Patent Publication Number: US-9411016-B2

Title: Testing of a transient voltage protection device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to testing of voltage protection devices, in particular, during the in-service life of a product. 
     2. Description of the Prior Art 
     Transient voltage protection devices are relied upon to protect sensitive electronic circuits from lightning strikes, faults and other transients which could cause damage to the circuits. Any faults in the protection devices themselves can have serious safety implications, particularly on aerospace products. 
     Conventionally, transient voltage protection devices are only thoroughly tested by the device manufacturer. They may not generally be re-tested, even when they are assembled into a printed circuit board (PCB). Consequently, this could allow some types of faults to remain undetected leaving the circuits unprotected. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a method of testing a voltage protection device in a circuit is provided. The circuit comprises a source and load and a detector is provided in parallel with the protection device. The method comprises opening a switching device provided in the circuit. The method further comprises detecting a property of a voltage spike caused by the rate of change of current in the circuit inductance produced by the opening of the switching device to determine the condition of the protection device. 
     According to another embodiment of the present invention, a protection device tester for testing a protection device in a circuit is provided. The circuit comprises a source and load. The protection device tester comprises a detector provided in parallel to the protection device and configured to detect a property of the voltage spike produced by opening a switching device provided in the circuit. The protection device tester further comprises a controller configured to determine the condition of the protection device based on the detected property of the voltage spike produced by the opening of the switching device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a circuit provided with a protection device tester in accordance with an embodiment of the present invention; 
         FIG. 2  shows voltage spikes at point T produced by the closing and opening of a switching device in the circuit of  FIG. 1 ; 
         FIG. 3  is an example of a voltage protection detection device for a circuit with a DC current; 
         FIGS. 4 a  and 4 b    show two examples of voltage protection devices for a circuit with an AC current; 
         FIG. 5  is an example of a peak voltage detector which may be used in accordance with an embodiment of the present invention; 
         FIG. 6  illustrates an embodiment of the present invention the present invention; 
         FIG. 7  shows a modified version of the circuit shown in  FIG. 1  provided with a short circuit arranged to be applied across an output of the circuit; 
         FIG. 8  shows a modified version of the circuit shown in  FIG. 6  provided with a short circuit arranged to be applied across an output of the circuit; and 
         FIG. 9  shows a switching device and protection device provided, as a single component block. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an example of a voltage protection device tester provided in a circuit. The circuit  10  has a voltage source  20 , a load  30  and a switching means  40  provided therebetween. The circuit  10  may, for example, be provided on an aircraft such that the voltage source  20  may be provided by an engine generator and the load  30  may be a component on the aircraft, such as to actuate a component of the aircraft such as a flap or the under carriage or a component within the aircraft such as instrumentation or in-flight entertainment. The switching means  40  may be an individual switch, a plurality of switches or a power switching circuit for example, such as a solid state power controller. A solid state power controller may, for example, comprise one or more parallel connected semiconductor devices. The switching circuit  40  is used to connect the power source  20  to the load  30 . The voltage source  20  and its associated cabling or wiring will have an inherent inductance  21 . Likewise, the load  30  and its associated cabling and wiring will have an inherent inductance  31 . 
     A protection device  50 , in this example a transient voltage protection device, is provided across the voltage source  20 . A detector  60  is provided in parallel with the protection device  50  for detecting a property of the voltage spike produced by opening the switching device  40 . The detector  60  which may be provided, as hardware or software detects a property such as peak voltage, duration, slope, etc. of the voltage spike which is used to determine a condition of the protection device  50  such as whether it is working satisfactorily. The detected property may also be used to determine whether there is any non-ideal or potential problems developing in the protection device  50  or a part of the circuit  10 . 
     The switching device  40  may, as explained above, comprise a solid state power controller (SSPC) which may comprise one or a plurality of connected semiconductor devices. If it comprises a plurality of parallel connected semiconductor devices, then they may be operated individually or in any combination to obtain a voltage spike. 
