I am currently designing a battery powered LED project, that will be charged and/or operated over 25W PoE+. In my current design, I am using a flyback converter to convert the 48V to 12V and for establishing galvanic isolation. For optimisation purposes, I would really like to kick out that chunky and expensive flyback transformer and replace it by a non isolated DC/DC converter. However, in all >25W PoE PD reference designs I saw from IC manufacturers, they are always using isolated converters.

My product has a plastic housing and is not grounded nor is it further connected to any other device that is grounded. Only in case of mechanical damage (e.g. housing cracked open) will the 48V be exposed to the environment.

Another issue could be my onboard battery charger IC, which bucks the 12V from the PoE flyback converter output further down to charge a 2S Li-Ion Pack. The IC has a max input voltage of 24V, so in case of failure of a non isolated DC/DC, 48V will be applied to charger IC and damage it.

I already read another post about isolated vs non isolated PoE but I did not come to a conclusion for my project.

  • Is there anything I missed why an isolated PoE converter would still be necessary or the better choice for my application?
  • If yes, could you please give an example for this failure case?
  • I have not seen any standard that PoE with higher power requires galvanic isolation. Am I wrong about that?

If this is just for a personal project you will probably be fine.

Ethernet wiring has to be isolated because the cables get run across buildings and to other buildings Each side of the buildings ground can be many volts different than the other side. Different buildings can have even more difference in potential in the grounds. Not having isolation can blow up devices on either side.

  • \$\begingroup\$ It will not be only a personal project, it will most likely become a real product. However, i am more interested if additional isolation on PD side is necessary and what are the failure modes. I am also aware of ethernet wiring isolation (magnetics) but the real question is: do i have to isolate the 48v from the PD to the rest of the device? \$\endgroup\$ – F. Heisenberg Apr 16 '20 at 12:11

Looks like isolation is a requirement: http://embeddedpowerlabs.com/Downloads/APEC_2006_PoE_Electrical_Isolation_Requirements.pdf

This electrical isolation shall withstand at least one of the following electrical strength tests:

a) 1500V rms steady-state at 50-60Hz for 60 seconds applied as specified in subclause 6.2 of IEC 60950-1:2001

IEEE link

  • \$\begingroup\$ I am aware of this paper, but it does not state clearly if isolation is required on PD side. In general an isolation is required on PSE side. \$\endgroup\$ – F. Heisenberg Apr 16 '20 at 5:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.