing from the invention as set forth in the following claims.