Technical Field: This invention relates to power supply circuits, to power line termination cards for insertion into a backplane, and having such power supply circuits, to external circuits for coupling between a power line termination card and a DC power supply, and having such a power supply circuit, and to methods of operating a power supply circuit for disconnecting a DC power supply from a backplane.

Background:

It is known to design telecommunications equipment to recover automatically after losing power temporarily caused by power supply interruptions, for any duration. Usually the equipment recovers automatically, but different pieces of equipment may recover and restart after different delays, or may survive different durations of interruption without powering down enough to trigger the restart properly. This can lead to unintended operation internally or at interfaces between such equipments, even if the equipment meets relevant standards. This can lead to electrical units within a network element ending up in a latched state that renders them non- functional.

Telecom Australia have a particular test whereby the power interruption time can be increased from 17ms in arbitrary increments up to 5 seconds to determine the equipment susceptibility.

Telecommunications equipment in many cases is powered from 48V Direct Current supplies, which can be a 48V distributed DC supply, shared by multiple racks. The point of power entry to each equipment rack is usually facilitated by a slide-in unit known as a Power Line Termination Unit, which plugs into a backplane and provides the following functions in order to satisfy ETSI 300-132 as referenced by the R&TTE (Radio and Telecommunications Terminal Equipment) directive: -

1. Common mode and differential mode voltage transient suppression

2. Conducted Emissions Filtering associated with EMC

It should be noted that the R&TTE directive is the main route to compliance for Radio and Telecommunications equipment sold into Europe. There are other governing bodies like Federal Communications Commission (FCC), which is the United States telecommunications regulator. Specification 1555 Power & Earthing of Transmission Equipment Part 1, Issue 2 (48V Power Source for Transmission Equipment) is required for equipment sold into Australia. For such equipment there can be instances when the supply is interrupted for short periods of time, e.g. milliseconds. During these interruption periods, electronic units that make up the system may not recover properly, particularly if there is legacy equipment amongst the equipment in the rack. If this happens, typically it requires manual intervention in order to resume normal operation. This is usually in the form of card extraction and re-insertion after a period of time, or power cycling the complete Network Element at the main power supply feed for a period of time. This can add considerably to operational maintenance costs, and to loss of revenue from down time, particularly if the equipment is at remote unmanned locations.

Summary:

An object of the invention is to provide improved apparatus or methods. According to a first aspect, the invention provides a power supply circuit for disconnecting a DC power supply from a backplane in the event of a fault and having a switch coupled between the power supply and the backplane, for carrying out the disconnection of the DC power supply from the backplane. The power supply circuit can also have a delay controller for controlling the switch according to an indication of the fault, and a diode protection circuit, coupled in series between the switch and the DC power supply for protecting the switch from reverse voltage transients in the DC power supply. Notably the delay controller is arranged to control the switch to disconnect the DC power supply based on the fault indication, and to keep the backplane disconnected for at least the minimum disconnection duration.

By enforcing a minimum disconnection duration of power down, the cards supplied with power via the backplane can be guaranteed to be powered down properly. The minimum disconnection duration can be set to be long enough to ensure a proper restart and avoid incorrect operation caused by an incomplete power down. By providing diode protection, the switch can be implemented using less rugged technologies, suitable for carrying enough current for all the equipment which might be supplied via the backplane.

Another aspect of the invention can involve a power line termination card for insertion into a backplane, the card having such a power supply circuit. This can enable incorporation into widely used format with no need for more space. Another aspect can provide an external circuit for coupling between a power line termination card and a DC power supply, and having such a power supply circuit. This enables use with existing conventional power line termination cards for example.

Another aspect of the invention provides a method of operating a power supply circuit for disconnecting a DC power supply from a backplane in the event of a fault, the circuit having a switch coupled between the power supply and the backplane, for carrying out the disconnection of the DC power supply from the backplane, and a diode protection circuit coupled in series between the switch and the DC power supply for protecting the switch from reverse voltage transients in the DC power supply. The method has the steps of supplying power to the backplane through the switch and through the diode protection circuit, and controlling the switch to disconnect the DC power supply based on an indication of a fault. Furthermore the switch is controlled to keep the backplane disconnected for at least the minimum disconnection duration.

