Another integrated PoE option

A project log for IoT Ethernet Sensor using Power over Ethernet

This solution has a number of unique benefits, a sensor platform with webserver, powered over Ethernet, accessible and programmed by browser

deandobdeandob 09/27/2015 at 07:340 Comments

I'm currently working on my Raspberry Pi Windows 10 IoT solution which will use a 5" IPS capacitive touch display on top of the Pi and have a peripherals board which will host a RGB24 to HDMI converter TFP401, audio amp, ambient light sensor, LED, IR receiver. But to power all that using PoE will exceed the 802.3af specs which is only good for about 5W. So I need to go to the more powerful 802.3at spec which can do up to 25w and will be enough to power the Pi and all the peripherals.

But the Silicon Labs PoE Si3402 as a fully integrated IC can't handle that much power through it, so I have to find another solution. Most of the large semi companies aren't interested in fully integrated PoE as they can't sell you as many ICs (eg. you need 2x diode bridges, a transient suppressor, PoE logic, switcher and mosfet to make a PoE solution) however this gets expensive and makes the circuitry complex needing more PCB real estate. So I'd still prefer an integrated solution and there is a small company called Akros who sell an integrated solution very similar to the Si3402 except it needs an external MOSFET as the switching dissipation is higher, the AS1124. It is currently out of production but ample can be found on aliexpress for about $2 each and also has a reference circuit for a buck converter (which avoids having to purchase transformers which are designed specifically for PoE chips and expensive) - so suits my needs perfectly.

I will still use the Si3402 and this design for small sensors but for the meantime I'll be concentrating on the Raspberry Pi solution.