Every Ethernet port has a network transformer. Most engineers spec the OCL and move on. But isolation voltage is the specification that actually protects your system from the outside world — and it's worth understanding in depth.
The Standard Requirement
IEEE 802.3 mandates 1500V AC minimum isolation between the PHY-side winding and the cable-side winding. This is verified by a hipot (high potential) test:
Test parameters:
Voltage: 1500V AC
Frequency: 50/60 Hz
Duration: 60 seconds
Ramp time: 3–5 seconds recommended
Pass limit: Leakage current < 5mA (IEC 62368-1)
No arc, no breakdown
This test should be run at the component level as part of incoming inspection. It's a production qualification test — not just an engineering sample test.
Withstand vs. Breakdown
Datasheets list withstand voltage, not breakdown voltage.
PCB Layout: Creepage and Clearance
Specifying the right part is only half the problem. Your PCB layout must maintain adequate physical separation between isolated circuits.
Per IEC 62368-1 (Pollution Degree 2, Material Group IIIa):
Working voltage 1500V:
Creepage distance: 6.4mm minimum
Clearance distance: 4.0mm minimum
Working voltage 2500V:
Creepage distance: 10.0mm minimum
Clearance distance: 6.0mm minimum
Check both sides of the board. Vias that pass through the isolation zone can violate clearance requirements even if the top-layer copper looks fine.
PoE-Specific Considerations
PoE magnetics must handle DC bias without core saturation. The isolation requirement stays at 1500V, but thermal behavior changes:
PoE power dissipation in transformer winding
P = I^2 * R (DCR matters a lot here)
poe_current_af = 0.35 # A (802.3af, 15.4W)
poe_current_at = 0.60 # A (802.3at, 30W)
poe_current_bt = 0.96 # A (802.3bt Class 6, 51W)
dcr_typical = 0.5 # Ohms (typical winding DCR)
p_af = poe_current_af*2 * dcr_typical # 0.061 W
p_at = poe_current_at2 * dcr_typical # 0.180 W
p_bt = poe_current_bt*2 * dcr_typical # 0.461 W
print(f"PoE af dissipation: {p_af:.3f}W")
print(f"PoE at dissipation: {p_at:.3f}W")
print(f"PoE bt dissipation: {p_bt:.3f}W")
Standard transformers saturate with DC bias — specify PoE-rated magnetics explicitly.
Hipot Test Setup
Instrument: Chroma 19032 / Hioki ST5520 (or equivalent)
Primary: Connect to PHY-side pins
Secondary: Connect to cable-side pins (chassis ground reference)
Voltage: 1500V AC
Dwell: 60 sec
Accept if: No breakdown, leakage < 5mA
Ask your supplier for the hipot test report — it should be available for every production lot.
Source
Voohu Technology (www.voohuele.com) — network transformers with 1500V and 2500V isolation ratings, 50pcs MOQ, DHL 3–5 days. Full component specs and test reports available on request.
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