Single-Port PSEs Put Power in More Places

Mar 23, 2007 4:06 PM
By Dilian Reyes, Applications Engineer, Linear Technology, Milpitas, Calif.

PoE Power Forwarder

A low-power Class 1 VoIP phone consumes less than 3.84 W of power. However, if configured for a Class 3 PD, the maximum power at the PD input is 12.95 W. The PSE would reserve the full port power for the device leaving close to 9 W of unused power at the PD. This unused power can go toward additional phone features such as a camera for video conferencing. Here in the phone, an LTC4263 is used as a power forwarder to the extension device (Fig. 4).

Initially, a PSE detects, classifies and powers on the device through the PD interface controller (shown with the LTC4257-1 in Fig. 4). The PD is set for Class 3 with a resistor at RCLASS. Power is used for the phone and the remaining power passes through a dc-dc converter to boost the voltage (dropped due to cable loss and interfacing) backup to the correct PSE output voltage. The LTC4263, powered up by this new voltage, provides the detection and safe power on of the extension device. Power to the camera is applied on the spare pairs while data passes through the Ethernet transformer on the data pairs.

Since less than 9 W is available for the new device, the power-forwarder PSE only has enough power available for a Class 1 (4-W) or Class 2 (7-W) device. Power management of the LTC4263 is then set for 8 W by sizing the resistor at the PWRMGT pin. This allows for either of these two class-type PDs to turn on, while a Class 3 (15.4 W) is denied power. The design of the extension device must meet either Class 1 or Class 2 power. With its class enforcement enabled, the LTC4263 will remove power to the extension device should the PD violate its power class. The power-forwarder device itself has to ensure that its power does not exceed the equivalent of a Class 1 PD. To remain on, the total power, consisting of the power-forwarder device power plus the extension device power, must not exceed a Class 3 PD.

High-Power Solutions

The 12.95-W power level is suitable for many PD applications. But, there is an ongoing need for devices with higher power. Wireless access points with additional radios can use 18 W or more. Cameras with pan, tilt and zoom (PTZ) can consume 60 W. In these cases, the limitation of the PoE PD to 12.95 W is inadequate. This conservative power level was initially selected for the standard, but there is increasing demand for more power for many devices that connect to Ethernet lines.

The IEEE has formed a committee to define a new high-power PoE standard. In the meantime, high-power PoE is already being implemented in prestandard systems. An extension of the LTC4263 is a high-power version, LTC4263-1, that is used in high power pre-standard solutions. This PSE controller provides the same PD detection and port power on and off, but with current limits that exceed those set in the IEEE 802.3af standard, nearly doubling port power on two pairs.

The standard was defined to work with Ethernet products at the time. But with the higher currents, one must also pay attention to devices in series with the power paths. One example is the Ethernet transformer. A special transformer must be used that can account not only for the higher power, but also any imbalance in the power lines.
Fig. 5 shows a high-power PSE, four-pair option, in a gigabit Ethernet system. On the first set of data pairs, the LTC4263-1 controls the power to a high-power PD. On the other end, a high-power PD interface controller (LTC4264) receives the power and passes it through to a dc-dc converter.

An option for more power is to have an additional LTC4263-1 on the second set of data pairs and an additional LTC4264 with its own dc-dc converter on the receiving end. The two dc-dc converters are then summed together, providing the current balancing to account for differences in resistance of the transformer, cable and connects, as well as diode drop differences.

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© 2007 Penton Media, Inc.