An Ethernet transformer, also known as LAN magnetics or network isolation transformer, is a small passive magnetic component mounted between the Ethernet PHY chip and RJ45 connector on network PCB boards. Unlike power transformers such as site transformers, its turns ratio is normally 1:1. It does not step up or step down power voltage; it transmits high-speed differential network signals through magnetic coupling.
Galvanic isolation is its primary function. It cuts off DC and low-frequency current paths between internal circuit and external twisted pair cable. It isolates ground potential difference between interconnected devices, eliminates ground loop damage, and provides high voltage isolation, typically 1500Vrms isolation rating, to protect PHY chips from surge, ESD and lightning induced transient voltage.
It performs impedance matching to match the 100Ω differential characteristic impedance of Ethernet twisted pair, reduces signal reflection and guarantees signal integrity for long-distance transmission. It also contains integrated common-mode choke to suppress EMI interference on network lines, improves communication stability and reduces packet loss.
Ethernet transformers are widely used in switches, routers, industrial PLC, gateway, servo drive, PoE equipment and all Ethernet-enabled devices. It can be packaged as standalone SMD magnetic modules or integrated inside shielded RJ45 jacks.
