Unified Digital Current Mode Control Tuning With Near Optimal Recovery in a CCM Buck Converter

Abstract

Linear small-signal models of a dc–dc converter often ignore switching dynamics; thus such models are insufficient to fully explore the performance objective under large signal transients. Linear/nonlinear hybrid controllers are promising alternatives; however, they require structurally different hardware resources along with extra antiwindup arrangements. This paper proposes a geometric tuning method in a digitally current-mode-controlled buck converter under continuous-conduction mode. This considers a proportional-integral voltage controller along with a load current feedforward in the digital domain, while the inductor current has a traditional analog implementation. This resembles a first-order switching surface with near load-invariant regulation; thus a (fixed) small integral gain is sufficient to minimize the steady-state error. Using phase-plane geometry, the objective is to tune the controller gain in a way to achieve proximate time optimal recovery using a fixed-frequency pulse-width modulator and also to retain the large-signal stability. The effects of parameter variation and finite sampling are analyzed. The proposed tuning is implemented using an FPGA device.