Any additional features can be added to these aspects, or disclaimed from them, and some are described in more detail below. Any of the additional features can be combined together and combined with any of the aspects. Other effects and consequences will be apparent to those skilled in the art, especially over compared to other prior art. Numerous variations and modifications can be made without departing from the claims of the present invention. Therefore, it should be clearly understood that the form of the present invention is illustrative only and is not intended to limit the scope of the present invention.

Brief Description of the Drawings:

How the present invention may be put into effect will now be described by way of example with reference to the appended drawings, in which:

Fig. 1 shows a schematic view of a circuit for supplying a backplane according to a first embodiment,

Fig. 2 shows a schematic view of a circuit for supplying a backplane according to another embodiment having a fault detection part,

Fig. 3 shows a schematic view of a circuit for supplying a backplane according to another embodiment in the form of a power line termination unit,

Fig. 4 shows a schematic view of a circuit for a fault detection part for use in embodiments, Fig. 5 shows a schematic view of a circuit for supplying a backplane according to another embodiment in the form of an external circuit for use with a conventional power line termination unit,

Fig 6 shows a view of method steps of operation of a power supply circuit according to an embodiment,

Fig. 7 shows a schematic view of an example of a delay controller part for use in embodiments,

Fig. 8 shows a schematic view of an example of a diode protection circuit for use in embodiments,

Fig. 9 shows a schematic view of an example of a switch for carrying out the disconnection, for use in embodiments,

Fig. 10 shows a schematic view of a hold up capacitor circuit for use in embodiments, Fig. 11 shows a schematic view of an external unit according to an embodiment without a hold up capacitor, also having a PLTU, a back plane, and a slide in unit having a hold up capacitor and

Fig. 12 shows operational steps for the delay controller according to another embodiment, for use where all units have their own hold up capacitance.

Detailed Description:

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

Definitions

Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps and should not be interpreted as being restricted to any means listed thereafter.

Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

Elements or parts of the described circuits may comprise logic encoded in media for performing any kind of information processing. Logic may comprise software encoded in a disk or other computer-readable medium and/or instructions encoded in an application specific integrated circuit (ASIC), field programmable gate array (FPGA), or other processor or hardware.

References to software can encompass any type of programs in any language executable directly or indirectly on processing hardware.

References to circuits, hardware, processing hardware or circuitry can encompass any kind of logic or analog circuitry, integrated to any degree, and not limited to general purpose processors, digital signal processors, ASICs, FPGAs, discrete components or logic and so on.

Introduction

By way of introduction to the embodiments, some issues with conventional designs will be explained.

Some embodiments relate to power supply circuits for deployment in a Network Element Power Line Termination Unit that specifically meets the performance criteria of ETSI 300-132 as detailed in section 4.3.2 to 4.3.4 inclusive, and also section 4.1.4 of Telecom Australia specification 1555 for example.

Section 4.3.2 indicates that the power conversion and management systems at the load side shall automatically restore service following restoration of supply to the normal voltage range, when the voltage is -40.5 or greater. Section 4.3.3 specifies that voltage transients may occur and that performance should be verified against a specified transient pulse. Section 4.3.4 specifies recovery from voltage transients without requiring manual intervention, in particular that no circuit breakers, fuses or such devices should be triggered.

Section 4.1.4 of specification 1555 indicates conditions for testing power source drop out immunity to either short circuit or open circuit in the power source feed. It refers to the equipment showing no effect for durations of 2- 16ms, and that equipment must recover automatically as in clause 4.1.6, for drop outs greater than 17ms.

However it has been found that it is difficult to guarantee automatic recovery from any duration of drop out, since different equipments may not power down completely if the drop out is short.

Introduction to features of embodiments:

Accordingly, a power supply circuit can have a delay controller for controlling a switch according to an indication of the fault. A diode protection circuit is coupled in series between the switch and the DC power supply for protecting the switch from reverse voltage transients in the DC power supply. Notably the delay controller is arranged to control the switch to disconnect the DC power supply as soon as the fault indication starts, and to keep the backplane disconnected for at least the minimum disconnection duration, so that the cards supplied with power via the backplane can be guaranteed to be powered down properly. The minimum disconnection duration can be set to be long enough to ensure a proper restart and avoid incorrect operation caused by an incomplete power down. By providing diode protection, the switch can be implemented using less rugged technologies, suitable for carrying enough current for all the equipment which might be supplied via the backplane. Figure 1 shows a schematic view of an example having a power supply feeding a diode protection part 30 in series with the switch 10, for connecting power, or disconnecting power from a backplane 40. Cards 50 are shown, inserted into the backplane to be supplied with power. A delay controller 20 provides a control signal to the switch to maintain the minimum duration of disconnection. A fault indication is fed to the delay controller to trigger the disconnection.

