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The nonlinearity due to the switching action in pulse-width-modulated (PWM) DC-to-DC converters, DC-to-AC inverters, or amplifiers and input-current-shaping AC-to-DC converters can often conveniently be confined to three-terminal structure referred to as the PWM switch. The PWM switch represents a static nonlinearity for which circuit models can easily be derived for frequencies harmonically related to the frequency of perturbation. Converter analysis can thus be approached in a way analogous to ordinary transistor circuit analysis whereby the nonlinear three-terminal device is replaced by its circuit model. A first-order approximation of the model results in the small-signal model.>
Tymerski et al. (1989) studied this question.