Methods to Estimate Load-Transient Response of Buck Converter under Direct-Duty-Ratio and Peak-Current-Mode Control
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This paper provides a simple and effective method to model the closed-loop output impedance of a buck converter under direct-duty-ratio (DDR) and peak-current-mode (PCM) control as well as under PCM control with output-current-feedforward (OCF), which can be used to study analytically the time-domain response of the output voltage to a change in the load current. The method is based on the modified small-signal models of the open-loop output impedance and the classical control-engineering method to model the closed-loop dynamics of a system by using the crossover frequency and phase margin of the feedback control loop. The method is validated by using an experimental buck converter as well as simulations. The derived time-domain load-response functions can be effectively used to study in detail the factors determining the transient process of the response. According to the time-domain functions, it can be observed that the initial phase of the transient is determined by the feedback-loop design, and the settling process is determined by the lowest-frequency zeros of the controller together with the open-loop zeros of the output impedance.