foil windings in power inductors: methods of reducing ac resistance weyman lundquist president and...
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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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xxooooooooooooo
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.