The modelling of a multirate system-composed of components with heterogeneous time constants-can be done using fixed-point method. This method allows a time-discretization of each subsystem with respect to its own time constant. In an optimisation process, executing the loop of the fixed-point at each model evaluation can be time consuming. By adding one of the searched waveform of the system to the optimisation variables, the loop can be avoided. This strategy is applied to the optimisation of a transformer