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System poles approach for pulsewidth-modulated (PWM) dc/dc converters analysis is developed making use of the sensitivity matrix. It combines both small-signal analysis for stability assessment and large-signal analysis for wide operating condition. This paper is particularly focused in stability study on the output capacitor capacitance and equivalent series resistance (ESR) in a buck regulator using the system poles approach. Using the system-poles approach in the output capacitance example it is found that unstable operating point possibly exists between two stable operating points for some conditions. Tends of stable-to-unstable or unstable-to-stable are observed by the z-domain poles movement with different changing circuit parameters. Making use of these tends, it is exemplified that the unstable operating point can be stabilized. This paper shows that output capacitance and ESR are critical to the stability and stability can be improved by modification on the output capacitor.