Ultrafast Transient Response in 48 V Automotive VRMs: An Auxiliary-Assisted Adaptive Slew-Rate Control Scheme

This article presents an auxiliary-assisted hybrid dc-dc converter for 48 V automotive voltage regulator modules (VRMs) that enable adaptive inductor-current slew-rate control for ultrafast transient response. The main stage, a 4:1 dual-inductor hybrid (DIH) dc-dc converter, delivers the dc load power, while the output voltage is regulated by a GaN-based auxiliary-buck stage that is accoupled by a buffer capacitor. The main stage regulates the output of the ac-coupled auxiliary stage using an average-current-mode-control (ACMC) scheme to achieve adaptive control of the auxiliary-inductor-current slew rate. The auxiliary ac-coupled buck (ACB) converter regulates the output voltage based on an output-capacitor current-based hysteretic-current-mode-control (HCMC) scheme. An adaptive-voltage-positioning (AVP) scheme is proposed for the auxiliary capacitor, which preemptively positions the ACB output voltage for improved transient response. A small-signal model of the auxiliary-assisted converter is presented for stability analysis and validated with simulation results. A 40 W proof-of-concept prototype was fabricated to demonstrate the feasibility of the adaptive slew-rate control technique and AVP scheme. The prototype achieves a peak efficiency of 90.6% with an output capacitance of 500 μF and an auxiliary capacitance of 22 μF, while maintaining the output voltage deviation within 50 mV.