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 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.

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

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

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.

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.

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

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

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.

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

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

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

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

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

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

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

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

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?

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

Weyman Lundquist, President 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 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.