---
title: "Does it matter how a 4.7 µH inductor is built?"
topic: "Power inductors"
answer: "Yes. Built five common ways, the same 4.7 µH puts anywhere from 0.44 mV to 7.5 mV on a trace 1 mm away, and loses 30% of its inductance at anywhere from 0.18 A to 4.7 A. How it’s built decides where its field goes and how much current it takes."
url: "https://nearfieldnotes.com/notes/inductor-types/"
site: "Near Field Notes"
author: "Michael Walker"
note: 12
family: "Power"
published: "2026-11-06"
revised: "2026-11-06"
revision: 1
video: "https://nearfieldnotes.com/videos/inductor-types/"
---

Note 12 · [Power](https://nearfieldnotes.com/families/power/) · Rev 1 · [Watch the video · 2:14](https://nearfieldnotes.com/videos/inductor-types/)

Power inductors

# Does it matter how a 4.7 µH inductor is built?

**Short answer:** Yes. Built five common ways, the same 4.7 µH puts anywhere from 0.44 mV to 7.5 mV on a trace 1 mm away, and loses 30% of its inductance at anywhere from 0.18 A to 4.7 A. How it’s built decides where its field goes and how much current it takes.

The figure runs the same current ramp through two of them. The lines are the simulated field, darker where it’s stronger, and each trace glows with the voltage it picks up.

The unshielded drum’s field loops out over the board. The molded powder part’s stays in its block, and its trace picks up a ninth as much.

**Note 12’s opening figure: Unshielded drum, molded powder.** Both 4.7 µH, the same ramp through each

![Note 12’s opening figure: Unshielded drum, molded powder](https://nearfieldnotes.com/notes/inductor-types/fig-opening.png)

*What the figure shows:* Two 4.7 µH inductors on a board, each cut open by a quarter, with a trace running past each 1 mm away. On the left an unshielded drum: its simulated field lines loop far out over the board, and the trace beside it picks up 7.5 mV. On the right a molded powder part: its lines stay close inside the block, and its trace picks up 0.83 mV, a ninth as much.

*What the figure shows:* Unshielded drum, Sumida CD43NP-4R7MC: 7.5 mV on the trace. Molded powder, Coilcraft XAL4030-472: 0.83 mV on the trace.

Both in the same 12 V to 3.3 V buck. Each part is cut open by a quarter to show its field lines.

## Five 4.7 µH inductors

*Cut open, at true size*

Power inductors come in five common builds. These are real parts of each, all 4.7 µH. Three are ferrite drums, one bare, one with magnetic resin around its winding and one inside a ferrite ring. The other two are a coil molded into metal powder and a coil printed between layers of ferrite.

Each is modeled from its datasheet’s outline. Makers don’t publish the insides, so the post, winding and materials are estimates, and each model’s turns are set so it simulates 4.70 µH.

**Fig. 12: The five.**

![Fig. 12: The five](https://nearfieldnotes.com/notes/inductor-types/fig-12.png)

*What the figure shows:* Figure 12: five 4.7 µH power inductors side by side at true size, each cut open by a quarter. 1, unshielded drum, Sumida CD43NP-4R7MC, 4.5 by 4.0 by 3.2 mm. 2, semi-shielded drum, Bourns SRN4018BTA-4R7M, 4.0 by 4.0 by 1.6 mm. 3, shielded drum, Bourns SRR4018-4R7Y, 4.8 by 4.8 by 1.8 mm. 4, molded powder, Coilcraft XAL4030-472, 4.0 by 4.0 by 3.1 mm. 5, multilayer chip, Murata LQM2HPN4R7MG0L, 2.5 by 2.0 by 0.9 mm.

Rendered by one camera at one distance, so they compare at true size.

## The circuit

*A 12 V to 3.3 V buck*

Each part sits in the same [buck converter](https://nearfieldnotes.com/glossary/#g-buck), making 3.3 V from 12 V. While the top switch is on, the inductor has 8.7 V across it, and its current climbs 1.85 A every µs.

A signal trace runs past the part, 1 mm from its body and 10 mm long, with its return in the ground plane 0.2 mm below. Some of the inductor’s field passes between the trace and the plane. That couples the two through a [mutual inductance](https://nearfieldnotes.com/glossary/#g-mutual-inductance), M.

