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Reduce winding loss by designing for the inductor’s actual ripple spectrum and magnetic-field geometry—not by choosing the conductor with the lowest DC resistance. Skin effect, proximity effect and core-gap fringing can make a seemingly low-resistance winding run hot at high frequency.
What causes high-frequency winding loss?
Winding loss has a DC component and a frequency-dependent AC component. A useful first approximation is:
- DC copper loss: PDC = IDC2 × RDC.
- Ripple-related AC loss: PAC ≈ IAC,rms2 × RAC(f).
For a nonsinusoidal ripple, calculate AC loss across its significant frequency components rather than assuming one frequency represents the waveform. The winding’s effective AC resistance varies with frequency, so a useful model sums the loss associated with each harmonic. The total copper loss is approximately the DC and AC contributions added together; it is separate from core loss.
Skin effect
A conductor’s changing magnetic field pushes current toward its surface. The resulting current crowding reduces the effective copper cross-section and raises AC resistance. Skin depth decreases as frequency rises; calculate it for the conductor material at the fundamental and at harmonics that carry meaningful ripple energy.
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Proximity effect and gap fringing
Fields from adjacent turns redistribute current within a conductor. This proximity effect can be significant in multilayer windings and flat conductors, where neighboring surfaces face one another. A core gap adds another concern: its fringing field can be intense near the gap and cause severe local current crowding in nearby turns. Average field or DC resistance alone will not reveal that hot spot.
Why the lowest DC resistance may not be the lowest-loss choice
A large solid wire or thick foil may have low RDC, yet incur high RAC at the operating frequency because current uses only part of the copper or crowds into particular regions. Compare candidates at the intended DC current, ripple RMS and waveform, switching-frequency spectrum, winding geometry and thermal conditions.
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Dowell-style models estimate AC-to-DC resistance using factors such as skin depth, conductor thickness or diameter, frequency and layer count. The Wiley treatment covers foil, strip, round and multistrand conductors, including harmonic currents. Use a suitable model or field simulation to compare realistic winding arrangements; do not treat a conductor’s nominal resistance as a winding-loss prediction.
How to choose a winding conductor
There is no universally lowest-loss conductor. The best fit depends on AC resistance at the relevant harmonics, DC resistance and temperature rise, ripple capacity, gap-field exposure, parasitic capacitance, fill and insulation, manufacturing repeatability and cost.
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| Winding option | Potential advantage | Main loss or design concern | When to consider it |
|---|---|---|---|
| Solid round wire | Simple, inexpensive and capable of low DC resistance when generously sized. | Large diameter and adjacent winding layers can increase skin and proximity loss. | When frequency is low enough, or the winding geometry limits field exposure. |
| Litz wire | Individually insulated, transposed strands can reduce skin and proximity effects. | Strand insulation adds complexity and can increase DC resistance and cost relative to foil. At very high frequency, proximity within the bundle can make AC resistance greater than that of solid wire. | When strand diameter and strand count suit the operating frequency and RMS current, and the bundle’s behavior is checked at the actual frequency. |
| Plain foil | Can provide low DC resistance and high current capacity. | Each turn behaves as a layer; proximity and gap-fringing fields may sharply increase AC resistance. | When a layered winding’s AC loss and gap-field exposure have been evaluated for its geometry. |
| Foil-cut or shaped foil | Modified copper near the gap can help produce a more uniform current distribution. | Performance depends on the specific cut, shape, core and winding geometry; test results are not transferable guarantees. | When concentrated gap fields make ordinary foil loss problematic and a suitable design can be built. |
| Single-layer or distributed-gap structure | Can reduce current crowding or exposure to concentrated fringing fields when properly matched to the core. | Requires a compatible core and winding layout; no universal loss advantage is established independent of geometry. | When the design can avoid an intensely fringing gap beside winding turns or distribute the gap effect. |
Specify litz by frequency and current
Litz wire combines many individually insulated strands that are transposed along the bundle. Choose strand diameter with frequency and skin depth in mind, and choose strand count to carry the required RMS current. New England Wire Technologies describes its selection process as starting with frequency and the engineer’s RMS-current requirement. More strands are not automatically better: at very high frequency, bundle proximity can outweigh the benefit of finer strands. Verify strand diameter, count, insulation and temperature rating against the winding design rather than relying on a generic “high-frequency” label.
Treat foil-loss claims as design-specific
West Coast Magnetics reported up to 68% lower winding loss than full foil for a modified cut design tested at 100 kHz, 30% ripple and 30 A DC. The same supplier reported foil-cut outperforming solid wire, litz and full foil in its tested design above 10 kHz. These are results for the tested designs and conditions, not a guarantee for a different core, winding, waveform or thermal setup.
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Keep the core gap from becoming a winding hot spot
Choose gap placement as part of winding design. Turns immediately beside a concentrated gap may experience strong fringing fields and high local proximity loss. Where the core and electrical requirements allow, consider moving turns away from the gap, using a distributed or quasi-distributed gap, or adopting a winding shape that better equalizes current density. Single-layer, foil-cut and shaped-foil structures can help only when their geometry is matched to the field and core.
An IEEE design paper from 2019 discusses single-layer or multilayer alternatives to conventional litz and the use of quasi-distributed gaps to mitigate fringing-field loss. Its example was an approximately 15 µH inductor with Q≈720 at 3 MHz and 2 A peak. That is an example of a particular design, not a general performance target.
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A practical workflow for reducing winding loss
- Define the electrical and thermal case. Record switching frequency and significant ripple harmonics, DC current, ripple RMS and peak, allowable temperature rise, inductance, saturation margin and available winding geometry.
- Calculate relevant skin depths. Check the fundamental and significant harmonics using the conductor material. This indicates whether a candidate’s diameter or thickness is large relative to the depth in which current can usefully flow.
- Estimate AC resistance for actual layouts. Use a Dowell-style model or FEM to account for conductor dimensions, layer count, neighboring turns and frequency. Compare solid round wire, single-layer wire, foil, litz, foil-cut and shaped foil where they are manufacturable.
- Include the construction constraints. Account for insulation thickness, fill factor, turn length, thermal path, parasitic capacitance and self-resonance, repeatability and manufacturing difficulty—not just copper cross-section.
- Check the gap field against the winding. Inspect whether turns sit in intense fringing fields. Evaluate alternative gap placement, distributed or quasi-distributed gaps, and field-compatible winding shapes.
- Prototype or simulate with the real waveform. Validate predicted winding loss under the intended ripple waveform and operating conditions. Where possible, distinguish winding loss from core loss so an improvement in one is not mistaken for an improvement in the other.
Validate estimates before committing to a design
Analytical models are useful for narrowing candidates, but geometry and current waveform matter. Confirm the design with field simulation or loss measurements on the intended construction. A supplier’s measured result is informative only when its core, winding layout, frequency, ripple, current and measurement conditions are comparable to yours. If those details differ or are not stated, treat the result as a design lead rather than a transferable performance figure.
The reliable decision is the winding that meets current, temperature and electrical constraints with acceptable total loss in the actual inductor—not the one that merely has the lowest quoted DC resistance.
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