Some additional features

Additional features of some embodiments can include any of the following:

1. High switching current capability, typically 10A to 100A.

2. High transient Disturbance Immunity up to 32J at a peak voltage of 300V as per BTNR 2511 in conjunction with the high switching current capability.

3. Fast switching in the order of microseconds using rapid Under/Over voltage detection in conjunction with high current sink/source drive capability, incorporating a delayed start feature in the order of seconds. This delay needs to be in the order of seconds typically, to allow all units within the equipment to completely power down prior to power being re-applied. For the purpose of the examples described the chosen duration is five seconds, though other durations can be set to achieve similar advantages for different applications or according to the equipment being supplied.

Some embodiments have a fault detection circuit for providing the fault indication. It can be better to include this circuit rather than rely on an externally generated signal, though in principle it can be generated externally and can indicate various kinds of faults. The fault detection circuit can have one or more comparators for detecting an overvoltage or undervoltage. It can be better to detect voltage than other indicators such as current as it is simpler and cost effective, though in principle other types of fault detection could be used.

The diode protection circuit can have an active circuit to provide a diode function in series with the switch. This can enable lower losses than conventional diodes, thus enabling higher currents to be handled for a given heat dissipation level, though in principle other types of diode protection could be used.

The switch and the diode protection circuit can have a current carrying capacity of at least 10 amps. This can mean the power dissipation is a significant constraint.

The minimum disconnection duration can be at least 3 seconds. This can enable a wide variety of different legacy cards to power down reliably, though in some cases 5 seconds may be selected.

The circuit can have a hold up capacitor coupled to the supply, to maintain the supply for a period of at least 5 milliseconds, to ensure that interruptions shorter than this do not trigger the fault indication. This can help reduce the number of unnecessary power downs and is essential in meeting the system hold-up requirements associated with European 48V DC Telecommunication Power standards and the like. E.g. if a power interruption between zero and 5 milliseconds is seen by the equipment its functionality shall remain unchanged.

The circuit can have a common mode and differential mode filter and transient suppression circuit coupled to the supply side of the switch. This enables the circuit to meet the conducted emissions requirements associated with European 48V DC Telecommunication Power standards and the like, by not returning noise back to the main distributed supply.

The circuit can be for use with a power supply of 48v DC which is widely used for telecoms applications and so is commercially significant.

The circuit can be incorporated in power line termination card for insertion into a backplane, which is a widely used format and so avoids a need for more space for the circuit.

The circuit can be in the form of an external circuit for coupling between a power line termination card and a DC power supply. This enables existing conventional power line termination cards to be used without alteration, though it may need more space to be provided.

The method of operation can have the step of generating the fault indication by detecting an overvoltage or undervoltage. The step of supplying power can involve supplying at least 10 amps. The operation can have a minimum disconnection duration set to at least 3 seconds.

The method of operation can have the step of using a hold up capacitor coupled to the supply, to maintain the supply for a period of at least 5 milliseconds, to ensure that interruptions shorter than this do not trigger the fault indication.

There can be a step of detecting a duration of the fault and delaying the disconnection until the fault has lasted beyond a minimum fault duration. This can help avoid unnecessary interruptions caused by short transient faults, if the equipment can withstand such short faults.

Fig 2, further embodiment having a fault detection part

Figure 2 shows a similar embodiment to that of figure 1, but adding a fault detection part 60 for generating the indication of the fault which is fed to the delay controller. This can be for example a voltage detector for detecting whether the supply voltage has dropped, or been dragged down by a downstream short circuit fault in other equipment for example. There is also shown a hold up capacitance 70 coupled to the supply line to the load. The energy stored in this capacitance is used to keep the load supplied and running for up to 5ms of power supply interruption. This can enable the circuit to meet the performance criteria as described in ETSI 300-132 for Europe. There are other standards like Telecom Australia 1555 as mentioned above that require the system to keep running when subjected to a power interruption up to 16ms, but the principle is the same.

