Abstract:
An apparatus and method for protecting an AC load from half-wave rectified power detects a half-wave rectified voltage or current, sets a status indication indicating the type of asymmetric fault causing the half-wave rectified power, and selectively turns the SSPC ON or OFF depending on the type of fault. There are two example circumstances that can cause an asymmetric fault: when the SSPC is OFF and a switch is shorted (“SSPC Failed Shorted”), or when the SSPC is commanded ON and a switch remains open (“SSPC Failed Open”). If the SSPC is OFF when the half-wave rectified AC voltage or current is detected, an “SSPC Failed Shorted” status indication is set, and the SSPC is turned ON to clear the fault. If the SSPC is commanded ON when the half-wave rectified AC voltage or current is detected, an “SSPC Failed Open” status indication is set, and the SSPC is turned OFF to clear the fault.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates generally to vehicle power systems and, more particularly, to AC solid state power controls. 
         [0002]    Vehicles, such as aircraft, require electrical wiring protection. The most common form of aircraft electrical wiring protection is a thermal circuit breaker. Also common is an all electronic switch or a solid state power controller (SSPC). An SSPC uses transistors to provide a switching function and uses electronics to provide a circuit breaker function. The SSPC has found widespread use because of its desirable status capability, reliability, and packaging density. 
         [0003]    The usual waveform of AC electric power is a sine wave, which has both positive and negative portions. In half-wave rectified AC, either the positive or negative portions of the AC waveform are blocked. When distributing AC electrical power through an AC SSPC, fault modes exist that can result in half-wave rectified AC power being transmitted to a load. These fault modes are known as asymmetric faults. Half-wave rectified waveforms can damage aircraft circuitry and can create hazardous conditions for an aircraft. 
         [0004]    There is a need for an SSPC that can detect asymmetric fault conditions and that can protect a load from a half-wave rectified AC power. 
       SUMMARY OF THE INVENTION 
       [0005]    In the disclosed embodiments of this invention, an apparatus and method for protecting AC loads from a half-wave rectified power is provided. An SSPC detects a half-wave rectified voltage, sets a status indication indicating the type of asymmetric fault causing the half-wave rectified power, and selectively turns the SSPC ON or OFF depending on the type of asymmetric fault. There are two example circumstances that can cause an asymmetric fault: when the SSPC is OFF and a switch is shorted (“SSPC Failed Shorted”), or when the SSPC is commanded ON and a switch remains open (“SSPC Failed Open”). If the SSPC is OFF when the half-wave rectified AC is detected, an “SSPC Failed Shorted” status indication is set, and the SSPC is turned ON to clear the fault. If the SSPC is commanded ON when the half-wave rectified AC is detected, an “SSPC Failed Open” status indication is set, and the SSPC is turned OFF to clear the fault. 
         [0006]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates a simplified AC solid state power controller according to one embodiment of the present invention. 
           [0008]      FIG. 1A  illustrates a power section of a solid state power controller according to a second embodiment of the present invention. 
           [0009]      FIG. 2  illustrates a regular AC voltage or current waveform and a positive half-wave rectified AC voltage or current waveform. 
           [0010]      FIG. 3  illustrates one embodiment of the present invention in an example environment of an aircraft. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0011]      FIG. 1  illustrates selected portions of an example solid state power controller (SSPC)  52  for use in a vehicle, such as an aircraft. In this example, the SSPC  52  includes a logic section  54  and a power section  56 . The logic section  54  comprises a power supply  58 , a microcontroller  10 , a gate drive  12 , a signal conditioning and logic module  14 , and a communications interface  16 . The power section  56  comprises a first switch  20  that has a reverse-biased intrinsic body diode  24 , a current sense resistor  28 , and a second switch  22  that has a forward-biased intrinsic body diode  26 . Although body diodes  24  and  26  are not actual physical diodes, they behave as diodes in parallel to the switch with which they are associated. In one example, switches  20  and  22  can be MOSFETs. 
