The article A simple Simsmith model for exploration of a common EFHW transformer design – 2t:14t proposed a simple model.

## The previous proposal

Above is the equivalent circuit used to model the transformer. The transformer is replaced with an ideal 1:n transformer, and all secondary side values are referred to the primary side.

The model works quite well for low leakage inductance / low ratio transformers but falls down for the higher leakage inductance / higher ratio transformers.

## An improved model

The improved model is similar, but Cse in the model above is distributed to the outer sides of the lumped constant model.

Above is the equivalent circuit used to model the transformer. The transformer is replaced with an ideal 1:n transformer, and all secondary side values are referred to the primary side.

- Secondary side leakage inductance Lls is divided by n^2 to obtain the value primary referred leakage inductance in the circuit diagram.
- Cse is an equivalent shunt capacitance to partially model self resonance effects, it is distributed in the ratio 1:n^2 to both sides of the network.
- Bm is the magnetising susceptance (calculated from other parameters).
- Gm is the magnetising conductance (calculated from other parameters).
- Llp is the primary side leakage inductance.
- Ccomp is a compensation capacitance.

A Simsmith model was built to implement the transformer model above, and a prototype transformer measured.

Above is the prototype transformer, 2:14t on a Fair-rite 2643251002 core.

- Complex core permeability is captured from a permeability data file.
- np is the number of turns on the primary.
- ratio is the turns ratio.
- cores is the number of cores in a stack.
- cse is Cse per the circuit diagram.
- Ll is the value of Llp and Lls’ (which are assumed equal).

The chart above compares the model on the left with measured impedance from a .s1p file. The model is calibrated by adjusting Ll and cse for best fit of calculated VSWR to the measured VSWR.

## References

Sample Simsmith model for download: EFHW-2643251002-43-2020-2-14-40-v103.7z. (Compressed with 7zip.)