Abstract:
A device for protecting electronic equipment having an output terminal includes an electronic breaker switch linked between the output terminal and ground, and a protection means which protects the electronic breaker switch against overvoltages applied to the output terminal. The protection means includes a means for detecting a current above a given threshold flowing in the electronic breaker switch, a means for prohibiting the closing of the electronic breaker switch when the current flowing in the electronic breaker switch is above the given threshold, and a means for permitting the closing of the electronic breaker switch after a given duration following a prohibition.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to foreign French patent application No. FR 10 01393, filed on Apr. 2, 2010, the disclosure of which is incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to the protection of electronic equipment against possible overvoltages applied to terminals thereof and, more precisely, to possible overvoltages applied to output terminals of the equipment. 
       BACKGROUND OF THE INVENTION 
       [0003]    Aboard aircraft, for example, standard supply networks deliver 28V DC, or 115V AC voltages at a frequency of 400 Hz. The application of such voltages directly to an output terminal of electronic equipment has a tendency to destroy components of the electronic equipment associated with the output terminal. The application of the voltages may be due, for example, to a failure of another piece of equipment intended to dialogue with the equipment through the output terminal. 
         [0004]    At present, protection of the output terminal is attempted by means of a diode, called Transient Voltage Suppressor diode, or TVS diode, connected between the output terminal and an electrical earth, e.g., ground, of the equipment, as well as by a fuse connected in series between the output terminal and the components of the equipment that are associated with this terminal. This protection is in place to attempt to avoid destruction of the components. Nonetheless, in the event of an overvoltage, the fuse and the diode are destroyed and require an intervention on the equipment. Moreover, a TVS diode cannot be tested without the application of a calibrated voltage wave to the diode. This application is difficult and cannot be carried out without removing the equipment, such as during a maintenance operation. In equipment aboard aircraft, maintenance operations are scheduled with an occurrence of every several thousand hours, for example. It is therefore possible for a situation to arise in which the TVS diode is out of service with no possibility of this being discovered. Moreover, in the event of doubt regarding the operation of the TVS diode, the equipment has to be reinstalled to test the TVS diode, thus requiring the replacement of the equipment to prevent immobilizing the aircraft. This replacement entails significant costs and efforts. 
       SUMMARY OF THE INVENTION 
       [0005]    Embodiments of the invention alleviate at least the problems described above by providing a way of protecting an output terminal of electronic equipment without requiring the use of a fuse and/or a TVS diode. The invention finds utility in electronic equipment, such as equipment aboard aircraft, for example, that could be subjected to lightning, or to which supply voltages might be inadvertently applied to the output terminal. 
         [0006]    As used herein, the expression output terminal is understood to mean a terminal at which an item of information exits the equipment. 
         [0007]    The present invention provides protection for electronic equipment, having an output terminal, including an electronic breaker switch linked between the output terminal and an earth, e.g., ground, of the equipment, and means for protecting the electronic breaker switch against possible overvoltages applied to the output terminal. The protection means includes means for detecting a current above a given threshold flowing in the electronic breaker switch and means for prohibiting the closing of the electronic breaker switch when the current flowing in the electronic breaker switch is above the given threshold. 
         [0008]    In an embodiment, the protection means includes means for permitting the closing of the electronic breaker switch after a given duration following a prohibition. This duration is fixed and begins at the start of the prohibition. 
         [0009]    The invention allows frequent testing of the means for protecting the electronic breaker switch without requiring the removal of the equipment, such as by simulating a current above the given threshold, for example. This test operation can be performed each time the equipment is powered up, or even on a simple request from an operator, for example, in the event of doubt regarding the fact that there has been an overvoltage. 
         [0010]    One or more embodiments of the invention also avoid the required use of a fuse. Therefore, changing a fuse is not required after the occurrence of an overvoltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention will be better understood and the above and other aspects, features, and advantages will become more apparent by reading the accompanying detailed description, given by way of example, and in view of the appended drawing, in which: 
           [0012]      FIG. 1  represents an exemplary basic diagram implementing the invention; and 
           [0013]      FIG. 2  represents a more complete diagram implementing the invention. 
           [0014]    For the sake of clarity, the same elements will bear the same labels in the various figures. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  represents components associated with an output terminal  10  of an electronic equipment  11 . During normal operation, e.g., in the absence of an overvoltage, the output terminal  10  may be either linked to an earth  12 , e.g., a ground  12 , of the equipment  11  or isolated. Linking to the earth  12  is done by means of an electronic breaker switch M 1  formed, for example, by a negative-channel field-effect transistor. The drain, denoted D, of the transistor M 1  is connected to the terminal  10  by way of a diode D 1  making it possible to permit the flow of the current only from the terminal  10  to the earth  12 . The source, denoted S, of the transistor M 1  is connected to the earth  12  by way of a resistor R 8  allowing the measurement of current flowing in the electronic breaker switch M 1  between its drain D and its source S. 