         [0012]    The power supply  58  channels power from a power source  18  to the microcontroller  10  and the gate drive  12 . The microcontroller  10  controls the gate drive  12 , which opens and closes the first switch  20  and the second switch  22 . When the gate drive  12  is turned on, the first switch  20  and the second switch  22  are closed and the SSPC power section  56  turns ON. When the gate drive  12  is turned off, the first switch  20  and the second switch  22  are opened and the SSPC power section  56  turns OFF. When the SSPC  52  is ON, and the switches  20  and  22  are closed, current with a normal AC waveform is transmitted to a load  37 . 
         [0013]    As shown in  FIG. 1A , it is possible that the first switch  20  and second switch  22  can be groups of switches. In the example shown in  FIG. 1A , the first group of switches includes switch  20   a  and  20   b , and body diodes  24   a  and  24   b , and the second group of switches includes switches  22   a  and  22   b  and body diodes  26   a  and  26   b . All switches in the first group are in parallel, and all switches in the second group are in parallel. The quantity of first switches is the same as the quantity of second switches. In addition, all switches in the first group have reverse-biased body diodes, and all switches in the second group have forward-biased body diodes. Of course, three or even more switches can be used in a group. 
         [0014]    A signal conditioning and logic module  14  has four inputs  30 ,  32 ,  34 , and  36 . The signal conditioning and logic module  14  uses inputs  32  and  34  to sense current across the current sense resistor  28 , and uses inputs  30  and  36  to detect voltage at the SSPC input and the SSPC load output. Microcontroller  10  transmits data to an external microprocessor module  38  via communications interface  16 . Communications interface  16  transmits data to the microprocessor module  38  via redundant communication lines  40  and  42 . 
         [0015]    Even if switches  20  and  22  are open (SSPC is OFF), the body diodes  24  and  26  are still able to permit a flow of current. Body diode  24  is forward-biased with negative AC voltage. Thus, even if switch  20  is open, negative AC current will flow from the V Line  input through the body diode  24  and through the current sense resistor  28 . Body diode  26 , however, is only forward-biased with positive AC voltage. Therefore, if both switches are off, and negative AC current flows through the body diode  24  and the current sense resistor  28 , no current will flow to the load  37  because body diode  26  only permits a flow of positive AC current. 
         [0016]    There are two example conditions in which an asymmetric fault can occur. In a first, “SSPC Failed Shorted” example fault condition, the SSPC power section  56  is OFF and one of the switches  20  or  22  is shorted. In this example, current flows through the shorted switch in series with the body diode of the open switch. If this failure is detected, then an “SSPC Failed Shorted”, status indication is set in the microprocessor module  38 . There are three example detection methods for an “SSPC Failed Shorted” fault condition. A first example detection method involves detecting a voltage on the output of the SSPC power section  56  when the SSPC power section  56  should be OFF, and determining if the voltage is greater than or equal to a threshold value. An example threshold value is 40 V RMS . A second example detection method would involve performing a comparison between the V Line  voltage and the voltage at the load output of the SSPC, and determining if the difference between these values, also known as a voltage drop, is greater than or equal to a threshold value. An example threshold value is 40 V RMS . A third example detection method would involve performing a comparison between alternate half cycles of voltage or current and determining if the difference between these values is greater than or equal to a threshold value. 
         [0017]    It is common for an SSPC to have a rating based upon the switches it contains. If the “SSPC Failed Shorted” fault detection is true and there is a current greater than or equal to a percentage of the SSPC rating, the SSPC  52  will override any existing switch commands and turn the output on. Using an example SSPC rating of 2.5 Amps and an example percentage of 10%, if the “SSPC Failed Shorted” fault detection is true and there is a current of 250 mA through the current sense resistor  28 , the SSPC would resolve this asymmetric fault condition by turning ON. Once both the first switch  20  and second switch  22  are closed (SSPC ON), the half-wave rectified power will be eliminated as a regular AC waveform will flow to a load  37 . 
