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several questions for using PD/PSE

TS Yang
Genius 4800 points
Other Parts Discussed in Thread: TPS23861

Hi,

There are some questions about the PoE.

(1) What happens when the PD / PSE classes are not equal to each other?
- If PD is Class3 and PSE is Class4, does PSE send out the proper power after Detecting PD to Class3?
- If PD is Class4 and PSE is Class3, does PSE block the power supply to Detecting PD to Class4?
Or does it only supply up to 15W, the Class 3 Max value?
- Class 3 (PoE): 12.95W (Max 15.4W)
Class 4 (PoE +): 25.50W (Max 30.0W)

(2) What is the behavior when the PD is designed as Class 3, but momentarily consumes power exceeding Class 3?
- The current consumption of Max terminal is 12.55W (1.35A / 9V)
- Very close to the Max value of Class 3

(3) The Max3 power of Class3 is written as 15.4W. Is it possible to keep the consumed power constantly with Max?

(4) When power is supplied by PoE, what is the approximate percentage of power loss in the cable?
- Does Class 3 look like 12.95W in Max, considering power loss?

(5) Does we need any PoE related software work?
- If PoE is connected for the first time, is the connected device a PD supporting PoE?
Detecting to detect any class.
- Are there any changes to the SW? Or does it only work in HW?
(BB will develop PoE (PD) rather than PSE)

thanks,

TS

  • 0
    TI__Intellectual 1695 points
    Hi TS,

    (1) If the power of PSE is set higher than PD, it will apply power according to PD class result.
    If the PSE is set to class 3 but connected with a class 4 PD, there will be a power demotion for PSE. PSE will give PD 15.4W with 1-event class finger.
    Our TPS23861 is type 2 PSE, so we won't have this phenomenon for your question.


    (2)For class3 the ICUT limit is 374mA, If the port current of PD is over the ICUT limit, it will trigger the ICUT or ILIM, the PSE will count the time of the over current period, if the time is beyond the ICUT or ILIM timeout, the PSE will shut down the port. After a cold time, it will begin to detect the port again if working in auto or semi-auto mode.

    (3)It is possible depending on the port voltage and cable length, as long as the port current is lower than ICUT value, for class3 374mA, for class4 645mA, the port power can be higher than 12.95W.

    (4)This power loss is calculated according to standard 100m CAT5 cable, whose resistance is 12.5ohms. If you use shorter cable, the max power can be higher but not much. So the class3 power can be higher than 12.95W.

    (5) It depends on whether you need software to control PSE working in different modes like semi-auto or manual mode.
    If you don't need that, our TPS23861 has auto mode which don't need MCU to control it. It can power any valid PD automatically.
    You can refer to our TPS23861EVM, www.ti.com/.../TPS23861EVM-612
    It can work without software and also be controlled by software.

    Thank you
    Best Regards
    Jack Chen