We opted to use the TI TPS23756 to be compliant with the standard 802.3at PoE+ (30W).
We used as reference the design of the evaluation board TPS23756EVM (http://www.ti.com/lit/ug/slvu329a/slvu329a.pdf)
Paragraph 6.2 page 9 of this user guide are reported the emissions measurements of the evaluation board.
We measured the emissions on the board developed by us obtaining very different results then the expected ones, so we brought to the test laboratory the development board to better understand the origin of the emissions and we found that the evaluation board has very similar issues to the ones encountered on our own developed board, such results differ considerably from the ones outlined in the user guide.
We tried to limit the polarising voltage Vc of the gates at 12V. We still have to understand the reason why, in our design, Vc is normally around 15-17V against 11-12V measured on the EVM.
As expected, after having measured EMC on the EVM, decreasing the gate voltage helps in reducing the emissions but it does not solve the problem.
The conclusion is that the TPS23756EVM has some failures at circuit level which causes the issues we’ve seen on the emissions.
We ask your support to verify the status of this board and give us the instructions needed on the fix to be implemented.