Patent Application
20090230771
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.
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.
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
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
It may be seen that the packaged hold-up circuit 10 shown in
Referring now to
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.
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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
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.
