Powered device for power over Ethernet system with increased cable length

A Powered Device (PD) in a Power Over Ethernet system that supports increased cable lengths of more than 100 m is provided herein. The proposed PD design requires no modifications at the PSE side. Embodiments include example modifications of IEEE 802.3af PD system rules, including example modifications of PD classification and port voltage ranges to enable increased cable length PoE.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to Power over Ethernet (PoE), and more particularly to a PD controller for PoE systems with increased cable length.

2. Background Art

Ethernet communications provide high speed communications between data terminals.

Power over Ethernet (PoE) systems enable power transmission over the same transmission lines that carry data in an Ethernet. Generally, power is generated at a Power Source Equipment (PSE) side of the PoE system and is carried over the data transmission lines to a Powered Device (PD) side of the PoE System.

A PSE controller is typically used at the PSE side to enable power management functions of the PD. For example, a PSE controller may be used to detect whether a valid PD device is active and to manage power flow to the PD. Further, at either side of a PoE system, a transceiver physical layer (PHY) is available to transmit and receive data over the transmission lines.

Current IEEE compliant PoE systems support cable lengths up to approximately 100 meters only. There is a need therefore for improved PoE systems that support greater cable lengths.

BRIEF SUMMARY OF THE INVENTION

Increased cable length Power over Ethernet (PoE) methods and systems are provided herein.

Embodiments can be designed for compliance with IEEE 802.3af, future IEEE 802.3at, or legacy PoE standards.

Embodiments include a PD controller design enabled for increased length PoE without modification on the existing PSE design.

Embodiments include example modifications of IEEE 802.3af PD system rules revised for increased cable length applications, including example modifications of PD port voltage ranges.

Embodiments include PDs supporting increased length PoE.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Overview

Current IEEE 802.3af compliant PoE systems support cable lengths up to approximately 100 meters only. There is a need therefore for improved PoE systems that support greater cable lengths.

Increased cable length Power over Ethernet (PoE) methods and systems are provided herein. Embodiments can be designed to support IEEE 802.3af, future IEEE 802.3at, and/or legacy PoE standards. Embodiments include PD designs enabled for increased length PoE.

Introduction

FIG. 1illustrates a high level diagram of a conventional Power over Ethernet (PoE) system100that provides DC power over a common data communications medium. Referring toFIG. 1, power source equipment102provides DC power over conductors104,110to a powered device (PD)106having a representative electrical load108. Accordingly, the power transfer between the PSE102and the PD106occurs simultaneously with the exchange of high speed data over the conductors104,110. In one example, the PSE102when used with a switching and PHY chip is a data switch having multiple ports that is communicating with one or more PD devices, such as Internet phones, wireless access points, etc.

The conductor pairs104and110can carry high speed differential data communications. In one example, the conductor pairs104and110each include one or more twisted wire pairs, or any other type of cable or communications media capable of carrying the data transmissions and DC power transmissions between the PSE and PD. In Ethernet communications, the conductor pairs104and110can include multiple twisted pairs, for example four twisted pairs for 1 Gigabit Ethernet. In 10/100 Ethernet, only two of the four pairs carry data communications, and the other two pairs of conductors are unused. Herein, conductor pairs may be referred to as Ethernet cables or communication links or structured cabling for ease of discussion. The conductor pairs may be CAT-5 cable for example. Alternatively, the conductor pairs may be CAT-3, CAT-5e, CAT-6, CAT-6a, or CAT-7 cable.

FIG. 2provides a more detailed circuit diagram of the PoE system100, where PSE102provides DC power to PD106over conductor pairs104and110. PSE102includes a transceiver physical layer device (or PHY)202having full duplex transmit and receive capability through differential transmit port204and differential receive port206. (Herein, transceivers may be referred to as PHYs.) A first transformer208couples high speed data between the transmit port204and the first conductor pair104. Likewise, a second transformer212couples high speed data between the receive port206and the second conductor pair110. The respective transformers208and212pass the high speed data to and from the transceiver202, but isolate any low frequency or DC voltage from the transceiver ports, which may be sensitive to large voltage values.

The first transformer208includes primary and secondary windings, where the secondary winding (on the conductor side) includes a center tap210. Likewise, the second transformer212includes primary and secondary windings, where the secondary winding (on the conductor side) includes a center tap214. The DC output voltage is applied across the respective center taps (e.g.210,214) of the transformers208and210on the conductor side of the transformers. An example DC output voltage for the DC supply218is 48 volts, but other voltages could be used depending on the voltage/power requirements of the PD106.