         [0016]    The electronic breaker switch M 1  is controlled by its gate, denoted G. More precisely, when the voltage between gate G and source S of the transistor M 1  has a low value, the electronic breaker switch is open. Stated otherwise the transistor M 1  is off. When the voltage between gate G and source S of the transistor M 1  has a high value, the electronic breaker switch is closed. Stated otherwise the transistor M 1  is on. 
         [0017]    During normal operation, to deliver an item of information at the output terminal  10 , the gate G of the transistor M 1  is controlled by a second electronic breaker switch, for example formed by a negative-channel field-effect transistor M 2 , making it possible to optionally link the gate G of the transistor M 1  to the earth  12 . When the second breaker switch M 2  is open, the gate G voltage of the transistor M 1  is at a high voltage value which is for example fixed by a voltage divider formed by two resistors R 1  and R 2 . The resistor R 1  is connected between a positive supply voltage  13  of the equipment  11  and the gate G of the transistor M 1 . The resistor R 2  is connected between the gate G of the transistor M 1  and the earth  12 . For this high value of gate G voltage, the transistor M 1  is on. 
         [0018]    When the transistor M 2  is on, the gate G voltage of the transistor M 1  is low. This voltage is about that of the earth  12 . For this low value of gate G voltage, the transistor M 1  is off. 
         [0019]    The transistor M 2  is for example driven by its gate by way of a resistor R 3  by means of a programmable logic circuit not represented in  FIG. 1 . 
         [0020]    It was seen above that the resistor R 8  allows the measurement of the current flowing in the breaker switch M 1  between its drain D and its source S. The common point of the transistor M 1  and of the resistor R 8 , that is to say the source S of the transistor M 1 , makes it possible to drive protection means  14  for the breaker switch M 1 . The protection means  14  comprise a third breaker switch M 3  making it possible to connect the gate G of the transistor M 1  to the earth  12  in the event of an overvoltage occurring at the level of the output terminal  10 . More precisely, an overvoltage present on the output terminal  10  when the breaker switch M 1  is closed causes the voltage across the terminals of the resistor R 8  to climb. The comparison of this voltage with a predefined threshold makes it possible to control the breaker switch M 3 . As long as the voltage across the terminals of the resistor R 8  remains below the predefined threshold, the breaker switch M 3  remains open and the transistor M 1  is driven by the transistor M 2 . On the other hand, if the voltage across the terminals of the resistor R 8  rises beyond the threshold, the breaker switch M 3  closes and forces the transistor M 1  to turn off. The measurement of current flowing in the resistor R 8  takes place only when the transistor M 1  is on. When the transistor M 2  turns off the transistor M 1 , the measurement of current flowing in the resistor R 8  is not performed. This absence of measurement does not have any consequence since, once turned off, the transistor M 1  is not sensitive to any overvoltage. 
         [0021]    By using the resistor R 8 , placed between the source S of the transistor M 1  and the earth  12 , to measure the current flowing in the breaker switch M 1 , it is possible, at the moment of the occurrence of an overvoltage on the terminal  10 , to cause the potential present at the source S of the field-effect transistor M 1  to climb. This increase in potential reduces the potential difference between the gate G and the source S of the transistor M 1 , thereby increasing the internal resistance of the transistor M 1  between its drain D and its source S. This drop in conduction of the transistor M 1  constitutes a first protection of the transistor M 1 . This protection remains partial and is insufficient to open the breaker switch M 1  completely. Complete opening is obtained by means of the breaker switch M 3 . 
         [0022]      FIG. 2  represents a more complete diagram implementing the invention. Depicted once again are the transistors M 1  and M 2 , the diode D 1 , the divider bridge formed by the resistors R 1  and R 2  and the resistors R 3  and R 8 . An exemplary embodiment of the protection means  14  is represented in greater detail in  FIG. 2 . The protection means  14  comprise a low-pass filter making it possible to filter the detection of the current flowing in the electronic breaker switch M 1  so as to prevent overvoltages of too small a duration from prohibiting the closing of the electronic breaker switch M 1 . The low-pass filter comprises for example a resistor R 9  and a capacitor C 1  linked in series between the source S of the transistor M 1  and the earth  12 . A common point  15  of the resistor R 9  and of the capacitor C 1  forms the output of the low-pass filter. 
         [0023]    The voltage present at the point  15  is inverted and amplified by means of a bipolar transistor Q 1 , the base of which is linked to the point  15 , the emitter to the earth  12  and the collector to the positive supply voltage  13  by way of a resistor R 6  and of a capacitor C 2  arranged in parallel. 
