Datasheet

AN3180 Electrical equivalent circuit models of coupled inductors and transformers
Doc ID 17273 Rev 1 35/39
is associated to the magnetic flux that flows mostly through the magnetic core and so, is
affected by the non-linearity typical of magnetic materials. If the flux density exceeds the
saturation level, the permeability of the magnetic material µ drops dramatically almost to the
free space value µ0; as a result L
M
tends to be a much lower value and shunts the ideal
transformer, so that, essentially, only L
l1
and L
l2
are visible from the terminals. Moreover, the
windings are nearly decoupled and energy transfer is extremely low.
Finally, although transformers and coupled inductors work in a substantially different way
(coupled inductors store energy, transformers ideally do not; in coupled inductors the
magnetizing current -
i (t) in Figure 24 - is directly related to the winding currents i
1
(t) and
i
2
(t), in transformers it is related to the winding voltages v
1
(t) and v
2
(t) and to time, and this
difference reflects also in the core saturation mechanism), they may be described by the
same equivalent circuits. Therefore, the basic model of
Figure 24 and its derivatives of
Figure 25 to Figure 29 can be indifferently considered as both coupled inductor models and
transformer models.