Ethernet & LAN Transformers

In this context, both terms refer to signal magnetics for copper Ethernet interfaces. Still distinguish discrete transformers, RJ45 connectors with integrated magnetics and PoE power transformers. This page is for selecting discrete LAN transformers.

Matching the rate is only an initial filter. Check the PHY requirements, ports, PoE, temperature, internal connections and PCB footprint. Start with the rate for a new design; check the footprint and pinout alongside it when replacing an existing part. Share the PHY or current part number to help us narrow the options.

There is no universal connection for every PHY. Distinguish PHY-side and cable-side center taps and follow the target PHY reference circuit, transformer schematic and PoE architecture for connections and decoupling. Do not copy another PHY’s circuit without checking it. Share your PHY and schematic for an FAE review.

PoE-capable Ethernet magnetics provide signal coupling and isolation while accommodating the port’s power path under specified connection and current conditions. A PoE power transformer serves the associated DC-DC power conversion stage. They are not interchangeable. Tell us the target PHY, PSE/PD role and power requirements.

Those two dimensions do not establish compatibility. Check row spacing, footprint, pin functions, center taps, turns ratio, magnetic structure and electrical limits. FAE can review the existing part and identify differences requiring validation. This page does not provide an unchecked drop-in replacement verdict.

Align frequency range, test voltage, temperature, DC bias and Min/Max/Typ definitions before comparing the relevant limits. Read negative-valued or formula-based return-loss specifications using the datasheet’s convention. An OCL test’s DC bias is not a PoE current rating. Use the complete datasheets when the comparison conditions are incomplete.

A discrete design lets you select the magnetics and RJ45 jack separately. A MagJack integrates magnetics into the connector. Choose based on PCB space, interface construction, PHY requirements, PoE and assembly constraints—not an assumption that either approach always has better signals or lower cost. Do not add a separate transformer to an already magnetically integrated interface without reviewing the design.

For a known part, send the part number, quantity and destination to request samples or pricing. If no part is selected, start with the application, target PHY or existing component. A sales engineer handles the request and involves FAE for matching or customization. Stock, price and lead time are confirmed per part and order; custom designs also require electrical, dimensional and validation requirements.