A network switch

Abstract

A Powered Single Pair Ethernet (SPE) switch (1s) comprises first and second trunk cable interfaces (P1, N1, P2, N2), at least one pair of inductors (5, 29, 7, 27) connected in series with one another between the trunk cable interfaces, an internal power supply (18) for powering the SPE switch, a switching circuit (SC) connected to the internal power supply and the trunk cable interfaces, a respective voltage and / or current measurement circuit (8, 14, 11, 22, 23, 26) for each trunk cable interface, and a decision circuit (9, 10, 16, 24, 25) configured to signal whether one of the trunk cable interfaces has been shorted based on the voltage and / or current measurement circuits. The switching circuit (SC) supplies power to the internal power supply (18) and has a first state allowing bidirectional flow of current between the first and second trunk cable interfaces, and a second state interrupting flow of current between the first and second trunk cable interfaces when the decision circuit signals a short circuit.

Description

[0001] A NETWORK SWITCH

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a network switch, for example a powered single-pair Ethernet switch such as an Ethernet-APL or PoDL switch.

[0004] BACKGROUND OF THE INVENTION

[0005] Network switches are commonly used to connect different portions or segments of a network to one another, to allow data to be passed between them. Historically most network switches have been powered from a power supply input that is separate from the connected network cables. However, in recent years network standards have been developed that allow transmission of power along the network cables themselves, enabling switches to be powered from the network cables without the need for a separate power input. It is common for such switches to be integrated into larger devices, such as industrial control instruments, and for the devices to be powered by the power delivered through the network cables.

[0006] A common methodology involves sending both power and data together along just two conductive wires, in the form of a voltage differential between the two lines that includes both a DC power component and higher frequency signal component that carries the data. Such two-wire power and data solutions, for example Fieldbus (Foundation Fieldbus® or Profibus PA®) and Ethernet-APL, typically comprise one power source and one or more daisy-chained network switches that are interconnected by two-wire point-to-point, or multi-dropped, trunk cables. Each network switch may contain one or more spur-ports, again connected by two-wire point-to-point spur cables, to one or more devices (which are typically industrial control instruments) powered from the said power source through the same said two-wires.

[0007] The power sources and sinks connected between the two wires are separated from the data carrying portions of the two wires by series inductors that act as high frequency filters, to allow modulation of the higher frequency data component onto the data carrying portions. This is illustrated in Fig. 1 , which shows a power supply 50 and a switch 1 connected by a 2-wire network cable segment 3. The switch 1 has a power supply 18 connected between the series inductors, and the power supply 18 draws power from the two wires to power the functions of the switch 1 and any device the switch 1 is part of. The switch 1 also has two transceivers 6, 28 for sending and receiving data over the network segments 3 and 31 , the transceivers being AC coupled to the two wires at either side of the series inductors. Since the switch 1 takes power from the network cable, it does not require a separate power supply input, which is a significant advantage especially for applications in potentially explosive environments where devices have to be intrinsically safe.

[0008] One of the problems with sending data over a network having multiple cables and interconnected switches / devices, is that a short-circuit fault at one of the cables or switches / devices can cause the whole network to stop functioning correctly. The consequences of this are even worse when the network is being used to power the switches / devices, because the switches / devices cease to function entirely, which is unacceptable for many applications. Looking again at Fig. 1 , if either of the cable segments 3 or 31 is short circuited, for example via a low resistance or electrical arcing between the conductors of the cable segment, or by connecting the conductors in reverse polarity due to a connection, cable or component fault, then power and / or communication for all participating devices and / or switches 1 will / can be lost. This is typically due to either over-current tripping of the power supply 50 and / or the prolonged, disruption to the data signal due to the reactive design of two-wire power and communication systems comprising inductors Za.

[0009] For conventional networks in which the network switches / devices are powered separately, it is possible to provide a ring-redundant system such as shown in Fig. 2, in which the network switches / devices are arranged in a ring and so can receive data from either direction around the ring to provide redundancy. Ring-redundancy is used where higher system integrity is required in order to maintain communication when a single trunk cable 42 is short-circuited or a component port fault occurs. For classic industrial Ethernet 4-wire or 8-wire applications, comprising multiple switches 41 with their own separate power supplies 43, ring-redundancy can be employed by providing switch ports 40 and 45 at either end of the network and also a ring completion segment 47 connecting the switch ports 40 and 45 together. In this case, if any cable segment 42 between one of the switches 41 and the switch port 40 is short-circuited, then communication to that switch 41 will be re-routed via the ring completion segment 47 and the switch port 45. Ring redundancy is simple to implement for conventional Ethernet because each switch or device is separately powered.

[0010] Standards such as PoE (Power over Ethernet) can be used to deliver power to devices, but cannot typically be used to deliver power to multiple devices spread over wide areas in a ring redundant network, as there is too much voltage drop due to the resistance of the CATx cables. The cable spans are limited to 100m, mechanically protected cable is expensive, and it is expensive to install in industrial and / or hazardous environments. Even if the switch ports 40 and 45 of Fig. 2 were modified to add power supplies for delivering power to the devices 44 alongside the data, upon occurrence of a short-circuit the data being sent along the ring would become corrupted enough to cause prolonged communication failure, resulting in a reset (restart) of the whole system. This is not acceptable for more critical real-time (continuous) industrial control and measurement, where all participating devices, and therefore process control and monitoring functionality, can be lost for some time until the system recovers or ‘heals’. Also, the current can increase to values where the current limitation of the power sources is exceeded, tripping the power sources and causing the complete ring to lose power.

[0011] It is therefore an aim of the invention to provide a network switch that can be powered from the network cables, and that can still provide ring-redundancy without whole system failure in the event of a short-circuit fault of one of the network cables.

[0012] SUMMARY OF THE INVENTION In accordance with the invention there is provided a network switch comprising first and second trunk cable interfaces, as defined in the appended claim 1 . The network switch may be a Powered Single Pair Ethernet (SPE) switch, for example an Ethernet-APL Ethernet switch. As is known in the art, powered single pair Ethernet provides both power and data over a single pair of conductors. Accordingly, each trunk cable interface comprises a positive and a negative connection for connecting to a corresponding trunk cable. The network switch comprises at least one pair of inductors connected in series with one another, the at least one pair of inductors comprising a pair of inductors connected in series within a positive line connecting between the positive connections and / or a pair of inductors connected in series within a negative line connecting between the negative connections. That is, there may be a pair of inductors connected within the positive line and not the negative line, or a pair of inductors connected within the negative line and not the positive line, or a pair of inductors connected within the positive line and another pair of inductors connected within the negative line.

