GB2448971A

GB2448971A - Measuring the resistance of data cable conductors used for supplying power

Info

Publication number: GB2448971A
Application number: GB0807433A
Authority: GB
Inventors: Iain Christopher Crouch
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-05-02
Filing date: 2008-04-24
Publication date: 2008-11-05
Also published as: WO2008135774A1; GB0807433D0; GB0708443D0

Abstract

Measuring the resistance of conductors within data cables over which power is supplied, where equipment connected at the remote end presents a DC short circuit. The invention has particular application in determining power loss in Power over Ethernet (PoE) systems, wherein Power Sourcing Equipment (PSE) delivers power to Powered Devices (PD). The invention is concerned with accurately determining the cable resistance, and thus power loss, in order for a PSE to supply more of its power budget to a PD when required. Apparatus according to the invention may be incorporated within PSE or PD. The DC short circuit may be presented by transformers in the PoE apparatus.

Description

APPARATUS AND METHOD FOR ESTIMATING POWER LOSSES IN DATA

CABLES

Field of the Invention

This invention relates to systems for supplying power over cables such as a communication cable and, in particular, to those systems in the field of Power over Ethernet (POE). The present invention particularly relates to systems where power levels are classified into ranges based on assumed worst-case power losses in the cables over which power is delivered.

Background to the Invention

POE is used to deliver power to electronic equipment via standard network cable, usually CAT-5 or a related type. For example, in a surveillance application where a digital IP camera is located remotely from a Cclv control room, its power supply and data connection can both be made over the same cable. Rather than requiring a power source near the camera, all power can be supplied and controlled at the CCTV control room end.

The Institute of Electrical and Electronic Engineers (IEEE) has standardised POE as 802.3af. In this standard, the circuitry for supplying power is termed the Power SQurcing Equipment (PSE), and the circuitry for receiving power at the other end of the cable is termed the Powered Device (PD).

A typical circuit showing a PSE and PD connected across a cable is shown in Fig. 1.

CAT-5 type cable 1 consists of eight conductors arranged as four twisted pairs. Two of these pairs are used for data transmission and are referred to as the data pairs.

The other two pairs are referred to as the spare pairs.

The POE supply treats each pair as a single conductor, To deliver power the PSE 2 presents a DC common-mode voltage across the data pairs via a centre tap on the cable side of data isolation transformers 3 and 4. Alternatively, the PSE may deliver power over the spare pairs, by short-circuiting the conductors in each pair and applying the POE voltage directly, shown by broken lines 5 and 6.

Note that Gigabit Ethernet uses all four pairs for data, so a DC voltage applied to the spare pairs would also be connected via data isolation transformers, otherwise the implementation is the same.

The PD 7 must be able to receive power from either the data pair or the spare pair, in either polarity. To allow this, the centre taps of data isolation transformers 8 and 9 and the shorted spare pair connections 10 and 11 are connected to diode bridges 12 and 13.

The POE power supply is shown at 16, and can be connected to the cable by PSE controller 14 when power sourcing is enabled. The power supply to PD equipment at 17 may be enabled by PD controller 17.

The IEEE 802.3af standard specifies a POE supply voltage in the range 44V to 57V DC, nominally 48V DC. Before this can be applied to the cable, the PSE must carry out a signature detection stage to ensure a compatible PD is connected, followed by an optional classification stage to determine the power requirements of the PD.

The PSE can choose not to supply power to a detected PD if its maximum power requirement, determined by its power class, is too great. For example, a multi-port PSE may have a limited power budget that would be insufficient to power all of its ports to the highest power class.

The PD power classes are defined as follows: Class 0 devices draw between 0.44W and 12.95W, requiring the PSE to allocate 15.4W to the PD. This is the default class.

Class 1 devices draw between 0.44W and 3.84W, requiring the PSE to allocate 4.0W to the PD.

Class 2 devices draw between 3.84W and 6.49W, requiring the PSE to allocate 7.0W tothePD.

Class 3 devices draw between 6.49W and 12.95W, requiring the PSE to allocate 15.4W to the PD.

Class 4 is reserved.

The difference between the power the PD can draw and the power the PSE must be able to deliver accounts for power losses between the PSE and PD, mainly in the cable. The IEEE 802.3af standard assumes a maximum resistance between PSE and PD of 20 Ohms, where the cable length is lOOm, the upper limit for Ethernet.

