# Low AC and DC Resistance Inductor Technology (PowerPoint)

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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.
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
Resistive loss                             Eddy-current loss

Ptotal  I Rdc  I2
dc
2
ac,rms     Rdc  Pskin  Pproximity
dc loss                                               ac loss
Pac  I ac,rms Rac
2

Pdc  I dc Rdc
2

“ac resistance”
Pac
Rac      2
I ac ,rms

Source: J. Pollock Thayer School of Engineering at Dartmouth
Skin Effect
B-Field                             Induced
x                   Current

J                 Main
Current               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.
Proximity Effect
B-Field
Induced
oo    xx
ooo   xxx                       Current
ooo   xxx
ooo   xxx
oo    xx

J
Current                              Main
Density
Current
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.
Magnetic Field Inside Core
Window

Ungapped E-core   Gapped E-core
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
Full Foil:                                Shaped Foil:
Ungapped Core                             Gapped Core

AC current evenly                        AC current pulled to
distributed on surface                   small copper cross
of foil across full width                section in the vicinity
of foil.                                 of the gap.

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
   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

Full Foil                 Long Cut         Prototype Cut
20/32 Litz                              400/40 Litz
Solid Wire

20/32 Litz                Solid Wire        50/40 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
Solid Wire

40 awg litz

Full Foil

32 awg litz

Shaped Foil Tech.
Shaped Foil Tech.
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
Source: J. Pollock Thayer School of Engineering at Dartmouth
1.4
Inductance = 97 μH
Number of turns, N = 15
1.2       Core size: E71/33/32
Full-width Foil
Ripple ratio = 20%

1
Rac, Ω

0.8

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
Temperature Rise Measurement:
Results at 15% Ripple
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
Weyman Lundquist, President
West Coast Magnetics
4848 Frontier Way, Ste 100
Stockton, CA 95215

www.wcmagnetics.com
800-628-1123

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.

```
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 views: 132 posted: 12/30/2010 language: English pages: 21
Description: Low AC and DC Resistance Inductor Technology