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This paper presents a stability study of a voltage-mode buck regulator using system poles approach. Under different input and output conditions the different operating points are first found individually. Sensitivity matrices of these operating points are studied in order to estimate whether the points are stable or not. When system poles (i.e., eigenvalues of the sensitivity matrix) go into an unstable region, oscillating or bifurcation would occur. Hence, the system behavior of the voltage-mode buck regulator under a variation of input and output conditions is demonstrated. This paper also investigates the stability caused by the feedback capacitor of the buck regulator through this system poles approach. Making use of the system poles movement, stable operation can be extended and explained systematically.