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I'm currently working on a project were I need to cascade several transformers (the core we use does not nearly work as good as it ought to..). I'm using the equivalent circuit shown in (A), transformed it into an ideal transformer and equivalent circuit (B) by dragging over the circuit on the secondary side, where $$\sigma_2 L_2 \rightarrow \sigma_2 L_2 N_1^2\\R_2\rightarrow R_2 N_1^2\\C_2 \rightarrow \frac{C_2}{N_1^2}$$

This works very well for a single transformer. When I use the same method for 2 cascaded equivalent circuits of transformers (C) to transform them into the e.c. (D)

the inductors, resistors and capacitors from the second e.c. transformer are modified accordingly (from C to D) :

$$\sigma_2 L_2 \rightarrow \sigma_2 L_2 N_1^2\\R_2\rightarrow R_2 N_1^2\\C_2 \rightarrow \frac{C_2}{N_1^2}\\C_3 \rightarrow \frac{C_3}{N_1^2}\\R_3\rightarrow R_3 N_1^2\\ \sigma_3 L_3 \rightarrow \sigma_3 L_3 N_1^2\\ R_{fe} \rightarrow R_{fe} N_1^2\\ L_3 \rightarrow L_3 N_1^2\\ \sigma_4 L_4 N_2^2\rightarrow \sigma_4 L_4 N_1^2 N_2^2\\ \sigma_4 R_4 N_2^2\rightarrow \sigma_4 R_4 N_1^2 N_2^2\\ \frac{C_4}{N_2^2} \rightarrow \frac{C_4}{N_1^2 N_2^2} $$

The LTSpice simulation of (D) works as one would expect it to, the problem is: the LTSpice simulation of (C) does not. It shows a decrease in amplification after the second ideal transformer in instead of an increase.

I suspect my method might be flawed, but I can't really pin it down right now.

Does anyone have an idea what could have gone wrong / where I made a mistake ?

If you can't open the schematic, I made a screenshot, hopefully that helps? http://imgur.com/pLg1v83

Thank you!

schematic

simulate this circuit – Schematic created using CircuitLab

ASC FILES:

https://github.com/mmelchger/transformers

EDIT:

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I'm limited by the number of turns (primary <= 10 turns) and the voltage (voltage through primary inductivity < 0.2V), but require primary inductivities > 270uH with low losses and low saturation between 0.5MHz and 1.5MHz. The transformer should have a ratio of ~ 40, which is why the number of turns on the primary side matters and I'm considering cascading multiple transformers, otherwise I won't have enough space and might run into trouble with capacitive coupling and other effects. I'm using ferrite cores (Al ~ 2700nH/[turn^2] delivers a primary inductivity of 270uH for 10 turns). Ideally the B-H curve should be linear for my use, but that's another question.

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    \$\begingroup\$ The most convenient way for anyone to see what you see is to provide the .asc file \$\endgroup\$ – PlasmaHH Nov 13 '14 at 16:32
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    \$\begingroup\$ In circuit D, did you remember to scale the second transformer's secondary components by n**4? \$\endgroup\$ – Brian Drummond Nov 13 '14 at 19:32
  • \$\begingroup\$ If you click "simulate this circuit" you should be able to see the whole circuit in a higher resolution. I scaled the second transformer's components, but I might have done it wrong. \$\endgroup\$ – mmoment Nov 14 '14 at 9:49
  • \$\begingroup\$ I added the ASC files to my original post. \$\endgroup\$ – mmoment Nov 14 '14 at 10:05
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    \$\begingroup\$ What are you trying to achieve? What did you find that was problematic with cascading the transformers? Why do you want to cascade transformers? What bandwidth are you talking about? \$\endgroup\$ – Andy aka Nov 28 '14 at 9:48
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There are several issues here.

  1. Simulation (C) is failing not because of a mistake in your circuit, but due to a limitation in LTSpice itself. The secondary inductance of your transformer windings is too high! Reduce primary and secondary inductances by a factor of at least 10000 (eg. to 100 and 10000 Henries) and then the results should come out correct.

  2. Your formulas are correct but your calculations for circuit (D) are wrong. From secondary to primary the transformers are stepping down 10:1, so N is 0.1 not 10. Therefore (for example) L3 should be 2.7uH, not 27000uH.

  3. You are driving the circuit with a current generator, so input voltage is directly proportional to impedance. Is that what you really wanted? If not then perhaps you should be using a voltage generator with an appropriate series resistance (eg. 50 Ohms).

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