The IEEE 802.3af Standard for PoE allows data terminal equipment (DTE) to receive power over the same cabling used for data in an Ethernet network. The Standard specifies the protocol for delivery of a nominal 48Vdc over unshielded twisted-pair cables (such as CAT-5). This eliminates the need for a local power source. The 802.3af Standard presents the requirements for providing and receiving power over the existing Ethernet cabling. It involves Power Sourcing Equipment (PSE) that provides the power on the cable for the Powered Device (PD).

PoE power sources support either the low voltage or higher voltage requirements for these systems. The low voltage power supplies operate with the PD and the higher voltage power supplies work with the PSE, as shown in the Figure.

One series of power modules provides an isolated, low-voltage power source to a remote PD in PoE applications. These modules incorporate all the necessary interface requirements to comply with IEEE Std. 802.3af, for managing the input power to the PD from the PoE Power Sourcing Equipment (PSE). This includes PD detection and PD classification current signatures required for the PSE. The modules are compatible with PD classifications, class 0 through class 3.

Besides a fully integrated dc-dc converter, this power module incorporates internal input diode bridges to support both Data Line and Spare Line pair standard Ethernet power connections, an integral transient suppressor for input over-voltage protection, and an EMI filter to ensure noise compatibility with Ethernet data signals. Other features include an input undervoltage lockout (UVLO), over-current and short-circuit protection, an output voltage adjust/trim, and over-temperature protection. An “Output Inhibit” control allows the output voltage to be turned off to support an idle condition or power saving mode. The output voltage options are for 3.3V, 5V, or 12V output. Its target applications are small low-power remote IP appliances, such as security card readers and cameras, test dongles, and IP phone consoles.

A higher voltage PSE power source is a dc-dc converter that operates over an input voltage range of 36VDC to 75VDC and provides regulated 52.5V output rated up to 3.8 amps (200 watts) of continuous output current. The output is fully isolated from the input, which allows a positive or negative output voltage configuration. The standard feature set includes remote on/off, remote sensing, undervoltage lockout, overtemperature protection, output overvoltage protection, output trim, and a dedicated pin that connects to the baseplate. The highly efficient topology and thermally-optimized construction allow this unit to provide high output current over a wide operating temperature range while maintaining a safe guardband of electrical and thermal component deratings. The addition of an external heat sink further increases the unit’s power handling capacity.