Foil Windings in Power Inductors:

Methods of Reducing AC Resistance

Weyman Lundquist

President and CEOWest Coast Magnetics

SMPS Inductor Winding Resistance Includes AC and DC Resistance

Every successful design requires minimization of total AC plus DC winding losses.

Comparison of DC Resistance:Foil, Solid Wire & Litz Wire

Foil windings: Fast and easy to wind Do not require bobbins or other supports

DCR = 1.9mΩ DCR = 4.3 mΩ DCR = 22.9mΩ

FOIL SOLID WIRE 50/40 awg LITZ WIRE

Components of Winding Losses

proximityskindcrmsacdcdctotal PPRIRIP 2,

2

2,rmsac

acac I

PR

dcdcdc RIP 2

Resistive loss Eddy-current loss

dc loss ac loss

acrmsacac RIP 2,

“ac resistance”

Source: J. Pollock Thayer School of Engineering at Dartmouth

x

Skin Effect

Current Density

InducedCurrent

B-Field

J MainCurrent

Skin Effect An isolated conductor carrying high-

frequency current which generates a field in itself that forces the current to flow near the surface of the conductor.

Proximity Effect

J

x

B-Field

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Current Density

InducedCurrent

MainCurrent

Proximity Effect An isolated conductor is placed in an uniform

external field External field results from other wires and windings

near the conductor but mainly from the field present in the core window.

Ungapped E-core Gapped E-core

Magnetic Field Inside Core Window

Gapped E-core

Magnetic Field Inside Gapped Inductor Core Window

Legend:

Red: strong field

Blue: weak field

Lines: constant field magnitude

Current Distribution:Ungapped E-Core and Gapped E-Core

AC current pulled to small copper cross section in the vicinity of the gap.

Full Foil: Ungapped Core

Shaped Foil: Gapped Core

AC current evenly distributed on surface of foil across full width of foil.

Shaped Foil is a patented technology developed by Professor Charles Sullivan and Dr. Jennifer Pollock at Dartmouth College.

Patented Inductor Technology Very Low DCR, High Window Utilization

Foil winding

Low AC Resistance AC loss reduction comparable to litz wire

SIGNIFICANTLY LOWER TOTAL WINDING LOSSES

Shaped Foil is a patented technology developed by Professor Charles Sullivan and Jennifer Pollock at Dartmouth College.

Experiment: Is the New Technology Really Better?

Objective: A conclusive comparison of the new technology to conventional windings

Step 1: Define the Inductor Inductance: 90 uH Current: 40 Adc Ripple: Triangle wave at 50 kHz Core: E70/33/32 Epcos N67 material Gap: 2.64 mm (1.32 mm each center leg) Turns: 15

Experiment: Is the New Technology Really Better?

Step 2: Wind inductors with conventional windings using best practices Full window Single layer

Step 3: Determine winding losses for each inductor as a function of ripple magnitude

Winding Cross Sections

20/32 Litz Solid Wire400/40 Litz

Full Foil Long Cut

50/40 Litz

Prototype Cut

Solid Wire20/32 Litz

Method of Estimating Losses DC Resistance

Measure voltage drop under 5 Amp DC load Core Losses

Derived from Epcos loss curves AC resistance

Sweep from 10 kHz to 200 kHz with Agilent 4294A network analyzer

Use Fourier decomposition to translate sinusoidal sweep data to triangular waveform

Total Loss Comparison:50 kHz

40 awg litz

Full Foil

Solid Wire

Shaped Foil Tech.Shaped Foil Tech.

32 awg litz

Total Loss Comparison:200 kHz

Shaped Foil Tech.Shaped Foil Tech.Shaped Foil Tech.Shaped Foil Tech.Shaped Foil Tech.

40awg litz

Full Foil

Solid Wire

Experimental Verification

Ra

c, Ω

0 50 100 150 200 2500

0.2

0.4

0.6

0.8

1

1.2

1.4

Frequency, kHz

Full-width Foil

Prototype Notched Foil #1Prototype Notched Foil #2

Prototype Notched Foil #3

FEA of Notched Foil

Optimization Program Loss

Inductance = 97 μHNumber of turns, N = 15Core size: E71/33/32Ripple ratio = 20%

Source: J. Pollock Thayer School of Engineering at Dartmouth

Temperature Rise Measurement: Results at 15% Ripple

Additional Development Work Completed:

Optimization of Foil Shape (cutout) for minimum loss.

Simulation program to predict copper losses in foil windings for full foil and shaped foil windings.

Simulation program to plot inductance vs. Idc for gapped cores.

Optimization of foil shape in distributed gap (powdered cores).

Foil Shape Optimization in Distributed Gap, Powdered Cores

Source: Jennifer Pollock. Optimizing Winding Designs for High Frequency Magnetic Components. PHD thesis, 2008

Thank you for your time

Weyman Lundquist, PresidentWest Coast Magnetics

4848 Frontier Way, Ste 100Stockton, CA 95215

www.wcmagnetics.com800-628-1123

The author gratefully acknowledges Professor Charles Sullivan, ThayerSchool of Engineering at Dartmouth and Jennifer Pollock, PHD EE for their work and contributions to this presentation.