       FIG. 2  illustrates voltage spikes at point T in  FIG. 1  produced by closing and opening the switching device  40  illustrated in  FIG. 1 . As can be seen, when the switching device  40  is closed or turned “on” at about 250 μs, the current through the switching device  40  quickly rises to that provided by the source  20 , in this example approximately 100 Amps. As can also be seen in  FIG. 2 , as the current through the switching device  40  increases, a first negative spike  101  is seen by the detector  60 . When the switching device  40  is opened or turned “off”, in this example at just before 500 μs, a positive voltage spike  102  is seen by the detector  60 . 
     The voltage protection device  50  does not conduct when the switching device  40  is closed or turned “on”. It only conducts when the switching device  40  is opened or turned “off”. For a DC circuit, the positive side of voltage protection device  50  is at point T so it conducts when the switching device  40  is opened or turned “off” which generates the positive voltage spike  102 . 
     If the switching device  40  comprises a plurality of switches, then the switches may be operated individually or in any combination to obtain a voltage spike. 
       FIG. 3  illustrates an example of a voltage protection device  50  which may be used in an embodiment of the present invention. In this example, the voltage protection device  50  comprises a transorb which is similar to a Zener diode which may be suitable for a DC supply  20 . 
       FIG. 4 a    illustrates a voltage protection device  50  which comprises a bidirectional transorb which is similar to two reverse connected Zener diodes. This may be used in the circuit of  FIG. 1  with an AC supply  20 . An alternative AC voltage protection device which performs the same function is a Metal Oxide Varistor as shown in  FIG. 4   b.    
     Although the detector  60  may detect any desired property of the voltage spike to determine the condition of the protection device, it has been found that detecting the peak voltage of the spike provides a reliable indication of the condition of the protection device  50  and is also able to be measured precisely and repeatedly. The peak voltage of the voltage spike may be measured in any suitable way, such as using software or hardware.  FIG. 5  illustrates a suitable hardware peak spike voltage detector which may be used in an embodiment of the present invention. The peak voltage detector has a charging resistor  110  connected in series with a diode  111  and a capacitor  112  which is charged by the voltage spike. The charge on the capacitor  112  is measured at the point P providing an indication of the peak voltage of the spike. The charged capacitor  112  is then discharged through the resistor  113  which is generally much larger than the charging resistor  110 . 
     A suitable controller,  70 , such as a microprocessor which may, for example, be provided within the detector arrangement  60  shown in  FIG. 1  or within other controlling electronics associated with the circuit  10  may be used to determine a condition of the protection device  50  based on the detected property of the voltage spike. For example, when measuring the peak voltage detected, a controller may use a look-up table or an algorithm to determine whether the detected peak voltage is within an expected predetermined range such that the voltage protection device  50  may be considered to be working satisfactorily or whether the detected peak voltage is outside a predetermined expected range indicating that the voltage protection device is working unsatisfactorily or has a fault. Whether the measured property is below or above the predetermined range or a predetermined value may indicate the nature of the fault. For example, if a measured peak voltage is below a predetermined value or range this may indicate a partial short circuit in the voltage protection device  50 . Conversely if a measured peak voltage is above a predetermined value or predetermined range this may indicate a high impedance or an open circuit in the voltage protection device  50 . Furthermore, how much a measured property is above or below an expected predetermined value or range may also indicate how serious a fault is. 
     In the example voltage spike  102  shown in  FIG. 2 , the peak of the spike  102  is about 90V. Different circuits and applications will have different determined ranges to indicate satisfactory operation. For example, if a detected voltage peak is above 100 V or below 70 V, this may be indicative of a fault. If a peak voltage is above or below the expected value, in this case 90 V, this may be indicative of non-ideal operation of the circuit or a component even if it is within the expected range and so may suggest further investigation. 
     The embodiment of the present invention described above with reference to  FIG. 1  generally has some inductance  21  in the input power supply  20  and its associated cabling or wiring. If this inherent inductance  21  is too small to produce a voltage spike with measurable properties, then a suitable inductor may be provided in the cabling or wiring associated with the source  20 . 
