Patent Publication Number: US-2009230771-A1

Title: Simple and passive solution for providing power interruption capability with controlled inrush current in critical power supply

Description:
BACKGROUND OF THE INVENTION 
     The present invention is in the field of electrical power supply control and, more particularly, control of electrical power supply systems that may be subject to input-power interruptions. 
     Some power supply systems are constructed to provide uninterrupted power for critical applications. For example, in some aerospace applications, power supplies may be required to supply regulated voltage to critical loads during power interruptions at their input. Such critical loads may include, for example, avionics control. 
     In some direct current (DC) to DC converters and alternating current (AC) to DC converters, bulk capacitors are used at DC inputs or in a DC link to provide “hold-up” time to mitigate input power interruptions. In many aerospace applications, high hold-up time (typically more than 50 milliseconds (ms)) may be required. High hold-up time may require use of very high energy-storage, leading to large value of the bulk capacitor. Large value bulk capacitors demand very high inrush current during start up of a power supply. Additionally, when input power resumes after a power interruption period, high inrush currents may develop. 
     In the prior art, various passive mechanisms have been employed to limit inrush current. In the simplest case, a current limiting resistor may be placed in series with a power conduction path. In some cases, the current limiting resistor may be a Negative Temperature Coefficient (NTC) resistor. NTC resistors may reduce undesirable power loss associated with use of conventional current limiting resistors. But NTC resistors may not be readily employed in high power applications. 
     Furthermore, prior-art passive systems may not be capable of limiting inrush current after a power interruption period when input power resumes while a system output is under load. Additionally, prior art passive systems may not be capable of limiting inrush current during initial starting under load. 
     Various non-passive mechanisms have been employed in the prior art to overcome passive system limitations. For example, the current limiting resistor may be shorted by a time-delayed switch. Also, constant current charging of hold-up capacitors has been employed in the prior art to address the above issue. However, these non-passive, or active, solutions are typically complex and difficult to implement. 
     As can be seen, there is a need to provide a power supply system which may provide uninterrupted power output if and when input power is interrupted. In particular, there is a need to provide such a system that may operate passively and at high power levels with low power loss. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention an electrical power supply apparatus comprises a hold-up capacitor connected to at least one bus, a resistor interposed between the capacitor and a second bus, and a rectifying device interposed between the capacitor and the second bus. 
     In another aspect of the present invention a hold-up circuit for an electrical power supply comprises a hold-up capacitor connected to a resistor and a rectifying device. The resistor and the rectifying device are connected in parallel with one another. 
     In still another aspect of the present invention a method for providing hold-up protection for an electrical power supply comprises the steps of reverse biasing a rectifying device when input power is supplied at a desired voltage to the power supply, forward biasing the rectifying device in the event of input power failure, and discharging a capacitor through the rectifying device to a load of the power supply when the rectifying device is forward biased. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an illustrative embodiment of a hold-up circuit in accordance with the invention; 
         FIG. 2  is a block diagram of a power supply system in accordance with an embodiment of the invention; 
         FIG. 3  is a block diagram of an illustrative embodiment of a power supply system in accordance with the invention; 
         FIG. 4  is a block diagram of a second illustrative embodiment of a power supply system in accordance with the invention; 
         FIG. 5  is a block diagram of a third illustrative embodiment of a power supply system in accordance with the invention; 
         FIG. 6  is a block diagram of a fourth illustrative embodiment of a power supply system in accordance with the invention; 
         FIG. 7  is a block diagram of a second illustrative embodiment of a hold-up circuit in accordance with the present invention; 
         FIG. 8  is a block diagram of a third illustrative embodiment of a hold-up circuit in accordance with the present invention; and 
         FIG. 9  is a flow chart of a method for providing hold-up protection in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Broadly, the present invention may be useful in electrical power supply systems which provide uninterrupted power to a load. More particularly, the present invention may provide such a power supply system with a capacitor based “hold-up” time that may mitigate adverse effects of input power interruptions. The present invention may be particularly useful in vehicles such as aircraft which may employ power systems to supply regulated voltage to critical loads such as avionics control. The present invention may provide requisite hold-up time and may provide a passive system for limiting inrush current. 
