Network Transformer Cheat Sheet: 30-Day Technical Reference — Specs, Layout Rules, PoE, Brand Matrix, and Sourcing Checklist

Complete technical reference card — one article covering the key numbers, rules, and decision criteria from 30 days of network transformer engineering content.
Core Specification Quick Reference
pythonNETWORK_TRANSFORMER_SPECS = {
"OCL": {
"100BASE_TX": {"min_uH": 350, "test": "100kHz / 0.1V RMS / 0A bias"},
"1000BASE_T": {"min_uH": 1000, "test": "100kHz / 0.1V RMS / 0A bias"},
"PoE_af": {"min_uH": 350, "test": "100kHz / 0.1V RMS / 350mA bias"},
"PoE_at": {"min_uH": 350, "test": "100kHz / 0.1V RMS / 600mA bias"},
"PoE_bt_T4": {"min_uH": 1000, "test": "100kHz / 0.1V RMS / 960mA bias"},
},
"isolation_voltage_VAC": {
"standard_802_3": 1500, # IEEE 802.3, 60 sec
"industrial": 2500, # IEC 61000 series, 60 sec
"medical_1MOPP": 1500, # IEC 60601-1, 60 sec
"medical_2MOPP": 4000, # IEC 60601-1 peak AC, 60 sec
},
"insertion_loss_dB_max": 1.0, # S21, 100Ω diff, 1-100 MHz
"return_loss_dB_min": 16.0, # S11, 100Ω diff, 1-100 MHz
"turns_ratio": "1CT:1CT", # center tap both sides
"bob_smith": {
"resistor_ohm": 75,
"capacitor_pF": 1000,
"connect_to": "CHASSIS ground (NOT signal ground)",
"count_100FX": 2, # 2 networks for 10/100BASE-TX
"count_GbE": 4, # 4 networks for 1000BASE-T
},
}
PCB Layout Rules — Minimum Compliance
Rule Value Consequence of violation
──────────────────────────────────────────────────────────────────────────────
Transformer to RJ45 distance ≤ 10 mm Increased insertion loss
Bob Smith to center tap pin ≤ 5 mm Reduced common-mode rejection
Copper-free moat around transformer 4 mm EMI radiation from traces
Creepage distance (standard 802.3) ≥ 6.4 mm Isolation failure risk
Creepage distance (medical 2 MOPP) ≥ 12.8 mm Regulatory non-compliance
Signal traces through transformer zone NONE EMC / isolation failure
PoE Transformer Requirements by Type
Type Standard Max I/pair DCR/winding target Core type OCL condition
──────────────────────────────────────────────────────────────────────────────────
Type 1 802.3af 350 mA < 2.0 Ω Air-gapped At 350mA bias
Type 2 802.3at 600 mA < 0.8 Ω Air-gapped At 600mA bias
Type 3 802.3bt 600 mA < 0.8 Ω Air-gapped At 600mA bias
Type 4 802.3bt 960 mA < 0.3 Ω ← CRITICAL Air-gapped At 960mA bias
Part Compatibility Matrix — Common Confusion Points
pythonCOMPATIBILITY = {
"HX1188NL vs H1102NL": {
"electrical": "IDENTICAL — both 10/100BASE-TX, same OCL/DCR/isolation",
"package": "DIFFERENT — HX1188NL=8-pin SMD, H1102NL=16-pin SMD",
"footprint": "INCOMPATIBLE — cannot substitute without PCB redesign",
"pin_assign": "DIFFERENT — verify against your land pattern",
"verdict": "NOT INTERCHANGEABLE despite same electrical spec",
},
"standard vs PoE_transformer": {
"electrical": "OCL may match at zero bias",
"core": "DIFFERENT — PoE needs air-gapped core",
"at_bias": "Standard core saturates at PoE DC bias currents",
"verdict": "NOT INTERCHANGEABLE for PoE PSE/PD designs",
},
"100BASE_TX vs automotive_100BASE_T1": {
"pairs": "100BASE-TX=2 pairs, 100BASE-T1=1 pair",
"standard": "Different IEEE standards entirely",
"verdict": "NOT INTERCHANGEABLE, completely different products",
}
}
Application Selection Matrix
Application Speed Isolation Temp Range Package Key Spec
──────────────────────────────────────────────────────────────────────────────────
Standard Ethernet 100M/1G 1500V AC 0/+70°C SMD OCL at 0A
PoE PSE/PD 100M/1G 1500V AC 0/+70°C SMD OCL at bias
Industrial Ethernet 100M/1G 2500V AC -40/+85°C THT IEC 61000
Medical IT port 100M/1G 1500V AC 0/+70°C SMD 1 MOOP CoC
Medical patient ckt 100M/1G 4000V AC 0/+70°C THT large 2 MOPP CoC
Automotive 100M 100BASE-T1 1500V AC -40/+125°C AEC-Q200 CISPR 25
30-Day Article Index
Day 1: What is a Network Transformer
Day 4: Isolation Voltage — 1500V AC explained
Day 5: OCL — the most important spec (test conditions matter)
Day 6: DCR — DC Resistance and PoE implications
Day 9: PoE Transformer Design (802.3af/at)
Day 11: Turns Ratio 1CT:1CT explained
Day 12: Common Mode Choke vs Network Transformer
Day 13: Bob Smith Termination — complete guide
Day 14: PCB Layout Rules
Day 15: Insertion Loss measurement
Day 16: Return Loss and impedance matching
Day 17: IoT device selection
Day 18: Industrial Ethernet requirements
Day 19: Datasheet reading guide
Day 21: Brand comparison (Pulse/TDK/Würth/Halo/Voohu)
Day 22: HX1188NL deep dive
Day 23: H1102NL vs HX1188NL — NOT pin compatible
Day 24: Automotive Ethernet (100BASE-T1 / AEC-Q200)
Day 25: PoE++ 802.3bt Type 3 & Type 4 — 960mA design
Day 26: Thermal design — DCR derating, self-heating
Day 27: Medical devices — IEC 60601-1 MOPP isolation
Day 28: China supply chain — manufacturer vs trader
Day 29: Sample evaluation protocol
Day 30: This article
Voohu Technology — www.voohuele.com
Network Transformers · LAN Magnetics · RJ45 Connectors | MOQ 50pcs | DHL 3–5 days | Japan / Korea / SE Asia