[0013] The network switch also comprises an internal power supply for powering the network switch and a switching circuit, the internal power supply having a power supply input that is connected to the switching circuit, wherein the switching circuit is further connected in series within the positive line between the pair of inductors in the positive line and / or connected in series within the negative line in between the pair of inductors in the negative line. That is, the switching circuit may be connected between and in series with the pair of inductors in the positive line and not connected between and in series with the pair of inductors in negative line, or the switching circuit may be connected between and in series with the pair of inductors in the negative line and not connected between and in series with the pair of inductors in positive line, or the switching circuit may be connected both between and in series with the pair of inductors in the positive line and between and in series with the pair of inductors in the negative line.

[0014] The network switch also comprises a respective voltage and / or current measurement circuit for each trunk cable interface. So, the network switch may comprise for each trunk cable interface a voltage measurement circuit but not a current measurement circuit, a current measurement circuit but not a voltage measurement circuit, or both a current measurement circuit and a current measurement circuit. The network switch comprises a decision circuit configured to signal a short-circuit fault between the positive and negative connections of each trunk cable interface, based on changes in voltage and / or current measured by the voltage and / or current measurement circuits. The signal of the short-circuit fault is an instruction for the switching circuit to change state from a first state to a second state. The switching circuit allows flow of current between the power supply and at least one of the trunk cable interfaces in the first state and the second state, and so current may flow into the power supply sourced from the first and / or the second trunk cable interface in either of those states The first state allows bidirectional flow of current between the first and second trunk cable interfaces, and so allows current from the first trunk cable interface to flow through to the second trunk cable interface and current from the second trunk cable interface to flow through to the first trunk cable interface. The second state prevents flow of current between the first and second trunk cable interfaces through the switching circuit, and so current cannot flow through the switching circuit from the first trunk cable interface into the second trunk cable interface, and cannot flow through the switching circuit from the second trunk cable interface into the first trunk cable interface.

[0015] Thus, power is supplied from at least one of the trunk cable interfaces to the internal power supply in normal circuit operation when both trunk cable interfaces are powered, and power is supplied from one of the trunk cable interfaces when the other trunk cable interface has a short-circuit fault, maintaining the operation of the network switch. In addition, the interruption of the flow of current between the first and second trunk cable interfaces upon occurrence of a short-circuit fault prevents current flowing from the un-shorted trunk cable interface into the shorted trunk cable interface, thereby maintaining data communications between the network switch and the un-shorted trunk cable interface.

[0016] The power to the internal power supply will be supplied from the un-shorted trunk cable interface. Current may be prevented from flowing between the power supply input and the shorted trunk cable interface using one or more switches or diodes of the switching circuit. The switching circuit may only allow unidirectional flow of current between the power supply input and one or both of the trunk cable interfaces in both the first state and the second state. The unidirectional flow of current may be achieved by using one or more rectifier diodes, or by using body diodes which are embedded in solid-state semiconductor switches, for example the body diode in a MOSFET. The unidirectional flow means that power can be supplied to the power supply input from either trunk cable interface, but cannot be supplied from the power supply input to either of the trunk cable interfaces. Then, when the switching circuit is in the second state no power may be supplied from the switching circuit into the trunk cable interfaces, the supply of power from the power supply input being blocked by the diodes and the supply of power from one of the trunk cable interfaces to the other trunk cable interface being blocked by one or more switches that are open in the second state of the switching circuit. Power may only be supplied from the switching circuit to the power supply input when power is available to be supplied to the switching circuit from one or more of the trunk cable interfaces.

[0017] The decision circuit may be configured to process the measured voltage and / or current, to identify a short-circuit voltage change and / or current change on the first trunk or the second trunk, which differs from the DC and / or AC voltage and / or current on the first and the second trunk when the system, for example a powered SPE ring redundant system, is functioning correctly within, or at, the operating extremes of DC and / or AC voltage and / or current. Thus, the presence of out-of-specification currents or voltages at one of the trunk cable interfaces may be used to determine that a short-circuit fault has occurred between the positive and negative connections of the trunk cable interface. The short-circuit fault may be at the trunk cable interface, or it may be at another location between the positive and negative connections of the trunk cable interface, for example at some point along a trunk cable that is connected to the trunk cable interface where the positive and negative conductors of the trunk cable have been shorted together with one another.

[0018] The decision circuit and / or the measurement circuits may be configured to filter out, or ignore, all of and / or part of, the static and / or dynamic, AC and / or DC operating components / disturbances, for example, the data signal and / or the DC voltage or DC voltage changes, present on the first or second trunk, when functioning correctly within, or at, the operating extremes of DC and / or AC voltage and / or current, that are not found in the AC and / or DC components of a short- circuit. Then the decision circuit is able to discern a short circuit voltage and / or current amplitude or change, from the operational AC or DC voltage and / or current changes, where the decision circuit can optionally be manually or automatically reconfigured or adjusted for each given installation, which may have greater and / or lesser operational AC or DC components, thus providing an improved short-circuit detection option for a given network, where the said filtering will comprise analogue and / or digital low-pass and / or band-pass and / or high-pass filters with fixed or variable corner / filter frequencies in Hz and / or fixed or variable filter orders, or roll-off rates, in dB / Octave.

[0019] When a short-circuit occurs the fault current will take time to fully develop due to the reactance of the passive inductors Za described previously in relation to Fig. 1 . During the time for the current to develop, and to be detected as a cable short circuit based on the current level exceeding a threshold, communication throughout the ring section between the two power sources will be affected. Therefore it is desirable to detect the short-circuit as soon as possible, to minimise the negative impact of the short-circuit on the data signals being sent along the overall ring.

[0020] The decision circuit may be configured to filter out DC components of the measured voltage to provide a filtered voltage, and to signal a short-circuit fault when the filtered voltage drops beneath a voltage threshold. For example, the decision circuit may comprise a high-pass filter that filters the measured voltage from the measurement circuit. Filtering out the DC components removes the steady-state power supply component of the measured voltage, which can vary significantly from system to system and across any given network, and allows checking of whether a sudden drop in voltage has occurred that may be indicative of a short-circuit. The magnitude of the voltage threshold should be greater than the voltage amplitude of the data signals on the trunk cable, to prevent ordinary changes in voltage due to data communications being mistaken for a short-circuit. This technique allows very fast detection of short-circuits, much faster than could be accomplished by looking at the magnitude of the current flow and waiting for it to rise to an excessively high level, since the magnitude only increases slowly because of the inductors in the circuit.