     In contrast to prior-art power control systems, among other things, the present invention may perform control of inrush current to a hold-up capacitor without employing an active current control scheme. The present invention, instead of an active switching system, may utilize a passive resistor-based system for inrush current control. The resistor-based system may employ a unique combination of a resistor, a rectifying element and a capacitor to passively limit inrush current without diminishing hold-up capability in a power supply. 
     Referring now to  FIG. 1 , a hold-up circuit is designated generally by the numeral  10 . The hold-up circuit  10  may comprise a resistor  12 , a rectifying device such as a diode  14  and a hold-up capacitor  16 . The circuit  10  may be interposed between a DC input  20  and a load  22 . In some embodiments of the present invention an isolating rectifying device or diode  24  may be interposed on a power conductor or bus  26 . 
     A current path  28  may be provided between the power conductor  26  and the resistor  12 . The current path  28  may continue through the resistor  12  to the hold-up capacitor  16  and the diode  14 . A current path  30  may be provided between the diode  14  and the power conductor  26 . A second power conductor or bus  32  may interconnect the input  20  and the load  22 . The hold-up capacitor  16  may be interposed between the current path  28  and the bus  32 . 
     It may be seen that in operation, whenever a voltage (Vin) is applied between the buses  26  and  32  at the input  20 , an output current (Io) may develop in the conductor  26 . A charging current (Ic) may pass through the current path  28  and the resistor  12  to provide charging of the capacitor  16 . Even though the capacitor  16  may be charging at initiation of voltage Vin at the input  20 , inrush current (Iin) may be only slightly larger than Io. The resistor  12  may substantially limit the current Ic. 
     It may also be seen that the diode  14  may be reverse-biased unless and until voltage Vc exceeds Vo. In this context, the capacitor  16  may become charged with the current Ic because the diode  14  may preclude current flow from the resistor  12  to the load  22 . Unlike prior-art systems, current Io may pass directly to the load without passing through an inrush current control resistor. Consequently the resistor  12  may be selected to have a large value. When the capacitor  16  becomes fully charged, current Ic may cease and there may be no further power loss through the resistor  12 . Thus, even if the resistor  12  has a high resistance, power loss associated with the resistor  12  is limited. 
     In the event of power interruption at the input  20 , the voltage Vin may drop and the voltage Vo may fall to a level below Vc. In that case, the diode  14  may become forward-biased and the capacitor  16  may discharge to maintain an output voltage equal to Vc. The capacitor  16  may be selected to provide a desired hold-up time during its discharge. In a typical aircraft application, hold-up time for a critical load may be about 50 milliseconds (ms) or more. 
     Referring now to  FIG. 2 , the hold-up circuit  10  may be seen positioned on a power system  34  constructed in accordance with the present invention. The power system  34  may comprise a DC/DC converter  36 . The hold-up circuit  10  may be configured and assembled in a package  38  as a two-terminal device. The hold-up circuit  10  may be provided with terminals  40  and  42  which may be connected to DC input buses  44  and  46  respectively. 
     It may be seen that the packaged hold-up circuit  10  shown in  FIG. 2  may be readily and conveniently installed on the power system  34 . The packaged circuit  10  may be installed during initial construction of the power system  34  or it may be installed as a retrofit on a pre-existing one of the power systems  34 . In this regard, the packaged hold-up circuit  10  may be utilized as an optional upgrade device for one of the power systems  34 . If the power system is to be employed in a critical application such as avionics power supply, then the hold-up circuit  10  may be installed. If, on the other hand, the power system  34  is to be employed in a non-critical application, a cost savings may be achieved by forgoing installation of one of the hold-up circuits  10 . 
     Referring now to  FIG. 3 , an alternate embodiment of an inventive power system  50  may comprise a DC/DC converter  52  with multiple outputs. A first output  54  may supply power to a critical load  56 . A second output  58  may supply power to a non-critical load  60 . For simplicity, the power system  50  is illustrated with two outputs. It should be understood that the power system  50  may be provided with any number of outputs which may supply power to any combination of critical and non-critical loads. 
