GB2486493A

GB2486493A - Testing operation of a switching device by using a pulldown device to short a load

Info

Publication number: GB2486493A
Application number: GB1021453.4A
Authority: GB
Inventors: Peter Michael Tyler; John Oliver Collins
Original assignee: GE Aviation Systems Ltd
Current assignee: GE Aviation Systems Ltd
Priority date: 2010-12-17
Filing date: 2010-12-17
Publication date: 2012-06-20
Anticipated expiration: 2030-12-17
Also published as: BRPI1105402A2; CN102608526A; GB201021453D0; JP6018377B2; DE102011056541B4; FR2969427B1; CN102608526B; JP2012147655A; CA2762475A1; DE102011056541A1; GB2486493B; CA2762475C; FR2969427A1

Abstract

A switching circuit 1 connecting a load 15 to a voltage source 2 comprises one or more switching devices 6, 7, for switching on and off power to the load, a pulldown device 4 for shorting out the load thereby isolating it from the voltage source, and a controller 3 operable while the source is isolated from the load to activate at least one of the switching devices at a time, wherein a current passes through the or each activated switching device and is measurable to test whether the or each activated switching device is operating correctly. The switching devices are thus tested in situ. The switching devices may form part of a circuit in an aircraft, where the source 2 would be an engine generator.

Description

SWITCHING CIRCUITS AND METHODS OF TESTING

The present invention relates to switching circuits and methods of testing thereof.

Often, switching circuits include several switching devices connected in parallel with one another whereby the current capacity of the switching circuit is the sum of the capacities of each switching device. This is particularly useful for applications in which the power required by the load exceeds the capacity of a single switching device. One example of such switching circuits is found in aircraft power distribution systems, wherein for example 8 solid-state switching devices may be provided in parallel. Generally, the switching devices can fail open or closed. Each switching device has a driver which can be the cause of the failure, if one or more of the switching devices fails open the others can act as backup switches, but potentially may be subjected to current overload, if one or more of the switching devices fails closed, it would not be possible to switch off the switching circuit and it would be readily apparent that such a failure had occurred. if failed open, the failure may be undetectable. Solid-state switching devices are tested thoroughly at the manufacturing stage, but it is desirable to be able to test them in service to ensure that they remain fully functional. This is also known as a built-in-test (BIT).

The present invention provides a switching circuit for connection to a load and to a voltage source, comprising one or more switching devices for switching on and off power to the load, a pulldown device for shorting out the load thereby isolating it from the voltage source and a controller operable while the load is shorted to activate at least one of the switching devices at a time, wherein a current passes through the or each activated switching device and is measurable to test whether the or each activated switching device is operating correctly.

Advantageously, the pulldown device allows the switching devices to be tested without the load, so that the test can be carried out before or after installation of the switching circuit.

Further, the present invention provides a method of testing a switching circuit that connects a load to a voltage source, the switching circuit including one or more switching devices, the method comprising shorting out the load by activating a puildown device, activating one or more of the switching devices, measuring the current through the or each activated switching device and determining from the measured current signal whether the activated switching devices is/are operating correctly.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows schematically a circuit including a switching circuit exemplifying the present invention; and Figure 2 is a graph of current against time during a test procedure carried out on the circuit of Figure 1.

Figure 1 illustrates an example of a circuit comprising a switching circuit 1 connected to a voltage source 2. The circuit has an output 5 which is connected to a load 15. The circuit may, for example, be provided on an aircraft such that the voltage source 2 may be provided by an engine generator and the load 15 may be a component on the aircraft, such as to actuate a landing gear flap or the under carriage or a component within the aircraft such as instrumentation or in-flight entertainment. The switching circuit 1 may comprise an individual switching device 6, or a plurality of switching devices 6, 7, ..., n. The switching devices are connected in parallel. The switching devices may each comprise any suitable solid-state switching device such as a field effect transistor. The switching circuit 1 is used to connect the power source 2 to the load 15. The voltage source 2 and its associated cabling or wiring will have an inherent inductance 11. Likewise, the load and its associated cabling and wiring will have an inherent inductance 14.

The switching circuit further includes a controller 3 which is connected to each of the switching devices 6, 7, ..., n via respective control lines 8,9,10. The controller 3 is also connected to a pulidown device 4 via a pulldown control line 12. The pulldown device 4 is further connected to the load output 5 and the power return line 13. When the pulldown 4 is closed it shuts off the load from the switching circuit, and diverts current through the pulidown 4. The puildown device, also referred to as a puildown circuit or simply a puildown, can include any appropriate switch, including electronic, electromechanical and mechanical switches.

The switching circuit I may 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, each device may be switched on and off sequentially so that each of the plurality of devices may be tested individually. Other test sequences can be envisaged, such as activating more than one switching device at once. Individual activation of the switching devices allows fault isolation to an individual switch. The testing sequence can be carried out at any convenient time, such as in between flights. The testing sequence could also be carried out in-flight during times when the load 15 is not required.

Each of the switching devices 6, 7, ..., n includes a respective current limiter 61, 71,..., ni. The current limiters restrict the current passing through the switching devices to on the order of five to ten times the normal maximum operating current to avoid damage to the switching devices. Alternatively, a single current limiter for all of the switching devices could be provided. In a further alternative, a hard current trip could be provided to turn off the switching devices when the trip limit is exceeded.

