Window utilisation — the fraction of the bobbin window actually filled with copper — is the number that decides whether a design fits. It is also the number most often assumed rather than calculated, which is why prototypes come back with a winding that will not close or a DCR 30% above target.
1. The definitions
Window area (Aw) is the cross-section available inside the bobbin, in mm². Copper area is the sum of the bare copper cross-sections of all turns, primary and secondary. Fill factor Ku = copper area / window area.
Practically achievable values:
| Winding type | Typical Ku | Constraint |
|---|---|---|
| Single-layer, large wire | 0.55–0.65 | Round wire packing |
| Multi-layer, enamelled wire | 0.40–0.55 | Layer insulation and crossover |
| Foil winding | 0.60–0.75 | Insulation between foils |
| Triple insulated wire | 0.25–0.40 | Thick insulation on the wire |
| Margin tape design | 0.30–0.45 | Dead margin on both sides |
2. Why the number is smaller than it looks
Three losses are usually forgotten. Layer insulation between every layer of wire removes 10–20% of the window. The bobbin wall and the creepage margin remove more. And round wire does not tile: in a single layer of round conductors, the theoretical packing limit is about 0.785 before any insulation is counted.
So a calculation that yields Ku = 0.6 with bare copper is not a design that will wind. Aim for 0.4 with insulation included, and check the actual wound part.
3. Calculating the required window
For each winding: copper area = N × (π/4) × d², where d is the bare conductor diameter, or N × strand area for Litz. Add the insulation build-up and layer tape, then sum the windings, then divide by the target Ku. If the result exceeds the bobbin’s window, the options are a larger core, finer wire (which raises DCR), foil instead of round wire, or a lower fill design with a bigger core.
4. The trade-off nobody wants to make
Finer wire fits but raises DCR and copper loss. A larger core fits but raises cost, height and weight. Foil fits well but is difficult below about 10 turns and awkward on small bobbins. The correct choice depends on which constraint is actually binding: in a 1U power supply it is height; in a cost-driven appliance it is material; in a medical isolation transformer it is the insulation barrier, which consumes window before anything else.
5. Improving utilisation without changing the core
- Reduce layer insulation where the voltage stress allows — a thinner tape or a single layer instead of two.
- Interleave rather than stack: fewer layers for the same turns.
- Use rectangular or foil conductor where the turn count is low.
- Replace margin tape with triple insulated wire where the volume justifies the wire cost.
- Move to a bobbin with a taller, narrower window that matches the wire diameter.
6. Verify on the wound part
Cut one sample and measure the copper area and the remaining window. A cross-section also reveals whether the layers are compact or whether crossover is eating space. Shaanxi SiRui Technology checks window utilisation at the design-review stage for Magnetic Core series wound parts, so a design that cannot be wound is rejected before tooling rather than after the first pilot lot.
Send your turns, wire gauge and bobbin to sales@sirui-tech.com for a windability check.