The IEEE 802.3af Power over Ethernet (PoE) standard specifies the delivery of a nominal 48 V at up to 350 mA over Cat 3 or higher cabling. The equipment that transmits the 48-V power is described as power-sourcing equipment (PSE), and the network devices that receive this dc power are referred to as powered devices (PDs). PSE controller ICs based on the standard perform PD detection, classification, port turn on, fault monitoring and power disconnect.
These chips are typically multichannel controllers, most commonly quad controllers, and are conventionally designed into large 12-, 24- or 48-port systems, requiring more than one IC. Some controllers run autonomously, but most applications use a microcontroller to step through the IEEE functions. A microcontroller is also used to read back from the PSE controller the port’s status and PD class. This information is then interpreted by the microcontroller for operations such as user interfacing and power management.
Multichannel controllers are suitable for large multiple-port equipment, either midspans that inject power into systems with legacy switches or, more frequently, endspan equipment such as switches, routers or hubs with the PSE function built in. However, there are many applications that only require a single port or a just a few ports with PoE capability. For example, in environments where PoE is not available, a single-port PSE powered from the ac outlet would allow users to power PDs. And in applications where a PD requires less than the allotted 12.95 W per port, a single-port PSE can serve as a “power forwarder,” taking the unused power and processing it to power a second PD.
PoE Power Classification
Before exploring the single-port PSE applications, it’s necessary to review the IEEE 802.3af specification as it relates to power classification. A PSE port is required to output 15.4 W for a Class 3 PD (Table), so providing full power to each port requires a large supply (15.4 W 3 24 ports = 370 W). To address this potential problem (i.e., the need for oversizing the power supply), the IEEE 802.3af standard gives the option for a PD to provide the PSE with power class information based on the maximum power the PD will ever consume. This allows a reduction in the size of the power supply with the use of a microcontroller performing power management.
Fig. 1 shows an example of a basic power-management operation. After setting the initial available power and detecting a PD, the power class is read by the PSE and then compared to the available power. If there is enough power for the PD, the port is turned on and power is subtracted from the available power. When the PD is disconnected, the port is turned off and power is added back to the available power.By keeping track of the power allocated at each port according to the PD’s power class, the power remaining determines if there is enough left to power up additional PDs. If a fault occurs at a port or an insufficient amount of power is left, preventing the PD power on, this status can be read back through a user interface if one is available.
Single-Port PSE Controller
In many applications, a single port or just a few ports with PoE capability are required. For these applications, one of the multichannel controllers is typically used, and unused channels are left floating. However, Linear Technology’s LTC4263, an autonomous single-channel PSE controller, permits an alternative approach in such applications (Fig. 2).
The LTC4263 provides fully compliant IEEE 802.3af PD detection, classification, port turn on, fault monitoring, and ac or dc disconnect sensing. In addition, an efficient LED driver provides the user with port status, indicating port on or faults through a single LED. Selectable options include a midspan back-off timer, legacy detection and power class enforcement set by tying their respective pin to VSS or VDD5. The LTC4263 features simple power management, programmed with an RC that works across multiple LTC4263s in a multiple-port application. An analog approach is used through a common PWRMGT pin with multiple LTC4263s to perform the power management described previously. The key advantage of the LTC4263 is its independent operation without the need for a microcontroller.
In addition to eliminating the microcontroller and its peripheral components, the LTC4263 also has an onboard power MOSFET that properly switches the negative rail, and a built-in ac disconnect signal and digital supply. Fig. 2 shows a minimum component circuit for operation with the LTC4263. An isolated 48-V supply comes in and provides power to the LTC4263. Bypass capacitors are placed at the input supply pin VDD48 as well as the internally generated 5-V supply brought out at VDD5. The output lines, the 48-V positive rail and the switched negative rail, go out to the RJ-45 port connection. If going through data pairs, these lines are connected to the center taps of the Ethernet transformer. Otherwise, they are connected directly to the spare pairs. A 58-V transient voltage suppressor (TVS) and a 0.1-F capacitor help protect against transients, overvoltage and reverse voltage at the port. An optional 1-A fuse can also be used to meet safety requirements.
The low-component-count solution allows for a layout area smaller than a single RJ-45 connector. Another layout advantage of the single-port PSE over multichannel PSEs is the parts placement. The individual controller circuits can be placed behind or close to the Ethernet port as opposed to long traces to meet at the shared controller. A 58-V transient voltage suppressor can also be placed close to the controller it is designed to protect. Furthermore, a simplified layout helps meet isolation requirements mentioned in the IEEE 802.3af standard, an area sometimes overlooked in PSE design.
Separating the controller and MOSFET to the individual ports also benefits the PSE thermally. Space between components provides better thermal management than multichannel controller solutions, where the power paths are close to each other. The payoffs are most apparent in areas where the PSE is enclosed in a tight space such as an integrated RJ-45 connector.
Other options (not shown in Fig. 2) are the LED connection and ac disconnect. An LED with a resistor to set the current goes to the LED pin. The LED driver can also act as a switching current source to increase power efficiency, only needing an additional inductor and diode. In applications where ac disconnect is needed in place of dc disconnect, the LTC4263 configuration is changed simply with a capacitor at the OSC pin to set the internal oscillator, an RC at ACOUT to apply the ac signal to the port, and an ac blocking diode in the negative power path.
Portable devices have different voltage and power requirements. Devices that are PoE capable (PDs) have their own dc-dc conversion built in and thus do not rely on connecting to the correct-voltage wall adapter. When traveling with such devices, a wall adapter is no longer needed; only an Ethernet cable is required. However, in situations where PoE is unavailable, but power is needed for a device such as a battery charger, a simple wall-adapter PSE could be handy to plug into a wall outlet to output PoE power to a PD. Such a device plugs directly into a wall socket. Power goes through an ac-dc converter to provide the PoE supply. This simple device does not require any smarts and only needs to provide power. The LTC4263 provides the PSE control function, enabling safe detection and power on of the PD. Users can also read out an LED driven by the LTC4263 for basic faults should a non-PoE device connect to this wall adapter.
If you need more than one port on a wall adapter, you can use multiple LTC4263s to implement a PoE wall-adapter expander. Power management can be provided by the LTC4263s and set according to the power limits from the supply. This would limit the number of ports of particular power classes that are powered on. Fig. 3 shows a three-port wall adapter configured for 30 W of available power for PD power on. Various combinations of PD power classes can be powered on as long as the set available power is not exceeded. For example, if a Class 3 PD is first powered on, the 15.4-W allocated power for the device leaves enough power for Class 1 or 2 PDs.
In the multiport solution, each port acts independently of the other ports, with the exception of power management. This is also useful for cases where the ports need to be isolated from each other. Here, a port would have its own isolated supply and the power management would not be used.
The wall-adapter PSE does not have to be limited to power. This device can be permanently installed in the wall and connect to an Ethernet data line from a legacy (non-PoE) switch and run through the wall. A PD plugged to the wall port can then receive both data and power. The wall adapter can have an assigned address, which is helpful information in an emergency when a call is made from a VoIP phone connected to that location. A response team would immediately know the location of that call.
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.
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.