       FIG. 6  illustrates a second example of a circuit illustrating an embodiment of the present invention. This example differs from the circuit of  FIG. 1  by the voltage protection device  50  being provided between the source  20  and the output load  30 . As in the example of  FIG. 1 , the switching device  40  may be a single switch or a plurality of switches. If the switching device  40  is a plurality of switches, the plurality of switches may be provided in parallel and may, for example, be a plurality of parallel connected semiconductor devices, such as a solid state power controller (SSPC) used to connect the source  20  to the load  30 . As in the example of  FIG. 1 , the transient voltage protection device  50  has a detector  60  connected across it in parallel. As before, the protection device may be tested by measuring a property of the voltage spike produced by the opening of the switching device  40  or by opening and closing the parallel connected semiconductor devices individually or in any combination to determine the condition of the protection device  50 . 
     As in the examples shown in  FIG. 1 , if there is insufficient inherent inductance in the circuit to provide a large enough voltage spike to make the protection device conduct, then an inductor may be added accordingly. 
     An advantage of embodiments of the present invention is that the voltage spikes produced are larger than those that occur at normal load currents and therefore provide clearly measurable values which may be used to determine the condition of the protection device, in particular whether or not it has a fault or is likely to develop a fault. 
     The size of the positive voltage spike  102  which is detected to determine the condition of the protection device  50  is dependent upon the inductance L in the circuit and the rate of change of current dI/dt when the switching device  40  is opened. The size of the voltage spike V upon opening the switching device  40  is given by the following equation
 
 V=L×dI/dt  
 
     The inherent inductance in the circuit and typical rate of change of current upon opening the switch have been generally found to be suitable to produce a voltage spike of sufficient size for clear and repeatable measurements. However, if required, larger voltage spikes may be produced when required by either or both of including an additional inductor L in the circuit or providing a higher current I. 
     As illustrated in  FIG. 7 , which corresponds to  FIG. 1  with the inclusion of a short-circuiting switch  120  to the output, a higher current, typically 5 to 10 times its normal current is provided when the switching device  40  is closed after previously closing the short-circuiting switch  120 , thus inducing a considerably larger voltage spike to test the protection device, even if the circuit inductance L is quite low. A considerable advantage of the use of the output short circuit  120  is that a current much higher than the normal load current can be used for the test without applying a significant voltage to the output load. 
       FIG. 8  shows a circuit equivalent to that of  FIG. 6  also provided with a short circuit pull-down  120  on the output. 
     Although the examples described above show the switching device  40  and protection device  50  as separate components, they could be combined into a single component block. For example, as shown in  FIG. 9 , the switching device  40  may include one or more transistors  41 , such as one or more field effect transistors or MOSFETs. The protection function may be provided by two small, low cost zener diodes  51 ,  52  connected between the gate (G) and drain (D) of the or each transistor  41 . These zener diodes force the transistor  41  to conduct heavily when an excess voltage is applied between the drain and source thus preventing this voltage from exceeding the breakdown voltage of the transistor. By appropriate choice of zener diodes, this configuration provides effective self-protection against excess voltage across the or each transistor  41  in the same way as with the protection devices  50  illustrated above, whilst still being able to be tested as in the above examples with a detector  60  connected around it to the drain (D) and source (S). The transistor  41  is turned on or off by an appropriate signal applied to the gate (G) of the transistor  41 . 
     An advantage of embodiments of the present invention is that the voltage spike produced by the opening of a switch is generally larger and more well defined than when normal load currents are switched. Consequently, a sufficiently large voltage spike to test a voltage protection device can be generated even in circuits with relatively low inductances. If necessary, additional inductance may be provided in the circuit or a short circuit may be applied across an output of the circuit to increase the rate of change of current in the circuit inductance when the switching device is opened and so produce a large enough voltage spike to make the protection device conduct. 
     Whilst examples have been described in detail above, many variations may be made to these examples without departing from the present invention. For example, the switching device  10  may be an individual switch or a plurality of switches. If used with a plurality of switches, these may be provided in parallel and each switch may be a solid state device such that the switching device  40  is a solid state power controller (SSPC). Embodiments of the present invention may be used in DC systems or AC systems. If used with an AC system and with the switching device  40  as a plurality of semiconductor devices, these semiconductor devices would be AC switches and the voltage protection device would be bidirectional as shown in  FIG. 4 a    or  FIG. 4 b    and may provide symmetrical positive and negative protection.