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[FAQ] PoE: Understanding Power Demotion

Daniel Campano
Intellectual 470 points
Part Number: TPS2372

Hello,

I'm understand the concept of demotion but I don't understand the Table 3 on the datasheet for TPS2372. On page 16 we can see Table 3. It states that a PSE type 3-4 can just allocate 12.95w on a type 4-8 PD. If a Type 3 PSE can supply  60w ( 51w at the PD), I don't understand why the PSE can just allocate 12.95w according to the table. I would agree with Table 3 if on the PSE column the first position was for Type 1 PSE, second position for Type 2 PSE and third position for Type 3 PSE. I'm probably missing something  and that is why I don't understand it , if somebody could explain it would be really appreciated. 

Regards,

Daniel.

  • 0
    TI__Expert 5615 points
    Greetings Daniel. Have you watched our video training on power demotion? That might help you. You can find the direct video link here --> training.ti.com/understanding-power-demotion-tis-ieee8023bt-powered-devices-pd . It is part of our larger IEEE 802.3bt training portal found here --> training.ti.com/power-over-ethernet-getting-started-new-standard .

    If that doesn't clear things up for you, our PoE PD Application Engineer Darwin Fernandez (the voice behind the video!) will assist you further on Monday.

    Thank you for considering PoE solutions from Texas Instruments,
  • 0 in reply to Thomas Lewis
    TI__Mastermind 41035 points
    Hi Daniel,

    I hope this video was helpful in answering your demotion question. If so, please click resolved for this thread. If not, I'd be happy to clarify any questions you may have. Thanks!

    Regards,
    Darwin
  • 0 in reply to Darwin Fernandez
    Intellectual 470 points

    Hi Darwin,

    Thanks for your help in advance. I understand the concept of demotion ( or at least that is what I think), it basically is when  a PD requests more power than the PSE can supply, then the PD can power up with power limitations and the TPH and TPL output a specific code to communicate to the PD application what to do. What I don't understand is why a PSE type 3-4 fall in demotion mode when PD class 4 is requesting power?, as a example. The minimum power available for a type 3 is 45w and a PD Class 4 requests 25.5w, so I don't understand why it is demotion?. I can see that the PSE send just 1 class cycle and it means that the PSE allocates as a maximum 12.95w, but why a type 3 PSE send just 1 class cycle?. If you could explain why this happens it would be very appreciated.

    Regards,

    Daniel.

  • 0 in reply to Daniel Campano
    TI__Mastermind 41035 points

    Hi Daniel,

    A PSE can be operated in a power-limited mode. For example, most multi-port PSE switches will have some power port management such that a Type 2 PSE can be power-limited because it can only budget 15W on one of its port. Therefore, it will only send 1 class finger and demote a Class 4 PD to Class 3 power levels. This is an example for the current IEEE802.3at standard.

    This behavior will extend to the new .bt standard in that a PSE switch with power management can power-limit a Type 4 PSE port to 60W if that's what it can budget in its current operating state. So a Class 8 PD will then get demoted to Class 6 when the port only delivers the 4 class cycles.

    To clarify Table 3, it is showing that a Type 3 or 4 PSE can demote a PD to a certain power level depending on the number of class cycles it delivers to the PD.

    Please let me know if this helps.

    Thanks! 

  • 0 in reply to Darwin Fernandez
    Intellectual 470 points

    Great help Darwin, It resolves my doubt, thanks.

    Regards,

    Daniel.