Toroidal Power Inductors1 min read
Bay note 06 — Toroidal when the field must stay home
Why a closed magnetic path changes EMI, mounting and winding cost — with a comparison strip against bobbin ferrite for the same inductance.
A toroid does not make inductance "better". It makes the field finish inside the doughnut. That is a board-level decision dressed as a magnetics one.
When the bay reaches for a toroid
- Nearby sense traces or RF sections hate drum-core leakage
- You need higher L in a short height with many turns
- Differential choke where a closed path cuts stray
Trade strip — 100 µH, 2 A class
| Bobbin ferrite (gapped) | Toroidal ferrite | |
|---|---|---|
| EMI | Open; needs keep-out | Much quieter |
| Winding | Fast, layered | Slower, often hand/sector |
| Mounting | Pins, easy wave-solder | Leads / glue / clip |
| Cost at volume | Usually lower | Higher labour |
| Soft-sat options | Gap / powder | Material choice |
Calculation note — turns on a toroid
N = √(L / AL). If AL = 100 nH/turn² and L = 100 µH = 1×10⁵ nH:
N = √1000 ≈ 32 turns.
Wire gauge from Irms and temperature class; watch inner circumference — thick wire eats window fast on small OD cores.
What to put on the print
- L, Ipk/Isat, DCR max
- OD / height max
- Mounting: radial leads, vertical, clip
- EMI constraint in one line ("keep H-field off ADC section")