**Fig. 12b: The buck.**

![Fig. 12b: The buck](https://nearfieldnotes.com/notes/inductor-types/fig-12b.png)

*What the figure shows:* Figure 12b: a buck converter from 12 V to 3.3 V. Two switches take turns connecting a 4.7 µH inductor to 12 V or to ground, and the inductor feeds the 3.3 V output and its capacitor. A trace runs past the inductor 1 mm away, coupled to it by a mutual inductance M.

The trace stands for any signal that happens to run past. M is how much of the inductor’s field its loop catches, per amp.

## The voltage on the trace

*Where the field crosses it*

A changing current in the inductor puts a voltage on the trace:

`V = M dI/dt`

where V: voltage on the trace (V); M: mutual inductance between the inductor and the trace’s loop (H); I: current through the inductor (A); t: time (s).

`= 7.5 mV for the unshielded drum, 0.83 mV for the molded powder part`

The current’s slope is the same for every part, so M decides. It counts the field that crosses the thin strip between the trace and the plane.

Cut through the middle, the unshielded drum’s field leaves its flanges and arcs back through the air, and some of it crosses that strip. The molded powder part’s field closes inside its block, and little reaches the trace.

**Fig. 12c: Cut through the middle.**

![Fig. 12c: Cut through the middle](https://nearfieldnotes.com/notes/inductor-types/fig-12c.png)

*What the figure shows:* Figure 12c: each part cut through its middle, with its simulated field lines in that cut, over a board whose ground plane is 0.2 mm down. The unshielded drum's lines leave its flanges and loop far out over the board, many of them through the narrow strip between the trace and the plane: 4.03 nH of mutual inductance. The molded part's lines close inside its block, and few reach the trace: 0.45 nH.

The simulated field lines that lie in the cut, darker where the field is stronger. The orange strip is the trace’s loop, between the trace and the plane.

## Unshielded drum

*Sumida CD43NP-4R7MC*

The plainest build is wire wound on a ferrite spool, open to the air between its flanges. Nothing carries the field from one flange back to the other, so it returns through the air around the part.

**Fig. 12d: Unshielded drum, cut open.**

![Fig. 12d: Unshielded drum, cut open](https://nearfieldnotes.com/notes/inductor-types/fig-12d.png)

*What the figure shows:* Figure 12d: the unshielded drum, Sumida CD43NP-4R7MC, 4.5 by 4.0 by 3.2 mm, cut open by a quarter on a board, with its simulated field lines and a trace 1 mm away that picks up 7.5 mV.

Field lines darker where the field is stronger, faint where the part hides them. The trace glows by the voltage it picks up.

Unshielded drum

### Late to saturate, loud to its neighbors

**Field marks.** A ferrite spool with copper showing between its two flanges. The Sumida CD43 is 4.5 × 4.0 × 3.2 mm.

**Look-alike.** The semi-shielded drum is the same spool with dark resin where the copper would show.

**Range.** 7.5 mV on the trace, the most here. Its inductance stays within 10% to about 3.3 A and is 30% down by 3.7 A, though Sumida rates it at 1.62 A. The winding is 108.7 mΩ max, and the part costs $1.58. Hot, its knee comes about 0.8 A sooner.

**Habitat.** Converters with nothing sensitive to field nearby. Würth calls unshielded drums generally the worst builds for EMI.

## Semi-shielded drum

*Bourns SRN4018BTA-4R7M*

Filling the space between the flanges with resin full of magnetic powder gives the field part of a path back, outside the winding.

**Fig. 12e: Semi-shielded drum, cut open.**

![Fig. 12e: Semi-shielded drum, cut open](https://nearfieldnotes.com/notes/inductor-types/fig-12e.png)

*What the figure shows:* Figure 12e: the semi-shielded drum, Bourns SRN4018BTA-4R7M, 4.0 by 4.0 by 1.6 mm, cut open by a quarter on a board, with its simulated field lines and a trace 1 mm away that picks up 3.8 mV.

The model’s resin is eight times as magnetic as air, so some field still leaves through it.

Semi-shielded drum

### Half the noise, for less than a full shield

**Field marks.** The spool with dark, slightly hollowed resin between its flanges, so no copper shows. The Bourns SRN4018BTA is 4.0 × 4.0 × 1.6 mm.

