How can I make a PoE circuit that is compatible with 24V passive PoE and 802.3at active PoE?

Question

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?

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.