[0021] The decision circuit may be configured to measure a rate of change of the measured current and signal a short-circuit fault if the rate of change is both higher than a first current threshold and / or lower than a second current threshold. The current that connected devices are allowed to draw for power supply purposes is not allowed to change too quickly, to prevent the changes in current flow from corrupting the data that is being sent over the trunk cable. Thus, the first current threshold may be set as the maximum rate of change of current that connected devices are allowed to create for non-data (i.e. power supply) purposes. The second current threshold may be set as the minimum rate of change of current that can be expected due to the data communications that are modulated on the trunk cable. Thus, if the measured current rate of change is higher than the first current threshold and lower than the second current threshold, then the current is out-of-specification which may be indicative of a short-circuit fault. Looking at the current rate of change also allows faster detection of a short-circuit fault than can be achieved by waiting for the magnitude of the current to rise to an excessively high level.

[0022] The decision circuit may be configured to transition the switching circuit from the first state to the second state following the instant the first or second trunk cable’s short-circuit voltage and / or current change is first detected, before the voltage and / or current in the second or first trunk cable respectively has had time to develop to cause a system failure or reset. The time to develop to cause a network failure or reset, is primarily defined by the reactance of the switch and / or the system.

[0023] A system failure or reset would cause an instrumented system for an automated process control application to be unavailable for an unacceptable length of time, for example greater than one second or greater than ten seconds. Thus, the transition from the first state into the second state happens quickly enough to prevent excessive disturbance at the un-shorted trunk cable interface, so that communication via the un-shorted trunk cable interface is maintained and the system does not fail. The time to transition from the switching circuit’s first state to the second state, from the instant a short-circuit is applied to the first or second trunk interface, is ideally less than said time to develop to cause a system failure or reset, for example in a 10BASE-T1 L Powered SPE, and is preferably less than about 10 micro seconds.

[0024] Thus, the current and / or voltage sensor in combination with the decision circuit quickly detects short circuits occurring on two-wire powered networks, and the switching circuit interrupts the current flowing into the short-circuited trunk cable. Then, disturbance to communications in a ring-redundant network can be minimised / reduced, and one or more participating powered switch(es) can remain powered, operational and communicating with the system and with all the connected devices.

[0025] The decision circuit may be configured to signal a short-circuit fault resolution between the positive and negative connections of each trunk cable interface, based on changes in voltage and / or current measured by the voltage and / or current measurement circuits, wherein the said signal of the short-circuit fault resolution is an instruction for the switching circuit to change state from the second state back to the first state. Thus, if a short-circuit spontaneously resolves, or is manually repaired, then the decision circuit will detect this via the voltage and / or current measurement circuits and change the switching circuit back to the first state so that communications via the previously shorted trunk cable interface can resume. This allows the network circuit to automatically cope with intermittent short-circuit faults, which can be common when trunk cables become slowly degraded over time.

[0026] The network switch may comprise a bypass resistor connected between the first and second trunk cable interfaces and in parallel with the switching circuit. The bypass resistor allows a current to flow from an un-shorted one of the trunk cable interfaces into a shorted one of the trunk cable interfaces, and so once the short-circuit has been repaired, the voltage on the originally shorted trunk will recover to normal levels. This can be detected by the voltage and / or current measurement circuits, causing the decision circuit to change the switching circuit back to the first state, whereupon communications via the previously shorted trunk cable interface can resume.

[0027] The bypass resistor typically has a high resistance value, for example at least about 10Kohm, so that the current flow through the resistor is minimal. Thus, if a short-circuit develops between the positive and negative connections of one of the trunk cable interfaces, then the resistor limits the current that is supplied from the un-shorted trunk cable interface into the shorted trunk cable interface to a low value, so that data communications on the un-shorted trunk cable interface are not adversely affected.

[0028] The bypass resistor may be a variable resistor, having a resistance which may for example be varied by the decision circuit. The resistance may be set at a high level by default, and then reduced by the decision circuit for the purposes of testing whether or not a detected short-circuit fault has been cleared yet or not. The variable resistor may comprise a series switch that is opened to increase the resistance to a very high level, or that is closed to reduce the resistance to a level for testing whether or not a detected short-circuit fault has been cleared.

[0029] The switching circuit may comprise three ports, a first port connected to the inductor that is between the switching circuit and the first trunk cable interface, a second port connected to the inductor that is between the switching circuit and the second trunk cable interface, and a third port connected to the power supply input. Thus, the first and second ports may connect the switching circuit between and in series with the pair of inductors in the positive line, or the first and second ports may connect the switching circuit between and in series with the pair of inductors in the negative.

[0030] In one embodiment, the switching circuit may comprise a 2-pole switch connected in series between the pair of inductors in the positive or negative lines, the first and second ports connected to the two poles of the 2-pole switch. The decision circuit may be configured to close the 2-pole switch in the first state of the switching circuit and open the 2-pole switch in the second state of the switching circuit. Accordingly, current can flow bi-directionally between the trunk cable interfaces in the first state of the switching circuit, and is blocked from flowing between the trunk cable interfaces in the second state of the switching circuit.

[0031] The switching circuit may comprise a first diode connected from a first pole of the 2-pole switch to the third port and a second diode connected from a second pole of the 2-pole switch to the third port, the third port being connected to the power supply input. Thus, power can still be supplied to the power supply from the un-shorted trunk cable interface regardless of the state of the 2-pole switch. The first and second diodes are typically arranged back-to-back so that current cannot flow from one trunk cable interface into the other trunk cable interface via the diodes. This also means that current flow between the power supply input and the trunk cable interfaces is unidirectional.

[0032] In another embodiment, the switching circuit may comprise a first switch connected between the third port (which is connected to the power supply input) and the first port (which is connected to a first inductor of the pair of inductors), and comprise a second switch connected between the third port (which is connected to the power supply input) and the second port (which is connected to a second inductor of the pair of inductors), wherein the decision circuit is configured to close the first and second switches in the first state of the switching circuit and open only one of the first and second switches in the second state of the switching circuit. The decision circuit may open the one of the first and second switches that connects the power supply input to the trunk cable interface that has been detected as having the short between its positive and negative connections. Thus, current will not flow between the power supply and the shorted trunk cable interface, and continues to flow between the power supply and the un-shorted trunk cable interface via the other one of the first and second switches that remains closed.

