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TPS2378: OR-ing two TPS2378 from different ethernet ports

Matthieu SPEDER
Prodigy 60 points
Part Number: TPS2378

I’m creating a PD which has two Ethernet ports.
Device will be able to receive PoE either by any of the two ports or by the two simultaneously (in that case load sharing would be expected).
Under normal condition device should consume around 0.8W. Short term (few seconds) spikes up to 4W are however possible and normal.

My initial idea was to implement this using 2 x TPS2378 and tying their RTN pins together to GND.

However by reading the two docs below, I understand this would probably not be the right solution (because of the back-bias) :

Here are my questions :

Q1/ Is inserting a diode between GND and RTN enough to achieve my requirement ? What are the required characteristics of these diodes ?

Q2/ From the app report slva625 , I understand an additional mosfet could be an alternative. This would increase complexity but maybe lead to a better efficiency. However compared to the app note, I don’t need in my case to always have both ports providing power, since one in enough. What is the correct way to wire the gate from the mosfet in that case ?

Q3/ In the case where both ports would be providing power, it seems 0.8W consumption could trigger the MPS disconnect. Is there a simple/efficient way to fix this without implementing the complete feedback loop with an optocoupler ?

Thanks !

Matthieu

  • 0
    TI__Mastermind 41790 points
    Hi Matthieu,

    Since 4W max is still within the standard for PoE, why not use one PoE PD?

    If current sharing is needed, you will need to implement two PDs and two converters that are sharing with current sharing controller on the feedback. Similar to PMP5236.
  • 0 in reply to Darwin Fernandez
    Prodigy 60 points
    Hi Darwin,

    Thanks a lot for taking the time to answer !

    Regarding the dual ports...

    There are in fact two points here :

    1/ in certain configurations, device will be chained (I'm talking about ethernet here, not PoE). In that configuration the device can be powered by a local AC adapter). So I definitively need two ethernet ports. For ease of use, I want to have similar behavior on the two ports, meaning, when their are used in a PoE config, I'd like to have the device powered indifferently by any of the two ports.

    2/ the idea behind having the two ports simultaneously receiving PoE was to have some kind of active redundancy where the failure of the first port would be someway transparent. This is desirable but not mandatory. Alternative is to have only the first port really drawing current and in case of failure the second port would start power negotiation. However this means I would have a temporary power disruption and the device will reboot.

    PMP5236 is clearly what I'd like to achieve. Thanks for pointing it, since I didn't saw it.
    However in my case, I have a strong space constraint and it is hardly possible to have two transformers since they are quite space consuming.
    If there is no way to power OR-ing the two ports before the transformer stage, what would be your recommendation to achieve at least point 1 ? Tying the two RTN together and rely on the bias to block 2nd port ? Or maybe some wiring between TPS2378 #1 and TPS2378 #2's APD to disable it ?

    Thanks again !

    Matthieu