The PSE102further includes a PSE controller216which performs the power management functions based on the dynamic needs of the PD106. More specifically, the PSE controller216measures the voltage, current, and temperature, etc so as to characterize the power requirements of the PD106.

Further, the PSE controller216detects and validates a compatible PD, determines a power classification signature for the validated PD, supplies power to the PD, monitors the power, and reduces or removes the power from the PD when the power is no longer requested or required. During detection, if the PSE finds the PD to be non-compatible, the PSE can prevent the application of power to that PD device, protecting the PD from possible damage. The IEEE has imposed standards on the detection, power classification, and monitoring of a PD by a PSE in the IEEE 802.3af™ standard, which is incorporated herein by reference.

Still referring toFIG. 2, the contents and functionality of the PD106will now be discussed. The PD106side includes a transceiver physical layer device219having full duplex transmit and receive capability through differential transmit port236and differential receive port234. A third transformer220couples high speed data between the first conductor pair104and the receive port234. Likewise, a fourth transformer224couples high speed data between the transmit port236and the second conductor pair110. The respective transformers220and224pass the high speed data to and from the transceiver219, but isolate any low frequency or DC voltage from the sensitive transceiver data ports.

The third transformer220includes primary and secondary windings, where the secondary winding (on the conductor side) includes a center tap222. Likewise, the fourth transformer224includes primary and secondary windings, where the secondary winding (on the conductor side) includes a center tap226. The center taps222and226supply the DC power carried over conductors104and106to the representative load108of the PD106, where the load108represents the dynamic power draw needed to operate PD106. A DC-DC converter230may be optionally inserted before the load108to step down the voltage as necessary to meet the voltage requirements of the PD106. Further, multiple DC-DC converters230may be arrayed in parallel to output multiple different voltages (e.g. 3 volts, 5 volts, 12 volts) to supply different loads108of the PD106.

The PD106further includes a PD controller228that monitors the voltage and current on the PD side of the PoE configuration. The PD controller228further provides the necessary impedance signatures on the return conductor110during initialization, so that the PSE controller216will recognize the PD as a valid PoE device, and be able to classify its power requirements.FIG. 2also illustrates a signature resistor248and a classification resistor250connected to PD106. Signature resistor248is used to validate the PD106, and the classification resistor250is used for classifying PD106and to limit current for classification.

During ideal operation, a direct current (IDC)238flows from the PSE Controller216through the first center tap210, and divides into a first current (I1)240and a second current (I2)242that are carried over conductor pair104. The first current (I1)240and the second current (I2)242then recombine at the third center tap222to reform the direct current (IDC)238so as to power PD106. On return, the direct current (IDC)238flows from PD106through the fourth center tap226, and divides for transport over conductor pair110. The return DC current recombines at the second center tap214, and returns to the DC power supply218.

As discussed above, data transmission between the PSE102and the PD106occurs simultaneously with the power as described above. Accordingly, a first communication signal244and/or a second communication signal246are simultaneously differentially carried via the conductor pairs104and110between the transceivers or PHY of PSE102and the PD106. It is important to note that the communication signals244and246are differential signals that ideally are not effected by the DC power transfer described above. However, the signaling used by the PSE controller is based on common mode signaling so it does not interfere with data transmission.

In order to conduct its management and control of PD106, PSE102analyzes certain characteristics of PD106, and the system as a whole, based on measurements taken at PD106. Based on those characteristics, PSE102can determine certain attributes of PD106as well as attributes of the system. Example attributes determined by PSE102can include, but are not limited to, the following: valid device detection, power classification, AC disconnect information, short circuit detection, PD load variations, various current measurements, overload conditions, and inrush conditions.

Increased Length Power Over Ethernet (PoE) System

Current IEEE compliant PoE systems support cable lengths up to approximately 100 meters only. As such, to support greater cable lengths, modified PSE and PD interfaces are needed.

In the teachings herein, several example embodiments for enabling increased length PoE systems are provided. These embodiments are provided for the purpose of illustration and are not limiting of the scope of the present invention. Further, embodiments will be described with respect to IEEE compliant systems (e.g., IEEE 802.3af and IEEE 802.3at). However, embodiments of the present invention are not limited to IEEE compliant systems and can be extended to legacy PoE systems having their own defined system rules.

