Fast-Response Variable-Frequency Series-Capacitor Buck VRM Through Integrated Control Approaches

TL;DR

This paper presents an integrated control scheme for variable-frequency Series-Capacitor Buck VRMs, achieving rapid voltage regulation suitable for AI workloads by combining linear and nonlinear controllers with advanced modeling and control strategies.

Contribution

It introduces a novel integrated control approach combining small-signal modeling, time-optimal control, and transition logic for fast voltage regulation in VRMs.

Findings

Achieves over ten times faster voltage recovery than linear control alone.

Demonstrates stable disturbance rejection with zero steady-state error.

Provides a comprehensive modeling framework for digital control design.

Abstract

Fast-response voltage regulation is essential for data-center Voltage Regulation Modules (VRMs) powering Artificial Intelligence (AI) workloads, which exhibit both small-amplitude fluctuations and abrupt full-load steps. This paper introduces a control scheme that integrates a linear controller and a nonlinear controller for variable-frequency Series-Capacitor Buck (SCB) converters. First, an accurate small-signal model is derived via a Switching-Synchronized Sampled State-Space (5S) framework, yielding discrete-time transfer functions and root-locus insights for direct digital design. A critical concern for SCB converters is series-capacitor oscillation during heavy load steps if the strict switching sequence is not maintained. To accelerate large-signal transients, a time-optimal control strategy based on Pontryagins Maximum Principle (PMP) relaxes the switching constraints to compute time-optimal switching sequences. A transition logic is then proposed to integrate the high-bandwidth small-signal controller and the large-signal controller. Simulations demonstrate a rapid output voltage recovery under a heavy load step-up, over ten times faster than a linear controller-only design. Preliminary hardware tests indicate a stable rejection to heavy load disturbances with zero steady-state error.