LTspice: Simple Steps for Simulating Transformers

Here is the simple approach to simulate a transformer in LTspice IV:

  1. Draft an inductor for each transformer winding
  2. Couple them using a single mutual inductance (K) statement via a SPICE directive:

    K1 L1 L2 L3 1

    The last entry in the K statement is the coupling coefficient, which can vary between 0 and 1, where 1 represents no leakage inductance. For practical circuits, it is recommended you start with a coupling coefficient of 1.

    Only a single K statement is needed per transformer; LTspice applies a single coupling coefficient to all inductors within a transformer. The following is an equivalent to the statement above:

    K1 L1 L2 1
    K2 L2 L3 1
    K3 L1 L3 1

  3. Adjust the inductor positions to match the transformer polarity by using move (F7), rotate (Ctrl + R) and mirror (Ctrl + E) commands. Adding the K statement displays the phasing dot of the included inductors.
  4. LTspice simulates the transformer using individual component values, in this case, the inductance of the individual inductors, not the turns ratio of the transformer. The inductance ratio corresponds to the turns ratio as follows:

    Inductance & Turns Ratio

    Inductance to Turns Ratios

    For example, for a 1:3 and 1:2 turns ratios, enter inductance values to produce a one to nine and one to four ratios:


    K Statements Example

    Example of a 1:3 and 1:2 Turns Ratios

For more information on how to simulate a transformer you can view the Using Transformers video

Author

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Gabino Alonso