Power Inductors1 min read
Bay note 05 — Power inductor: Isat against DCR
A buck/boost inductor strip that faces saturation current, ripple ratio and copper loss as one decision — not three separate catalogue filters.
Catalogue rows sort by L, Isat or DCR. Real converters lose on the corner where all three meet. This note sizes that corner.
Worked strip — 12 V → 5 V buck, 4 A, 400 kHz
Target ripple ΔI = 30% of Iout → ΔI = 1.2 A.
L = (Vin − Vout) × D / (fsw × ΔI) with D = Vout/Vin = 5/12 ≈ 0.42
L = (12 − 5) × 0.42 / (400e3 × 1.2) ≈ 2.94 / 480000 ≈ 6.1 µH
Use 6.8 µH standard value.
Peak current: Ipk = Iout + ΔI/2 = 4 + 0.6 = 4.6 A
Specify Isat ≥ 5.5–6 A to the L-drop you allow (for example L falls to 80%), not a marketing knee alone.
DCR and temperature
Pcu = Irms² × DCR. For continuous buck Irms ≈ Iout = 4 A.
- DCR = 15 mΩ → Pcu = 0.24 W — comfortable on a drum core with airflow
- DCR = 60 mΩ → Pcu = 0.96 W — expect a hot part and a derated Isat
Ferrite drum vs powder vs toroid
| Style | Isat behaviour | EMI | When we suggest it |
|---|---|---|---|
| Ferrite drum (gapped) | Sharper knee | Open field | Cost, vertical boards |
| Iron powder / sendust | Soft sat | Lower | Wide load range |
| Toroidal ferrite | Closed path | Quiet | Noise-sensitive rails |
What to put on the print
- L, fsw, Iout, ΔI or ripple %
- Isat definition (e.g. L drops to 80%)
- Max DCR or max temperature rise
- Height limit and preferred style
Browse power inductors and toroidal power inductors.