When should I choose Litz wire over solid wire or copper foil?

Litz wire is ideal for multi-layer high-frequency windings above 20 kHz to 50 kHz where proximity effect dramatically increases AC resistance in solid conductors. Copper foil is superior for high-current single-turn or low-turn secondaries, while Litz wire dominates multi-turn primaries and high-voltage secondaries.

What is the Dowell proximity effect and why does it scale with m²?

In a multi-layer transformer winding, each successive layer is immersed in the cumulative magnetomotive force (MMF) produced by all preceding inner layers. This intense perpendicular magnetic field induces circulating eddy currents whose power loss scales with the square of the layer number (m²).

Why use Extruded TIW wire instead of standard enamel wire with Margin Tape?

Extruded Triple Insulated Wire (TIW) provides reinforced insulation meeting IEC 61558 safety standards without needing physical creepage margin tape at the bobbin flanges. This reclaims 4 mm to 6 mm of traverse window length, allowing more turns per layer, fewer total layers, and significantly lower proximity loss.

Universal Winding & Window Fit Studio — Magnetics Design Toolkit

Hi, I'm a power electronics engineer.

Designing high-frequency magnetic components is often a frustrating cycle of guessing wire sizes, wrestling with complex Dowell formulas in Excel, and finding out during prototype winding that the bobbin window overflows.

I built this Universal Winding Studio to solve the complete problem domain: from Sullivan-Zhang strand optimization and strand twisting factorization to exact bobbin fit and AC resistance. Every equation has been verified against peer-reviewed IEEE papers and IEC standards.

Questions, feedback, or need custom core geometries? Contact me at:contact.engineeringdesigntools@gmail.com

Conductor Technology:

Winding Window Space (Shared Window)

Window Width Ww (mm)

Window Height Hw (mm)

Number of Turns (N)

Conductor Specification & Insulation

Outer Serving Type

Enamel Insulation Grade (IEC 60317)

Strand Diameter d (mm)

Strand Count N

Electrical & Thermal Parameters

Operating Frequency f (kHz)

RMS Current I_rms (A)

Target Current Density J (A/mm²)

Winding Temperature T_op (°C)

Proposed Spec 375/0.100 (25 × 3 × 5) Standard SKU: LZ-375-0100-SN

Outer Diameter (OD) 2.72 mm Strand wire: ⌀0.100 mm

AC Loss Factor F_R (AC/DC) 3.92× Skin depth δ: 0.240 mm

Copper Loss P_Cu 19.50 W R_AC = 60.2 mΩ (R_DC = 15.4 mΩ)

Conductor Cross-Section & Current Density Adaptive Cross-Section

Window Build Utilization WARNING

82.7 % (≤ 80% Safe)

0 % Warn: 80 Max: 100

Ratio of total winding build thickness W_build to usable window width W_usable.

AC Proximity Factor (F_R) FAIL

3.9 × (≤ 1.6 Optimal)

1 × Warn: 1.6 Max: 2.5

AC resistance multiplier due to proximity and skin effect (Dowell).

Current Density (J) PASS

6.1 A/mm² (Target: 6 A/mm²)

0 A/mm² Warn: 6.8999999999999995 Max: 8.100000000000001

Actual RMS current density in bare copper cross-section.

🩺

Winding Health & Diagnostics

1 Notice

TIP

Alternative Conductor Recommendation

On this shared core window, switching to Planar PCB Coil yields a 60% reduction in AC copper loss (7.70 W vs. 19.50 W).

Recommendation: Switch to the Planar PCB Coil tab to evaluate detailed winding parameters.

📊

Compare 5 Conductor Technologies on Shared Window

Shared Window Evaluation · Dowell 1D Loss Comparison

Conductor Technology	Proposed Spec	R_DC (mΩ)	F_R (×)	R_AC (mΩ)	Loss P_Cu (W)	Cu Fill (Ku)	Build Util (%)	Window Fit	Action
Litz Wire Active	375/0.100 (25 × 3 × 5)	15.4	3.92×	60.2	19.50 W	27.2%	83%	⚠ TIGHT	Active
Solid / Multifilar Wire	2filar × ⌀1.37 mm	13.3	29.95×	398.9	129.24 W	27.2%	48%	✓ PASS
Flat / Edgewise Wire	2.97 × 0.99 mm (Edgewise)	14.2	16.72×	236.6	76.65 W	27.2%	62%	✓ PASS
Copper Foil	23.4 mm × 126 µm	13.7	4.94×	67.5	21.87 W	27.2%	54%	✓ PASS
Planar PCB Coil	12-Layer Stackup, 3 oz (28.0 mm trace)	16.1	1.48×	23.8	7.70 W	27.2%	95%	⚠ TIGHT

Foundational Physics & Formulas

⚡

Skin Depth & Copper Resistivity

δ = √(2ρ(T) / (2π f μ₀))

The skin effect confines AC current to the outer perimeter of the conductor. Resistivity ρ(T) is temperature-corrected with copper temperature coefficient α = 0.00393/°C.

🌀

Sullivan-Zhang Strand Optimization

d_opt = δ(T) · (πη/4)^-1/4 · [3/(5m² - 1)]^1/4

Balances DC conduction loss (favoring larger strands) against AC proximity eddy loss (favoring smaller strands). In multi-layer windings (large m), the optimal strand diameter is significantly smaller than skin depth δ.

📐

Generalized Dowell 1D Method

F_R = Δ · [ M(Δ) + (2/3)(m² - 1) D(Δ) ]

Solves 1D magnetic field distribution parallel to the winding axis to calculate the AC-to-DC resistance ratio F_R = R_AC / R_DC across all 5 conductor technologies.

🛡

Extruded TIW vs. Margin Tape

H_usable = H_win - 2·t_flange - 2·d_margin

Extruded Triple-Insulated Wire (TIW) provides reinforced insulation compliant with safety standards, eliminating margin tape (d_margin = 0 mm) and reclaiming up to 25% of bobbin winding length.

High-Frequency Transformer Winding Guide

1. Four-Step Industrial Design Workflow

Determine Bobbin Window Space: Select standard ferrite core geometry (ETD, PQ, EE, or Planar) and verify usable bobbin height H_usable after flange clearance and safety margins.
Select Conductor Technology: For operating frequencies > 50 kHz with layer countm ≥ 2, Litz wire is standard. For high-current secondaries (> 20 A), evaluate Copper Foil or Flat wire.
Calculate Strand Size & Stranding Structure: Apply Sullivan's equation for optimal strand diameterd_bare, solve strand count N = I_RMS / (J · A_strand), and verify cabling stage decomposition (e.g. 375 = 25 × 3 × 5).
Verify Window Fit & Dowell Proximity Factor: Ensure total build thickness W_build ≤ W_usable(< 80% utilization for manufacturability) and AC proximity factor F_R ≤ 1.6.

Related Design Tools

Feedback & Engineering Inquiries

Collaborate & Improve

Found a calculation discrepancy, need a specific ferrite core library, or want to suggest a new tool? I actively welcome feedback from fellow hardware and power electronics engineers.

Send Feedback Email