Electronic – Flyback Transformer Primary Inductance Calculation

The proper test is applying the 120V AC and if this implies an inductance of 106 H then that is what I'd assume. So why does the meter indicate a lower inductance? One thing that might indicate a "problem" is the measurement of 7.7nF. If you take the inductance reading of 106 H and calculate the resonant frequency with 7.7 nF you get 176 Hz and this is much to close to make sense of any inductance measurements on your LCR bridge.

Another factor is that the eddy current induction into the laminations rise with some factor of frequency (I think it's like skin effect so it rises with \$\sqrt{F}\$ from memory). Eddy currents are like partially shorted turns and therefore have the effect of lowering the perceived magnetization inductance of the core.

The easiest way is to connect the open-circuited transformer and observe the current circulating in the primary side. The frequency is adjusted so that the current remains discontinuous over a switching period. The demagnetization time is quite long as it is given by the off-slope \$V_f/L_p\$ therefore the switching frequency will probably be a few hundred of Hz. The generator will drive the MOSFET on and off with a square-wave signal featuring a peak value of 10-12 V to ensure the right turn-on sequence. When you apply the dc voltage across the inductance by turning the MOSFET on, the current starts from 0 (the current is discontinuous with a low frequency) and grows to a peak value with a slope equal to \$\approx\frac{V{_{in}}}{L_p}\$ where \$L_p\$ is the inductance you want. If you observe the current across the \$1-\Omega\$ resistance, you should see something like below:

You select a portion of the current and measure \$\Delta t\$ and \$\Delta I\$. You compute the slope by \$S=\frac{\Delta I}{\Delta t}\$. This slope is equal to \$S=\frac{V{_{in}}}{L_p}\$. You can now extract the inductance value following \$L=\frac{V_{in}}{S}\$. Assume you measure a slope equal to 10 kA/s (10 V applied across a 1-mH inductance) and the source is biased to 10 V. Then the inductance is \$L=10/10000=1\;mH\$. You have to know that depending on the transformer model, the measurement may include the leakage inductance term \$l_l\$. In this case, the measured slope is \$S=\frac{V{_{in}}}{L_p+l_l}\$.

With this circuit, you can now either increase the source voltage or the on-time (keep the current discontinuous) and check the saturation current \$I_{sat}\$ as shown below. The best is to do this experiment when the core is at its highest operating temperature.