TPS2372: 802.3bt with 100BaseTX topology

Hello Bruce,

So POE++ and 802.3.bt are not the same standard. You mention both in your post, which one are you designing for?

The TPS2372 is a powered device, so i cannot comment on the availability/options of integrated magnetics for the data. 

As for the requirement, 802.3.bt is meant to support 100baseTX. When you say you want to tie the pairs together, where do you intend to tie them?

The pairs are tied together after the rectifier. 

if you mean to tie them together before sending the power over the wire, that would result in more power loss in the cable and probably require a higher class. I am not sure if  PSE can even support that. .bt PSE's can support two pair if it is sending .at or .af power levels..

Additionally, why could you not send data over two pairs and power over four?

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards, 

Michael P.

Applications Engineer

Texas Instruments 

Michael,

I have read documents online that equate POE++ and the IEEE standard as the same. 

(ex: www.analog.com/.../jumpstarting-ieee-802-3bts-poe.html)

I take it that POE++ is used to describe anything above the 802.3at power levels - 802.3bt being a specific standard.

I uploaded a diagram that shows what I am describing. In Type 2, 2 pairs are used to send power. Two pair are for data and power, and the other pairs are "spare" and are tied together at the RJ45 Jack. -  all pairs  are rectified.

In Type 3, all 2 or 4 pairs carry power otionally, and all designs I have seen terminate at the PD side at transformers before being rectified. In a 100BaseT system these transformers are not used to pass data so they only there to hook up the "spare" pairs.  

My example depicts sending data over two pairs and power over all pairs as you pointed out. How these should be terminated is my question.

Also, gicen a 4 pair power system, I am trying to understand how the currents travel through each pair and at what level. 

I did not mean to say that 802.3bt  and POE++ are the "same". And I do not mean to question your position on this. I wished to point out that the term POE++ has been commonly used to refer to 802.3bt. 

"Q: What is PoE++? A: PoE++ (IEEE 802.3bt Type 3 and Type 4) is a form of Power over Ethernet in which 49 watts (Type 3) or 96 watts (Type 4) of power is transmitted over all 4 pairs of conductors in a category cable. This standard is currently in development."

Yes, it's outdated, but this is a common statement I am seeing online as I do my research. 

The confusion in the industry is understandable since POE and POE+ describe 802.3af and 802.3at - it appears that earlier articles use POE++ as a way to describe anything that might be higher power - before the bt standard and yet there is a lot of material out there that links POE++ with the bt standard - which is actually "4PPOE" while POE++ refers to Cisco's "UPOE" (i think). Often one cannot tell when an article was written. This is quite unfortunate. To the extent that nonstandard implementations may be compatible with 802.3bt, I regret using the term POE++ in this discussion - I should have used "4PPOE" instead. 

FYI, I am not new to POE. I have designed several PD products that are currently on the market that fall under 802.3 Type 1 and 2 range, but I have not designed a PD or PSE POE product of Type 3 or 4, so I am adding this knowledge to my previous experience and wish to apply it to my current 50W project.    

I believe you may have answered my question regarding the extras transformers - All fours pairs need to be balanced for proper detection. 

That brings up a couple other questions:

- If only 2 pairs are used for power and data in type 1 and 2, then why are the "spare pairs" rectified if they do not carry any current - or might they?

-  I am seeing all 802.3bt design examples where all 4 pairs are being used for data transmission while 802.3af/at design examples seem to use only 2 pairs. I assume this difference is in regard to the ethernet physical layer used: 2 pair for 100Base-Tx and 4 pair for Gigabit Ethernet. In my application, I only need two actual transformers for 100Base-TX data isolation - in a 4 element package, the other two are only used for impedance balancing so I am wondering if I can get by with a cheaper fast ethernet LAN transformer and use cheaper/smaller inductors to balance the other pairs. Might this be compatible in a network?  

Like this.

Thanks

Bruce

The Wiki article on Power over Ethernet has a table that lists 802.3bt and "4PPOE" and  "POE++" in the same header.

If this is incorrect, someone needs to change the Wiki.