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Foil Windings in Power Inductors: Methods of Reducing AC Resistance. Weyman Lundquist President and CEO West Coast Magnetics. SMPS Inductor Winding Resistance Includes AC and DC Resistance. Every successful design requires minimization of total AC plus DC winding losses.

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foil windings in power inductors methods of reducing ac resistance

Foil Windings in Power Inductors:Methods of Reducing AC Resistance

Weyman Lundquist

President and CEO

West Coast Magnetics

smps inductor winding resistance includes ac and dc resistance
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
Comparison of DC Resistance:Foil, Solid Wire & Litz Wire

FOIL

SOLID WIRE

50/40 awg LITZ WIRE

DCR = 1.9mΩ

DCR = 4.3 mΩ

DCR = 22.9mΩ

  • Foil windings:
    • Fast and easy to wind
    • Do not require bobbins or other supports
components of winding losses
Components of Winding Losses

Resistive loss

Eddy-current loss

dc loss

ac loss

“ac resistance”

Source: J. Pollock Thayer School of Engineering at Dartmouth

slide5

Skin Effect

B-Field

InducedCurrent

x

MainCurrent

J

Current Density

  • 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.
slide6

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xx

ooo

xxx

ooo

xxx

ooo

xxx

xx

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Proximity Effect

B-Field

InducedCurrent

J

MainCurrent

Current Density

x

  • 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.
slide7

Magnetic Field Inside Core Window

Ungapped E-core

Gapped E-core

slide8

Magnetic Field Inside Gapped Inductor Core Window

Legend:

Red: strong field

Blue: weak field

Lines: constant field magnitude

Gapped E-core

current distribution ungapped e core and gapped e core
Current Distribution:Ungapped E-Core and Gapped E-Core

Full Foil: Ungapped Core

Shaped Foil: Gapped Core

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

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

Shaped Foil is a patented technology developed by

Professor Charles Sullivan and Dr. Jennifer Pollock at Dartmouth College.

patented inductor technology
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
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 better1
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

400/40 Litz

20/32 Litz

Solid Wire

Long Cut

Prototype Cut

Full Foil

20/32 Litz

Solid Wire

50/40 Litz

Winding Cross Sections
method of estimating losses
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
Total Loss Comparison:50 kHz

Solid Wire

40 awg litz

Full Foil

32 awg litz

Shaped Foil Tech.

Shaped Foil Tech.

total loss comparison 200 khz
Total Loss Comparison:200 kHz

Full Foil

Solid Wire

40awg litz

Shaped Foil Tech.

Shaped Foil Tech.

Shaped Foil Tech.

Shaped Foil Tech.

Shaped Foil Tech.

experimental verification

Experimental Verification

Source: J. Pollock Thayer School of Engineering at Dartmouth

1.4

Inductance = 97 μH

Number of turns, N = 15

Core size: E71/33/32

Ripple ratio = 20%

1.2

Full-width Foil

1

0.8

Rac, Ω

Prototype Notched Foil #3

0.6

Prototype Notched Foil #2

Prototype Notched Foil #1

0.4

Optimization Program Loss

0.2

FEA of Notched Foil

0

0

50

100

150

200

250

Frequency, kHz

additional development work completed
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
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, President

West Coast Magnetics

4848 Frontier Way, Ste 100

Stockton, CA 95215

www.wcmagnetics.com

800-628-1123

Thank you for your time

The author gratefully acknowledges Professor Charles Sullivan, Thayer

School of Engineering at Dartmouth and Jennifer Pollock, PHD EE for their work and contributions to this presentation.