TPS23841: Common mode noise on the PoE lines

Jagbir

Part Number: TPS23841

Hi,

I want to filter the common mode noise on the PoE lines by inserting a filter on the CAT 5A cable. I have designed filter circuit(shown in red rectangle) consisting of common mode chokes on each of the differential pairs. Now I got the filter board and if add this inline filter to the RJ45 cable, the PoE circuit stops working. What could be causing this issue.

Thanks,

Jagbir

11 months ago

0 Brandon Beader 11 months ago

TI__Genius 14645 points

Hi Jagbir,

Do you have I2C capability to communicate with TPS23841? This would enable to see the exact fault reporting.

Regards,

Brandon

0 Diang Xing 11 months ago in reply to Brandon Beader

TI__Mastermind 25041 points

Hi Jagbir,

Besides the fault report.

The PoE current for two wires per pair (like 1&2, or 3&6) is common-mode current. When you added CM choke the inductance will increase. It is recommended to have the CM choke after the data transformer or make sure your total wire + PD inductance less than 100 uH.

Best regards,

Diang

0 Jagbir 11 months ago in reply to Diang Xing

Expert 1525 points

I got the the idea to have the CM choke after the data transformer. I am using the following CM choke. the inductance is not listed on the datasheet.

https://www.murata.com/en-us/products/productdetail?partno=DLW21SN921SK2%23

0 Jagbir 11 months ago in reply to Brandon Beader

Expert 1525 points

No, I don't have an I2C interface.

0 Diang Xing 11 months ago in reply to Jagbir

TI__Mastermind 25041 points

Hi Jagbir,

Thanks for your updates.

We currently have a relative busy schedule and may replay later for the E2E. Sorry for the inconvenience.

Best regards,

Diang

0 Diang Xing 11 months ago in reply to Diang Xing

TI__Mastermind 25041 points

Hi Jagbir,

Looks like the inductance is about ~0.3 uH at 1MHz.

I would recommend you to do an ABA swap: using a different type of PSE and PD to check if this issue follows with PD or PSE. Besides, if you use a short Ethernet cable, will you still see this issue?

Best regards,

Diang

Because of the Thanksgiving holiday in the U.S., TI E2E™ design support forum responses may be delayed from November 25 through December 2. Thank you for your patience.

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TPS23841: Common mode noise on the PoE lines