1. Foil Windings in Power Inductors: Methods of Reducing AC Resistance
Weyman Lundquist
President and CEO
West Coast Magnetics

2. SMPS Inductor Winding Resistance Includes AC and DC Resistance
Maybe: Every successful design requires minimization of both AC and DC winding losses.
Maybe mention configurations the have a large inductor current ripple --- soft switching
Every successful design requires minimization
of total AC plus DC winding losses.

3. Comparison of DC Resistance: Foil, Solid Wire & Litz Wire
50/40 awg LITZ WIRE
Could consider using R, dc instead o DCR
DCR = 1.9mΩ
DCR = 4.3 mΩ
DCR = 22.9mΩ
Foil windings:
Fast and easy to wind
Do not require bobbins or other supports

4. Components of Winding Losses
Resistive loss
Eddy-current loss
dc loss
ac loss
Adjusted formatting of equations
“ac resistance”
Source: J. Pollock Thayer School of Engineering at Dartmouth

5. Skin Effect B-Field Induced Current x Main Current J Skin Effect
Current Density
Which gives more loss LF or HF in
Different field origins
Skin effect – internal currents
Proximity effect – external field
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.

6. Proximity Effect B-Field Induced Current J Main Current x
xxx xx oo
ooo J
Main Current
Current Density x
Different field origins
Skin effect – internal currents
Proximity effect – external field
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.

7. Magnetic Field Inside Core Window
Need better pictures here! Hmmm…winding window or core window???
A legend would then be very helpful
Ungapped E-core
Gapped E-core

8. Magnetic Field Inside Gapped Inductor Core Window
Legend:
Red: strong field
Blue: weak field
Lines: constant field magnitude
Added formatting back
Gapped E-core

9. Current Distribution: Ungapped E-Core and Gapped E-Core
Full Foil: Ungapped Core
Shaped Foil: Gapped Core
JDP: hard to see current distribution in pictures??
Should consider adding core to figure
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.

10. 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.

11. 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
Saturation spec?? Is that why 15 turns is needed?

12. 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
Changed capitalization for consistancy

13. Winding Cross Sections
20/32 Litz
Solid Wire
400/40 Litz
Full Foil
Long Cut
50/40 Litz
Prototype Cut
O could look for these figures to improve quality

14. 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
Maybe say calculating in title instead of estimate? Or determine? Estimate seems too approximate

15. Total Loss Comparison: 50 kHz
Solid Wire
40 awg litz
Full Foil
32 awg litz
Shaped Foil Tech.
I can regenerate this figure so we can make the line thicker and much easier to see

16. Total Loss Comparison: 200 kHz
Full Foil
Solid Wire
40awg litz
Shaped Foil Tech.
Same here: I can regenerate this figure so we can make the line thicker and much easier to see

17. 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 50
100
150
200
250
Frequency, kHz

18. Temperature Rise Measurement: Results at 15% Ripple
Same here: I can regenerate this figure so we can make the line thicker and much easier to see

19. 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).
Not crazy about the title…maybe just: Additional Development work completed?

20. Foil Shape Optimization in Distributed Gap, Powdered Cores
Same here: I can regenerate this figure so we can make the line thicker and much easier to see
Source: Jennifer Pollock. Optimizing Winding Designs for High Frequency Magnetic Components. PHD thesis, 2008

21. Weyman Lundquist, President
Thank you for your time
Weyman Lundquist, President
West Coast Magnetics
4848 Frontier Way, Ste 100
Stockton, CA 95215
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.