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