         [0018]    In a second, “SSPC Failed Open” example fault condition, the SSPC  52  is commanded ON, and one of the switches  20  or  22  remains open. In this example current flows through the closed switch in series with the body diode of the open switch. If this failure is detected, then an “SSPC Failed Open” status indication would be set in the microcontroller  10 . There are two example detection methods for detecting this condition. A first example detection method for this condition would involve an increased voltage across the SSPC when the SSPC should be ON. This detection method involves performing a comparison between the V Line  input voltage and the voltage at the output of the SSPC, and determining if the difference between these values, also known as a voltage drop, is greater than or equal to a threshold value. An example threshold value is 40 V RMS . A second example detection method for this condition would involve measuring a voltage or current of alternating half cycles of AC, and evaluating the measurements. One example way of evaluating the measurements is to average the measured values, and determine if the average value is less than or equal a threshold value. An example threshold value is 100 V RMS . In order to resolve the “SSPC Failed Open” fault condition, the microcontroller  10  would override existing switch commands and turn off the SSPC power section  56  by opening the first switch  20  and the second switch  22 . This would remove the half-wave rectified power from the load output by permitting both the positive and negative portions of the AC current waveform to pass to a load  37 . 
         [0019]    A single SSPC typically handles only a single phase of AC. In an application requiring multiple phases of AC power, such as in an aircraft, it is possible to use multiple SSPCs. If one phase of a multi-phase AC waveform is open, this is an acceptable scenario, for aircraft loads can be designed to handle an open phase condition. However an aircraft load can not handle an “SSPC Failed Open” asymmetric fault, because of the high DC content that accompanies such a fault. Thus, it is therefore desirable to distinguish between an acceptable open phase and an unacceptable “SSPC Failed Open” asymmetric fault. 
         [0020]    An example method of distinguishing between an open phase and an “SSPC Failed Open” asymmetric fault involves measuring the difference in voltage between an input voltage (V Line ) and an output voltage, and determining if the difference is greater than or equal to twice the value of an “SSPC Failed Open” threshold value, where the threshold value varies depending on the number of phases involved. If the difference is greater than, say twice the value of the threshold value, the fault is considered an open phase and no action is required of the SSPC  52 . A second example method of distinguishing between an open phase and an “SSPC Failed Open” asymmetric fault involves measuring a voltage or current of alternating half cycles of AC, and evaluating the measurements. 
         [0021]    In addition, although an SSPC is shown in  FIG. 1 , one skilled in the art could apply the various features and advantages of this invention to other electronic switching elements that use MOSFETs or devices with diodes. 
         [0022]      FIG. 2  illustrates both a regular AC waveform  60  and a half-wave rectified AC waveform  62 . The half-wave rectified waveform  62  is a positive half-cycle waveform, and corresponds to a condition in which current flows through the body diode  26  and through the first switch  20 , which is either shorted or closed. The waveform  62  has both AC content  62   a  and DC content  62   b . While not shown in  FIG. 2 , it is possible that negative half-wave rectified voltage can occur. If current flowed through the second switch  22 , which was shorted or closed, and through the body diode  24 , the waveform  62  would show a negative half-cycle. A negative half-wave rectified waveform would resemble waveform  62 , except the positive peaks would be absent and negative peaks would be present. 
         [0023]      FIG. 3  illustrates one embodiment of the present invention in an example environment of an aircraft  70 . An SSPC  52  uses a power source  18  to control a flow of current from V Line  to a load  37 . The SSPC  52  transmits data to a microprocessor module  38  via redundant communication lines  40  and  42 . 
         [0024]    While all example voltages have been RMS voltages, it is understood that peak voltage measurements could also be used for asymmetric fault detection. Further, it should be understood that specific values disclosed in this application are examples only, and are not intended to be limiting. 
         [0025]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.