Transformer design for high coupling: To achieve a high coefficient of coupling k (i.e., strong magnetic linkage between windings), fixed transformers are typically wound on a __________ so that the primary and secondary share the same magnetic path.

Introduction / Context:
A transformer transfers energy from primary to secondary by magnetic coupling. The fraction of magnetic flux from the primary that links the secondary is captured by the coefficient of coupling k, which ideally approaches 1. Understanding how construction affects k is essential for minimizing leakage inductance and improving regulation in power and signal transformers.

Given Data / Assumptions:

• The transformer is a fixed (not air-gapped tunable) unit.
• Goal: maximize flux linkage between windings.
• Windings and core style can be selected to enhance coupling.

Concept / Approach:
Magnetic coupling improves when both windings share the same concentrated magnetic path. Winding the primary and secondary on a common core leg (or interleaving them) ensures most primary flux passes through the secondary window, raising k and reducing leakage. Construction choices like core material and lamination style (EI, CI, toroidal) matter, but the key design idea is a common core path for both windings.

Step-by-Step Solution:

Verification / Alternative check:
Interleaved windings on a common core leg (including toroidal and EI with both windings on the center leg) are standard practice for high coupling, confirming the principle independent of a specific lamination shape.

Why Other Options Are Wrong:

• EI core / CI core: These identify lamination shapes, not the coupling principle. Either can be common or not, depending on winding layout.
• Iron core: A material choice that increases permeability but does not, by itself, guarantee high k without shared path.
• Aluminum core: Non-magnetic; would not support efficient coupling.

Common Pitfalls:
Equating core material or alphabet shape (EI, CI) with coupling quality; ignoring winding placement and interleaving which dominate k.

Final Answer:
common core