Load invariant buck converter - analysis and implementation

Abstract

Direct-load-current feedback in a peak-current-mode-controlled buck converter is used to obtain near zero open-loop output impedance. Theoretical formulation is derived utilizing two-port modeling technique. The ideal unity-feedback output impedance would resemble the output impedance of a voltage-mode-controlled buck converter but without the resonant behavior. The maximum output impedance would be the equivalent series resistances of the inductor and capacitor branches. It will be shown that the output impedance would approach the magnitude of the output impedance of the original converter, i.e., the load invariance would be gradually lost, if the equivalent current sensing resistors differ from each other sufficiently. In addition, the output impedance would resemble the negative incremental resistor at low frequencies, and may cause excess peaking during the output load transients.