well, I've designed PoE supplies for work. The problem is that it's NOT just a resistor between a few wires.
There are a multitude of PoE types, AND classes (I think 5 if I'm remembering) in each PoE type.
If I'm not mistaken (been a few years) there are 3 types of PoE. There's PoE (14.7W), High Power (27W), Ultra High Power (I don't know the name of this one but...) (which is just the 27W on all 8 wires...so 41ish due to losses across multiple pairs and how the standard asks you to re-combine the DC voltage).
Basically, you need to have a dynamic resistance, that is only there when it's needed, during the initial negotiating period. This tells the HUB what class you are in. It tells the hub how much power to budget for you (as the hubs can NOT produce enough power for the highest power class over all ports).
So, you need a controller. It's generally a chip that can negotiate with the PoE hub to tell it how much power it needs. It also needs to control WHEN the power supply is ON...as well as needs to know what to do when it is plugged into a NON-PoE port.
Then you need a buck converter. Realistically, anything that will buck the voltage down to what you want will work (for most applications).
The history (as I understand it), is that PoE came out, but 14.7watts (it's a strange number due to the line loss at 48V over the max ethernet distance of 24AWG twisted pair). People that DID use it wanted more. Initially designs were there to support the power over the "dead" lines. This left Gigabit out in the cold...but who used Gigabit? Then Gigabit started to get popular. So, engineers devised a different way to get the DC offset out of the data (remember Ethernet is current based, not voltage based). So, the power supplies had to get smarter.
Linear Tech has a good design as well as Maxim-IC. I can't really say much about them, as work is what work is...but I've used them and would suggest you head that route. You might be able to get a reference design board from Linear Tech for not too much (pre-build board). If you go the Maxim-IC route, they also have reference design boards...but you'll need two. The Controller and power supply. The Linear Tech one is an all in one circuit.
Now to cost...why does PoE cost so much? It's because of HOW you have to inject and how you have to pull the power out of the line without disturbing the communications. PLUS you need to be 100% isolated (due to FCC rules on lightning, etc.). There are ways you can cheapen it up if you ignore some of those...or bend some of them (you can ignore some of them if you plug into a HUB that acts as your isolation barrier)...which will SIGNIFICANTLY increase your efficiency, lower cost and lower boards space.
Keep in mind, the hub has to know IF something is PoE, how much power to give it, has to be isolated, has to know what to do if something IS PoE and it's unplugged, then plugged in, then (you get the idea)...or if you plug into a PoE device, then plug into a legacy device! The circuits need to be very smart. The smarter the circuit, the more it costs...
As for the other end...the reason the power supplies cost so much, is they can't bend any rules...they don't know WHAT you are plugging into. So, from an end-point...point of view, they can't assume anything!
Anywhoo...it's not just a matter of "sucking" the 48VDC off the line for real PoE. That's why (at least for what we are mostly doing)...if you didn't already OWN the PoE hub...I would have recommended just applying the necessary voltage across the dead pairs with a "fake" power injector (there is a decent write-up on the Homeseer forums). Not really PoE...but hey, they are spare wires...use them if you aren't doing Gigabit. The problem is, if you are doing Gigabit...you have to use REAL PoE!
One last note...the reason things cost so much...most of the parts / power supplies etc. that you would use for this are expensive due to QTY pricing. There are TONS and TONS of parts that are rated for 12/24 V input. Not so many for 48V (needs to be rated for 96V input that are rated for lightning strikes).
HTH!
--Dan