So, for example, a PSE delivering a maximum continuous current of 350mA, at the minimum voltage of 44V, to a PD requiring up to 12.95W, must allocate 15.4W of available power to that port, to allow up to 2.45W of resistive losses.

Power losses between PSE and PD are usually lower than this however, as the cable's length, and its resistance per unit length, will often be significantly lower than the worst case.

It would therefore be desirable for a PSE with a restricted power budget, or a PD whose power requirements are close to the class thresholds, to be able to account for cable losses more accurately.

It would also be desirable for the system by which the PSE or PD accounts for these losses to be compatible with the majority of PD or PSE equipment, without requiring any modification to that equipment, or requiring that equipment to display any characteristics beyond those specified in the IEEE 802.3af standard.

Summary of the Invention

According to a first aspect of the present invention there is provided apparatus for receiving power comprising: means of connecting to a data cable, means of receiving power via said cable, and measuring means for determining the resistance of at least one conductor within said cable.

The present invention proposes a novel apparatus which may supply power over a communication cable, or be supplied power over a communication cable, or both, and which includes a method to determine or predict power losses between itself and the remotely connected equipment, in order to account for those losses in determining limits for the power supplied or drawn by the device.

In particular, the apparatus may be used where power is carried over at least two pairs, at least four pairs, or a plurality of conductors within the communication cable.

In particular, the invention may be applied to equipment supplying or receiving Power over Ethernet. Preferably, such equipment will be compatible with IEEE standard 802.3af, or future standards.

The apparatus may incorporate a measurement means for determining the resistance of the DC path used for transferring power from PSE to PD. This measurement means may measure this resistance directly, or allow it to be approximated to sufficient accuracy, for example by measuring the resistance of other conductors in the communication cable and assuming any variation in resistance between conductors, and the resistance of other conducting components which may not have been measured, is negligible. The resistance of said DC path may be referred to as the "combined resistance" of the conductors of said cable which carry said power and one skilled in the art would realise that it may include resistances external to said conductors.

The measurement means may not usually require any modification to the equipment connected the other end of the cable, or interfere with its operation.

The measurement means may measure, for example, the resistance of another DC path consisting of the two conductors in a twisted pair, and other conductors which may include connectors, PCB tracks, and transformer windings. This path is formed as most conventional POE-compatible equipment will provide a short-circuit to DC current between the conductors in each pair when the equipment is connected to the cable, either through direct connection of conductors in spare pairs, or via a transformer winding. The resistance of said DC path may be referred to as the "series resistance" of the conductors within said pair.

In embodiments where the remote POE equipment is not suited to the measurement means, for example, where it does not provide a DC return path between conductors, the apparatus may use a greater or upper limit of resistance value for the cable, ensuring the appropriate power limit is not exceeded.

In an alternative embodiment of the invention, where the apparatus is a PD, the apparatus may sample the voltage and current delivered to it, for a range of known loads, in order to calculate a model of the PSE and cable, for example, the PSE and cable's voltage and resistance characteristics. This would be used to test regulation of the PSE voltage, or to determine losses external to the DC loop used for measurement, for example, the on-resistance of a switching metal-oxide---semiconductor field-effect transistor (MOSFET) in the PSE. These may be included in said combined resistance.

Typically, the apparatus may take inputs such as the measured resistances and local voltages and currents, alongside known constants such as power requirements and losses internal to the system, and use them to calculate how much power is available from, or would be required by, connected POE equipment. Methods for performing this in either analogue or digital circuitry as appropriate would be apparent to anyone skilled in the art.

The apparatus may use this information to ensure compatibility with any connected equipment before drawing more power from a PSE or enabling power delivery to a PD.

It should be noted that "compatibility" herein is intended to be interpreted as not just in terms of power availability but also of delivered voltage level.

For example, an apparatus connected between a PSE and a PD could calculate what the total power requirement from the PSE and input voltage to the PD would if it were to enable power to the PD, based on the measured cable resistances, voltages and device classes, and only enable power to the PD if both were within the range of the

IEEE specification.

