Most copper twisted-pair Ethernet ports use magnetics between the Ethernet PHY and the cable interface. That small transformer module or integrated RJ45 connector is not just a passive part on the bill of materials. Ethernet magnetics help provide galvanic isolation, support the 100 ohm differential signal path, reduce common-mode noise, and protect the device side from cable-side electrical stress.
Ethernet magnetics selection should start before layout is complete. The right part depends on the PHY, speed tier, PoE class, isolation rating, electrical specs, package, temperature range, and whether the design needs discrete LAN magnetics or an RJ45 connector with integrated magnetics.
Allied Components supplies both LAN / Telecom Magnetics and RJ45 connectors with integrated magnetics, so the right path depends on whether your design needs maximum electrical and layout control or a compact Ethernet front end with fewer line items.
| Design question | What to check | Why it matters | Product category to review |
|---|---|---|---|
| What speed does the PHY support? | 10/100, 1000BASE-T, 2.5G, 5G, or 10G | Speed changes bandwidth, insertion loss, return loss, and crosstalk requirements. | LAN / Telecom Magnetics |
| Do you need a compact connector and magnetics module? | RJ45 with integrated magnetics | Reduces BOM count and PCB area. | RJ45 with Integrated Magnetics |
| Do you need topology flexibility? | Discrete LAN magnetics plus separate RJ45 | Lets you tune magnetics and jack choices independently. | LAN / Telecom Magnetics plus RJ45 connectors without magnetics |
| Does the port carry PoE? | PoE, PoE+, or PoE++ class | Changes center-tap routing, DCR, current handling, thermal rise, and saturation margin. | PoE, PoE+, or PoE++ options |
| Is EMC margin tight? | CMC, CMRR, layout, shield strategy | Magnetics and layout both affect radiated and conducted emissions. | Start with parts that include CMC or shielding options. |
What Ethernet Magnetics Do In The Signal Chain
In a typical copper Ethernet port, the signal path runs from the Ethernet PHY to the magnetics, then to the RJ45 connector and twisted-pair cable. The magnetics sit at the media-dependent interface, where board-level signals meet the outside cable environment.
The magnetics usually perform three jobs:
- Provide galvanic isolation between the device circuitry and cable-side wiring.
- Couple differential Ethernet signals through isolation transformers.
- Help suppress common-mode noise, often with integrated common-mode chokes.
IEEE 802.3 isolation testing is commonly expressed as 1500 V rms for 60 seconds, 2250 V dc for 60 seconds, or an equivalent impulse test using ten 2400 V impulses of alternating polarity. When you compare a LAN magnetic or RJ45 integrated-magnetics module, check the actual isolation or HiPot rating on the datasheet instead of assuming all Ethernet parts are equivalent.
For more background on why the RJ45 interface uses magnetic coupling, see Allied's related post on magnetic coupling of RJ45 sockets.
Discrete LAN Magnetics vs RJ45 With Integrated Magnetics
One of the first Ethernet magnetics selection decisions is architecture. You can use a discrete LAN magnetic with a separate RJ45 jack, or choose an RJ45 connector with the magnetics integrated into the connector body.
Neither option is automatically better. The decision depends on board space, qualification plan, sourcing flexibility, EMC margin, rework strategy, and the mechanical constraints around the enclosure opening.
| Factor | Discrete LAN magnetics plus separate RJ45 | RJ45 with integrated magnetics |
|---|---|---|
| PCB area | Uses more board area because the magnetic module and connector are separate. | Reduces the Ethernet front-end footprint. |
| BOM count | More line items and more placement decisions. | Fewer line items and a simpler front-end BOM. |
| Layout control | More control over magnetics placement, CMC strategy, and differential-pair routing. | Less flexible, but often faster to implement. |
| Rework | Magnetics and jack can be replaced independently during debug or production test. | Connector and magnetics are replaced as one module. |
| EMC tuning | More room to tune common-mode filtering and placement. | Metal shielding may help, but compact internal cores can reduce some margins. |
| Sourcing | Easier to swap the jack or magnetics independently if one part becomes constrained. | One qualified integrated module handles the connector and magnetics together. |
| Product category | LAN / Telecom Magnetics plus RJ45 connectors without magnetics. | RJ45 with Integrated Magnetics. |
Use discrete LAN magnetics when the PHY vendor recommends a specific topology, when EMC margin is tight, when the enclosure forces a specific RJ45 jack, or when the supply strategy benefits from keeping the connector and magnetics independent.
Use RJ45 with integrated magnetics when board area is limited, BOM count needs to be reduced, port density is high, or the project needs a faster path from schematic to qualified Ethernet front end.
Ethernet Magnetics Selection Flow
The safest workflow is to start with the PHY and narrow the part list step by step.
