BCM: The Ideal Power Component to Enable PoE

January 28, 2018

Power over Ethernet (PoE) refers to several standard and proprietary systems that use twisted pair Ethernet cabling to provide both power and a network connection. The technology allows for networked devices such as webcams, VoIP phones and wireless access points to operate without the need for separate cabling to carry mains electricity or a power distribution rail.

With twisted pair Ethernet cabling using thin conductors, current must be minimized to prevent a large voltage drop before reaching the powered device. IEEE 802.3af-2003, the original PoE standard, specified a voltage range of 44V to 57V at the output of the power sourcing equipment (the device that provides the power on the ethernet cable, for example a network switch). Voltage drops in the cable mean that the voltage at the powered device could drop as low as 37V. With a maximum current of 350mA, PoE could deliver 12.95W on each port.

In 2009, IEEE 802.3at-2009 was published, defining a PoE standard also known as PoE+ or PoE plus. This enhanced standard provides up to 25.5 W of power at the powered device, and limits the output voltage to a range of 50V to 57V.


The Ideal Power Component for PoE

Designing a PoE switch or router presents several challenges for the power engineer. Firstly, the equipment can operate in a range of different environments ranging from the heat of a data center rack to a cold outdoor installation. The components used in a PoE power system must be able to cope with a harsh environment.

BCM power over ethernet PoEEfficiency is usually very important as the system will need to keep the internal temperature as low as possible to enable a wide operating temperature. Dissipating heat can add both weight and cost to the product. The system must also operate efficiently over a wide range of load currents: it must work equally well when none of the Ethernet ports are providing power to a device as it does when all ports are supplying the maximum load.

The equipment must also be protected from problems associated with powering long runs of cable. It is important to isolate the switch or router from the cabling to prevent noise being transferred back into the system.

The ideal power component would therefore look like a transformer, although transformers don’t work with DC power.


The BCM: A DC-DC “Transformer” for PoE

Vicor’s Bus Controller Module (BCM) operates as if it were a DC-DC transformer. These components provide isolation, high efficiency and have a minimal voltage drop over a wide range of load currents. The BCM will also convert the input voltage at a fixed ratio, allowing PoE to be delivered from supply rails that do not fall into the range required by the standard, as well as offering versions with a 1:1 ratio that will provide isolation for existing PoE rails.


Application Example

Vicor has worked with a number of different IT system manufacturers to help them develop PoE solutions using the BCM. For more information, read our Power over Ethernet case study, or analyze the performance of the BCM in a PoE application on the PowerBench Whiteboard.

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