Typically, the apparatus may assess compatibility of connected devices based on the assumption that the devices may not have been designed or tested to the full extent of the appropriate standard. For example, a PSE only has to deliver 15.4W or 350mA if its supply voltage is 44V. For higher PSE voltages, maximum current and cable losses both fall, so the PSE may have been designed and tested to deliver a lower power. The apparatus can calculate the supply voltage at the PSE and account for this. Another example is that of a relatively low-power PD, which has not been designed and tested to account for the worst-case PD voltage, as the voltage drop across the cable will be lower. The apparatus can ensure the PD voltage will be within the range the PD would have been designed for before enabling it. The voltage supplied by the apparatus may drop below 44V while the voltage at the PD is still within its designed range.

Preferably, the apparatus may also incorporate a method of detecting when it is connected to a PSE which is capable of supplying power greater than that of standard POE equipment. For example, if the PSE were designed to deliver up to the maximum rated current and voltage combined, 35OmA at 57V, the apparatus could detect this and increase its power limit accordingly Further examples for the present invention are where a PSE connected to the apparatus may be a network switch or midspan power injector, or where a PD connected to the apparatus may be an Internet Protocol (IP) surveillance camera, an IP telephone, or a Wireless Access Point (WAP).

According to a second aspect of the present invention there is provided apparatus for delivering power comprising: means of connecting to a data cable; means of delivering power via said cable; and measuring means for determining the resistance of at least one conductor within said cable.

According to a third aspect of the present invention there is provided a method for measuring resistance, comprising: means of connecting to a data cable; measuring means for determining the series resistance of a pair of wires within said cable; wherein equipment connected to the remote end of said cable would present a short-circuit to DC current across said pair of wires within said cable

Brief description of the drawings

The invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 2 shows an example of a power-forwarding network switch or repeater which incorporates the present invention in measuring and accounting for cable losses in both the PSE and PD connections; Figure 3 shows in further detail the connection between the invention and a PD, including an exemplary embodiment of the resistance measuring circuit; Figure 4 shows a further exemplary embodiment of the resistance measuring circuit, applied to a data pair which may also be connected to the POE power supply; Figure 5 shows a further exemplary embodiment of the resistance measuring circuit, applied to a spare pair which may also be connected to the POE power supply; and Figure 6 shows in further detail the connection between the invention and a PSE, including an exemplary embodiment of the resistance measuring circuit.

Figures 1 to 6 have been simplified in order to describe the invention. The design of any elements such as electronic circuits or microprocessor code that are not shown would be obvious to anyone skilled in the art. Anyone skilled in the art would also realise that elements such as over-voltage protection components, noise suppression components, and common-mode AC termination are not shown in the diagrams, and that the inclusion of these elements would not affect the operation of the invention as described in Figures 1 to 6.

In some of following description, for clarity, the resistance that is being measured is simply referred to as being the resistance of the cable. One skilled in the art would realise that the total resistance being measured may include the resistance of transformer coils, connector contacts, electronic switches such as MOSFETs, or other elements. One skilled in the art would also realise that these resistances are negligible in comparison to the cable resistance and required accuracy of measurement, and also that their inclusion would actually improve accuracy for applications where it is the total resistance of the DC power supply path, rather than just the cable conductors, that is required by the apparatus. The resistance of the DC power supply path may therefore be referred to as the "combined resistance" of the conductors of said cable which carry said power, and the resistance measured across a pair of conductors which may include the resistance of such elements may therefore be referred to as the "series resistance" of said pair.

The examples given are for application in POE systems, however one skilled in the art would realise that the invention may be applied to any system where power is transferred via communication cable.

Detailed description of the drawings

Figure 2 is a simplified block diagram of a POE system including a PSE 18, connected to a switch 19 via cable 20. The switch is connected to PDs 21, 22, 23 and 24 via cables 25, 26, 27 and 28, respectively. This embodiment covers, for example, a two-port repeater, a three or five-port switch or POE between two ports and adds a low-voltage output.

The switch 19 may be a power-forwarding Ethernet switch. A five-port switch is shown in the example, but the invention may be applied to switches with any combination of ports connected to PSEs, PDs, or non-POE equipment, or where some switch ports are disconnected. If the switch has only two ports, one for connection to a PSE and one for connection to a PD, the switch may function as a power-forwarding Ethernet repeater.