Fig. 3 embodiment in the form of a power line termination unit

Fig. 3 shows a schematic view of a circuit for supplying a backplane according to another embodiment in the form of a power line termination unit 90. This has a 48vDC power supply having a negative feed and a return path, for carrying heavy currents, and a ground. Other types of supply can be envisaged. The negative feed is coupled to a diode protection part in the form of an ideal diode circuit 32 in series with the switch 10, for connecting power, or disconnecting power from a backplane 40 to be supplied with power. A delay controller 20 provides a control signal to the switch to maintain the minimum duration of disconnection. A fault indication is fed to the delay controller from a fault detection part 60 to trigger the disconnection. A hold up capacitor circuit C 70 is shown coupled between the feed and return supply lines. Also coupled between the lines ahead of the power supply circuit is a common mode and differential mode transient suppression and filtering circuit 80, which can be implemented according to established practice using circuitry as used in conventional power line termination units. This part can contain differential, common mode filtering and transient voltage protection that is required to meet well known Telecommunications 48V DC standards in terms of conducted emissions and transient immunity. This functionality is met using passive inductive and capacitive filtering. System differential transient immunity is met using a single Bi-directional high energy transient voltage suppressor which has a breakdown voltage of 100V +1-5% @ 1mA. A data sheet on this suppressor can be obtained from the manufacturer, and is available currently at this URL:

http ://www.mdesemiconductor. com/MAX- 100%2010-24-02.pdf

Further details of one example implementation of the ideal diode, fault detection part, the switch, and the delay controller are shown in further figures described below.

Fig. 4 fault detection part

Fig. 4 shows a schematic view of a circuit for a fault detection part 60 for use in embodiments. This shows one example implementation, based on under or over voltage detection, others are conceivable in principle, such as current detection. The supply voltage as sent out to the load is also fed to a potential divider 66. Under voltage and over voltage detection signals are output, each being a proportion of the supply voltage, the under voltage being a larger proportion than the over voltage. A reference voltage is generated, at a level between the under and over voltages when the supply is at the nominal level. The fault detection part 60 has a pair of comparators 64 for comparing the reference to the under voltage and the over voltage signals. If the supply voltage increases, the over voltage will eventually reach the level of the reference and the over voltage comparator output will change. If the supply voltage decreases, the under voltage will eventually drop to the level of the reference and the under voltage comparator output will change. The comparator outputs are fed to a gate 68 which can be an OR gate or other logic and outputs a fault signal.

Fig. 5, an external circuit for use with a conventional power line termination unit. Fig. 5 shows a schematic view of a circuit for supplying a backplane according to another embodiment in the form of an external circuit for use with a conventional power line termination unit PLTU 92. This shows the features of figure 1 as an external circuit coupled to the back plane via the power line termination unit. This means there may be some parts such as the hold up capacitor circuit C 70 and the transient suppression and filtering circuit 80, implemented in the conventional PLTU. In some cases the transient suppression and filtering circuit 80 may need to be duplicated in the external circuit to protect the power supply circuit. The external circuit can be for example incorporated in a cable or connector or box between the DC supply and the PLTU card.

Fig 6 steps of operation of a power supply circuit

Fig 6 shows a view of method steps of operation of a power supply circuit according to an embodiment. At step 205, DC voltage is supplied. A fault is detected at step 200. The switch is turned off to disconnect the supply at step 210. At step 220, the end of the fault is detected. If the fault is shorter than a minimum disconnection duration, the method involves waiting for the end of the minimum disconnection duration at step 230. Then the switch is turned on again at 240.

Fig. 7 delay controller part

Fig. 7 shows a schematic view of an example of a delay controller part for use in embodiments. This is for embodiments where if the incoming supply falls below a particular voltage threshold (typically 40.5V), the delay controller holds the switch off for an additional five seconds, after the point at which the supply returns to normal (40.5V to 75V). This permits all downstream units to be completely powered down for five seconds, thus preventing any unwanted behaviour resulting from short power interruptions.

