Based on the sequence of circuit topologies, a symbolic sequence method is proposed for analyzing the bifurcations and chaos in a class of piecewise smooth systems like DC/DC converters. The largest subsequence(LS) is used to distinguish the type of bifurcation and detect border collision bifurcation e. g., when a perioddoubling bifurcation occurs, the LS is unchanged; when border collision bifurcation occurs, the LS is changed; and when chaos occurs,the LS does not exist.It is shown that the duty ratio is an essential variable for describing the nonlinear dynamics of a class of piecewise smooth systems like DC/DC converters， and saturating nonlinearity is the root cause of border collision bifurcation.