- Identify the Ethernet speed tier: 10BASE-T, 100BASE-TX, 1000BASE-T, 2.5GBASE-T, 5GBASE-T, or 10GBASE-T.
- Decide whether the design needs discrete LAN magnetics or an RJ45 connector with integrated magnetics.
- Read the PHY vendor's magnetics recommendation before choosing by speed label alone.
- Match the PoE class, center-tap routing, current path, and thermal requirement.
- Verify isolation rating and safety requirement.
- Compare OCL, insertion loss, return loss, leakage inductance, interwinding capacitance, DCR, crosstalk, and common-mode rejection.
- Check the package, port count, shield, LED, temperature, and layout constraints.
- Shortlist Allied parts by speed, PoE tier, package, and architecture.
Datasheet Specs To Check
Do not compare Ethernet magnetics by product title alone. A "10/100/1000" label is a starting point, not a full design check. The PHY datasheet and the magnetics datasheet need to agree on topology and test conditions.
| Spec | What to look for | Why it matters | Notes |
|---|---|---|---|
| Isolation / HiPot | 1500 V rms for 60 seconds, 2250 V dc for 60 seconds, or equivalent impulse rating | Confirms the isolation barrier between cable side and device side. | Verify the exact compliance requirement for the product. |
| Turns ratio | Often 1 CT:1 CT for common 10/100/1000 copper ports | Matches the PHY and MDI interface requirement. | Follow the PHY vendor recommendation. |
| OCL | 350 uH minimum in common PHY-vendor checklists | Supports low-frequency response and baseline stability. | Compare OCL only when voltage, frequency, bias current, and temperature conditions match. |
| OCL test condition | Example condition: 100 mV, 100 kHz, 8 mA bias | Prevents misleading comparisons between datasheets. | Treat this as PHY-vendor guidance, not a universal value for every Ethernet design. |
| Insertion loss | Around 1 dB maximum in referenced PHY checklists | Preserves signal amplitude through the magnetic interface. | Frequency range depends on the PHY and speed tier. |
| Leakage inductance | 0.4 uH maximum in one Microchip 10/100 reference table | Affects high-frequency response and signal integrity. | Check the exact PHY reference design. |
| Interwinding capacitance | 12 pF maximum in one Microchip reference table | Lower capacitance reduces common-mode noise coupling across the isolation barrier. | Lower is generally better, but compare within the same topology. |
| DCR | 0.9 ohm maximum in one Microchip reference table | Higher resistance increases voltage drop and heat, especially in PoE designs. | Check per winding or per pair, not just a headline number. |
| CMRR / common-mode rejection | Compare dB values across frequency | Helps EMC margin and cable-side noise rejection. | Useful when the cable is long, noisy, or exposed. |
| Operating temperature | Commercial or industrial range, such as 0C to 70C or -40C to 85C | Industrial and outdoor-adjacent equipment often needs more margin. | Match the system environment, not just the prototype bench. |
The most common mistake is comparing two magnetics by a single spec while ignoring test conditions. A 350 uH OCL value measured at one bias current is not the same as a 350 uH value measured under another condition. The same caution applies to insertion loss, return loss, crosstalk, and common-mode rejection because the relevant frequency range changes with Ethernet speed.
PoE, PoE+, And PoE++ Considerations
PoE changes Ethernet magnetics selection because power is no longer separate from the data interface. The magnetics and connector path must tolerate the current, center-tap routing, DCR, thermal rise, and saturation conditions created by the selected PoE class.
| PoE type / class | PSE power | PD power | Magnetics implication | Product category to review |
|---|---|---|---|---|
| PoE / Type 1, Class 1 to 3 | 4 W to 15.4 W | 3.84 W to 13 W | Lower power, but still verify center taps, DCR, and isolation. | PoE LAN magnetics |
| PoE+ / Type 2, Class 4 | 30 W | 25.5 W | More current and thermal load than basic PoE. | PoE+ LAN magnetics |
| PoE++ / Type 3, Class 5 to 6 | 45 W to 60 W | 40 W to 51 W | Higher current, often with 4-pair power delivery. | PoE++ LAN magnetics |
| PoE++ / Type 4, Class 7 to 8 | 75 W to 90 W | 62 W to 71.3 W | Highest thermal and current stress. Verify rating carefully. | PoE++ LAN magnetics |
Do not choose the Ethernet front end from a non-PoE part list and assume the PoE controller will solve the rest. For PoE and PoE+ designs, DCR and heat rise become more important. For PoE++ designs, 4-pair power delivery can raise the thermal burden across the connector, magnetics, and PCB routing.
If the reader needs a basic PoE refresher before selecting parts, Allied's post on PoE for high power applications is a useful companion.
PHY Recommendations And Layout Constraints
The PHY datasheet should be the first technical reference, not the last. It will usually define the transformer ratio, center-tap connections, common-mode choke recommendation, termination strategy, and any required relationship between pairs.