In order to achieve the under and over voltage feature a standard Hot-Swap controller integrated circuit is used, which minimises the number of components required. In this example the MAX5948 from Maxim has been selected. The circuit designation for this device is Nl . Data sheet for this device can be obtained from the manufacturer, and is available currently at the following URL: - http ://datasheets .maxim-ic.com/eri/ds/MAX5948 A-MAX5948B .pdf

This datasheet explains that the MAX5948A/MAX5948B are hot-swap controllers that allow a circuit card to be safely hot plugged into a live backplane. The MAX5948A/MAX5948B operate from -20V to -80V and are well-suited for -48V power systems. The MAX5948A is pin- and function-compatible with both the LT1640AL and LT1640L. The MAX5948B is pin- and function-compatible with both the LT1640AH and LT1640H. The MAX5948A/MAX5948B provide a controlled turn- on to circuit cards preventing glitches on the power-supply rail and damage to board connectors and components.

The MAX5948A/MAX5948B provide undervoltage, overvoltage, and overcurrent protection. These devices ensure the input voltage is stable and within tolerance before applying power to the load. Both the MAX5948A and MAX5948B protect a system against overcurrent and short-circuit conditions by turning off the external MOSFET in the event of a fault condition. Other features of this device that reduce component count are as follows: -

• Typical gate pin source current of 45uA of which can be used to implement the five second delayed start.

Gate pin sink current of typically 50mA. This is used to rapidly discharge C20 via V94 when an under or over voltage conditions occurs.

• Propagation delay from an under or over voltage condition and the gate pin being driven low is typically less than 0.5us

The under voltage when falling trip point has a minimum range of 33.5V to 34.9V as set on pin 3 by Rl, R3 and RIO, and the over voltage when rising trip point has a maximum range of 77.3V to 80.5V as set on pin 2 by R6, R7 and R8. Resistor Rl 1 is coupled to pin 6 of Nl, and to R12 and R13. R12 leads to the negative supply rail. R13 leads to a base of transistor V8. R 14 is provided to couple a collector of V8 to a positive supply rail. C27 is provided between the positive supply rail and negative supply rail. R9 and C8 form a low pass filter which increases the transient voltage immunity of Nl .

The outputs of this figure from pins 2 and 4 of N5 are coupled to the inputs of the circuit of fig 9. If the DC supply is not within these limits, the equipment is isolated from the supply by the switch in the form of transistors VI 9, V21 and V25 shown in figure 9 which are turned hard off until the supply returns to normal limits, i.e. greater than 35V and less than 75.0V. The turn off mechanism is achieved by pin 6 of Nl sinking current until 0V is achieved, turning V7 off, V94 on, thus discharging C20 quickly, and switching V8 which feeds pin3 of N5. Pin 3 of N5 then switches to +12V, which results in pins 4 and 5 of N5 turning V19, V21 and V25 off within lOuS. The turn on mechanism of VI 9, V21 and V25 after a five second delay is achieved as follows: -

The delay is set by Rl 1, R12, R13, C20 and the typical 45uA source current of Nl pin 6 in conjunction with the switching threshold of V8. When V8 turns on, pin 3 of N5 is switched to 0V, thus causing pins 4 and 5 to source current in the order of amps which turns V19, V21 and V25 on within lOus. C27 is required to provide a high current burst when pins 4 and 5 of N5 are sourcing current to charge the gate source capacitance of VI 9, V21 and V25 within this short period of time. This feature is required to minimise the time VI 9, V21 and V25 remain in their linear region during switching, thereby reducing the energy being dissipated. N5 also contains logic input hysteresis to prevent circuit oscillation under certain conditions. There is also a resistor R105 between V7 and a positive supply rail. The data sheet for this device can be obtained from the manufacturer, and is available currently at the following URL: - http :/'/datasheets .maxim-ic.com/eri/ds/MAX 15070 A-M AX 15070B .pdf

This datasheet explains that the MAX15070A/MAX15070B are high-speed MOSFET drivers capable of sinking 7A and sourcing 3A peak currents. The ICs, which are an enhancement over MAX5048 devices, have inverting and noninverting inputs that provide greater flexibility in controlling the MOSFET. They also feature two separate outputs working in complementary mode, offering flexibility in controlling both turn-on and turn-off switching speeds. The ICs have internal logic circuitry that prevents shootthrough during output- state changes. The logic inputs are protected against voltage spikes up to +16V, regardless of V+ voltage. Propagation delay time is minimized and matched between the inverting and noninverting inputs. The ICs have a very fast switching time, combined with short propagation delays (12ns typ), making them ideal for high-frequency circuits. The ICs operate from a +4V to +14V single power supply and typically consume 0.5mA of supply current. The MAX15070A has standard TTL input logic levels, while the MAX15070B has CMOS-like high-noise-margin (HNM) input logic levels.

