Ethernet Magnetic Module: Guide to Specs & Selection

An Ethernet magnetic module (also called LAN magnetics) sits between the Ethernet PHY and the RJ45/cable and provides galvanic isolation, differential coupling, and common-mode noise suppression. Correct magnetics selection—matching OCL, insertion/return loss, isolation rating and footprint—prevents link instability, EMI issues and safety test failures.

This is an authoritative guide to Ethernet magnetic modules: functions, key specs (350µH OCL, ~1500 Vrms isolation), 10/100 vs 1G differences, layout and selection checklist.

★​ What an Ethernet Magnetic Module Does?

An Ethernet magnetic module performs three tightly related roles:

1. Galvanic isolation. It creates a safety barrier between the cable (MDI) and digital logic, protecting devices and users from surges and meeting safety test voltages. Industry practice and IEEE guidance typically require an isolation withstand test on the port — commonly expressed as ~1500 Vrms for 60 s or equivalent impulse tests.
2. Differential coupling & impedance matching. Transformers provide the center-tapped differential coupling required by Ethernet PHYs and help shape the channel so the PHY meets return-loss and mask requirements.
3. Common-mode noise suppression. Integrated common-mode chokes (CMCs) reduce differential-to-common conversion and limit radiated emissions from twisted pair cables, improving EMC performance.

These roles are interdependent: isolation choices influence winding insulation and creepage; OCL and CMC parameters affect low-frequency behavior and EMI; footprint and pinout determine whether a part can be a drop-in replacement.

★ Key Specifications of Ethernet Magnetic Module

Below are the attributes that engineering teams and procurement use to compare and qualify magnetics. Treat these as the minimum checklist for any selection or replacement decision.

Electrical specifications

Practical note: When comparing datasheets, ensure OCL test frequency, voltage, and bias current match — these variables change the measured inductance substantially.

Mechanical and Package Specifications

• Package type: SMD-16P, integrated RJ45 + magnetics, or discrete through-hole.
• Body dimensions & seated height: Important for chassis clearance and mating connectors.
• Pinout & footprint: Pin compatibility is essential for drop-in replacements; verify recommended land pattern and pad dimensions.

Environmental, Materials & Compliance

• Operating / storage temperature ranges (commercial vs industrial).
• RoHS & halogen-free status and peak reflow rating (e.g., 255 ±5 °C typical for RoHS parts).
• Lifecycle / availability: For long-lifecycle products, verify manufacturer support and obsolescence policies.

 ★ 10/100Base-T vs. 1000Base-T LAN Magnetics — Core Differences

Understanding these differences avoids costly mistakes:

• Signal bandwidth & pair count. 1000Base-T uses four pairs simultaneously and operates at higher symbol rates, so magnetics must meet tighter return-loss and crosstalk masks. 10/100 designs are lower bandwidth and often tolerate higher OCL values.
• Common-mode choke integration and performance. Gigabit modules typically require CMCs with stricter impedance across wider bands to control pair-to-pair coupling and meet EMC. 10/100 modules have simpler CMC needs.
• Interoperability. A 1000Base-T magnetics assembly can often satisfy 10/100 requirements electrically, but may be more expensive. Conversely, a 10/100 magnetics assembly is usually unsuitable for gigabit operation. Validate with PHY vendor guidelines and lab testing.

When to choose which: Use 10/100 magnetics for cost-sensitive Fast Ethernet devices; use 1000Base-T magnetics for switches, uplinks and products where full gigabit throughput is required.
 

★ Why OCL Matters and How to Read its Spec

Open-Circuit Inductance (OCL) is the transformer's primary inductance measured with the secondary open. For 10/100Base-T designs, a higher OCL (commonly ≈350 µH minimum under IEEE test conventions) ensures the magnetics provide enough low-frequency energy storage to prevent baseline wander and droop during long frames. Baseline wander and droop affect receiver tracking and can lead to increased BER if unchecked.

Key reading tips:

• Check test conditions. OCL is often given at a specific test frequency, voltage and DC bias; different labs report different numbers.
• Look at OCL vs bias curve. OCL falls with increased unbalanced bias current — manufacturers often plot OCL across bias levels; examine worst-case values that apply in your system.

