I'm designing a PoE circuit that should be compatible with both active (802.3af/at) PoE and also 24V passive PoE. How can I do this?

Can I just use a PD chip that doesn't include a DC-DC converter? It looks like most of them are low-side MOSFET switches that only switch on when the PoE negotiation has successfully happened. But these PD chips support external adapters:

  • TI TPS2379
  • AD LTC4265
  • ONsemi NCP1094

So maybe I could wire up the external input in some way to support passive PoE?

There are also plenty of integrated PD and DC-DC converter controller chips that are compatible with both active / standards-based PoE and external adapters. Maybe it would be possible that they can they be wired to work with non-standards based (passive) PoE?

  • TI TPS23754 / TPS23756
  • AD LTC4269 / LTC4278
  • Microsemi PD70201 (has an 'external enable' pin)
  • ONsemi NCP1083
  • Akros AS1135

Alternatively, maybe I can wire my own circuit that performs the PD classification, inrush current limiting, etc. Is that difficult?


1 Answer 1


802.3 PD devices are required to support mode A (1,2/3,6) or mode B (4,5/7,8). Passive POE is pretty much always provided on the spare pairs (4,5/7,8) for 10/100. The easiest solution to supporting both is to use the mode A pairs for 802.3 (1,2/3,6) and use the "spare" pairs for passive POE (4,5/7,8). That way no synchronization or control coupling is required between the two systems.

  • \$\begingroup\$ Do all 802.3af/at PSE devices support both mode A and mode B? \$\endgroup\$
    – Tom Bull
    Apr 18, 2018 at 18:46
  • \$\begingroup\$ My recollection of the standard is that power source devices can implement either but powered devices are required to support both. With the POE devices I have seen, only devices which support both passive (24V) poe and 802.3af/at don't support both mode A and B (e.g. Ubiquiti unifi UAP-AC-M access points). \$\endgroup\$ Apr 19, 2018 at 2:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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