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PoE+ grounding/isolation conundrum

Sammy Peiren62
Intellectual 310 points
Other Parts Discussed in Thread: TPS2378

Hello all. For one of our customers, we are working on a system that has both PoE+ and an AC input as an option to power the system. There is a bit of confusion regarding the isolation requirements because the standard is imho not very clear on this. I was wondering if anyone here is familiar enough with the standards to help us out.


First of these are the relevant specs of our system:

  • We have two available variants, both using the same system and enclosure but a different power supply.
    • Version A has a power supply with PoE+ and 24-36VAC input options
    • Version B has a power supply with 240VAC input only
  • The system has a metal enclosure
  • We are using the TPS2378 PD controller
  • We are using an isolated DC-DC converter that is connected to both the PoE+ input and rectified AC input

Here is a rough diagram of both situations:

So my exact question is: what grounds should be connected and where do I need to isolate grounds? For safety reasons it seems logical that the metal enclosure (SHIELD) should be connected to EARTH. For EMC it seems best to connect the negative output of the DC/DC converter (SYSTEM GND) to the metal enclosure, however the PoE+ standard requires 1500V isolation between the PoE+ inputs and the rest of the system. Lastly there is also the metal shielding on the RJ45 connector.

It's also not entirely clear to me if the PoE+ pairs can be connected 'in parallel' to the rectified AC signal because this would create a path between the AC input and the PoE+ input.

  • 0
    TI__Mastermind 41800 points

    Hi Sammy,

    It is best to connect the shielding to earth ground as it will present to lowest resistance path for unwanted energy (ESD etc) from reaching any circuitry inside the enclosure and might damage it.

    As for the system ground, note that the PoE EMI performance should be better when left floating as common mode noise does not have a return path through the chassis (if secondary connects to chassis).

    I recommend reading the below app note that discusses this on page 9.

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Intellectual 310 points

    Thanks Darwin,


    The application note was helpful, however I'm wondering if radiated noise won't be worse in the case of a floating output. We are using a metal enclosure which van be grounded, however the front uses a glass plate which is only partially covered by the LCD shielding.

  • 0 in reply to Sammy Peiren62
    TI__Mastermind 41800 points
    Hi Sammy,

    The earth ground shielded enclosure will help a lot. You can also try adding ferrite beads which help more at higher frequencies to mitigate the higher frequency noise on the floating output.

    Having good margin at the high frequency end of the conducted emissions is a good initial test to see how low the radiated noise from the PD will be.