Sectioned windings reduce leakage inductance, voltage stress, and AC loss. They improve EMI behavior and thermal distribution. OEM designs in audio and LED drivers apply this for compact, reliable high-frequency transformers.

    At high frequencies (kHz to MHz), parasitic effects dominate. Sectional winding, also called interleaving or splitting, is a critical technique to control these effects.

Primary Purposes:

  1. Reduce Leakage Inductance ($L_{lk}$): Leakage inductance is the magnetic energy stored in the space between windings that does not couple. By splitting one winding and placing the sections adjacent to the other winding (e.g., Primary-Half-Secondary-Primary-Half-Secondary for a two-section interleave), the magnetic coupling is dramatically improved. The opposing ampere-turns from adjacent layers cancel the stored magnetic field in the gap between them.

    • Benefit: Essential for high-efficiency, high-frequency power converters (e.g., LLC, forward, flyback) to minimize switching loss and voltage spikes.

  2. Reduce High-Frequency Winding Loss (Skin & Proximity Effect Loss):

    • Proximity Effect: In a solid multilayer winding, the AC current in one layer induces eddy currents in adjacent layers, forcing current to crowd to the sides of the conductor facing the gap, drastically increasing effective resistance ($R_{ac}$).

    • How Sectioning Helps: By reducing the number of layers per section and placing primary and secondary layers close together, the transverse magnetic field (the field perpendicular to the conductor surface) is reduced. This mitigates the severity of the proximity effect, lowering $R_{ac}$ and copper loss at high frequency.

  3. Improve Thermal Distribution: Sectioning spreads the heat-generating windings more evenly across the bobbin window, avoiding a single “hot spot” in a thick multilayer winding. This aids in heat dissipation.

  4. Reduce Inter-Winding Capacitance: While interleaving increases capacitance between the interleaved layers, it can allow for a more optimal overall capacitive structure, sometimes helping to shape the resonant frequency in certain designs.

Handbook’s Design Rule: The choice of interleaving strategy (e.g., 1:1, 2:1, 4:2) is a detailed trade-off between minimizing $L_{lk}$ and minimizing $R_{ac}$ for a specific frequency and duty cycle. Simulation and careful measurement are often required.