Hi all,
I'm starting a new design of an ETHERNET SPD. I have looked over the internet a common design and I found that most of the designs use TVS diodes to the ground on each ETHERNET line. Also, some of the designs use a gas discharge tube between each ethernet pair.
( i have attached a diagram)


why do I need to use a TVS and gas discharge tube? TVS doesn't do the job( without gas discharge tube)?


thanks

 

GDTs can't be fused in a dangerous state, and can handle a much higher voltage and current than a diode. Consider the GDT as a backup (insurance) against liability.

 

The TVS diodes suppress differential transients - stopping spikes within the pairs of data lines. The balanced GDTs protect against common mode transients - those affecting both lines of a data pair simultaneously. Due to the fast data rates of Ethernet the TVS and GDT parts need to be very low capacitance (often sub-1 pF). TVS diodes should break over at just a bit more than the data swing voltage, they protect the "phy" chip. The GDT breakdown voltage may be near 100V as it protects the insulation of the coupling transformer.

 

The TVS diodes suppress differential transients - stopping spikes within the pairs of data lines. The balanced GDTs protect against common mode transients - those affecting both lines of a data pair simultaneously. Due to the fast data rates of Ethernet the TVS and GDT parts need to be very low capacitance (often sub-1 pF). TVS diodes should break over at just a bit more than the data swing voltage, they protect the "phy" chip. The GDT breakdown voltage may be near 100V as it protects the insulation of the coupling transformer.

thank you for your explanation

does the use of the inductor necessary ?
there are any low cost TVS and GDT that you can recommend to use in my design ?

 

The inductor helps to de-couple the capacitance of the TVS diode. Allows use of higher capacitance and/or beefier TVS parts. Probably not needed for 100Mb/s, likely needed for gigabit, definitely for anything faster. Sorry, can't recommend parts as I have not done Ethernet designs in the last few years.