         [0024]    The collector of the transistor Q 1  is linked to a positive input of a comparator U 1  by way of a resistor R 7 . The positive input of the comparator U 1  is moreover linked to the gate of the transistor M 1  by way of a resistor R 4 . A negative input of the comparator U 1  is linked to a reference voltage  16  by way of a resistor R 5 . The reference voltage  16  forms a threshold making it possible to define the maximum current flowing in the resistor R 8 , beyond which it is necessary to force the transistor M 1  to turn off so as to ensure its protection. 
         [0025]    The output of the comparator U 1  is linked to the gate G of the transistor M 1 . The output of the comparator M 1  is either open or connected to the earth as a function of the potential difference between its positive and negative inputs. This type of comparator is known in the literature by the name “open/ground”. The comparator U 1  forms the breaker switch M 3  represented in  FIG. 1 . The comparator U 1  compares an image of the voltage across the terminals of the resistor R 8  with the reference voltage  16 . The image of the voltage across the terminals of the resistor R 8  is the voltage present at the level of the collector of the transistor Q 1 . The output of the comparator U 1  is either open or connected to the earth as a function of the result of the comparison between the image of the voltage across the terminals of the resistor R 8  and the reference voltage  16 . 
         [0026]    More precisely, when the current flowing in the source S of the transistor M 1  remains below the tolerable threshold for this transistor, stated otherwise when the voltage across the terminals of the resistor R 8  remains below a given value, the transistor Q 1  is off, the potential of the positive input of the comparator U 1  remains above the reference voltage  16  present on the negative input of the comparator U 1  and the output of the comparator U 1  is in the open state. In this case, the transistor M 1  may be driven normally by the transistor M 2 . 
         [0027]    In the converse case, when the current flowing in the resistor R 8  goes above the tolerable threshold for the transistor M 1 , the transistor Q 1  turns on, the potential of the positive input of the comparator U 1  becomes less than the reference voltage  16  present on the negative input of the comparator U 1 , the voltage present on the output of the comparator U 1  is about that of the earth  12 , thereby forcing the gate G voltage of the transistor M 1  to the voltage of the earth  12 . 
         [0028]    When the gate G voltage of the transistor M 1  is forced to the earth  12 , the current flowing in the resistor R 8  vanishes thereby interrupting the forcing to the earth of the gate G of the transistor M 1 . To avoid too fast an interruption of the forcing, the protection means advantageously comprise means for permitting the closing of the electronic breaker switch M 1 , that is to say for interrupting the forcing, after a given duration following a prohibition. This given duration begins at the instant at which the prohibition starts. The given duration is fixed. It is defined by means of passive components and more precisely by means of the value of the capacitor C 2  in conjunction with the values of the resistors R 4 , R 6  and R 7  which make it possible to defer the return to the normal state of the equipment, that is to say with no forcing of the gate G of the transistor M 1 . 
         [0029]    Advantageously, the equipment  11  comprises test means  17  for the protection means  14 . The test means  17  make it possible to force the image of the voltage across the terminals of the resistor R 8  to a value obtained in the event of an overvoltage applied to the output terminal  10 . 
         [0030]    The test means  17  make it possible for the base potential of the bipolar transistor Q 1  to be taken to a voltage sufficient to turn it on independently of the current flowing in the resistor R 8  and therefore without calling upon an overvoltage on the terminal  10 . The test means  17  comprise for example a positive-channel field-effect transistor M 4  whose drain is linked to the point  15  by way of a resistor R 10 , whose source is linked to a voltage source  18  which may be used for logic applications of the electronic equipment  11 , such as for example a voltage source of 3.3V. The gate of the transistor M 4  is linked to a terminal  19  of a logic circuit that can deliver either a voltage of 3.3V or a voltage of 0V. More generally, the transistor M 4  forms an electronic breaker switch making it possible to force the turning on of the bipolar transistor Q 1 . 
         [0031]    When it is not desired to perform any test on the protection means  14 , a logic voltage of 3.3V is applied to the gate of the transistor M 4  and opens the latter. The voltage of the point  15  is then formed solely across the resistor R 9  and only an overvoltage applied to the terminal  10  can turn on the transistor Q 1 . 
         [0032]    On the other hand, when it is desired to perform a test on the protection means  14 , a zero logic voltage is applied to the gate of the transistor M 4  and the transistor M 4  turns on. It makes it possible to apply to the point  15  the voltage of the source  18  across the resistor R 9 , thereby forcing the transistor Q 1  to turn on and taking the gate voltage of the transistor M 1  to the earth  12  by way of the comparator U 1 .