I am designing a
power supply cicruit. The circuit will get it power from PoE (preferably) and
external adapter (7 - 12V). For the designing I have been researching online to understand PoE splitter design. From my current understanding I have designed a abstract circuit and want to valid my assumption before proceeding further.
Initially, I was planning to have just DC-DC converter and a full-wave rectifier to get the DC out of the
PoE but after reading this, I got to know PoE has some signalling mechanisms and I will be needing a
controller (or make the controller on my own using a cheap microcontroller) for signalling part of
IEEE 802.3af PoE standard.
The abstract design is below:
- Question Regarding Rectifier design using Diode.
I have been considering the diode design (after
magjack) as given in this answer. I have included design picture given in the answer.
I am considering the
Mode A and
Mode B only. I have heard about
4 pair mode in which power is applied on
Data pair and
Spare pair simultaneously. But I think
4-pair mode wouldn't be beneficial with the current diode design as DC+ applied at two pairs (it will reduce the resistance as well) because diode will block one of the pair and allow only DC+ and DC- from single pair to supply the current. Am I correct? How can I solve this issue with little complexity?
I am planning to use schottky diodes to reduce power wastage in the circuit.
- Question Regarding ORing power supply from PoE and Adapter.
I am planning to support
7-12V Adapter. I have placed diodes so that single power supply powers my circuit at a single time. To make
PoE preferable, the DC-DC converter for PoE outputs
13V and the external adaptor range would be
7-12V. Is my design correct? I have seen this design somewhere and I think it should work. I am not worried about the dropout with the diode but however worried with power dissipation in the diode (and rectifier diodes as well).
For the over-voltage protection, I am planning to use
cross-bar or simple 14V
zener diode circuit for over-voltage protection.
- Question Regarding MAGJACK
I want to use this magjack as I have few samples with me. I am not sure if it work with PoE as nothing has been mentioned in the datasheet. Please help. My doubts are If it can handle that much current (400 mA)?