The right thing of measuring leakage inductance?

In your document, A can be used to measure leakage inductance and B can be used to measure inter-winding capacitance.

Thanks a lot.

What are you working on with the cute pancake coils? What are some of your typical coil parameters and their measured inductance and leakage inductances?

Method A - method B only measures the inter-coil capacitance.

The thinking underlying method A is that the voltages across coils 1 and 2 are given by

V1 = L1. dI1/dt + M. dI2/dt

V2 = L2. dII2/dt + M. dI1/dt

where M is the mutual inductance of the two coils. If the coils are

M = (L1. L2 )^0.5

Method A forces V1 to be zero

L1.d1/dt + M. dI2/dt = 0

This can be re-arranged to gives us an expression for d I1/dt

dI1/dt = - (M/L1) /dI2/dt

which can be substituted into the second equation to give us

V2 = L2. dI2/dt - M^2/L1. dI2/dt

The method is imperfect, because slightly more real coils have resistance, which means that

V1 = L1. dI1/dt + M. dI2/dt + R1.I1

V2 = L2. dI2/dt + M. dI1/dt + R2.I2

An even more realistic mathematical model would take the parallel capacitace of each coil into account, and a serious model would include a third single turn "coil" to represent the current induced in the transformer core ...

Hope this helps.

As others have mentioned, 2nd circuit measures interwinding capacitance. Note that using the 1st circuit for leakage inductance frequently yields incorrect results if the measurement frequency is too low. Common LCR bridges use 50/60 Hz or 1kHz, and the resistance of the windings and the shorting link 2a-2b dominate the measurement. If a bridge with a higher test freq is unavailable, best to use another method of measuring inductance, e.g., di/dt with V step or ring-down methods. These have been described many times on this group. Paul Mathews

Probably a homework assignment.

John