7.1.9 LL or LLO - Leakage Inductance
Leakage inductance is important in many applications. One example is flyback designs for high frequency switched mode power supplies, where the leakage inductance must be less than a specified critical value for proper operation.
Leakage inductance is tested by measuring the inductance of a 'primary' winding when one or more 'secondary' windings are shorted out. In performing the calculation at the end of the test to extract the inductance value from the measured winding impedance, the tester uses a series equivalent circuit.
In making the measurement, the tester automatically compensates for the impedance of the wiring, the connections and the relays in the shorting path.
Leakage inductance can be measured using a test current in the range 20μA to 100mA at a frequency of 20Hz to 1MHz.
You may choose a suitable test current and frequency based on the expected value of the leakage inductance using the following table:
|Leakage Inductance range
||Preferred test signal
|100nH → 1uH
1uH → 10uH
10uH → 100uH
100uH → 1mH
1mH → 10mH
10mH → 100mH
100mH → 1H
1H → 10H
The Test Conditions for Leakage Inductance Measurement
NOTE: Because leakage inductance is measured with a secondary winding shorted out, be careful to choose a test signal that will not cause excessive currents to flow. This is particularly significant in transformers where the turns ratio is very high and the shorted winding has only a few turns.
If, for example, the primary winding has 300 turns, and the secondary 3 turns, a test current of 10mA flowing through the leakage inductance on the primary side will give rise to a current of 1 Amp flowing in the shorted secondary winding.
In order to protect transformer windings, the test current when measuring leakage inductance is limited in the table to 50mA maximum.
In addition, the problem of self-resonant frequency listed under the primary inductance test also applies when measuring leakage inductance, so always use the lower of the available band of frequencies.
User Offset (LLO test only)
This is used to offset the result returned by the AT by a user defined amount.
Result returned = Measured result + User offset
Normally a user offset would not be required. However for small values of leakage inductance (<1μH) when there are a number of additional low terminals, it is not always possible during fixture compensation for the tester to remove all of the parasitic errors of the test fixture. In this case, the user offset may be required to fully remove the errors. The LLO test provides this additional feature.