[0033] The first switch may be a MOSFET switch and the second switch may also be a MOSFET switch. The MOSFET switches may each comprise a body diode, so that each MOSFET switch enables bidirectional flow of current through the MOSFET switch when the MOSFET switch is closed, and enables unidirectional flow of current through the corresponding MOSFET switch via the body diode when the MOSFET switch is open. This means that current can flow bidirectionally between the trunk cable interfaces via the MOSFET switches when the switches are both closed in the first state, and that current cannot flow into a shorted trunk cable interface when the MOSFET switch connected to the shorted trunk cable interface is opened, since the body diode will be in reverse bias in that case. The MOSFET switches are typically arranged with the body diodes back-to- back with one another.

[0034] The invention is primarily intended for Ethernet-APL, but it can be implemented for other two-wire power and communication solutions.

[0035] DETAILED DESCRIPTION

[0036] Embodiments of the invention will now be described by way of non-limiting example only and with reference to the accompanying drawings, in which:

[0037] Fig. 1 shows a schematic diagram of a known 2-wire power and data circuit configuration, including a power source and network switch;

[0038] Fig. 2 shows a schematic diagram of a known ring-redundancy system incorporating separately powered network switches;

[0039] Fig. 3 shows a schematic diagram of a network switch in accordance with an embodiment of the invention;

[0040] Fig. 4 shows a schematic diagram of voltage measurement circuitry and filters forming part of the network switch of Fig. 3;

[0041] Fig. 5 shows graphs illustrating a voltage Vm present across a trunk cable interface of the network switch of Fig. 3 and a filtered voltage Vn;

[0042] Fig. 6 shows a graph of a voltage signal that is output from a high pass filter forming part of a decision circuit of the network switch of Fig. 3;

[0043] Fig. 7 shows a schematic diagram of an alternative switching circuit for use in the network switch of Fig. 3; and

[0044] Fig. 8 shows a more detailed schematic diagram of the switching circuit of

[0045] Fig. 7. The figures are not to scale, and same or similar reference signs denote same or similar features.

[0046] The schematic diagram of Fig. 3 shows a network switch 1 s in accordance with a first embodiment of the invention. The network switch 1 s comprises a first trunk cable interface defined by first positive and negative connections P1 and N1 respectively, and a second trunk cable interface defined by second positive and negative connections P2 and N2 respectively. The first trunk cable interface P1 , N1 may be connected to one end of a trunk cable 3, and an opposite end 2 of the trunk cable 3 may be connected to a further network switch and / or device. The second trunk cable interface P2, N2 may be connected to one end of a trunk cable 31 , and an opposite end 32 of the trunk cable 31 may be connected to a further network switch and / or device. The network switch comprises a positive line PL connecting from the first positive connection P1 to the second positive connection P2, and a negative line NL connecting from the first negative connection N1 to the second negative connection N2.

[0047] The network switch 1s may comprise a first transceiver 6 for sending and receiving data on the trunk cable 3 and a second transceiver 28 for sending and receiving data on the trunk cable 31 .

[0048] The network switch 1s may comprise a pair of inductors 5 and 29 connected in series within the positive line PL between the first and second positive connections P1 and P2, and a switching circuit SC that is connected between the two inductors 5 and 29, and to an internal power supply 18. The switching circuit SC has a first port 35 connected to the inductor 5, a second port 36 connected to the inductor 29, and a third port 37 connected to the internal power supply 18 for supplying power to the internal power supply 18. The internal power supply 18 may power the functions of the switch 1 s and any device the switch 1 s is part of. The two inductors 5 and 29 present a high impedance to the data signals that are modulated on the trunk cables 3 and 31 by the first and second transceivers 6 and 28, and so the data signals are largely unaffected by the amount of power that is supplied though the switching circuit to the internal power supply 18. The network switch 1 s may comprise a pair of inductors 7 and 27 connected in series within the negative line NL, between the first and second negative connections N1 and N2. A negative input 18b of the power supply may be connected between the two inductors 7 and 27. The inductors 7 and 27 balance the impendence of the inductors 5 and 29, to maintain a balanced system for differential signalling. The inductors 5 and 7 may each comprise a winding on a respective core, or may each comprise a winding on a core that is shared between the inductors 5 and 7. The inductors 29 and 27 may each comprise a winding on a respective core, or may each comprise a winding on a core that is shared between the inductors 29 and 27.

[0049] The switching circuit may comprise a two-pole isolation switch 21 connected in series within the positive line PL between the two inductors 5 and 29, and two diodes 13 and 20 that may be connected in parallel with one another from either pole of the switch 21 to a power supply input 18a of the internal power supply. The switch 21 has closed and open states, respectively connecting or disconnecting the inductors 5 and 29 to one another. A first one 13 of the diodes may be connected from intermediate the inductor 5 and the switch 21 , and be biased towards the internal power supply 18, and a second one 20 of the diodes may be connected from intermediate the switch 21 and the inductor 29, and also be biased towards the internal power supply 18. Thus, the diodes 13 and 20 are connected back-to-back with one another, and mean that current can only flow unidirectionally into the power supply input 18a. A high impedance resistor 12 may be connected in parallel with the switch 21 , and so allow a small current to bypass the switch 21 even when the switch 21 is opened.

[0050] The network switch 1 s also comprises a decision circuit that may include a controller 16 and filters 9, 10, 24 and 25. The controller 16 has an output 15 for controlling the state of the switch 21 . When the switch 21 is closed by the decision circuit I controller, the first and second positive connections P1 and P2 are connected to one another via the inductors 5 and 29 and the isolation switch 21 , and so current can flow in either direction through the switch 21 . Thus, current may flow into the first positive connection P1 from the trunk cable 3 and out of the second positive connection P2 to the trunk cable 31 , or current may flow into the second positive connection P2 from the trunk cable 31 and out of the first positive connection P1 to the trunk cable 3, depending on the relative voltages of those connections. In either case, current may flow though the diodes 13 and 20 to supply power to the internal power supply 18 of the network switch 1 s. The decision circuit and the first and second transceivers 6 and 28 are powered by the power supply 18 (circuitry not shown for clarity).