**Look-alike.** The shielded drum, whose sides are flat ferrite and whose top shows a round seam.

**Range.** 3.8 mV on the trace, half the unshielded drum’s. Its inductance slides from the start: 10% down by about 0.95 A, 30% by 2.0 A. Rated 1.7 A for a 40 °C rise, 70 mΩ, $0.42.

**Habitat.** Cost-sensitive converters whose neighbors aren’t badly sensitive to field, which is where Würth places it too.

## Shielded drum

*Bourns SRR4018-4R7Y*

A ferrite ring around the drum closes the field’s path back. The only air left in it is a thin gap between the drum and the ring.

**Fig. 12f: Shielded drum, cut open.**

![Fig. 12f: Shielded drum, cut open](https://nearfieldnotes.com/notes/inductor-types/fig-12f.png)

*What the figure shows:* Figure 12f: the shielded drum, Bourns SRR4018-4R7Y, 4.8 by 4.8 by 1.8 mm, cut open by a quarter on a board, with its simulated field lines and a trace 1 mm away that picks up 0.67 mV.

What little field escapes leaves through the gap on top.

Shielded drum

### A closed path, and an early knee

**Field marks.** A round drum inside a square ferrite ring. From the top, a circle shows inside the square. The Bourns SRR4018 is 4.8 × 4.8 × 1.8 mm.

**Look-alike.** The molded powder part, also a square block, but solid, with no seam on top.

**Range.** 0.67 mV on the trace. The unshielded drum puts 11 times as much. Flat to about 0.6 A, then 30% down by 1.3 A. Rated 1.72 A for a 30 °C rise, 82 mΩ max, $0.65.

**Habitat.** Next to radio, audio and sensors, and in cars, where ROHM recommends closed parts. Its ferrite is brittle and can chip if knocked.

## Molded powder

*Coilcraft XAL4030-472*

Pressing the coil inside a block of metal powder and resin wraps the field’s whole path in magnetic material. The resin between the grains spreads the gap through the whole block.

**Fig. 12g: Molded powder, cut open.**

![Fig. 12g: Molded powder, cut open](https://nearfieldnotes.com/notes/inductor-types/fig-12g.png)

*What the figure shows:* Figure 12g: the molded powder, Coilcraft XAL4030-472, 4.0 by 4.0 by 3.1 mm, cut open by a quarter on a board, with its simulated field lines and a trace 1 mm away that picks up 0.83 mV.

The lines close inside the block, around the coil.

Molded powder

### The most current, with a soft knee

**Field marks.** A plain block with its terminals underneath. Cut open, the coil sits buried in the powder. The Coilcraft XAL4030 is 4.0 × 4.0 × 3.1 mm.

**Look-alike.** The shielded drum, which shows a round seam on top.

**Range.** 0.83 mV on the trace. Its inductance falls gradually, 30% by 4.7 A, with no sharp knee. Rated 3.9 A for a 20 °C rise and 40.1 mΩ, the most current and the least resistance here, for $2.18, the highest price.

**Habitat.** High-current bucks, tight spaces and cars. Coilcraft rates the build for shock and low hum. Its powder loses more energy than ferrite when the ripple is large.

## Multilayer chip

*Murata LQM2HPN4R7MG0L*

The smallest build prints the coil in silver on thin sheets of ferrite and stacks them into one chip, the turns joined through holes in the sheets.

**Fig. 12h: Multilayer chip, cut open.**

![Fig. 12h: Multilayer chip, cut open](https://nearfieldnotes.com/notes/inductor-types/fig-12h.png)

*What the figure shows:* Figure 12h: the multilayer chip, Murata LQM2HPN4R7MG0L, 2.5 by 2.0 by 0.9 mm, cut open by a quarter on a board, with its simulated field lines and a trace 1 mm away that picks up 0.44 mV.

Drawn larger than the others. The scale bar gives its size.

Multilayer chip

### Tiny and quiet, for small currents

**Field marks.** A chip with metal end caps, the shape of a ceramic capacitor. The Murata LQM2HPN is 2.5 × 2.0 × 0.9 mm.

**Look-alike.** A ceramic chip capacitor of the same 2520 size, with the same metal ends.

**Range.** 0.44 mV on the trace, the least here. It has lost 12% by 0.1 A and 30% by 0.18 A, and is down to 0.66 µH at its 1.1 A rating, which is set by heating. 110 mΩ, $0.28.

