• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 40 subscribers
  • Views 294 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TIDA-010083: Physical trace and plane separation to ensure sufficient electrical isolation

Jorgen Dahlback
Prodigy 130 points
Part Number: TIDA-010083

Hi,

I'm trying to get my head around the isolation requirements for PoE, and was wondering if you could help out with some comments on the design choices in component selections and layout for the TIDA-010083 reference design.

We're working on a design with a metal enclosure, where there is only one user accessible port, the RJ45. We're using a shielded RJ45 connector.

My understanding is that we need to ensure proper isolation between the chassis ground (and PI) and the ethernet signals carrying power. Based on the safety specification, we've been using 3mm as the isolation barrier distance between PGND / PoE input power and digital GND. But, I'm a bit confused about the layout around the RJ45 in the reference design. The connector GND is filled underneath the connector in L2 and L3, and then the ethernet pairs carrying power are routed on top/bottom in the same area. Wouldn't this mean that the distance from the input ethernet pairs with power to chassis GND is too small for the 1500V isolation requirement?

Also, you've opted to use 1206 size 1nF-2kV capacitors between the Bob Smith terminations and the connector GND, as well as 1210 size 2.2nF-2kV caps between PGND and digital GND. This would also lead to quite a bit smaller isolation distance than 3mm. We have used a 1812 size capacitor to get the required physical distance.

What is the thinking behind the component selections and routing distances? Are we being overly cautious?

Is the reference design relying on having a non-conductive housing, or can the RJ45 be connected to the chassis?

Hoping to hear from you soon.

Best regards,

Jørgen

  • 0
    TI__Intellectual 1005 points

    Hi Jørgen,

    to be honest I'm not an expert on isolation requirements, however here are my thoughts on that topic.

    As for my understanding PoE specifies to have an isolation between Ethernet and any connection to the outer world. This means if you have any connection to any other system it has to be isolated. This can either be achieved by isolating power and data on the Ethernet side, or by isolating the connection to the other system.

    As you don't have any connector other than the RJ45 for Ethernet, you don't need the isolation at all. Only relevant in that case should be the isolation to your housing.

    Design related questions:

    The GND plane underneath the RJ45 connector and transformer is there to have the correct impedance for the signals. The electric strength of a typical FR4 material (Isola IS420) is specified with 54 kV/mm. The distance between 1st and 2nd layer is specified with 0.23 mm, so it should withstand roughly 12 kV. So I don't see this as a problem.

    The capacitors: are chosen to be 1206 just because of the size, to guarantee the 1500 V they probably should be coated.

    I would not connect the RJ45 connectors shield to the chassis. By doing that it would not be isolated anymore and if connecting the devices to earth on both sides you would have a large ground loop. A lot of Ethernet RJ45 cables also have no shield at all, so you can not rely on that.

    This design is not meant to have a housing but is also not a finished product, it should shown the technical capabilities and provide access to the components for testing.

    I hope this helps you.

    Best regards,

    Steffen