[0051] When the switch 21 is opened by the decision circuit, current can no longer flow out of the first positive connection P1 to the trunk 3 nor out of the second positive connection P2 to the trunk 31 , aside from a small current flowing via the resistor 12. The diodes 13 and 20 regulate the power to flow from the first positive connection P1 or the second positive connection P2, and subsequently into the internal power supply 18 to power the network switch 1 s. Thus, if a short-circuit 4 occurs across the trunk cable 3, then the opening of the switch 21 blocks any significant power from flowing into the short circuit 4 from the second positive connection P2, and the power supply 18 is powered by power flowing into the second positive connection P2 from the trunk 31 . Conversely, if a short-circuit 30 occurs across the trunk cable 31 , then the opening of the switch 21 blocks any significant power from flowing into the short circuit 30 from the first positive connection P1 , and the internal power supply 18 is powered by current flowing into the first positive connection P1 from the trunk 3. The diodes 13 and 20 also provide reverse polarity protection to the power supply 18 should the network switch 1 s accidently be connected to the first and second trunk cable interfaces in reverse polarity.

[0052] The controller 16 determines whether or not a short-circuit fault has occurred between the positive and negative connections of either of the trunk interfaces, and may operate the switch 21 accordingly. When a plurality of the network switches 1 s are chained together in a ring-redundant configuration, for example if the network switches 41 of Fig. 2 are replaced by the network switches 1s of Fig. 3, then a short- circuit fault in one of the trunk cables causes the switches 21 of the two network switches connected at opposing ends of the trunk cable to open, cutting off power from the short-circuited trunk cable. Then, the two network switches 1 s are powered from another one of their trunk cable interfaces connected with another trunk cable. It would alternatively be possible to connect the switching circuit including the switch 21 and the diodes 13 and 20 in the negative side of the circuit rather than the positive side, so that the switch 21 was connected within the negative line NL and the diodes connected between the negative input 18b of the power supply and the poles of the switch 21. In such a case the diodes 13, 20 would be biased to allow current flow from the negative input 18b to the switch 21 .

[0053] The decision circuit may comprise one or more filters or sensors connected to the controller 16 to help quickly determine whenever a short-circuit fault has occurred, and to switch the switch 21 accordingly. As shown in Fig. 3, the network switch 1 S may comprise voltage measurement circuits including a difference amplifier 8 having inputs connected to the first positive and negative connections P1 and N1 , and a difference amplifier 26 having inputs connected to the second positive and negative connections P2 and N2. An output of the difference amplifier 8 may be passed through high and low pass filters 9 and 10, and on to the controller 16. An output of the difference amplifier 26 may be passed through high and low pass filters 25 and 24, and on to the controller 16. The difference amplifiers 8 and 26 monitor the differential voltage across the positive and negative connections of the first and second trunk cable interfaces.

[0054] The current measurements taken by the difference amplifiers 8 or 26, and / or the voltage measurements from the low pass filters 10 or 24, may also be used to assess any potential overload of the network power supplies that provide power to the connected network switches via the positive and negative connections of the trunk interfaces. Therefore, an additional low voltage and / or high current isolating means may be fitted to the power supply 18 to reduce the load on the network power supplies.

[0055] The schematic diagram of Fig. 4 shows example circuity for the difference amplifier 26 and the high and low pass filters 25 and 24. The difference amplifier 26 makes use of the differential transient voltage developed across both positive and negative connections P2 and N2 to provide common mode rejection. The high and low pass filters are shown as simple resistor-capacitor single-pole filters having outputs 25A and 24A that are connected to the controller 16, but in alternate embodiments each filter may comprise a two (or more)-pole filter, for example a capacitor-inductor filter, and upwards, which may also comprise a semiconductor based active filter. The filters 25 and 24 may be biased by a reference voltage, for example a Ground Reference voltage GR.

[0056] The purpose of the amplifier 26 and the high pass filter circuit 25 is to isolate, and optionally amplify or attenuate, both the data signal and any fault transient, with an emphasis on revealing more of the fault transient, and to eliminate any DC component or slowly changing DC (low frequency) component. In practice, a low pass filter (not shown) may be added in series with the high pass filter between the output 25A and the controller 16 to provide some filtering of the higher frequency components of the communication signal and for EMC purposes so that short transient, for example occurring during an Electrical Fast Transient (EFT) event, will not unintentionally trigger the detection circuit. Also the low pass filter provides at least some filtering of the higher frequency components of the communication signal. Thus, the high pass filter circuit 25 may form part of a band pass filter circuit when combined with a low pass filter connected in series.

[0057] The purpose of the amplifier 26 and the low pass filter circuit 24 is to isolate, and optionally amplify or attenuate the DC component or slowly changing DC (Low frequency) component, and eliminate both the data signal and any fault transient, with an emphasis on revealing more of the DC component and providing this to the controller 18

[0058] The difference amplifier 8 and the high and low pass filters 9 and 10 of Fig. 3 may be constructed in the same manner as the difference amplifier 26 and the high and low pass filters 25 and 24 shown in Fig. 4.

[0059] It has been observed that when a short-circuit fault occurs, although the current takes a relatively long time to exponentially increase through the inductors, the voltage across the positive and negative connections instantaneously changes and so voltage measurements by the difference amplifiers 8 and 26 can enable fast detection of short circuit faults. The graph shown in Fig. 5 (not to scale) illustrates the voltage Vm that is present across the two connections of each trunk cable interface, for example across the first positive and negative connections P1 and N1 , or across the second positive and negative connections P2 and N2. The graph shows how the trunk voltage Vm may vary slowly due to both load and power supply variations etc., and also more quickly due to data modulations DS. If an absolute, static trip point m was set to detect a short-circuit fault, then the static trip point could easily be breached by normal DC or low frequency variations, and be incorrectly detected as a fault. However, the voltage Vn at the output from the high-pass filters 9 or 25 eliminates any DC or low frequency component, and centres the signal about the bias GR to allow an absolute trip point n to be used for detection of short-circuit faults. Thus, the decision circuit 16 may be configured to determine that a short- circuit fault has occurred when the voltage outputs from the high pass filters drop below the absolute trip point n.

[0060] Alternatively, the trip point m could be modified dynamically dependent on the DC or lower frequency components of voltage Vm to enable detection of a short- circuit fault. The trip points m and / or n could be modified on demand, for example dependent on the measured power supply voltage or based on the length of the trunk cable as detected by a TDR measurement, or to provide a higher margin for external disturbances.

[0061] The trip point voltages m and / or n are selected / chosen, or adjusted / adjustable, to be beyond the level of the data signal DS, so that the data signal will not exceed any, or one, of the trip points, but so the trip points will allow detection of short-circuit faults. For example, at least a zero-ohm fault on a full- length cable of permitted maximum resistance (at the highest cable core temperature), or a reverse connection, up to a fault that has resistance.

