Reading an inductor

Three characters are printed on the part, and they are the resistor code with the units changed.

The rest of what you need is on the datasheet rather than the part.

L, and how much of it you actually get

The marked inductance is measured at a small current and usually at a stated frequency — 100 kHz is common. It is a starting value, not a promise. Tolerance on a power inductor is often ±20 %, and the next section is about the other way it drops.

DCR: the resistance of the wire

The copper has resistance, and the datasheet calls it DCR. A 100 µH power inductor might be 0.15 Ω. At 1 A that is 0.15 V lost and 0.15 W of heat, which in a 5 W supply is three per cent of your efficiency spent on nothing.

Finer wire means more turns in the same space, so the parts with the best inductance-per-millimetre have the worst DCR. That trade is the whole reason there are twenty parts in a catalogue at the same value.

I_sat: the one that surprises people

Most parts in this track fail by getting hot, so a current rating reads like a heat rating. This one is not.

Past the knee the core has no domains left to line up, the inductance falls away, and the part stops slowing its own current down. In a switching supply that is a runaway: less inductance means a steeper ramp, a steeper ramp means a higher peak, and a higher peak means even less inductance. What stops it is the regulator's current limit, if the chip has one, or the switch, if it does not.

Manufacturers do not agree on where to draw the line — some quote the current at which the inductance has fallen 10 %, some 20, some 30 — so two parts with the same I_sat on the front page are not necessarily comparable. The number is in the small print.

The second rating, I_rms, is the thermal one: how much current the wire will carry before the part reaches some stated temperature rise. Whichever of the two is lower is the one that limits you, and on small parts it is usually I_rms, while on the ferrite drums in switching supplies it is usually I_sat.

SRF: where it stops being an inductor at all

Every turn is a small capacitor to the turn beside it, so a coil is an inductor in parallel with a few picofarads. At the self-resonant frequency those cancel, and above it the part behaves like a capacitor — impedance falling with frequency instead of rising.

You want the SRF comfortably above whatever you are using the part for. A 100 µH part with an SRF of 3 MHz is fine in a 500 kHz buck converter and useless as a filter at 10 MHz.

Next: the circuit all of this has been for.