How to Select LAN Transformers for PoE+ and PoE++ Applications

Power over Ethernet (PoE) has evolved significantly since the original IEEE 802.3af standard delivered 15.4 W per port. Today's PoE+ (IEEE 802.3at, 30 W) and PoE++ (IEEE 802.3bt, up to 90 W) standards place substantially higher demands on the LAN transformer at the heart of every Ethernet port. Selecting the wrong transformer can lead to excessive insertion loss, inadequate isolation, or thermal failure under full load.

This guide walks through the key electrical parameters, form-factor considerations, and compliance requirements engineers must evaluate when specifying LAN transformers for PoE-capable designs, with practical references to Shareway Technology's SC series.

Understanding PoE Power Classifications

The IEEE 802.3 standards define power delivery over the same twisted-pair cabling used for data. The transformer must pass both the differential data signal and the common-mode power injection without saturating or distorting the signal.

As power levels increase, the DC bias current through the transformer center tap rises proportionally. This bias current shifts the operating point on the B-H curve, reducing effective inductance and potentially causing saturation — which manifests as signal distortion and increased bit-error rates.

Turns Ratio: The First Decision

The turns ratio of the LAN transformer determines how the PHY and the line interface are matched. For standard 10/100/1000BASE-T applications:

• 1:1 turns ratio — used for standard PoE (802.3af) applications where the PHY drives the line directly. The 1:1 ratio provides impedance matching between the 100 Ω differential line and the PHY while allowing common-mode DC injection via the center tap.
• 1:1 with center tap (1CT:1CT) — the most common configuration for PoE+ (802.3at). The center tap on the line side carries the DC supply voltage from the PSE, while the chip-side center tap connects to the PHY's common-mode voltage or termination network.
• 1.25:1 or 1:1.25 ratios — sometimes used for reduced transmit voltage swing or specific PHY requirements. Always verify the PHY vendor's recommended turns ratio.

For PoE++ applications drawing 60-90 W, the 4-pair power delivery requires two transformers per port (one for each pair set), or a single quad transformer module. The turns ratio remains 1CT:1CT for each channel, but the transformer core must handle nearly 1 A of DC bias without saturation.

Isolation Voltage Requirements

LAN transformers provide galvanic isolation between the PHY and the Ethernet cable. The IEEE 802.3 standard and Telcordia GR-1089 specify minimum isolation requirements:

• 1500 Vrms (hipot test) — the baseline requirement for most LAN transformers. This is tested as a 1-minute dielectric withstand between primary and secondary windings.
• 2250 Vdc — equivalent to 1500 Vrms for DC hipot testing.
• 60 s ramp time — the voltage is typically ramped from 0 to the test voltage over a few seconds to avoid transient damage.

Shareway's SC series transformers, such as the SC110-series, are designed and tested to meet 1500 Vrms isolation with margin. For designs requiring reinforced isolation (e.g., medical equipment), look for parts rated at 3000 Vrms or higher, though these are less common in standard LAN filter product lines.

Insertion Loss and Return Loss

Insertion loss limits the amount of signal power that reaches the receiver. For 1000BASE-T applications, the total channel insertion loss budget is tight, and the transformer typically contributes 0.5-1.5 dB of that budget.

• Typical specification: ≤1.0 dB at 1-100 MHz for gigabit transformers
• PoE impact: Under DC bias, the core permeability drops (incremental permeability effect), which can increase insertion loss by 0.1-0.3 dB. Always verify insertion loss under worst-case DC bias conditions.
• Return loss: Typically ≥16 dB at 1-30 MHz, ≥10 dB at 30-100 MHz, matching the 100 Ω differential impedance requirement.

Common Form Factors: SMD vs. Through-Hole

LAN transformers are available in two primary package styles:

Surface-Mount Device (SMD)

SMD LAN transformers dominate new designs due to automated assembly and lower cost. Common footprints include:

• 12-pin DIL (dual in-line) — 12.7 x 7.0 x 5.0 mm typical, compact, suitable for 1-port designs
• 16-pin DIL — larger footprint, supports 2-channel (dual port) transformers
• 24-pin or 32-pin QUAD — for 4-port or integrated RJ45+transformer modules

Through-Hole

Still preferred for high-reliability industrial and PoE++ designs where thermal dissipation and mechanical robustness are priorities. Through-hole parts handle higher DC bias currents with lower temperature rise due to larger core and thicker wire.

Shareway's SC series includes both SMD (SC110, SC111 packages) and through-hole options, giving design engineers flexibility across different board assembly workflows.

Cross-Reference with Shareway SC Series

Shareway Technology Co., Ltd. manufactures a broad range of LAN filters under the SC prefix that are drop-in compatible with industry-standard footprints from Pulse, HALO, Bel, TDK, and other major brands. For PoE+/PoE++ designs, the SC110 and SC111 series offer:

• 1CT:1CT turns ratio for PoE+ (2-pair, 30 W) and PoE++ (4-pair, 60-90 W)
• 1500 Vrms isolation verified per IEEE 802.3
• Insertion loss ≤1.0 dB typical across 1-100 MHz
• Operating temperature range -40°C to +85°C
• RoHS-compliant, REACH-compliant construction

Rather than redesigning your bill of materials, use the lanfilter.com cross-reference tool to find the Shareway SC-series equivalent for your existing Pulse, HALO, or Bel part number.