**Habitat.** Phones and other small, low-current converters, which ROHM places at about 3 MHz and up.

## Noise on the trace

*All five, in the same buck*

Side by side, the unshielded drum puts 9 times the molded powder part’s voltage on the trace, and 11 times the shielded drum’s. Resin around the winding halves it.

The three closed builds land within a factor of two of each other. The multilayer chip is the quietest, and the smallest.

**Fig. 12i: On a trace 1 mm away.**

![Fig. 12i: On a trace 1 mm away](https://nearfieldnotes.com/notes/inductor-types/fig-12i.png)

*What the figure shows:* Figure 12i: the voltage each part puts on a trace 1 mm away, in the same buck. Unshielded drum: 7.5 mV. Semi-shielded drum: 3.8 mV. Shielded drum: 0.67 mV. Molded powder: 0.83 mV. Multilayer chip: 0.44 mV.

While the top switch is on, from each part’s simulated mutual inductance. With it off, the current falls 0.70 A every µs, so each voltage is about 0.38 times as big, the other way.

## Inductance under load

*From each datasheet*

The other half of the choice is current. As the current through a part rises its core saturates, and its inductance falls.

The unshielded and shielded drums hold flat, then fall at a knee. The semi-shielded drum and the molded powder part slope the whole way. The multilayer chip falls steeply from the start.

Each datasheet defines its rated current its own way, so the curves compare better than the ratings.

**Fig. 12j: Inductance against current.**

![Fig. 12j: Inductance against current](https://nearfieldnotes.com/notes/inductor-types/fig-12j.png)

*What the figure shows:* Figure 12j: each part's inductance against the steady current through it, from its datasheet, at 20 °C where stated. Each dot marks where it has lost 30% of its inductance. The unshielded drum at 3.7 A, the semi-shielded drum at 2.0 A, the shielded drum at 1.3 A, the molded powder at 4.7 A, the multilayer chip at 0.18 A.

Each dot marks 30% below the part’s inductance with no current. Each curve is read from its datasheet’s chart at 20 °C, except the XAL4030’s, whose temperature Coilcraft doesn’t give.

## Side by side

*The five in numbers*

**Figure: The five.**

| Build | Part | Size, mm | On the trace | 30% down at | Rated | Resistance | Price |
|---|---|---|---|---|---|---|---|
| 1 Unshielded drum | Sumida CD43NP-4R7MC | 4.5 × 4.0 × 3.2 | 7.5 mV | 3.7 A | 1.62 A | 108.7 mΩ max | $1.58 |
| 2 Semi-shielded drum | Bourns SRN4018BTA-4R7M | 4.0 × 4.0 × 1.6 | 3.8 mV | 2.0 A | 1.7 A | 70 mΩ | $0.42 |
| 3 Shielded drum | Bourns SRR4018-4R7Y | 4.8 × 4.8 × 1.8 | 0.67 mV | 1.3 A | 1.72 A | 82 mΩ max | $0.65 |
| 4 Molded powder | Coilcraft XAL4030-472 | 4.0 × 4.0 × 3.1 | 0.83 mV | 4.7 A | 3.9 A | 40.1 mΩ | $2.18 |
| 5 Multilayer chip | Murata LQM2HPN4R7MG0L | 2.5 × 2.0 × 0.9 | 0.44 mV | 0.18 A | 1.1 A | 110 mΩ | $0.28 |

Rated currents as each maker defines them. Prices for one part on 3 November 2026, from Digi-Key or Mouser.

If something sensitive sits close, skip the unshielded drum. If the current is high, the molded powder part holds its inductance best. The multilayer chip suits only small currents.

