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We are designing a PoE design to power our camera. The block diagram below is the basic design we are planning to implement.

enter image description here I want to get some advice to make sure we will not be damaging our camera or harming the user before going any further. From my understanding the PoE Magnetics will isolate the earth ground thus after the PoE Ethernet Magnetics, there is a different ground lets call it ground A. The PoE PD interface IC has a DCDC converter that will output 12V, I read that it is a good idea to add an isolation transformer after the output of the DCDC converter to keep anything downstream of the transformer protected.

If I add the isolation transformer then the ground to my camera is different from ground A, lets call it ground B.

I have two questions:

  1. The MDI Pairs will be going directly to the ethernet PHY on the camera. Does it not send ground A to ground B directly? Ground A and ground B will be the same because of the MDI pairs going to the PHY on the camera. If this is the case, would it not defeat the purpose of the isolation transformer?

  2. Our camera is mounted against a metal chassis that the user will touch when connecting power. What effects will this have on the user and the camera since the user would have a different ground than ground B. We want to make sure the camera does not get damaged.

Please feel free to provide any suggestions or improves to the design to better protect the camera. Thank you in advance for any help or suggestions. I really appreciate it!

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  • \$\begingroup\$ Ethernet lines are differential pairs, I am not sure why it would create a problem \$\endgroup\$ – Umar Aug 21 at 19:51
  • \$\begingroup\$ Hi Umar, the MDI differential pairs has a reference to ground A, but then directly connects to the PHY chip on the camera which references ground B. I am just wondering if they are the same ground then and if we need the isolation transformer after the DCDC converter. \$\endgroup\$ – k051819 Aug 21 at 20:52
  • \$\begingroup\$ The poe magnetics do not offer isolation for power but only for data, you know that right? \$\endgroup\$ – Jeroen3 Aug 21 at 20:54
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The MDI Pairs will be going directly to the ethernet PHY on the camera. Does it not send ground A to ground B directly? Ground A and ground B be the same because of the MDI pairs going to the PHY on the camera? If this is the case, would it not defeat the purpose of the isolation transformer?

The Ethernet magnetics can provide isolation, depending on the transformer you buy, I've seen some go up to 4.5kV. The MDI pairs are isolated from the cable at the ethernet transformer.

However, if the ground from the MDI pairs was connected to the same ground as the POE circuit before the isolation transformer, it would provide a direct pathway to the data lines.

If you isolate the data pair ground from the Powered Device (PD) ground and isolate the PD controller with the isolated DC to DC converter (as discussed in your post) then you should achieve the isolation you want. (in short, keep the PD controller on its own separate ground that is shared with the cable).

Our camera is mounted against a metal chassis that the user will touch when connecting power. What effects will this have on the user and the camera since the user would have a different ground than ground B. We want to make sure the camera does not get damaged.

You can use isolation or a plastic box. Tying the ground to the chassis isn't going to be a good idea, ESD needs a return pathway and sending it back thorugh the POE power lines is not good either (the ESD will then be on the power lines of the PD and PSE). This is why isolation is required.

Terminal side (DTE) - The IEEE 802.3af standard calls for 1500 VAC galvanic isolation between the link, the chassis ground and the DTE circuits. Since in many systems the power supply’s return is tied to the chassis ground, for enhanced EMI reduction, a conflict may occur. The required Isolation may be achieved using the following two methods: - Power that is fed over Ethernet is isolated from DTE circuits by an isolated DC/DC converter that withstands 1500 VRMS. - The terminal is housed in a non-conductive casing, with no external connections other than the RJ-45 data and power connector, resulting in the internal power supply input-output no longer needing to be electrically isolated.

Source: https://portal.chippc.com/support/downloads/files/PoE_and_IEEE802_3af.pdf

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