Critical Parameters in the Transient Response of Synchronous Buck Converters

The transient behavior of synchronous buck converters, using critical parameters, is investigated in this work under high current slew-rate load transients. Critical parameters are defined as the combination of parameters for which the peak transient response is dominated by the effect of the capacitor Equivalent Series Resistance (ESR). Given the importance of the effect of ESR in low-voltage, high- current synchronous buck converters, an effective normalization method is developed in this paper to reduce the dependence on other parameters, thus providing valuable generality to the transient analysis. A set of critical normalized expressions valid for any possible synchronous buck converter is derived, which can be used as a useful design tool to meet transient specifications. By using critical parameters, both transient recovery time and peak voltage regulation can be met while keeping L and C to a minimum. A detailed characterization of the effect of ESR is presented, and the normalized approach clarifies the role of ESR in the transient response. A critical design example is presented where the parameters of the converter are obtained by performing a simple denormalization. Finally, the critically designed converter is evaluated using boundary controllers as well as classical current- and voltage-mode controllers to assess the influence of the control scheme on the transient response of the converter.