One problem that is encountered in supporting increased length IEEE compliant PoE systems includes the increased resistance of the Ethernet cable (e.g., a CAT-5 cable) that connects the PSE and the PD. Indeed, IEEE compliant PSEs are designed to operate with a CAT-5 cable having a maximum length of approximately 100 meters. As such, IEEE compliant PSEs are designed to compensate for a maximum cable resistance of approximately 20 Ohms when supporting IEEE compliant PDs in current PoE systems. This compensation includes compensating for the voltage drops that could occur across the 20 Ohms resistance during each of the different operational phases (e.g., detection, classification, etc.) of the PoE system.

Accordingly, when the cable length (and correspondingly the cable resistance) is increased, compliance with IEEE requires modification of system rules at the PSE to enable the same operational voltages at the PD.

Examples of modification of PSE systems rules according to an embodiment of the present invention will now be described with respect toFIGS. 3-6. These examples are provided with respect to two exemplary PoE embodiments having increased cable length of 350 meters. These examples are provided for the purpose of illustration and are not limiting of the scope of the present invention. As would be understood by a person skilled in the art based on the teachings herein, these examples can be extended to support PoE systems having different cable lengths depending upon the PSE port voltages and PD class and load currents.

FIG. 3illustrates PD classification voltage ranges for two exemplary PoE embodiments with increased cable length.FIG. 3uses Class 4 as an example for which maximum current is up to 51 mA.FIG. 3also illustrates the required PD classification voltage range in a typical IEEE compliant PoE system. This range according to IEEE 802.3af should be between a minimum of 14.5 volts and a maximum of 20.5 volts on the PD side. It is noted that this required classification voltage range is specified based on a maximum cable length of 100 meters (or equivalently a 20 Ohms resistance) to provide valid classification voltages at the PD (typically, a valid classification voltage at the PD is between 14.5 volts and 20.5 volts).

As such, when the cable length is increased, the PD classification voltage range needs to be modified to compensate for the increased cable resistance. In an embodiment, this includes decreasing the minimum end of the classification voltage range according to the maximum possible added voltage drop due to the increase in cable resistance. In other words, this includes decreasing the minimum end of the classification voltage range by the added voltage drop due to the maximum possible current during the classification phase of an IEEE compliant PoE system.

Table 33-4 (not shown) of the IEEE 802.3af standard specifies the maximum current during classification to be equal to 51 milliamps in a PoE system that supports PDs of class 0 to 4. Accordingly, the resulting classification voltage ranges for the exemplary PoE systems with cable length of 350 and 500 meters are as shown inFIG. 3. For example, for a cable length of 350 meters, the minimum end of the classification voltage range at the PD is equal to 15.5−(51 milliamps×70 Ohms)≈11.93 volts. Similarly, for a cable length of 500 meters, the minimum end of the classification voltage range at the PSE is equal to 15.5−(51 milliamps×100 Ohms)≈10.4 volts.

It is noted, however, that current IEEE compliant PoE systems do not allow the usage of PDs of class 4, which is specified as “reserved” in the IEEE 802.3af standard. To be IEEE802.3af compliant, a PSE therefore only requires support of PDs of class 0 to 3. As such, the maximum current during classification with respect to which added voltage drops need to be calculated is only 35 milliamps, instead of 51 milliamps when class 4 is used. Based on that, compliance with IEEE can also be achieved using the PD classification voltage ranges illustrated inFIG. 4, which are calculated according to a maximum current during classification of 35 milliamps.

It is noted that in other aspects classification in increased length PoE systems remain substantially similar to classification in current PoE systems. For example, the classification phase duration remains as specified in IEEE 802.3af (less than or equal to 75 milliseconds). Also, the classification current limit (less than or equal to 100 milliamps) remains as specified in IEEE 802.3af. Similarly, the detection phase remains as specified in IEEE 802.3af, with the increased cable resistance added to the PD resistor signature and compensated for at the PSE.

As with classification voltage ranges, however, post start up voltage ranges need to be modified to enable increased cable length IEEE compliant PDs.FIG. 5illustrates PD post start up voltage ranges for the two exemplary PoE embodiments with increased cable length.FIG. 5also illustrates the required PD post start up voltage range in a typical IEEE compliant PoE system. This range according to IEEE 802.3af should be between a minimum of 36 volts and a maximum of 57 volts. As illustrated, this required post start up voltage range is specified based on a maximum cable length of 100 meters (or equivalently a 20 Ohms resistance) to provide valid post start up voltages at the PD (typically, a valid post start up voltage at the PD is between 36 volts and 57 volts).

