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I'm looking into doing a Bob Smith termination for a Power over Ethernet (POE) PSE. The TI app note shows the following termination: enter image description here

But all of TI's reference designs (and later figures in the same document) use the following termination with two ESD capacitor: enter image description here

Are these circuits equivalent? What is the purpose of having two ESD capacitors?

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  • \$\begingroup\$ I don't much like that as an ESD protection scheme, whether there are one or two caps. \$\endgroup\$
    – Matt Young
    Oct 16 '14 at 15:44
  • \$\begingroup\$ I'm not sure about the ESD aspects, but in the 2nd configuration, the 10 nF caps are doing essentially nothing, since 10 nF in series with 1 nF is just 0.9 nF. \$\endgroup\$
    – The Photon
    Oct 16 '14 at 16:11
  • \$\begingroup\$ FYI, TI's E2E forum might be a good place to ask this question. The relevant subforum is Power Interface. \$\endgroup\$
    – Null
    Oct 16 '14 at 18:17
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I found the following examples of POE termination from around the internet:

After posting the same question on TI's E2E forum, I got the following answer:

The majority of termination schemes will follow the examples you've cited and not the SLVU126 (TPS2384) and SLUU269 (TPS23841) TI EVM designs. These EVM designs will tend to have more reference evaluation circuits than any final product design would have. So, in this case a single high voltage ESD capacitor for each port.

The 10nF capacitors act as a DC block for the PoE port voltage and appear as a short at high frequencies. These capacitors are required on the ethernet pairs which carry the DC voltage but could be removed for the other pair (at least on the PSE end). On the PD end, the 10nF capacitors are required on each pair because the PD can receive DC voltage from either pair set. The 10nF capacitors can be lower cost, voltage and size.

Other good references are located within vendor datasheets who make magnetic's modules such as the Pulse PN JK0-0177NL which is used on http://www.ti.com/lit/pdf/sluuay8. In this case, a single HV ESD cap and 4x 22nF LV caps are used.

So, the second ESD 1nF capacitor is superfluous and I think diminishes the effects of the Bob Smith termination.

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  • \$\begingroup\$ All your 3 example links have broken over time \$\endgroup\$
    – Ferrybig
    Mar 11 '20 at 14:04

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