WHAT IS LEAKAGE INDUCTANCE?

Leakage inductance is an inductive component present in a transformer that results from the imperfect magnetic linking of one winding to another. Any magnetic flux that does not link the primary winding to the secondary winding acts as inductive impedance in series with the primary, therefore this "leakage inductance" is shown on a schematic diagram as an additional inductance before the primary of an ideal transformer.

In certain applications, such as switched-mode power supplies and lighting ballasts, leakage inductance of the transformer may play a critical function in the product design. For this reason, accurate measurement of leakage inductance is often an important test function for transformer manufacturers. In order to avoid confusion with other transformer characteristics, this technical note will not refer to other components of loss such as winding resistance or inter-winding capacitance.

Ideal transformer

For a theoretical, ideal transformer, there are no losses. Voltages are transformed in the direct ratio of the turns; currents in the inverse ratio of turns (figure 1).

Real transformer

In a real transformer, some of the flux in the primary may not link the secondary winding. This "leakage" flux takes no part in the transformer action and can be represented as an additional inductive impedance that is in series with the primary winding (figure 2).

Real transformer plus an air gap

In certain transformer designs, leakage inductance must be a greater proportion of the total inductance and is specified within a tight tolerance. The increased proportion of leakage inductance is usually achieved by introducing an air gap in the core design, thus reducing the permeability of the core and therefore the value of primary inductance. The ratio of flux that does not link the primary winding to the secondary winding will therefore increase relative to the flux that links both windings (figure 3).