App note from Microchip about their MIC2145 boost switching regulator to do POE Flayback conversion. Link here (PDF)
The MIC2145 skip mode controller is used to implement a flyback converter that is intended for use in nonisolated, low power, POE applications. The circuit has a nominal 48V input and supports the POE voltage range of 36V to 57V. The output voltage is 2.5V at 350mA.