[0062] The graph shown in Fig. 6 illustrates the voltage signal output from the high pass filters 9 or 25. The voltage signal is biased about the ground reference voltage GR, and the data signal is shown at 11 a. A potentially disruptive short-circuit causes the filter output signal to drop at 11 b, and to pass through a lower voltage threshold TL which corresponds to trip point n Fig. 5. Upon detecting the drop of the voltage below the lower threshold TL, the controller 16 determines that a short-circuit fault has occurred and opens the switch 21 as described further above.

[0063] The voltage signal output from the high pass filters 9 or 25 may also be used to detect open circuit faults, since the sudden drop in the current caused by the open-circuit causes the inductors to instantaneously increase the voltage across the positive and negative connections, and this increase results in a voltage peak from the high-pass filters as shown at 11c. Upon detecting the increase in the voltage above the upper threshold TU, the decision circuit determines that an open-circuit fault has occurred. Since an open-circuit is a fail-safe fault, it is not normally as important to detect open-circuits as short-circuits, and detection of an open-circuit may cause the controller to issue an advisory alarm.

[0064] Each threshold TL and TU is fixed and / or adjustable closer to, or further away, from the bias voltage GR. The bias voltage GR is typically a negative voltage bias, or a zero-voltage bias. The threshold voltages TL and TU are selected / chosen, or adjusted / adjustable, to be far enough beyond the level of the data signal 11 a, so that the data signal will not exceed any, or one, of the threshold voltages, but near enough to be able to detect at least a zero-ohm fault on a full-length cable of permitted maximum resistance (at the highest cable core temperature), or a reverse connection, up to a fault that has resistance.

[0065] The difference amplifiers 8 and 26 are directly connected to the positive and negative connections in this embodiment, however it would alternatively be possible to provide inductive couplings to the positive and negative connections instead, which would automatically filter out the DC voltage component.

[0066] Referring again to Fig. 3, the network switch 1 s may also comprise current measurement circuits including a first current sense resistor 14 connected in series between the first negative connection N1 and the internal power supply 18, and a difference amplifier 11 that has inputs connected either side of the first current sense resistor 14, and an output connected to the controller 16. The output of the difference amplifier 11 provides a signal representative of the current flowing through the first negative connection N1. The current measurement circuits may also comprise a second current sense resistor 22 connected in series between the second negative connection N2 and the internal power supply 18, and a difference amplifier 23 that has inputs connected either side of the second current sense resistor 22, and an output connected to the controller 16. The output of the difference amplifier 23 provides a signal representative of the current flowing through the second negative connection N2. The current sensing resistors 14 and 22 could be connected in series with the positive connections P1 and P2 rather than the negative connections N1 and N2 in alternative embodiments if desired.

[0067] A conventional approach to short-circuit detection would be to compare the currents that flow through the positive and / or negative connections to a threshold, and determine that a fault has occurred if the current exceeds the threshold. However, the inductors (e.g. 5, 7, 28 and 29) that are present in powered single-pair Ethernet switches will slew any transient current demand, and the current slew can take tens of microseconds to reach a current reading that would be indicative of a fault. During this time of a low impedance fault load between the positive and negative connections of the trunk cable, the data signal will be corrupted or distorted or attenuated, which can instigate a data link and / or power loss. To re-establish communication or power would take an unacceptable length of time for an operational industrial control system, and therefore comparing the current level detected by the difference amplifiers 11 or 23 to a threshold, may not enable the controller 16 of the decision circuit to provide a sufficiently fast response.

[0068] The current level changes quickly in synchronism with the data signal that is modulated on the positive and negative connections, and the current level also changes when new switches and / or devices are added to the network. To ensure that the addition of a new switch or device to the network does not result in a sudden change in current that corrupts the data signal, new switches / devices are required to slowly ramp up their current demands when first connected to the network or turned on, so that data integrity is maintained. Therefore, if a short-circuit fault results in a rate of change of the current that is faster than what is permissible, normal or expected, but slower than the rate of change expected due to data signals, then the fault may be detected quickly before communication has been affected or before the current has adversely developed. Accordingly, the current measurements taken by the difference amplifiers 11 and 23 is may provide an alternative or additional measurement option for the controller 16 to take into account when determining whether a short-circuit fault has occurred, besides the voltage measurements taken by the difference amplifiers 9 and 26.

[0069] Once the short-circuit fault has been detected and the switch 21 has been opened by the decision circuit, annunciation of the fault may be given by way of a local audio and / or visual indication at the network switch, and / or annunciation of the fault may be transmitted to other systems, for example, the controller 16 may have a data connection to the 2-wire ring, for example via the transceivers 6 or 28, to announce the fault to the control system or maintenance system.

[0070] The short-circuit fault may be caused by a low resistance or electrical arcing between the conductors of the trunk cable, or by connecting the conductors in reverse polarity due to a connection, cable or component fault. In one example, a vehicle drives over an old or degraded trunk cable, compressing the conductors into electrical contact with one another and / or allowing arcing through broken insulation, which causes a fault until the vehicle moves off the cable and the conductors move back apart from one another again.

[0071] Since it may be possible for the short-circuit fault to clear without any intervention, for example in the case above where the fault occurs intermittently when a vehicle drives over the cable, the network switch 1s may have the ability to automatically check whether the fault has cleared, and to close the switch 21 to reinstate the trunk cable where the fault occurred once the fault has cleared. The high impedance resistor 12 that is connected in parallel with the switch 21 (see Fig. 3) allows a small current to bypass the switch 21 even when the switch 21 is open, and this may be used to test whether the fault has cleared. Specifically, the fault will form a potential divider comprising resistor 12 and the fault’s resistance, in combination with any DC coupled measuring resistances across the trunk. If the current from the resistor 12 causes the voltage across the positive and negative connections that are connected to the faulted trunk cable to rise to a voltage that is above a threshold voltage indicative of an acceptably high resistance / impedance across the trunk cable, then the fault can be considered to have cleared. The voltage across the positive and negative connections that are connected to the faulted trunk cable is monitored by the controller 16, based on the output of the low pass filter 10 or 24, and if the voltage rises above the threshold then the decision circuit 16 determines the fault has cleared and closes the switch 21 again so that power can flow bidirectionally between the first and second trunk cable interfaces again.

