Patent Application
20130285449
The present invention is related to power interrupts, and in particular to a system and method for providing hold-up power during a power interrupt.
Power interrupts often occur on aircraft systems due to, for example, transfer of power between a battery and a primary generator. These systems, including auxiliary power unit (APU) systems, are required to operate through these power interrupts. Several approaches have been used to meet this requirement. For example, permanent magnet generators (PMGs) have been installed on APUs to provide back-up power to the APUs' control electronics. This implementation adds significant cost, weight, complexity and circuitry to the APU.
Alternately, controlled flameout has been used to handle power interrupts for APU electronics. Controlled flameout involves extinguishing a flame in the combustor when a power interrupt occurs. This is done in order to eliminate the need for the APU electronic controls during the power interrupt. When power is restored following the interrupt, the APU re-lights the combustor and resumes operation. Utilizing a controlled flameout requires a complex system approach which necessitates vigorous system level testing to ensure reliability.
Additionally, bulk capacitors have been added to the system to provide hold-up power during an interrupt. Traditionally, an input voltage is simply applied to a capacitor, and stored until a power interrupt occurs. During the power interrupt, the voltage stored on the capacitor is directly used to power the APU's electronic controls and other external loads. As the length of the interrupt increases, the size of the capacitance must increase. These capacitors are heavy and add significant cost to the design of the APU system.
A system and method for providing hold-up power includes a power source, one or more loads, a boost circuit, a capacitor, and a buck circuit. The boost circuit boosts an input voltage from the power source, the capacitor stores a boosted voltage from the boost circuit, and the buck circuit steps down a voltage from the capacitor. The buck circuit provides a stepped down voltage to the one or more loads when the power source is unavailable.
The present invention describes a system and method for providing hold-up power during a power interrupt. The system includes a boost circuit, a storage capacitor, and a buck circuit. The boost circuit receives a direct current (DC) input voltage from a primary power source. This primary power source is also used to power loads during normal system operation. The boost circuit boosts the input voltage to a stepped up output voltage that is provided to charge the storage capacitor during normal system operation. By stepping up the voltage using the boost circuit, the capacitor charges to a higher voltage, therefore storing more energy. Upon loss of power from the primary power source, the storage capacitor is discharged through the load in order to provide hold-up power to the load. The buck circuit steps down the voltage from the storage capacitor and provides the stepped down voltage to the load.
The energy stored in a capacitor is equal to ½(C)(V2), where C is the value of capacitance of the storage capacitor, and V is the value of voltage across the storage capacitor. During a power interrupt, the amount of energy drawn from the storage capacitor is dependent upon the load. Some external loads act like a simple resistance. Thus, the higher the load voltage, the higher the current draw and therefore, the higher the required energy. By boosting the voltage from the primary power source from the typical 28 volts to a higher voltage and lowering the output voltage delivered to the load during the power interrupt, the required capacitance of the storage capacitor can be greatly reduced. Therefore, by adding the boost circuit and buck circuit to the system, the necessary capacitance to provide hold-up power for a given time to the load is reduced. Because boost circuits and buck circuits have become relatively light, inexpensive and reliable this greatly reduces the overall weight and cost of the system.
Boost circuit 14, storage capacitor 16, and buck circuit 18 operate to provide hold-up power to loads 22 during normal system operation. Normal system operation is any time that power interrupt protection must be provided to loads 22. For example, if system 10 is an APU system, normal system operation involves the APU running at an operational speed. During normal system operation, controller 32 enables boost circuit 14 and buck circuit 18 using boost circuit enable line 34 and buck circuit enable line 36 respectively. Boost circuit 14 steps up the DC voltage from primary power source 12. This stepped up voltage is used to charge storage capacitor 16. Boost circuit 14 may be implemented in a number of ways, all of which are known in the art. Boost circuit 14 may step up the voltage, for example, from an input of 28 volts to an output of 48 volts. Storage capacitor 16 is charged up to the voltage on boost output line 24. The capacitor remains charged while primary power source 12 is providing power to system 10.
Buck circuit 18 steps down the voltage across storage capacitor 16. Buck circuit 18 may be implemented in a number of ways, all of which are known in the art. The voltage may be stepped down, for example, from 48 volts to 12 volts. The value of the stepped down voltage is selected to provide loads 22 with a functional voltage while minimizing the power dissipation through loads 22 during a power interrupt. The stepped down voltage will typically be lesser than the voltage provided by primary power source 12. Because of this, diodes 20a and 20b can be used to diode OR buck output line 28 and primary power source 12. During normal system operation, primary power source 12 will provide a voltage larger than the voltage on buck output line 28, forward biasing diode 20a, and therefore providing primary power to loads 22. When primary power source 12 is unavailable, the voltage on buck output line 28 will be larger than the voltage provided by primary power source 12, forward biasing diode 20b, and therefore providing voltage to loads 22 from buck circuit 18.
The amount of time for which power can be provided to loads 22 from storage capacitor 16 is based upon the energy stored in storage capacitor 16 and the power dissipation of loads 22. Because energy is equal to power multiplied by time, ½(C)(V2)=(P)(t), where P is the power dissipated by loads 22 and t is time. Thus, the amount of time for which storage capacitor 16 can provide power to loads 22 is equal to (C)(V2)/(2P). Prior hold-up circuits simply charged the capacitor to the input voltage of, for example, 28 volts and then provided that 28 volts to the loads during a power interrupt. This results in a necessary capacitance of approximately 0.067 farads in order to accommodate a 220 millisecond interrupt. Boosting the voltage such that the capacitor charges to 48 volts, and stepping the voltage of storage capacitor 16 down to 12 volts, results in a necessary capacitance of approximately 0.0067 farads for a 220 millisecond interrupt. Reducing the required capacitance in this way allows the weight and cost of system 10 to be greatly reduced.
Controller 32 is used in conjunction with reference resistor 38 to test the functionality of storage capacitor 16. This test can be done at any time when power interrupt protection is not required for system 10. If system 10 is an APU system, for example, the test may be performed during startup of the APU. Certain capacitors, such as aluminum electrolytic capacitors must be tested regularly to ensure proper functionality. To test storage capacitor 16, controller 32 disables boost circuit 14 and buck circuit 18 using boost circuit enable line 34 and buck circuit enable line 36 respectively. When both boost circuit 14 and buck circuit 18 are disabled, storage capacitor 16 will discharge through reference resistor 38. Controller 32 monitors the voltage across reference resistor 38 during discharge of storage capacitor 16. Because the value of resistance of reference resistor 38 is known, controller 32 can monitor the amount of time it takes for the voltage across reference resistor 38 to drop a predetermined amount. This allows controller 32 to calculate the actual capacitance of storage capacitor 16 in order to ensure its proper functionality.
In this way, the present invention describes a system and method for providing hold-up power during a power interrupt. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
