2.5 LSB, LPB - Inductance With Bias Current
With the LSB and LPB tests, the AT5600 offers two further ways to confirm
that the transformer has been assembled properly, with the appropriate
number of turns, the right grade of magnetic material for the core, and
the correct air gap if required.
These tests would normally be used for transformers designed to be used
in applications where a large dc current is flowing through one or more of
To measure inductance with bias, the tester firstly applies the
programmed bias current to the winding under test.
When this has stabilised, it then applies an ac voltage across the
selected winding, and measures the voltage across and the current through
the winding using harmonic analysis. The measured voltage is divided by
the harmonic current to obtain a complex impedance and the inductance is
The bias current can be programmed in the range 10 mA to 400 mA. The (ac)
test signal can have a frequency between 20Hz and 3 MHz, and an amplitude
from 1 mV to 5 V.
Usually, the signal frequency and voltage are chosen so that, with the
expected inductance, the resulting signal current is less than 20% of the
Recommended test signals generally would be the same as given in section
1.11 for the normal inductance test. However, if the recommended levels
correspond to too high a signal current, then use a corresponding smaller
It is not normally recommended that the test frequency be increased in an
attempt to reduce the signal current, as this may lead to other problems,
such as those caused by parasitic inductance and capacitance of the test
fixture, and the self-resonant frequency of the transformer itself.