High Efficiency Transformers for PoE Applications available Off-the-Shelf

Applications include Voice over Internet Protocol (VoIP) phones, security cameras, web cameras, wireless routers, gaming machines, retail point of information systems, building access control systems, and battery chargers for mobile phones and PDAs.

The PoE series consists of 22 powered device side PoE transformers for 3, 7 and 13W output power levels. The Pulse transformers have 50 percent lower leakage than other PoE power transformers, reducing the need for inefficient snubber circuits and increasing overall power supply efficiency. Power supply efficiency is crucial for PoE applications because limited input power is available over the Ethernet cable. Higher efficiency transformers directly increase the amount of power that can be delivered to the powered device.

The Power Sourcing Unit (PSU) delivers a loosely regulated 48V (33 to 75V) input to the powered device over the Ethernet cable. The powered device converts the 48V input to a regulated output voltage at the required power level. The PoE standard allows the PD to request one of three levels or classes of power: 3W/Class 1, 7W/Class 2, 13W/Class 3.

Pulse has designed 3.3V, 5.0V, and 12.0V output voltage transformers at each of the three power levels. Three sizes of power transformers and three output voltage levels for each power class allow designers to select the smallest and most efficient package for the class of PoE device they are designing and to use standard, off-the-shelf components.
The transformers are designed for continuous mode flyback topologies operating between 200 and 400kHz. This topology has been chosen by all the major power IC vendors because it is the least expensive and produces 90%+ efficiency. The 1500Vrms isolation between primary and secondary windings meets the isolation voltage specification for PoE applications as defined in the IEEE 802.af standard. Each transformer has a primary side auxiliary winding to supply input power to the power IC, further reducing power loss.

These PoE series transformers maximize the usage of available height and minimize PCB footprint. The footprint of the 13W/Class 3 output transformers is 40% smaller than competitive packages. Industry standard footprints provide the opportunity to develop a second source, if required. The footprints are 13.2mm x 10.7mm for the EP7 (8-pin SMT, 3W/Class 1), 15.2mm x 12.7mm for the EP10 (10-pin SMT, 7W/Class 2), and 17.8mm x 12.7mm for the EP13 (10-pin SMT, 13W/Class 3) transformer.