Bob Smith Termination with POE

But all of TI's reference designs <EBW> Please provide the reference design link so I can look it up.

Are these circuits equivalent? What is the purpose of having a second ESD capacitor? Doesn't this break the current path between the two 75 Ohm resistors that are supposed to terminate the common mode signals? <EBW> Since it is an "AC" termination, the 1x and 2x ESD capacitor circuit is equivalent. But due to cost, most applications will only use one ESD capacitor.

Here are the two reference designs: http://www.ti.com/lit/pdf/slvu126  http://www.ti.com/lit/pdf/sluu269

What I'm confused about is that the addition of the 1nF capacitor in series with the 10nF DC blocking cap eliminates the need for the 10nF cap and lowers the total capacitance. What would be the rational for keeping the 10nF cap?

It seems to me that separating the lower node of the 75 Ohm resistors (as in the second schematics) puts two additional 1nF caps in the path of a common mode transients. Is that not correct?

Other examples of POE termination from around the internet seem to follow the first scheme. e.g.

• http://tinyurl.com/mrhv9xg (Figure 5)
• http://tinyurl.com/p5yozxh (Figure 4)
• http://tinyurl.com/mb9fdos (Figure 5)

Thanks Ali,

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.

Thanks Eric,

I understand. Do you think the addition of the second ESD capacitor diminishes the effect of the Bob Smith termination? 

No, the second ESD capacitor will not affect the termination but will add a bit of cost.