1, What Is DC Bias And When Should It Be Tested?
In the context of a transformer or choke, DC bias relates to a constant
current element that is added to the AC signal.
Many wound components must operate with DC currents flowing through
them and, during the design stage, it is necessary to establish that the
component will function correctly with the specified current.
In the case of production testing however, it is possible to confirm
the correct assembly, and therefore correct operation of a wound
component, without applying a DC bias.
However, For best confidence, you should check with DC bias present,
using a DC bias unit such as DC1000A or at least perform an audit.
The Voltech AT5600 allows for integration of DC1000A and automatic
audit of DC performance.
This tech note discusses the issue in some detail and explains how
production facilities can sometimes identify good components with complete
confidence in the absence of a DC bias generator.
2, Low-current And High-current Applications
In some cases, the DC bias current is small (under
As, for example, in telecom transformers where a winding
is in series with the DC power supply current to the telephone.
In other cases, the DC bias current is much larger, such
as in inductors used as output filters on power supplies:
In all these cases, the wound component must retain a
specified inductance with the rated dc current flowing in the winding.
3, Design Considerations
Magnetic materials such as iron and ferrite generally
have a high value of permeability, i.e., a coil of a given number of turns
will have much more inductance than the same core in air.
However, a wound component with a high-permeability core
has a very steep B-H curve and, therefore, can tolerate only a very small
DC bias current or the core will saturate.
If the core saturates, the inductance will fall to a very
In order to make a coil that will operate with higher
values of dc bias current, it is necessary to reduce the permeability of
This is done by introducing air gaps in the magnetic
circuit, either by using a physical spacing or by using a core made of a
composite of magnetic and non-magnetic materials (providing the effect of
Cores with air gaps have a much lower overall
permeability and can tolerate much larger dc currents before saturating:
4, DC Bias Testing
4.1 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.
4.2 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.
5 DC Bias Conclusions
All DC chokes use low-permeability cores, either powdered
iron or ferrite cores with a substantial air gap.
A low permeability is essential to prevent the core
saturating with a large dc current.
Inductance is a measure of the slope of the B-H curve. A
core with high permeability can have a slope or inductance value with a
The low permeability of cores with an air gap or made of
powdered iron causes these cores to exhibit an inductance that can be
specified within very tight limits.
Cores for low dc bias currents (<400mA)
Cores for high dc bias currents (>400mA)
| Measure Inductance
with specified dc current in winding.
Accept wide limits on range of inductance values but result must
be greater than a certain minimum value.
| Measure inductance without dc
Set limits as tight as possible e.g. 5% as means to verify gap.
It is essential during design testing to confirm that a
dc choke exhibits the proper inductance at rated dc current.
In production testing, however, some dc chokes can be
tested by checking the inductance without dc bias, but
Specifying tight limits will verify that the core has the
correct turns and, therefore, the correct slope to give the required
inductance at the specified dc current.