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Confusing behavior in LCR/stepup transformer circuit - Help?
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me_engineer:
I have a stepup transformer driving a test load (46nF + 16 ohms), with a 1:10 turns ratio. The idea is that near resonance (~54kHz here) it will draw minimal current/power.

I've measured the transformer using an LCR meter (at the frequency range of interest) and with parasitics (leakage inductance), have the following circuit:


(I measured leakage by shorting one side of the transformer, and measuring inductance of the other, and vice versa; is that right?)

I'm sensing current using a small resistor on the ground side. (0.05ohm)

This all makes sense, the simulation shows a dip in current at around 54kHz. I would have expected the output to be flatter, but I guess the leakage inductance has a big effect. The result is here:



Anyway, when I build and measure it (source is an old power amp, not quite flat but OK), I get the following:



The gain looks reasonable, but why isn't current dipping near resonance? It's also trending up with frequency, rather than down.

Clearly something is wrong with my model, or my build, any ideas?  I've double-checked everything I could think to. Waveforms are clean and undistorted.
iMo:
It could be you have to add some parasitics into your simulation - your primary/secondary windings have got certainly a capacitance and the 47nF capacitor is rather an ideal one. Such a capacitor is usually modeled by several R, L, C.

You may get the Spice model for your capacitor as the output of this online app:
http://ksim.kemet.com/
me_engineer:
Thanks.  Interwinding capacitance is supposed to be less than 75pF.  The leakage inductance is (supposedly) included accurately.  The capacitors are film type, so they should be very clean.  What other parasitics should I be looking for?
me_engineer:
I measured the resistor's inductance as 2.5uH, and added that to the load (series).  I also added 75pF of capacitance between the primary and secondary (worst case).  The simulation had no noticeable change.

Maybe I'm getting my leakage wrong?  I'm pretty sure I did it right but I'm a little new to this.
iMo:
See above the capacitor model. Yours parasitic values may differ (depend on the capacitor type).
The coil's windings have the capacity in parallel to its L.
Do measure the self resonance freqs of the primary and secondary and you will get Cprimary and Csecondary.
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