The switch 19 may include a measuring means to determine the resistance of cable 20, and other resistive components in the POE current path between PSE 18 and switch 19. The switch 19 may also be capable of measuring other variables such as its own supply voltage. The switch 19 may then use these measurements in calculating, for example, the supply voltage at the PSE, how much power it can draw from the PSE without exceeding any specified or designed limits at the PSE, or how its supply voltage will vary as the power drawn from the PSE changes.

The switch 19 may include measuring means to determine the resistance of cables 25, 26, 27, or 28, and other resistive components in the POE current paths between the switch and the respective PDs. The switch may use such measurements, as well as other measurements such as its own supply voltage, in calculating the power that must be allocated to the PD, given the PD's power class.

The switch 19 may use resistance measurements for all cables, and other measurements, to compute a model for the POE system in Figure 1. It may then use this model to determine whether power can be enabled for any connected PDs, while ensuring that power and voltage levels at all connected PSEs and PDs will remain within specified limits. For example, the switch 19 could appear as a class 0 PD to the PSE 18, and could be connected to two class 2 PDs. Without accounting for cable losses, the switch can only draw 12.95W and must be able to supply approximately 7.0W to each PD, so would not be able to enable both PDs. However if cable resistances are known, the switch may be able to draw up to 15.4W from the PSE and may only need to supply 6.49W to each PD, so depending on the cable resistances it may be possible to power both PDs and the switch itself. Examples of PDs include but are not limited to IP cameras, IP phones and WAPs.

As shown in Figure 3, a PSE may include a measuring means for measuring cable resistance when a typical PD is connected at the other end of the cable. In this example, the data pairs of cable 29 are used for POE power transfer via the centre taps of transformers 30, 31, 32, and 33. The PD can accept power from either the data pairs via diode bridge 34, or from the spare pairs via diode bridge 35.

The PD 36 short circuits the conductors within each pair of cable 29 to DC current.

The data pairs are short-circuited to DC via transformers 31 and 33, and the spare pairs are short-circuited directly at points 37 and 38.

The PSE 39 includes a 48V DC power supply, whose positive terminal is connected to node 40, and whose negative terminal may be connected to node 41 by the PSE when POE is to be enabled. In this example, cable resistance measurement takes place before POE is enabled, with node 41 floating. The PSE also provides 3.3V at nodes 42 and 4310 supply power to microcontroller 44.

To measure resistance in cable 29, microcontroller 44 may enable switch 45 and current source 46. A constant current will then flow through the two conductors of pair 47 in series, resulting in a voltage drop between nodes 40 and 48 in proportion to the series resistance of the conductors in pair 41. This voltage may then be translated to an appropriate range by level shifter 49 and measured by an analogue to digital converter within microcontroller 44. The microcontroller may then use this measured voltage to determine the resistance of pair 41, and thus determine, to an acceptable degree of accuracy, the resistance of the path that POE current will take over the cable.

The resistance measurement method shown in Figure 3 may be modified to allow the resistance of a data pair to be measured, optionally with the same pair being used to supply POE power. Figure 4 shows an example of how a resistance measuring circuit may be connected. When the PD 50 is connected to cable 51, the conductors in pair 52 are short-circuited together to DC current via transformer 53. The transformer 54 in PSE 55 has a split winding which is connected to the resistance measuring circuit 56. The resistance measuring circuit 56 is therefore able to pass a DC current through the conductors of pair 52 and the cable-side windings of transformers 53 and 54, in order to measure their series resistance.

When the resistance is not being measured, the PSE controller 57 may connect the POE supply to the conductors of pair 52 via a separate switch for each pair. The cable-side windings of transformer 54 are connected via capacitor 58 to provide a direct path for the AC data signal.

Figure 5 shows an example of how a resistance measurement circuit which uses AC current may be connected to a spare pair which is also used for carrying DC current for the POE supply.

When the PD 59 is connected to cable 60, it short-circuits the conductors of pair 61 at point 62. In the PSE 63, the POE DC supply is connected, optionally via PSE controller 64, to pair 61, via inductors 65 and 66.

The resistance measurement circuit 67 may pass an AC current across the conductors of pair 61, in parallel with the series inductance of inductors 65 and 66, and use, for example, a phase or voltage measuring technique to determine the resistance of the conductors of pair 61.

In the example shown in Figure 6, the PSE 68 has already enabled POE power to PD 69, which is connected via cable 70.