★ Common-mode Chokes (CMC) — Selection and PoE Considerations

A CMC is a core element of Ethernet magnetics. It provides high impedance to common-mode currents while allowing the desired differential signal to pass. When selecting CMCs, pay attention to:

• Impedance vs frequency curve — ensures suppression in the problem frequency band.
• DC saturation rating — critical for PoE applications where DC current flows through center taps and can bias/saturate the choke, reducing CMRR.
• Insertion loss and thermal performance — high currents (PoE+) create heat; parts must be derated or verified under expected PSE current.

 ★ Ethernet Magnetic Module Compatibility & Replacement

When a product page claims “equivalent” or “drop-in replacement,” follow this checklist before approving the substitution:

1. Pinout & footprint match. Any mismatch here can force a PCB redesign.
2. Turns ratio & center-tap connections. Confirm center-tap usage matches PHY biasing.
3. OCL and insertion/return loss parity. Ensure equal or better electrical performance — and confirm test conditions match.
4. Hi-Pot / isolation margin. Safety ratings must equal or exceed original. ﹘1500 Vrms is a common reference.
5. Thermal and DC bias behavior (PoE). Validate DC saturation and thermal derating under PoE currents.

Practical workflow: compare datasheets line-by-line, request samples, run PHY link stability, BER and EMC pre-scans on the target board before volume replacement.

★ Ethernet Magnetic Module PCB layout

Good layout avoids defeating the magnetics you just picked:

• Keep a GND keepout beneath the magnetics body where recommended — this preserves the choke’s common-mode performance and reduces unintended mode conversion. Follow PHY vendor application notes and magnetics datasheet guidance.
• Minimize stub lengths from PHY to magnetics — stubs increase reflections and can break return-loss masks. This is especially important for gigabit designs.
• Route center taps correctly — typically to the DC bias network (Vcc or bias resistors) and decoupling per PHY reference.
• Thermal and creepage planning for PoE: maintain sufficient creepage/clearance and verify thermal rise when PoE currents flow.

★ Testing & validation checklist 

Before approving a magnetics part for production, run these checks:

• PHY link test: link up at required speeds across representative cables and lengths.
• BER / stress test: sustained data transfer and long frames to reveal baseline wander issues.
• Return-loss / insertion-loss sweep: validate against PHY masks or vendor application notes.
• Hi-Pot / insulation test: verify isolation withstand levels per target standard.
• EMC pre-scan: quick radiated and conducted checks to spot obvious failures.
• PoE thermal & DC saturation test: if PoE/PoE+ applies, verify CMC saturation and temperature rise under full PSE current.

★ FAQ About LAN Magnetic Module

Q – What does OCL mean and why is 350 µH specified?

A – OCL (open-circuit inductance) is the inductance measured on a primary with the secondary open. In 100Base-T normative guidance, ~350 µH minimum (under specified test conditions) helps control baseline wander and guarantee receiver tracking for long frames.

Q – Is 1500 Vrms isolation required?

A – IEEE guidance and referenced safety standards commonly use 1500 Vrms (60 s) or equivalent impulse tests as a target isolation test for Ethernet ports; designers should confirm the version of the applicable standard for their product category.

Q – Can I use a gigabit magnetics part in a fast Ethernet design?

A – Yes, electrically a gigabit part usually meets or exceeds 10/100 masks, but it may be costlier and its footprint/pinout must be compatible. Verify vendor guidance and test in your system.

Q – How do I verify a claimed “equivalent” part?

A – Line-by-line datasheet comparison, sample testing (PHY, BER, EMC), and pinout validation are required. Marketing claims alone are insufficient.

Quick selection checklist 

1. Confirm required speed (10/100 vs 1G).
2. Match turns ratio and center-tap scheme.
3. Verify OCL and test conditions (350 µH min for many 100Base-T cases).
4. Check insertion & return loss across the PHY frequency band.
5. Confirm isolation (Hi-Pot) rating (~1500 Vrms target).
6. Validate footprint/pinout and package height.
7. For PoE, check CMC DC saturation and thermal behavior.
8. Request samples and run PHY + EMC pretests.

Conclusion

Choosing the right Ethernet magnetic module is a design decision that combines electrical performance, safety and mechanical compatibility. Use OCL, insertion/return loss, isolation rating and pinout as your primary gates; validate claims with datasheets and sample testing on your actual PHY and board layout.

download the datasheet, request a footprint file, or order engineering samples to run PHY/BER and EMC pre-validation on your target board.