While designing high voltage output Flyback converter, we need to ensure that the primary inductance of flyback transformer is same as required with an air gap for DCM mode. But when we use LCR meter, it measures primary inductance different at different frequencies especially when you have large secondary turns for high voltage purpose. So what should be the LCR set frequency at which it would measure true inductance?
Thanks in advance.
Hi Imran,
generally, the best frequency for measurement is the frequency you expect your converter to work at. However, flyback converters produce a spectrum of frequencies, so you could take also the nearest harmonics into account.
With increasing number of turns of the secondary winding the capacitance of the secondary coil (and perhaps the capacitance between primary and secondary coils) increases also. (So, apart from its own capacitance, by coupling the primary coil is loaded by an additional capacitance even if the secondary coil is open.) Since XC and XL point in opposite directions, they compensate each other partially; therefore I would expect to see lower inductance with increasing number of turns, and generally lower inductance at higher frequencies. (If your LCR meter would provide high enough frequencies, you might even see a pure R with no inductance at all if you tune exactly into resonance.)
Because your circuit sees the same effect, there is no "true" inductance. Even if you devise a model containing R, L, and C which behaves like your transformer, and call that L the true inductance, your circuit does not interact with an isolated L of this magnitude but with the whole model.
Another cause of different results at different frequencies is the material of the core of the transformer. Usually, the permeability (and hence the inductance) decreases with increasing frequency while the loss increases.
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