Performance and Stability Analysis of a Multiphase Buck Converter under Mixed-Signal Current Mode Control for Mobile and Automotive Applications

Abstract

Multiphase buck converters find widespread ap-plications in automotive, mobile phones, and other portable devices for energy-efficient power management for high computational processors, in which fixed-frequency current mode control (CMC) is frequently used in many commercial products. In this context, a mixed-signal CMC (MCMC) architecture remains an attractive solution with fast changing (phase) current feedback loops in the analog domain for current balancing and voltage controller in the digital domain for real-time controller tuning. In existing MCMC, the output voltage is sampled once per switching cycle clock using a uniform sampling clock. This paper shows that a higher proportional (voltage) controller gain while achieving fast transient performance leads to fast-scale instability with duty ratio saturation, resulting in much higher ripple parameters and RMS current with higher conduction losses. Further, an event-based (voltage) sampling technique is proposed, which significantly enhances the stability boundary and achieves a fast transient response. With the DAC output, programmable voltage biasing is considered to generate customized current references for individual phases for asymmetric phase current balancing reducing conduction loss in the case of widely varying per-phase DC equivalent resistance. A discrete-time PI voltage controller is considered, and a comparative study of fast-scale stability and transient performance is carried out using uniform and event-based voltage-loop sampling methods. The improved performance using the latter is demonstrated using simulation results. A four-phase buck converter prototype is fabricated, and tested and experimental results are presented.