[0072] Alternatively, or additionally, the switch 21 may be closed automatically after a predetermined time or intermittently after successively lengthening periods of time, in a retry approach, without the need to necessarily test the faulted cable using the said resistor 12 and function. A local manual reset may also be provided, or a remote isolation switch reset (e.g. via Ethernet), if the fault is assessed or tested (using test equipment) to be clear by a maintenance engineer.

[0073] The schematic diagram of Fig. 7 shows an alternative switching circuit for use in the network switch of Fig. 3, instead of the switch 21 and diodes 13 and 20. The inductors 5, 7, 29 and 27, and the internal power supply 18 are shown in Fig. 7 for context, but the remainder of the network switch of Fig. 3 has been omitted for clarity.

[0074] The switching circuit may comprise a first switch 21 a that is connected in series between the power supply input 18a and the inductor 5, within the positive line PL, and a second switch 21 b that is connected between the power supply input 18a and the inductor 29, also within the positive line PL. The switching circuit may have a first port 35a connected to the inductor 5, a second port 36a connected to the inductor 29, and a third port 37a connected to the internal power supply 18 for supplying power to the internal power supply 18. The state of the first switch 21 a may be controlled by a signal 15a from the controller 15 of the decision circuit, and the state of the second switch 21 b may be controlled by a signal 15b from the controller 15 of the decision circuit. The power supply input 18a may be connected to the first and second switches 21a and 21 b via an optional diode 40x that ensures unidirectional flow of current between the power supply 18 and the trunk cable interfaces, and protects the power supply 18 from any reverse polarity voltages.

[0075] The negative input 18b of the power supply may be connected to the negative line NL at a point between the two inductors 7 and 27. The diode 40x is biased to allow current flow from the switches 21a and 21 b into the power supply input 18a, however the diode could alternatively be connected between the negative input 18b and the point of the negative line NL between the two inductors 7 and 27, and biased to allow current flow from the negative input 18b to the inductors 7 and 27. It would also possible to connect the switches 21a and 21 b within the negative line NL rather than within the positive line PL if desired.

[0076] The decision circuit may be configured to close the first and second switches 21a and 21 b in the first state of the switching circuit via the signals 15a and 15b, which corresponds to the normal operation of the circuit in which power can be supplied to the power supply from either of the trunk cable interfaces.

[0077] The decision circuit may be configured to open only one of the first and second switches 21a and 21 b in the second state of the switching circuit via the signals 15a and 15b. That is, to open whichever one of the first and second switches is connected between the power supply 18 and the trunk cable interface that has been detected as having the short-circuit fault. Then, current will not flow into the shorted trunk cable interface and power will be supplied from the unshorted trunk cable interface to the power supply 18.

[0078] Fig. 7 also shows a reservoir capacitor 51 and an optional resistor 52 connected in series with one another across the positive and negative lines PL and NL. The components 51 and 52 are connected to the positive line PL at a point between the two inductors 5 and 29, and the components 51 and 52 are connected to the negative line NL at a point between the two inductors 7 and 27. The components may also include a rectifier diode connected across the terminals of the resistor 52, in parallel with the resistor 52. These components are not shown in Fig. 3 for clarity, and may or may not be implemented in Fig. 3. The reservoir capacitor 51 helps to smooth the voltage between the positive and negative inputs of the power supply 18, and combines with the inductors 5, 29, 7, and 27 to block the high frequency data signals at the trunk cable interfaces. The reservoir capacitor 51 may supply current to a short-circuit between the positive or negative connections of one of the trunk cable interfaces, instead of the short circuit drawing a disruptive current from the un-shorted trunk cable interface.

[0079] The reservoir capacitor 51 may be damped by the resistor 52 to help prevent disruptive oscillation caused by a short circuit, or the release of a short circuit, which can lead to the switching circuits of one or more network switches of the network unacceptably switching from the first state to the second state. When the resistor 52 is present, the current flow from the reservoir capacitor into the short-circuit may be increased by providing the optional rectifier diode across the resistor 52.

[0080] Advantageously, the first and second switches 21a and 21 b may be respective MOSFET switches 41 x and 42x that each comprise a body diode 45x and 44x, as shown in the more detailed schematic diagram of Fig. 8. The signals 15a and 15b are supplied to the gates 40x and 43x of the MOSFETS, respectively, in order to control the state of the switches. Each MOSFET switch enables bidirectional flow of current through the MOSFET switch when the MOSFET switch is closed, and each body diode 45x or 44x enables unidirectional flow of current through the corresponding MOSFET switch when the MOSFET switch is open. Thus, there is no need for the diode 40x shown in Fig. 7 since the function of the diode 40x is instead accomplished by the body diodes 45x and 44x of the MOSFETS.

[0081] Fig. 8 also illustrates further details of the bypass resistor 12. The bypass resistor 12 may be a variable resistor, and may incorporate a series switch, as shown. The series switch allows the resistance of the variable resistor to be increased to a maximal value when the switch is opened. The variable resistor may be controlled by a signal 15x from the controller 15, and the controller may reduce the resistance of the variable resistor 12 in order to test whether a detected short circuit between the positive and negative connections of a trunk cable interface has been resolved or not, before the switching circuit is moved back to the first state and normal circuit operation is resumed.

[0082] In summary, the invention provides a network switch comprising first and second trunk cable interfaces, at least one pair of inductors connected in series with one another between the trunk cable interfaces, an internal power supply for powering the network switch, a switching circuit connected to the internal power supply and the trunk cable interfaces, a respective voltage and / or current measurement circuit for each trunk cable interface, and a decision circuit configured to signal whether one of the trunk cable interfaces has been shorted based on the voltage and / or current measurement circuits. The switching circuit supplies power to the internal power supply and has a first state allowing bidirectional flow of current between the first and second trunk cable interfaces, and a second state interrupting flow of current between the first and second trunk cable interfaces when the decision circuit signals a short circuit.

[0083] Many other variations of the described embodiments falling within the scope of the invention will be apparent to those skilled in the art.

