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TPS23756EVM: Hot Swappable

Genius 5355 points
Other Parts Discussed in Thread: TPS23756, TPS23756EVM, TPS23754

Hi Support,

From my understanding, the TPS23756 only features hot swappable between its POE power and its external power adapter only.

Can you help to advise whether the TPS23756EVM circuit also supports hot swap amount different class 4 LAN cables?

Lastly, please help to advise how the TPS23756EVM circuit will behave with other class POE circuit outputs, like class 3, 2 and 1.

Thanks.

  • Hi Support,

    Any feedback?

    Thanks.

  • Hi Shaun,

    The TPS23756 can be considered a hotswap for PoE. There is some bulk cap at the input of the converter so when the PSE port turns on to 48V, the inrush current can be quite high. However, the PD controller mitigates the inrush with its inrush current limit and also has its own normal operation current limit and protection during faults. This is similar functionality to a hotswap. The reason that I say hotswap for PoE is that the IEEE802.3at standard has specific specs on the sourcing side (slew rates etc) in which the PD is designed around these specs to protect. This includes all Classes 0-4. As such the PD will behave the same way for each class during inrush and normal operation.

    The thing that would be different is during classification (based on the classification resistor). The PSE will change its own current limit after determining the class current of the PD.

    For aux solutions, the aux input bypasses the PD controller (it connects between VDD and RTN of the IC). In this configuration, there is no protection (inrush/current limit) from the PD controller. If using the PPD feature of the TPS23756, the aux can be inserted at the PD controller front end (VDD and VSS). In this configuration, you will have the protection features of the PD controller. This is described on page 33 of the datasheet.

    Regards,
    Darwin
  • Hi Darwin,

    Found a problem during testing of the TPS23756 EVL Board circuit:

     

    When I supply power from both external adapter(24V, 3A) and Ethernet power(55V, 2A) to the TPS23756 EVL board, the EVL board will take the power from external adapter by default with my E-load(5V, 3A).

    But when I turned off the power from the adapter (24V, 3A) with the Ethernet power still on, there will be a 1 to 2 seconds delay for the power to be swapped from adapter back to PoE power.

    This will cause the output power to be totally shut down for 1 to 2 seconds. May I know if there is any solution for this?

     

    Thanks.

  • Hi Support,

    Any feedback?

    Thanks.

  • Hi Shaun,

    It is VERY difficult to implement smooth transition successfully especially with 24V since the voltage difference between the PoE and adapter will cause a high inrush of the bulk cap to transition back up to PoE voltage. The conditions must be limited to do this type of application. The best circuit to try and implement is what is shown on page 33 of the datasheet. It discusses a smooth transition circuit with a 48V adapter.

    Regards,
    Darwin
  • Hi Darwin,

    Thanks.

    The TPS23756 datasheet says I can use a diode to isolate VDD1 from VDD so that the reference design supports PoE priority operation.

    Does this mean when I plug in both PoE power source and external adapter, the system will use the power from PoE first?

    Please advise what is the part number for this diode.

     

    Thanks, Shaun.

  • Hi Darwin,

    In addition, for the TPS23756 reference board design, please advise the difference in using D21 alone vs. the whole replacement efficient diode circuit.

    Does that mean I can use D21 to replace the whole circuit which consists of R27, R28, C27, Q6, R30, Q7, R32 and D23? (The reference design does not mount D21) What will be the efficiency different between these two solutions?

    Thanks.

  • Hi Shaun,

    That's correct adding the diode between VDD and VDD1 will help block the adapter voltage from the PoE circuitry such that the PSE can performance detection and classification successfully. A typical diode used is B1100 like in the TPS23754 EVM. This will work well with adapter voltages lower than PoE voltage like 12-24V adatpers. If 48V adapters are being used, there will be some extra logic circuitry to achieve PoE priority as described on page 7 from below app note:
    www.ti.com/lit/an/slva306a/slva306a.pdf

    For "Efficient diode" circuit, you are correct, this was used to avoid any diode voltage drop on the already low adapter voltage capability of 12V. Yes, this can be replaced with the D21. The efficiency drop would be ~1-2% less at the adapter with a diode rather than the FET circuit.

    Regards,
    Darwin
  • Hi Support,

    For the TI POE reference design circuit, can I get a 5.5V at the output?

    I may have some voltage drop at the switching circuit and I do not want that to affect my USB voltage input.

     

    Thanks.

  • Hi Shaun,

    Yes you can try slightly decreasing the lower feedback resistor to increase the output voltage to 5.5V.