2026-07-09
An inductor's core material is arguably more important than its winding. The core sets the achievable inductance density, the saturation current, the core losses, and the frequency range where the part actually behaves like an inductor. Pick wrong and your buck converter runs hot, your RF filter mysteriously loses Q, or your EMI choke stops filtering above 1 MHz.
Air-core inductors have no magnetic material — just a coil of wire. Permeability is exactly 1, so inductance per turn is tiny (you need lots of turns for even a few microhenries). But they have zero core loss, zero saturation, and perfectly linear behavior. Air-core coils dominate RF work above ~30 MHz where core losses in other materials would destroy Q. Classic use: the tank coil in a 100 MHz oscillator, wound as a self-supporting helix of tinned copper.
Iron-core (laminated silicon steel) inductors have very high permeability (μᵣ = 4000–10,000), giving enormous inductance in a small volume. Laminations break up eddy currents, but they only work up to ~1–20 kHz. Above that, hysteresis and eddy losses explode. This is 60 Hz territory: line-frequency transformers, mains chokes, audio output transformers.
Powdered iron cores (Micrometals, Magnetics Kool-Mu) are the SMPS workhorse. Iron particles are individually insulated and pressed into a toroid, giving a distributed air gap. Permeability is moderate (μᵣ = 10–100), and the distributed gap prevents saturation — inductance rolls off gracefully with DC bias instead of collapsing. Use these for 20 kHz to a few MHz: buck/boost inductors, PFC chokes, output filters.
Ferrite (covered obliquely in prior lessons) sits between: much higher frequency capability than powdered iron (up to ~100 MHz for NiZn), but sharp saturation. Great for signal chokes and small SMPS transformers; needs a physical gap for high-DC-bias applications.
Rule of thumb — pick your core by frequency:
Concrete example: Design a 100 µH inductor for a 200 kHz buck converter carrying 5 A DC with 1 A ripple. Powdered iron toroid (Micrometals T50-26, AL = 32 nH/turn²) needs N = √(100000/32) ≈ 56 turns. Check DC bias: at 5 A, the T50-26 shows ~30% inductance rolloff — still acceptable. Try the same design with a ferrite toroid without a gap and it saturates hard at 2 A. That's the powdered iron advantage.
Watch the Curie temperature too: ferrites lose their magnetic properties around 200–450°C, and even mild self-heating shifts permeability. Powdered iron degrades permanently above ~125°C — a stressed inductor slowly loses inductance over months.