Fig 8 diode protection circuit

Fig. 8 shows a schematic view of an example of a diode protection circuit for use in embodiments. It is fundamentally an improved diode function that is useful to protect the circuitry in Fig 7 and Fig 9 from positive transient events which might otherwise damage the switch or damage downstream circuitry. In practice, due to the high currents involved ( 1 OA to 100 A), it will typically be implemented by an active circuit arrangement in order to reduce unwanted power losses. It should be noted this implementation could be an external box placed in line with a 48V battery feed on legacy equipment, thus solving power interruption issues that may exist on different slide in unit types within a system.

The example shown uses an Active OR'ING ZXGD3102 Controller N3 from Diodes Incorporated to drive three N Channel MOSFETS V2, V3 and V23 configured in parallel as ideal diode replacements. A 12 v linear regulator is also shown for supplying the various circuits. The key functional features of this diode protection circuit are the 180V blocking voltage and the 105ns typical turn-off time of the ZXGD3102 controller itself. This blocking voltage is important in protecting VI 9, V21 and V25 that are shown in fig 9, from any transient level that is not completely removed by earlier suppression and filtering. The ideal diode characteristic is also useful in keeping power dissipation to a minimum. The three N-Channel MOSFETS V2, V3 and V23 act as the ideal diode. There are ancillary parts including capacitor C14 across the supplies to N3, and capacitor CI 7 coupled to pin 7, R30 between pin 7 and the positive supply rail, and R31 between pin 2 and the supply rail. R32 is shown as having no resistance, thus being equivalent to a wire coupled between pin 4 and the gate of V2. A data sheet of this Active Controller can be obtained from the manufacturer, and is available currently at this URL: - http:// ^' www.diodes.com' ^' datasheets/ZXGD3102.pdf

This datasheet explains that the ZXGD3102 is intended to drive MOSFETs configured as ideal diode replacements. The device is comprised of a differential amplifier detector stage and high current driver. The detector monitors the reverse voltage of the MOSFET such that if body diode conduction occurs a positive voltage is applied to the MOSFET's Gate pin. Once the positive voltage is applied to the Gate the MOSFET switches on allowing reverse current flow. The detectors' output voltage is then proportional to the MOSFET Drain-Source reverse voltage drop and this is applied to the Gate via the driver. This action provides a rapid turn off as current decays.

Fig 9, switch example

Fig. 9 shows a schematic view of an example of a switch for carrying out the disconnection, for use in embodiments. This example comprises typically of an N- Channel Power MOSFET, but is not limited to this and could be electromechanical or the like. Negative supply rail AC is fed to the main current path of each of the three transistors VI 9, V21 and V25 in parallel. Their gates are tied together and driven by signal AB from N5 in figure 7.

There are various ways of arriving at the functional requirements of these blocks, however the implementation detailed is what has been realised in the intended application.

Fig 10 hold up capacitor

Fig. 10 shows a schematic view of a hold up capacitor circuit for use in embodiments. A capacitor 320 is coupled in series with a current limit circuit 300 for charging. A diode 310 is coupled in parallel with the other parts. The diode is required to extract the energy during a hold-up event as the current travels in the opposite direction, which effectively shunts the current limiting charging circuitry. The hold-up capacitance can be implemented either as part of the power supply circuit in the PLTU or in the external unit, or elsewhere such as on individual units that plug into the backplane. However the implementation elsewhere can in some cases cause a further problem which will be discussed in relation to figure 11.

