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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?

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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.

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  • \$\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

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