Claims

CLAIMS1 A Powered Single Pair Ethernet (SPE) switch, for example an Ethernet-APL Ethernet switch, wherein the SPE switch comprises first and second trunk cable interfaces, each trunk cable interface comprising a positive and a negative connection for connecting to a corresponding trunk cable, at least one pair of inductors connected in series with one another, the at least one pair of inductors comprising a pair of inductors connected in series within a positive line connecting between the positive connections and / or a pair of inductors connected in series within a negative line connecting between the negative connections; a switching circuit and an internal power supply for powering the SPE switch, the internal power supply having a power supply input that is connected to the switching circuit, wherein the switching circuit is further connected: between and in series with the pair of inductors in the positive line, and / or between and in series with the pair of inductors in the negative line; a respective voltage and / or current measurement circuit for each trunk cable interface; a decision circuit configured to signal a short-circuit fault between the positive and negative connections of each trunk cable interface, based on changes in voltage and / or current measured by the voltage and / or current measurement circuits, wherein the said signal of the short-circuit fault is an instruction for the switching circuit to change state from a first state to a second state, the switching circuit allowing flow of current between the internal power supply and at least one of the trunk cable interfaces in the first state and the second state, the first state allowing bidirectional flow of current through the switching circuit from the first trunk cable interface to the second trunk cable interface and from the second trunk cable interface to the first trunk cable interface, and the second state preventing flow of current through the switching circuit from the first trunk cable interface to the second trunk cable interface and from the second trunk cable interface to the first trunk cable interface.

2. The SPE switch of claim 1 , wherein the decision circuit is configured to process the measured voltage and / or current, to identify a short-circuit voltage change and / or current change on the first trunk or the second trunk, which differs from the DC and / or AC voltage and / or current on the first and the second trunk when the system, for example a powered SPE ring redundant system, is functioning correctly within, or at, the operating extremes of DC and / or AC voltage and / or current.

3. The SPE switch of claim 1 or 2, wherein the decision circuit is configured to transition the switching circuit from the first state to the second state following the instant the first or second trunk cable’s short-circuit voltage and / or current change is first detected, before the voltage and / or current in the second or first trunk cable respectively has had time to develop to cause a system failure or reset, for example, a system failure that would cause an instrumented system for an automated process control application to be unavailable for an unacceptable length of time, for example greater than one second or greater than ten seconds.

4. The SPE switch of claim 4, wherein the time to transition from the switching circuit’s first state to the second state, from the instant a short-circuit is applied to the first or second trunk interface, is less than said time to develop to cause a system failure or reset, for example in a 10BASE-T1 L Powered SPE, and is preferably less than about 10 micro seconds.

5. The SPE switch of any preceding claim, where the said decision circuit and / or the measurement circuits are configured to filter out, or ignore, all of and / or part of, the static and / or dynamic, AC and / or DC operating components I disturbances, for example, the data signal and / or the DC Voltage or DC Voltage changes, present on the first or second trunk, when functioning correctly within, or at, the operating extremes of DC and / or AC voltage and / or current, that are not found in the AC and / or DC components of a short-circuit, so that the decision circuit is able to discern a short circuit voltage and / or current amplitude or change, from the operational AC or DC voltage and / or current changes, where the said filtering will comprise analogue and / or digital low-pass and / or band-pass and / orhigh-pass filters with fixed or variable corner / filter frequencies in Hz and / or fixed or variable filter orders, or roll-off rates, in dB / Octave.

6. The SPE switch of any preceding claim, wherein the decision circuit can be manually or automatically re-configured or adjusted for each given installation, which may have greater and / or lesser operational AC or DC components, thus providing an improved short-circuit detection option for a given network.

7. The SPE switch of any preceding claim, wherein each inductor comprises a winding on a single core, or two or more of the inductors comprise windings on one or more shared cores.

8. The SPE switch of any preceding claim, wherein the switching circuit allows only unidirectional flow of current between the power supply input and one or both of the trunk cable interfaces in the first state and the second state, and wherein the unidirectional flow of current is preferably achieved by using one or more rectifier diodes, or by using body diodes which are embedded in solid-state semiconductor switches, for example the body diode in a MOSFET.

9. The SPE switch of any preceding claim, wherein one or more reservoir capacitors, with optional series damping, are positioned across the negative and positive lines at one or more positions between the inductors.

10. The SPE switch of claim 8, wherein the damping, for example a series resistor, is used to help prevent disruptive oscillation caused by a short circuit, or the release of a short circuit, which can lead to one or more participating field switch’s switching circuit to unacceptably or inadvertently switch from the first state to the second state, where the reservoir capacitor is used to supply current to a short-circuit on the first or second trunk interface, instead of the short circuit drawing a disruptive current from the second or first trunk respectively, where if a series resistance is used for damping, the current flow from the reservoir capacitor into the short circuit can be increased by connecting a rectifier diode across the resistor.11 . The SPE switch of any preceding claim, wherein the decision circuit is configured to filter out DC components of the measured voltage to provide a filtered voltage, and to signal a short-circuit fault when the filtered voltage drops beneath a voltage threshold.

12. The SPE switch of any preceding claim, wherein the decision circuit is configured to measure a rate of change of the measured current and signal a short-circuit fault if the rate of change is both higher than a first current threshold and lower than a second current threshold.

13. The SPE switch of any preceding claim, comprising a bypass resistor connected between the first and second trunk cable interfaces and in parallel with the switching circuit.

14. The SPE switch of claim 13, wherein the bypass resistor is a variable resistor.

15. The SPE switch of any preceding claim, wherein the decision circuit is configured to signal a short-circuit fault resolution between the positive and negative connections of each trunk cable interface, based on changes in voltage and / or current measured by the voltage and / or current measurement circuits, wherein the said signal of the short-circuit fault resolution is an instruction for the switching circuit to change state from the second state back to the first state.

16. The SPE switch of any preceding claim, wherein the switching circuit comprises a 2-pole switch connected in series between the pair of inductors in the positive or negative lines, wherein the decision circuit is configured to close the 2- pole switch in the first state of the switching circuit and open the 2-pole switch in the second state of the switching circuit.

17. The SPE switch of claim 16, wherein the switching circuit comprises a first diode connected from a first pole of the 2-pole switch to the power supply input and a second diode connected from a second pole of the 2-pole switch to the power supply input.

18. The SPE switch of any one of claims 1 to 15, wherein the switching circuit comprises a first switch connected between the power supply input and a first inductor of the pair of inductors, and a second switch connected between the power supply input and a second inductor of the pair of inductors, wherein the decision circuit is configured to close the first and second switches in the first state of the switching circuit and open only one of the first and second switches in the second state of the switching circuit.

19. The SPE switch of claim 18, wherein the first and second switches are respective MOSFET switches that each comprise a body diode, wherein each MOSFET switch enables bidirectional flow of current through the MOSFET switch when the MOSFET switch is closed, and wherein each body diode enables unidirectional flow of current through the corresponding MOSFET switch when the MOSFET switch is open.

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