# LTspice How to model coupled inductors in SMPS ?

#### DarthVolta

Joined Jan 27, 2015
499
I'm pretty new to LTspice, I made a main's-transformer once and using the k command or whatever(real new).

I have taken apart a 335W computer SMPS, it's a forward converter configuration I do believe. With an LCR meter I have the Ls, Rs, Rp, Cp for all the magnetic parts. I have no idea about the cores of any of them, but they must be pretty standard, off the shelf parts as far as these go.

I figured out the turns ratios, and rough currents I think

So how do I make simple models of coupled inductors/"transformer" , and not make a regular transformer ? Right now I have made almost the whole schematic in LTspice, but I'm adding various voltages instead of making them.

I'll watch this, but I doubt I'll remember

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#### DickCappels

Joined Aug 21, 2008
6,532
From the Analog Devices website:
https://www.analog.com/en/technical-articles/ltspice-basic-steps-for-simulating-transformers.html

spice: Simple Steps for Simulating Transformers
by Gabino Alonso

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:

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:

Inductance to Turns Ratios

https://www.analog.com/en/education/education-library/videos/5579254291001.html

#### DarthVolta

Joined Jan 27, 2015
499
Ok I have the inductors and correct phase dots in the schematic, but what info is needed to act like a coupled inductor that stores energy until released, rather than a transformer that transfers the energy in a magnetic circuit.

#### DickCappels

Joined Aug 21, 2008
6,532
It already works that way; it is an inductor. Or maybe I don't understand your question -is there anything special you want to happen?