Average current-mode control of buck dc-dc converter with reduced control voltage ripple

This paper proposes a technique to track and regulate the “true average” current in any branch of a pulse-width modulated dc-dc power converter. An example buck converter in continuous-conduction mode is considered. A comprehensive characterization is presented for the proposed scheme. An overall dc and small-signal analysis of the inner current loop is performed. The current average current-mode control methods suffer from switching instability at low duty ratios due to a large ripple in the sensed current. Moreover, the current error amplifier's high-frequency pole located at the switching frequency neither alleviates the stability issue nor improves the dynamic response of the converter. In the proposed approach, a low-pass filter, which attenuates any high-frequency ripple is placed in the feedback path of the current loop. Consequently, the control voltage is nearly dc and proportional to the actual average value of the inductor current. The inner current loop gain transfer function of the presented circuit has been derived. The following critical path closed-loop transfer functions have been derived: reference voltage-to-inductor current and reference voltage-to-output voltage. Using practical specifications of a buck dc-dc converter, a theoretical framework to design the inner loop is presented. Verification of the theoretically predicted transfer functions and transient analysis is performed through simulations.