Converter Models

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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