Layout also affects whether the selected magnetics perform as expected. PHY-vendor layout guidance commonly warns designers to keep the magnetics, RJ45 connector, common-mode chokes, ground-plane strategy, and cable-side routing under control. One Microchip/SMSC layout guide, for example, places common-mode chokes within 10 mm, about 400 mils, of an integrated RJ45 module in the applicable topology and gives 2 kOhm at 100 MHz as typical common-mode choke impedance guidance.
Practical layout checks include:
- Keep differential-pair routing short, balanced, and impedance controlled.
- Do not extend the digital ground plane under the magnetics, RJ45 connector, or the cable-side isolation area unless the PHY vendor layout guide calls for it.
- Keep cable-side and PHY-side regions visually and electrically distinct.
- Confirm shield, chassis ground, and termination strategy before layout release.
- Treat common-mode choke placement as part of the Ethernet front-end design, not as a procurement substitution.
For a deeper related topic, see Allied's post on common-mode chokes.
Industrial Ethernet Selection Notes
Industrial Ethernet designs often need more margin than office equipment. The speed label still matters, but so do operating temperature, vibration exposure, enclosure constraints, surge/ESD strategy, cable length target, and supply continuity.
For industrial equipment, check:
- Temperature range, often -40C to 85C for industrial parts.
- Port count and panel layout, especially single, dual, quad, stacked, vertical, or right-angle RJ45 formats.
- Shielding and EMI finger requirements.
- Whether the design uses discrete LAN magnetics for more layout control or integrated RJ45 magnetics for a compact front end.
- Lifecycle and availability for production builds, not just prototypes.
Allied's LAN magnetics catalog includes 137 SKUs covering 10/100 Base-T through 10G, with SMD and through-hole options, industrial-temperature options, and PoE through PoE++ paths. Allied also carries jack-only RJ45 connectors for designs that use separate magnetics, including tab-up, tab-down, stacked, LED, and EMI finger options.
What To Send Allied For A Shortlist
The fastest way to narrow Ethernet magnetics selection is to send the constraints that actually determine fit. A useful RFQ or engineering support request includes:
| Input | Example | Why it matters |
|---|---|---|
| PHY part number | Microchip LAN9668, ADIN1300, or another selected PHY | Determines transformer topology and center-tap guidance. |
| Speed tier | 10/100, 1000BASE-T, 2.5G, 5G, or 10G | Narrows bandwidth and loss requirements. |
| PoE tier | Non-PoE, PoE, PoE+, or PoE++ | Narrows current, thermal, and center-tap requirements. |
| Port count / form factor | Single, dual, quad, stacked | Narrows package and connector family. |
| Mounting / orientation | SMD, through-hole, vertical, right angle | Confirms mechanical fit. |
| Operating temperature | 0C to 70C or -40C to 85C | Narrows commercial vs industrial options. |
| Annual volume and lifecycle | Prototype, pilot, or production | Helps choose standard, custom, or supply-focused paths. |
If you already know the PHY, Ethernet speed, PoE class, and port layout, Allied can help narrow the catalog to a short list of LAN magnetics or RJ45 integrated-magnetics options. Start with LAN / Telecom Magnetics, RJ45 with Integrated Magnetics, or request a quote with the design inputs above.
FAQ
Do all Ethernet ports need magnetics?
Most copper twisted-pair Ethernet ports use magnetics between the PHY and the cable interface for galvanic isolation and signal coupling. Some specialized transformerless designs exist, but they are not the default path for exposed RJ45 Ethernet ports. Check the PHY datasheet and compliance requirements before assuming magnetics can be removed.
Should I choose discrete LAN magnetics or an integrated RJ45 connector?
Choose discrete LAN magnetics when you need layout control, sourcing flexibility, independent jack selection, or a PHY-specific front-end topology. Choose RJ45 with integrated magnetics when you need a smaller BOM, less PCB area, higher port density, or a faster qualified front-end path.
What is the most important Ethernet magnetics spec?
There is no single spec that makes the part correct. Start with the PHY vendor's recommended topology, then verify isolation, turns ratio, OCL, insertion loss, return loss, leakage inductance, DCR, crosstalk, common-mode rejection, temperature range, and PoE rating where applicable.
How does PoE affect magnetics selection?
PoE adds power current to the Ethernet front end. That makes center-tap routing, DCR, heat rise, saturation margin, and connector rating more important. PoE+ and PoE++ designs need extra care because available PSE power rises from 30 W for Class 4 to as high as 90 W for Class 8.
Can I use a 10/100 magnetic in a gigabit design?
Do not assume that a 10/100 LAN magnetic is valid for 1000BASE-T. Gigabit Ethernet uses different signaling requirements and all four pairs. Match the part to the PHY and speed tier, then verify the datasheet specs and reference design.