In an embodiment, the minimum end of the PD post start up voltage range is decreased according to the maximum possible added voltage drop due to the increase in cable resistance.

IEEE 802.3af specifies the maximum load current during post start up to be equal to 350 milliamps. Accordingly, the resulting PSE post start up voltage ranges are as shown inFIG. 5. For example, for a cable length of 350 meters, the minimum end of the start up voltage range is equal to 44 v−(350 milliamps×70 Ohms)−1 v (approximate diode voltage drop)=18.5 volts. Similarly, for a cable length of 500 meters, the minimum end of the start up voltage range is equal to 44 v−(350 milliamps×100 Ohms)−1 v (approximate diode voltage drop)=8 volts, where 44 v is the minimum port voltage at PSE side. These exemplary PD start up voltage ranges guarantee valid post start up port voltages at the PD. Since IEEE802.3af allows a PSE port voltage range of 44 v-57 v, it would be recommended to use a higher value of port voltage at the PSE side (e.g., 56 v). This will ensure a minimum power delivery of 10.67 W at the PD side for a cable length of 350 m.

In addition to ensuring valid port voltages at the PD, IEEE compliance also necessitates compliance with power requirements at the PD. IEEE 802.3af specifies a maximum power level of 12.95 Watts at the PD for PDs of class 0 to 3. With a cable length of 100 meters, this implies that the PD should be able to support a maximum of 12.95 Watts, as approximately a maximum of 2.45 Watts could dissipate in the cable (7 volts×350 milliamps=2.45 Watts) due to cable resistance. However, with increased cable length it may not be possible to support the maximum 12.95 W at the PD side due to increased power dissipation in the cable.

With greater cable length, larger power dissipation in the cable will occur. Consequently, the PSE will be required to support greater power outputs. This is shown inFIG. 6, which illustrates required PD power requirements and cable power loss in example increased length PoE systems. For example, for a cable length of 350 meters, the PSE will be required to support 21.52 Watts as a maximum of 8.57 Watts (24.5 volts×350 milliamps=8.57 Watts) could dissipate in the cable. Similarly, for a cable length of 500 meters, the PSE will be required to support 25.2 Watts as a maximum of 12.25 Watts (35 volts×350 milliamps=12.25 Watts) could dissipate in the cable due to cable resistance. This basically requires increased port voltage from the PSE side. Since the current IEEE802.3af standard supports a voltage range of 44 v-57 v, a port voltage at the high end (e.g., 57 v) should be preferably used. However, other port voltage values can also be used. If the port voltage at PSE side is not increased, the power delivered to the PD will be reduced due to the dissipation on the increased cable length.

Note that similar cable power loss figures (8.57 Watts and 12.25 Watts) as described above are currently being studied by the IEEE 802.3at task force in approving a cable plant that propose supporting up to 720 milliamps, which approximates a cable loss of 10.3 Watts for a cable of length 100 meters.

The above described example embodiments describe PD IEEE compliance requirements to enable increased length PoE systems. As noted above, similar PD requirements can be created for increased length PoE systems that comply with legacy PoE standards. These PD requirements ensure that the PD works properly in view of the maximum possible cable current.

FIG. 7illustrates an IEEE compliant PoE system700. PoE system700includes an IEEE compliant PSE702and an IEEE compliant PD704, connected by a CAT-5 cable706having a length of 100 meters.

A maximum compliant current of 350 milliamps is allowed over cable706during the post start up phase. As such, a maximum voltage drop of 7 volts can occur over cable706. Accordingly, PSE702is required to support a minimum port voltage of 44 volts to enable a minimum port voltage of 36 volts at PD704(a drop of approximately 1 volt occurs across a diode prior to the PD input port).

Note, however, that the required maximum PSE port voltage is only 57 volts, which is also approximately the maximum voltage supported by current IEEE compliant PD processes. As such, notwithstanding the value of the cable current (or equivalently the voltage drop across cable706), the PD port voltage will always be within an allowable maximum voltage of the PD process.

FIG. 8illustrates an example increased length PoE system800. PoE system800includes a PSE802and PD804, connected by a CAT-5 cable806having a length of 350 meters. This example uses a PSE minimum port voltage of 44 v.