The pulldown circuit is activated whenever any of the switching devices are activated for the BIT testing routine. Each switching device and its drive circuitry may be fully tested by using the controller 3 to activate one device at a time while simultaneously checking that the current flowing through it and through the pulldown device is within the correct limits. The current measuring circuitry is not shown in Figure 1. In one particular embodiment, the minimum time for which each switching device is activated is chosen to be at feast the time required for the current to become relatively constant to allow consistent measurement of this current to the accuracy required by the BIT system. The time for which each switching device is activated is generally dependent on the maximum total inductance ii in the input power source cable.

However, the system can be operated on shorter timescales if desired.

The pulldown device is designed so that when it sinks the current from a single main switching device during the BIT test, the voltage developed at the output 5 to the load is negligible compared to the normal output voltage when the system is on. This ensures that the load is not subjected to significant voltage when the system is supposed to be off. This is achieved by using a pulidown device 4 that has a lower impedance than each switching device, preferably much lower. In practice, when the current limiters 61, 71, ..., ni are in operation, the switching devices 6, 7, ..., n have a higher effective impedance than the pulldown device 1. During normal system operation, the controller 3 which activates the switching devices 6, 7, ..., n would generally be expected to turn all of the switching devices on and off simultaneously.

Embodiments of the invention apply not only to DC systems but also AC systems. In this case the voltage source 2 would be an alternating voltage. The switching devices would be AC switches (optionally with AC current limiters) and the pulldown circuit would be capable of sinking AC currents. In a DC embodiment, the pulldown device comprises a field effect transistor (PET) or a bipolar transistor, or an insulated gate bipolar transistor for example. Other types of switching device can be used. In an AC embodiment, the pulldown device may comprise a triac or a solid state relay for

example.

Figure 2 illustrates the output current through a switching device when a test procedure is performed. Only one peak is shown, but in an exemplary embodiment eight switching devices are provided so the test would result in a total input current from the voltage source 2 which shows 8 sequential pulses like the single one shown in Fig. 2. The correct operation of each switching device 6, 7, ,.., n may be determined by checking the amplitude of each individual current pulse is within the correct limits.

The test procedure begins by shorting out the load by activating the pulldown device 4. The pulldown device will generally remain on for the duration of the BIT test. The first switching device 6 is closed at about 250 p.s in the example shown, and the current through the switching device 6 quickly rises to that provided by the source 2, in this example approximately 100 Amps. The first switching device 6 is then opened and a short time later, the second switching device 7 is closed and the current through the second switching device 7 is measured. This procedure is repeated in sequence until all of the switching devices have been tested. Differences in the current profiles to the ones shown in Figure 2 may indicate a fault with the corresponding switching device.

Advantageously, embodiments of the invention allow individual assessment of the operation of the switching devices. A simple BIT test which just checks the overall solid state power controller operation would not normally be able to detect a faulty device which was stuck in the open circuit state. A technical advantage of this invention is that not only does it detect single device failures but also can check the current limiting performance of each device individually. The invention can therefore provide complete BIT coverage during in-service operation.

In an alternative embodiment the switching devices do not have current limit control.

In this case the inductance of the power source 2 and cable inductance 11 can be relied on in combination with a fast current trip circuit to prevent the current rising to dangerous levels during the test pulse. In other words, the controller 3 in this embodiment would operate to activate the sequence of opening and closing the switching devices quickly enough to avoid current overload.

Claims

CLAIMS: 1. A switching circuit for connection to a load and to a voltage source, comprising one or more switching devices for switching on and off power to the load, a pulldown device for shorting out the load thereby isolating it from the voltage source and a controller operable while the load is shorted to activate at least one of the switching devices at a time, wherein a current passes through the or each activated switching device and is measurable to test whether the or each activated switching device is operating correctly.
2. A switching circuit according to claim 1, wherein the pulldown device includes a switch disposed in parallel with the load, and closure of the switch shorts out the load.
3. A switching circuit according to claim 2, wherein the puildown device includes an electronic, electromechanical andlor mechanical switch, including any one or more of a field effect transistor, a bipolar transistor, an insulated gate bipolar transistor, a triac, and a solid state relay.
4. A switching circuit according to any of the preceding claims, wherein the or each switching device is activated individually.
5. A switching circuit according to any of the preceding claims, further comprising a current limiter for restricting the current through the or each switching device.
6. A switching circuit according to claim 5, wherein the or each switching device has a current limiter disposed therein.
7. A switching circuit according to any of claims 1 to 4, wherein system inductance is used to restrict the current through the or each switching device.
8. A switching circuit according to any of the preceding claims, wherein the impedance of the puildown device is less than the impedance of the or each switching device.
9. A switching circuit according to any of the preceding claims, wherein an output voltage of the switching circuit under test conditions is negligible compared to the output voltage in normal operating conditions.
10. A switching circuit according to any of the preceding claims, wherein the circuit is connectable to an AC or a DC voltage source.
11. A switching circuit according to any of the preceding claims, wherein the or each switching device comprises a solid state switching device.
12. A method of testing a switching circuit that connects a load to a voltage source, the switching circuit including one or more switching devices, the method comprising shorting out the load by activating a pulidown device, activating one or more of the switching devices, measuring the current through the or each switching device that is activated and determining from the measured current signal whether the activated switching devices is/are operating correctly.
13. A method according to claim 12, wherein the switching devices are activated sequentially one after another.
14. A method according to claim 12, wherein the one or more switching devices are activated for a time sufficient for the current flowing therethrough to become substantially constant.
15. A method according to any of claims 12 to 14, further comprising restricting the current that passes through each switching device.
16. A switching circuit substantially as herein described with reference to the accompanying drawings.
17. A method of testing a switching circuit substantially as herein described with reference to the accompanying drawings.