#### Conditions

- **Parts:** Sumida CD43NP-4R7MC, Bourns SRN4018BTA-4R7M, Bourns SRR4018-4R7Y, Coilcraft XAL4030-472 and Murata LQM2HPN4R7MG0L, all 4.7 µH, with outlines from their datasheets
- **Models:** Posts, windings and the ring’s gap estimated, with permeabilities of 200 for the drums’ and the ring’s ferrite, 8 for the magnetic resin, 30 for the metal powder and 100 for the multilayer’s ferrite. Each model’s turns set so it simulates 4.70 µH.
- **Board:** A ground plane 0.2 mm under the top copper, a perfect conductor at the ripple’s frequency
- **Trace:** 10 mm long, 1 mm from the body’s side, centered on the part, its return in the plane
- **Buck:** 12 V to 3.3 V. While the top switch is on, the current climbs 1.85 A every µs.
- **Curves:** Read by eye from each datasheet’s chart of inductance against current, at 20 °C, except the XAL4030’s, whose temperature Coilcraft doesn’t give
- **Solver:** Elmer, 3D magnetostatics at 1 A. An air coil checks it: 0.896 µH against Wheeler’s formula’s 0.906 µH.

Simulated only. Nothing on this page has been measured on a board yet.

#### Limits

- The insides are estimates, and real ones will move each number. The ring’s gap, for one, is taken as 0.1 mm, from a patent’s range of 0.1 to 0.5 mm.
- The materials are linear, so the field is the one at low current, before any core saturates.
- Only the magnetic field is counted. The switch node’s voltage also reaches nearby traces through its electric field.
- Ratings are defined differently. The CD43’s is the lower of a 10% drop and a 40 °C rise. The others are a 20, 30 or 40 °C rise.
- The CD43’s 1.62 A rating sits below every limit its own charts show.

#### Files

- sim/parts.py The five models’ sizes, turns and permeabilities
- sim/run.py The Elmer solve and each model’s inductance
- sim/lines.py The field lines, traced through the solved field
- sim/noise.py The trace’s mutual inductance and voltage
- render/scene.py The renders, and the lines projected into them

#### Sources

- [Sumida, CD43 datasheet](https://products.sumida.com/products/pdf/CD43.pdf)
- [Bourns, SRN4018BTA datasheet](https://www.bourns.com/docs/Product-Datasheets/SRN4018BTA.pdf)
- [Bourns, SRR4018 datasheet](https://www.bourns.com/docs/Product-Datasheets/SRR4018.pdf)
- [Coilcraft, XAL4000 series datasheet](https://www.coilcraft.com/pdfs/xal4000.pdf)
- [Murata, LQM2HPN4R7MG0 data sheet](https://www.farnell.com/datasheets/4799566.pdf)
- [Würth Elektronik, ANP047c, radiation from power inductors](https://we-online.com/components/media/o109027v410%20ANP047c_The%20Behavior%20of%20Electro-Magnetic%20Radiation%20of%20Power%20Inductors%20in%20Power%20Management.pdf)
- [Würth Elektronik, SN018b, molded power inductors](https://www.we-online.com/components/media/o400296v410%20SN018b_Cracks%20on%20Molded%20Power%20Inductors_EN.pdf)
- [Coilcraft, Doc1733, ferrite against composite](https://www.coilcraft.com/getmedia/5196d074-40d5-4f53-ac0b-bf458214187d/Doc1733_Ferrite_Vs_Composite.pdf)
- [ROHM, choosing a buck converter’s inductor](https://fscdn.rohm.com/en/products/databook/applinote/ic/power/switching_regulator/buck_pwr_ind_app-e.pdf)
- [TDK, B82477P2 datasheet, on handling ferrite](https://de.tdk-electronics.tdk.com/inf/30/ds/b82477p2.pdf)
- [Murata, how a multilayer inductor is built](https://www.digikey.be/en/ptm/m/murata-electronics-north-america/power-inductors/tutorial?slide=4)
- [Toko, US 2013/0307657, the drum-to-ring gap](https://patents.google.com/patent/US20130307657A1/en)
- Wheeler, simple inductance formulas for radio coils, Proc. IRE, 1928
- [Elmer, the field solver](https://www.elmerfem.org/)

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---

Source: https://nearfieldnotes.com/notes/inductor-types/

Cite as: Michael Walker, “Power inductors”, Near Field Notes, Note 12, Rev 1, 6 Nov 2026. https://nearfieldnotes.com/notes/inductor-types/

Figures: free to reuse under CC BY 4.0 (https://nearfieldnotes.com/license/). Each one's image is linked under its name. Credit Michael Walker, Near Field Notes, and link the figure: this address plus #fig- and its number, like https://nearfieldnotes.com/notes/inductor-types/#fig-12.