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TPS2373-4EVM-758: How to determine the minimum capacitance of C28 of the EVM?

Part Number: TPS2373-4EVM-758

Tool/software:

Hi,

Good day.

I am currently designing a POE product with TPS2373-4+UCC2897ARGPR and encountered a lack of space to place the electrolytic capacitor. 

So, could you please guide me in determining the minimum capacitance of C28 of the EVM?

Sincerely

webber

  • Hi,

    Our PoE team is currently out of office and will be back tomorrow. Sorry for the delay.,

    BR,

    Ke Wang

  • Hi Webber

    802.3BT 最worst的250us transient 没有input current,加上100m线 + PSE最低输出电压 + PD 满载load。这时为了不让PSE - PD port关断我们推荐如下的cap。但实际情况这个很难发生,很多设计不会用这么大的cap。

    Power                  Bulk Cap

    13W                      10-22uF

    25W                      33-47uF

    50W                      68-100uF

    70W                      100uF-200uF

    802.3BT还有个和Cport相关的最低要求。考虑到4-paris是两个port,可能需要Bulk >= 40uF for Class-7,8。其余的Cap要求就是和DC/DC相关了。

    Best regards,

    Diang

  • HI Diang,

    感謝解釋.
    請問EVM C28的目的是用於儲存電流,避免當PD抽重載導致PSE關閉電源?

    如果是,有方式可以計算我需要多少電容才能達到這目的?

    謝謝

    webber

  • Hi Webber,

    正常工作时C28是为了稳定DC/DC input电压,保证DC/DC在优化的输入电压下运行。这时和SW 250kHz频率、start up时给Vout电容充电也有关系。

    PSE-PD inrush Cport需要40uF。如果有250us PSE-PD 断电transient,C28是为了独立提供能量保证PD不落到低于undervoltage (36V)的情况。这时需要100uF-200uF的cap。

    Best regards,

    Diang