PSE802is enabled to support increased cable length up to 350 meters. For example, PSE802supports a minimum port voltage of 44 volts during the start up phase to provide a minimum port voltage of 18.5 volts at PD804(a maximum voltage drop of 24.5 volts occurs across cable806).

PD804is an increased cable length PD with a process that supports voltages up to 57 volts.

Note that the minimum port voltage at PD804(18.5 volts) is less than the minimum supported voltage of an IEEE802.3af compliant PD (36 v-57 v). Therefore, adjustment in PD design is needed for PD804.

FIG. 9illustrates an example increased length PoE system900. PoE system900includes a PSE902and PD904, connected by a CAT-5 cable906having a length of 350 meters. This example uses a PSE maximum port voltage of 57 v.

PSE902is enabled to support increased cable length up to 350 meters. For example, PSE902supports a maximum port voltage of 57 volts during the start up phase to provide a minimum port voltage of 31.5 volts at PD904(a maximum voltage drop of 24.5 volts occurs across cable906).

PD904is an increased cable length PD with a process that supports voltages up to 57 volts.

Note that the minimum port voltage at PD904(31.5 volts) is less than the minimum supported voltage of an IEEE802.3af compliant PD (36 v-57 v). Therefore, adjustment in PD design is needed for PD904.

Note that power class at PD side is used as a factor to calculate the voltage drop across the cable, as different power classes (0-3) will cause different voltage drops across the cable due to different load currents. Accordingly, there is a need to detect the PD power class at the PD side in order to calculate the Von/Voffvoltage at the PD. The PD Vonvoltage as per IEEE802.3af is 42 v at maximum. The PD Voffvoltage as per IEEE802.3af is 30 v at minimum. These levels of Von/Voffare defined based on a 100 m cable length, accounting for only 7 v of cable drop. Therefore, to accommodate increased cable lengths and increased cable voltage drops, these levels need to be adjusted.

One solution according to the present invention is to design the PD chip such that it is capable of detecting its own power class signature. Accordingly, the PD chip can calculate its own Vonand Vofflevels.

FIG. 10illustrates an example powered device (PD) chip1000usable in an increased length PoE system. Example PD chip1000includes a plurality of circuits1002,1004,1006, and1008and a Pulse Width Modulation (PWM) Controller1010.

Circuits1002,1004,1006, and1008are each connected to a port voltage Vport1012of PD chip1000. Each of these circuits1002,1004,1006, and1008is triggered when Vport1012is within the voltage range associated with it. For example, the circuit1002is triggered when Vport1012is within the voltage range of 2.7-10.1 volts to connect Vport1012through resistor Rdetect1014to the ground terminal. Similarly, the circuit1004is triggered when Vport1012is within the voltage range of 13-20.5 volts to connect Vport1012through resistor Rclass1016to the ground terminal.

Circuits1002and1004are used during the detection phase and the classification phase, respectively, which occur prior to power transfer in a PoE system. As such, Rdetect1014and Rclass1016correspond respectively to the signature resistor and the classification resistor of PD chip1000. In example embodiment1000, Rdetect1014and Rclass1016are shown as implemented externally to PD chip1000. In other embodiments, Rdetect1014and Rclass1016can be integrated within PD chip1000.

Circuit1006is used during the start up phase in PD chip1000. Typically, as described above, IEEE 802.3af specifies a minimum PD shut down voltage (Voff) of 30 volts, i.e., the PD chip must shut down if Vport1012is approximately 30 volts and falling. Similarly, IEEE 802.3af specifies a maximum PD turn on voltage (Von) of 42 volts, i.e., the PD must turn on at this voltage. However, according to this embodiment of the present invention, circuit1006is used to calculate the Vonand Voffvoltages of the PD based on its power class. Accordingly, an increased cable length PD is capable of calculating its own Vonand Vofflevels based on the power class it is using.

When triggered, circuit1006connects Vport1012through a Hot Swap MOSFET1020to a PD load (not shown inFIG. 10). Hot Swap MOSFET1020can be implemented within or outside PD chip1000.

In current IEEE compliant PoE systems having a cable length of 100 meters or less, circuits1002,1004, and1006are sufficient for required PD operation. This is because Vport1012cannot be less than 36 volts, and consequently no changes are needed to deal with Vport1012levels lower than 36 volts. This does not take transients into considerations.

CONCLUSION