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Can you help me on how to model or calculate the inductance of a step down transformer which supplies 230V to produce 0V-15V in LTspice I'm really confused.

Below is my circuit which I want to make the transformer for, I've already built it in practical and its working but in LTspice its giving me headaches.

Thank you.

Below is my circuit which i want to make the transformer for , I've already built it in practical and its working but in LTspice its giving me headaches.

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  • \$\begingroup\$ It's rarely a parameter you need to worry about. Are you about to wind your own transformer? If so, you only need to think about saturation and thermal, where inductance will follow. If you just want to get going for a simulation, assume a high primary value in the Henry range and adjust your secondary to give a suitable ratio. \$\endgroup\$ – winny May 2 '17 at 7:54
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The only significant figure for simulating that sort of power supply is the transformer leakage inductance, which will range from negligible to very negligible.

You should assume a figure of 0, which will mean that the peaks of inductor current will be limited only by the winding resistance of the transformer (much easier to get a real figure for that, measure with a meter, then double it for the effect of the primary as well), and the residual resistance of your bridge rectifier diodes.

As you allow the leakage inductance to rise (try figures of 1uH, 10uH, 100uH etc in your simulation), you will find the peaks of rectifier current become slightly less in amplitude, and slightly longer. Basically good things. But you need to design for zero, which is worst case for current peaks.

The transformer has other defined inductances of course, primary and secondary inductance. However, the primary inductance only defines the magnetising current the transformer takes from the supply. The secondary inductance is short-circuited by the primary and the supply connection, and so does not appear in the output. To the extent that the secondary inductance is not 100% short-circuited by the primary, that's the little bit of leakage inductance left over!

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Hopefully this answers all of your questions. It's not perfect, but I tweaked a few things. Click for full-size image. The plot is current into a dummy load of 120/12/6/4/3/2.4Ω. It should give 0.1/1/2/3/4/5A but there is some error.

Note that you need to consider the power dissipation limits of the devices. Q2 for instance might be limited to 5W, and drawing 5A from this supply will push it way over this rating. The addition of current limiting would be a good idea.

LTSpice simulation of linear PSU including transformer.

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