TPS23730EVM-093: Ground/Shield connection when using a PoE connector with integrated Magnetics

hello, 

First and foremost, any questions about how to connect your magnetics should be directed towards your magnetics vendor. Typically the datasheet of said magnetic part should include the proper grounding and terminations, but if not then they should be contacted. 

For the grounding scheme, it really depends on your application system. First thing to understand is that TI considers the "ground" of the input transformer as a Bob-Smith Plane. then, for isolated systems, there is a primary ground, which is what the IC connects to (like RTN and GND pins). Then there is a secondary ground which is the output ground of the DCDC, like the secondary side of the transformer. This secondary is where an MCU/CPU is, so that is typically where the TX/RX lines eventually go. 

So now that those different terms are understood, let's discuss the different options here. So first decision to make is if the Bob-Smith plane is going to connect to the secondary ground. Keeping them isolated helps reduce EMI since the radiated EMI from the cable can connect to a smaller bob-smith plane vs. both the Bob-Smith plane and the secondary ground. Keeping them isolated also helps with surge to keep the ground separated. However, when planes are isolated, there is an amount of leakage current called common mode current. To help filter it, it will require common mode capacitors (like 2kV caps) and common mode chokes. So it is a more expensive way to do things. 

Please check out this paper on EMI Compliant PoE Designs. It walks through a typical flyback implementation. 

The other decision you need to make is if you want the system to be connected to a true Earth Ground or a Cassis Ground. Some applications require this based on the end equipment or country of deployment. For example, I think nurse call stations in Chicago need to be connected to earth ground, but factory lights in West China probably do not. I don't know for sure, but you get the point I am trying to make?

Floating designs are easier to implement and reduce the chance of surge since it is floating and there is no path for the surge current to flow. But floating designs have higher risk of ESD since there can be a voltage potential between the device, the user and the earth ground. 

So once you decide if the system is somehow connected to Earth Ground or not, then you need to decide which grounds connect to this Earth Ground. Typically the Bob-Smith plane is always connected to this system Earth ground. Then the secondary is more dependent on the application. 

So I cannot tell you exactly what you need to do since I do not know all the ends and outs of every application space, but your design will either have bob-smith and secondary connected or isolated, and it will be connected to earth ground or floating. 

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

Regards, 

Michael P.

Applications Engineer

Texas Instruments 

You could use a digital signal isolator, but if that is cost prohibitive then you could connect the Bob Smith Plane and the secondary ground. 

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

Regards, 

Michael P.

Applications Engineer

Texas Instruments