7.1.6 LSB or LPB-Inductance With Bias Current (Series or Parallel Circuit)
With the LSB and LPB tests, the AT3600 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 the windings.
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 calculated.
The bias current can be programmed in the range 10mA to 400mA. The (ac) test signal can have a frequency between 20Hz and 1MHz, and an amplitude from 1mV to 5V.
Usually, the signal frequency and voltage are chosen so that, with the expected inductance, the resulting signal current is less than 20% of the bias current.
Recommended test signals generally would be the same as given in section 7.1.8 for the normal inductance test. However, if the recommended levels correspond to too high a signal current, then use a corresponding smaller test voltage.
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.