Cores for small dc bias currents
Wound components for small dc bias currents are generally
constructed with cores having medium to high permeability.
The value of permeability of such cores varies from batch
to batch, as it depends on the manufacturing process of the core itself.
This variation results in a wide tolerance of the
measured inductance of the winding, which is seen in the wide tolerance of
inductance constant (AL) of core manufacturers specifications.
This variation in inductance results in the possibility
that some coils will be able to tolerate the specified dc bias current and
some will not:
The only sure way of verifying whether the coil can
operate with specified dc current is to measure the inductance with
this small dc bias current flowing, ensuring that the inductance is
at least the specified minimum value.
Cores for larger dc bias currents
As mentioned earlier, coils for higher dc bias currents
(greater than about 400mA) have a low permeability core due to air gaps.
As the air gap is increased the permeability and,
therefore, inductance falls, and the dc current capability increases, as
shown for a typical air-gapped ferrite E-Core below. (The number of turns
is the same for each value.)
DC Current Capability
Provided that the core does not saturate, which is
established during the design phase, as described above, the value of
inductance for any transformer will be the same with or without dc bias
To illustrate this, the graph below shows inductance
measurements obtained from the transformer in the table above with no dc
bias, compared to the same transformer with the specified dc bias applied.
For cores with larger air gaps, the permeability and,
therefore, the inductance are determined predominately by the size of the
gap and are much less affected by variations in the core material.
This results in the variation in inductance being much
smaller with a gapped core, as the gap has a much more constant
permeability than the magnetic material itself.
The value of inductance will therefore be predictable
within a tight tolerance.
It follows, therefore, that a measurement of inductance
(without dc bias) of such a coil provides the necessary check that the
core has the correct air gap and, therefore, has the ability to operate at
the specified dc current.