IEEE 802.3bt PoE++ Magnetics: Type 3 vs Type 4 Requirements, DCR Thermal Math, OCL Under Bias

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Complete technical reference for 802.3bt PoE++ transformer selection — the specs that change from 802.3at and why they matter.
PoE Standard Progression
Standard Type PSE Power PD Power Max I/pair Pairs Transformer
──────────────────────────────────────────────────────────────────────────────
802.3af 1 15.4W 12.95W 350 mA 2 100BASE-TX OK
802.3at 2 30W 25.5W 600 mA 2 100BASE-TX OK
802.3bt 3 60W 51W 600 mA 4 GbE 4-pair ← key
802.3bt 4 90W+ 71.3W 960 mA 4 GbE 4-pair ← key
DCR Thermal Calculation — Why Type 4 Needs Different Magnetics
pythondef poe_transformer_thermal(dcr_per_winding, current_A, num_windings, rth_ja_C_per_W):
"""
Calculate transformer temperature rise from PoE DC copper loss.

dcr_per_winding: DC resistance per winding (Ohms)

current_A:       DC bias current per winding (Amps)

num_windings:    Total powered windings (2 for 100BASE-TX, 4 for GbE)

rth_ja_C_per_W:  Thermal resistance junction-to-ambient (°C/W)

"""

power_per_winding = current_A**2 * dcr_per_winding

total_copper_loss = power_per_winding * num_windings

temp_rise_C = total_copper_loss * rth_ja_C_per_W

return power_per_winding, total_copper_loss, temp_rise_C

802.3at transformer (DCR=1.0Ω) used in 802.3bt Type 4 application:

p_w, p_t, dt = poe_transformer_thermal(1.0, 0.96, 4, 60)
print(f"Power/winding: {p_w:.2f}W | Total: {p_t:.2f}W | ΔT: {dt:.0f}°C")

→ Power/winding: 0.92W | Total: 3.69W | ΔT: 221°C ← DANGEROUS

802.3bt Type 4 rated transformer (DCR=0.25Ω):

p_w, p_t, dt = poe_transformer_thermal(0.25, 0.96, 4, 60)
print(f"Power/winding: {p_w:.2f}W | Total: {p_t:.2f}W | ΔT: {dt:.0f}°C")

→ Power/winding: 0.23W | Total: 0.92W | ΔT: 55°C ← ACCEPTABLE

Rule: For 802.3bt Type 4, target DCR per winding ≤ 0.3Ω

OCL Under DC Bias — What to Verify
Standard OCL test: LCR meter at 100kHz / 0.1V RMS, zero DC offset
PoE OCL test: LCR meter at 100kHz / 0.1V RMS, WITH DC bias superimposed

Required minimums (must hold at operating DC bias current):
100BASE-TX: ≥ 350 µH at rated PoE current
1000BASE-T: ≥ 1000 µH at rated PoE current

Ask supplier for:
"OCL vs DC current curve from 0mA to rated maximum"
"OCL value specifically at 350mA / 600mA / 960mA"

Red flag: datasheet shows OCL only at zero bias for a "PoE rated" part
Green flag: datasheet has OCL vs I curve with values at 600mA and 960mA

Typical OCL rolloff for air-gapped core (illustrative):
0 mA: 1200 µH (above minimum — headroom)
350 mA: 1100 µH ✅ 802.3af OK
600 mA: 900 µH ✅ 802.3at / 802.3bt Type 3 OK

960 mA: 750 µH ✅ 802.3bt Type 4 OK (> 1000µH minimum? NO — need better design)
960 mA: 1050 µH ✅ Type 4 OK (properly designed air-gap)
Center Tap Current Rating Check
For PoE PSE: DC current injected at transformer center taps
For PoE PD: DC current extracted at transformer center taps

Design checklist for 802.3bt Type 4 (960mA):
[ ] Center tap pin current rating ≥ 1000mA (check Maximum Ratings table)
[ ] PCB trace width for center tap nets (IPC-2221):
960mA, 1oz copper, 10°C rise: ≥ 0.5mm trace width
960mA, 1oz copper, 5°C rise: ≥ 0.8mm trace width
[ ] Via current capacity: single via rated ~500mA → use 2 vias in parallel
[ ] Thermal relief on center tap pads: avoid (increases resistance, adds heat)
Type 3 vs Type 4 Specification Requirements
Requirement 802.3at (Type 2) 802.3bt Type 3 802.3bt Type 4
─────────────────────────────────────────────────────────────────────────────
Transformer type 100BASE-TX or GbE GbE 4-pair GbE 4-pair
Max DC per winding 600 mA 600 mA 960 mA
OCL spec condition At 600mA DC bias At 600mA DC bias At 960mA DC bias
Target DCR per winding < 0.8Ω < 0.8Ω < 0.3Ω ← tighter
CT pin current rating ≥ 700mA ≥ 700mA ≥ 1000mA
Thermal verification Recommended Required CRITICAL
Compatible PSE/PD Controller ICs
PSE Controllers (Power Sourcing Equipment):
TI TPS2388x — 802.3bt Type 3/4, 4-pair
Microchip PD69208 — 802.3bt, managed
Silvertel AG9900M — 802.3bt

PD Controllers (Powered Device):
TI TPS2373 — 802.3bt Type 3/4 PD
Microchip PD69100 — PD controller
Silvertel Ag5450A — 802.3bt PD
Voohu Technology (www.voohuele.com) — PoE-rated transformers with OCL vs bias curves and thermal data.
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