Inductance is an important physical quantity that determines a transformer’s response characteristics against electric signals. Inductance is generally categorized into self-inductance and leakage inductance. Self-inductance is an indicator of to what extent the transformer can produce magnetic flux, and leakage inductance is an indicator of how much magnetic flux the transformer can send from the primary coil to the secondary coil without leaking. This is why self-inductance and leakage inductance are important items for transformer design requirements.
The amount of inductance is dependent on the magnetic circuit, but the nonlinear characteristics of the magnetic properties make it so that the magnetic circuit changes when the operating point changes. The leakage inductance has all of the same properties, but it also has a flux path in non-magnetic regions, making it easily affected by the arrangement and geometry of the winding in addition to the core. This is why a magnetic field analysis using the finite element method (FEM) is necessary when evaluating these types of inductance.
This Application Note explains how to obtain self-inductance and leakage inductance for two types of secondary coiling in a transformer: uniform coiling and close coiling.

The inductance of the transformer when the secondary coil has a uniform coiling and close coiling is indicated in table 1. The model that has a secondary coil with a closed coiling has a larger leakage inductance.

The leakage inductance is obtained from the flux linkage of the primary coil when the secondary coil has a short circuit.
The flux linkage of the primary coil is evaluated as the flux leakage because the magnetic flux produced by the primary coil eliminates the flux linkage of the secondary coil. The variations of the flux leakage versus time can be evaluated from the voltage terminal of the primary circuit because they are induced voltage.