How is Leakage Inductance tested and what precautions should be taken?
Manual test Method:
When testing Leakage Inductance manually, a soldered short circuit is applied to the secondary of the transformer to be tested and the value of inductance on the primary is measured. This value of inductance will be recorded as the ‘true’ leakage inductance (e.g. 150μH).
The inductance will then be measured on the same transformer after the soldered short circuit has been replaced by either a shorting clip or a fixture with relay-operated short circuit, depending on the technique that will be chosen for production. The measured inductance is again recorded (e.g. 180μH). This value will, of course, be greater than the original because it includes the true leakage inductance plus the short-circuit error inductance. The difference between these two values (in our example 30μH) is then used in production testing as a fixed offset that is programmed into a production LCR meter to obtain an approximation of the correct value in the presence of an imperfect short circuit.
The Voltech solution:
Voltech have developed an architecture and processing ability to remove the short-circuit error from the primary inductance measurement during each and every test.
Before applying a short circuit, the Voltech AT series testers measure the primary to secondary turns ratio. The AT testers then automatically apply a short circuit, using an internal relay matrix, and measure the short-circuit voltage at the transformer secondary pins.
The vector of this short circuit voltage is automatically multiplied by the turns ratio, producing an 'error vector' that is equal to the short-circuit error voltage reflected into the primary measurement. The leakage inductance is then computed from the total primary inductance value less the primary error vector that has been calculated. This process enables Voltech AT series testers to provide the true leakage inductance value, irrespective of short circuit variability.
Please see attachment “Measuring Leakage Inductance” for a more detailed explanation.
Also see attachment “Schematic when using LL”