Standard network transformers fail in PoE circuits because DC bias saturates the core. Here's the complete technical breakdown of what to specify and how to calculate thermal performance.
Why Standard Transformers Fail Under DC Bias
Standard transformer: designed for AC signals only
→ No DC current → no DC flux → core stays in linear region
→ OCL stable at rated value (e.g., 1000µH for 1000BASE-T)
PoE transformer: DC bias current from PoE PSE/PD
→ DC current → DC flux consumes core's flux capacity
→ AC signal flux has less headroom
→ Core saturates → OCL collapses → link fails
Typical OCL derating in a standard transformer under 600mA DC bias:
Rated OCL (0mA bias): 1000µH
Effective OCL (600mA): ~150–300µH ← BELOW 1000BASE-T minimum
Result: link unstable, drops under load
Key PoE Transformer Specifications
Spec to check What you need
──────────────────────────────────────────────────────
OCL at rated DC bias ≥ 1000µH (1000BASE-T) under max PoE current
DCR per winding As low as possible — drives thermal load
Current rating ≥ max PoE class current per winding
Core type Air-gapped (prevents saturation under DC bias)
Isolation voltage ≥ 1500V AC (unchanged from standard)
Operating temperature Match to deployment environment
OCL Under Bias: What to Look for in a Datasheet
Good PoE transformer datasheet shows:
OCL @ 0mA: 1200µH (headroom above minimum)
OCL @ 350mA: 1050µH (still above 1000µH → valid for 802.3af)
OCL @ 600mA: 1010µH (barely above minimum → acceptable for 802.3at)
OCL @ 960mA: 980µH (below minimum → NOT suitable for 802.3bt Type 4)
If the datasheet only shows zero-bias OCL: ask for the bias curve,
or test it yourself at rated current before committing to the part.
DCR Thermal Calculation
PoE transformer self-heating calculator
def poe_thermal_dissipation(current_A, dcr_ohms, num_pairs):
"""
current_A: DC bias current per pair (A)
dcr_ohms: DCR per winding (Ω)
num_pairs: number of pairs carrying DC (2 for af/at, 4 for bt)
"""
power_per_winding = current_A ** 2 * dcr_ohms
# Each pair has 2 windings (primary + secondary)
total_power = power_per_winding * 2 * num_pairs
return total_power
Examples:
print(poe_thermal_dissipation(0.35, 0.5, 2)) # 802.3af: 0.245 W
print(poe_thermal_dissipation(0.60, 0.5, 2)) # 802.3at: 0.720 W
print(poe_thermal_dissipation(0.96, 0.5, 4)) # 802.3bt T4: 3.686 W
Alt A vs Alt B Power Delivery
Alternative A: DC on data pairs (1-2, 3-6) via center taps
→ Transformer windings carry both data signal AND DC power
→ Center tap used for both Bob Smith termination AND PoE injection
→ Most common in modern PoE equipment
Alternative B: DC on spare pairs (4-5, 7-8)
→ Data transformer windings carry signal only (no DC bias)
→ Separate magnetics or choke may be used on spare pairs
→ Not usable in 1000BASE-T (no spare pairs)
Design Checklist for PoE Magnetics
[ ] Transformer is explicitly PoE-rated (not just "1000BASE-T rated")
[ ] OCL at rated DC bias ≥ 1000µH (get the bias curve)
[ ] Current rating per winding ≥ PoE class maximum
[ ] DCR thermal load calculated and within package rating
[ ] PoE class matches transformer rating (don't underrate)
[ ] For 802.3bt: all 4 pairs PoE-rated
[ ] Standard 1500V AC isolation still verified
Source
Voohu Technology (www.voohuele.com) — PoE-rated 1000BASE-T transformers for 802.3af/at/bt. OCL-under-bias specs provided. 50pcs MOQ, DHL 3–5 days.
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