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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
       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



  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
    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, 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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Description: Low AC and DC Resistance Inductor Technology