The PSE's 48V power supply at nodes 71 and 72 connects to the data pairs of cable via transformers 73 and 74, via switch 75, which has been enabled by PSE controller 76.

The PD's power supply at nodes 77 and 78 connects to the data pairs of cable 70 via transformers 79 and 80, diode bridge 81, and switch 82, which has been enabled by PD controller 83.

Microcontroller 84 may measure the PD supply voltage using an internal Analogue to Digital Converter connected via the potential divider formed by resistors 85 and 86.

The microcontroller 84 may also connect load 87 to the PD supply via switch 88 and measure the PD supply voltage again.

The microcontroller may then use these measured voltages, along with known load characteristics for the PD and load 87, to calculate a model of the characteristics of the PSE 68 and cable 70, for example as a fixed supply voltage and loop resistance of the DC current path including transformer windings and switch 75.

The microcontroller may also switch in additional loads to allow a more complicated or accurate model to be determined. By using this means to measure total resistance and another means such as those described to measure cable resistance only, the microcontroller may be able to determine the extent of characteristics such as poor regulation of the PSE power supply.

Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.

Claims

Claims 1. Apparatus for receiving power comprising: means of connecting

to a data cable; means of receiving power via said cable; and measuring means for determining the resistance of at least one conductor within said cable
2. Apparatus according to claim 1, and further including means to calculate the combined resistance of the conductors of said cable which carry power supply current to said apparatus.
3. Apparatus according to claim 2, and further including means to calculate the maximum power available for supply to said apparatus, given said combined resistance.
4. Apparatus according to any preceding claim, wherein all additional electronic circuitry for said measuring means is contained within said apparatus
5. Apparatus according to any preceding claim, wherein said measuring means comprise means of measuring the voltage of a power supply received via said cable.
6. Apparatus according to any preceding claim, wherein said measuring means comprise means of varying the power drawn by said apparatus.
7. Apparatus according to any preceding claim, wherein said measuring means measure the series resistance of the conductors within at least one pair of wires within said cable.
8. Apparatus for delivering power comprising: means of connecting to a data cable; means of delivering power via said cable; and measuring means for determining the resistance of at least one conductor within said cable.
9. Apparatus according to claim 8, and further including means to calculate the combined resistance of the conductors of said cable which carry power supply current from said apparatus.
10. Apparatus according to claim 9, and further including means to calculate the minimum power required for supply via said cable, given said combined resistance.
11. Apparatus according to any of claims 8 to 10, wherein all additional electronic circuitry for said measuring means is contained within said apparatus
12. Apparatus according to any of claims 8 to 11, wherein said measuring means measure the series resistance of the conductors within at least one pair of wires within said cable.
13. Apparatus for forwarding power comprising: at least one power receiving apparatus according to any of claims 1 to 7, and at least one power delivering apparatus according to any of claims 8 to 12.
14. Apparatus according to claim 13, and further including means of determining whether power delivery can be enabled for equipment connected to said power delivering apparatus, given inputs comprising: power available from equipment connected to said power receiving apparatus; power required by equipment connected to said power delivering apparatus; measured cable resistances; local voltage measurements; and local power requirements.
15. Method for measuring resistance, comprising: means of connecting to a data cable; and measuring means for determining the series resistance of a pair of wires within said cable, wherein equipment connected to the remote end of said cable would present a short-circuit to DC current across said pair of wires within said cable
16. Method according to claim 15, including the step of calculating the resistance of the wires of said pair in parallel, given that the wires of said pair are of approximately equal resistance.
17. Method according to any of claims 15 or 16, wherein a current is passed across the wires of said pair, and the voltage across said wires is measured.
18. Method according to any of claims 15 to 17, wherein an alternating current is passed across the wires of said pair, and a phase change is measured.
19. Method according to any of claims 1510 18, wherein said pair is also used for carrying data.
20. Method according to any of claims 15 to 18, wherein said pair is not used for carrying data.
21. Method according to any of claims 15 to 20, wherein said pair is used to carry power supply current.
22. Method according to claim 21, wherein the power supply connection to the wires of said pair is made via a separate switching element for each wire.
23. Method according to claim 21, wherein the power supply connection to the wires of said pair is made via a separate inductor for each wire.
24. Method according to any of claims 15 to 20 wherein said pair is not used to carry power supply current.