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I am getting confused in selecting TVS diode for Ethernet Interface.

Should I connect one bidirectional TVS diode across a pair (one pin connecting to D+ and other to D-)? I think if transient is coupled to the Ethernet cable, then the two wires of a pair will be having same transient and it doesn't make any sense to connect a bidirectional diode across a pair.

Should I connect one TVS diode for each wire of the pair (other end of the diode connected to Ground)? Should it be bidirectional or unidirectional?

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Ethernet is not that different from USB. Consider two of Littlefuse PGB1010603NR from Digikey, one on each signal to Gnd. These are found on Arduino boards, good for USB 2.0 speeds. Keep the signal path on the board from connector to receiver chip short. https://www.littelfuse.com/data/en/data_sheets/littelfuse_pulseguard-esd_pgb1.pdf

PulseGuard ESD Suppressors help protect sensitive electronic equipment against electrostatic discharge (ESD). They supplement the on-chip protection of integrated circuitry and are best suited for low-voltage, high-speed applications where low capacitance is important. Data ports utilizing such high-speed protocols as USB 2.0, IEEE1394, HDMI and DVI can benefit from this new technology.

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    \$\begingroup\$ Except it is different. Ethernet is transformer isolated which means you can clamp on the secondary and then you need smaller rated TVS. This is assuming the primary can float to the strike potential \$\endgroup\$ – JonRB Dec 17 '18 at 22:56
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The different boards I worked on have the TVS diodes between PHY and transformer and not on cable side.

That said, there are several TVS diode design possible. You can put a single TVS diode to the ground on each line or use more advanced design proposed by manufacturers: Semtech RClamp2594N, Protek SRV05-4, etc.

Single TVS on each line can be a challenge to implement on a 4 differential pairs design, so manufacturer developped specific package that are soldered "in-line". For uni or bidirectional, choose the second one if you're not familiar with TVS diodes.

The main parameter to look at is the capacitance which is usually under 4-5pF for Gigabit Ethernet.

Analog Front End (AFE) of PHY can be different between manufacturer and series and may not react in the same way to TVS diode and sometime to specific TVS diode series. On one design we needed to use some resistor trick to make it work.

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