TPS23753A: PoE power loop with diode bridge

Hello Chin,

My name is Michael and I am one of the applications engineers for PoE. 

Thank you very much for your question. The Ethernet Alliance created a standard for power delivery other ethernet CAT cables. The standard can be found on their website: https://ethernetalliance.org by searching for "IEEE802.3 bt standard"

(Note you might need to create an account to view the PDF)

A summary of the standard can be found on our website, "Whitepaper: Overview of the 802.3bt Power over Ethernet Standard" on page 40 it will give the pair sets. 

https://training.ti.com/power-over-ethernet-poe-training-series

In this standard they outline PSE pinout configuration and permissible power supply polarity on page 40. If you do not follow this standard, then the device will need a specialized power sourcing equipment, and neither will be IEEE802.3bt compliant. 

If you would like a reference for how it is done according to the standard, any of our EVMs or TI reference designs can be used to compare. Below I provided a PDF link to one of our schematics that includes the diode bridge input:

Please let me know if you have any questions, or if you feel that this solved your question please indicate to us by clicking the resolved button. Thank you!

Regards,

Michael Pahl 

Hello Chen,

Thank you for the clarification. I thought you were mixing the power pairings, but I see now that you will only be using a 2-pair solution. 

So if the diode bridges are integrated, then the power connection 2 would theoretically have better thermal performance since the power through each bridge would be split in half. However, if the bridge is not integrated, then there is no improvement since it is the same amount of power being dissipated through the same components. 

My only concern is that you would need to ensure that the PSE only powers the correct two pairs every time. My understanding is that it should be checked/designed, and do not assume the power will be delivered that way. Other than that, I do not have any other concerns!

Regards,

Michael Pahl 

Hello Chin,

Ethernet cables have four pairs of wires. Usually one of the pairs is colored (green, blue, orange, brown) and the other is white. They are twisted together in the cable shielding. Power can be delivered with two pairs, or with all four pairs of wires. Since you are using two, we call it a 2-pair solution. 

If you would like to know more about PoE, I would highly encourage you to visit our Training Series webpage:

https://training.ti.com/power-over-ethernet-poe-training-series

Or our Support & Training Page:

http://www.ti.com/power-management/power-over-ethernet-poe/support-training.html

Both have videos and links to useful information pages such as blogs or whitepapers. 

Regards,

Michael Pahl 

Hi Michael,

Thanks for the clarification.

Because we design TPS23753A solution on our PD and the solution only support IEEE 802.3at type_1, so the power path is 2-pair.(1&2+3&6 or 4&5+7&8).

It means we will not use 4-pair power path because our PD doesn't support IEEE 802.3bt.

So any concern of 2-pair power path of connection_2 of attached schematic to improve thermal performance on integrated diode bridge?

Thanks!

Chin

4174.PoE power connection_1.docx

Hello Chin,

Correct, IEEE802.3at only supports 2-pair.

No concerns from me at this point.

Regards,

Michael Pahl