Fig 11 , external unit and hold up capacitor located elsewhere

Fig. 1 1 shows a schematic view of an external unit 745 according to an embodiment without an on board hold up capacitor, also having a conventional PLTU 92, a back plane, and a slide in unit having a hold up capacitor 774. This shows some of the features of figure 3 incorporated in an external circuit coupled to the back plane via a conventional power line termination unit 92. The negative feed is coupled to a diode protection part in the form of an ideal diode circuit 32 in series with the switch 10, for connecting power, or disconnecting power from a backplane 40 to be supplied with power. A delay controller 20 provides a control signal to the switch to maintain the minimum duration of disconnection. A fault indication is fed to the delay controller from a fault detection part 60 via a timer 772 to trigger the disconnection. No hold up capacitor circuit is provided in the external unit as it is located elsewhere. Also coupled between the lines ahead of the power supply circuit is a differential mode transient suppression circuit 81, which can be implemented according to established practice using similar circuitry as used in conventional power line termination units. The external circuit can be for example incorporated in a cable or connector or box between the DC supply and the PLTU card. Each slide in unit in this example contains sufficient hold-up capacitance to keep the unit functioning for a short 5ms power interruption. There is a diode 330 and an inrush circuit 790 on each slide in unit in series with the capacitor to prevent the energy from discharging into the low impedance source as well as protecting down stream circuitry such as the isolated DC-DC converter 795 as shown, from reverse voltage transients. Fundamentally the main distributed supply does not always go open circuit during a power interruption. It can become low in voltage, for example approximately 10V as a result of down stream equipment developing a low impedance fault causing a rack mounted fuse to blow, and whilst the fuse is blowing the supply becomes low in voltage for a period of approximately 5ms. The ideal diode here has the same function when hold-up capacitance is present on the PLTU. In the example shown, if there was a short power interruption of say 2ms, the fault detector 60 would be triggered and would instigate a delayed start of 3 seconds which would disrupt system operation. This is not necessarily what is wanted, if such short interruptions do not affect the operation of slide in units that have their own hold-up capacitance. Thus the 3 second delay and restart may cause unnecessary loss of service and loss of revenue, compared to the case where the bulk hold-up capacitance is located in the external unit when used in conjunction with a voltage based fault detector.

To address this problem of such embodiments, another different option for the external unit or the PLTU implementation has been developed, which will be described with reference to figure 12. This option enables better operation with some legacy systems that contain slide in units incorporating hold-up capacitance.

This option provides Differential mode DM transient protection as Common mode CM is not required, as there is no 0V connection at the external unit at this point, voltage based fault detector and there is a further function, to measure the time the fault lasts, as well as the 3 second delayed restart. As before there is an ideal diode 32, solid state switch 10, but the bulk hold-up capacitance is elsewhere. In principle the timer can be part of the fault detector or part of the delay controller or outside these but coupled to one of these. The operation will now be described in more detail.

Fig 12, operational steps to avoid disconnection for short transient faults

Fig. 12 shows operational steps for the delay controller according to another embodiment, for use where all slide in units have hold up capacitance in the system. At step 205, DC voltage is supplied. A fault is detected at step 200. A timer is triggered at step 400 to determine a duration of the fault. At step 410, the timer is stopped when the end of the fault is detected. At step 420, if the recorded time is less than or equal to 5ms, then as shown by step 425, the state of the switch is left unchanged, and at step 430 the timer is reset and the method can be repeated from step 200. Otherwise, if the time of the fault is greater than 5ms, the switch is turned off to disconnect the supply at step 210. If the fault is shorter than a minimum disconnection duration, the method involves waiting for the end of the minimum disconnection duration at step 230. Then the switch is turned on again at 240, and the timer is reset at step 430. This method is useful to avoid the supply being disconnected unnecessarily by short transient faults which trigger the fault detection (because for example the hold up capacitor is located remotely), but are too short to affect operation of the units in the system (less than or equal to 5ms).

Concluding remarks

The various approaches described can solve many Network Element power interruption problems on legacy equipment by modifying the existing Power Line Termination Unit, or by having an external box containing this function. This external box can be connected in line with the battery feed itself which may prove more cost effective. It can be easy to implement within a new system architecture. It can save time and money by negating equipment failures of this nature, in particular on telecommunications products such as optical nodes, routers, wireless base stations, and land line based transmission products amongst others, and including other equipment that is powered from a DC 48V power distribution network for example.

Other variations and embodiments can be envisaged within the claims.