     The output  54  may be connected to the critical load  56  load through buses  62  and  64 . One of the hold-up circuits  10  may be connected between the buses  62  and  64  to provide hold-up protection for the critical load  56 . 
     The output  58  may be connected to the non-critical load  60  through buses  66  and  68 . The power system  50  is illustrative of a versatility of the packaged hold-up circuit  10 . The circuits  10  may be placed between output buses only as needed, for hold-up protection of critical loads. Space and cost savings may be achieved by forgoing installation of the hold-up circuits  10  in outputs that may supply non-critical loads. 
     Referring now to  FIG. 4 , an inventive power supply system  70  may comprise a DC/AC converter  72  with an input  72 - 1  and an output  72 - 2 . The converter  72  may provide power to a resistive load  74 . The converter  72  may provide single or multiple phase AC output. The converter may be a variable voltage variable frequency (VVVF) type or a fixed voltage and frequency type. When the hold-up circuit  10  is connected between DC input buses  76  and  78 , the power system  70  may provide AC power to the load  74 . 
     Referring now to  FIG. 5 , an inventive power system  80  may comprise an AC/DC converter  82 . The converter  82  may provide DC power to a critical load  84 . For simplicity, the power system  80  is illustrated with a single set of output buses  86  and  88 . One of the hold-up circuits  10  may be connected between the buses  86  and  88  to provide hold-up protection for the load  84 . It should be understood, however, the power system  80  may comprise multiple outputs (not shown) and each of the multiple outputs may or may not be protected with one of the hold-up circuits  10 . 
     Referring now to  FIG. 6 , an inventive power system  90  may comprise an AC/DC converter  92  and a DC/AC converter  94 . One of the hold-up circuits  10  may be interposed between the converters  92  and  94 . The hold-up circuit  10  of  FIG. 6  may provide hold-up protection for a load  96 . 
     Referring now to  FIG. 7 , an alternative embodiment of a hold-up circuit  100  comprises rectifying devices  102  and  104  which may be controlled switching devices such as metal-oxide field effect transistors (MOSFET&#39;s) or Bipolar Junction Transistors or equivalent. In operation the switches  102  and  104  may function in a manner similar to the diodes  14  and  24  of  FIG. 1 . In the context of the hold-up circuit  100 , the controlled semiconductor switches  102  and  104  may be kept continuously on or may be operated with synchronous switching. At certain current levels, a forward-voltage drop of a MOSFET may be lower than that of a diode rectifier. Under these conditions, the MOSFETS  102  and  104 , as compared to the diodes  14  and  24  of  FIG. 1 , may provide for a reduction of rectifier conduction loss. 
     Referring now to  FIG. 8 , an alternative embodiment of a hold-up circuit  200  may comprise a resistor-inductor (R-L) device  202  to provide inrush current control. An inductor  202 - 1  may be placed in series with a resistor  202 - 2  to slow down a rate of change of inrush current. 
     In one embodiment of the present invention, a method is provided for providing hold-up protection in a power system with passive control of inrush current. In that regard the method may be understood by referring to  FIG. 9 . In  FIG. 9 , a flow chart portrays various aspects of an inventive method  900 . In a step  902 , startup input current may be applied to a power system (e.g., Iin may be applied to buses  26  and  32  at a voltage Vin). In a step  904 , a capacitor-charging portion of the input current may be passed through a resistor (e.g., the current Ic may pass through the resistor  12 ). In a step  906 , a rectifying device may be reverse biased to block the capacitor-charging current from passage to an output of the power system (e.g., the diode  14  may be reverse biased when the voltage Vin exceeds Vc, thereby blocking current along the conduction path  30 ). In a step  908  a hold-up capacitor may be charged (e.g., the capacitor  16  may be charged with the current Ic). 
     In a step  910 , input power failure may occur. In a step  912  the rectifying device may become forward biased (e.g., power failure may drop Vin to a level below Vc resulting in forward biasing of the diode  14 ). In a step  914 , the hold-up capacitor may discharge to an output of the power system and thus provide hold-up for the power system (e.g., the capacitor  16  may discharge through the forward biased diode  14  into the load  22 ). 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.