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| Confusing behavior in LCR/stepup transformer circuit - Help? |
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| me_engineer:
Quote from: The Electrician on Yesterday at 11:35:44 pm Can you post the transformer's spec sheet (or a link to it)? Is it spec'd as an audio transformer? Measure and plot the impedance at the primary with the load connected over the frequency range 10 kHz to several hundred kHz. --- End quote --- Thank you. I see now this is the correct approach and is very helpful. I should have done this from the start. Published Specs: (@100kHz) Sec Inductance: 156-173uH Leakage (sec) 70uH max IWC: 75pF max Turns ratio: 1:10 Measurement and simulation: These seem quite close, at least qualitatively (I did reduce IWC for the above plot for a better fit) The big discrepancy must be the amplifier's output impedance, having a nontrivial reactive component. This cleanly confirms it I think. |
| The Electrician:
Does the spec indicate a recommended working impedance for the windings? Something like 100:10k ohms? Is this transformer wound on a ferrite core? |
| me_engineer:
--- Quote from: The Electrician on May 31, 2019, 07:41:03 pm ---Does the spec indicate a recommended working impedance for the windings? Something like 100:10k ohms? Is this transformer wound on a ferrite core? --- End quote --- Nope, it only specified what's above (and some maximum resistances). The values I used in the simulation were ones I measured myself though. (I'm still a little uncertain if I did leakage correctly). It is indeed wound on a ferrite core (over a bobbin). |
| me_engineer:
--- Quote from: MagicSmoker on May 31, 2019, 04:39:06 pm ---Hmmm... what you have drawn is a series resonant LCR network, hence you should see a maximum in current at the resonant frequency. Also, the preferred way to represent leakage inductance in LTSpice is by reducing the coupling coefficient, K, from 1 (leakage fraction is then equal to 1-K^2). The simulation will sometimes run faster if you model leakage as an external inductor, as you have done, but then you should only put it on one winding as it will be correctly reflected to the other windings by K=1. Thirdly, inductance scales with the square of the turns ratio, so the secondary inductance should be 250uH, not 168uH. Finally, yes, the practical way to measure leakage is to short out the primary (secondary) and measure the inductance across the secondary (primary), assuming there are just two windings; if there are more than two then technically you need to short out all of them except for one to measure the leakage, but unless the leakage is really high then just shorting any winding with a high VA rating (e.g. - the primary or the main secondary) will suffice. --- End quote --- Thanks. I prefer having discrete L's to show leakage, as long as it's correct. Why wouldn't there be leakage on both coils separately? The values in my simulation are all measured using the methods I described; the 168uH is correct. Perhaps the difference is from leakage? |
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