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
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
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).
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).