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2:14 · With sound

Power inductors

Does it matter how a 4.7 µH inductor is built?

Read Note 12

Transcript

Both of these are 4.7 µH inductors.

The one on the left puts 9 times more noise on the trace next to it.

There are 5 common kinds of power inductor. How each one is built decides where its field goes.

Each one here is a real part, in a 12 V to 3.3 V buck, with a trace 1 mm away.

First, the unshielded drum. Wire wound on a ferrite spool, open to the air.

Its field loops out over the board, and the trace picks up 7.5 mV.

It saturates late, and ferrite has low loss.

But it sprays field everywhere, and its knee is sharp and drops when it's hot.

Use it where nothing nearby is sensitive.

Second, semi-shielded. The same drum, with magnetic resin filling in around the wire.

The resin pulls some of the field back in. The trace sees half as much, 3.8 mV.

It's cheap, at 42 cents.

But field still leaks out the sides, and it starts losing inductance early.

Use it on low cost boards where the neighbors don't mind.

Third, the shielded drum. A ferrite ring closes the path around the outside.

Only a little field escapes, at the gap on top. The trace sees 0.67 mV.

That's 11 times quieter than the open drum.

But it saturates sooner, and the ferrite is brittle.

Use it next to radio, audio, or sensors, and in cars.

Fourth, molded metal powder. The coil is pressed inside a solid block of it.

The powder wraps the whole coil. The trace sees 0.83 mV.

It carries the most current, it saturates softly, and it's quiet and rugged.

But the powder has more core loss, and it costs the most.

Use it for high current bucks, tight spaces, and cars.

Fifth, the multilayer chip. The coil is printed between layers of ferrite.

It's tiny and closed. The trace sees 0.44 mV.

It's the smallest and the cheapest.

But it loses 30% of its inductance by 0.18 A, and its resistance is high.

Use it in phones and other tiny, low current designs.

Here's the noise on that trace for all five.

If something sensitive sits close, skip the open drum.