Capacitor current fixed off-time control for buck converter with fast response and output capacitor ESR independence

Fixed off-time (FOT) control technique has been widely used in various industry applications due to its fast transient response. However, when output capacitor with small equivalent series resistance (ESR) is used, output capacitor voltage phase lagging behind inductor current phase can cause instability in FOT controlled buck converter. Although using large output capacitor ESR can ensure stable operation, which will cause large output voltage ripple. To eliminate the instability in FOT control switching dc-dc converter with small output capacitor ESR, a capacitor current FOT (CC-FOT) control is proposed in this paper. The operational principle of CC-FOT controlled buck converter is elaborated. By the describing function method, the control-to-output voltage transfer function of CC-FOT controlled buck converter is derived and its stability is studied. The study results indicated that CC-FOT control has a better control performance than that of FOT control, and its stability is independent of output capacitor ESR, which are verified by PSIM simulation results.