PWL Transformer Model

Transformers for switching power converters are typically constructed with discrete circuit components in SIMPLIS. The saturation effects of the transformer are modeled with a PWL inductor, and the leakage inductance parasitics are typically lumped into a single leakage inductor. It is important to note that this leakage inductor must not be treated as an ideal inductor or possible slow simulation results will occur. Instead, a "Lossy Inductor" is used which has a resistor in parallel with the inductance. The resistor limits the smallest time constant present in the circuit by limiting the L/R time constant. The winding resistances are modeled with one resistor per winding.

The following example is taken from the self oscillating converter.

Example files:

The PWL transformer model contains the following:

  • L4: PWL Inductor for primary magnetizing inductance - includes saturation effects
  • RW1: Primary winding resistance
  • Lleak: Primary leakage inductance
  • S1: Ideal DC Transformer (infinite magnetizing inductance)
  • RW2: Secondary winding resistance
  • RW3: Secondary winding resistance

L4 is based on the SIMPLIS primitive PWL inductor, which has the points defined on a X-Y plane.

An example circuit which uses this transformer can be downloaded here:

To run the simulation for this saturable inductor, follow these steps:

  1. Download the schematic zip file.
  2. Unzip and open the schematic.
  3. Run the simulation by pressing F9.
  4. Confirm the expected saturation at about 400mA.