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TPS23754-1: Regarding TPS23754-1(TPS23754PWPR-1) 802.3at POE design question

shermen fang
Prodigy 120 points
Part Number: TPS23754-1
Other Parts Discussed in Thread: PMP6812, TPS23754

Tool/software:

Hi Sir,

According to previous TPS23754-1(TPS23754PWPR-1) WAPAX09 project POE issue we raised by another thread, we create this new thread to discuss more questions.

Based on WAPAX09 TPS23754-1 schematic and TI PMP6812 schematic, you had provided the Cap calculation formula shown below. 

Using I=Cdv/dt with I=25.5W/42.5V=0.6A, dv=42.5-36=6.5V and dt=250usec results in C=~23.1uF

Power Bulk Cap
13W 10-22uF
25W 33-47uF
50W 68-100uF
70W 100uF-200uF

25.5W 推薦33-47 uF.   22uF 比較worst的250us 0-current transient不行，但99.9%的情况影響不大

From your reply and 145.3.8.6 PD behavior during transient at the PSE PI information, we have some questions.

1.It shows PSE PI has to maintainan output voltage and the voltage transient  longer than 30us and less than 250us to PD.

As you said above, 22uF cap is not appropriate for voltage transient that is 250us condition.So, you suggest 33-47 uF for 25.5W.

Is it correct?

2. As you said above,what is the meaning of this 但99.9%的情况影響不大? Did you mean 22uF can meet 250us 0-current voltage transient?

Is it correct?

3. If 22uF is appropriate for 250us 0-current voltage transient, why TI suggest 33-47 uF? We are confused.

Please help to explain more information for us to realize.

4.How can we measure the 250us 0-current voltage transient? Is it at detection procedure?

Can we avoid 250us 0-current voltage transient happened? 

5. As you mentioned, If the PD cannot pass the handshake, can you double check all caps between VDD-VSS is less than 120nF?

How can we check all caps value? It is calculate by schematic or how can we check the 120nF? Please provide the guideance and method.

6. As you mentioned, If the PD is connected then power off, it may be related to the loss of maintain power signature. Could you try with a load > 1W?

Where we have to add a load >1W? please based on attached WAP-AX09 schematic to provide the guideance how to implement.

thanks

TPS23754-1_PSE voltage transient to PD question20250304.pptx

  • 0
    TI__Mastermind 37071 points

    Hi shermen,

    PoE team is doing internal tests and the reply on E2E may be delayed for 3 - 5 business days. Sorry for the delay and thanks for your patience.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 120 points

    Hi Sir,

    We don't realize below calculation formula. Please help to provide the detail definition and explain what is the definition and theorem. thanks

    Using I=Cdv/dt with I=25.5W/42.5V=0.6A, dv=42.5-36=6.5V and dt=250usec results in C=~23.1uF

  • 0 in reply to shermen fang
    TI__Mastermind 37071 points

    Hi Shermen,

    PoE team is doing internal tests and the reply on E2E may be delayed for 3 - 5 business days. Sorry for the delay and thanks for your patience.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 37071 points

    Hi Shermen,

     I=Cdv/dt with I=25.5W/42.5V=0.6A, dv=42.5-36=6.5V and dt=250usec results in C=~23.1uF

    dt = 250us from

    36V from

    42.5V from

    25.5W from

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 120 points

    Hi Diang,

    We have an importnat question for  C22 cap value is 22uF in TI PMP6812 schematic reference design.

    Our customer is asked why TI PMP6812 schematic reference design is adopted 22uF.

     I=Cdv/dt with I=25.5W/42.5V=0.6A, dv=42.5-36=6.5V and dt=250usec results in C=~23.1uF

    Form this equation, the calculation result C is ~23.iuF.  

    Since a ceramic cap may have DC bias which could has effective lower cap when voltage is high. Did yiu consider that for above 

     calculation result?

    Please help to advise.

    BTW, please proivde you wechat contact information, we would like to discuss with you by wechat. Can you provide?

    thanks

  • 0 in reply to shermen fang
    TI__Mastermind 37071 points

    Hi shermen,

    Thanks for reply. We will get back to you in 3-7 business days. Thanks for your patience.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 37071 points

    Hi shermen,

    Thanks for your patience. 

    The calculation does not consider capacitance changes with higher voltage (DC bias).

    I think other parts of your concerns about caps are answered in "">e2e.ti.com/.../5715484

    We could not provide personal contact in E2E but if you still have questions I can reach out in email. 

    Best regards,
    Diang