3.8 DC Chokes
Multi winding chokes are becoming increasingly popular in multi-output
switched mode power supplies, offering enhanced regulation characteristics
compared to single chokes. Multi-winding chokes make good use of the
versatile testing capability of the AT SERIES.
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.
High Permeability High Inductance -------------------Low Permeability Low
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 wide tolerance.
The low permeability of cores with an air gap or made of powered iron
causes these cores to exhibit an inductance that can be specified within
very tight limits.
It is essential during design testing to confirm that a dc choke exhibits
the proper inductance at rated dc current. In production testing, however,
dc chokes can be tested by checking the inductance without dc bias -
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.
Recommended measurements for dc chokes.
||Checks correct wire size has been used
||Checks for correct turns, and correct
permeability or air gap
|Checks turns and phasing of all windings
|Checks that winding have been correctly
positioned on core or bobbin.
|Checks working insulation between windings.