Where Used

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.

Measurement Conditions

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 20mA 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:

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.