Converter Models

Depending on the analysis to be carried on, several choices are offered to the designer.   Small-Signal Model (SSM)- It assumes that the variations of concern (e.g. output or input voltages) are small around a steady-state DC operating point. In this case, second-order AC cross-products can be neglected and the model is linear around its operating point. The SSM is then usually employed for harmonic simulations where the AC transfer functions are of interest. Do not use a SSM to simulate a 100% transient load span, the result would be wrong since AC cross-products could no longer be negligible. Some SSM models can find their DC point alone, some not and the operating point must then be fed by the designer   Vincent BELLO has been the first in the 80’s to port MIDDLEBROOK’s non-linear state-space average models to the SPICE domain [1]. In BELLO’s models, the previous AC terms were no longer neglected but rather dynamically multiplied by some POLY SPICE2 statements. Large-signal variations could then be simulated and allowed the user to visualize the effects of a 0 to 100% duty-cycle sweep. These large-signal models (LSM) are best suited for TRANSIENT runs.   State-space averaging (SSA) method PWM switch model