Toroid Core Inductance

Formula

Description

The inductance of a toroidal inductor is the product of the core AL value and the square of the number of turns. The AL value (inductance factor) is a property of the core material and geometry, specified in nH per turn squared. Toroidal cores provide excellent magnetic shielding since the flux is largely contained within the core, minimizing radiated EMI. This makes them the preferred choice for EMI filters, common-mode chokes, and power inductors in noise-sensitive applications.

Variables

• L — Inductance (H)
• AL — Core inductance factor (nH/turn²)
• N — Number of turns (dimensionless)

Practical Notes

AL values are specified at low flux density and decrease as the core approaches saturation. Common toroid cores: T37-2 (iron powder, AL = 4.0), T68-2 (AL = 5.7), FT37-43 (ferrite, AL = 885). The nH/turn² unit means the result of AL × N² is in nanohenries, which is internally converted to henries. Doubling the turns quadruples the inductance but also increases DCR and may require a larger core.